Comparative DFT Study of K₂AgSbBr₆ and K₂NaScBr₆: Exploring the Role of B^′B^″ Cation Substitution on Material Properties

Abstract

The effects of cation substitution are the main emphasis of this investigation into the structural, mechanical, electronic, and optical properties of double perovskites K₂AgSbBr₆ and K₂NaScBr₆. Outwardly favorable tolerance and octahedral factors and negative formation energy confirmed structural stability and thermodynamic feasibility. Mechanical analysis showed that K₂AgSbBr₆ possesses greater volumetric stability and rigidity, while K₂NaScBr₆ exhibits greater ductility and isotropic characteristics. The electronic properties determined based on density functional theory (DFT) calculations indicate that K₂AgSbBr₆ has an indirect bandgap of 0.857 eV, making it suitable for applications using visible light, and K₂NaScBr₆ has a direct bandgap of 3.107 eV, making it ideal for UV-specific technologies. Optical analyses demonstrate complementary characteristics, particularly in terms of the dielectric function, absorption, reflectivity, energy loss function, refractive index, extinction coefficient, and optical conductivity. K₂AgSbBr₆ exhibits strong visible light absorptivity.

1. Introduction

The recently emerged organic–inorganic hybrid halide perovskites ($APb{X}_3$), more specifically ${CH}_3{NH}_3{PbI}_3$, have attracted immense attention in optoelectronics and photonics due to their unique properties such as an optimal optical bandgap, high photoluminescence quantum yield, large absorption coefficient, long carrier diffusion length, good carrier mobility, defect tolerance, and low-cost synthesis options [1,2]. The strides we have made since their emergence have been extraordinary. Photovoltaic devices using lead halide perovskite materials improved power conversion efficiency from $3.8\mathrm{\%}$ to $25.5\mathrm{\%}$. These improvements have excited scientists everywhere, and materials with these properties seem to be promising candidates for alternatives to silicon-based solar cells [3].

In-depth experimental and theoretical studies have highlighted certain exceptional structural and electronic properties that are behind the remarkable photovoltaic characteristics of ${CH}_3{NH}_3{PbI}_3$. First and foremost, the high symmetry of the perovskite structure serves as a basis for its functionalities, and its defect-tolerant behavior maximizes efficiency. Moreover, the $Pb$-$6s$ and $I$-$5p$ orbitals have strong interactions at the edge of the valence band, leading to an antibonding configuration, which, overall, contributes to the remarkable photovoltaic properties of ${CH}_3{NH}_3{PbI}_3$ [4]. In quantum physics, the intrinsic thermodynamic instability of ${CH}_3{NH}_3{PbI}_3$ may result from the weak chemical bonding of the organic cation ${CH}_3{{NH}_3}^{+}$ and its intrinsic instability [5]. Organic constituents and the toxic properties of lead (${Pb}^{2+}$), both relevant properties of the halide-based organic–inorganic hybrid perovskites that play a pivotal role in stability issues, present important challenges for chemical stability. While having remarkable properties such as low-cost solution processing, high defect tolerance, and tunability in light emission across visible wavelengths, ${CH}_3{NH}_3{PbI}_3$-based perovskites are limited in some critical aspects [6,7]. These involve susceptibility to extended exposure to light, sensitivity to humidity, thermal instability under environmental conditions, and the toxicological effects of the ${Pb}^{2+}$ ions that can bioaccumulate within ecosystems. Moreover, the issues presented by the properties can potentially be amplified by restrictions from the Restriction of Hazardous Substances ($RoHS$) directive, greatly limiting their use [8,9,10].

Tackling stability issues has become a part of the research agenda, building on partial success in replacing the organic cation with an inorganic one, potassium. With this step, researchers want to examine the potential of alternative metal halide perovskites, which deliver optoelectronic performance similar to that of the original structure, low toxicity, and enhanced structural stability. Recent studies have uncovered a new type of lead-free halide double perovskite ($HDP$) that has generated a lot of attention. This nanomaterial has shown impressive properties, including excellent environmental stressor resistance, intrinsic thermodynamic stability, and low effective mass for charge carriers; these double perovskites appear to be excellent, non-toxic candidates for usage as photovoltaic absorbers as a substitute for $APb{X}_3$ in halide perovskite solar cells [11,12]. $HDPs$ are also being investigated for assorted optoelectronic applications, including $X-ray$ sensing, thermally stable scintillators for nuclear monitoring, broad-spectrum white phosphors, and $UV$ detection devices [13,14].

Although $HDP$ compounds have been synthesized and studied since the 1970s, earlier methods were limited. Clearly, the synthesis of Pb-free $HDPs$, which can be written with the chemical formula $A2B\prime B″Cl6$, where (one of each) $B^{\prime 1+}$ and $B^{″3+}$ replace two toxic Pb²⁺ ions, is now worthy of considerable attention. This research aims to develop non-toxic perovskite structures, paving the way for eco-friendly and non-toxic material solutions. $HDPs$ are broadly categorized into two types based on the presence or absence of lone-pair electrons in the B^′ cations. Type $I$ $HDPs$ are characterized by $B^{\prime }$ cations that lack lone-pair s-electrons ($s^0$), while Type $II$ $HDPs$ involve $B^{\prime }$ cations with lone-pair $s$-electrons ($s^2$). These lead-free $HDPs$ also exhibit significant potential for ecological sustainability [3].

Recent reviews have provided an extensive assessment of research on a series of $CAIC$ and $CASC$, which have many uses [15,16]. Further, these materials exhibit optoelectronic properties similar to those of lead-based organic–inorganic $HDPs$, thus increasing their utility for various applications [17,18,19,20,21].

In a previous work, we examined the structural and optoelectronic properties of $K_{2}AgSb{Br}_6$ [22], and in this research we will provide a comparative study between $K_{2}AgSb{Br}_6$ and the most recently proposed double perovskite $K_{2}NaSc{Br}_6$. We propose $K_{2}NaSc{Br}_6$ as a new candidate for UV transparent and lead-free optoelectronics. This general idea was previously considered in terms of $K_{2}AgSb{Br}_6$ but never with an in-depth examination of replacing the B^′B^″ site with Na⁺/Sc³⁺. We will primarily examine what the effects of cation substitution at the $B\mathrm{ʹ}B″$ sites ($B\mathrm{ʹ}B″=AgSb,NaSc$) are on the structural, optoelectronic, and mechanical properties of these compounds. We center on the first-principles density functional theory ($DFT$) methodology to fully understand the structural characteristics, electronic conductivity, optical properties, elastic constants, and thermodynamic stability of these double perovskite candidates.

This thorough theoretical study utilizes $DFT$ based on $GGA-PBE$ to expose the essential relationships between the electronic properties and optical properties, as well as additional mechanical parameters like elastic moduli, Poisson’s ratio, the anisotropy required to characterize these materials, hardness, mean sound velocity, and the Debye temperature of $K_{2}AgSb{Br}_6$ and $K_{2}NaSc{Br}_6$.

2. Results and Discussions

2.1. Structural Properties and Stability

The structural properties of K₂AgSbBr₆ and K₂NaScBr₆ were meticulously optimized in the cubic phase, adopting the space group $Fm-3m$ (no. 225), using the $PBE-GGA$ functional. The computational method ramps up the dependability of our prediction on stability and lattice parameters, which facilitate an understanding of the structural considerations in these double perovskites [22,23].

Both K₂AgSbBr₆ and K₂NaScBr₆ crystallize in a face-centered cubic structure with the cations at B^′ and B^″ showing some order at their sites. The B^′ and B^″ sites are occupied by Ag and Sb in K₂AgSbBr₆ and by Na and Sc in K₂NaScBr₆, contributing to unique structural forms. Replacing AgSb with NaSc should dramatically change the lattice geometry, and while the geometry may only be subtly altered, it still changes the crystal structure. The optimized lattice parameters shown in

Table 1. Selected bond distances of K₂B^′B^″Br₆ (B^′B^″ = AgSb, NaSc) double perovskite.

Compound K2B′B″Br6	$\mathit{r}({\mathit{B}}^{\mathbf{″}}–\mathit{B}\mathit{r})/\mathsf{\AA }$	$\mathit{r}({\mathit{B}}^{\mathbf{\prime }}–\mathit{B}\mathit{r})/\mathsf{\AA }$	$\mathit{r}(\mathit{K}–\mathit{B}\mathit{r})/\mathsf{\AA }$
K2AgSbBr6	2.81	2.82	3.98
K2NaScBr6	2.65	2.89	3.92

facilitate the structural comparisons associated with this cation substitution [24].

Figure 1 illustrates the $3D$ structure of K₂AgSbBr₆ and K₂NaScBr₆ and their complex arrangements of alternating corner-sharing octahedra which have B^′Br₆⁻⁵ and B^″Br₆⁻³ units, where the $B^{\prime }$ and $B^{″}$ cations create local electronic and bonding environments. The octahedra connect to form a strong $3D$ framework, with $K$ cations occupying the $A$ sites within cuboctahedral voids [22,23].

The bond lengths present in the octahedra are displayed in Table 1, and they mirror those of the bond lengths observed in the two compounds and may represent slight structural deviations in both compounds. Such differences underline the critical role of ${\mathrm{B}}^{\prime }{\mathrm{B}}^{″}$ site substitution in shaping the material’s geometry and stability.

The lattice parameters and atomic positions of K₂AgSbBr₆ and K₂NaScBr₆, detailed in

Table 2. Selected geometrical (lattice, volumetric, density….) properties of K₂B^′B^″Br₆ (B^′B^″ = AgSb, NaSc) double perovskite.

	K2AgSbBr6	K2NaScBr6
$\mathit{a}=\mathit{b}=\mathit{c}$	11.26 Å	$11.09 \mathrm{\AA }$
$\mathit{\alpha }=\mathit{\beta }=\mathit{\gamma }$	90.00 º	$90.00 \mathrm{º}$
$\mathit{V}\mathit{o}\mathit{l}\mathit{u}\mathit{m}\mathit{e}$	$1426.65 {\mathrm{\AA }}^3$	$1363.43 {\mathrm{\AA }}^3$
$\mathit{N}\mathit{u}\mathit{m}\mathit{b}\mathit{e}\mathit{r} \mathit{o}\mathit{f} \mathit{A}\mathit{t}\mathit{o}\mathit{m}\mathit{s}$	40	$40$
$\mathit{D}\mathit{e}\mathit{n}\mathit{s}\mathit{i}\mathit{t}\mathit{y}$	$3.67 \mathrm{g}·\mathrm{c}{\mathrm{m}}^{-3}$	$3.05 \mathrm{g}·\mathrm{c}{\mathrm{m}}^{-3}$
$\mathit{D}\mathit{i}\mathit{m}\mathit{e}\mathit{n}\mathit{s}\mathit{i}\mathit{o}\mathit{n}\mathit{a}\mathit{l}\mathit{i}\mathit{t}\mathit{y}$	$3D$	$3D$
$\mathit{P}\mathit{o}\mathit{s}\mathit{s}\mathit{i}\mathit{b}\mathit{l}\mathit{e} \mathit{O}\mathit{x}\mathit{i}\mathit{d}\mathit{a}\mathit{t}\mathit{i}\mathit{o}\mathit{n} \mathit{S}\mathit{t}\mathit{a}\mathit{t}\mathit{e}\mathit{s}$	Ag+, Br−, Sb3+, K+	Na+, Br−, Sc3+, K+

and

Table 3. Atom positions in the crystal structure of the K₂B^′B^″Br₆ (B^′B^″ = AgSb, NaSc) double perovskite.

$\mathbf{W}\mathbf{y}\mathbf{c}\mathbf{k}\mathbf{o}\mathbf{f}\mathbf{f}$	$\mathbf{E}\mathbf{l}\mathbf{e}\mathbf{m}\mathbf{e}\mathbf{n}\mathbf{t}$	$\mathbf{x}$	$\mathbf{y}$	$\mathbf{z}$
4a	Sb, Sc	0	0	0
4b	Ag, Na	1/2	0	0
8c	K	1/4	1/4	3/4
24e	Br 1	0.244754	0	0
24e	Br 2	0.239138	0	0

^1,2 in Br elements means that this element coordinates in the structures of K₂AgSbBr₆ and K₂NaScBr₆, respectively.

, further emphasize the influence of cationic substitution on the overall structural framework.

The perovskite structure of compounds such as $AB{O}_3$ [25], $AM{X}_3$, and $A_2{MM}^{\prime }{X}_6$ [26] is often determined by evaluating the values of the octahedral factor ($\mu$) and the Goldschmidt tolerance factor ($t$), as defined by Equation (1):

$$\mu ={\displaystyle \frac{r_B}{r_{Br}}};t={\displaystyle \frac{r_K+r_{Br}}{\sqrt{2}\left(r_B+r_{Br}\right)}},$$

(1)

Here $r_K$, $r_{{\mathrm{B}}^{″}}$, $r_{{\mathrm{B}}^{\prime }}$, and $r_{Br}$ are the corresponding ionic radii of the ions of the K, B^″ (Sb, Sc), B^′ (Ag, Na), and Br atoms, respectively, and $r_B={\displaystyle \frac{r_{{\mathrm{B}}^{\prime }}+r_{{\mathrm{B}}^{″}}}{2}}$.

In perovskite structures (AMX₃ and A₂MM′X₆), the octahedral factor ($\mu$) and the tolerance factor ($t$) play key roles in assessing structural stability. Stable perovskites typically fall within the ranges of $0.415<\mu <0.895$ and $0.813<t<1.107$. Those with a $t$ value near $1.0$ are more likely to adopt a cubic phase. If these values deviate significantly from these ranges, it can suggest distortion or instability in the perovskite structure [26].

Following the calculation of the tolerance factor ($t$) and octahedral factor ($\mu$) for the two double perovskites, K₂AgSbBr₆ and K₂NaScBr₆, a comparative study of their structural, electronic, and optical properties reveals important insights. The values of $t$ and $\mu$ for the two compounds are presented in

Table 4. $r$ of ions, $\mu$, $t$, and $E_f$ for K₂B^′B^″Br₆ (B^′B^″ = AgSb, NaSc) double perovskite.

Compound K2B′B″Br6	rK/Å	rB′/Å	rB″/Å	rB/Å	rBr/Å	μ	t	Ef (eV/atom)
K2AgSbBr6	1.64	1.15	0.76	0.955	1.96	0.48	0.86	−4.791
K2NaScBr6	1.64	1.02	0.745	0.882	1.96	0.45	0.84	−5.643

. Both materials exhibit structural stability, with tolerance factors of 0.86 for K₂AgSbBr₆ and 0.84 for K₂NaScBr₆, placing them within the stable range for perovskite structures. However, the slightly lower tolerance factor of K₂NaScBr₆ suggests a minor deviation from the ideal cubic phase, potentially leading to more subtle structural distortions.

The octahedral factors are 0.48 for K₂AgSbBr₆ and 0.45 for K₂NaScBr₆, showing that both materials have relatively stable octahedral geometries, but there may be slightly more structural distortion in K₂NaScBr₆ because of the smaller cations being used. Overall, both of these materials are still relatively stable and comparable to the typical ranges of stability for perovskite phases, particularly K₂AgSbBr₆, which is defined as being at least closer to an ideal cubic structure.

Looking at their electrical behavior, both materials seem promising, but existing differences in cationic sizes (cations exhibiting different structural influencing behaviors) may still affect respective electronic functionality and conductivity characteristics. For optical characteristics/filtering applications, both materials should perform actually somewhat similarly to one another based on our observations regarding their feasibility, but K₂AgSbBr₆ should produce more consistent optical behavior when compared to K₂NaScBr₆ due to its relatively more stable structure.

The geometric stability is verified through computed tolerance and octahedral factors; however we recognize that intrinsic point defects such as halide vacancies or B site antisites could have an effect on the synthesis and function of these materials. A defect formation energy analysis in detail would be more than the present study is able to conduct and is left for another study.

Thermodynamic stability was confirmed through the calculation of formation energy ($E_f$) [27], which was determined using the formula provided in [22,28]:

$$E_f=E_{K_2{B}^{\prime }{B}^{″}{Br}_6}-(2E_K+E_{B^{\prime }}+E_{B^{″}}+6E_{Br})$$

(2)

Here, $E_{K_2{B}^{\prime }{\mathrm{B}}^{″}{Br}_6}$ represents the total energy of $K_2{B}^{\prime }{\mathrm{B}}^{″}{Br}_6$ (B^′B^″ = AgSb, NaSc) compounds, while $E_K$, $E_{B^{\prime }}$, $E_{{\mathrm{B}}^{″}}$, and $E_{Br}$ denote the individual energies of the K, Ag, B^′ (Ag, Na), B^″ (Sb, Sc), and Br atoms, respectively. The calculated formation energy ($E_f$) values are negative for all compounds, confirming their thermodynamic stability, as shown in Table 4.

While phonon dispersion or AIMD at finite temperatures could yield more information about dynamic stability, our discussion relies solely on structural criteria and formation energies, consistent with this comparative study performed at 0 K.

2.2. Mechanical Properties

The study of elastic behavior holds considerable relevance in both academic research and industrial applications due to its relevance in various applications [29,30]. The parameters of elastic behavior are essential for evaluating a material’s ductility, brittleness, atomic bonding, anisotropy, and stiffness. For cubic crystals, only three independent elastic constants ($C_{11}$, $C_{12}$, and $C_{44}$) are needed to define the stiffness matrix $C_{ij}$, which describes the material’s directional mechanical responses to different applied forces. The longitudinal distortion constant, C₁₁, corresponds to hardness as it relates to longitudinal compression. The transverse constant, C₁₂, is associated with transverse expansion and is tied to $Poisson’s ratio$. Lastly, the $shear$ constant, C₄₄, reflects the crystal’s resistance to shearing and is tied to the shear modulus [31,32].

Our calculations confirm that all $eigenvalues$ ${\lambda }_i$ of the $C_{ij}=C_{ji}$ matrix are real and positive for both K₂AgSbBr₆ and K₂NaScBr₆ systems. Furthermore, the Born stability $criteria$ for cubic systems are satisfied: (i) $C_{11}–C_{12}>0$; (ii) $C_{11}+{2C}_{12}>0$; and (iii) $C_{44}>0$ [31,33]. Together with the real and positive eigenvalues of the stiffness matrix, these results indicate that both K₂AgSbBr₆ and K₂NaScBr₆ meet the mechanical stability conditions.

According to the elastic parameters reported in

Table 5. Calculated mechanical parameters $C_{ij}\left(\mathrm{Gpa}\right)$ of K₂B^′B^″Br₆ (B^′B^″ = AgSb, NaSc) double perovskite.

Material	${\mathit{C}}_{11} \left(\mathbf{G}\mathbf{P}\mathbf{a}\right)$	${\mathit{C}}_{12} \left(\mathbf{G}\mathbf{P}\mathbf{a}\right)$	${\mathit{C}}_{44} \left(\mathbf{G}\mathbf{P}\mathbf{a}\right)$
K2AgSbBr6	36.98	11.94	9.02
K2NaScBr6	37.22	7.92	5.65

, we observe notable differences between the mechanical properties of K₂AgSbBr₆ and K₂NaScBr₆. Both materials have similar $C_{11}$ values, with K₂AgSbBr₆ at 36.98 GPa and K₂NaScBr₆ slightly higher at 37.22 GPa, indicating comparable resistance to longitudinal compression.

The transverse modulus, C₁₂, differs more significantly: 11.94 GPa for K₂AgSbBr₆ and 7.92 GPa for K₂NaScBr₆. This variation suggests that K₂NaScBr₆ is less resistant to transverse strain, potentially influencing its elastic behavior under applied stress.

For the shear modulus, C₄₄, K₂AgSbBr₆ shows a higher value of 9.02 GPa compared to 5.65725 GPa for K₂NaScBr₆. This suggests that K₂AgSbBr₆ has more resistance to shear deformation which indicates greater mechanical preparedness when experiencing shear stress.

These differences in mechanical performance, shown in Table 5, demonstrate the differing mechanical properties of K₂AgSbBr₆ and K2NaScBr₆, which in turn conveys valuable information about their structural stability and where they could work best in applications.

The mechanical properties of K₂AgSbBr₆ and K₂NaScBr₆ were analyzed to gain information about their structural stability, stiffness, and ductility. All mechanical properties were based on elastic constants $C_{11}$, $C_{12}$, and $C_{44}$, which provide information on the mechanical behavior of cubic crystals (see

Table 6. Calculated values of $E$, $A$, $G$, B, $v$, $B/G$, $C_p$, $\theta$, $v_m$, λ, and $\beta$.

Material	$\mathit{E} \left(\mathrm{GPa}\right)$	$\mathit{G} \left(\mathrm{GPa}\right)$	$\mathit{B} \left(\mathrm{GPa}\right)$	$\mathit{\nu }$	$\mathit{B}/\mathit{G}$	${\mathit{C}}_{\mathit{p}}$ (GPa)	$\mathit{A}$	$\mathit{\theta } \left(\mathbf{K}\right)$	${\mathit{v}}_{\mathit{m}} (\mathbf{m}/\mathbf{s})$	$\mathit{\lambda } \left(\mathrm{GPa}\right)$	$\mathit{\beta } (1/\mathrm{GPa})$
K2AgSbBr6	26.69	10.42	20.29	0.28	1.95	2.92	0.72	169.90	1877.65	13.34	0.0493
K2NaScBr6	21.70	8.37	17.68	0.29	2.11	2.27	0.38	165.40	1783.52	12.10	0.0565

). Young’s modulus ($E$) calculated using $E=9BG/(3B+G)$ quantifies the stiffness of a material [23,34]. While K₂AgSbBr₆ (26.69 GPa) is stiffer than K₂NaScBr₆ (21.70 GPa), this is consistent with its greater resistance to tensile strain which can be attributed to the greater ionic size of ${Ag}^{+}$ and ${Sb}^{3+}$ compared to ${Na}^{+}$ and ${Sc}^{3+}$, which aids in the bond network stability.

The $bulk$ $modulus$ ($B=(C_{11}+2C_{12})/3$) indicates resistance to uniform compression [34,35], with K₂AgSbBr₆ (20.29 GPa) slightly outperforming K₂NaScBr₆ (17.68 GPa). This finding is consistent with K₂AgSbBr₆ due to its stronger bonding and tighter packing resisting volumetric deformation. The shear modulus ($G=(C_{11}-C_{12}+3C_{44})/5$) also confirms that K₂AgSbBr₆ has a greater resistance against shear deformation (10.42 GPa) than K₂NaScBr₆ (8.37 GPa) in accordance with its greater stiffness.

Poisson’s ratio ($\nu =(3B-2G)/2(3B+G)$) was found to be 0.28 for K₂AgSbBr₆ and 0.29 for K₂NaScBr₆, suggesting comparable ductility. With respect to Pugh’s ratio ($B/G$), both materials fall into the ductile category (i.e., $B/G>1.75$) and have values of K₂AgSbBr₆ = 1.95 and K₂NaScBr₆ = 2.11, with K₂NaScBr₆ being more ductile than K₂AgSbBr₆. The positive Cauchy pressure ($C_p=C_{12}-C_{44}$) values in K₂AgSbBr₆ at 2.92 GPa and in K₂NaScBr₆ at 2.27 GPa also support ductility. The Cauchy pressure parameters suggest that metallic bonding contributions are important for both systems [23,34].

The anisotropy factor ($A=2C_{44}/(C_{11}-C_{12}$) reveals that K₂AgSbBr₆ (0.72) is more anisotropic than K₂NaScBr₆ (0.38). This indicates that K₂NaScBr₆ has relatively more isotropic mechanical behavior and may be useful for targeted applications that are insensitive to crystallographic directionality and require homogeneous mechanical performance [23,35].

The Debye temperature $\theta$ calculated from elastic constants and sound velocity is an index of vibrational stability and thermal conduction [36,37]. K₂AgSbBr₆’s Debye temperature θ of 169.90 K is larger than K₂NaScBr₆’s Debye temperature $\theta$ of 165.40 K, which indicates that K₂AgSbBr₆ is capable of better phonon propagation and thermal transport. Accordingly, the average sound velocity ($v_m$) of K₂AgSbBr6 (1877.65 $\mathrm{m}/\mathrm{s}$) is larger than that of K₂NaScBr₆ (1783.52 $\mathrm{m}/\mathrm{s}$), corroborating the increased stiffness and denser structure of K₂AgSbBr₆ compared to K₂NaScBr₆.

Lame’s parameter ($\lambda =(B-2G)/3$) further confirms the volumetric stability of K₂AgSbBr₆ (13.34 GPa) over K₂NaScBr₆ (12.10 GPa). On the other hand, compressibility ($\beta =1/B$) indicates that K₂NaScBr₆ (0.05653 1/GPa) is more compressible than K₂AgSbBr₆ (0.0493 1/GPa), which correlates with its lower bulk modulus and thus a softer bonding environment [38,39].

In conclusion, K₂AgSbBr₆ has better stiffness, resistance to shape change, and thermal and acoustic properties and is appropriate for applications that require mechanical stability and thermal transport. Conversely, K₂NaScBr₆ has better ductility, isotropy, compressibility, and robustness of applicable bending and deformity, while K₂AgSbBr₆ has stiffness and rigidity better suited to applications where pliancy is not a requirement for effectiveness. The noted differences are thus a consequence of substituting Ag⁺/Sb³⁺ with Na⁺/Sc³⁺ at the B^′ and B^″ sites, which relates directly to bond strength and lattice ties.

The relatively low bulk and shear moduli values determined from both K₂AgSbBr₆ and K₂NaScBr₆ are in agreement with previously reported DFT values for lead-free halide double perovskites featuring Ag⁺, Bi³⁺, Sb³⁺, or In³⁺ cations and showing weak inter-atomic bonding with open frameworks, i.e., softness. The Cs₂AgSbCl₆ and Cs₂AgInCl₆ compounds have also shown similar mechanical softness, which have been shown to have bulk moduli below 25 GPa [Ref]. In addition, the elastic constants predicted here satisfy the Born mechanical stability criteria for cubic systems of which both materials are confirmed stable. The stress convergence criterion used for the structural relaxations was 0.1 GPa, which should be sufficiently small to provide numerically stable and physically meaningful results for the size of systems considered. However, it is acknowledged that by further tightening the stress convergence, the elastic moduli values will be better estimates, and this will be addressed in future work [23,40,41,42].

2.3. Electronic Properties

A material’s performance in various applications is influenced by its electronic and optoelectronic properties. The electronic properties of K₂AgSbBr₆ and K₂NaScBr₆ were studied using their band structures [22,23], total density of states (DOS), and projected density of states (PDOS). These analyses provide important information about the electronic bandgap, electronic transition, and orbital participation of the contributing orbitals near the Fermi level, which is important to understand their functional properties [43,44].

The band structure of K₂AgSbBr₆ (Figure 2a) exhibits an indirect bandgap of $0.857 \mathrm{eV}$, a value representing semiconducting behavior and ideal for optoelectronic applications such as photovoltaics and photodetection. This indirect transition is due to the valence band maximum $VBM$ aligning with the conduction band minimum $CBM$ at different k-points in the $Brillouin$ zone. K₂NaScBr₆ (Figure 2b), on the other hand, exhibits a direct bandgap of $3.107 \mathrm{eV}$ at the $\Gamma$ point, which is indicative of a material favorable for wide-bandgap applications, such as $UV$ photodetectors and insulating layers. Its direct bandgap suggests that electronic transitions are optically allowed for, implying suitable potential for light-based applications.

It is well-known that the GGA-PBE approximation systematically underestimates bandgap values as it conducts an approximate treatment of the exchange–correlation effects when compared to other methods. Many other calculations use more robust levels of theory such as hybrid functionals (i.e., HSE06) or GW, and generally, the calculated values are closer to the experimental values. For example, in other similar compounds such as Cs₂AgBiBr₆ or Cs₂AgInCl₆, the bandgap typically increases from about 1.3 eV (PBE) to approximately 2.0 eV (HSE06) [3,19]. Therefore, while absolute values can be better corrected with higher levels of theory, the relative comparison of K₂AgSbBr₆ vs. K₂NaScBr6 was valid because both materials were treated at the same theory level.

While spin–orbit coupling (SOC) was not considered in this study, it has been shown to have a non-negligible effect on halide perovskites that contain heavy atoms (such as Sb and Br). In general, SOC will reduce the bandgap through the splitting of the valence and conduction band edges and will also, in some instances, change the nature of the bandgap. For example, for Cs₂AgBiBr₆, the inclusion of SOC lowers the bandgap by approximately 0.4 eV and shifts the extrema of the bands slightly [19]. In this study, we excluded SOC to be consistent with our previous GGA-PBE study of K₂AgSbBr₆ [22]. However, SOC may be included in future studies to improve the quantitative accuracy of the band structure, especially for applications where precise knowledge of the electronic transitions is required.

The $DOS$ (Figure 3) highlights the distribution of electronic states for both materials [45,46]. For K₂AgSbBr₆, the states near the VBM are primarily composed of contributions from the $p$ orbitals of $Br$ and $Sb$, with additional input from $Ag$’s $d$ orbitals. The $CBM$ is characterized by antibonding interactions involving the Sb and Br orbitals. In K₂NaScBr₆, the $DOS$ reveals that the $VBM$ is predominantly composed of $Br-p$ states, while the $CBM$ is influenced by $Sc-d$ states, resulting in a less dense distribution of states near the $Fermi$ level compared to K₂AgSbBr₆.

The $PDOS$ (Figure 4a,b) provides a deeper understanding of the atomic contributions to the electronic structure [45]. For K₂AgSbBr₆, the $VBM$ shows strong contributions from the $p$ orbitals of $Br$ and $Sb$, alongside the $d$ orbitals of $Ag$, resulting in a denser and more hybridized electronic structure. The $CBM$ is primarily dominated by Sb-p and Br-p states, reflecting strong interactions. Conversely, for K₂NaScBr₆, the $VBM$ is mainly driven by $Br-p$ orbitals, while the $CBM$ features $Sc-d$ states with minimal hybridization. This reduced interaction leads to a noticeable increase in the bandgap and associated effects on electronic transitions in the material.

The differences between K₂AgSbBr₆ and K₂NaScBr₆ arise because of the substitution of ${Na}^{+}$ and ${Sc}^{3+}$ at the $B^{\prime }$ and $B^{″}$ sites. K₂NaScBr₆ significantly reduced the overlapping orbitals between the metal and halide orbitals, which correlates to a larger direct bandgap and a different electronic structure. These observed differences demonstrate the extent to which cation substitution can change the electronic properties of double perovskites.

As a final point, K₂AgSbBr₆’s relatively smaller indirect bandgap and high density of electronic states near the Fermi level make it quite amenable to integration into optoelectronic systems, for example, as light absorbers in photovoltaic cells or photodetectors. On the other hand, K₂NaScBr₆, with its larger direct bandgap and weaker hybridization, will be useful in wide-bandgap applications including $UV$ photodetectors or insulating layers in electronic devices. These results demonstrate the plethora of tunable electronic properties in halide double perovskites and how they can be used to solve complications in engineering for application-specific compositions.

2.4. Optical Properties

The optical properties of materials, particularly their response to electromagnetic radiation, are critical for determining their suitability in optoelectronic applications. These properties are commonly described by the dielectric function $\epsilon \left(\omega \right)$ [47,48]:

$$\epsilon \left(\omega \right)={\epsilon }_1\left(\omega \right)+i{\epsilon }_2\left(\omega \right),$$

(3)

where ${\epsilon }_1\left(\omega \right)$ is the real part (representing dispersion), and ${\epsilon }_2\left(\omega \right)$ is the imaginary part (representing absorption). The dielectric function provides insights into electronic transitions, energy loss mechanisms, and light–matter interactions.

The real part ${\epsilon }_1\left(\omega \right)$ describes the ability of the material to store energy under an applied electric field from outside, while the imaginary part ${\epsilon }_2\left(\omega \right)$ represents the use of electromagnetic radiation through absorption, which comes from interband electronic transitions. These quantities are generated from first-principles calculations based on the $Kubo-Greenwood$ formalism, which connects $\epsilon \left(\omega \right)$ to the electronic structure of the material [49].

Figure 5a shows the calculated dielectric functions for K₂AgSbBr₆ and K₂NaScBr₆. The static dielectric constant ${\epsilon }_1\left(0\right)$, which is equal to ${\epsilon }_1\left(\omega \right)$ at the zero-frequency limit, is greater for K₂AgSbBr₆ than for K₂NaScBr₆. This is due to the greater hybridization of the $Ag-d$ and $Sb-p$ orbitals in K₂AgSbBr₆ and simply a larger polarizability. K₂NaScBr₆ has lower values for the static dielectric constant because of a larger energy bandgap and less hybridization of the $Sc-d$ and $Br-p$ orbitals.

The imaginary part ${\epsilon }_2\left(\omega \right)$ reflects the range of energies where absorption occurs due to interband transitions. Primarily, for K₂AgSbBr₆, the largest absorption peak is found at close to $2.5 \mathrm{e}\mathrm{V}$, which 2.5*corresponds to electronic transitions from the maximum of the valence band (with states that are heavily $Sb-p$ and $Br-p$) to the minimum of the conduction band (involving $Sb-p$ states). K₂NaScBr₆ on the other hand has its first absorption peak near 4 eV, consistent with its larger bandgap and electronic transitions involving mainly the $Br-p$ and $Sc-d$ orbitals.

The larger value of ${\epsilon }_2\left(\omega \right)$ for K₂AgSbBr₆ in the visible region indicates that it absorbs more light in the visible range than K₂NaScBr₆, making it a more favorable option for growing visible light-absorbing layers in photovoltaic devices. In contrast, the smaller amount of $\mathsf{\alpha }\left(\omega \right)$ in the visible range for K₂NaScBr₆ suggests that the material is well-suited for devices that require transparency or an active material that interacts with ultraviolet light, like ultraviolet photodetectors or insulating layers.

The absorption coefficient $\mathsf{\alpha }\left(\omega \right)$ conveys how well a material absorbs light as a function of its photon energy and is important in evaluating materials for optoelectronic applications. The absorption coefficient is derived from the dielectric function as follows [50,51]:

$$\mathsf{\alpha }\left(\omega \right)=\sqrt{2}\omega {[\sqrt{{\epsilon }_1^2\left(\omega \right)+{\epsilon }_2^2\left(\omega \right)}-{\epsilon }_1(\omega \left)\right]}^{{\displaystyle \frac{1}{2}}},$$

(4)

As shown in Figure 5b, the absorption spectra $\mathsf{\alpha }\left(\omega \right)$ of K₂AgSbBr₆ and K₂NaScBr₆ are depicted. For the K₂AgSbBr₆ material, strong absorption is observed from energies of approximately $1.5 \mathrm{e}\mathrm{V}$, consistent with its narrow bandgap material and relevant transitions involving the hybridized $Sb-p$ and $Br-p$ orbitals. The absorption intensity increases quickly through the near-visible region and peaks in the visible regime, around $2.5 \mathrm{e}\mathrm{V}$, which is what makes it great for applications that harvest light energy, such as photovoltaics and photodetectors.

K₂NaScBr₆, on the other hand, starts absorbing at a higher energy ($3.2 \mathrm{e}\mathrm{V}$), which is expected from the larger direct bandgap. The absorption $\mathsf{\alpha }\left(\omega \right)$ steadily increases and peaks around $4 \mathrm{e}\mathrm{V}$, primarily due to the transitions involving $Br-p$ and $Sc-d$ states. Given this behavior, the K₂NaScBr₆ material is better suited for applications that absorb high-energy UV light energy, such as UV photodetectors and UV transparent electronics.

The broader absorption range of K₂AgSbBr₆ in the visible spectrum confirms its potential for effective solar energy conversion applications, while the narrower range and higher energy onset of K₂NaScBr₆ reflect its potential to be a material for optical applications in which it could exhibit transparency or UV activity.

The optical spectra that are detailed in this paper were calculated in the independent-particle approximation, which does not include any excitonic effects or scissor correction. Hence, we undoubtedly biased some calculated absorption onset, which appears to be underestimated for the PBE functional, largely due to the intrinsic bandgap of the PBE functional approximated for this material class. This will somewhat limit the quantitative accuracy for optical constants, but the relative comparison of K₂AgSbBr₆ to K₂NaScBr₆ remains valid and helps to consolidate ideas. Note, previous work on Cs₂AgBiBr₆ and Cs₂AgInCl₆ has indicated that scissor shifts and hybrid functionals such as HSE06 can very much improve agreement with experiments [3,42]. Including many-body effects such as electron–hole interactions (using the Bethe–Salpeter equation) would provide an even more accurate representation of the predicted spectra and would be a suitable target for future work.

Reflectivity $R\left(\omega \right)$ quantifies the amount of incident light rejected from a material at its surface and is important for applications like photovoltaics where the goal is to reduce reflection energy loss to improve efficiency. Reflectivity is determined from the real and imaginary parts of the dielectric function (${\epsilon }_1\left(\omega \right)$ and ${\epsilon }_2\left(\omega \right)$) using the following [22,47]:

$$R\left(\omega \right)={\left|{\displaystyle \frac{\sqrt{\epsilon \left(\omega \right)}-1}{\sqrt{\epsilon \left(\omega \right)}+1}}\right|}^2,$$

(5)

The reflectivity spectra of K₂AgSbBr₆ and K₂NaScBr₆ are shown in Figure 5c. K₂AgSbBr₆ has a higher reflectivity in the visible region, with a peak near $2.5 \mathrm{e}\mathrm{V}$, which corresponds to the strong optical transitions after hybridizing the contribution of the $Ag-d$, $Sb-p$, and $Br-p$ orbitals. This behavior is consistent with its high absorption in the visible range, meaning that only a very small amount of light is reflected, and the remaining light is absorbed, which makes it a good candidate for applications in light-harvesting devices.

K₂NaScBr₆, on the other hand, shows a relatively lower reflectivity $R\left(\omega \right)$ across the visible spectrum, with peaks occurring at higher photon energies (~4 eV) due to transitions involving the Br-p and Sc-d orbitals. The reduced reflectivity $R\left(\omega \right)$ at visible wavelengths highlights its potential for transparency-focused applications or UV-sensitive devices, where minimal reflection in the visible range is desirable.

At higher energies (>5 eV), the reflectivity of both materials converges, indicating similar optical behavior in this region, likely dominated by high-energy interband transitions. The distinct reflectivity profiles of the two materials further emphasize their diverse optical applications.

Following the discussion on reflectivity, the energy loss function $L\left(\omega \right)$ offers additional insight into the electronic excitation behavior of K₂AgSbBr₆ and K₂NaScBr₆. The loss function $L\left(\omega \right)$ is defined as follows [45,47]:

$$L\left(\omega \right)=Im\left[{\displaystyle \frac{-1}{\epsilon \left(\omega \right)}}\right].$$

(6)

$L\left(\omega \right)$ represents the energy dissipation of fast-moving electrons interacting with the material. Peaks in $L\left(\omega \right)$ are directly linked to plasmon resonances, revealing energy ranges of significant collective electronic oscillations.

The loss spectra for both compounds, depicted in Figure 5d, exhibit distinct behaviors. For K₂AgSbBr₆, the primary loss peak occurs at approximately 6.2 eV, corresponding to the plasmon resonance frequency. This peak is associated with the collective oscillations of free electrons, strongly influenced by the hybridized Ag-d, Sb-p, and Br-p orbitals. The sharp and intense nature of the peak indicates lower dielectric screening and strong electronic excitation within this energy range.

Conversely, K₂NaScBr₆ shows a plasmon resonance peak at a higher energy of $~7.5 \mathrm{e}\mathrm{V}$. The shift in ${Ag}^{+}/{Sb}^{3+}$ to ${Na}^{+}/{Sc}^{3+}$ increases the bandgap and changes the electronic environment. The broader and less intense peak of K₂NaScBr₆ suggests reduced plasmonic activity and higher dielectric screening, consistent with its lower free carrier density.

The differences in the loss spectra align with the contrasting dielectric properties of the two materials. K₂AgSbBr₆, with its smaller bandgap and higher polarizability, exhibits a lower-energy, more pronounced plasmonic resonance. Meanwhile, the higher-energy resonance of K₂NaScBr₆ reflects its wider bandgap and reduced electronic excitations.

Building on the energy loss function $L\left(\omega \right)$, the refractive index $n\left(\omega \right)$ and the extinction coefficient $k\left(\omega \right)$ (complex refractive index), as optical parameters, depend on the dielectric function. These parameters describe how light propagates through a material, with $n\left(\omega \right)$ representing the material’s ability to bend light and $k\left(\omega \right)$ characterizing the material’s absorption of light. The refractive index and extinction coefficient are expressed as follows [52,53]:

$$n\left(\omega \right)={\displaystyle \frac{1}{\sqrt{2}}}{[\sqrt{{\epsilon }_1^2\left(\omega \right)+{\epsilon }_2^2\left(\omega \right)}+{\epsilon }_1(\omega \left)\right]}^{{\displaystyle \frac{1}{2}}},$$

(7)

$$k\left(\omega \right)={\displaystyle \frac{1}{\sqrt{2}}}{[\sqrt{{\epsilon }_1^2\left(\omega \right)+{\epsilon }_2^2\left(\omega \right)}-{\epsilon }_1(\omega \left)\right]}^{{\displaystyle \frac{1}{2}}}.$$

(8)

Figure 5e illustrates the calculated $n\left(\omega \right)$ and $k\left(\omega \right)$ spectra for K₂AgSbBr₆ and K₂NaScBr₆. The static refractive index $n\left(0\right)$, corresponding to the zero-frequency limit, is higher for K₂AgSbBr₆ compared to K₂NaScBr₆, reflecting its enhanced polarizability due to the hybridized $Ag-d$, $Sb-p$, and $Br-p$ orbitals. The peak value of $n\left(\omega \right)$ for K₂AgSbBr₆ occurs at approximately 2.5 eV, indicating strong dispersion in the visible range, which is beneficial for light-harvesting applications.

The refractive index $n\left(\omega \right)$ of K₂NaScBr₆ is lower than that of other compounds throughout the visible spectrum, with a maximum at roughly 4 eV. This trend in $n\left(\omega \right)$ is due its larger energy bandgap and lower polarizability, as ${Ag}^{+}/{Sb}^{3+}$ in K₂AgSbBr₆ is replaced by ${Na}^{+}/{Sc}^{3+}$ which induces less hybridization of the $Sc-d$ and $Br-p$ orbitals.

The extinction coefficient $k\left(\omega \right)$, which characterizes the absorption of a material, displays the same trends as absorption coefficient measurements. $k\left(\omega \right)$ for K₂AgSbBr₆ exhibits a considerable increase around 2.5 eV, which suggests electronic transitions from $Sb-p$ and $Br-p$ orbital contributions to the conduction band. The calculated K₂NaScBr₆ shows a lower $k\left(\omega \right)$ in the visible range and a noticeable increase around $4 \mathrm{eV}$, confirming the limiting of transparent conductors for UV applications.

The complex optical conductivity $\sigma \left(\omega \right)$ reflects the response of the material to optical excitations and accounts for the contribution of the material to conduction under an applied electromagnetic field. It is expressed as follows [54,55]:

$$\sigma \left(\omega \right)={\sigma }_1\left(\omega \right)+i{\sigma }_2\left(\omega \right),$$

(9)

In optoelectronic studies, optical conductivity is consistently expressed with the real and imaginary parts of complex conductivity, defined by the following, where ${\sigma }_1\left(\omega \right)$ is the real part (dissipative (conductive), and ${\sigma }_2\left(\omega \right)$ is the imaginary part (energy storage). These components are directly derived from the dielectric function using the following [40,56]:

$${\sigma }_1\left(\omega \right)={\displaystyle \frac{\omega {\epsilon }_2\left(\omega \right)}{4\pi }},{\sigma }_2\left(\omega \right)={\displaystyle \frac{\omega ({\epsilon }_1\left(\omega \right)-1)}{4\pi }},$$

(10)

Figure 5f shows the complex optical conductivity $\sigma \left(\omega \right)$ spectra for K₂AgSbBr₆ and K₂NaScBr₆.

For K₂AgSbBr₆, the real part ${\sigma }_1\left(\omega \right)$ shows a strong peak at about 2.5 eV that arises from very strong optical transitions involving the $Sb-p$ and $Br-p$ orbitals at the valence band and the conduction band minimum which correspond to the high absorption and high extinction coefficient of visible light where this material is consistently capable of conducting excitation energy from light. The imaginary part ${\sigma }_2\left(\omega \right)$ reached its peak slightly below an energy of 2.5 eV, in part representing the energy stored by interband transitions.

The real part ${\sigma }_1\left(\omega \right)$ for K₂NaScBr₆ shows a much lower intensity peak at the expected higher energy (~4 eV) based on a larger bandgap and expected UV-active behavior. This results in a weaker ability to conduct excitation energy in the visible range for K₂NaScBr₆ based on the observed lower hybridization of $Sc-d$ with $Br-p$. The imaginary part ${\sigma }_2\left(\omega \right)$ also has a peak in UV with a trend similar to its real part, which corresponds to the electronic transitions corresponding to higher energy states.

With its higher conductivity and wider response range in the visible spectrum, K₂AgSbBr₆ is an attractive option for optoelectronic devices, especially those requiring the efficient conversion of incident light to electrons, such as photodetectors and solar cells. In contrast, K₂NaScBr₆ has a narrower response range spectrum, with its conductivity being active in the UV range, making it more applicable in devices that utilize higher-energy light, such as UV photodetectors and transparent layers for optoelectronic devices.

The optical spectra in this work were computed using the Kubo–Greenwood formalism, which does not include excitonic effects arising from electron–hole (e–h) interactions. In bulk 3D halide double perovskites such as K₂AgSbBr₆ and K₂NaScBr₆, the relatively large dielectric screening typically results in low exciton binding energies, suggesting that excitonic effects may only slightly shift the absorption edge without significantly altering the overall spectral profile. Previous studies on related materials, such as Cs₂AgBiBr₆ and Cs₂AgInCl₆, have shown exciton binding energies on the order of tens of meV, indicating that the independent-particle approximation remains reasonable for qualitative optical analysis in these systems. Nonetheless, future work involving many-body perturbation theory (e.g., BSE) would help clarify the precise excitonic influence [50].

3. Materials and Methods

The $DFT$ calculations employed the Projected Augmented Wave ($PAW$) pseudopotential method incorporated in the $CASTEP$ code (CAmbridge Serial Total Energy Package) [57,58,59]. The model configurations of $K_{2}AgSb{Br}_6$ and $K_{2}NaSc{Br}_6$ were systematically relaxed, with periodic boundary conditions applied to the system. For exchange and correlation (xc) functionals, we utilized the $generalized gradient approximation$ with $Perdew$–$Burke$–$Ernzerhof$ ($GGA-PBE$) in our computational approach [60,61]. The interactions between the core ions and valence electrons were accurately represented using norm-conserved pseudopotentials. The pseudopotentials considered the valence states of $K$ (${4s}^1$), $Ag$ ($4d^{10}{5s}^1$), $Na$ ($2s^{2}2p^{6}3s^1$), $Sb$ ($4d^{10}{5s}^{2}5p^3$), $Sc$ ($3d^{10}{4s}^2$), and $Br$ ($4s^2{4p}^5$) for the materials under study. Geometry optimization was performed using the $Broyden$–$Fletcher$–$Goldfarb$–$Shanno$ minimization procedure [62,63]. The atomic configurations were relaxed using a $Hellmann–Feynman$ force convergence criterion of $0.01 \mathrm{e}\mathrm{V}/\mathrm{\AA }$, with the conjugate gradient minimization algorithm. The $Brillouin$ zone was sampled using a $Monkhorst–Pack$ $k-pointgrid$ of 4 × 4 × 4 for the optimization of the structures [64]. Energy calculations were carried out with a total energy tolerance of $5.0\times {10}^{-6} \mathrm{e}\mathrm{V}$ per atom, stress tolerance of $0.01 \mathrm{e}\mathrm{V}/\mathrm{\AA }$, ionic displacement of $0.02 \mathrm{G}\mathrm{P}\mathrm{a}$, and an accuracy of $5.0\times {10}^{-4} \mathrm{\AA }$ for ionic relaxation. A plane wave energy cut-off of $500 \mathrm{e}\mathrm{V}$ was employed for the calculations.

4. Conclusions

This study outlines a thorough analysis of the structural, mechanical, electronic, and optical properties of K₂AgSbBr₆ and K₂NaScBr₆, with a focus on how cation substitution impacts the materials’ properties. The structural stability of both materials was verified through reasonable tolerance and octahedral factors, as well as negative formation energies, indicating good thermodynamic feasibility. The mechanical properties of each material reveal K₂AgSbBr₆ to have higher stiffness and volumetric compressibility, while K₂NaScBr₆ possesses better ductility and nearly isotropic mechanical properties. This finding indicates that by varying the cation, there is a trade-off between rigidity and flexibility, depending on the intended application. From an electronic perspective, K₂AgSbBr₆ has an indirect bandgap of 0.857 eV, which would be useful for visible light-harvesting technologies, while K₂NaScBr₆ has a direct bandgap of 3.107 eV, which would work well for ultraviolet applications. The density of states analysis and orbital hybridization data further highlighted the mere fact that the choice of cation changes the nature of the electronic transitions and influences the overall band structure.

With respect to optical performance, K₂AgSbBr₆ shows strong absorption in the visible range, and its reflectivity and plasmonic properties could allow for its use in photovoltaics and photodetectors. In contrast, K₂NaScBr₆ shows considerable UV absorption and conductivity, suggesting that it could have applications in UV photonic devices and transparent optoelectronic applications.

Ultimately, the tunable properties afforded by the cation engineering of K₂AgSbBr₆ and K₂NaScBr₆ indicate the potential for their use in many optoelectronic applications. The customizability of structural integrity, electronic properties, and optical properties positions K₂AgSbBr₆ and K₂NaScBr₆ as strong candidates to usher developments in the photonic and electronic space.