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Article

Phonon Structure Engineering for Intrinsically Spectrally Selective Emitters by Anion Groups

by
Rui Zhang
,
Enhui Huang
,
Wenying Zhong
* and
Bo Xu
*
Key Laboratory of Biomedical Functional Materials, School of Science, China Pharmaceutical University, Nanjing 211198, China
*
Authors to whom correspondence should be addressed.
Photonics 2025, 12(6), 597; https://doi.org/10.3390/photonics12060597
Submission received: 28 March 2025 / Revised: 17 May 2025 / Accepted: 9 June 2025 / Published: 11 June 2025
(This article belongs to the Special Issue Infrared Optoelectronic Materials and Devices)
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Abstract

Spectrally selective emitters (SSEs) have attracted considerable attention, because of radiative cooling, which could dissipate the heat from earth to outer space through the atmospheric window without any energy input. Intrinsically inorganic SSEs have significant advantages to other SSEs, such as the low fabrication cost due to the extremely simple structures and long life span under solar exposure. However, few inorganic materials can act as intrinsic SSEs due to the limited emissions in the atmospheric window. Here, we propose a strategy to design intrinsic SSEs by complementing the IR-active phonons in atmospheric window with anion groups. Accordingly, we demonstrate borates containing both [BO3]3− and [BO4]5− units can exhibit high emissivity within the whole atmospheric window, because the IR-active phonons of [BO3]3− units usually locate around 8 and 13 μm, while those of [BO4]5− units distribute in 9~11 μm. Furthermore, K3B6O10Cl and BaAlBO4 are selected as two examples to display their near-unity emissivity (>95%) within the whole atmospheric window experimentally. These results not only offer a new strategy for the design of intrinsic SSEs, but also endow wide band-gap borates containing both [BO3]3− and [BO4]5− units with great potential applications for radiative cooling.

1. Introduction

With the acceleration of global warming, about 17% of worldwide electricity consumption is spent on the building cooling every year [1]. Conventional cooling systems, such as air-conditioners, further exacerbate global warming by increasing the energy consumption and CO2 emission [2]. Fortunately, radiative cooling, which relies on the natural infrared (IR) emission of terrestrial objects to the cold (3 K) outer space through the atmosphere window (AW) (8~13 µm), has been considered as an appealing technology for alleviating these problems, because it is eco-friendly, and does not require any input energy [3,4,5,6,7,8,9,10,11,12]. The performance of radiative cooling is strongly dependent on the optical properties of spectrally selective emitters (SSEs), which should have a high reflectance in the solar spectral region (0.3–2.5 µm) and a high emissivity in AW [13,14,15].
Up to now, various engineered SSEs with tailored spectrum responses have been developed to achieve the effect of radiative cooling [7,10,11,16,17,18,19,20,21,22,23,24,25,26,27,28], which can be mainly divided into two strategies: promoting the solar reflectance based on IR emitters and enhancing the IR emission for solar reflectors. Most polymers [10,11,16,17,18,19,20,21,22,23], such as PDMS [10], PVDF [11], PLA [23], and PMMA [19], are intrinsic IR emitters, because the fingerprint vibrations of many functional units are just full in the AW region, such as C–O (1260~1110 cm−1), C–OH (1239 ~1030 cm−1), and C-F (1148 cm−1). With a reasonable device design, such as backed with high reflective metals [16], hybridized with inorganic particles [10,18], or scattering with micro- and nano-structures [11,17,19,21,22,23], high solar reflectance can be realized based on these polymer emitters. Although polymer-based emitters have great advantages on low-cost fabrication, they are usually non-selective emitters, and also suffer the aging problem for the long-time exposure of UV light. In contrast, inorganic-material-based emitters have significant durable property compared to polymer-based emitters. Meanwhile, they have a low absorption in the solar spectral region, which is favored to achieve high solar reflectance. But, different from polymers, none of them can possess near-unity emission within the entire AW, because of their limited IR phonon vibrations in AW. Many methods have been explored to enhance the emission in AW for the applications of radiative cooling, such as composite materials, multilayered films, photonic crystals and metasurfaces [7,24,25,26,27,28]. For example, Fan et al. [7], firstly, demonstrated SiO2/HfO2-based photonic crystals, in which SiO2 has a strong phonon resonance near 9 µm, while HfO2 has a broad absorption in AW, can yield a cooling power of 40.1 W/m2 and a temperature drop of 4.9 °C. However, such emitters usually require high fabrication costs, because they have to be prepared by complicated high-vacuum deposition methods or nanofabrication techniques, such as PVD, CVD, and e-beam lithography, which restricts their large-scale applications in radiative cooling.
Recently, single inorganic materials with intrinsically spectrally selective optical characteristics have been suggested to overcome these issues [29]. A single-layer material with intrinsic spectral selectivity, i.e., intrinsic SSE, would extremely simplify the device structures for radiative cooling, and further facilitate the reduction in fabrication cost. Moreover, inorganic materials, especially oxides, usually show better durable properties than polymers for outdoor environments. However, until now, no single inorganic material can exhibit selective emission in such a wide spectrum, especially the emission within the entire AW. According to Kirchhoff’s law [30], under thermal equilibrium, the emissivity is equal to the absorptivity, which is determined by the energy level at the corresponding wavelength. As we know, the energy levels in AW, from 8 to 13 µm, are usually composed of phonon vibrations [31]. We notice that, for inorganic materials, the phonon resonances above 700 cm−1 are usually related to their anion groups. Here, we propose to design intrinsic SSEs by the phonon structure engineering with anion groups. Accordingly, we find that wide band-gap borates containing both triangular [BO3]3− and tetrahedral [BO4]5− (or other tetrahedral analogues, such as [BO3F]4−) units can be potential candidates for intrinsic SSEs. Accordingly, two borates, K3B6O10Cl and BaAlBO4, are selected to display their spectrally selective optical characteristics experimentally.

2. Results and Discussions

2.1. Design Concept for Intrinsic SSEs with Anions

As we know, if a material contains some anions, it should have the characteristic IR absorption peaks, and also the corresponding emission around these peaks based on Kirchhoff’s law [30]. The emission in AW can be designed based on anion groups accordingly. However, as we see in Table 1, no single anion can have the IR absorptions over the whole AW. Thus, we think about designing SSEs by complementing the phonon absorptions with different anions in AW, as illustrated in Figure 1. Table 1 summarizes the IR characteristic absorption peaks for typical anions. For [SO4]2−, there are usually two IR absorption peaks in AW, located at 950~1000, and 1025~1210 cm−1, while, for [SiO4]4−, there is only one IR absorption peak in AW, around 1100 cm−1. Both of them cannot cover the whole AW. But, if some materials contain both [NO3]− and [SO4]2− units, they should have IR absorption peaks over all of AW. As a result, they should have considerable emissivity over AW. But the stability of nitrates restricts the applications for outdoor environments.
Among these anions, we notice that, for borates, there are two kinds of anions: triangular [BO3]3− and tetrahedral [BO4]5−. As shown in Table 1, the typical IR absorption peaks of triangular [BO3]3− units are mainly located around 750 and 1250 cm−1, while those of tetrahedral [BO4]5− units are mainly between 850~1100 cm−1. They can complement the IR absorptions for each other in AW (700~1300 cm−1) just right. More importantly, borates containing both triangular [BO3]3− and tetrahedral [BO4]5− units have been widely investigated for deep-UV nonlinear optical crystals, which also exhibit very low solar absorption for their wide band gaps [32,33,34]. It seems that wide band-gap borates containing both triangular [BO3]3− and tetrahedral [BO4]5− units are born for SSEs. As a result, we do not need to design specific borates for intrinsic SSEs, but only screen the intrinsic SSEs based on these reported borates which contain both triangular [BO3]3− and tetrahedral [BO4]5− units. Therefore, we choose triangular [BO3]3− and tetrahedral [BO4]5− units to demonstrate the method of designing intrinsic SSEs by complementing the phonon absorptions with anions.

2.2. Phonon Structures for Borates Only with [BO3]3− or [BO4]5− Units

To display how to complement phonon absorptions by anions in AW, we firstly perform the density functional calculations on the phonon structures of the borates which only contain triangular [BO3]3− units or tetrahedral [BO4]5− units. We take KMgBO3 as the example for the borates only containing triangular [BO3]3− units [35]. As we discussed above, the emission in AW is dependent on the phonon vibrations between 700~1300 cm−1. Thus, we mainly focus on the phonon spectra of borates from 600 to 1400 cm−1 of these borates. The structure of KMgBO3 and triangular [BO3]3− units are displayed as the inset of Figure 2a. By the B-atom-projected phonon structure (red dots in Figure 2a), we find that the phonons around 800 and 1350 cm−1 are mainly contributed by the vibrations of [BO3]3− units in KMgBO3. Similar phonon distributions have been found in Be2BO3F [36], which also only contains [BO3]3− units (Supplementary Figure S1). In condensed matters, the phonons can be divided into three modes: static, Raman-active, and IR-active. But only IR-active phonons can induce the emission in AW. Thus, we further analyze IR-active phonons between 600 and 1400 cm−1. As shown in Figure 2b and Supplementary Figure S1, for KMgBO3 and Be2BO3F, IR-active phonons are distributed at both ends of AW. The IR-active phonons of KMgBO3 around 716 and 1308 cm−1 are attributed to the vibrations of [BO3]3− units, which agree with previous experimental reports [35]. But, between 716 and 1308 cm−1, it is almost empty for IR-active phonons. Not only these two borates, but other borates with only triangular [BO3]3− units, such as CsBaYB6O12 [37] and NaSr4(BO3)3 [38], also display similar IR-active phonon distributions. In contrast, LiGeBO4 only contains tetrahedral [BO4]5− units (inset of Figure 2d) [39]. By the B-atom-projected phonon structure of LiGeBO4 (Figure 2c), the vibrations of [BO4]5− units are located between 800 and 900 cm−1. According to the IR-active phonon analysis, two strong IR-active phonons attributed to the vibrations of tetrahedral [BO4]5− units are found around 900 cm−1 (Figure 2d). We find that, only with [BO3]3− or [BO4]5− units, it is hard to have the IR-active phonons filling the AW. But IR-active phonons of [BO4]5− units can just fill the phonon gap of triangular [BO3]3− units in AW. These calculated IR-active phonons are very consistent experimental observations. As mentioned above, IR-active phonons could be used to identify the vibrations of bonds or units, because, even though their chemical environments or localized structures have been changed, the frequencies of phonon vibrations would only be shifted around their origins. Thus, it is reasonable that the borates containing both triangular [BO3]3− and tetrahedral [BO4]5− units can result in enough IR-active phonons in AW, and, then, a broad and high emission in AW.

2.3. Complementing Phonon Resonances in AW with [BO3]3− and [BO4]5− Units

We further take K3B6O10Cl as an example to illustrate how borates containing both [BO3]3− and [BO4]5− units can complement the phonon absorptions in AW [40]. K3B6O10Cl contains hexaborate [B6O10]2− units, which consist of three tetrahedral [BO4]5− units shared by the oxygen vertex and three triangular [BO3]3− units attached to the terminal vertices of these tetrahedra, as illustrated in Figure 3a. Based on our calculations, the average bond length of B-O in the [BO3]3− unit is about 1.37 Å. The bond angles of O-B-O are within 2° of the three-fold symmetrical 120° angle. The localized structure of [BO3]3− units in K3B6O10Cl has very minor distortions compared to other [BO3]3− units in KMgBO3. The [BO4]5− units also show very small distortions with that in LiGeBO4. The bond lengths of B-O vary from 1.45 to 1.52 Å, with an average of 1.48 Å, and the bond angles range from 107° to 110°. These minor structural distortions in [BO3]3− and [BO4]5− units indicate that K3B6O10Cl should have both IR-active phonons due to the [BO3]3− and [BO4]5− units. We confirm these by phonon spectrum calculations and an IR-active phonon analysis. Figure 3b,c show the B-atom-projected phonon spectrums between 600 and 1400 cm−1. Red and blue circles indicate the phonons originated from the vibrations of B atoms in the [BO3]3− and [BO4]5− units, respectively. As we see, the phonon vibrations of the [BO3]3− units are distributed at both ends of AW, while those of the [BO4]5− units appear in 850~1000 cm−1. We further analyze the IR-active phonons in Figure 3d. We find that there are several strong IR-active phonons between 600 and 1400 cm−1: 731, 828, 864, 972, 1007, 1186 and 1330 cm−1. In details, IR-active phonons at 1330, 1186, and 731 cm−1 are attributed to vibrations of [BO3]3− units, while those at 828, 864, 972, and 1006 cm−1 are vibrations of B-O bonds in [BO4]5−. We notice that the strength of four main IR-active phonons is much stronger than those in other SSEs. Their strengths are higher than 4, and, for the phonon vibrations at 1330 cm−1, they can even reach 10. The stronger strength of the IR-active phonon would result in a higher absorption, and then a higher emission. Moreover, several very weak IR-active phonons are found in this range. Even though they are weak, they can also enhance the emission in AW. More importantly, these IR-active phonons are almost evenly distributed in AW, not localized in some ranges. This is very important in order to obtain a near-unity emissivity within the whole AW.
BaAlBO4 is also a borate that contains both [BO3]3− and [BO4]5− units [41], in which two [BO4]5− units are linked by sharing a common edge to form a B2O6 ring, and two [BO3]3− units are connected to this B2O6 ring to form [B4O10]8− units, as displayed in Figure S2a. According to the projected phonon spectra of B atoms (Figure S2b,c), BaAlBO4 shows similar phonon distributions with K3B6O10Cl in AW. Most phonons between 850 and 1000 cm−1 belong to the vibrations of [BO4]5− units, while others in AW are attributed by the vibrations of [BO3]3− units. By analyzing the IR-active phonons, we find more IR-active phonons in AW compared to K3B6O10Cl (Figure S2d), because of its lower symmetry. Although these two inorganic borates have different structures and element compositions, they have similar IR-active phonon distributions in AW, which are strongly related to the [BO3]3− and [BO4]5− units. Thus, [BO3]3− and [BO4]5− units could complement IR-active phonons in AW for each other.
IR-active phonons indicate that they can interact with the IR photons, and then absorb them. By complementing IR-active phonons with both [BO3]3− and [BO4]5− units, AW can be filled with enough IR-active phonons. We can expect the broad and high emissions in AW can be realized in the borates containing both principle [BO3]3− and [BO4]5− units. It is better to demonstrate their selective IR emissions by experiments. We further synthesize K3B6O10Cl and BaAlBO4 to examine their spectrally selective optical characteristics experimentally. Polycrystalline K3B6O10Cl and BaAlBO4 powders are synthesized by the solid-state reaction method. The XRD results (Supplementary Figure S3) indicate that they are in a pure phase as reported in [40,41]. Figure 4a displays the emission curves of K3B6O10Cl and BaAlBO4 powders between 4 and 18 μm. As shown, both of them show a high emissivity in AW (8~13 μm), up to 0.95, which is higher than most reported SSEs. The emissivity quickly drops below 7 μm due to the absence of phonons. To detect the origin of the high and broad emissivity in AW, we also include the IR absorption spectrum from 400 to 1400 cm−1 and the calculated IR-active phonons in Figure 4b,c. For K3B6O10Cl, we can find 14 IR absorption peaks between 400 and 1400 cm−1: 492, 569, 596, 633, 684, 732, 822, 873, 985, 1008, 1177, 1317, and 1338 cm−1, which are in agreement with the previous experimental reports. More than half of these peaks are distributed in AW. Interestingly, IR absorption peaks also coincide with the calculated IR-active phonons (green lines in Figure 4b) very well. More importantly, as displayed by the blue dashed lines in Figure 4b, we can find the corresponding emission peak for each IR absorption peak. Similar results can be found in BaAlBO4 (Figure 4c). These results indicate that the high and broad emissivity in AW are mainly due to the vibrations of [BO3]3− and [BO4]5− units in K3B6O10Cl and BaAlBO4. Thus, by inducing [BO3]3− and [BO4]5− units into single materials, we can easily obtain near-unity emission in the whole AW.
Apart from the near-unity emission in AW, we also find that K3B6O10Cl and BaAlBO4 have very large band gaps. The band structures of K3B6O10Cl and BaAlBO4 are calculated by the PBE functional as shown in Supplementary Figure S4. As calculated, the band gaps of K3B6O10Cl and BaAlBO4 can still reach 5.5 and 4.4 eV. As displayed in Supplementary Figure S5, the reflectance of the K3B6O10Cl powder suggests that the band gap is about 5.3 eV, which indicates low optical absorption in the solar spectrum. The band gap of BaAlBO4 is in agreement with the calculation by Guo [41]. In their report, they also demonstrated a high reflectance between 300 nm to 2500 nm for the BaAlBO4 powder. Due to their wide band gaps, high solar reflectance can be easily achieved if backed with high reflective layers. Thus, by complementing IR-active phonons with [BO3]3− and [BO4]5− units, K3B6O10Cl and BaAlBO4 can be potential candidates for intrinsic SSEs. We also notice that borates containing both [BO3]3− and [BO4]5− units have been widely investigated because of their applications in deep-UV nonlinear optics. Usually, the unit cells of these kinds of borates are very large, containing dozens of or even hundreds of atoms. It is impractical to perform the phonon calculations for all of these potential borates which contain both [BO3]3− and [BO4]5− units. Here, we carry out an intensive survey on the experimental IR spectra of them. We find they have similar IR-active phonon distributions with K3B6O10Cl and BaAlBO4: IR-active phonons of [BO3]3− units usually located around 700~800 and 1200~1300 cm−1, while those of [BO4]5− units are distributed between 850~1100 cm−1, such as KSeB3O7 [42], RbGeB3O7 [43], YBe2B5O11 [44], K7SrY2(B5O10)3 [45], and KZnB3O6 [46]. Meanwhile, they also have very little solar absorption because of their wide band gaps. Thus, all of these wide band-gap borates with both [BO3]3− and [BO4]5− units are potential candidates for intrinsic SSEs.

2.4. Complementing Phonon Resonances in AW with [BO3]3− and [BO3F]4− Units

As we see, by complementing the IR-active phonons with [BO3]3− and [BO4]5− units, we can easily discover potential intrinsic SSEs based on borates. But we believe that the anions used to design potential intrinsic SSEs should not be limited in [BO3]3− and [BO4]5− units, but also other anions. The [BO3F]4− unit is a tetrahedral analogue of the [BO4]5− unit, with one O atom replaced by an F atom in the [BO4]5− unit. Although distorted, the [BO3F]4− unit should still have similar IR-active phonon distributions as the [BO4]5− unit. Thus, we also suggest that [BO3F]4− units can also be used to complement the phonon blank of [BO3]3− in AW.
Hexagonal LiB2O3F features a two-dimensional layered structure with the inset of Li ions [47]. In LiB2O3F, B atoms exhibit two forms of coordination in each layer: three-coordinated BO3 and four-coordinated BO3F. By the atomic-projected phonon spectra and analyzing IR-active phonons (Supplementary Figure S6 and Figure 4a), the IR-active phonons of [BO3F]4− units are mainly located at 881 and 944 cm−1, while those of [BO3]3− units are at both ends of AW. The building block in CaB5O7F3 is [B5O9F3] units, which is composed of three tetrahedral [BO3F]4− and two triangular [BO3]3− units (Supplementary Figure S7 and Figure 5) [48]. Due to the different crystal structures, more IR-active phonons appear in AW compared to LiB2O3F. But they have similar phonon distributions: between 850 and 1100 cm−1, most of the IR-active phonons are attributed to the vibration of the [BO3F]4− units, while others are due to the [BO3]3− units. These calculated IR-active phonon distributions of CaB3O7F3 are in good agreement with these experimental results [48]. Such IR-active phonon distributions can also be found in other fluorooxoborates, such as CsKB8O12F2 [49], and RbB4O6F [49] (Figure 5c,d, and Supplementary Figures S8 and S9). Thus, we expect that, by complementing the IR-active phonons with [BO3]3− and [BO3F]4− units, these fluorooxoborates can also be intrinsic SSEs, and this further proves that it could be a routine strategy to design intrinsic SSEs by complementing phonon resonances with anions.
Apart from borates and fluorooxoborates, we also calculated the IR-active phonons for reported inorganic SSEs, such as SiO2 [7], Si3N4 [24], Al2O3 [28], HfO2 [7], and BaSO4 [50,51]. We found that none of them can have IR-active phonons cover the AW, as shown in Supplementary Figure S10. For example, BaSO4 is a typical radiative pigment, but it has three strong IR-active phonons around 1000~1100 cm−1. Thus, as reported, the IR emission among AW is limited [50]. Fan et al. [7] demonstrated multilayer-structured SiO2/HfO2-based photonic crystals for radiative cooling. From Figure S10, we found that SiO2 has a strong phonon resonance near 1100 cm−1, while HfO2 has a phonon resonance near 720 cm−1. Thus, multilayer-structured SiO2/HfO2-based photonic crystals can have broad and high emissions in AW. Therefore, by complementing phonon resonances with anion groups, we could design a broad and high emission in the atmospheric window.
In summary, we have shown that, by complementing phonon resonances with anion groups, near-unity emission in the atmospheric window can be easily realized in the borates with both [BO3]3− and [BO4]5− units. Combined with their wide band gaps, they are candidates for intrinsic SSEs and display great potential applications in radiative cooling. Such broad emissions over the entire atmospheric window in K3B6O10Cl and BaAlBO4 are mainly due to the phonon vibrations of anions groups, suggesting that the intrinsic SSEs can be designed by not only [BO3]3− and [BO4]5− units, but also other anions. More importantly, these intrinsic SSEs designed by complementing phonon resonances with anions have great advantages in reducing the fabrication cost and simplifying device structures. We hope our studies will promote the experimental realization of radiative cooling on borates and attract more attention to the design of intrinsic SSEs by the phonon structure engineering of anions.

3. Methods

3.1. Phonon Structure and Electronic Structure Calculations

The calculations were performed using the plane-wave pseudopotential density-functional theory formalism, as implemented in the Vienna ab initio Simulation Package (VASP5.3.2) code [52]. The exchange–correlation interactions were treated by the generalized gradient approximation (GGA) in the form of Perdew–Burke–Ernzerhof (PBE) [53,54]. The cutoff energy for plane-wave basis sets is 500 eV. The convergence thresholds for force and total energy are 10−2 eV/Å and 10−5 eV, respectively. Phonon structures were calculated by using the Phonopy code with VASP as the force calculator [55,56]. The IR intensities of the Γ-point modes were calculated with the software of Phonopy-Spectroscopy (https://phonopy.github.io/phonopy/) [57].

3.2. Synthesis of K3B6O10Cl and BaAlBO4 Powder and Their IR Emission

Polycrystalline samples of K3B6O10Cl and BaAlBO4 were prepared via solid-state reaction techniques with stoichiometric amounts of BaCO3, Al2O3, K2CO3, KCl, and B2O3. The samples were preheated at 500 °C for 10 h to decompose the carbonate and eliminate the water. After they were grounded, the mixtures were gradually raised to the appropriate temperatures (K3B6O10Cl, 730 °C; and BaAlBO4, 810 °C) for two days with several intermediate grindings. The powder X-ray diffraction (XRD) data were recorded by using a Bruker D2 PHASER diffractometer (Billerica, MA, USA) equipped with a monochromatized Cu Kα radiation source (λ = 1.5418 Å) at room temperature. The UV–Vis–NIR diffuse reflectance spectra for K3B6O10Cl were measured on a Shimadzu SolidSpec-3700DUV spectrophotometer (Kyoto, Japan) with a wavelength range from 180 to 2000 nm at room temperature. Infrared (IR) spectra were measured using a Shimadzu IR Affinity-1 Fourier transform IR spectrometer (Kyoto, Japan) in the range from 400 to 4000 cm−1 with a resolution of 2 cm−1 at room temperature. KBr was used as the reference pellet, and the samples were mixed thoroughly with it. The mid-infrared emissive spectra of K3B6O10Cl and BaAlBO4 powder were calculated from the reflectance spectra measured by Fourier-transform infrared spectroscopy (FT-IR) (Nicolet IS-50, Thermo Scientific, Waltham, MA, USA) with an integrating sphere (Mid-IR IntegratIR, PIKE Technologies, Madison, WI, USA). The emissivity of K3B6O10Cl and BaAlBO4 powders were calculated by the equation: ε = 1 − R, according to Kirchhoff’s law of thermal radiation.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/photonics12060597/s1, Figure S1. (a) B atoms project phonon structure of Be2BO3F (red lines), which contains only triangular [BO3]3− units. (b) The IR-active phonons of Be2BO3F between 600 and 1400 cm−1. Figure S2 (a) Crystal structure of BaAlBO4. Triangular [BO3]3− and tetrahedral [BO4]5− units are illustrated by green triangles and blue tetrahedrons. (b) and (c) B atoms (in triangular [BO3]3− units and tetrahedral [BO4]5− units, respectively) projected phonon structure of BaAlBO4; d) The IR-active phonons of BaAlBO4 between 600 and 1400 cm−1. The red and blue lines are the IR-active phonons due to triangular [BO3]3− and tetrahedral [BO4]5− units, respectively. Figure S3 XRD results of K3B6O10Cl (a) and BaAlBO4 (b). Figure S4 The band structures of K3B6O10Cl (a) and BaAlBO4 (b) by PBE functional. Figure S5 The reflectance of K3B6O10Cl powder. Figure S6 (a) B atoms projected phonon structure of LiB2O3F. The red and blue lines are due to B atoms in triangular [BO3]3− and tetrahedral [BO3F]4− units, respectively. (b) The IR-active phonons of LiB2O3F between 600 and 1400 cm−1. The red and blue lines are the IR-active phonons due to triangular [BO3]3− and tetrahedral [BO4]5− units, respectively. Figure S7 (a) and (b) B atoms projected phonon structure of CaB5O7F3. The red and blue lines are due to B atoms in triangular [BO3]3− and tetrahedral [BO3F]4− units, respectively. (c) The IR-active phonons of CaB5O7F3 between 600 and 1400 cm−1. The red and blue lines are the IR-active phonons due to triangular [BO3]3− and tetrahedral [BO4]5− units, respectively. Figure S8 (a) and (b) B atoms projected phonon structure of CsKB8O12F2. The red and blue lines are due to B atoms in triangular [BO3]3− and tetrahedral [BO3F]4− units, respectively. (c) The IR-active phonons of CsKB8O12F2 between 600 and 1400 cm−1. The red and blue lines are the IR-active phonons due to triangular [BO3]3− and tetrahedral [BO4]5− units, respectively. Figure S9 (a) and (b) B atoms projected phonon structure of RbB4O6F. The red and blue lines are due to B atoms in triangular [BO3]3− and tetrahedral [BO3F]4− units, respectively. (c) The IR-active phonons of RbB4O6F between 600 and 1400 cm−1. The red and blue lines are the IR-active phonons due to triangular [BO3]3− and tetrahedral [BO4]5− units, respectively. Figure S10. The calculated IR-active phonons of SiO2, BaSO4, Si3N4, Al2O3, and HfO2 between 600 and 1400 cm−1. The shaded region represents the atmospheric window.

Author Contributions

B.X. and W.Z. conceived the initial idea of this research. R.Z. and E.H. performed the calculations and the experimental test of optical properties. B.X. and W.Z. wrote the paper and all authors commented on it. All authors have read and agreed to the published version of the manuscript.

Funding

B.X. acknowledges the support from “Double First-Class” University project of China Pharmaceutical University (CPU2018GFY25).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All relevant data are available from the authors.

Acknowledgments

This work was also supported by the advanced computing resources provided by the High-Performance Computing Center of Nanjing University.

Conflicts of Interest

The authors declare no conflict of interest.

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Photonics 12 00597 g001
Figure 1. Design intrinsic SSEs with anion groups. Red line is the emissivity of ideal SSEs. By filling the atmospheric window with enough IR-active phonons from different anion groups (blue and orange lines), it is expected that a high and broad emission (green dashed line) can be obtained.
Figure 1. Design intrinsic SSEs with anion groups. Red line is the emissivity of ideal SSEs. By filling the atmospheric window with enough IR-active phonons from different anion groups (blue and orange lines), it is expected that a high and broad emission (green dashed line) can be obtained.
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Figure 2. (a) B-atom-projected phonon structure of KMgBO3 (red lines), containing only one kind of anion: triangular [BO3]3− unit, as the green triangles in the inset. (b) The IR-active phonons of KMgBO3 between 600 and 1400 cm−1; (c) B-atom-projected phonon structure of LiGeBO4, containing only tetrahedral [BO4]5− unit, as the blue tetrahedrons in the inset of (d); and (d) the IR-active phonons of LiGeBO4 between 600 and 1400 cm−1.
Figure 2. (a) B-atom-projected phonon structure of KMgBO3 (red lines), containing only one kind of anion: triangular [BO3]3− unit, as the green triangles in the inset. (b) The IR-active phonons of KMgBO3 between 600 and 1400 cm−1; (c) B-atom-projected phonon structure of LiGeBO4, containing only tetrahedral [BO4]5− unit, as the blue tetrahedrons in the inset of (d); and (d) the IR-active phonons of LiGeBO4 between 600 and 1400 cm−1.
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Figure 3. (a) Crystal structure of K3B6O10Cl. Triangular [BO3]3− and tetrahedral [BO4]5− units are illustrated by green triangles and blue tetrahedrons. Red balls are oxygen atoms. (b,c) B-atom (in triangular [BO3]3− units and tetrahedral [BO4]5− units, respectively)-projected phonon structure of K3B6O10Cl; and (d) the IR-active phonons of K3B6O10Cl between 600 and 1400 cm−1. The red and blue lines are the IR-active phonons due to triangular [BO3]3− and tetrahedral [BO4]5− units, respectively.
Figure 3. (a) Crystal structure of K3B6O10Cl. Triangular [BO3]3− and tetrahedral [BO4]5− units are illustrated by green triangles and blue tetrahedrons. Red balls are oxygen atoms. (b,c) B-atom (in triangular [BO3]3− units and tetrahedral [BO4]5− units, respectively)-projected phonon structure of K3B6O10Cl; and (d) the IR-active phonons of K3B6O10Cl between 600 and 1400 cm−1. The red and blue lines are the IR-active phonons due to triangular [BO3]3− and tetrahedral [BO4]5− units, respectively.
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Figure 4. (a) Emissivity of K3B6O10Cl (red line) and BaAlBO4 (black line) powder in the range from 3 to 18 μm. (b,c) Comparison of IR absorption (black line), emissivity (red line), and calculated IR-active phonons (green lines) from 400 to 1400 cm−1 for K3B6O10Cl and BaAlBO4, respectively.
Figure 4. (a) Emissivity of K3B6O10Cl (red line) and BaAlBO4 (black line) powder in the range from 3 to 18 μm. (b,c) Comparison of IR absorption (black line), emissivity (red line), and calculated IR-active phonons (green lines) from 400 to 1400 cm−1 for K3B6O10Cl and BaAlBO4, respectively.
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Figure 5. The calculated IR-active phonons of LiB2O3F (a), CaB5O7F3 (b), CsKB8O12F2 (c), and RbB4O6F (d). The insets display the triangular [BO3]3− (green triangles) and tetrahedral [BO3F]4− units (blue tetrahedrons) in these fluorooxoborates. The red and blue lines are the IR-active phonons due to triangular [BO3]3− and tetrahedral [BO3F]4− units, respectively.
Figure 5. The calculated IR-active phonons of LiB2O3F (a), CaB5O7F3 (b), CsKB8O12F2 (c), and RbB4O6F (d). The insets display the triangular [BO3]3− (green triangles) and tetrahedral [BO3F]4− units (blue tetrahedrons) in these fluorooxoborates. The red and blue lines are the IR-active phonons due to triangular [BO3]3− and tetrahedral [BO3F]4− units, respectively.
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Table 1. Typical IR absorption peaks for anion groups.
Table 1. Typical IR absorption peaks for anion groups.
Anion GroupsTypical IR Absorption Peaks (cm−1)
[SO4]2−1210~1025, 1000~950
[PO4]3−1100~1050, 970~940
[NO3]−1510~1210, 1060~1020, 840~800, 760~715
[SiO4]4−1100~1000
[BO3]3−1375~1150, 750~600
[BO4]5−~1100, 1045~1000, 954~800
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Zhang, R.; Huang, E.; Zhong, W.; Xu, B. Phonon Structure Engineering for Intrinsically Spectrally Selective Emitters by Anion Groups. Photonics 2025, 12, 597. https://doi.org/10.3390/photonics12060597

AMA Style

Zhang R, Huang E, Zhong W, Xu B. Phonon Structure Engineering for Intrinsically Spectrally Selective Emitters by Anion Groups. Photonics. 2025; 12(6):597. https://doi.org/10.3390/photonics12060597

Chicago/Turabian Style

Zhang, Rui, Enhui Huang, Wenying Zhong, and Bo Xu. 2025. "Phonon Structure Engineering for Intrinsically Spectrally Selective Emitters by Anion Groups" Photonics 12, no. 6: 597. https://doi.org/10.3390/photonics12060597

APA Style

Zhang, R., Huang, E., Zhong, W., & Xu, B. (2025). Phonon Structure Engineering for Intrinsically Spectrally Selective Emitters by Anion Groups. Photonics, 12(6), 597. https://doi.org/10.3390/photonics12060597

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