Pigments and Near-Infrared Phosphors Based on Mn⁵⁺

Abstract

The optical properties of Mn⁵⁺ ions, which are responsible for the intense green–turquoise–blue coloration of Mn⁵⁺-based pigments and the near-infrared emission of phosphors, are the focus of this article. Mn⁵⁺ ions enter crystalline matrices in four-fold coordinated positions and can maintain their 5+ valence state when crystalline hosts meet the conditions described in this work. Mn⁵⁺ ions have [Ar]3d² electronic configuration and always experience a strong crystal field due to a high electric charge; therefore, their lower electronic states have the ³A₂ < ¹E < ¹A₁ < ³T₂ < ³T₁ progression in energy. We present the properties of several Mn⁵⁺-based pigments and discuss the electronic transitions responsible for their coloration. Specifically, we show that the color is determined by the spin-allowed ³A₂ → ³T₁(³F) absorption, which extends across the orange–red–deep red spectral region and is strongly influenced by crystal field strength. The narrow-band emission Mn⁵⁺-activated near-infrared phosphors arise from the spin-forbidden ¹E → ³A₂ transition, whose energy is independent of the crystal field strength and determined by the nephelauxetic effect. We demonstrate the linear relationship between ¹E state energy and the nephelauxetic parameter β₁ using Racah parameter literature data for Mn⁵⁺ phosphors. Lastly, we address the recent applications of these Mn⁵⁺ phosphors in luminescence thermometry.

1. Introduction

The vivid colors of materials have captivated individuals since antiquity. The principal factor influencing a material’s color is its interaction with light in the visible spectrum (380–750 nm) perceptible to the human eye. Unlike organic dyes, inorganic pigments have superior resistance to heat, light, weathering, solvents, and chemicals, rendering them preferable for artist pigments, exterior coatings, and heat-reflective paints. Inorganic pigments come in a wide variety of colors, from subtle earth tones to vivid blues, greens, and reds. Their colors are highly saturated and fade-resistant, ensuring long-term use in a wide range of applications [1], such as automotive and industrial coatings, paints, plastics, printing inks, cosmetics, and construction materials. Furthermore, pigments play a vital part in the creation of paper, rubber, glass, porcelain, and glazes. Inorganic pigments have high chemical stability, making them resistant to degradation from light, heat, and chemical interactions. This stability makes them ideal for outdoor applications where weather resistance is critical. They also provide a variety of opacity levels, from wholly opaque to transparent, allowing for fine control of visual effects in paints, varnishes, and other materials. Although the fundamental principles of color science for gemstones and minerals are comprehended, precisely predicting the color of inorganic solids continues to be difficult until they are experimentally produced [2].

Pigments are classified based on their different properties. Color is one approach to categorize; white pigments, colorful pigments, and black pigments are the three primary divisions, each with its own set of components. Among white pigments, titanium dioxide, in both rutile and anatase forms [3,4]; zinc sulfide, including lithopones [5]; and zinc oxide take precedence. Colored pigments come in a variety of colors, ranging from blues like complex metal oxides, ultramarine, and Prussian blue [6] to greens like chromium(III) oxide. Yellows include iron(III) oxide hydroxide [7], lead chromate, bismuth vanadate [8], and cadmium sulfide, and reds vary from iron(III) oxide to lanthanum tantalum oxynitride. Carbon black represents the realm of black pigments [9]. Classification based on chemical composition highlights a pigment’s underlying characteristics. Oxides, such as iron oxide and titanium dioxide, are known for their opacity and durability. Sulfides, such as cadmium and zinc sulfide, provide bright colors and resistance to heat. Carbonates, silicates, and hydrated oxides, along with pigments such as zinc carbonate and ultramarine blue, create a flexible palette suitable for a variety of applications. Furthermore, various pigments, such as carbon black and metallic powders, have specialized applications, particularly in the ink and decorative finishes industries.

Transition metal ions are crucial as pigments, offering a broad spectrum of vibrant colors and exceptional stability [10,11]. These ions possess unique properties that make them indispensable in applications such as coatings, plastics, ceramics, and cosmetics. In terms of properties, transition metal pigments exhibit a remarkable variability in color, ranging from deep blues and greens to vivid reds and yellows. Their high opacity and coverage ensure uniform coloration with minimal application, while their chemical stability ensures longevity and resistance to environmental factors like light, heat, and chemicals [12]. Additionally, their compatibility with various binders and substrates facilitates their incorporation into different materials and formulations. Transition metals having a coordination environment with less symmetry and more mixing between the p and d orbitals are more likely to produce bright colors due to the relaxation of selection rules for d–d transitions. Charge transfer transitions typically have high transition probabilities (Laporte permitted), resulting in colorful hues, and they tend to dominate crystal field transition colors when both are present.

Cobalt, copper, iron, nickel, and chrome compounds are commonly used transition metal pigments, each with a unique color and application. Cobalt is known for its brilliant blue and purple hues; copper is prized for its vibrant green and blue shades; iron produces reds, browns, and yellows; nickel provides yellow, green, and gray tones; and chrome, bright yellows and greens. Despite their broad application, these pigments have several drawbacks. Cobalt pigments have been linked to health risks, as cobalt compounds can cause skin sensitivity and respiratory problems in vulnerable persons. Moreover, cobalt pigments’ color may fade or change, especially when exposed to extreme climatic conditions or reactive substances. Copper-based pigments may corrode over time. They can oxidize when exposed to environmental elements such as moisture and oxygen, resulting in color changes and damage to the substance to which they are applied. Iron pigments, while durable and stable, can exhibit a relatively limited color range compared to other pigments. Additionally, they may lack the vibrancy and intensity found in pigments derived from other metals. Moreover, iron-based pigments can be prone to color fading over time, particularly when exposed to harsh environmental conditions such as prolonged sunlight or moisture. Nickel-based pigments pose health risks due to the carcinogenic properties of nickel compounds. Prolonged exposure to nickel pigments, whether through inhalation or skin contact, can lead to allergic reactions and respiratory issues. Additionally, nickel pigments may exhibit limited chemical stability, making them prone to degradation when exposed to certain environmental conditions or chemical agents. Chromium-based pigments raise environmental and health concerns due to the toxicity of chromium compounds. Exposure to chromium pigments can adversely affect human health and the environment, prompting regulatory restrictions and environmental management protocols. Despite these drawbacks, ongoing research and development efforts focus on mitigating the adverse effects of chromium, copper, nickel, and cobalt pigments. Strategies include exploring alternative ions, enhancing production processes, and developing safer substitutes to address environmental and health concerns while maintaining the desired color properties.

One of the solutions is the use of manganese, since it is an essential trace element in the human body and has good biocompatibility. Manganese is known for its ability to stabilize in different oxidation states in crystalline solids, ranging from 1+ to 7+. Each manganese oxidation state has unique electronic and optical properties, which provide opportunities to develop pigments having a wide range of coloration. Well-known examples include manganese brown pigments based on Mn₃O₄, manganese violet (NH₄MnP₂O₇), manganese pink (Al₂O₃:Mn), orange/red Mn⁴⁺-activated oxide phosphors [13], black pigments with high reflectivity in the near-infrared spectral range [14], and Mn⁵⁺-based pigments which are discussed in detail in this paper. Important progress has been achieved recently with the development of blue pigment based on Mn³⁺ optical centers in the hexagonal YInO₃ and, similarly, in the trigonal bipyramidal sites of hexagonal ScGaZnO₄, LuGaZnO₄, and LuGaMgO₄ [15]. The Mn³⁺ optical centers provide red coloration when introduced into indium sites of the monoclinic Li₃InB₂O₆ [16], purple in YGaO₃, brown in YAlO₃ [17] and CaAl₁₂O₁₉ [18], and violet in LaAlGe₂O₇ [19]. Mn²⁺ can facilitate brown coloration when it is introduced to the Zn site in Zn₂SiO₄ [20].

Mn⁵⁺-based pigments are distinguished by their ability to generate vibrant blue and turquoise/green colors [21,22]. Moreover, Mn⁵⁺-based pigments often exhibit excellent opacity, leading to substantial coverage and consistent coloration in paints, coatings, and other substances. This characteristic is effective for attaining desired aesthetic outcomes with minimal application. Mn⁵⁺ pigments provide significant stability against deterioration from conditions including light exposure, heat, and chemical interactions. This guarantees the durability and color retention of items containing these pigments, enhancing their lasting attractiveness. Mn⁵⁺ pigments exhibit compatibility with various binders and substrates, enabling their integration into multiple formulations and materials. This adaptability increases their usefulness across multiple industries and applications. Mn⁵⁺ pigments play an important part in the creation of eye-catching glazes, tiles, and decorative pottery. Beyond traditional applications, Mn⁵⁺ pigments may exhibit unique optical and magnetic properties in specialized matrices, offering opportunities for innovation in fields such as electronics, optics, and magnetic materials. These matrices enable the development of novel materials and technologies that leverage the distinctive properties of Mn⁵⁺ pigments for diverse applications.

Upon activation with low concentrations of Mn⁵⁺ ions, certain phosphors exhibit light emission in the near-infrared (NIR) spectral range, specifically at wavelengths exceeding 1100 nm and characterized by narrow spectral bands. The narrowband near-infrared luminescence of Mn⁵⁺ is beneficial for near-infrared lasers [23,24,25] and for narrow-band near-infrared light sources designed for the targeted detection of chemical substances [26]. Recent studies demonstrate that nanoparticles activated by Mn⁵⁺ function as effective probes for luminescence imaging in deep tissues and luminescence thermometry within the second biological transparency window (1000–1350 nm). Moreover, these nanoparticles demonstrate significant resistance to photochemical degradation [27,28].

This article discusses the electronic processes that regulate the color and emission properties of Mn⁵⁺ pigments and phosphors, demonstrates the use of crystal field engineering for the control of their color and emission, and reviews the current landscape of Mn⁵⁺ pigments and phosphors.

2. Electronic and Optical Properties of Mn⁵⁺ Ions

The Mn⁵⁺ ion has two electrons in the outer 3d electron shell, so its electron configuration is [Ar]3d², where [Ar] denotes the electron configuration of argon with completely filled electron shells 1s²2s²2p⁶3s²3p⁶. Coulomb interaction between two 3d electrons produces 45 degenerated microstates, which are grouped in five LS terms: two spin-triplets—³F and ³P—and three spin-singlets—¹D, ¹G, and ¹S. Here, the ^2S+1L notation is used, where S is the total spin, and L is the orbital momentum. According to Hund’s rule, the ³F term is the ground state. The degree of degeneracy of these terms is as follows: 21 for ³F, 9 for ³P, 5 for ¹D, 9 for ¹G, and 1 for ¹S.

The quantitative description of the energy level scheme of a free Mn⁵⁺ ion requires knowledge of the so-called Racah parameters B and C, which have dimensions of energy and are linear combinations of the Slater integrals. In terms of the Racah parameters, the energies of the above-given terms are as follows (the energy of the ground term ³F is taken as zero): 15B for the ³P term, 5B + 2C for the ¹D term, 12B + 2C for the ¹G term, and 22B + 7C for the ¹S term. The values of B and C fitted to the experimentally observed energy levels of free Mn⁵⁺ ions are B₀ = 1223 cm⁻¹ and C₀ = 4613 cm⁻¹ [29]. The Hartree–Fock calculated values (which are typically 15–20% overestimated if compared to the experimental values) of the same parameters are B₀ = 1436 cm⁻¹ and C₀ = 5450 cm⁻¹ [11].

The degenerated energy levels will undergo splitting in the crystal field, with the splitting pattern governed by the point group symmetry of the impurity ion’s site and the magnitude of the splitting reliant on the interionic distances and charges of the adjacent ions. Mn⁵⁺ ions often occupy four-fold coordinated sites in solids. If such a position has the ideal tetrahedral symmetry described by the T_d point group, then the splitting patterns of all above-mentioned LS terms are as follows (the notation of the irreducible representations of the T_d group is used): ³F → ³A₂ + ³T₁ + ³T₂, ³P → ³T₁, ¹D → ¹E + ¹T₂, ¹G → ¹A₁ + ¹E + ¹T₁ + ¹T₂, and ¹S → ¹A₁. If the true symmetry of the Mn⁵⁺ site is reduced, namely resembling a deformed tetrahedron, the orbitally degenerate states will experience additional splitting, consistent with the symmetry characteristics of the corresponding point group.

The conventional method for analyzing the spectra of transition metal ions in crystals relies on Tanabe–Sugano diagrams [30], which illustrate the splitting of the terms of free ions within cubic crystal fields (in fact, the term “cubic” is a very general one and covers both tetrahedral and octahedral symmetries). These diagrams are usually plotted for a fixed ratio of C/B; the horizontal axis corresponds to the non-dimensional ratio Dq/B (Dq is the crystal field strength), and the vertical axis is the energy E of the crystal field states in terms of the Racah parameters B and E/B.

Figure 1 shows the Tanabe–Sugano diagram for the 3d² electron configuration in the tetrahedral crystal field (since the Mn⁵⁺ ions occupy the four-fold coordinated sites). In a majority of crystal field books, the Tanabe–Sugano diagrams are plotted for the octahedral crystal field, but the tetrahedral Tanabe–Sugano diagrams can easily be obtained from the octahedral ones if the conjugate electron configurations dⁿ and d^10-n are considered. Thus, the Tanabe–Sugano diagram for Ni²⁺ (3d⁸ configuration) in the octahedral crystal field corresponds to the Tanabe–Sugano diagram for Mn⁵⁺ or Cr⁴⁺ (3d² configuration) in the tetrahedral crystal field. The vertical dashed line separates the diagram into two sections designated as the “weak” and “strong” crystal fields. In the former case, the first excited state is the orbital and the spin-triplet ³T₂, and the emission will correspond to the broad spin-allowed ³T₂ → ³A₂ transition. This can be the case with the ions with a smaller electric charge, V²⁺ for example. In the latter case, the first excited state is the orbital doublet and spin-singlet ¹E, and the emission will be due to the sharp spin-forbidden ¹E → ³A₂ transition. The Mn⁵⁺ ions, because of their high electric charge, are always in a strong crystal field situation. Figure 1 also shows a typical way of excitation of the Mn⁵⁺ emission: at first, the absorption takes place to the ³T₂ state, then the non-radiative relaxation to the ¹E state occurs, and, finally, the ¹E → ³A₂ emission transition is realized in the near-infrared spectral range.

A particular feature of the 3d² electron configuration is that the energy difference between the ³A₂ and ³T₂ states coming from the ³F term is equal to the 10Dq, which gives an easy way of estimating the crystal field strength from the position of the lowest in energy excitation (absorption) peak.

3. Spectroscopic Properties, Color, and Photoluminescence of Mn⁵⁺ in Crystalline Solids

The Mn⁵⁺ ions tend to enter crystalline matrices in four-fold coordinated positions. From the point of view of charge compensation, the easiest way would be if they substituted for the pentavalent ions, such as P⁵⁺, V⁵⁺, or As⁵⁺. Therefore, the phosphates, vanadates, and arsenates are the most promising materials for doping with the Mn⁵⁺ ions. With certain charge compensation, it is possible to substitute Mn⁵⁺ ions for Si⁴⁺ ions in various silicate hosts [31].

The typical absorption (or excitation) spectrum of Mn⁵⁺ ions in solids is dominated by two broad structured bands, Figure 2, which are due to the spin-allowed transitions ³A₂ → ³T₂ (³F) at about 10,000–12,000 cm⁻¹ and ³A₂ → ³T₁ (³F) at about 15,000–17,000 cm⁻¹, depending on the host. The intensity of the former is usually somewhat lower because, in the T_d symmetry, the ³A₂ → ³T₂ transition is forbidden by the group selection rules. The electric dipole-allowed ³A₂ → ³T₁ (³P) transition that corresponds to a two-electron jump occurs at higher energies of about 24,000–27,000 cm⁻¹. The absorption from the spin-forbidden transitions to the singlet state ³A₂ → ¹A₁ (¹G) occurring around 13,500 cm⁻¹ is weak, sharp, and does not depend on the crystal field strength. It is only weakly dependent on the host material’s properties because of the covalent effects (the nephelauxetic effect). The ³A₂ → ¹E (¹D) spin-forbidden transition is barely seen in the absorption spectra. Charge transfer bands (CTBs) occur at high energies and are strictly host-dependent.

The color of Mn⁵⁺ pigments is primarily determined by the ³A₂ → ³T₁ (³F) absorption, which extends across the orange–red–deep red spectral region. The energy of the barycenter of this absorption band is significantly influenced by the crystal field strength surrounding Mn⁵⁺ ions; refer to Figure 1. Therefore, it is possible to tune the pigment color through crystal field engineering, which involves modifying the host crystal structure to alter the distances between Mn⁵⁺ ions and ligand ions, as well as changing the type of ligand ions. The absorption strength of this transition is influenced by the Mn⁵⁺ concentration, and any change in this concentration also affects the intensity of pigment color. The CT and ³A₂ → ³T₁ (³P) transitions allow absorption in the violet spectral region, with the extent of absorption influenced by the crystal field strength. As a result, the color of the Mn⁵⁺ pigments is a blend of green and blue, with a slight addition of violet in certain cases.

The ¹E (¹D) → ³A₂ transition determines the emission spectra of the Mn⁵⁺ ions. It occurs at about 8200–8900 cm⁻¹. In certain cases, the weak and broad emission from the ³T₂ (¹D) → ³A₂ transition can be observed at above-room temperature, usually in systems with low crystal field splitting. The ¹E emission band is narrow due to weak electron–phonon interaction (the lateral displacement between ³A₂ and ¹E states is quite small). The ¹E lifetime is relatively long (half a millisecond) due to the spin-forbidden character of the ¹E → ³A₂ transition. The Tanabe–Sugano diagram for the 3d² configuration in a tetrahedral crystal field (Figure 1) indicates that the energy difference between the ground state ³A₂ and the first excited state ¹E is unaffected by the crystal field strength, as both states are parallel to one another. This energy difference closely approximates the energy interval between the ³F and ¹D states of the free ion, defined using the Racah parameters as 8B + 2C, where B and C differ among various hosts due to covalent effects. Figure 3a shows a perfect correlation between the energy of the ¹E state and (8B + 2C) for Mn⁵⁺ in different hosts listed in Table 1. Thus, the nephelauxetic effect predominantly influences the energy of the ¹E state.

For the quantitative analysis of the nephelauxetic effect, a new nephelauxetic parameter β₁ was introduced earlier for the ions with the 3d³ electron configuration [32,33] and derived in Ref. [34]:

$${\beta }_1=\sqrt{{\left(B/B_0\right)}^2+{\left(C/C_0\right)}^2}$$

(1)

It has been demonstrated in those references that the dependence of energy of the ²E→⁴A₂ spin-forbidden transition for the 3d³ electron configuration is a linear function of the β₁ parameter. Later on, the energy of the ¹E→³A₂ spin-forbidden transition of the Ni²⁺ ions in the octahedral coordination (whose energy level sequence is identical to the case of the Mn⁵⁺ ions in the tetrahedral coordination, considered in the present work) was also shown to be a linear function of the β₁ parameter. Following that approach, the dependence of the Mn⁵⁺ ions’ ¹E state energy on the β₁ parameter is shown in Figure 3b for data taken from Table 1; it appears to be linear (R² of 0.94),

$$\mathrm{E}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y} \mathrm{o}\mathrm{f} {}^1\mathrm{E} \mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{e} \left[{\mathrm{c}\mathrm{m}}^{-1}\right]=11418·{\beta }_1+1101.1.$$

(2)

Table 1. Crystal field strength Dq, Racah parameters B and C, nephelauxetic parameter β₁, and the energy of the ¹E state for the tetrahedrally coordinated Mn⁵⁺ ions in different crystals.

Host Material	Dq [cm−1]	B [cm−1]	C [cm−1]	β1	1E [cm−1]	Reference
Y2SiO5	1133	550	2255	0.6642	8754.0	[35]
Sr10(VO4)6F2	1088	518	2321	0.6577	8642.4	[36]
Sr5(PO4)3Cl	1053	510	2407	0.6679	8710	[37]
Ca2PO4Cl	1162	455	2657	0.6857	8849.6	[37]
Li3PO4	1208	475	2556	0.6767	8802.0	[37]
Ca2AsO4Cl	1030	530	2245	0.6516	N.A. *	[38]
Ca2VO4Cl	1000	535	2253	0.6557	N.A.	[38]
Li3VO4	1049	646	2006	0.6842	8950.9	[39]
YAlO3	1100	485	2256	0.6296	8267.2	[40]
Ba3(VO4)2	1000	530	2250	0.6525	8499.1	[41]
Ca6Ba(PO4)4O	1060	544	2292	0.669	8773.9	[42]
Ba2SiO4	1131	419	N.A.	N.A.	8403.4	[43]

* N.A.—not applicable.

Table 1. Crystal field strength Dq, Racah parameters B and C, nephelauxetic parameter β₁, and the energy of the ¹E state for the tetrahedrally coordinated Mn⁵⁺ ions in different crystals.

Host Material	Dq [cm−1]	B [cm−1]	C [cm−1]	β1	1E [cm−1]	Reference
Y2SiO5	1133	550	2255	0.6642	8754.0	[35]
Sr10(VO4)6F2	1088	518	2321	0.6577	8642.4	[36]
Sr5(PO4)3Cl	1053	510	2407	0.6679	8710	[37]
Ca2PO4Cl	1162	455	2657	0.6857	8849.6	[37]
Li3PO4	1208	475	2556	0.6767	8802.0	[37]
Ca2AsO4Cl	1030	530	2245	0.6516	N.A. *	[38]
Ca2VO4Cl	1000	535	2253	0.6557	N.A.	[38]
Li3VO4	1049	646	2006	0.6842	8950.9	[39]
YAlO3	1100	485	2256	0.6296	8267.2	[40]
Ba3(VO4)2	1000	530	2250	0.6525	8499.1	[41]
Ca6Ba(PO4)4O	1060	544	2292	0.669	8773.9	[42]
Ba2SiO4	1131	419	N.A.	N.A.	8403.4	[43]

* N.A.—not applicable.

It can be noted from the data collected in Table 1 that both Racah parameters B and C are strongly reduced from their free ion values. This is a clear indication of a high degree of covalency of the Mn⁵⁺–O²⁻ chemical bonds, which is caused by a high electric charge of the manganese ions pulling the charge density from the oxygen ions towards manganese and thus enhancing the overlap between their wave functions. Higher values of the β₁ parameter are indications of the more ionic nature of the “impurity ion–ligand” chemical bonds, whereas lower β₁ values correspond to enhanced covalent interactions between the impurity ions and ligands.

Oxide complexes including two or more metals represent an important category of thermally stable pigments. The color spectrum of these pigments is generally determined by the presence of various 3d transition metals (V, Cr, Mn, Fe, Co, Ni, or Cu) and a number of coordination polyhedra that can accommodate 3d transition metal ions (tetrahedral, pentahedral, octahedral, or prismatic sites). Phosphate compounds of alkali or alkaline earth metals doped with transition metal ions have been thoroughly examined across many fields, including catalysis [44,45,46], energy research (notably with lithium phosphate cathodes currently under intense study [47,48,49]), and healthcare (where calcium phosphate materials are pivotal biomaterials and continue to be the preferred option for biomedical applications, as substantiated by recent reviews [50,51,52,53]). Moreover, these compounds play a significant role in the coloration of inorganic substances, functioning as dyes or pigments based on their resistance to corrosive environments [54,55,56,57].

Oxide matrices activated in tetrahedral sites by 3d² transition-metal ions, such as Cr⁴⁺, Mn⁵⁺, and Fe⁶⁺, showcase vibrant colors thanks to their significant absorption cross sections in the visible spectral region. Furthermore, these ions exhibit exceptional luminescent characteristics. While Mn⁵⁺ and Fe⁶⁺ activated materials display sharp line luminescence in the NIR range [58], compounds activated with Cr⁴⁺ display a broad emission due to a much weaker nephelauxetic effect. This feature renders them appropriate for use as tunable lasers in the NIR region [59].

Manganese blue, an industrial pigment, is formed by the introduction of Mn⁵⁺ ions into BaSO₄. Nonetheless, due to environmental issues, production of this pigment has almost come to a halt. The high price of indium-containing pigments activated by Mn⁵⁺ has limited their commercial use. To reintroduce Mn⁵⁺ in commercial pigments, new host material ought to be found [60]. There is a clear need to find cheaper blue pigments with comparable optical characteristics to those currently in use. The main limitation of Mn⁵⁺ is the need for a host material that can stabilize manganese ions in their 5+ oxidation state. According to Shannon [61], Mn⁵⁺ exists only in a tetrahedral coordination, with a small effective ionic radius of 33 pm. The (VO₄)³⁻ group in Ba₃(VO₄)₂ is a good example of a structure for the successful Mn⁵⁺ ion incorporation at V⁵⁺ sites (V⁵⁺ ion radius in tetrahedral coordination is approximately 35.5 pm). Isovalent tetrahedral molecular ions such as (PO₄)³⁻ or (VO₄)³⁻ can be easily substituted with (MnO₄)³⁻ ions, with no charge compensation required because of the same electric charge and similar effective ionic radius (see

Table 2. Effective ionic radii in tetrahedral coordination of selected metals, according to Shannon [57].

Ion	Mn5+	V5+	P5+	As5+	Si4+
Radius	33 pm	35.5 pm	17 pm	33.5 pm	26 pm

). These replacements produce phosphate and vanadate pigments in bright, vivid colors ranging from blue to green [62,63]. These newly developed pigments are easy to synthesize, demonstrate resistance to heat and acids, and are environmentally friendly, offering significant potential for diverse applications where thermal resistance is critical, such as roofing materials. Similar substitutions can be made to (AsO₄)³⁻ without charge compensation and to (SiO₄)²⁻ with charge compensation, for example, with Al.

Several compounds with Mn⁵⁺ have been documented for their bright coloration. For instance, Mn takes 5+ valence and occupies tetrahedral sites in the brownmillerite-type Ba₂In_2-xMn_xO_5+x, providing a light yellow (x = 0), intense turquoise (x = 0.1), green (x = 0.2, 0.3), or dark green (x ≥ 0.4) color [60]. The distinct color of these compounds arises from the substantial absorption of visible light at 500 nm. Under reducing conditions, Mn⁵⁺ is converted to Mn³⁺, resulting in Ba₂In_2−xMn_xO_5+x turning black. The colors and diffuse reflectance spectra of Ba₂In_2−xMn_xO_5+x phases are shown in Figure 4. The light-yellow appearance of pure Ba₂In₂O₅ results from the band edge tailing into the visible spectrum. Replacing indium with manganese produces a turquoise to green hue due to increased optical absorption in both high- and low-energy parts of the visible spectrum. The high-energy peak predominantly originates from the In–O charge-transfer transitions, whereas a low-energy shoulder results from the Mn–O charge transfer, affecting the visible hue.

The apatite-type structure A₅(MO₄)₃X with A as Ca²⁺, Sr²⁺, Ba²⁺, or Pb²⁺; M as P⁵⁺, Mn⁵⁺, Cr⁵⁺, or V⁵⁺; and X as F⁻, Cl⁻, OH⁻, provides substantial possibilities for the combining of various A-, M-, and X-site elements, facilitating the creation of compounds with a wide range of advantageous features [64,65]. Notably, apatite-type compounds featuring Mn⁵⁺ tetrahedrally coordinated by oxygen exhibit promising characteristics as materials for pigments that produce vibrant blue and green colors [66,67].

The 5+ oxidation state of manganese is infrequent in inorganic oxides because of its instability, as Mn⁵⁺ typically takes more stable oxidation states like 4+ and 7+. However, apatite-type compounds that include Mn⁵⁺ in tetrahedral coordination at the M-site with alkaline earth elements, especially Ba²⁺, at the A-site exhibit stability because of the high first ionization potential of alkaline earth elements (approximately 5 eV for Ba²⁺), which promotes the preservation of this uncommon oxidation state of Mn [68]. A variety of compounds with an apatite structure have been synthesized, and their structural, optical, and magnetic characteristics and coloration have been studied [69].

Table 3. Synthesized materials doped with Mn⁵⁺ used as pigments.

Material	Synthesis	Structure	Color	Reference
Ba5Mn3−xMxO12Cl (M = V5+, P5+) (x = 0–3.0)	Solid-state Sol-gel process	hexagonal P63/m	White (x = 3.0) Turquoise (x = 1.5) Dark green (x = 0)	[69]
Ba3(V1−xMnxO4)2 (0 < x ≤ 1.0)	Conventional ceramic route	hexagonal R3m	Turquoise (x = 0.10) Green (x = 0.20, 0.25) Dark green (x ≥ 0.50)	[70]
Ba2Ti1−xMnxO4+x/2 (0 < x ≤ 0.15) Ba2Si1−xMnxO4 x = 0.10 and 0.15	Conventional ceramic route	Ba2Ti1−xMnxO4+x/2 orthorhombic P21nb Ba2Si1−xMnxO4 orthorhombic Pmcn	Turquoise (x = 0.10, 0.15) Turquoise (x = 0.10, 0.15)	[70]
Ba3(P1−xMnxO4)2 x = 0, 0.02, 0.05, 0.10, 0.25, 0.30, 0.40, 0.50, 0.75, 0.90, 1.00	Conventional ceramic route	hexagonal R3m	Sky blue (x = 0.02–0.10) Turquoise blue (x = 0.25–0.40) Green and dark green (x ≥ 0.50)	[54]
Ca6Ba(P1−xMnx)4O17 (0.005 ≤ x ≤ 0.13)	Solid-state	monoclinic C2/m (No. 12)	Sky blue color near the green	[71]
BaAl2−xMnxO4+y (0 ≤ x ≤ 0.10)	Sol-gel process	ferroelectric phase P63 when T increase paraelectric phase P6322	White (x = 0.05) Green blue (0.01 ≤ x ≤ 0.03) Green yellow (0.04 ≤ x ≤ 0.10)	[22]
Ba7Al2−xMnxO10+y (0 ≤ x ≤ 0.7)	Solid-state		From cyan to ocean blue	[72]
Ba2In2−xMnxO5+x (x = 0.1−0.7)	Solid-state	Ba2In1.9Mn0.2O5.1 Orthorhombic Icmm Ba2In1.8Mn0.2O5.2 tetragonal I4/mcm Ba2In2−xMnxO5+x (x = 0.4, 0.5, 0.6) cubic phases Pm3m	Light yellow (x = 0) Turquoise (x = 0.1) Green (0.2, 0.3) Dark green (x ≥ 0.4)	[60]
Ba10(P1−xMnxO4)6F2 x = (0.1–1)	Solid-state	hexagonal P63/m	Turquoise blue to dark green	[73]

presents the compounds doped with Mn⁵⁺ and the synthesis techniques, structural characteristics, and powder color variations resulting from Mn⁵⁺ ion doping.

The optical properties of Mn⁵⁺-doped Ba₅Mn_3-xM_xO₁₂Cl (M = V, P) apatite structures have been presented by Medina et al. [69]. The doped samples show bright colors from light to dark turquoise and dark green, while undoped Ba₅(PO₄)₃Cl and Ba₅(VO₄)₃Cl are white, Figure 5a. L*, a*, and b* denote the coordinates of the CIELAB color space, with L* indicating lightness, the a*-axis representing the green–red range, and the b*-axis spanning from blue (−b*) to yellow (+b*) [74]. The color coordinates (L*, a*, b*) exhibit an increase in L* and a* values alongside a decrease in b* values with higher Mn content (larger x values) in Ba₅Mn_3-xM_xO₁₂Cl (M = V, P) samples (Figure 5). The diffuse reflectance spectra of Ba₅Mn_3-xV_xO₁₂Cl pigments, presented in Figure 5b, reveal substantial absorption of red/orange light (around 630 nm) and purple light (around 400 nm), with low absorption in the green/blue spectral region (around 500–520 nm). These optical properties result in the manifestation of green or turquoise colors. Ba₅V₃O₁₂Cl and Ba₅P₃O₁₂Cl are white because they do not absorb visible light. The high-energy absorption peak in the UV spectral region is caused by the Mn⁵⁺–O²⁻ charge transfer. Reduction of manganese concentration decreases absorbance and lightens samples. The authors measured near-infrared reflectance spectra of pigments to assess their cool pigments (Figure 5c). We found out that the reflection intensity decreases with an increase in Mn content, i.e., for pigments of a darker color. Reflectance in the 750–2500 nm range is about 70–85%, making all compounds promising cool pigments.

Sun Woog Kim et al. [71] analyzed novel inorganic sky-blue pigments, Ca₆Ba(P_1-xMn_x)₄O₁₇ (0 ≤ x ≤ 0.13), produced using a solid-state reaction technique, emphasizing its chromatic characteristics alongside thermal and chemical stability. The most vivid sky-blue hue, Figure 6, was obtained for Ca₆Ba(P_0.99Mn_0.01)₄O₁₇, which has CIE color parameters of L* = 75.47, a* = 11.67, b* = 10.29, C = 15.6, and H = 221.4 (see the upper table on Figure 6.)

To evaluate the color thermal stability of this pigment, the authors utilized the Ca₆Ba(P_0.99Mn_0.01)₄O₁₇ sample. Firstly, the authors heated the sample at 300 °C, 600 °C, and 1000 °C for 6 h to evaluate the thermal stability of its color performance. Using the equation, the color difference (ΔE) of the samples was determined [74] thus:

$$∆E=\sqrt{{∆L}^{*2}+{∆a}^{*2}+{∆b}^{*2}}$$

(3)

The samples heated at 300 °C and 600 °C showed color difference (ΔE) values of 1.61 and 1.12, respectively, below the minimum difference in color perceptible by humans (ΔE_min = 2) [75]. The sample heated at 1000 °C had a ΔE value of 2.27. The result suggests that the pigment possesses excellent thermal stability within the 0–600 °C region; however, its stability reduces as temperature rises into the 600–1000 °C temperature range. These pigments displayed some chemical color instability in acidic and alkaline solutions; see the upper table in Figure 6. Despite the necessity for enhancements in chemical color stability, this pigment may serve as a promising option for a novel ecologically friendly inorganic sky-blue pigment.

In comparison to Mn²⁺ and Mn⁴⁺ optical centers, emissions from Mn⁵⁺ centers have been observed in a much smaller number of host materials. These are, for example, Li₃PO₄ [76], Sr₅(VO₄)₃F₄^,[25,77], Ba₅(PO₄)₃Cl [78], Sr₅(PO₄)₃Cl [78,79,80], Ca₂PO₄Cl [78,79], Ca₂VO₄Cl [78,79], Sr₂VO₄Cl [78,79], Ca₆Ba(PO₄)₄O [42], Y₂SiO₅ [31], and M₂SiO₄ (M  =  Ba, Sr, Ca) [43].

The early application of Mn⁵⁺ emission has been concentrated on the development of NIR-emitting solid-state lasers [23,25]. The laser action is characterized using a three-level laser scheme, and it has been demonstrated that the long lifetime of the ¹E state, together with its substantial visible absorption, makes the Mn⁵⁺ system suitable for flashlamp pumping. However, the Mn⁵⁺ laser’s tunability is severely constrained due to its narrow emission. The typical internal quantum efficiencies (IQE) of Mn⁵⁺-activated phosphors vary from 20% to 40%, with new findings indicating a potential enhancement in IQE with Bi³⁺ co-doping [81].

4. Luminescence Thermometry with Mn⁵⁺ Ions

Mn⁵⁺ has the benefit of activating nanoparticles, making them ideal probes for luminescent imaging in deep tissues and luminescent thermometry in the second window of biological transparency (1000–1350 nm). Recently, Piotrowski et al. [82] demonstrated Mn⁵⁺ lifetime-based thermal imaging in the optical transparency windows through skin-mimicking tissue phantoms. Dramicanin et al. [42] presented and explained the near-infrared luminescence of Ca₆Ba(PO₄)₄O:Mn⁵⁺ and demonstrated its use for temperature sensing in the near-infrared spectral region. When excited within a broad and strong absorption band spanning from 500 to 1000 nm, this phosphor provides an ultra-narrow emission (FWHM = 5 nm) centered at 1140 nm, originating from the spin-forbidden ¹E→³A₂ transition. They discovered that the ¹E emission is quenched due to thermally assisted crossover with the ³T₂ state and that the relatively high Debye temperature of 783 K for Ca₆Ba(PO₄)₄O facilitates efficient emission. A high Debye temperature indicates the material’s rigid structure. The increased structural rigidity restricts the non-radiative transition of photons, resulting in enhanced quantum efficiency. This phosphor has been effective in luminescence intensity ratio thermometry, with a relative sensitivity of 1.92% K⁻¹ and temperature resolution of 0.2 K within the physiological temperature range. The relative sensitivity of a thermometer is defined as the rate of the temperature-induced change in a measured luminescence feature divided by the magnitude of this feature, and the temperature resolution is the smallest change in a temperature that causes a perceptible change in the measured luminescence feature [83]. While there are no established thresholds at which the materials are considered promising for thermometers since different applications require different measurement performances and temperature operating ranges, it is generally assumed that thermometers with relative sensitivities exceeding 1% K⁻¹ and temperature resolution better than 0.5 K around room temperature may be considered promising [84,85].

As illustrated in Figure 7a, an increase in temperature results in the enhancement of the broad emission peak from the ³A₂ level within the 950 nm to 1030 nm range, while the narrow emission peak from the ¹E level around 1140 nm decreases. This phenomenon occurs due to thermalization between the ¹E and ³T₂ levels, with the energy difference (ΔE_T = 1216 cm⁻¹) being overcome by thermally excited electrons. To experimentally determine the uncertainty in the luminescence intensity ratio (LIR), 50 emission spectra were recorded at each temperature. The mean of these measurements was used as the LIR value, and the standard deviation (σ_LIR) was considered the uncertainty in LIR, as depicted in the inset of Figure 7b (distribution measured at 30 °C). Figure 5c shows that the relative sensitivity value, represented by blue dots for measurements taken at 30 °C, ranges from 2.35%K⁻¹ to 1.26%K⁻¹ over the temperature range, with a value of 1.92% K⁻¹ at 30 °C. This sensitivity is among the highest recorded for luminescent thermometers operating within the second biological transparency window.

5. Conclusions

The color of Mn⁵⁺ pigments is influenced by the spectral positions and intensity of absorptions from the spin-allowed ³A₂ → ³T₁ (³F) (the orange–deep red spectral region) and ³A₂ → ³T₁ (³P) (the violet spectral region) electronic transitions. The former can be controlled by engineering the crystal field surrounding the Mn⁵⁺ ions (by varying manganese-ligand bond lengths, bond angles, and ligand type), whereas the latter can be altered by changing Mn⁵⁺ concentration. The charge transfer between Mn⁵⁺ and ligands also has an effect on violet absorption. The turquoise-blue coloring of the Mn⁵⁺ pigments is a result of these absorptions. The narrow-band near-infrared emission with a long lifetime, typically around a half millisecond, occurs in low-doped materials. It is composed of emission from the spin-forbidden ¹E → ³A₂ transition and its vibrational sidebands. The energy of the ¹E emission is strongly influenced by the nephelauxetic effect (covalency) and is not dependent on the crystal field strength. We showed here that the energy of the ¹E state is linearly dependent on the value of the nephelauxetic parameter β₁. The weak and broad emission from the spin-allowed ³T₂ (¹D) → ³A₂ transition can be observed at temperatures above room temperature. This phenomenon is typically observed in systems with small crystal field splitting when the energy difference between ¹E and ³T₂ is sufficiently small to allow thermalization. Despite the difficulty of maintaining manganese’s 5+ valence state when added into solids, the number of reported Mn⁵⁺ pigments and phosphors is growing over time. Suitable hosts for Mn⁵⁺ doping typically consist of (PO₄)³⁻, (VO₄)³⁻, or (SiO₄)²⁻ groups, where Mn⁵⁺ replaces P⁵⁺, V⁵⁺, or Si⁴⁺ in tetrahedral coordination, and contain electropositive alkaline earth metals such as Ba, Sr, and Ca. Mn⁵⁺ pigments are generally non-toxic and possess favorable stability, which presents a significant opportunity for their future application. Their narrow-band near-infrared emission was initially evaluated for solid-state laser development but has recently been applied in bioimaging and biothermal imaging within the second biological transparency window. Luminescence thermometry sensors utilizing Mn⁵⁺ emission typically exhibit a large relative sensitivity of approximately 2% K⁻¹ at room temperature. This, combined with small uncertainties in the measurements of intense ¹E emissions, results in high precision and accuracy, achieving around 0.2 K in measurements.