Introduction

In recent years, an overwhelming amount of new phosphor materials has emerged to meet versatile industrial and scientific requests. Most of these phosphors are Ce³⁺- or Eu²⁺-doped oxide or nitride-based ceramics, which show luminescence based on electric dipole-allowed 4f^N−15d¹→4f^N transitions (N=1 for Ce³⁺, N=7 for Eu²⁺).^1-6 In order to meet the numerous requirements for applications in light-emitting devices and displays, the employed host compounds for these activator ions usually combine structural rigidity and high coordination number with high chemical and thermal resistance to induce desirable luminescence features like narrow-band emission, high quenching temperatures and high quantum efficiency. Famous commercialized examples are Y₃Al₅O₁₂ : Ce³⁺ (YAG : Ce³⁺),⁷ Sr₂Si₅N₈ : Eu²⁺,⁸ Sr[LiAl₃N₄] : Eu²⁺ (SLA : Eu²⁺)⁹ and Sr[Li₂Al₂O₂N₂] : Eu²⁺ (SALON : Eu²⁺).¹⁰ Besides those, Eu²⁺ activated CaAlSiN_3,¹¹ alkali lithooxidosilicates,^12-17 (oxo−)nitridoberyllates^18-21 and oxynitrides^{22, 23} have attracted attention due to their favorable luminescence characteristics. Despite the promising photoluminescence properties of Eu²⁺ for next-generation phosphor-converted white light-emitting diodes (pc-wLEDs), the more abundant transition metal ion Mn²⁺ could be a potential alternative in this field. Since the photoluminescence of Mn²⁺ relies on electronic transitions within the spatially less extended 3d subshell compared to the diffuse 5d orbitals of Eu²⁺, narrower and more color-pure emission bands can generally be expected from Mn²⁺ if appropriately optimized. Moreover, given the spatially extended nature of the 3d orbitals, the emission color of Mn²⁺ can be as easily controlled by the coordination geometry and type of surrounding ligands as for Eu²⁺. Tetrahedrally coordinated Mn²⁺ usually exhibits narrow-band green luminescence, e.g. in MgAl₂O₄ : Mn²⁺ (λ_em=525 nm),²⁴ Zn₂SiO₄ : Mn²⁺ (λ_em=525 nm),^{25, 26} ZnGa₂O₄ : Mn²⁺ (λ_em=500 nm),²⁷ BaZnAl₁₀O₁₇ : Mn²⁺ (λ_em=516 nm),²⁸ LaMgAl₁₁O₁₉ : Mn²⁺ (λ_em=517 nm),²⁹ Sr₂MgAl₂₂O₃₆ : Mn²⁺ (λ_em=518 nm),³⁰ Zn₄B₆O₁₃ : Mn²⁺ (λ_em=540 nm).³¹ In contrast, octahedrally coordinated Mn²⁺ typically shows red emission, e.g. MgSiO₃ : Mn²⁺ (λ_em=661 nm),³² Mg₄Ta₂O₉ : Mn²⁺ (λ_em=659 nm),²⁶ Ba₂M(BO₃)₂ : Mn²⁺ (M=Mg, Ca; λ_em=615 nm, 630 nm),^{33, 34} NaZn(PO₃)₃ : Mn²⁺ (λ_em=613 nm, 643 nm).³⁵ This is related to a stronger ligand field splitting in an octahedral field and accompanied by larger emission bandwidths as well as generally lower quenching temperatures (T₅₀).³⁶ The marked difference in the emission properties for tetrahedrally and octahedrally coordinated Mn²⁺ is understandable from the d⁵ Tanabe-Sugano diagram valid for both cases (see Figure 1). The lowest energetic excited ligand field state ⁴T_1(g)(⁴G) of a d⁵ ion decreases in energy relative to the ground state ⁶A_1(g)(⁶S) with increasing ligand field strength. The spin-forbidden nature of the optical transitions of Mn²⁺ in a 3d⁵ high-spin configuration leads to a generally long emission decay time, which may pose problems to high-power pc-wLEDs based on saturation effects (cf. the related emitter Mn⁴⁺).³⁷ The decay time can be, however, shortened upon incorporation into hosts with heavier atoms due to enhanced spin-orbit coupling or upon exploitation of magnetic exchange interaction in Mn−Mn pairs.³⁹ Often is only an ionic picture of the crystal field interaction considered and its strength is generally enhanced by decreasing Mn−ligand distances as well as a transition from tetrahedral to octahedral coordination geometry. However, the impact of covalency and a decrease in the interelectronic repulsion parametrized by the Racah parameters B and C is as important and can also be used to tune the ligand field strength. Nitrides with tetrahedrally coordinated doping sites for Mn²⁺ serve as an illustrative example here. AlN : Mn²⁺ (λ_em=600 nm),⁴⁰ ZnGeN₂ : Mn²⁺ (λ_em=610 nm)⁴¹ and MgSiN₂ : Mn²⁺ (λ_em=630 nm)⁴² are red-emitting phosphors despite tetrahedral coordination of the doped Mn²⁺ ions, which is a consequence of the strong σ-donating nature of the N³⁻ ligands and the resulting strong degree of covalency of the Mn−N bonds. In contrast, one would expect a blue-shifted luminescence of octahedrally coordinated Mn²⁺ ions if the surrounding ligands were weakly coordinating with a high degree of ionicity in the Mn-ligand bond. This strategy has so far only been scarcely regarded. An alternative approach to obtain narrow-band emitting phosphors based on Mn²⁺ with other emission colors than green is thus to embed them in an octahedral ligand field with only weakly coordinating ligands. Similar effects are already known for Eu²⁺ in selected borates e.g. SrB₄O₇^{43, 44} and aluminates⁴⁵ or very ionic fluorides such as AMgF₃ (A=K, Rb),^{46, 47} for which in the latter example, even the observation of 4f⁷-4f⁷ narrow-line luminescence of Eu²⁺ was possible.

Years ago, borosulfates as a new class of silicate-analogue compounds started to gain increasing interest and in recent years some borosulfate anions were characterized as weakly coordinating, making them promising candidates for our investigations.^48-50 Typically, the anionic substructures of borosulfates consist of vertex-connected (BO₄)- and (SO₄)tetrahedra formally substituting the (SiO₄)tetrahedra in silicates.⁵¹ The anionic substructures comprise e.g. oligomeric anions (like nesosilicates),^{52, 53} infinite chains (like inosilicates),^{53, 54} layers (like phyllosilicates)^{50, 55-57} or 3D network structures (like tectosilicates).^{58, 59} The weak coordination behavior was also demonstrated by the stabilization of (I₄)²⁺-cations.⁶⁰ The latter can also be obtained in presence of a sufficiently powerful oxidizer and weakly coordinating anions such as [AsF₆]⁻ and [SbF₆]⁻.⁶¹ The special reaction conditions during the synthesis as well as the weak coordination behavior of the borosulfate anions are most likely also the reason for the stabilization of the cluster-like heteropolycations in [Au₃Cl₄][B(S₂O₇)₂] and the chain-like heteropolycations in [Au₂Cl₄][B(S₂O₇)₂](SO₃).⁶² Spectroscopic evidence for the weak coordination behavior of borosulfates has so far been mainly related to the luminescence of lanthanoid ions such as Ce³⁺ or the UV/Vis absorption of transition metal ions such as Co²⁺ or Ni²⁺.^{48, 50, 63}

Based on these promising findings, it was our goal to demonstrate that weakly coordinating ligands can be beneficially employed to tune the photoluminescence properties of Mn²⁺ for alternative lighting sources in a desirable manner and that the covalency of the Mn-ligand bond plays an equally important role in controlling of the ligand field strength.

Results and Discussion

Zn[B₂(SO₄)₄] crystallizes in the space group P2₁/n (no. 14), with the lattice parameters a=7.8338(4), b=8.0967(4), c=9.0399(4) Å and β=111.26(1)°.⁵⁷ It features heteropolyanionic layers of alternating (BO₄)- and (SO₄)-tetrahedra sharing a common vertex. Each (BO₄)-tetrahedron is connected to four (SO₄)-tetrahedra, which themselves are connected only to two (BO₄)-tetrahedra, thus exhibiting two terminal oxygen atoms. The layers consist of vierer- and zwölfer-rings of alternating tetrahedra, a structural motif which can be found in a lot of conventional phyllosilicate-analogue borosulfates. Inside the twelve-membered rings the charge compensating Zn²⁺ cations reside, octahedrally coordinated by six oxygen atoms; four belonging to the surrounding ring and two denote to the adjacent polyanionic layers. All oxygen atoms coordinating the Zn²⁺ cation are terminal oxygen atoms of the (SO₄)-tetrahedra. Figure 2 depicts the structure and coordination of the cation in Zn[B₂(SO₄)₄] : Mn²⁺, both within the layer (Figure 2b) and by two oxygen atoms of adjacent layers, which are situated in a trans arrangement within the coordination octahedron (Figure 2a).

Figure 3 shows a representative Rietveld refinement of the solid solution series Zn[B₂(SO₄)₄]: x%Mn²⁺ (x=0.5, 1, 2, 3, 4, 5, 10). The percentage of dopant was fixed during the refinement. Further details of the synthesis and the fitting quality parameters of the Rietveld refinement are given in the Supporting Information.

Luminescence Spectroscopy

All of the Mn²⁺-activated Zn[B₂(SO₄)₄] samples, enclosed in quartz ampoules, with high optical quality, showed luminescence upon excitation with UV light. Figure 4a depicts the photoluminescence emission (PL) and excitation (PLE) spectra of Zn[B₂(SO₄)₄]: 10 % Mn²⁺ at room temperature. An intense, broad orange emission is observed under excitation with 290 nm. The emission band is located at 595 nm with a full width at half maximum (FWHM) of 2960 cm⁻¹ (≈103 nm in the wavelength domain) and the emission is assigned to the ⁴T_1g(⁴G)→⁶A_1g(⁶S) transition according to the Tanabe-Sugano diagram. The PL and PLE spectra were also recorded at 77K but no additional information could be gained due to limited emission intensity of the compounds in evacuated quartz ampoules. Figure 4b depicts the luminescence decay trace of the ⁴T_1g(⁴G)→⁶A_1g(⁶S) transition of Mn²⁺. It shows multi-exponential decay with an average decay time of 21 ms. The luminescence characteristics agree with the structural feature of octahedrally coordinated Mn²⁺. The decay time of 21 ms is longer than the commonly reported decay times for tetrahedrally coordinated Mn²⁺ (e.g. MgAl₂O₄: 3 % Mn²⁺: τ=6.5 ms at room temperature;^{24, 39} [Bz(Me)₃N]₂[MnCl₄]: τ=3.8 ms at room temperature ([Bz(Me)₃N]⁺: Benzyl-trimethylammonium);⁶⁴ Zn₂SiO₄: 0.05 % Mn²⁺: τ=13 ms at room temperature).⁶⁵ Even if the regarded high doping fraction of 10 % Mn²⁺ is taken into account, the extracted decay time of 19 ms at room temperature already indicates that violation of the spin selection rule cannot be the sole reason for the slow decay, but the transition must be also parity-forbidden. This goes in line with examples of octahedrally coordinated Mn²⁺ ions in inorganic host compounds such as KMgF₃: 0.5 % Mn²⁺ (τ=100 ms at room temperature) or selected perovskite-derived chlorides AMCl₃ : Mn²⁺ (A=K, Rb, Cs; M=Mg, Ca, Sr) (τ≈20 ms at room temperature) with slightly distorted octahedral doping sites for the Mn²⁺ ions.^{66, 67} In contrast, the FWHM of the Mn²⁺-related emission band in Zn[B₂(SO₄)₄] : Mn²⁺ is comparably small and in a similar order of magnitude compared to reported values for the FWHMs of emission bands for tetrahedrally coordinated Mn²⁺ (in halides: 1400 cm⁻¹–2000 cm⁻¹ at room temperature).⁶⁴ Moreover, the emission wavelength is comparably short for octahedrally coordinated Mn²⁺, which usually extends beyond 600 nm (e.g. MgSiO₃ : Mn²⁺: λ_em=661 nm,³² CsMnBr₃: λ_em=680 nm,⁶⁸ Ba₂Ca(BO₃)₂ : Mn²⁺: λ_em=630 nm³³) and only becomes orange in weak octahedral ligand fields (AMCl₃ : Mn²⁺: λ_em≈560 nm,⁶⁷ CsPbCl₃ : Mn²⁺: λ_em=586 nm,⁶⁹ MgF₂ : Mn²⁺: λ_em=586 nm⁷⁰). Altogether, these properties demonstrate that the Mn²⁺ ions are octahedrally coordinated in Zn[B₂(SO₄)₄] : Mn²⁺ but with only weakly coordinating ligands. This is in agreement with earlier findings by Höppe et al. on lanthanoid-activated borosulfates.^{48, 71} The weak coordination behavior is understandable given the presence of the strongly electron-withdrawing S^VI atoms that compete with Mn²⁺ about the electron density at the 2p lone pair of a coordinating O atom. In agreement with this interpretation, the luminescence decay time barely changes between 77 K and room temperature (see Figure 4b). The photoluminescence excitation spectra (PLE) of Zn[B₂(SO₄)₄]: 10 % Mn²⁺ shows characteristic transitions of Mn²⁺ that can be assigned according to the d⁵ Tanabe-Sugano diagram (see Figure 1). All these transitions are spin- and parity-forbidden in an octahedral ligand field. Furthermore, a weak bluish emission is also observed, which is attributed to host-related luminescence but could not specified further. Even in nominally undoped Zn[B₂(SO₄)₄], a broad-banded emission peaking at around 550 nm can be observed at room temperature (see Supporting Information). The intensity of this host emission is small upon excitation with 290 nm and is only dominant in samples with low Mn²⁺ content (3 mol % or below, see Supporting Information). Figure 5 shows the PL properties of Mn²⁺-activated Zn[B₂(SO₄)₄] with varying Mn²⁺-concentrations. The ⁴T_1g(⁴G)→⁶A_1g(⁶S)-based emission band of the Mn²⁺ ions is slightly red-shifted for the higher doped samples, which is a consequence of exchange interaction between different Mn²⁺ ions.³⁸ However, given a relatively long closest Zn−Zn distance of 6.2707(2) Å in Zn[B₂(SO₄)₄],⁵⁶ the effect of mutual interactions between the incorporated Mn²⁺ ions is limited. Exchange interaction between the Mn²⁺ ions is usually accompanied by an increasing FWHM and a gradual decrease of the decay time together with increasingly multi-exponential decay behavior.³⁸ Both the dependence of the emission FWHM and average luminescence decay times show that exchange coupling and concentration quenching is weak up to the regarded doping concentrations of 10 mol %.

It should be noted that also an effect due to a change in the average Mn−O bond length is imaginable, which could affect the ligand field splitting based on its R⁻⁵ dependence for d-elements.^{72, 73} Since Mn²⁺ in its 3d⁵ high-spin configuration has, however, a larger ionic radius than Zn²⁺ with a 3d¹⁰ configuration, one would expect an overall decreasing ligand field strength giving rise to an observable blue-shift of the emission based on the d⁵ Tanabe-Sugano diagram (see Figure 1). Since this is not experimentally observed, we conclude that this contribution is not the dominant one.

Conclusion

Mn²⁺ is a so-far underestimated potential emitter for inorganic phosphors given its less spatially extended 3d orbitals compared to the 5d orbitals exploited in the celebrated case of Eu²⁺. In most studies, the emission color is varied by changes in either the metal−ligand distances or the coordination geometry. Tetrahedrally coordinated Mn²⁺ typically emits in the green, octahedrally coordinated Mn²⁺ typically in the red range. An alternative concept to tune the emission color of transition metal ions can be electronic control of the nature of the metal-ligand bond. This concept was explicitly demonstrated in the phyllosilicate-analogous borosulfates Zn_1-xMn_x[B₂(SO₄)₄] (x=0, 0.03, 0.04, 0.05, 0.10) with weakly-coordinating borosulfate ligands. Zn[B₂(SO₄)₄]: x% Mn²⁺ (x=3, 4, 5, 10) shows an orange ⁴T_1g(⁴G)→⁶A_1g(⁶S) emission at≈590 nm, which is unusually high in energy for Mn²⁺ in an octahedral coordination of an oxidic compound and indeed implies a weak coordination behavior of the ligands according to the d⁵ Tanabe -Sugano diagram. By comparison with literature-known examples of Mn²⁺-activated symmetric fluorides and chlorides with octahedrally coordinated doping sites for this activator, we were able to place the borosulfate ligands into the commonly known spectrochemical series of ligands just between Cl⁻ and F⁻. These studies give an insight into more sophisticated possibilities to tailor the emission color of Mn²⁺ in inorganic phosphors by electronic means with chemically more uncommon ligands.

Conflict of interest

The authors declare no conflict of interest.