Synthesis and Crystal Structure of the Short LnSb₂O₄Br Series (Ln = Eu–Tb) and Luminescence Properties of Eu³⁺-Doped Samples

Abstract

Pale yellow crystals of LnSb₂O₄Br (Ln = Eu–Tb) were synthesized via high temperature solid-state reactions from antimony sesquioxide, the respective lanthanoid sesquioxides and tribromides. Single-crystal X-ray diffraction studies revealed a layered structure in the monoclinic space group P2₁/c. In contrast to hitherto reported quaternary lanthanoid(III) halide oxoantimonates(III), in LnSb₂O₄Br the lanthanoid(III) cations are exclusively coordinated by oxygen atoms in the form of square hemiprisms. These [LnO₈]¹³⁻ polyhedra form layers parallel to (100) by sharing common edges. All antimony(III) cations are coordinated by three oxygen atoms forming ψ¹-tetrahedral [SbO₃]³⁻ units, which have oxygen atoms in common building up meandering strands along [001] according to ${}_{\infty }{}^1\{{[{\mathrm{SbO}}_{2/2}^v{\mathrm{O}}_{1/1}^t]}^{–}\}$ (v = vertex-sharing, t = terminal). The bromide anions are located between two layers of these parallel running oxoantimonate(III) strands and have no bonding contacts with the Ln³⁺ cations. Since Sb³⁺ is known to be an efficient sensitizer for Ln³⁺ emission, photoluminescence studies were carried out to characterize the optical properties and assess their suitability as light phosphors. Indeed, for both, GdSb₂O₄Br and TbSb₂O₄Br doped with about 1.0–1.5 at-% Eu³⁺ efficient sensitization of the Eu³⁺ emission could be detected. For TbSb₂O₄Br, in addition, a remarkably high energy transfer from Tb³⁺ to Eu³⁺ could be detected that leads to a substantially increased Eu³⁺ emission intensity, rendering it an efficient red light emitting material.

1. Introduction

Lanthanoid compounds containing complex oxoanions are an interesting material class for application as light phosphors because the valence 4f-states of lanthanoids typically give rise to photoluminescence characteristic for the respective lanthanoid. However, direct photoexcitation into 4f-levels is an inefficient way to stimulate photoluminescence, because of the forbidden character of these transitions. For all-inorganic materials this problem can be circumvented through charge-transfer sensitization or by energy transfer (ET) from other metal cations, which are all allowed processes. Both processes are possible in lanthanoid(III) oxopnictogenates(III/V) [1,2,3,4], such as oxoantimonates, which are readily available through high-temperature solid-state reaction of the respective binary starting materials, i.e., the respective lanthanoid(III) and pnictogen(III) sesquioxides (Ln₂O₃ and Pn₂O₃). Moreover, the lanthanoid(III) oxoarsenates(III) or -antimonates(III) generally feature a quite high stability in water and air. Varying the host lattice and the lanthanoid(III) dopant in these systems reveals the possibility to fine-tune the selective emission properties in a wide range [2,5,6]. For example, current research on Y[PO₄] co-doped with Gd³⁺ and Sb³⁺ revealed that antimon(III) cations are appropriate to activate gadolinium(III) emission in the ultraviolet (UV-B) range [7]. This inspired us to synthesize lanthanoid(III) containing oxoantimonates(III) to have materials at hand equipped with intrinsic Sb³⁺ cations as sensitizer for Ln³⁺ cations and therefore being suitable as efficient luminescent material class.

The structural diversity of lanthanoid(III) oxopnictogenates(III) is quite large and varies not only with the lanthanoid, but also on the pnictogen (As or Sb). Structural complexity can be enlarged through the incorporation of halide anions. For ternary lanthanoid(III) oxoarsenates(III) the compositions LnAsO₃ (Ln = La, Ce) and Ln₂As₄O₉ (Ln = Pr, Nd, Sm) are known [8,9,10,11]. The Ln[AsO₃] representatives crystallize either with the α-Pb[SeO₃]- or the K[ClO₃]-type structure exhibiting coordination numbers of eight or nine for the lathanoid(III) cations and feature isolated ψ¹-tetrahedral [AsO₃]^3– anions without any bridging As–O–As contacts as the dominating structural feature. These anions occur as discrete triangular pyramids with stereochemically active lone pairs at the As³⁺ cations. In contrast, the oxoarsenates Ln₂As₄O₉ display both [As₂O₅]^4– pyroanions and cyclic [As₄O₈]^4– units as condensation products of ψ¹-tetrahedral [AsO₃]^3– entities according to Ln₄[As₂O₅]₂[As₄O₈]. Up to now, no other further ternary lanthanoid(III) oxoarsenates(III) are known.

The addition of fluxes, such as alkali-metal halides, to the oxide reaction mixtures in order to improve crystallization and reduce the reaction temperature often results in the formation of different halide-anion containing quaternary compounds, enriching the structural diversity. So far, three different compositions for lanthanide oxoarsenates(III) containing trivalent lanthanoids are known, i.e., Ln_{5 × 3}[AsO₃]₄, Ln₃OX[AsO₃]₂ and Ln₃X₂[As₂O₅][AsO₃]. Similar to their halide-free congeners, they have in common ψ¹-tetrahedral [AsO₃]³⁻ anions, and also feature pyroanionic [As₂O₅]⁴⁻ groups. In contrast to them, because of the presence of halide anions, mixed oxide-halide coordination polyhedra around the Ln³⁺ cations occur, providing coordination numbers as high as seven, eight or nine [12,13,14,15,16,17,18].

With antimony as the heavier congener of arsenic, lanthanoid(III) oxoantimonates(III) with three different compositions are known: LnSb₅O₁₂ (Ln = La), Ln₃Sb₅O₁₂ (Ln = Pr, Nd, Sm–Gd and Yb) and LnSbO₃ (Ln = Ce). While LaSb₅O₁₂ contains both Sb³⁺ and Sb⁵⁺ cations with coordination numbers of three and six, respectively, featuring pyramidal [SbO₃]^3– as well as octahedral [SbO₆]^7– anions, Ln₃Sb₅O₁₂ and CeSbO₃ include antimony(III) cations exclusively. In the Ln₃Sb₅O₁₂ series, coordination numbers of three and four for the Sb³⁺ cations with oxygen atoms are present in the shape of vertex-connected [SbO₃]^3– ψ¹-tetrahedra and [SbO₄]^5– ψ¹-square pyramids, while the unique central lanthanoid(III) cation is coordinated eightfold by oxygen atoms. In cubic CeSbO₃, coordination numbers of 8 for cerium and 6 for antimony are reported with identical interatomic distances (d(Ce‒O) = d(Sb‒O) = 234 pm). However, both crystallographically independent oxygen-atom sites have to be only occupied by about 85% to realize trivalent antimony, so the true crystal structure of CeSbO₃ remains puzzling [19,20,21,22,23].

Compared to the oxoarsenates(III), analogous compounds with the heavier congener antimony synthesized under halide-flux conditions seemed to be restricted to the formula type Ln₅X₃[SbO₃]₄ (tetragonal, P4/ncc for X = F; monoclinic, P2/c for X = Cl) [24,25]. For the heavier lanthanoids, divergent oxoantimonate(III) halides with the composition LnSb₂O₄X are encountered, which exhibit completely different structural features in their crystal structures known so far and can be interpreted as derivatives of the rare-earth metal(III) (RE) oxobismuthate(III) halides (REBi₂O₄X with RE = Y, Pr, Nd and Sm‒Lu for X = Cl‒I) [26,27,28,29]. For lanthanum being the largest rare-earth metal(III) cation, an antimony(III)- and bromide-containing crystal structure with the formula (La_0.75Sb_0.25)OBr related to LaOBr (PbFCl-type structure, tetragonal, P4/nmm) is also reported, which can be understood as heavily Sb³⁺-doped LaOBr [25]. Not in this compound, but in all the LnSb₂O₄X cases, the Ln³⁺ cations have only contact to oxygen atoms resulting in square [LnO₈]^13– square prisms. In Ln_1+nSb_2‒nO₄X with Ln = Nd, Sm and X = Cl, Br as the only known members, tetragonal crystal structures (P4/mmm) containing anionic layers of vertex-sharing square pyramidal [SbO₄]^5– units (square ψ¹-pyramids) according to ${}_{\infty }{}^2\{{[{\mathrm{SbO}}_{4/2}^v]}^{-}\}$ (v = vertex-sharing) are observed. The halide anions have little or no contact to the Ln³⁺ cations, but settle with far away Sb³⁺ cations at positions, where four of them encircle their stereochemically active lone pairs. However, no knowledge of further representatives and their crystal structures for the heavier lanthanoid(III) systems Ln‒Sb‒O‒X was available up to now.

In this work, we report on the single-crystal synthesis of the short LnSb₂O₄Br series (Ln = Eu–Tb) crystallizing in a novel structure type featuring layered polyhedra around the central lanthanoid(III) cations without any contact to the bromide anions. With the crystal structure at hand, photoluminescence measurements of bulk GdSb₂O₄Br were performed to investigate the luminescence processes caused by Sb³⁺. With this understanding, the photoluminescence investigations of EuSb₂O₄Br and TbSb₂O₄Br as well as doped GdSb₂O₄Br:Eu³⁺ and TbSb₂O₄Br:Eu³⁺ were undertaken to examine the consequence of incorporating multiple luminescent cations. The article discusses both the relevant excitation and emission spectra and deals with possible energy transfer mechanisms of the involved ions investigated with lifetime measurements to reach an understanding about the major luminescence processes taking place and the special role of Sb³⁺ cations in the crystal structure as intrinsic sensitizers and possible efficient alternative to the forbidden direct excitation via 4f-4f transitions of the Ln³⁺ ions.

2. Materials and Methods

2.1. Synthesis of LnSb₂O₄Br Representatives

The lanthanoid(III) oxoantimonate(III) bromides LnSb₂O₄Br (Ln = Eu–Tb) were synthesized via solid-state reactions from sesquioxides with anhydrous lanthanide tribromides (Ln₂O₃: ChemPur, Karlsruhe, Germany, 99.9%) at elevated temperatures. About 1.0 mg (0.0028 mmol) of Eu₂O₃ was applied for doping and 54 mg (0.154 mmol) for the pure europium compound, gadolinium sesquioxide (Gd₂O₃: 54 mg, 0.153 mmol) or in case of Ln = Tb a mixture of terbium metal (Tb: ChemPur, Karlsruhe, Germany, 99.9%: 8 mg, 0.050 mmol) and terbium(III,IV) oxide (Tb₄O₇: ChemPur, Karlsruhe, Germany, 99.9%: 49 mg, 0.066 mmol) for in-situ synproportionation of Tb₂O₃, lanthanoid(III) tribromide (EuBr₃, GdBr₃ and TbBr₃, Sigma-Aldrich, Taufkirchen, Germany, 99.99%: 61 mg, 0.153 mmol) and antimony sesquioxide (Sb₂O₃: ChemPur, Karlsruhe, Germany, 99.9%: 134 mg, 0.460 mmol) served as starting materials according to Equation (1). Eutectic mixtures of cesium bromide (CsBr: ChemPur, Karlsruhe, Germany, 99.9%: 530 mg, 2.490 mmol) and sodium bromide (NaBr: Merck, Darmstadt, Germany, suprapur: 127 mg, 1.234 mmol) were used as flux.

$$L{n}_2{\mathrm{O}}_3 + Ln{\mathrm{Br}}_3 + 3 {\mathrm{Sb}}_2{\mathrm{O}}_3 \stackrel{ \mathrm{CsBr} + \mathrm{NaBr} }{\to } 3 Ln{\mathrm{Sb}}_2{\mathrm{O}}_4\mathrm{Br} (\mathrm{Ln}= \mathrm{Eu}‒\mathrm{Tb})$$

(1)

The reactions were carried out in evacuated fused silica ampoules (Quarz- und Glasbläserei Müller, Berlin-Adlershof, Germany; inner diameter: 10 mm, wall thickness: 1 mm, length: 60 mm). The starting materials were transferred into the ampoules inside of an argon-filled glove box (Glovebox Systemtechnik, GS Mega E-Line, Malsch, Germany) and sealed under dynamic vacuum. The multi-stage heating program shown in Figure A1 was applied employing a Nabertherm L 9/12 muffle furnace (Nabertherm, Lilienthal, Germany). Generally, cooling rates of 2–3 °C h⁻¹ were chosen. An increase of the rates to 5 °C h⁻¹ upwards resulted in a significant decrease of the obtained crystal size.

After cooling to room temperature, the samples were washed with about 250 mL demineralized water and dried subsequently in a drying oven at 80 °C for at least 6 h. First visual inspection with a stereo microscope of the powder samples revealed a homogeneous, pale yellow powder with square tabular shaped crystals of the target compound. However, using crossed-polarized light indicated the tendency for the formation of aggregated and twinned crystals.

2.2. Single-Crystal and Powder X-ray Diffraction

For single-crystal X-ray diffraction (SCXRD) measurements, crystals of adequate size were isolated under a stereomicroscope and fixed with grease into a glass capillary (outer diameter: 0.1 mm, wall thickness: 0.01 mm). The diffraction experiments were performed on a Stoe StadiVari four-circle diffractometer (Stoe and Cie, Darmstadt, Germany) with Eulerian cradle and Mo-K_α radiation at room temperature. For data collection and integration, Stoe X-Area 1.86 (2018) was used [30]. The crystal structures of EuSb₂O₄Br and TbSb₂O₄Br were solved in the space group P2₁/c through direct methods and subsequently refined using the ShelX-1997 program package [31,32].

Powder X-ray diffraction (PXRD) experiments were performed on a Stoe Stadi-P diffractometer with monochromatic Cu-K_α radiation (λ = 154.06 pm) in transmission geometry and a Stoe linear PSD detector with a range of 5°/2θ. As monochromator, a curved germanium single crystal with (111) as diffractive face was used. About 5 mg of the slightly crushed sample were fixed between two layers of an amorphous adhesion tape.

Since it was not possible to grow single crystals of GdSb₂O₄Br in sufficient quality for single-crystal X-ray diffraction analysis, a Le Bail profile fit of the powder data for GdSb₂O₄Br to obtain the lattice constants with the Fullprof suite was performed [33,34,35,36]. EuSb₂O₄Br was used as the starting model.

2.3. Electron-Beam Microprobe Analysis

Scanning electron images as well as energy- and wavelength-dispersive X-ray spectrometry were performed on a Cameca SX-100 microanalyzer (Cameca, Gennevilliers, France) equipped with one energy-dispersive X-ray spectrometer (EDXS, ThermoScientific UltraDry, Waltham, MA, USA) and five wavelength-dispersive X-ray spectrometers (WDXS). Crystals of sufficient size were placed on a conductive carbon pad (Plano G3357, Wetzlar, Germany) and afterwards coated with a thin carbon layer to prevent surface charging. An acceleration voltage of 20 kV was chosen to improve the intensities of the emission lines of the heavier elements. Quantification of an europium(III)-doped sample of TbSb₂O₄Br was performed by using Eu[PO₄], Tb[PO₄], InSb and CsBr for instrument calibration. The oxygen content was calculated using the individual valences of all present ions. As matrix correction, the Pouchou and Pichoir algorithm “PaP” as implemented in the Cameca PeakSight 6.2 package was applied [37,38,39].

2.4. Photoluminescence Investigations

Photoluminescence measurements were carried out at room temperature on a Fluorolog-3 modular spectrofluorometer (Horiba JobinYvon, France), equipped with a double excitation monochromator, a single emission monochromator (HR320), and a R928P PMT detector. A continuous xenon lamp (450 W) was used as the excitation source for steady state measurements. For lifetime acquisition, the time-correlated single photon counting (TCSPC) technique was used, with a xenon microsecond-pulsed lamp for excitation. For both steady state and lifetimes measurements, the samples were measured as powders in front field geometry, by putting the sample holder plane perpendicular to the incoming ray, and collecting the emitted light at 22.5° from the incident light.

3. Results and Discussion

3.1. Crystal-Structure Description

All members of the LnSb₂O₄Br series (Ln = Eu–Tb) crystallize in the monoclinic space group P2₁/c with four formula units per unit cell. The lanthanoid(III) cations (Ln³⁺) occupy only one crystallographically unique position exhibiting a coordination number of eight. Being coordinated exclusively by oxygen atoms, distorted [LnO₈]^13– anti- or hemiprisms are formed (Figure 1). The observed lanthanoid(III)-oxygen bond lengths fall into the range between 227 and 258 pm, which are similar to those found in the respective sesquioxides (Eu₂O₃ (Sm₂O₃- or B-type: 224–262 pm and bixbyite- or C-type: 230–239 pm), Gd₂O₃ (Sm₂O₃- or B-type: 215–270 pm and bixbyite- or C-type: 228–239 pm) and Tb₂O₃ (Sm₂O₃- or B-type: 218–273 pm and bixbyite- or C-Type: 227–239 pm) [40,41,42,43,44,45]. These [LnO₈]^13– polyhedra are edge-connected by common oxygen atoms to form infinite layers spreading out parallel to the (100) plane according to the Niggli formula ${}_{\infty }{}^2\{{[Ln{\mathrm{O}}_{8/2}^e]}^{5–}\}$ (e = edge-sharing, Figure 2).

The Sb³⁺ cations occupy two different crystallographic positions. Both (Sb1)³⁺ and (Sb2)³⁺ are surrounded by three oxygen atoms each in the shape of [SbO₃]^3– ψ¹-tetrahedra (Figure 3). These pyramidal units share common oxygen atoms and build up infinite strands of vertex-connected [SbO₃]^3– ψ¹-tetrahedra with alternating (Sb1)³⁺ and (Sb2)³⁺ cations meandering along the c axis (Figure 4) according to ${}_{\infty }{}^1\{{[{\mathrm{SbO}}_{2/2}^v{\mathrm{O}}_{1/1}^t]}^{–}\}$ (v = vertex-sharing, t = terminal). The different bonding situation is also reflected well by the interatomic Sb³⁺–O^2– distances. While the terminal oxygen atoms reside at distances of about 193 pm to the central Sb³⁺ cations, the linking ones exhibit noticeable longer values with 205–212 pm. These different values are also illustrated in Figure 4 for the example of TbSb₂O₄Br and all lone pairs present at the Sb³⁺ cations are pointing into the same direction along [100].

In valentinite and senarmontite, the two natural crystalline modifications of Sb₂O₃, the antimony(III)-oxygen distances feature values of 198–202 pm and therefore fall in between the range of our refined distances for the LnSb₂O₄Br series with regards to the differences of the linking and the terminal oxygen atoms [46,47]. Consequently, the sharing of an oxygen atom between two antimony(III) cations leads to an expansion of the respective bond length by at least 10 pm.

The influence of the lone electron pair of a pnictogen(III) cation has recently been reported for comparable lanthanoid(III) oxopnictogenates(III), such as the lanthanoid(III) halide oxoarsenates(III) with the compositions Ln₅Cl₃[AsO₃]₄ and Ln₃X₂[As₂O₅][AsO₃] [12,14]. While the title compounds feature one-dimensional infinite strands of corner-connected [SbO₃]^3– units, the latter only display isolated [AsO₃]³⁻ ψ¹-tetrahedra or doubles of them in vertex-condensed [As₂O₅]⁴⁻ pyroanions. In these condensed units, the bridging oxygen atoms also show the longest distances to the arsenic(III) cations, much like in the monoclinic members of the series LnSb₂O₄Br, where the bond lengths to the corner-connecting oxygen atoms of the [SbO₃]³⁻ groups fall into a longer interval than the short terminal ones.

Comparing the structural features of the short LnSb₂O₄Br (Ln = Eu–Tb) series with the crystal structure of the long Na₂Ln₃Cl₃[TeO₃]₄ series (Ln = La–Nd, Sm–Lu) featuring Te⁴⁺ as cation isoelectronic to Sb³⁺ reveals several similarities [48,49,50]. The lanthanoid(III) cations also exhibit an eightfold coordination sphere exclusively by oxygen atoms building up [LnO₈]¹³⁻ hemiprisms and thus no Ln³⁺‒X⁻ bonds exist. These hemiprisms build up layers of the composition ${}_{\infty }{}^2\{{[Ln{\mathrm{O}}_{8/2}^e]}^{5–}\}$ parallel to the (001) plane. Furhtermore, the Te⁴⁺ cations are coordinated by three oxygen atoms forming [TeO₃]²⁻ anions in the shape of ψ¹-tetrahedra, but in this case, without any Te⁴⁺–O²⁻–Te⁴⁺ contacts. The halide anions X^– are only connected to the sodium cations located in between two layers of [LnO₈]¹³⁻ hemiprisms.

The crystallographically unique bromide anions do not exhibit any bonding contacts to the Ln³⁺ cations, since the shortest Ln³⁺···Br^– separations amount to 422 pm. The bromide anions are located between planes formed by edge-connected [LnO₈]^13– polyhedra. The antimony(III) cations connected to these oxygen atoms show distances of 318–320 pm up to 366–367 pm to the neighboring bromide anions (Figure 5). Comparing these values with ternary antimony oxide bromides seems appropriate. In Sb₄O₅Br₂, a well-known ternary antimony(III) oxide bromide, the Sb³⁺ cations also show only short interatomic distances to the oxygen atoms with a major contribution to chemical bonding (189–251 pm), while the bromide anions are partially even more removed away from the Sb³⁺ cations with distances of 302–366 pm [51]. Almost the same holds for a second antimony(III) oxide bromide with the composition Sb₈O₁₁Br₂, where Sb³⁺–O^2– distances of 190–257 pm and Sb³⁺···Br^– distances of 314–396 pm occur [52,53]. Therefore, in both these ternary compounds and LnSb₂O₄Br (Ln = Eu–Tb), the Sb³⁺–Br⁻ bond lengths illustrate the tendency of Sb³⁺ cations to be coordinated by bromide anions with far greater interatomic distances compared to the oxygen atoms.

In Figure 6, a section of the crystal structure with indication of the cell edges, coordination polyhedra around the lanthanoid(III) cations and the refined anisotropic displacement ellipsoids is shown. The lattice parameters for all members of the LnSb₂O₄Br series (Ln = Eu–Tb) are listed in

Table 1. Lattice parameters for the three members of the LnSb₂O₄Br series (Ln = Eu–Tb) as determined by means of powder X-ray diffractometry.

Compound	EuSb2O4Br	GdSb2O4Br	TbSb2O4Br
Crystal system	monoclinic
Space group	P21/c (no. 14)
a/pm	895.69(2)	895.67(2)	894.56(2)
b/pm	791.90(2)	789.27(2)	785.84(2)
c/pm	790.46(2)	788.44(2)	784.25(2)
β/°	91.743(1)	91.681(1)	91.625(1)

, while the crystallographic data as well as the positional atomic parameters and selected interatomic distances for EuSb₂O₄Br and TbSb₂O₄Br can be found in

Table A1. Crystallographic data of the end-members of the LnSb₂O₄Br series (Ln = Eu and Tb).

Formula	EuSb2O4Br	TbSb2O4Br
Crystal system	monoclinic
Space group	P21/c (no. 14)
Lattice constants, a/pm	895.69(6)	895.37(6)
b/pm	791.82(5)	786.14(5)
c/pm	790.38(5)	785.09(5)
β/°	91.817(3)	91.638(3)
Formula units, Z	4
Calculated density, Dx/g∙cm–3	6.394	6.569
Molar volume, Vm/cm3∙mol–1	84, 36	83, 17
Diffractometer	StadiVari (STOE, four-circle diffractometer)
Wavelength (λ/pm)	71.07 (Mo-Kα)
F(000)	928	936
2θmax/°	63.58	63.72
hkl range (±hmax, ±kmax, ±lmax)	13, 11, 11	12, 11, 11
Observed reflections	19853	12173
Unique reflections	1840	1438
Absorption coefficient, µ/mm–1	27.68	29.52
Absorption correction	numerical (Stoe X-Shape 2.21)
Rint/Rσ	0.076/0.041	0.078/0.045
R1/R1 with |FO| ≥ 4σ(FO)	0.055/0.034	0.047/0.033
wR2/GooF	0.082/1.047	0.071/1.019
Structure determination and refinement	Program package ShelX-1997 [31,32]
Extinction coefficient, ε/pm–3	0.00012(9)	0.00022(9)
Residual electron density, ρ/e– 10–6 pm–3	+1.91/−1.78	+2.02/−1.95
CCDC number	CSD-2016635	CSD-2016636

,

Table A2. Fractional atomic coordinates and coefficients of the equivalent isotropic displacement parameters of EuSb₂O₄Br (top) and TbSb₂O₄Br (bottom). All atoms occupy the general Wyckoff site 4e.

Atom	x/a	y/b	z/c	Ueq/pm2
Eu	0.48857(4)	0.23760(5)	0.50274(5)	104(1)
Sb1	0.78149(6)	0.05812(7)	0.75756(7)	108(1)
Sb2	0.21659(6)	0.00677(7)	0.79543(7)	103(1)
O1	0.6332(6)	0.0021(7)	0.5856(7)	147(12)
O2	0.3642(6)	0.1758(7)	0.7481(7)	123(11)
O3	0.6656(6)	0.0086(7)	0.9703(7)	112(11)
O4	0.6686(6)	0.2105(7)	0.2594(7)	107(10)
Br	0.02117(11)	0.23521(12)	0.50388(12)	205(2)
Tb	0.48990(4)	0.23728(5)	0.50185(5)	85(1)
Sb1	0.78039(6)	0.05548(7)	0.75626(7)	92(2)
Sb2	0.21888(6)	0.00810(7)	0.79380(7)	90(1)
O1	0.6321(6)	0.0028(7)	0.5804(7)	128(14)
O2	0.3675(6)	0.1777(7)	0.7465(7)	111(13)
O3	0.6617(6)	0.0069(7)	0.9696(7)	100(13)
O4	0.6686(6)	0.2102(7)	0.2597(7)	118(13)
Br	0.01915(12)	0.23617(11)	0.50403(11)	188(2)

and

Table A3. Selected interatomic distances (d/pm, e.s.d.: ±0.5 pm) for EuSb₂O₄Br (left) and TbSb₂O₄Br (right).

Contact	EuSb2O4Br	TbSb2O4Br
Ln–O1	228.7	226.6
Ln–O1′	235.2	231.4
Ln–O2	231.9	228.7
Ln–O2′	237.1	235.4
Ln–O3	256.4	253.0
Ln–O3′	257.7	254.9
Ln–O4	255.8	252.9
Ln–O4′	258.4	257.7
Sb1–O1	192.2	193.3
Sb1–O3	204.5	204.5
Sb1–O4	209.3	209.7
Sb2–O2	192.6	192.7
Sb2–O3	210.3	212.0
Sb2–O4	205.7	204.0
Br–Sb1	328.8	327.9
Br–Sb1‘	329.7	328.6
Br–Sb1‘‘	360.6	358.7
Br–Sb1‘‘‘	360.8	358.8
Br–Sb2	318.4	318.3
Br–Sb2‘	337.5	337.0
Br–Sb2‘‘	344.8	343.9
Br–Sb2‘‘‘	367.2	366.2

. In the case of GdSb₂O₄Br, it was also possible to synthesize very flat, square platelets of crystals with the desired composition, however, the quality of the obtained single crystals revealed to be too poor for a trustworthy structure refinement. Instead, it was possible to refine the unit-cell parameters of GdSb₂O₄Br both via single-crystal X-ray diffraction as well as powder X-ray diffraction methods (Section 3.2).

3.2. Powder X-ray Diffraction

For phase analyses of the obtained and water-washed product mixtures of all three systems, PXRD measurements were performed. Figure A2 shows the diffractogram of EuSb₂O₄Br and TbSb₂O₄Br, respectively. While good accordance among the positions of the simulated and measured reflections is observed, a remarkable variance of the relative reflection intensities occurs. This kind of texture effects can be attributed to the extreme platelet habit of the crystals (Figure 8). Intensive crushing of the powder samples and subsequent diffraction experiments showed an improvement of the relative intensities towards the theoretical values, however, strong reflex broadening was observed. Therefore, the samples were only slightly crushed to avoid these broadenings.

Since it was not possible to yield single crystals of GdSb₂O₄Br with sufficient quality for a reliable structure determination, the lattice parameters were refined from powder X-ray diffraction methods. For comparison, single crystals of GdSb₂O₄Br with unit-cell parameters of a = 895.54(6) pm, b = 788.91(5) pm, c = 787.67(5) pm and β = 91.725(3)° were observed. The experimental and simulated pattern as well as the plot of the calculated deviation and the Bragg positions are given in Figure 7. The refined cell parameters for GdSb₂O₄Br are also given in Table 1 and are in high accordance with the values deriving from the single-crystal measurement as well as with the expected ones due to the lanthanoid contraction.

3.3. Electron-Probe Microanalysis

To evaluate the crystal morphology of the three LnSb₂O₄Br representatives with Ln = Eu–Tb, scanning electron images were recorded. In Figure 8, representative micrographs of all three compounds illustrating the size and crystal habit of the synthesized crystals are given. A prominent feature for all three compounds is the formation of platelet-shaped crystals.

All members of the LnSb₂O₄Br series (Ln = Eu–Tb) were characterized with energy-dispersive X-ray spectroscopy to verify the purity and validate the absence of unintended elements in the gained crystals. As shown in Figure 9, each compound exhibits the emission lines of the intended elements and no additional ones are present in the examined samples.

For an europium(III)-doped sample of TbSb₂O₄Br as representative of the series, quantitative measurements of the single crystals were performed. This was of special interest, since previous structure studies about the crystal structures in the quaternary system Sm–Sb–O–X (X = Cl and Br) revealed a mixed-Ln³⁺/Sb³⁺ occupancy of the Sb³⁺ position and, therefore, an incoherency with the theoretical formula “SmSb₂O₄X” [26]. For this new structure type in the quaternary system Ln‒Sb‒O‒Br with similarities regarding structural features and synthesis methods, it seemed appropriate to apply a second method to confirm the determined composition of at least one representative with the formula LnSb₂O₄Br (Ln = Eu–Tb). Moreover, the method was applied to determine the degree of doping in TbSb₂O₄Br, since it is not possible to refine the europium(III) content in a terbium(III) host lattice by means of X-ray diffraction methods.

The results of the quantitation using wavelength-dispersive spectrometry are compiled in

Table A4. Quantitative electron-beam microprobe analysis of TbSb₂O₄Br doped with 1.5 at-% Eu³⁺ using wavelength-dispersive spectrometry and the Pouchou and Pichoir (PaP) matrix correction algorithm. The oxygen content was calculated stoichiometrically.

Ion	Emission Line	Content (wt-%)	Normalized Content (at-%)	Theoretical Content (at-%)
Tb3+	Lα	28.0(8)	12.3(3)	12.31
Eu3+	Lα	0.43(7)	0.20(3)	0.19
Sb3+	Lα	43.2(7)	25.0(4)	25.00
Br−	Kα	14.4(7)	12.6(6)	12.50
O2−	–	11.6(9)	49.9(9)	50.00

. When comparing the experimental values to the expected of TbSb₂O₄Br doped with 1.5 at-% Eu³⁺ regarding the total amount of lanthanoid(III) cations, the measurements validate formula TbSb₂O₄Br:Eu³⁺ with the desired doping content.

3.4. Photoluminescence Spectroscopy

Antimony(III) compounds are known to show an excitation band in the ultraviolet range (UV) with a maximum around 270 nm corresponding to the partially forbidden ¹S₀ → ³P₁ electronic transition. The Sb³⁺ emission ³P₁ → ¹S₀ is known to occur over a broad range (from 300 to 650 nm), and often overlaps the excitation range of trivalent lanthanoid cations, such as Eu³⁺ and Tb³⁺, rendering it potentially a suitable sensitizer for lanthanoid emission [54,55]. The luminescence of Sb³⁺ in LnSb₂O₄Br representatives with Ln = Eu–Tb can be best investigated in GdSb₂O₄Br as Gd³⁺ only emits in the UV (typically around 310 nm) due to a transition from the ⁶P_7/2 to ⁸S_7/2 levels. However, even efficient sensitization of Gd³⁺ has been reported in YPO₄:Sb³⁺, Gd³⁺ [7]. The excitation spectrum of GdSb₂O₄Br monitored at 455 nm shows a band with a maximum at 257 nm, which can be assigned to the the partially forbidden ¹S₀ → ³P₁ electronic transition of Sb³⁺ (Figure 10).

Upon excitation into 257 nm, a broad band is visible in the emission spectrum in the region between 400 and 500 nm (Figure 10), with a maximum at 455 nm, corresponding to the transition ³P₁ → ¹S₀. This band in the blue region of the electromagnetic spectrum occurs for all the investigated compounds upon excitation in ¹S₀ → ³P₁ (inset in Figure 11, left). For TbSb₂O₄Br:Eu³⁺ and GdSb₂O₄Br:Eu³⁺ its intensity is negligible compared to the Eu³⁺ transitions (Figure 11, left) and less intense than Tb³⁺ transitions in the terbium derivatives (inset in Figure 11, left), suggesting the presence of an energy transfer (ET) from Sb³⁺ to Ln³⁺. The different contribution of the Sb³⁺ emission (blue region) compared to the Eu³⁺ emission (red region) as well as Tb³⁺ role, are demonstrated by the chromaticity coordinates calculated for TbSb₂O₄Br:Eu³⁺ and GdSb₂O₄Br:Eu³⁺ (λ_ex = 257 nm) (Figure 11, right). In order to explain this difference, a deep investigation on the luminescence properties of both TbSb₂O₄Br:Eu³⁺ and GdSb₂O₄Br:Eu³⁺ was performed.

Excitation spectra of EuSb₂O₄Br, GdSb₂O₄Br:Eu³⁺, and TbSb₂O₄Br:Eu³⁺ are shown in Figure 12. For EuSb₂O₄Br, when monitoring at 611 nm, the most intense band appears at 464 nm, which belongs to the set of ⁷F₀ → ⁵D₂ hypersensitive dielectric dipole transitions of Eu³⁺. The other excitation lines correspond to ⁷F₀ → ⁵L₆ at 393 nm, ⁷F₀ → ⁵D₁ and ⁷F₁ → ⁵D₁ at 525 and 534 nm of Eu³⁺, respectively.

All the aforementioned transitions can also be observed in the two Eu³⁺-doped compounds, GdSb₂O₄Br:Eu³⁺ and TbSb₂O₄Br:Eu³⁺. Moreover, in both the Eu³⁺-doped compounds, two broad bands can be detected in the UV region. In addition to the band around 250 nm, a second broad band at higher wavelength can be observed, which is attributed to the 2p (O²⁻) → 4f (Eu³⁺) charge transfer which frequently is observed in Eu³⁺ oxide compounds [6,56,57,58]. Due to the eight-fold coordination of the lanthanide cations in GdSb₂O₄Br:Eu³⁺ and TbSb₂O₄Br:Eu³⁺ and to their similar radii, both compounds are supposed to have charge transfer bands in the region around 260–270 nm, similarly to Gd₂Zr₂O₇:Eu³⁺ and LaPO₄:Eu³⁺ [57]. Thus in the ultraviolet (UV) region, an overlap between the aforementioned ¹S₀ → ³P₁ transition of Sb³⁺ and the Eu³⁺ charge transfer band occurs. For GdSb₂O₄Br:Eu³⁺, the charge transfer band is the most intense. Conversely, it is negligible in the neat Eu-compound.

By monitoring the Eu³⁺ emission at 611 nm in TbSb₂O₄Br:Eu³⁺, besides the characteristic interconfigurational f–f transitions of Eu³⁺, albeit with comparatively weak relative intensity, the bands in the UV region of light belonging to Eu³⁺–O²⁻ charge transfer and the Sb^{3+ 1}S₀ → ³P₁ transitions can be seen. In addition, transitions characteristic for Tb³⁺ are present which points to Tb³⁺ → Eu³⁺ sensitization.

Excitation spectra monitored at the Tb³⁺ emission for TbSb₂O₄Br as well as TbSb₂O₄Br:Eu³⁺ are shown in Figure 12 (right). Both materials show the characteristic bands of Tb³⁺ which can be assigned to the ⁷F₆ → ⁵D₃ and ⁷F₆ → ⁵D₄ transitions. The presence of Tb³⁺ peaks in the excitation spectrum of TbSb₂O₄Br:Eu³⁺ when detecting the Eu³⁺ emission indicates the presence of the Tb³⁺ → Eu³⁺ energy transfer.

When excited at 283 and 464 nm,, corresponding to ¹S₀ → ³P₁ of Sb³⁺, charge transfer band, ⁷F₀ → ⁵L₆ and ⁷F₀ → ⁵D₂ of Eu³⁺, respectively (Figure 13), all Eu³⁺-containing compounds show emission from the ⁵D₀ level of Eu³⁺ transitions. The low site symmetry of Eu³⁺ in the compound series increases the number of observable transitions [6]. The hypersensitive (or electric dipole) band ⁵D₀ → ⁷F₂ with five sublevels, and the magnetic dipole transition ⁵D₀ → ⁷F₁ with all three possible sublevels can be observed.

The dominance of the ⁵D₀ → ⁷F₂ transition in the emission spectra relative to the ⁵D₀ → ⁷F₁ transition is compatible with the location of Eu³⁺ in a low-symmetry site [59]. The asymmetry ratio, defined as (⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁), is a parameter that indicates the distortion of Eu³⁺ site with respect to an inversion centre. As expected, the asymmetry ratio is significantly larger than 1 indicating a low-symmetry site, in agreement with a site symmetry of 1 for Ln³⁺ in LnSb₂O₄Br [59,60].

Very weak peaks corresponding to emission from higher levels are also visible: at 582 nm, attributable to the forbidden transition ⁵D₁ → ⁷F₀ and at 579 nm, that probably corresponds to the ⁵D₁ → ⁷F₃ transition and others even weaker in the 560–576 nm region [5]. The intensity of the ⁵D₀ → ⁷F₄ transition is higher than the magnetic dipole band, as already found in lanthanoid(III) orthoborates with C_s sites symmetry [61] or in the polyoxometalate Na₉[EuW₁₀O₃₆] · 14 H₂O with a distorted D_4d symmetry [62].

Due to the full Eu³⁺ concentration, a partial quenching of Eu³⁺ emission occurs in EuSb₂O₄Br. This is confirmed not only by the short lifetime of ⁵D₀ → ⁷F₂ detected upon excitation at 464 nm (10.7 μs), but also by the absence of the transitions from levels higher than ⁵D₀. As can be observed in the excitation spectrum (Figure 12, left) by detecting the most intense Eu³⁺ emission at 611 nm, no excitation transitions are present in the UV region, and for this reason, the sample does not show any emission upon excitation at 257 nm and was not included in the CIE (Commission internationale de l’éclairage) diagram. Nevertheless, upon excitation in the most intense excitation transition at 464 nm, corresponding to the ⁷F₀ → ⁵D₂ transition of Eu³⁺, EuSb₂O₄Br shows the characteristic luminescence features of Eu³⁺. This means that energy migration between ions in the bulk is not completely efficient upon excitation at lower energy.

The emission spectrum of the Tb³⁺-based materials upon excitation with λ_ex = 378 nm are shown in Figure 14. The spectrum of TbSb₂O₄Br shows ⁵D₄ → ⁷F₆, ⁵D₄ → ⁷F₅, ⁵D₄ → ⁷F₄, ⁵D₄ → ⁷F₃ transitions, resulting overall in an impression of green light. In the Eu³⁺-doped compound, TbSb₂O₄Br:Eu³⁺, emission from Eu³⁺ are dominant whilst those for Tb³⁺ are hardly visible. Together with the presence of Tb³⁺ excitation bands by monitoring the luminescence of Eu³⁺, the strong Eu³⁺ emission at the expense of Tb³⁺ emission, indicates the presence of notable Tb³⁺ → Eu³⁺ energy transfer. Moreover, this can explain the red shift in the CIE diagram with respect to GdSb₂O₄Br:Eu³⁺: in TbSb₂O₄Br:Eu³⁺, Sb³⁺ → Eu³⁺ is mediated by Tb³⁺, that in turn transfers almost all the excitation light to Eu³⁺, shifting the final emission towards the red, while in GdSb₂O₄Br:Eu³⁺ some Sb³⁺ contribution is still present, giving a combination of both red and blue contributions.

Analysis of the luminescence decays were performed in order to better investigate the interactions (i.e., radiative and non-radiative processes) involved between the emitting cations, such as energy transfer (ET) and concentration quenching.

The Eu³⁺ emission (611 nm), upon excitation with λ_ex = 257 nm (corresponding to the ¹S₀ → ³P₁ electronic transition of Sb³⁺) was probed in order to allow for a comparison of the optical properties of GdSb₂O₄Br:Eu³⁺ and TbSb₂O₄Br:Eu³⁺ (see also the CIE diagram, Figure 11, right).

Upon this excitation, an intensity rise time can be seen for both the decay curves of GdSb₂O₄Br:Eu³⁺ and TbSb₂O₄Br:Eu³⁺, indicating that Eu³⁺-emitting levels are fed by energy transfer processes. The corresponding lifetime values were calculated as a weighted average of the two components [63]. As previously discussed, a difference between excitation spectra of the two materials is noticeable when monitoring the Eu³⁺ emission at 611 nm: In TbSb₂O₄Br:Eu³⁺ the main peak occurs at 488 nm, followed by the peak at 378 nm, both belonging to Tb³⁺, while the bands in the UV region have a minor importance. This demonstrates the major role of Tb³⁺ in sensitizing the emission of Eu³⁺, allowing a color more shifted towards the red. By contrast, in GdSb₂O₄Br:Eu³⁺, Eu³⁺ emission can only be sensitized through Sb³⁺ energy transfer. This is related to the greater role played by the blue component in the final emission, as shown in the different color coordinates in the CIE diagram. (see Figure 11, right).

Moreover, the lifetimes of both doped materials, TbSb₂O₄Br:Eu³⁺ and GdSb₂O₄Br:Eu³⁺, are dependent on the excitation wavelength, as an example, a comparison between the decays of GdSb₂O₄Br:Eu³⁺ upon excitation in the ¹S₀ → ³P₁ electronic transition of Sb³⁺ at 257 nm vs. excitation in the Eu³⁺ transition at ⁷F₀ → ⁵L₆ at 393 nm is shown in Figure 15 (right).

Eu³⁺ lifetime corresponding to the main emission at 611 nm was also evaluated upon excitation in the most intense band at 464 nm (⁷F₀ → ⁵D₂ of Eu³⁺). In TbSb₂O₄Br:Eu³⁺, a single exponential intensity decay was observed and an excited state lifetime of about 0.8 ms could be calculated. The single exponential behavior reflects the occupation of only one crystallographic site by Eu³⁺. The observed lifetime is similar to the value observed for Eu³⁺ in zirconia samples [56], as well as with antimony–germanate–silicate glasses [63]. However, for excitation of GdSb₂O₄Br:Eu³⁺ with 464 nm, the decay curve needed to be fitted bi-exponential (Figure 15, middle and fitted components in

Table A5. Components and the average time of the multi-exponential decays reported in the manuscript. For the single-exponential decays, the fitted lifetime is reported as only the average value.

Sample	λex/nm	λem/nm	A1	τ1/s	A2	τ2/s	τavg/s
TbSb2O4Br:Eu3+	257	611	34.11	3.08 × 10−3	65.89	1.09 × 10−3	2.3 × 10−3
	464	611					0.8 × 10−3
	483	541					12.3 × 10−6
GdSb2O4Br:Eu3+	257	611	22.88	4.12 × 10−3	77.12	1.04 × 10−3	3.5 × 10−3
	464	611	33.45	2.87 × 10−3	66.55	7.74 × 10−4	2.1 × 10−3
	393	611	64.87	9.06 × 10−4	35.13	4.65 × 10−3	3.6 × 10−3
TbSb2O4Br	483	541	22.54	1.39 × 10−4	74.46	2.74 × 10−4	0.25 × 10−3

), pointing to two different Eu³⁺ emitting species, either with different environment, potentially Eu³⁺ in the bulk and Eu³⁺ as the surface (note that it was not possible to obtain single crystals of GdSb₂O₄Br of sufficient quality for single crystal X-ray diffraction (SCXRD) analysis).

The efficiency of Tb³⁺ → Eu³⁺ was evaluated through lifetime analysis, by using the formula:

η_ET = 1 − τ_DA τ_D⁻¹

(2)

where τ_DA is the lifetime of the donor (Tb³⁺) in presence of the acceptor (Eu³⁺), and τ_D is the lifetime of the donor in the undoped material.

Thus, the emission decays corresponding to the Tb³⁺ emission from the ⁵D₄ level were acquired by exciting into the most intense excitation band, namely 483 nm (corresponding to the ⁷F₆ → ⁵D₄ transition), for both TbSb₂O₄Br and TbSb₂O₄Br:Eu³⁺ (Figure 16). The undoped sample shows a double exponential decay. The total lifetime of 0.25 ms can be calculated as a weighted average of the two components as reported by Zmojda et al. [63], while the Eu³⁺-doped sample shows a value of 12.3 μs, fitted by a single exponential decay. The η_ET obtained from these values is around 95%.

4. Conclusions

LnSb₂O₄Br series (Ln = Eu–Tb) presents a new material class of quaternary lanthanoid(III) oxoantimonate(III) halides. With respect to the hitherto reported quaternary samarium(III) oxoantimonate(III) halides, the presented compounds show structural similarities, such as the eightfold oxygen-coordination sphere of the lanthanoid(III) cations, but no mixed-occupied sites for the Ln³⁺ and Sb³⁺ cations. Besides La₅F₃[SbO₃]₄, the reported compounds can be viewed as a new composition type in the quaternary systems Ln(III)–Sb(III)–O–X without mixed-occupation sites so far. As important structural features, the lanthanoid(III) cations are coordinated exclusively by eight oxygen atoms in the shape of [LnO₈]^13‒ hemiprisms. As second structural feature worth mentioning, the antimony(III) cations build up meandering strands of vertex-connected [SbO₃]^3‒ anions and, therefore, show a remarkable difference to the already reported lanthanoid(III) halide oxopnictogenates(III) such as La₅F₃[SbO₃]₄ or Ln₃X₂[As₂O₅][AsO₃], where mostly isolated or vertex-shared [PnO₃]³⁻ anions as dimers are present.

Whilst the bulk europium compound, EuSb₂O₄Br, is not suitable as luminescent material, both GdSb₂O₄Br and TbSb₂O₄Br doped with Eu³⁺ show strong photoluminescence thanks to Sb³⁺ which is acting as a blue-emitting species and sensitizer for lanthanide. TbSb₂O₄Br:Eu additionally benefits form strong energy transfer from Tb³⁺ to Eu³⁺. Since Sb³⁺ is emitting in the blue, Tb³⁺ in the green and Eu³⁺ in the red white-light emitting materials can be envisioned in this class of compound.