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

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 {[SbO2/2vO1/1t]–}∞1 (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.


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 (Ln 2 O 3 and Pn 2 O 3 ). Moreover, the lanthanoid(III) oxoarsenates(III) or -antimonates(III) generally feature a quite high (v = vertex-sharing) are observed. The halide anions have little or no contact to the Ln 3+ cations, but settle with far away Sb 3+ 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 LnSb 2 O 4 Br 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 GdSb 2 O 4 Br were performed to investigate the luminescence processes caused by Sb 3+ . With this understanding, the photoluminescence investigations of EuSb 2 O 4 Br and TbSb 2 O 4 Br as well as doped GdSb 2 O 4 Br:Eu 3+ and TbSb 2 O 4 Br:Eu 3+ 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 Sb 3+ cations in the crystal structure as intrinsic sensitizers and possible efficient alternative to the forbidden direct excitation via 4f -4f transitions of the Ln 3+ ions.

Synthesis of LnSb 2 O 4 Br Representatives
The lanthanoid(III) oxoantimonate(III) bromides 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.

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 Crystals 2020, 10, 1089 4 of 23 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 EuSb 2 O 4 Br and TbSb 2 O 4 Br were solved in the space group P2 1 /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 GdSb 2 O 4 Br in sufficient quality for single-crystal X-ray diffraction analysis, a Le Bail profile fit of the powder data for GdSb 2 O 4 Br to obtain the lattice constants with the Fullprof suite was performed [33][34][35][36]. EuSb 2 O 4 Br was used as the starting model.

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 TbSb 2 O 4 Br was performed by using Eu [PO 4 ], Tb[PO 4 ], 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].

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.

Crystal-Structure Description
All members of the LnSb 2 O 4 Br series (Ln = Eu-Tb) crystallize in the monoclinic space group P2 1 /c with four formula units per unit cell. The lanthanoid(III) cations (Ln 3+ ) occupy only one crystallographically unique position exhibiting a coordination number of eight. Being coordinated exclusively by oxygen atoms, distorted [LnO 8 ] 13anti-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 (Eu 2 O 3 (Sm 2 O 3 -or B-type: 224-262 pm and bixbyite-or C-type: 230-239 pm), Gd 2 O 3 (Sm 2 O 3 -or B-type: 215-270 pm and bixbyiteor C-type: 228-239 pm) and Tb 2 O 3 (Sm 2 O 3 -or B-type: 218-273 pm and bixbyite-or C-Type: 227-239 pm) [40][41][42][43][44][45]. These [LnO 8 ] 13polyhedra are edge-connected by common oxygen atoms   The Sb 3+ cations occupy two different crystallographic positions. Both (Sb1) 3+ and (Sb2) 3+ are rrounded by three oxygen atoms each in the shape of [SbO3] 3-ψ 1 -tetrahedra ( Figure 3). These     The Sb 3+ cations occupy two different crystallographic positions. Both (Sb1) 3+ and (Sb2) 3+ are surrounded by three oxygen atoms each in the shape of [SbO 3 ] 3ψ 1 -tetrahedra ( Figure 3). These pyramidal units share common oxygen atoms and build up infinite strands of vertex-connected [SbO 3 ] 3ψ 1 -tetrahedra with alternating (Sb1) 3+ and (Sb2) 3+ cations meandering along the c axis ( Figure 4) according to 1   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)    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  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 Ln 5 Cl 3 [AsO 3 ] 4 and Ln 3 X 2 [As 2 O 5 ][AsO 3 ] [12,14]. While the title compounds feature one-dimensional infinite strands of corner-connected [SbO 3 ] 3units, the latter only display isolated [AsO 3 ] 3ψ 1 -tetrahedra or doubles of them in vertex-condensed [As 2 O 5 ] 4pyroanions. 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 LnSb 2 O 4 Br, where the bond lengths to the corner-connecting oxygen atoms of the [SbO 3 ] 3groups fall into a longer interval than the short terminal ones.
The crystallographically unique bromide anions do not exhibit any bonding contacts to the Ln 3+ cations, since the shortest Ln 3+ ···Brseparations amount to 422 pm. The bromide anions are located between planes formed by edge-connected [LnO 8 ] 13polyhedra. 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 Sb 4 O 5 Br 2 , a well-known ternary antimony(III) oxide bromide, the Sb 3+ 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 Sb 3+ cations with distances of 302-366 pm [51]. Almost the same holds for a second antimony(III) oxide bromide with the composition Sb 8 O 11 Br 2 , where Sb 3+ -O 2distances of 190-257 pm and Sb 3+ ···Brdistances of 314-396 pm occur [52,53]. Therefore, in both these ternary compounds and LnSb 2 O 4 Br (Ln = Eu-Tb), the Sb 3+ -Br − bond lengths illustrate the tendency of Sb 3+ cations to be coordinated by bromide anions with far greater interatomic distances compared to the oxygen atoms. The crystallographically unique bromide anions do not exhibit any bonding contacts to the Ln 3+ cations, since the shortest Ln 3+ ···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 Sb 3+ 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 Sb 3+ cations with distances of 302-366 pm [51]. Almost the same holds for a second antimony(III) oxide bromide with the composition Sb8O11Br2, where Sb 3+ -O 2-distances of 190-257 pm and Sb 3+ ···Br -distances of 314-396 pm occur [52,53]. Therefore, in both these ternary compounds and LnSb2O4Br (Ln = Eu-Tb), the Sb 3+ -Brbond lengths illustrate the tendency of Sb 3+ 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, while the crystallographic data as well as the positional atomic parameters and selected interatomic distances for EuSb2O4Br and TbSb2O4Br can be found in Tables A1-A3. In the case of GdSb2O4Br, it was also possible to synthesize very flat, square platelets of crystals with the desired composition, 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 LnSb 2 O 4 Br series (Ln = Eu-Tb) are listed in Table 1, while the crystallographic data as well as the positional atomic parameters and selected interatomic distances for EuSb 2 O 4 Br and TbSb 2 O 4 Br can be found in Tables A1-A3. In the case of GdSb 2 O 4 Br, 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 GdSb 2 O 4 Br both via single-crystal X-ray diffraction as well as powder X-ray diffraction methods (Section 3.2).
Crystals 2020, 10, x FOR PEER REVIEW 8 of 23 refinement. Instead, it was possible to refine the unit-cell parameters of GdSb2O4Br both via singlecrystal X-ray diffraction as well as powder X-ray diffraction methods (Section 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) Figure 7. The refined cell parameters for GdSb2O4Br are also given in Table 1 and are in high accordance with

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 EuSb 2 O 4 Br and TbSb 2 O 4 Br, 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 GdSb 2 O 4 Br with sufficient quality for a reliable structure determination, the lattice parameters were refined from powder X-ray diffraction methods. For comparison, single crystals of GdSb 2 O 4 Br 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 GdSb 2 O 4 Br 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.

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 Xray 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-Ln 3+ /Sb 3+ occupancy of the Sb 3+ position and, therefore, an incoherency with the

Electron-Probe Microanalysis
To evaluate the crystal morphology of the three LnSb 2 O 4 Br 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.

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 Xray 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-Ln 3+ /Sb 3+ occupancy of the Sb 3+ position and, therefore, an incoherency with the theoretical formula "SmSb2O4X" [26]. For this new structure type in the quaternary system Ln-Sb- All members of the LnSb 2 O 4 Br 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. The results of the quantitation using wavelength-dispersive spectrometry are compiled in Table  A4. When comparing the experimental values to the expected of TbSb2O4Br doped with 1.5 at-% Eu 3+ regarding the total amount of lanthanoid(III) cations, the measurements validate formula TbSb2O4Br:Eu 3+ with the desired doping content.

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 1 S0 → 3 P1 electronic transition. The Sb 3+ emission 3 P1 → 1 S0 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 Eu 3+ and Tb 3+ , rendering it potentially a suitable sensitizer for lanthanoid emission [54,55]. The luminescence of Sb 3+ in LnSb2O4Br representatives with Ln = Eu-Tb can be best investigated in GdSb2O4Br as Gd 3+ only emits in the UV (typically around 310 nm) due to a transition from the 6 P7/2 to 8 S7/2 levels. However, even efficient sensitization of Gd 3+ has been reported in YPO4:Sb 3+ , Gd 3+ [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 1 S0 → 3 P1 electronic transition of Sb 3+ (Figure 10). For an europium(III)-doped sample of TbSb 2 O 4 Br 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-Ln 3+ /Sb 3+ occupancy of the Sb 3+ position and, therefore, an incoherency with the theoretical formula "SmSb 2 O 4 X" [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 LnSb 2 O 4 Br (Ln = Eu-Tb). Moreover, the method was applied to determine the degree of doping in TbSb 2 O 4 Br, 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. When comparing the experimental values to the expected of TbSb 2 O 4 Br doped with 1.5 at-% Eu 3+ regarding the total amount of lanthanoid(III) cations, the measurements validate formula TbSb 2 O 4 Br:Eu 3+ with the desired doping content.

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 1 S 0 → 3 P 1 electronic transition. The Sb 3+ emission 3 P 1 → 1 S 0 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 Eu 3+ and Tb 3+ , rendering it potentially a suitable sensitizer for lanthanoid emission [54,55]. The luminescence of Sb 3+ in LnSb 2 O 4 Br representatives with Ln = Eu-Tb can be best investigated in GdSb 2 O 4 Br as Gd 3+ only emits in the UV (typically around 310 nm) due to a transition from the 6 P 7/2 to 8 S 7/2 levels. However, even efficient sensitization of Gd 3+ has been reported in YPO 4 :Sb 3+ , Gd 3+ [7]. The excitation spectrum of GdSb 2 O 4 Br monitored at 455 nm shows a band with a maximum at 257 nm, which can be assigned to the the partially forbidden 1 S 0 → 3 P 1 electronic transition of Sb 3+ (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 3 P1→ 1 S0. This band in the blue region of the electromagnetic spectrum occurs for all the investigated compounds upon excitation in 1 S0 → 3 P1 (inset in Figure 11, left). For TbSb2O4Br:Eu 3+ and GdSb2O4Br:Eu 3+ its intensity is negligible compared to the Eu 3+ transitions (Figure 11, left) and less intense than Tb 3+ transitions in the terbium derivatives (inset in Figure 11, left), suggesting the presence of an energy transfer (ET) from Sb 3+ to Ln 3+ . The different contribution of the Sb 3+ emission (blue region) compared to the Eu 3+ emission (red region) as well as Tb 3+ role, are demonstrated by the chromaticity coordinates calculated for TbSb2O4Br:Eu 3+ and GdSb2O4Br:Eu 3+ (λex = 257 nm) (Figure 11, right). In order to explain this difference, a deep investigation on the luminescence properties of both TbSb2O4Br:Eu 3+ and GdSb2O4Br:Eu 3+ was performed.  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 3 P 1 → 1 S 0 . This band in the blue region of the electromagnetic spectrum occurs for all the investigated compounds upon excitation in 1 S 0 → 3 P 1 (inset in Figure 11, left). For TbSb 2 O 4 Br:Eu 3+ and GdSb 2 O 4 Br:Eu 3+ its intensity is negligible compared to the Eu 3+ transitions (Figure 11, left) and less intense than Tb 3+ transitions in the terbium derivatives (inset in Figure 11, left), suggesting the presence of an energy transfer (ET) from Sb 3+ to Ln 3+ . The different contribution of the Sb 3+ emission (blue region) compared to the Eu 3+ emission (red region) as well as Tb 3+ role, are demonstrated by the chromaticity coordinates calculated for TbSb 2 O 4 Br:Eu 3+ and GdSb 2 O 4 Br:Eu 3+ (λ ex = 257 nm) (Figure 11, right). In order to explain this difference, a deep investigation on the luminescence properties of both TbSb 2 O 4 Br:Eu 3+ and GdSb 2 O 4 Br:Eu 3+ was performed.  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 3 P1→ 1 S0. This band in the blue region of the electromagnetic spectrum occurs for all the investigated compounds upon excitation in 1 S0 → 3 P1 (inset in Figure 11, left). For TbSb2O4Br:Eu 3+ and GdSb2O4Br:Eu 3+ its intensity is negligible compared to the Eu 3+ transitions (Figure 11, left) and less intense than Tb 3+ transitions in the terbium derivatives (inset in Figure 11, left), suggesting the presence of an energy transfer (ET) from Sb 3+ to Ln 3+ . The different contribution of the Sb 3+ emission (blue region) compared to the Eu 3+ emission (red region) as well as Tb 3+ role, are demonstrated by the chromaticity coordinates calculated for TbSb2O4Br:Eu 3+ and GdSb2O4Br:Eu 3+ (λex = 257 nm) (Figure 11, right). In order to explain this difference, a deep investigation on the luminescence properties of both TbSb2O4Br:Eu 3+ and GdSb2O4Br:Eu 3+ was performed.    Figure 12. For EuSb 2 O 4 Br, when monitoring at 611 nm, the most intense band appears at 464 nm, which belongs to the set of 7 F 0 → 5 D 2 hypersensitive dielectric dipole transitions of Eu 3+ . The other excitation lines correspond to 7 F 0 → 5 L 6 at 393 nm, 7 F 0 → 5 D 1 and 7 F 1 → 5 D 1 at 525 and 534 nm of Eu 3+ , respectively. of the lanthanide cations in GdSb2O4Br:Eu and TbSb2O4Br:Eu and to their similar radii, both compounds are supposed to have charge transfer bands in the region around 260-270 nm, similarly to Gd2Zr2O7:Eu 3+ and LaPO4:Eu 3+ [57]. Thus in the ultraviolet (UV) region, an overlap between the aforementioned 1 S0 → 3 P1 transition of Sb 3+ and the Eu 3+ charge transfer band occurs. For GdSb2O4Br:Eu 3+ , the charge transfer band is the most intense. Conversely, it is negligible in the neat Eu-compound.
By monitoring the Eu 3+ emission at 611 nm in TbSb2O4Br:Eu 3+ , besides the characteristic interconfigurational f-f transitions of Eu 3+ , albeit with comparatively weak relative intensity, the bands in the UV region of light belonging to Eu 3+ -O 2-charge transfer and the Sb 3+ 1 S0 → 3 P1 transitions can be seen. In addition, transitions characteristic for Tb 3+ are present which points to Tb 3+ → Eu 3+ sensitization. Excitation spectra monitored at the Tb 3+ emission for TbSb2O4Br as well as TbSb2O4Br:Eu 3+ are shown in Figure 12 (right). Both materials show the characteristic bands of Tb 3+ which can be assigned to the 7 F6 → 5 D3 and 7 F6 → 5 D4 transitions. The presence of Tb 3+ peaks in the excitation spectrum of All the aforementioned transitions can also be observed in the two Eu 3+ -doped compounds, GdSb 2 O 4 Br:Eu 3+ and TbSb 2 O 4 Br:Eu 3+ . Moreover, in both the Eu 3+ -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 (O 2− ) → 4f (Eu 3+ ) charge transfer which frequently is observed in Eu 3+ oxide compounds [6,[56][57][58] Figure 12 (right). Both materials show the characteristic bands of Tb 3+ which can be assigned to the 7 F 6 → 5 D 3 and 7 F 6 → 5 D 4 transitions. The presence of Tb 3+ peaks in the excitation spectrum When excited at 283 and 464 nm" corresponding to 1 S 0 → 3 P 1 of Sb 3+ , charge transfer band, 7 F 0 → 5 L 6 and 7 F 0 → 5 D 2 of Eu 3+ , respectively (Figure 13), all Eu 3+ -containing compounds show emission from the 5 D 0 level of Eu 3+ transitions. The low site symmetry of Eu 3+ in the compound series increases the number of observable transitions [6]. The hypersensitive (or electric dipole) band 5 D 0 → 7 F 2 with five sublevels, and the magnetic dipole transition 5 D 0 → 7 F 1 with all three possible sublevels can be observed.
number of observable transitions [6]. The hypersensitive (or electric dipole) band 5 D0 → 7 F2 with five sublevels, and the magnetic dipole transition 5 D0 → 7 F1 with all three possible sublevels can be observed.
The dominance of the 5 D0 → 7 F2 transition in the emission spectra relative to the 5 D0 → 7 F1 transition is compatible with the location of Eu 3+ in a low-symmetry site [59]. The asymmetry ratio, defined as ( 5 D0 → 7 F2)/( 5 D0 → 7 F1), is a parameter that indicates the distortion of Eu 3+ 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 Ln 3+ in LnSb2O4Br [59,60].
Very weak peaks corresponding to emission from higher levels are also visible: at 582 nm, attributable to the forbidden transition 5 D1 → 7 F0 and at 579 nm, that probably corresponds to the 5 D1 → 7 F3 transition and others even weaker in the 560-576 nm region [5]. The intensity of the 5 D0 → 7 F4 transition is higher than the magnetic dipole band, as already found in lanthanoid(III) orthoborates with Cs sites symmetry [61] or in the polyoxometalate Na9[EuW10O36] · 14 H2O with a distorted D4d symmetry [62]. Due to the full Eu 3+ concentration, a partial quenching of Eu 3+ emission occurs in EuSb2O4Br. This is confirmed not only by the short lifetime of 5 D0 → 7 F2 detected upon excitation at 464 nm (10.7 µs), but also by the absence of the transitions from levels higher than 5 D0. As can be observed in the excitation spectrum (Figure 12, left) by detecting the most intense Eu 3+ emission at 611 nm, no The dominance of the 5 D 0 → 7 F 2 transition in the emission spectra relative to the 5 D 0 → 7 F 1 transition is compatible with the location of Eu 3+ in a low-symmetry site [59]. The asymmetry ratio, defined as ( 5 D 0 → 7 F 2 )/( 5 D 0 → 7 F 1 ), is a parameter that indicates the distortion of Eu 3+ 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 Ln 3+ in LnSb 2 O 4 Br [59,60].
Very weak peaks corresponding to emission from higher levels are also visible: at 582 nm, attributable to the forbidden transition 5 D 1 → 7 F 0 and at 579 nm, that probably corresponds to the 5 D 1 → 7 F 3 transition and others even weaker in the 560-576 nm region [5]. The intensity of the 5 D 0 → 7 F 4 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 Na 9 [EuW 10 O 36 ] · 14 H 2 O with a distorted D 4d symmetry [62].
Due to the full Eu 3+ concentration, a partial quenching of Eu 3+ emission occurs in EuSb 2 O 4 Br. This is confirmed not only by the short lifetime of 5 D 0 → 7 F 2 detected upon excitation at 464 nm (10.7 µs), but also by the absence of the transitions from levels higher than 5 D 0 . As can be observed in the excitation spectrum (Figure 12, left) by detecting the most intense Eu 3+ 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 7 F 0 → 5 D 2 transition of Eu 3+ , EuSb 2 O 4 Br shows the characteristic luminescence features of Eu 3+ . This means that energy migration between ions in the bulk is not completely efficient upon excitation at lower energy.
The emission spectrum of the Tb 3+ -based materials upon excitation with λ ex = 378 nm are shown in Figure 14. The spectrum of TbSb 2 O 4 Br shows 5 D 4 → 7 F 6 , 5 D 4 → 7 F 5 , 5 D 4 → 7 F 4 , 5 D 4 → 7 F 3 transitions, resulting overall in an impression of green light. In the Eu 3+ -doped compound, 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 7 F0 → 5 D2 transition of Eu 3+ , EuSb2O4Br shows the characteristic luminescence features of Eu 3+ . This means that energy migration between ions in the bulk is not completely efficient upon excitation at lower energy. The emission spectrum of the Tb 3+ -based materials upon excitation with λex = 378 nm are shown in Figure 14. The spectrum of TbSb2O4Br shows 5 D4 → 7 F6, 5 D4 → 7 F5, 5 D4 → 7 F4, 5 D4 → 7 F3 transitions, resulting overall in an impression of green light. In the Eu 3+ -doped compound, TbSb2O4Br:Eu 3+ , emission from Eu 3+ are dominant whilst those for Tb 3+ are hardly visible. Together with the presence of Tb 3+ excitation bands by monitoring the luminescence of Eu 3+ , the strong Eu 3+ emission at the expense of Tb 3+ emission, indicates the presence of notable Tb 3+ → Eu 3+ energy transfer. Moreover, this can explain the red shift in the CIE diagram with respect to GdSb2O4Br:Eu 3+ : in TbSb2O4Br:Eu 3+ , Sb 3+ → Eu 3+ is mediated by Tb 3+ , that in turn transfers almost all the excitation light to Eu 3+ , shifting the final emission towards the red, while in GdSb2O4Br:Eu 3+ some Sb 3+ 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. 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 Eu 3+ emission (611 nm), upon excitation with λ ex = 257 nm (corresponding to the 1 S 0 → 3 P 1 electronic transition of Sb 3+ ) was probed in order to allow for a comparison of the optical properties of GdSb 2 O 4 Br:Eu 3+ and TbSb 2 O 4 Br:Eu 3+ (see also the CIE diagram, Figure 11, right).
Upon this excitation, an intensity rise time can be seen for both the decay curves of GdSb 2 O 4 Br:Eu 3+ and TbSb 2 O 4 Br:Eu 3+ , indicating that Eu 3+ -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 Eu 3+ emission at 611 nm: In TbSb 2 O 4 Br:Eu 3+ the main peak occurs at 488 nm, followed by the peak at 378 nm, both belonging to Tb 3+ , while the bands in the UV region have a minor importance. This demonstrates the major role of Tb 3+ in sensitizing the emission of Eu 3+ , allowing a color more shifted towards the red. By contrast, in GdSb 2 O 4 Br:Eu 3+ , Eu 3+ emission can only be sensitized through Sb 3+ 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, TbSb 2 O 4 Br:Eu 3+ and GdSb 2 O 4 Br:Eu 3+ , are dependent on the excitation wavelength, as an example, a comparison between the decays of GdSb 2 O 4 Br:Eu 3+ upon excitation in the 1 S 0 → 3 P 1 electronic transition of Sb 3+ at 257 nm vs. excitation in the Eu 3+ transition at 7 F 0 → 5 L 6 at 393 nm is shown in Figure 15 (right). The Eu 3+ emission (611 nm), upon excitation with λex = 257 nm (corresponding to the 1 S0 → 3 P1 electronic transition of Sb 3+ ) was probed in order to allow for a comparison of the optical properties of GdSb2O4Br:Eu 3+ and TbSb2O4Br:Eu 3+ (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:Eu 3+ and TbSb2O4Br:Eu 3+ , indicating that Eu 3+ -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 Eu 3+ emission at 611 nm: In TbSb2O4Br:Eu 3+ the main peak occurs at 488 nm, followed by the peak at 378 nm, both belonging to Tb 3+ , while the bands in the UV region have a minor importance. This demonstrates the major role of Tb 3+ in sensitizing the emission of Eu 3+ , allowing a color more shifted towards the red. By contrast, in GdSb2O4Br:Eu 3+ , Eu 3+ emission can only be sensitized through Sb 3+ 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:Eu 3+ and GdSb2O4Br:Eu 3+ , are dependent on the excitation wavelength, as an example, a comparison between the decays of GdSb2O4Br:Eu 3+ upon excitation in the 1 S0 → 3 P1 electronic transition of Sb 3+ at 257 nm vs. excitation in the Eu 3+ transition at 7 F0 → 5 L6 at 393 nm is shown in Figure 15 (right). Eu 3+ lifetime corresponding to the main emission at 611 nm was also evaluated upon excitation in the most intense band at 464 nm ( 7 F0 → 5 D2 of Eu 3+ ). In TbSb2O4Br:Eu 3+ , 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 Eu 3+ . The observed lifetime is similar to the value observed for Eu 3+ in zirconia samples [56], as well as with antimonygermanate-silicate glasses [63]. However, for excitation of GdSb2O4Br:Eu 3+ with 464 nm, the decay curve needed to be fitted bi-exponential ( Figure 15, middle and fitted components in Table A5), pointing to two different Eu 3+ emitting species, either with different environment, potentially Eu 3+ in the bulk and Eu 3+ 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 Tb 3+ → Eu 3+ was evaluated through lifetime analysis, by using the formula: ηET = 1 -τDA τD -1 (2) where τDA is the lifetime of the donor (Tb 3+ ) in presence of the acceptor (Eu 3+ ), and τD is the lifetime of the donor in the undoped material.
Thus, the emission decays corresponding to the Tb 3+ emission from the 5 D4 level were acquired by exciting into the most intense excitation band, namely 483 nm (corresponding to the 7 F6 → 5 D4 transition), for both TbSb2O4Br and TbSb2O4Br:Eu 3+ (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 0 5 10 15 Intensity / a.u.  Eu 3+ lifetime corresponding to the main emission at 611 nm was also evaluated upon excitation in the most intense band at 464 nm ( 7 F 0 → 5 D 2 of Eu 3+ ). In TbSb 2 O 4 Br:Eu 3+ , 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 Eu 3+ . The observed lifetime is similar to the value observed for Eu 3+ in zirconia samples [56], as well as with antimony-germanate-silicate glasses [63]. However, for excitation of GdSb 2 O 4 Br:Eu 3+ with 464 nm, the decay curve needed to be fitted bi-exponential ( Figure 15, middle and fitted components in Table A5), pointing to two different Eu 3+ emitting species, either with different environment, potentially Eu 3+ in the bulk and Eu 3+ as the surface (note that it was not possible to obtain single crystals of GdSb 2 O 4 Br of sufficient quality for single crystal X-ray diffraction (SCXRD) analysis).

Time / ms
The efficiency of Tb 3+ → Eu 3+ was evaluated through lifetime analysis, by using the formula: where τ DA is the lifetime of the donor (Tb 3+ ) in presence of the acceptor (Eu 3+ ), and τ D is the lifetime of the donor in the undoped material. Thus, the emission decays corresponding to the Tb 3+ emission from the 5 D 4 level were acquired by exciting into the most intense excitation band, namely 483 nm (corresponding to the 7 F 6 → 5 D 4 transition), for both TbSb 2 O 4 Br and TbSb 2 O 4 Br:Eu 3+ (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 Eu 3+ -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%.
Crystals 2020, 10, x FOR PEER REVIEW 16 of 23 components as reported by Zmojda et al. [63], while the Eu 3+ -doped sample shows a value of 12.3 µs, fitted by a single exponential decay. The ηET obtained from these values is around 95%.

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 Ln 3+ and Sb 3+ 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- Whilst the bulk europium compound, EuSb2O4Br, is not suitable as luminescent material, both GdSb2O4Br and TbSb2O4Br doped with Eu 3+ show strong photoluminescence thanks to Sb 3+ which is acting as a blue-emitting species and sensitizer for lanthanide. TbSb2O4Br:Eu additionally benefits form strong energy transfer from Tb 3+ to Eu 3+ . Since Sb 3+ is emitting in the blue, Tb 3+ in the green and Eu 3+ in the red white-light emitting materials can be envisioned in this class of compound.
Author Contributions: F.C.G. synthesized the title compounds LnSb2O4Br (Ln = Eu-Tb). F.C.G. and T.S. solved their crystal structures. Funds provided to T.S. funded the chemicals, the materials, the scientific equipment and