The Synthesis, Structure, and Luminescent Properties of TmMgB5O10 Crystals

TmMgB5O10 spontaneous crystals were synthesized via the flux-growth technique from a K2Mo3O10-based solvent. The crystal structure of the compound was solved and refined within the space group P21/n. The first principles calculations of the electronic structure reveal that TmMg-pentaborate with an ideal not defected crystal structure is an insulator with an indirect energy band gap of approximately 6.37 eV. Differential scanning calorimetry measurements and powder X-ray diffraction studies of heat-treated solids show that TmMgB5O10 is an incongruent melting compound. A characteristic band of the Tm3+ cation corresponding to the 3H6 → 1D2 transition is observed in the photoluminescence excitation spectra of TmMg-borate. The as-obtained crystals exhibit intense blue emission with the emission peaks centered at 455, 479, 667, and 753 nm. The most intensive band corresponds to the 1D2 → 3F4 transition. TmMgB5O10 solids demonstrate the thermal stability of photoluminescence.


Introduction
In recent decades, a large amount of research has focused on the development of materials used as environmentally friendly light sources and phosphors, as well as laser and nonlinear optical materials.Future progress in science and technology is directly related to the search and further application of new compounds, as well as to the improvement of the parameters of existing materials.One of the promising classes of such compounds is rareearth borates, which demonstrate high chemical stability, thermal and radiation resistance, wide transparency area, high laser threshold, etc.In addition, borates have a wide variety of chemical compositions and crystal structures due to the ability of the boron atom to form various anionic and polyanionic groups [1].Nowadays, the search for borate materials that can be used as phosphors is an actual problem.There are many borate crystals that do not exhibit luminescent properties in their pure form, and their luminescent characteristics are associated with doping, such as the incorporation of rare-earth or transition metal cations into their structure.
In particular, among numerous borates, a group of rare-earth magnesium pentaborates can be used as phosphors to create powerful emitters of the visible range with ultraviolet (UV) excitation [2,3].Rare-earth magnesium borates with the general chemical formula LnMgB 5 O 10 (where Ln = La, Ce-Nd, and Sm-Er) were first synthesized by Saubat et al. in 1980 by the solid-phase method.LaMgB 5 O 10 single crystals were then obtained by melting a mixture of La 2 O 3 , MgO, and B 2 O 3 at 1200 • C with an excess of magnesium and boron oxides with respect to the stoichiometric amount to compensate for volatilization losses.The crystal structure of LaMgB 5 O 10 was solved and refined within the space group (sp.gr.) P2 1 /c [4].The powder patterns of other polycrystalline LnMgB 5 O 10 samples were also indexed in the same space group.These materials exhibit remarkable structural features: a weak atomic Ln/O ratio, well-isolated Ln chains separated by large polyborate anions, and a highly covalent matrix suggest the possibility of relatively low-concentration quenching.LnMgB 5 O 10 is expected to be an excellent material as a phosphor host and a matrix for solid-state lasers.
A review of previous studies on the synthesis and crystal growth of rare-earth pentaborates shows that the Li 2 O-B 2 O 3 -LiF mixed flux is the most commonly used to obtain LnMgB 5 O 10 compounds by the solution growth on dipped seeds technique [10,11].Recently, GdMgB 5 O 10 single crystals were obtained from K 2 Mo 3 O 10 flux [12].The authors of Ref. [13]  Spontaneous TmMgB 5 O 10 (TmMB) single crystals were obtained for the first time in the framework of the approach described in [13].The present work is focused on the synthesis and study of the crystal structure, thermal behavior, and luminescent properties of TmMgB 5 O 10 crystals.

Materials and Methods
Two subsequent routes were applied to obtain TmMg-borate: solid-state synthesis and flux-growth techniques.In both cases, a vertical resistance-heated furnace equipped with a Proterm-100 precision temperature controller and a set of Pt/Rh-Pt thermocouples was used.Tm 2 O 3 (99.996%),H 3 BO 3 (A.C.S. grade), and MgO (A.C.S. grade) were used as crystal-forming agents for the solid-state synthesis of polycrystalline TmMB.Calculated amounts of Tm 2 O 3 , MgO, and H 3 BO 3 were weighted, mixed together, and pressed into tablets 15 mm in diameter.The compact TmMB tablets were heated in an alundum crucible at the temperature of 900 • C for 72 h, and then a furnace was gradually cooled to room temperature.
In solid-phase synthesis, crystal-forming components were initially weighed corresponding to the composition of TmMgB 5 O 10 .As a result, only the TmBO 3 phase was formed.According to the Le Chatelier principle, an excess of one of the components shifts the direction of the reaction toward the formation of the desired phase.Further experiments were carried out using a non-stoichiometric load with 100% excess of MgO.This approach led to the formation of a nearly monophase TmMgB 5 O 10 sample at 900 • C.
Spontaneous TmMB crystals were obtained from a high-temperature K 2 Mo 3 O 10 -based flux melt.The K 2 Mo 3 O 10 solvent used was a stoichiometric mixture of K 2 MoO 4 (A.C.S. grade) and MoO 3 (A.C.S. grade).TmMB previously synthesized by the solid-state technique was taken as a crystal-forming component.A thoroughly mixed starting charge with a crystal phase-to-flux weight ratio of 30:70 wt.% was loaded into a 15 mL platinum crucible, placed in a furnace, heated to 900 • C and held for 24 h to ensure complete homogenization of the solution.Subsequently, the flux melt was cooled to 800 • C at a rate of 1 • C/h, followed by cooling at 10  [14].The temperature range for LnMB synthesis is restricted by the properties of K 2 Mo 3 O 10 -based high-temperature solutions and by borate stability in such a melt.Usually, the melting temperature of the charge does not exceed 1050 • C because of the significant increase in the borate decomposition rate.The lower temperature boundary (commonly ~800 • C) is determined by a significant increase in melt viscosity with a corresponding decrease in the crystal growth rate.
The morphological feature and elemental analysis were performed by analytical scanning electron microscopy (SEM) technique using a JSM-IT500 microscope, JEOL Ltd., Tokyo, Japan, equipped with energy dispersive X-ray (EDX) detector X-Max-50, Oxford Instruments Ltd. (Abingdon, UK), GB (Laboratory of Analytical Techniques of High Spatial Resolution, Dept. of Petrology, Moscow State University).
Differential scanning calorimetry (DSC) measurements were performed by means of a STA 449 F5 Jupiter (Netzsch, Selb, Germany) in the temperature range of 50-1200 • C with a heating rate of 20 • C/min in Ar gas flow.A PtRh20 crucible of 85 µL volume was used in the DSC experiments.
The band structure for TmMB was calculated within the framework of density functional theory using the pseudopotential plane wave basis of the Quantum Espresso v. 7.1 software package [16].The electronic exchange correlations were treated by the Perdew-Burke-Ernzerhof (PBE) approach under a generalized gradient approximation (GGA) [17].Ultrasoft pseudopotentials were used to describe the interaction between electrons and ions [18].The kinetic energy cut-off values of the wavefunctions were limited to 58 Ry.The integration calculation of the system in the Brillouin region uses the Monkhorst-Pack scheme, the k grid point is 3 × 3 × 3, and the cut-off energy of the plane wave of the system is set to 750 eV to ensure the convergence of energy and the configuration of the system at the level of quasi-complete plane wave base.In the self-consistent field operation, the Pulay density mixing method is adopted, and the self-consistent field is set to 1 × 10 −6 eV/atom.The calculations did not include spin-orbit coupling.Full relativistic effects were taken for the nucleus states, and the scalar relativistic approximation was used for the valence states.
A Cary Eclipse (Agilent Technologies) fluorescence spectrometer equipped with a 75 kW xenon light source (pulse length τ = 2 µs, pulse frequency ν = 80 Hz, wavelength resolution 0.5 nm; PMT Hamamatsu R928) was used to record photoluminescence emission (PL) and excitation (PLE) spectra.All measurements were performed at room temperature and corrected for the sensitivity of the spectrometer.The quantum yield defined as the ratio of the number of emitted photons to the number of photons absorbed (QY, %) for the visible region was measured on an Edinburgh Instruments FS5 spectrofluorometer equipped with a SC-30 integrating sphere module and a Hamamatsu PMT R928P.The measurement was performed at room temperature.
Luminescence emission spectra under heating upon 500 K with the excitation in the UV region were measured using a 150 W xenon lamp (Oriel Instruments, Stratford, ON, USA) as an excitation source, an MDR-206 primary monochromator (Lomo, Saint Petersburg, Russia), and a LOT-Oriel MS-257 spectrograph (Oriel Instruments, Stratford, ON, USA) equipped with a Marconi CCD detector (Marconi Applied Technologies Limited, Chelmsford, UK).Samples were mounted into a Cryotrade LN-120 vacuum optical cryostat (Cryotrade Engineering, Moscow, Russia).

Results and Discussion
Spontaneous isometric crystals up to 1 mm in size were obtained from a K 2 Mo 3 O 10 -based flux melt (Figure 1).Preliminary XRD studies were performed to find the sample with the best diffraction reflection profile for a single-crystal X-ray study.Based on these pre-experiments, all tested specimens were crystal clusters or twin crystals.field operation, the Pulay density mixing method is adopted, and the self-consistent field is set to 1 × 10 −6 eV/atom.The calculations did not include spin-orbit coupling.Full relativistic effects were taken for the nucleus states, and the scalar relativistic approximation was used for the valence states.A Cary Eclipse (Agilent Technologies) fluorescence spectrometer equipped with a 75 kW xenon light source (pulse length τ = 2 μs, pulse frequency ν = 80 Hz, wavelength resolution 0.5 nm; PMT Hamamatsu R928) was used to record photoluminescence emission (PL) and excitation (PLE) spectra.All measurements were performed at room temperature and corrected for the sensitivity of the spectrometer.The quantum yield defined as the ratio of the number of emitted photons to the number of photons absorbed (QY, %) for the visible region was measured on an Edinburgh Instruments FS5 spectrofluorometer equipped with a SC-30 integrating sphere module and a Hamamatsu PMT R928P.The measurement was performed at room temperature.
Luminescence emission spectra under heating upon 500 K with the excitation in the UV region were measured using a 150 W xenon lamp (Oriel Instruments, Stratford, ON, USA) as an excitation source, an MDR-206 primary monochromator (Lomo, Saint Petersburg, Russia), and a LOT-Oriel MS-257 spectrograph (Oriel Instruments, Stratford, ON, USA) equipped with a Marconi CCD detector (Marconi Applied Technologies Limited, Chelmsford, UK).Samples were mounted into a Cryotrade LN-120 vacuum optical cryostat (Cryotrade Engineering, Moscow, Russia).

Results and Discussion
Spontaneous isometric crystals up to 1 mm in size were obtained from a K2Mo3O10-based flux melt (Figure 1).Preliminary XRD studies were performed to find the sample with the best diffraction reflection profile for a single-crystal X-ray study.Based on these pre-experiments, all tested specimens were crystal clusters or twin crystals.Qualitative energy-dispersive X-ray analysis showed peaks corresponding to thulium, magnesium, boron, and oxygen (Figure 2).Qualitative energy-dispersive X-ray analysis showed peaks corresponding to thulium, magnesium, boron, and oxygen (Figure 2).The ratio of the two twin components is 75.1% and 23.0% of all registered reflections, respectively.The existence of two components for the TmMB single crystal slightly reduces the quality of the obtained structural model and leads to overestimated structure refinement factors.The model proposed is obtained from the experimental data of the first component (75.1% of the total number of reflections).It is not possible to correctly solve and refine the crystal structure of the second component due to the extremely high value of the parameter Rint2 = 79%.
The crystal structure model was determined and refined using the Jana2020 software package [19].Structural patterns similar to those described in the literature [20,21] were obtained using the SuperFlip utility [22] within the sp.gr.P21/n.The experimental and crystallographic data and structural refinements of TmMB are summarized in Table 1.The ratio of the two twin components is 75.1% and 23.0% of all registered reflections, respectively.The existence of two components for the TmMB single crystal slightly reduces the quality of the obtained structural model and leads to overestimated structure refinement factors.The model proposed is obtained from the experimental data of the first component (75.1% of the total number of reflections).It is not possible to correctly solve and refine the crystal structure of the second component due to the extremely high value of the parameter R int2 = 79%.
The crystal structure model was determined and refined using the Jana2020 software package [19].Structural patterns similar to those described in the literature [20,21] were obtained using the SuperFlip utility [22] within the sp.gr.P2 1 /n.The experimental and crystallographic data and structural refinements of TmMB are summarized in Table 1.
The model of the structure consists of 17 independent atomic positions: one thulium position (Tm1), one magnesium position (Mg1), five boron atom positions (B1-B5), and ten oxygen positions (O1-O10).The low quality of the crystal and, as a result, of the diffraction experiment does not allow one to refine the parameters of anisotropic atomic displacements, which affects the value of the structure refinement factors and the values of the residual electron density.The atomic displacement parameters were refined isotropically for all localized positions; the occupancy of all positions is 100% (Table 2).The principle interatomic distances are listed in Table 3.The thulium cations are coordinated by ten O atoms, forming distorted polyhedra that share a common edge and form one-dimensional zigzag chains extending along the b axis (Figure 4a).Magnesium atoms are located inside oxygen octahedra that form edgesharing dimeric Mg 2 O 10 groups.The adjacent Tm-O chains are linked together by dimeric Mg 2 O 10 , building Tm-Mg-O layers (Figure 4b).Boron atoms have a mixed coordination (3t + 2∆): Three B atoms are located inside oxygen tetrahedra and the other two B atoms are coordinated by three O atoms, forming a planar triangle.BO 4 tetrahedra and BO 3 triangles connected by corners form a pentaborate cluster [1].Each B 5 O 12 unit via a corner-sharing O atom between two neighboring clusters forms 4-membered rings, from which infinite twodimensional B-O layers are further assembled (Figure 4c).Countless boron-oxygen layers are linked together by Tm and Mg cations forming a rigid three-dimensional framework (Figure 4d).
To determine the behavior and melting temperature of TmMg-borate, DSC measurements were performed.The calorimetric data in the temperature range of 50-1200 • C are shown in Figure 5.The DSC heating curve for the sample features one sharp endothermic peak with an onset temperature of ~1020 • C, but no exothermal peak was observed on the cooling curve.After melting, the residue was characterized by PXRD analysis, which showed that it was different from the initial compound powder, and the decomposition products of TmMB compound are a mixture of TmBO 3 and Mg 2 B 2 O 5 .These results agreed with those previously obtained for Yb:YMgB 5 O 10 crystals [24].
The PXRD dataset was collected to confirm that the crystal structure is representative of the entire experimental sample.The pattern fits well with that calculated from the cif file obtained from the structural studies and calculated from the cif file of YMaB 5 O 10 (ICSD #4489) (Figure 6).To determine the behavior and melting temperature of TmMg-borate, DSC measurements were performed.The calorimetric data in the temperature range of 50-1200 °C are shown in Figure 5.The DSC heating curve for the sample features one sharp endothermic peak with an onset temperature of ~1020 °C, but no exothermal peak was observed on the cooling curve.After melting, the residue was characterized by PXRD analysis, which showed that it was different from the initial compound powder, and the decomposition products of TmMB compound are a mixture of TmBO3 and Mg2B2O5.These results agreed with those previously obtained for Yb:YMgB5O10 crystals [24].The PXRD dataset was collected to confirm that the crystal structure is representative of the entire experimental sample.The pattern fits well with that calculated from the cif file obtained from the structural studies and calculated from the cif file of YMaB5O10 (ICSD #4489) (Figure 6).The PXRD dataset was collected to confirm that the crystal structure is representative of the entire experimental sample.The pattern fits well with that calculated from the cif file obtained from the structural studies and calculated from the cif file of YMaB5O10 (ICSD #4489) (Figure 6).The band structure of the TmMB material was calculated to find out the electronic characteristics of the TmMgB5O10 model structure (Figure 7).The outcome of this analysis The band structure of the TmMB material was calculated to find out the electronic characteristics of the TmMgB 5 O 10 model structure (Figure 7).The outcome of this analysis shows that TmMB is an insulator, as evidenced by the determination of an indirect energy gap of about 6.37 eV.This gap was determined by the A point at the maximum of the valence bands (VBM) and the Γ point at the minimum of the conduction band (CBM).However, it should be noted that GGA calculations have been shown to underestimate band gaps, and therefore, the experimental band gap for MgTmB 5 O 10 may differ slightly from the calculated value.Notably, the VBM is a flat band, and the maximum values at Γ and A points in the Brillouin zone are quite close.This suggests that even a small concentration of defects in the simulated crystal structure could result in a direct band gap at the Γ point.
According to the calculated density of states (DOS) of MgTmB 5 O 10 (Figure 8), the electronic structure in the energy range from −28.0 to 22 eV mainly comprises the Tm-s and O-s/p states with smaller contributions from the B-s/p and Mg-s/d/p states.The significant contribution to the lower conduction band comes from the Tm-d state, while O-s in the upper VB has a considerable effect on the dispersion.
valence bands (VBM) and the Γ point at the minimum of the conduction band (CBM).However, it should be noted that GGA calculations have been shown to underestimate band gaps, and therefore, the experimental band gap for MgTmB5O10 may differ slightly from the calculated value.Notably, the VBM is a flat band, and the maximum values at Γ and A points in the Brillouin zone are quite close.This suggests that even a small concentration of defects in the simulated crystal structure could result in a direct band gap at the Γ point.According to the calculated density of states (DOS) of MgTmB5O10 (Figure 8), the electronic structure in the energy range from −28.0 to 22 eV mainly comprises the Tm-s and O-s/p states with smaller contributions from the B-s/p and Mg-s/d/p states.The significant contribution to the lower conduction band comes from the Tm-d state, while O-s in the upper VB has a considerable effect on the dispersion.The normalized photoluminescence excitation and photoluminescence emission spectra of the TmMgB5O10 compound are shown in Figure 9. On the PLE spectrum (Figure 9a) for TmMgB5O10 the typical transition of the Tm 3+ ion is observed in the near-ultraviolet region.The sharp line with maxima centered at 358 nm corresponds to the transition 3 H6 → 1 D2 [25].Since the indirect energy gap is about 6.37 eV, excitation with such higher transition energies is not possible in the 220-340 nm range.The standard sharp lines for Tm 3+ were registered for the PL spectrum (Figure 9b).The maxima centered at 455, 479, 667, and 753 nm correspond to the 1D2 → 3 F4, 1 G4 → 3 H6, 1 G4 → 3 F4, and 3 H4 → 3 H6 transitions according to [26].The most intensive transition is observed for 1 D2 → 3 F4, which emits in the blue region.So, MgTmB5O10 demonstrates photolumines-   The normalized photoluminescence excitation and photoluminescence emission spectra of the TmMgB5O10 compound are shown in Figure 9. On the PLE spectrum (Figure 9a) for TmMgB5O10 the typical transition of the Tm 3+ ion is observed in the near-ultraviolet region.The sharp line with maxima centered at 358 nm corresponds to the transition 3 H6 → 1 D2 [25].Since the indirect energy gap is about 6.37 eV, excitation with such higher transition energies is not possible in the 220-340 nm range.The standard sharp lines for Tm 3+ were registered for the PL spectrum (Figure 9b).The maxima centered at 455, 479, 667, and 753 nm correspond to the 1D2 → 3 F4, 1 G4 → 3 H6, 1 G4 → 3 F4, and 3 H4 → 3 H6 transitions according to [26].The most intensive transition is observed for 1 D2 → 3 F4, which emits in the blue region.So, MgTmB5O10 demonstrates photolumines- The normalized photoluminescence excitation and photoluminescence emission spectra of the TmMgB 5 O 10 compound are shown in Figure 9. On the PLE spectrum (Figure 9a) for TmMgB 5 O 10 the typical transition of the Tm 3+ ion is observed in the near-ultraviolet region.The sharp line with maxima centered at 358 nm corresponds to the transition 3 H 6 → 1 D 2 [25].Since the indirect energy gap is about 6.37 eV, excitation with such higher transition energies is not possible in the 220-340 nm range.The standard sharp lines for Tm 3+ were registered for the PL spectrum (Figure 9b).The maxima centered at 455, 479, 667, and 753 nm correspond to the 1 D 2 → 3 F 4 , 1 G 4 → 3 H 6 , 1 G 4 → 3 F 4 , and 3 H 4 → 3 H 6 transitions according to [26].The most intensive transition is observed for 1 D 2 → 3 F 4 , which emits in the blue region.So, MgTmB 5 O 10 demonstrates photoluminescence in the blue spectral region.Additionally, the integral intensity is significantly low and takes a few arb.units.Such behavior can be explained by the presence of a high concentration quenching effect in observed TmMgB 5 O 10 , which was registered in other hosts (see, for example, Refs.[27,28].This is confirmed by the minor value of QY of approximately 3%.Such extremely low QY can indicate concentration quenching through all the resonance channels of cross-relaxation "upward" and "downward" [29] in a studied host.The calculated Tm-Tm distance in the structure (approximate value is 3.94 Å) leads to registered concentration quenching.However, in spite of DFT calculations of the MgTmB 5 O 10 band structure for an ideal crystal model showing that this material is an insulator, contrasting these results with the experimental observations revealed the presence of PL peaks in the blue spectral range.Hence, it can be inferred that the presence of point defects in the crystal structure gives rise to additional electronic states.A similar conclusion was observed in [20], where the properties of the MgYB 5 O 10 compound were investigated.Furthermore, observed crystals exhibit thermal stability of photoluminescence (Figure 10).As the temperature increases, the total integral intensity of the 1 D 2 → 3 F 4 transition decreases slowly, indicating the standard thermal quenching of photoluminescence for MgTmB 5 O 10 .The thermal stability of photoluminescence in this Tm 3+ -activated host is higher than in other hosts, for example, in crystals BaY 2 F 8 [30] and powder YNbO 4 [31].It should be noted that other transitions in temperature dependence were not detected because of their low intensity.
presence of PL peaks in the blue spectral range.Hence, it can be inferred that the presence of point defects in the crystal structure gives rise to additional electronic states.A similar conclusion was observed in [20], where the properties of the MgYB5O10 compound were investigated.Furthermore, observed crystals exhibit thermal stability of photoluminescence (Figure 10).As the temperature increases, the total integral intensity of the 1 D2 → 3 F4 transition decreases slowly, indicating the standard thermal quenching of photoluminescence for MgTmB5O10.The thermal stability of photoluminescence in this Tm 3+ -activated host is higher than in other hosts, for example, in crystals BaY2F8 [30] and powder YNbO4 [31].It should be noted that other transitions in temperature dependence were not detected of their low intensity.presence of PL peaks in the blue spectral range.Hence, it can be inferred that the presence of point defects in the crystal structure gives rise to additional electronic states.A similar conclusion was observed in [20], where the properties of the MgYB5O10 compound were investigated.Furthermore, observed crystals exhibit thermal stability of photoluminescence (Figure 10).As the temperature increases, the total integral intensity of the 1 D2 → 3 F4 transition decreases slowly, indicating the standard thermal quenching of photoluminescence for MgTmB5O10.The thermal stability of photoluminescence in this Tm 3+ -activated host is higher than in other hosts, for example, in crystals BaY2F8 [30] and powder YNbO4 [31].It should be noted that other transitions in temperature dependence were not detected because of their low intensity.
performed experiments in Li 2 O-B 2 O 3 -LiF-and K 2 Mo 3 O 10 -based systems to determine the most suitable one for the growth of LnMgB 5 O 10 bulk crystals.It was shown that the K 2 Mo 3 O 10 flux is preferable.The strong tendency of Li 2 O-B 2 O 3 -LiF melts toward glass formation, and the high volatility and reactivity of fluorides at high temperatures make reproducible growth of high-quality crystals difficult.It was also proposed to use pre-synthesized LnMgB 5 O 10 tablets as a crystal-forming agent instead of the mixture of corresponding Ln 2 O 3 -MgO-B 2 O 3 oxides.

Figure 4 .
Figure 4. Structure of TmMgB5O10: (a) zigzag chains of edge-sharing TmO10 distorted polyhedral extending along the b axis; (b) Tm-Mg-O layers, formed by linked Tm-O chains and dimeric Mg2O10 clusters, projection along the b axis; (c) 4-membered ring assembled from B5O12 neighboring clusters; (d) two-dimensional B-O layers, linked together by Tm and Mg cations, projection along b axis.The blue, orange, green, and red balls represent Tm, Mg, B, and O, respectively.Visualization by VESTA [23].

Figure 4 . 13 Figure 5 .
Figure 4. Structure of TmMgB 5 O 10 : (a) zigzag chains of edge-sharing TmO 10 distorted polyhedral extending along the b axis; (b) Tm-Mg-O layers, formed by linked Tm-O chains and dimeric Mg 2 O 10 clusters, projection along the b axis; (c) 4-membered ring assembled from B 5 O 12 neighboring clusters; (d) two-dimensional B-O layers, linked together by Tm and Mg cations, projection along b axis.The blue, orange, green, and red balls represent Tm, Mg, B, and O, respectively.Visualization by VESTA [23].Materials 2023, 16, x FOR PEER REVIEW 9 of 13

Figure 5 .
Figure 5. DSC curve of TmMgB5O10 compound in the temperature range of 50-1200 °C.

Figure 6 .
Figure 6.PXRD patterns (a) of TmMgB5O10 solids, (b) calculated from the cif file obtained from the structural studies of a single crystal, and (c) calculated from the cif file of YMgB5O10 (ICSD #4489).

Figure 6 .
Figure 6.PXRD patterns (a) of TmMgB 5 O 10 solids, (b) calculated from the cif file obtained from the structural studies of a single crystal, and (c) calculated from the cif file of YMgB 5 O 10 (ICSD #4489).

Figure 7 .
Figure 7.The calculated band structure of MgTmB5O10.

Figure 8 .
Figure 8.The calculated density of states of MgTmB5O10.

Figure 7 .
Figure 7.The calculated band structure of MgTmB5O10.According to the calculated density of states (DOS) of MgTmB5O10 (Figure8), the electronic structure in the energy range from −28.0 to 22 eV mainly comprises the Tm-s and O-s/p states with smaller contributions from the B-s/p and Mg-s/d/p states.The significant contribution to the lower conduction band comes from the Tm-d state, while O-s in the upper VB has a considerable effect on the dispersion.

Figure 8 .
Figure 8.The calculated density of states of MgTmB5O10.

Figure 8 .
Figure 8.The calculated density of states of MgTmB 5 O 10 .

Figure 10 .
Figure 10.The temperature dependence of Tm 3+ photoluminescence (λ em = 358 nm); the right figure shows the relative emission intensity as a function of temperature.
• C/h to 300 • C. K 2 Mo 3 O 10 -based solvents seem to be the most suitable for the growth of different borate crystals.Nevertheless, LnMgB 5 O 10 solids dissolve incongruently in a K 2 Mo 3 O 10 melt, and reactions between K 2 Mo 3 O 10 and LnMgB 5 O 10 result in new crystalline phases, mostly crystal-forming oxides, as well as LnBO 3 .The formation temperatures of these co-crystallizing phases depend on the borate type and its concentration in a flux melt.Small crystals of co-existing phases that appeared at high temperatures become the nucleation centers of spontaneous LnMgB 5 O 10 crystals upon cooling.As a result of the incongruent dissolution of LnMg-borate, the melt is enriched in B 2 O 3 and Ln 2 O 3 .Rare-earth metal oxides, in turn, cause the formation of LnBO 3 with calcite-or vaterite-type structures depending on the temperature

Table 1 .
Crystallographic data, details relating to X-ray data collection, and structure refinements of TmMgB 5 O 10 .

Table 2 .
Atomic coordinates and equivalent isotropic thermal parameters U eq in the TmMgB 5 O 10 structure.