High-pressure/high-temperature synthesis of the ﬁ rst walstromite-analogue borate Tm 2 CrB 3 O 9

: Tm 2 CrB 3 O 9 was obtained by high-pressure/high-temperature syntheses at 7.5 GPa and 850 ° C. The compound crystallizes homeotypically to walstromite in the triclinic space group P ¯ 1 (no. 2) with the unit cell parameters a = 5.9118(4) Å , b = 5.9148(4) Å , c = 8.3575(6) Å , α = 83.64(1) ° , β = 71.19(1) ° , and γ = 79.93(1) ° , and two formula units per cell. Single-crystal and powder di ﬀ raction data are accompanied by an infrared spectroscopic investigation and the structural features of this new compound are discussed in detail.


Introduction
Walstromite was first described as BaCa 2 Si 3 O 9 by Alfors et al. as one of seven barium-containing minerals from Eastern Fresno County in California [1] and was found to be identical to a synthetic compound prepared in 1922 [2].A structural characterization of synthetic walstromite was later carried out by Dent-Glasser and Glasser using photographic X-ray diffraction data [3].The formal substitution of Ba by Ca leads to CaSiO 3 -walstromite [4], which also occurs naturally in the form of the mineral breyite [5][6][7].Furthermore, CaSiO 3wollastonite-II, a high-pressure polymorph of wollastonite, was reported to display similar structural features as walstromite [8][9][10][11].Structural differences between CaSiO 3 -walstromite and CaSiO 3 -wollastonite-II were discussed [9,12,13] until 2011 when Barkley et al. [11] pointed out that the crystal coordinates of walstromite, CaSiO 3 -walstromite, and CaSiO 3wollastonite-II can be transformed into each other by a linear transformation matrix and therefore all are isostructural.This also means that CaSiO 3 -walstromite and CaSiO 3 -wollastonite-II are identical compounds [11].In substance classes such as phosphates and germanates, walstromiteanalogue crystal structures were previously described with the discoveries of Na 2 KP 3 O 9 [14] and BaSrGe 3 O 9 [15], respectively.In this article, we report on the first borate Tm 2 CrB 3 O 9 within this structure family.Under ambient pressure, depending on the ionic radius of the lanthanoid, two structure types of lanthanoid chromium borates LnCr(BO 3 ) 2 (Ln = Ho-Lu) [16] and LnCr 3 (BO 3 ) 4 (Ln = Eu, Gd) [17,18] were previously described, while the new compound was obtained by high-pressure/hightemperature syntheses in a modified Walker-type multianvil device.
2 Experimental section 2.1 High-pressure/high-temperature synthesis The title compound was first encountered during explorative syntheses in the system Tm 2 O 3 -Cr 2 O 3 -B 2 O 3 at elevated pressures.Subsequently, a targeted synthesis of Tm 2 CrB 3 O 9 was carried out by mixing the starting materials Tm 2 O 3 (0.17 mmol, ChemPur, 99.90 %), Cr(NO 3 ) 3 •9H 2 O (0.17 mmol; Sigma Aldrich; 99 %), and H 3 BO 3 (0.52 mmol, Roth, >99.8 %) in the stoichiometric ratio 1:1:3.The sample was placed in a platinum capsule and centered in an 18/11 assembly of a modified Walker-type multianvil press (Max Voggenreiter GmbH, Mainleus, Germany) on which detailed information can be found in the literature [19][20][21].The sample was compressed to 7.5 GPa within 196 min and subsequently heated to 850 °C within 10 min.The temperature was kept constant for 25 min before it was slowly decreased to 600 °C within 45 min, where the reaction was quenched.After the decompression phase of 588 min, a dark violet product was obtained.For structure determination, this product did not contain any sufficiently sized crystals.By applying the same pressure and temperature on a mixture of the starting materials in the molar ratio 1:1:7, red crystals of Tm 2 CrB 3 O 9 embedded in a dark-gray, unidentifiable matrix were obtained.

X-ray diffraction structure determination
Measurements were carried out on a Bruker D8 Quest equipped with a Photon III C14 area detector.The programs SAINT (V8.40B) [22] and APEX4 (v2021.4.0) [23] were used for data collection and processing.A total of 18 tests and full measurements of different crystals of good optical quality were carried out.However, despite the good optical quality, all crystals displayed very strong signs of twinning, from which we deduce that intrinsic effects in Tm 2 CrB 3 O 9 lead to these results.Crystal structure determinations were therefore carried out on a non-merohedrally twinned crystal (BASF 0.3083).Three twin domains (second cell rotated 180.0°and third cell rotated 74.3°from the first domain) were identified in the data by the program CELL_NOW [24].A multi-scan absorption correction was carried out using the program TWINABS (2012/1) [25], and the structure was solved via SHELXT (2018/2) [26,27] algorithms.12150 reflections were found to only involve domain 1, 12441 reflections involved only domain 2, 21363 reflections were found to involve two domains, and 3 reflections were found to involve three domains.Of these, 2510, 2485, 3900, and 3 reflections, respectively, were considered to be unique.Due to the low number of reflections of the third domain, only domain one and two were considered in calculations of the structure refinements.During structure refinement, three reflections were omitted due to their high deviation.Structure refinements were calculated on the basis of a HKLF5 data file using JANA2020 [28].All atoms were refined anisotropically and the structural and refinement data are shown in Tables 1-4 below.Symmetry checks of the final structure solution using the ADDSYM-routine of PLATON [29][30][31][32][33] did not suggest any further change of the symmetry.
X-ray powder diffraction patterns (Figure 1) were collected on an STOE Stadi P Powder Diffractometer (STOE & CIE GmbH, Darmstadt, Germany) [34], equipped with a Ge(111) monochromator and an MYTHEN 1 K detector system (Dectris Ltd., Baden Daettwil, Switzerland) [35].The measurements were carried out overnight in transmission mode using a flat sample holder [36].While the reflection positions show good congruency with the calculated positions, the relative intensities differ due to intrinsic effects of preferred orientations, which were found on several independent samples.

Infrared spectroscopy
Infrared spectra of the new compound were collected on a Bruker Alpha Platinum attenuated total reflection (ATR) spectrometer on the bulk material.The spectra were measured in the range of 4000 to 400 cm −1 , and the data was processed and corrected for atmospheric influences employing the OPUS 7.2 [37] software.

Crystal structure description
The compound Tm 2 CrB 3 O 9 crystallizes with two formula units per cell in the triclinic crystal system in the space group P1̅ (no.[16-18, 39, 40].Furthermore, it is apparent from Table 4 that all bonding angles are in the expected range for a slightly distorted octahedron.Each corner of the [CrO 6 ] octahedron is linked to one [B 3 O 9 ] 9− anion and thus connects the isolated borate units within the crystal structure (Figure 3).Thereby, the O2 and O6 atoms of one [CrO 6 ] octahedron belong to the same [B 3 O 9 ] 9− anion.
Two crystallographically different thulium atoms are found in the voids within the crystal structure.For a graphical depiction of the respective coordination environments, see Figure 4.The Tm1-site is eight-fold coordinated by oxygen with interatomic distances between 2.171(3) Å and 2.607(3) Å. Around the Tm2-position, eight oxygen atoms are found at distances from 2.258(3) Å to 2.365(3) Å and two additional oxygen atoms are located at distances of 2.841(3) Å and 2.898(3) Å, leading to a ten-fold coordinated site.These values are reasonable, considering known thulium borates,   where similar interatomic Tm-O-distances ranging from 2.170 to 2.977 Å are reported [41][42][43][44].
Within the crystal structure, the isolated [B 3 O 9 ] 9− anions are arranged in layers, in which the apices of two adjacent rings point in opposite directions.While the Tm1 atoms are located within these layers, the Tm2-site and the chromium atoms are found between two layers.In its unit cell, the resulting crystal structure is shown in Figure 5.
Calculations of the bond valence sums according to the bond-length/bond-strength (BLBS) [45] and charge distribution (CHARDI) [46] concept show good congruency with the expected values of +3 for thulium, chromium, and boron, and −2 for oxygen.The results of these calculations are provided in Table 5.

Atoms Distance Atoms Distance
Tm-O   LnCr 3 (BO 3 ) 4 (Ln = Eu, Gd) [17,18], which were obtained under ambient pressure conditions.In these ambient pressure phases, the boron atoms are exclusively three-fold coordinated, while the crystal structure of the new high-pressure phase is exclusively built up by [BO 4 ] tetrahedra.This corresponds to the pressure-coordination rule [47], which can also be observed for the lanthanoid cations.In LnCr(BO 3 ) 2 (Ln = Ho-Lu) [16] and LnCr 3 (BO 3 ) 4 (Ln = Eu, Gd) [17,18], the lanthanoid atoms are six-fold coordinated in an octahedral or distorted trigonal prismatic geometry, while in Tm 2 CrB 3 O 9 , the coordination numbers of the thulium atoms are considerably higher with C.N. 8 for the Tm1-and C.N. 10 for the Tm2-site.As expected, the increase of the coordination number is accompanied by an increase in interatomic distances [16][17][18]47].The chromium atoms are octahedrally coordinated in all compounds discussed [16][17][18].During the investigation of the thermal behavior and stability of Tm 2 CrB 3 O 9 by powder X-ray diffraction, we have observed the formation of π-TmBO 3 [48,49], after heating to temperatures around 900 °C and subsequent cooling.The formation of π-TmBO 3 [48,49] was accompanied by a decrease of the reflection intensities assignable to the title compound, thus indicating its decomposition.However, at these temperatures, this reaction was very slow.We have therefore placed the quartz capillary in a high-frequency furnace and heated the sample up to 1200 °C.This resulted in a powder diffraction pattern which showed reflections of π-TmBO 3 [48,49], and weaker ones that could be assigned to cristobalite (ICDD collection code: 00-039-1425).Notably, no chromium species could be identified, despite the green color of the resulting product.Likely, this is due to a reaction with the quartz glass at such high temperatures.Syntheses carried out under ambient pressure did not yield Tm 2 CrB 3 O 9 .Although a detailed investigation was not yet possible, its status as a high-pressure phase seems likely, considering these observations.

Infrared spectroscopy
In Figure 6, the infrared spectrum of Tm 2 CrB 3 O 9 is shown.Notably, the absence of a broad absorption at wavenumbers around 3000 cm −1 confirms a hydroxyl-free crystal structure  Intensity / a.u.
Wavenumber / cm -1 for the title compound [50].Stretching vibrations of [BO 4 ] tetrahedra are observed in the expected wavenumber range of 800-1100 cm −1 [51], but are also precedented to contribute to signals at slightly higher wavenumbers by ab initio quantum chemical calculations [52].Bending modes of [BO 4 ] tetrahedra and vibrations involving the chromiumand thulium-centered polyhedra are expected in the lower wavenumber range [50][51][52][53][54]. Therefore, it can be said that the infrared spectroscopic characteristics of the sample match with our expectations from the single-crystal structure solution.

Conclusions
Tm 2 CrB 3 O 9 is the first borate that exhibits a crystal structure analogous to walstromite.It was synthesized in a modified Walker-type high-pressure device at 7.5 GPa and 850 °C and forms red, strongly twinned crystals with a structure built up of isolated [B 3 O 9 ] 9− anions.The structural model is further supported by an infrared spectroscopic investigation.

Figure 1 :
Figure 1: Comparison of the calculated powder X-ray diffraction pattern of Tm 2 CrB 3 O 9 with the measured data (λ = 70.93pm).Reflections marked with an asterisk stem from an unidentified side phase.

Figure 3 :
Figure 3: The chromium atoms are octahedrally coordinated.Each [CrO 6 ] octahedron is connected to a total of five [B 3 O 9 ] 9− anions via corner-sharing.

Figure 4 :
Figure 4: Graphical depiction of the coordination environments of the two crystallographically different thulium atoms.

Figure 5 :
Figure 5: The crystal structure of Tm 2 CrB 3 O 9 is shown in its unit cell.

Table  :
Structural and refinement data for Tm CrB  O  .

Table  :
Atom labels and corresponding Wyckoff-positions, atomic coordinates, and equivalent isotropic displacement parameters (U eq ) or isotropic displacement parameter (U iso ) for all crystallographically different atoms.U eq is defined as one third of the trace of the orthogonalized U ij tensor (standard deviations in parentheses).All atoms are located on Wyckoff-position i.

Table  :
Anisotropic displacement parameters for all crystallographically different atoms.