High-pressure synthesis and crystal structure of the samarium meta-oxoborate γ-Sm ( BO 2 )

γ-Sm(BO2)3 was obtained via a high-pressure/ high-temperature approach in a multi-anvil apparatus at 10 GPa and 1673 K. It crystallizes in the orthorhombic space group Pca21 (no. 29) with the lattice parameters a = 18.3088(8), b = 4.4181(2), and c = 4.2551(2) Å. The compound was analysed by means of X-ray diffraction and vibrational spectroscopy. The structure is isotypic to that of the already known meta-oxoborates γ-RE(BO2)3 (RE = La−Nd) and built up of a highly condensed borate framework containing three-, four-, six-, and ten-membered rings. Next to neodymium, samarium represents the second rare earth element that forms the α-, β-, and γ-modification of the four known rare earthmeta-oxoborate structure types.

In contrast to the α-phase, all three other modifications are built up exclusively of BO 4 tetrahedra, leading to complex borate frameworks with increasing condensation at higher pressures. This can be illustrated with the amount of threefold coordinated oxygen atoms within the structures, which concerns every sixth of the oxygen atoms in δ-RE(BO 2 ) 3 (RE = La, Ce) and every third of the oxygen atoms in γ-RE(BO 2 ) 3 (RE = La−Nd).
In this work, we present γ-Sm(BO 2 ) 3 , where samarium is now the fifth element of the lanthanoid series to exhibit this crystal structure and also only the second one to show the βas well as the γ-modification. The various syntheses conditions are compared and the results of the singlecrystal structure determination and the vibrational spectroscopic analysis are presented.  Table 1 shows all relevant details of the single-crystal structure refinement.
The crystal structure of γ-Sm(BO 2 ) 3 is built up exclusively of BO 4 tetrahedra, which are connected by common corners to form a complex borate network. Alongside [001], the BO 4 tetrahedra are arranged in ten-membered rings ( Figure 2), formed by alternating layers (blue polyhedra) and zig-zag chains of BO 4 tetrahedra (light blue polyhedra). The borate layers in the bc plane are extremely condensed, with one third of the oxygen atoms being threefold coordinated. The BO 4 tetrahedra within these layers form three-and four-membered rings ( Figure 3) with two of the three oxygen atoms in a three-membered ring being threefold coordinated and one of the oxygen atoms in a four-membered ring. Looking onto [010], sixmembered rings are developed between the borate layers  and the connecting zig-zag chains ( Figure 1, bottom). The B-O bond lengths in this structure range from 1.427(4) to 1.553(4) Å with the longest distances between the threefold coordinated oxygen atom O1 and the respective boron atoms (1.527(4)-1.553(4) Å), which was found in the isotypic compounds γ-RE(BO 2 ) 3 (RE = La−Nd) as well [14]. The average B-O distance of 1.476 Å is in good agreement with the value of 1.48(4) reported by Zobetz [17]. The O-B-O angles also lie within a rather wide range of 105.6(3) and 118.5(3)°, but the average value of 109.5°corresponds well with the expected tetrahedral angle. All the bond lengths and angles are listed in Table 2 and Table 3. The positional parameters can be seen in Table 4. The samarium cations in γ-Sm(BO 2 ) 3 are coordinated by 8 + 2 oxygen atoms and are located in the sixmembered rings along [010]. The ten-membered rings alongside [001] contain two Sm 3+ cations each. The Sm-O distances range from 2.342(6) to 2.838(3) Å, with two additional oxygen atoms at slightly longer distances of 2.970(3) and 3.025(3) Å, that only coordinate weakly to the rare earth cations, leading to the 8 + 2 coordination ( Figure 4).
This was confirmed by calculation of the bond valence sums (ΣV) [18,19] as well as the values based on the CHARDI (ΣQ) [20] concept, which show only a small contribution for these two oxygen atoms. The resulting formal ionic charges correspond well with the expected values of +3 for samarium and boron as well as −2 for oxygen as shown in Table 5.
The newly presented meta-oxoborate γ-Sm(BO 2 ) 3 is isotypic to the already known compounds γ-RE(BO 2 ) 3 (RE = La−Nd) and its lattice parameter fit well into this series, which is shown in Table 6 and Figure 5. The contraction of the structures due to the smaller radii for the lanthanide cations going from the left to the right in the periodic table (lanthanide contraction) is particularly distinct for the a parameter. The crystal structure seems to be more compressible in this direction because of the looser connectivity between the layers and the chains of BO 4 tetrahedra compared to the shrinkage within the layers in the bc plane. Samarium now represents the second lanthanide cation, the other being neodymium, to form the α-, β-, and γ-meta-oxoborate. The compound presented here was synthesized under quite extreme conditions of 10 GPa and 1673 K, while the β-compound γ-Sm(BO 2 ) 3 was first found at conditions of 7.5 GPa and 1323 K. Recent investigations in connection with the neighbouring phase β-Eu(BO 2 ) 3 have demonstrated that β-Sm(BO 2 ) 3 can already be formed at lower pressures of 4 GPa. This difference in the required pressure can also be observed for the compounds βand γ-Nd(BO 2 ) 3 . Here, the β-compound was found at 3.5 GPa and the γ-compound at 7.5 GPa.
This structure type has also been found for BiB 3 O 6 prepared under high-pressure conditions of 5.5 GPa and 1093 K, designated as δ-BiB 3 O 6 . The two compounds differ mainly in the coordination of the cation [21]. Because of the sterically active lone pair of Bi 3+ , the cation is only sevenfold coordinated by oxygen atoms and the Bi-O distances are slightly shorter (2.26-2.73 Å) than in the γ-Sm(BO 2 ) 3 form presented here (Sm-O = 2.342−3.025 Å).
Looking onto [010], the structure of γ-Sm(BO 2 ) 3 is very similar to that of the ambient pressure phase SrB 4 O 7 [22][23][24] and its isotypic compounds PbB 4 O 7 [23,25] and EuB 4 O 7 [26], as well as the high-pressure borates β-CaB 4 O 7 [27] and β-HgB 4 O 7 [28], exhibiting the same crystal structure. These compounds also feature borate layers in the bc plane that are condensed to form sixmembered rings in [010]. In contrast to the γ-meta-oxoborates, these layers are not separated by zig-zag chains of BO 4 tetrahedra but are connected directly via a mirror plane ( Figure 6). This symmetrical arrangement also leads to an increased cation coordination of CN = 15 by oxygen atoms.   Table : Atomic coordinates and equivalent isotropic displacement parameters U eq (Å  ) of γ-Sm(BO  )  . All atoms are located on Wyckoff positions a.      Compound  O, and B-O bending and stretching vibrations, as confirmed by quantum-chemical calculations for β-ZnB 4 O 7 , a high-pressure compound that also features exclusively BO 4 tetrahedra, and β-CaB 4 O 7 , which is isotypic to the aforementioned SrB 4 O 7 [30]. BO 4 bending vibrations are observed below ∼900 cm −1 .

Conclusion
The new γ-meta-oxoborate γ-Sm(BO 2 ) 3 has been synthesized under high-pressure/high-temperature conditions of 10 GPa and 1673 K. It is isotypic to the already known series of γ-RE(BO 2 ) 3 (RE = La−Nd), which were all formed at not so extreme conditions of 7.5 GPa and 1273 K. Besides neodymium, samarium now represents only the second rare earth element that is able to develop the α-, β-, and γphase of the meta-oxoborate structure family. The structure is built up of highly condensed BO 4 layers in the bc plane, which are connected via zig-zag chains of BO 4 tetrahedra. This borate network forms six-membered rings along [010] and ten-membered rings along [001], wherein one or two samarium cations are located, respectively. These Sm 3+ cations are coordinated by 8 + 2 oxygen atoms.
These findings indicate that the use of higher pressure can be a successful route to further extend the already large meta-oxoborate structure family. Ongoing experiments suggest that also for Eu 3+ , which is even smaller than Sm 3+ , the γ-meta-oxoborate crystal structure can be formed at sufficiently high pressure.

Experimental section 4.1 Synthesis
A high-pressure/high-temperature experiment in a hydraulic 1000 t press with a modified Walker-type module (both Max Voggenreiter GmbH, Germany) was performed to obtain the title compound γ-Sm(BO 2 ) 3 . A mixture of a stoichiometric ratio (1:6) of Sm 2 O 3 (Smart Elements, Wien, Austria, 99.9%) and H 3 BO 3 (Carl Roth, Karlsruhe, Germany, >99.8%) was grinded together in an agate mortar under ambient conditions and filled into a Pt capsule, that was placed in an α-BN crucible, closed with a lid of the same material (both Henze Boron Nitride Products AG, Germany) and subsequently centered in a pressure transmitting octahedron (MgO, doped with 5% Cr 2 O 3 ; Ceramic Substrates & Components Ltd, Newport, United Kingdom). The high-pressure/high-temperature experiment was carried out in an 18/11 assembly. A more detailed description of the experimental setup can be found in the literature [31][32][33].
The sample was compressed to 10 GPa in 270 min and then heated up to 1673 K in the following 15 min. This temperature was kept for the next 10 min, before it was lowered to room temperature in the following 25 min. Afterwards, the pressure was relieved in the following 810 min. The product was found to be light green crystal needles of γ-Sm(BO 2 ) 3 .

X-ray diffraction measurements
The reaction product was characterized by powder diffraction analysis on a STOE Stadi P powder  diffractometer (STOE & Cie GmbH, Darmstadt, Germany), carried out on a flat sample in transmission geometry. The measurement was performed with Ge(111)-monochromatized MoKα 1 radiation (λ = 70.93 pm) and detected with a Mythen 1 K detector (Dectris) in the 2θ range of 2-52°with a step size of 0.015°. The Rietveld refinement was performed employing the TOPAS 4.2 software [34].
For the single-crystal structure analyses, suitable crystals were separated under a polarization microscope and measured on a Bruker D8 Quest Kappa diffractometer equipped with an Incoatec microfocus X-ray tube, a multilayer optic to generate monochromatized MoKα radiation (λ = 0.7107 Å) and a Photon 100 CMOS detector. Reflections were measured in the range 3.3 ≤ θ ≤ 41.2°and the structure solution and parameter refinement were performed with Direct Methods using SHELXS/L-2017/1 [35,36] implemented in the program WINGX-2013.3 [37]. All atoms except the boron atom B2 could be refined with anisotropic displacement parameters. The atomic coordinates were standardized employing STRUCTURE TIDY [38] as implemented in PLATON [39].
Further details of the crystal structure investigation may be obtained from The Cambridge Crystallographic Data Centre CCDC/FIZ Karlsruhe deposition service via www.ccdc.cam.ac.uk/structures on quoting the deposition number CCDC 1995888 for γ-Sm(BO 2 ) 3 .

Vibrational spectroscopy
For further characterization of γ-Sm(BO 2 ) 3 , an FTIR-ATR (Attenuated Total Reflection) spectrum of a powder sample was acquired with a Bruker ALPHA Platinum-ATR spectrometer (Bruker, Billerica, USA). The spectrometer is provided with a 2 × 2 mm diamond ATR-crystal and a DTGS detector. 320 scans of the powder sample were obtained in the spectral range of 400-4000 cm −1 and afterwards corrected for atmospheric influences employing the OPUS 7.2 software [40].