(11)B MAS NMR and First-Principles Study of the [OBO3] Pyramids in Borates.

Borates are built from the [Bϕ3] planar triangles and the [Bϕ4] tetrahedral groups, where ϕ denotes O or OH. However, the [Bϕ4] groups in some borates are highly distorted to include three normal B-O bonds and one anomalously long B-O bond and, therefore, are best described as the [OBO3] pyramids. Four synthetic borates of the boracite-type structures (Mg3B7O13Br, Cu3B7O13Br, Zn3B7O13Cl, and Mg3B7O13Cl) containing a range of [OBO3] pyramids were investigated by multifield (7.05, 14.1, and 21.1 T) (11)B magic-angle spinning nuclear magnetic resonance (MAS NMR), triple quantum (3Q) MAS NMR experiments, as well as density functional theory calculations. The high-resolution (11)B MAS NMR spectra supported by theoretical predictions show that the [OBO3] pyramids are characterized by isotropic chemical shifts δiso((11)B) from 1.4(1) to 4.9(1) ppm and nuclear quadrupole parameters CQ((11)B) up to 1.3(1) MHz, both significantly different from those of the [BO4] and [BO3] groups in borates. These δiso((11)B) and CQ((11)B) values indicate that the [OBO3] pyramids represent an intermediate state between the [BO4] tetrahedra and [BO3] triangles and demonstrate that the (11)B NMR parameters of four-coordinate boron oxyanions are sensitive to local structural environments. The orientation of the calculated unique electronic field gradient tensor element Vzz of the [OBO3] pyramids is aligned approximately along the direction of the anomalously long B-O bond, corresponding to B-2pz with the lowest electron density.

Polymorphism of NaVO2F2: a P21/c superstructure with pseudosymmetry of P21/m in the subcell.

The ADDSYM routine in the program PLATON [Spek (2015). Acta Cryst. C71, 9-18] has helped researchers to avoid structures of (metal-)organic compounds being reported in an unnecessarily low symmetry space group. However, determination of the correct space group may get more complicated in cases of pseudosymmetric inorganic compounds. One example is NaVO2F2, which was reported [Crosnier-Lopez et al. (1994). Eur. J. Solid State Inorg. Chem. 31, 957-965] in the acentric space group P21 based on properties but flagged by ADDSYM as (pseudo)centrosymmetric P21/m within default distance tolerances. Herein a systematic investigation reveals that NaVO2F2 exists in at least four polymorphs: P21, (I), P21/m, (II), P21/c, (III), and one or more low-temperature ones. The new centrosymmetric modification, (III), with the space group P21/c has a similar atomic packing geometry to phase (I), except for having a doubled c axis. The double-cell of phase (III) arises from atomic shifts from the glide plane c at (x, », z). With increasing temperature, the number of observed reflections decreases. The odd l reflections gradually become weaker and, correspondingly, all atoms shift towards the glide plane, resulting in a gradual second-order transformation of (III) into high-temperature phase (II) (P21/m) at below 493 K. At least one first-order enantiotropic phase transition was observed below 139 K from both the single-crystal X-ray diffraction and the differential scanning calorimetry analyses. Periodic first-principles calculations within density functional theory show that both P21/c superstructure (III) and P21 substructure (I) are more stable than P21/m structure (II), and that P21/c superstructure (III) is more stable that P21 substructure (I).