Polymorphism of NaVO2F2: aP21/csuperstructure with pseudosymmetry ofP21/min the subcell

Abstract

The ADDSYM routine in the programPLATON[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-Lopezet al.(1994).Eur. J. Solid State Inorg. Chem.31, 957–965] in the acentric space groupP21based on properties but flagged by ADDSYM as (pseudo)centrosymmetricP21/mwithin default distance tolerances. Herein a systematic investigation reveals that NaVO2F2exists 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 groupP21/chas a similar atomic packing geometry to phase (I), except for having a doubledcaxis. The double-cell of phase (III) arises from atomic shifts from the glide planecat (x, 1/4,z). With increasing temperature, the number of observed reflections decreases. The oddlreflections 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 bothP21/csuperstructure (III) andP21substructure (I) are more stable thanP21/mstructure (II), and thatP21/csuperstructure (III) is more stable thatP21substructure (I).