title: Polymorphism of NaVO2F2: a P2₁/c superstructure with pseudosymmetry of P2₁/m in the subcell.
abstract: 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, ¼, 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).
authors.affiliation: Fujian Provincial Key Laboratory of Fire Retardant Materials, Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen, Fujian Province 361005, People's Republic of China.
authors.forename: Zi-Qun
authors.initials: ZQ
authors.lastname: Yu
authors.affiliation: Fujian Provincial Key Laboratory of Fire Retardant Materials, Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen, Fujian Province 361005, People's Republic of China.
authors.forename: Jing-Quan
authors.initials: JQ
authors.lastname: Wang
authors.affiliation: Fujian Provincial Key Laboratory of Fire Retardant Materials, Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen, Fujian Province 361005, People's Republic of China.
authors.forename: Ya-Xi
authors.initials: YX
authors.lastname: Huang
authors.affiliation: Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK, S7N 5E2, Canada.
authors.forename: Sanda M
authors.initials: SM
authors.lastname: Botis
authors.affiliation: Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK, S7N 5E2, Canada.
authors.forename: Yuanming
authors.initials: Y
authors.lastname: Pan
authors.affiliation: Fujian Provincial Key Laboratory of Fire Retardant Materials, Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen, Fujian Province 361005, People's Republic of China.
authors.forename: Jin-Xiao
authors.initials: JX
authors.lastname: Mi
keywords: ADDSYM; NaVO2F2; PLATON; crystal structure; density functional theory (DFT) calculations; polymorphism; pseudosymmetry; space-group determination