Abstract: | The ADDSYM routine in the program PLATON Spek (2015). Acta Cryst. C 71 , 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). |