Polymorphism of NaVO₂F₂: aP2₁/csuperstructure with pseudosymmetry ofP2₁/min 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).

In situ Raman investigation of the phase transition of NaVO2F2 under variable temperature conditions

“…At a low temperature of 93 K, the characteristic peak 2 merged from double peaks into a single peak and accompanied by a Raman shi decrease from 553 cm À1 to 445 cm À1 , which indicates that the mode of the V-O-V symmetric stretching vibration of the bridge bond has changed, and led the symmetry of the crystal structure to be transformed concurrently. This is also consistent with the DSC test results reported by Yu, 23 where a rst-order phase transition occurred when the temperature was lower than 139 K.…”

“…25 Moreover, our approach for the synthesis of NaVO 2 F 2 single crystals also by a hydrothermal method has been reported. 23 Briey, the description of materials and the synthetic procedure is shown in Fig. 1.…”

“…Up to now, some researchers assessed the phase structure of NaVO 2 F 2 from low temperature to high temperature by DSC, DTA and XRD experimental methods, [20][21][22][23][24] proving that this compound has the P2 1 /c phase with a symmetric center and the P2 1 phase with an asymmetric center simultaneously. Yu 23 proved by XRD that NaVO 2 F 2 has four crystal phases, namely P2 1 (I), P2 1 /m(II), P2 1 /c(III) and another low temperature phase. The rst principles calculations demonstrate that the substructures of P2 1 (I) and P2 1 /c(III) were more stable than the substructure of P2 1 /m(II).…”

“…DSC tests show that the compound will undergo more than one rst-order phase transition, when the temperature is lower than 139 K. However, the single crystal data under low temperatures failed to be analyzed due to the intensity error of the diffraction point caused by the occurrence of twin crystals. 23 DTA tests show that one or two transitions occur at low temperatures, but the temperature range cannot be accurately measured. 23 In addition, a few XRD consequences proclaimed that there is a reversible phase transition of NaVO 2 F 2 at high temperatures.…”

“…23 DTA tests show that one or two transitions occur at low temperatures, but the temperature range cannot be accurately measured. 23 In addition, a few XRD consequences proclaimed that there is a reversible phase transition of NaVO 2 F 2 at high temperatures. [21][22][23] However, although the examination of the crystal structure of NaVO 2 F 2 has been carried out via different methods, these are limited only to theoretical speculation or ex situ experiments, and the in situ measurements of the crystal structure and phase transformation for NaVO 2 F 2 are still lacking.…”