Nefedovite, Na5Ca4(PO4)4F: thermal evolution, phase transition and crystal structure refinement

Nefedovite, Na5Ca4(PO4)4F, has been investigated by in situ high-temperature powder (30–690 °C) and single crystal (27–827 °C) X-ray diffraction and Raman spectroscopy. Nefedovite is tetragonal, space group I-4, a = 11.6560(2), c = 5.4062(2) Å, V = 734.50(2) Å3 (R1 = 0.0149). Nefedovite is a 1D antiperovskite, since its crystal structure contains chains of corner-sharing anion-centered [FCa4Na2]9+ octahedra. The chains are parallel to the c direction. Nefedovite is stable up to 727 °C and undergoes a displacive phase transition in the temperature range 277–327 °C. With increasing temperature, the PO4 tetrahedra in the crystal structure of nefedovite gradually rotate around the imaginary fourfold inversion axes aligning the O2…O3 edge parallel to [110], which ultimately leads to the appearance of the mirror plane perpendicular to the c direction and the change of space group from I-4 (82) to I4/m (87). The crystal structure of nefedovite expands strongly anisotropically with the direction of the maximum thermal expansion oriented perpendicular to the chains of anion-centered octahedra. The information-based structural complexity analysis demonstrates that both low- and high-temperature modifications of nefedovite are structurally simple with the IG,total value less than 100 bits per unit cell. The structural complexity decreases along the phase transition, which is typical for displacive phase transitions.