High-temperature order–disorder phase transition in nacaphite, Na₂CaPO₄F

Abstract

The thermal behavior of nacaphite, Na₂CaPO₄F, was studied by the powder high-temperature X-ray diffraction method. A monoclinic-to-orthorhombic phase transition has been observed at 330 °C associated with the appearance of the Ca/Na disorder at one of the two crystallographically inequivalent Na sites. At room temperature, nacaphite is monoclinic, P2₁ /c, a = 13.3185(14), b = 7.0964(8), c = 10.6490(11) Å, β = 113.526(1)°, V = 922.81(17) Å³. The structure is based upon one-dimensional antiperovskite units consisting of face-sharing [FNa₄Ca₂]⁷⁺ anion-centered octahedra running parallel to the c axis. The structure is fully ordered and contains two Ca and four Na sites. The crystal structure of the high-temperature modification [refined by Rietveld method (R _B 0.025) at 400 °C from the powder X-ray diffraction data] is orthorhombic, Pnma, a = 5.4123(1), b = 7.1196(1), c = 12.3171(1) Å, V = 474.62(1) Å³. The structure has one fully occupied Na1 site and one mixed occupied Na2 site, the latter being equally occupied by Na and Ca. The Na1 and Na2 sites are coordinated by two F⁻ and four O²⁻ anions each. The phase transition has an order–disorder character and is associated with the decrease of structural complexity measured as an information content per unit cell (300.235 bits for the low- and 98.117 bits for the high-temperature modifications). Thermal expansion of both modifications has an anisotropic character with the degree of anisotropy increasing from the low- to the high-temperature phase. The direction of the strongest thermal expansion is parallel to the direction of chains of face-sharing anion-centered octahedra that can be explained by the temperature-induced expansion of the F–Na/Ca bonds.