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High-temperature phase transition of synthetic kogarkoite, Na 3 SO 4 F, has been studied by high-temperature X-ray powder and single-crystal diffraction. The temperature of the phase transition can be estimated as 112.5±12.5 °C. The low-temperature phase, α-Na 3 SO 4 F, at 293 K, is monoclinic, P 2 1 / m , a= 18.065(3), b= 6.958(1), c= 11.446(1) Å , β= 107.711(1)°, Z =12. The structure contains thirteen symmetrically independent Na sites with coordination numbers varying from 6 to 8, and six independent S sites. The high-temperature β-phase at 423 K is rhombohedral, R- 3 m , a= 6.94(1), c= 24.58(4) Å, Z =9. The crystal structure of both polymorphs of Na 3 SO 4 F can be described as a 9 R antiperovskite polytype based upon triplets of face-sharing [FNa 6 ] octahedra linked into a three-dimensional framework by sharing corners. In the α-modification, the SO 4 tetrahedra are completely ordered and located in the framework cavities. In the β-modification, there are only two symmetrically independent Na atoms in the structure. The main difference between the structures of the α- and β-phases is the degree of ordering of the SO 4 tetrahedra: in the α-modification, they are completely ordered, whereas, in the β-modification, the complete disorder is observed, which is manifested in a number of low-occupied O sites around fully occupied S sites. The phase transition is therefore has an order–disorder character and is associated with the decrease of structural complexity measured as an information content per unit cell [577.528 bits for the low- (α) and 154.830 bits for the high- (β) temperature modifications]. [ABSTRACT FROM AUTHOR]