Polymorphism of Na2CaPO4F: crystal structures, thermal stability and structural complexity

The compound Na2CaPO4F has at least three polymorphic modifications: monoclinic α (also known as the natural mineral nacaphite), orthorhombic β and rhombohedral γ. All three polymorphs have antiperovskite-type structures belonging to either 2H (α and β) or 15R (γ) polytypes. The β-phase was synthesized using CaF2, NaF and Na3PO4 as initial reagents at 800 ​°C. Its crystal structure is orthorhombic (Pnma, a ​= ​5.3542(1), b ​= ​7.0878(2), c ​= ​12.2560(3) Å, V ​= ​465.11(3) Å3, Z ​= ​4) and based upon the chains of fluorine-centered face-sharing octahedra running along [100]. Upon heating, the β form is stable up to 640 ​°C, when it melts and, in the temperature range of 640–800 ​°C, the γ form crystallizes. Its crystal structure (rhombohedral, R3¯m, a ​= ​7.0272(3), c ​= ​40.609(2) Å, V ​= ​1736.66(18) Å3, Z ​= ​15) consists of framework based upon pentamers of face-sharing [F(Na,Ca)6] octahedra connected to each other through common Na vertices. The strongest thermal expansion for both modifications is parallel to the modules of face-sharing anion-centered octahedra, whereas it is almost isotropic within the plane perpendicular to the modules. The information-theoretic structural complexity analysis points out to the possible metastable character of the β-polymorph. The proposed stability row of the Na2CaPO4F phases under ambient conditions corresponds to the following sequence: α ​> ​γ ​> ​β. This agrees well with the relations among their structural complexities, degrees of order, physical and information densities. The sequence of phase transitions α → β → γ proceeds via transitional metastable β-phase. The α → β transition is reversible and of the order-disorder type with the conservation of structural topology, whereas the β → γ transition is reconstructive and irreversible. The latter transition is associated with the transformation of the antiperovskite 2H polytype into the 15R polytype.