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The compound Na 2 CaPO 4 F 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 2 H (α and β) or 15 R (γ) polytypes. The β-phase was synthesized using CaF 2 , NaF and Na 3 PO 4 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, R 3 ¯ 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 Na 2 CaPO 4 F 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 2 H polytype into the 15 R polytype. The β-Na 2 CaPO 4 F transforms into γ-Na 2 CaPO 4 F in the temperature range 640–720 ​°C. The transition is not direct and involves melting of the initial phase and its re-crystallization. The crystal structure of γ-Na 2 CaPO 4 F is based on pentamers of face-sharing octahedra connected with each other through common Na1 vertices (instead of chains observed in other polymorphs). [Display omitted] • The possible metastable character of the β-polymorph is discussed on the base of the information-theoretic structural complexity analysis. • The α → β transition is reversible and has the order-disorder type, the β → γ transition is reconstructive and irreversible; • Thermal expansion of all three Na 2 CaPO 4 F polymorphs is anisotropic and has the same general features. [ABSTRACT FROM AUTHOR]

Borophosphates of transition 3d metals are based on the mixed heteropolyhedral MTT - or MTTTr -type frameworks formed by M Ø 6 octahedra, two types of T Ø tetrahedra (PO 4 and BO 4 tetrahedra), and, more rarely, Tr O 3 triangles (BO 3 triangles). Family of compounds with the general formula A 1 − x + B x 2 + { M x 2 + M 1 − x 3 + [BP 2 O 8 (OH)]} (where A + ​= ​K, Rb, Cs, NH 4 , B 2+ = Ba, Pb, M 2+ ​= ​Fe, Co, Zn, and M 3+ ​= ​Al, Ga, V3+, Fe3+, Sc, In; x ​= ​0, 1) includes monoclinic (space group P 2 1 / c) and triclinic (space group P 1 ¯) borophosphates. Single crystals of Rb{V[BP 2 O 8 (OH)]} were synthesized by a hydrothermal method. The crystal structure was studied by single-crystal X-ray diffraction analysis: space group P 2 1 / c (No. 15), a ​= ​9.3699(8), b ​= ​8.3162(7), c ​= ​9.6707(8) Å, β ​= ​102.073(9); V ​= ​736.89(11) Å3. The crystal structure of Rb{V[BP 2 O 8 (OH)]} was refined to the final R ​= ​4.52%. A topological description of both structure types of the A 1 − x + B x 2 + { M x 2 + M 1 − x 3 + [BP 2 O 8 (OH)]} compounds and related vanadium-containing phases is given. Because these compounds are characterized by the microporous crystal structures, the possible migration paths for different alkaline ions have been calculated. Family of compounds with the general formula A 1 − x + B x 2 + { M x 2 + M 1 − x 3 + [BP 2 O 8 (OH)]} includes monoclinic (space group P 2 1 / c) and triclinic (space group P 1 ¯) borophosphates.A topological description of both structure types and related vanadium-containing phases is given.The possible migration paths for different alkaline ions have been calculated. [Display omitted] • A topological description of both structure types of the A 1 − x + B x 2 + { M x 2 + M 1 − x 3 + [BP 2 O 8 (OH)]} compounds and related vanadium-containing phases is given. • The possible migration paths for different alkaline ions have been calculated. [ABSTRACT FROM AUTHOR]