结晶学
正交晶系
八面体
单斜晶系
晶体结构
化学
相变
多态性(计算机科学)
亚稳态
负热膨胀
材料科学
热膨胀
凝聚态物理
物理
生物化学
有机化学
基因型
基因
冶金
作者
Margarita S. Avdontceva,Maria G. Krzhizhanovskaya,Sergey V. Krivovichev,Andrey A. Zolotarev,Victor N. Yakovenchuk
标识
DOI:10.1016/j.jssc.2022.123779
摘要
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.
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