Research output: Contribution to journal › Article › peer-review
Polymorphism of Na2CaPO4F: crystal structures, thermal stability and structural complexity. / Avdontceva, Margarita S.; Кржижановская, Мария Георгиевна; Krivovichev, Sergey V.; Zolotarev, Andrey A.; Yakovenchuk, Victor N.
In: Journal of Solid State Chemistry, Vol. 319, 123779, 01.03.2023.Research output: Contribution to journal › Article › peer-review
}
TY - JOUR
T1 - Polymorphism of Na2CaPO4F: crystal structures, thermal stability and structural complexity
AU - Avdontceva, Margarita S.
AU - Кржижановская, Мария Георгиевна
AU - Krivovichev, Sergey V.
AU - Zolotarev, Andrey A.
AU - Yakovenchuk, Victor N.
PY - 2023/3/1
Y1 - 2023/3/1
N2 - 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.
AB - 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.
KW - Antiperovskites
KW - Complexity
KW - Nacaphite
KW - Phase transition
KW - Structural complexity
KW - Thermal behavior
KW - Thermal expansion
KW - X-ray diffraction studies
UR - https://www.mendeley.com/catalogue/d459b6a6-338f-32e4-acb0-6913adc0aae2/
U2 - 10.1016/j.jssc.2022.123779
DO - 10.1016/j.jssc.2022.123779
M3 - Article
VL - 319
JO - Journal of Solid State Chemistry
JF - Journal of Solid State Chemistry
SN - 0022-4596
M1 - 123779
ER -
ID: 102655856