High-pressure synthesis and crystal structure of non-centrosymmetric K2Ca3(CO3)4

Abstract

A new non-centrosymmetric K₂Ca₃(CO₃)₄ crystal (P2₁2₁2₁, a = 7.3905(2) Å, b = 8.8156(3) Å, c = 16.4817(5) Å) was synthesized in a multi-anvil press at a pressure of 3 GPa and temperature of 975 °C. The compound is homeotypic to A₃B₂(BO₃)₄ (A = Ca, Sr, Ba; B = REE, Y, Bi) orthoborates with the Pnma symmetry, but is characterized by the non-centrosymmetric arrangement of (CO₃) anions within the Pnma-pseudosymmetric cation sublattice. K₂Ca₃(CO₃)₄ represents a pure carbonate member of the extended M₅(XO₃)₄ (X = B, C, B/C) structural type, which also includes A₃B₂(BO₃)₄ orthoborates as well as recently synthesized mixed orthoborate–carbonates of Ba and Sr. Crystallochemical considerations suggest that cation-substituted (REE, Cs, Sr, and Ba) derivatives of K₂Ca₃(CO₃)₄ can probably be synthesized at significantly lower pressures than 3 GPa, which is required to stabilize the compound itself.