Synthesis and Crystal Structure of New Carbonate Ca3Na2(CO3)4 Homeotypic with Orthoborates M3Ln2(BO3)4 (M = Ca, Sr, and Ba)

Single crystals of the new double alkali carbonate Ca3Na2(CO3)4 were synthesized using a high-pressure large volume apparatus at 6 GPa by slow cooling of the stoichiometric carbonate mixture from 1400 to 1150 °C for 2.5 h. The structure was solved and refined to R = 0.044 using 7489 independent reflections: P1n1, Z = 8, a = 31.4421(8) Å, b = 8.1960(2)­Å, c = 7.4360(2) Å, and β = 89.923(2)°. The structure is characterized by the maximal number of nonparallel sets of CO3 groups among carbonates. The compound is homeotypic with the orthoborates M3Ln2(BO3)4 (where M = Ca, Sr, Ba and Ln = Er, Sm, Nd, Pr, La, Y, Gd, Eu, Dy, Ho, and Bi). No homeotypic analogs have been found among carbonates. The structure is described as a packing of two-capped trigonal prisms formed by Na+ and Ca2+ cations and centered by CO3 triangles.