Novel dense crystal structure of high-pressure synthetic K8Ca3(CO3)7 as described through cation-carbon nets.

Abstract

K₈Ca₃(CO₃)₇ was synthesized at 1200°C and 6 GPa. Its layered triclinic crystal structure presents a new type characteristic of high-pressure K-Ca carbonates. It is also the second example of K/Ca disorder coupled with orientational disorder of carbonate anions. Descriptions based on the most symmetric (tri/hexagonal) and the densest (krd-type) cation-carbon nets reveal its peculiarities and common features with the crystal structures of chemically related carbonates. The four-layered ABA'B packing in K₈Ca₃(CO₃)₇ is based on hexagonal cation nets, wherein carbon atoms occupy two types of positions. Most of the carbon atoms are localized close to net B, within the centers of triangular loops. This forms a bütschliite layer, which is found in other alkali-bearing carbonates and is composed of alkali cations. Its presence in the K₈Ca₃(CO₃)₇ crystal structure implies that the K/Ca disorder is incomplete. In nets A and A', carbon atoms replace the cations at some nodes, resulting in the aggregation and increased concentration of carbon atoms and their stronger mixing with cations in the densest (krd-type) nets. Thus, a new type of densest mixed net is formed in the K₈Ca₃(CO₃)₇ crystal structure. A quantitative measure of the packing density of high-pressure K-Ca carbonates is the reticular density of the symmetric atomic nets, as well as the distance between the densest nets.