Ab initio prediction of the low-temperature phase diagrams in the systems MBr–MCl (M = Li, Na, K)

We have studied the low-temperature phase diagrams of the systems MBr–MCl (M = Li, Na or K), via global exploration of the enthalpy landscapes for many different compositions, leading to candidates for solid solution-like and ordered crystalline phases. For all these candidates the free enthalpies are computed on ab initio level, and the low-temperature phase diagrams of the three chemical systems are derived. We find not only the expected stable solid solution in the rocksalt structure type, but also a metastable solid solution based on the CsCl-type for the KBr–KCl system, which becomes thermodynamically stable at elevated pressures. Furthermore, we calculated the P–T–x phase diagram up to the transition pressure for the rocksalt modification, and for 5 GPa for the CsCl-modification, for the KBr–KCl system.

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