Density Functional Theory Study of Lanthanide Monoxides under High Pressure: Pressure-Induced B1–B2 Transition

Using density-functional theory we have studied the influence of hydrostatic pressure in the crystal structure of lanthanide monoxides considering the monoxides formed by the fifteen ele-ments of the lanthanide series, from La to Lu. Calculations have been performed using two methods for the ambient-pressure B1 (NaCl-type) structure, the general-gradient approximation (GGA) and the local-density approximation (LDA). By a systematic comparison with existent experimental data, we have found that the first method agrees better with experiments. In addi-tion, considering other cubic structures previously reported for lanthanide monoxides, as B2 (CsCl-type) and B3 (ZnS-type), we have explored the possibility of the occurrence of pres-sure-induced phase transitions. Based on the better accuracy of GGA to describe the B1 phase at ambient conditions, we have exclusively used GGA for the high-pressure study. We have found for the fifteen studied compounds that at ambient pressure the B1 structure is the one with the lowest enthalpy, being therefore the thermodynamically most stable structure. We have also determined that at elevated pressures all the studied compounds undergo a structural phase transition to the B2 phase. We have finally established the relationship between pressure and the volume of the unit cell, along with the associated isothermal equation of state, determining the bulk modulus.