B1-to-B2 phase transition of transition-metal monoxide CdO under strong compression

This paper reports the experimental research on the compressibility and phase transition of CdO up to 176 GPa at room temperature using high-resolution angular-dispersive x-ray diffraction from synchrotron source combined with the diamond anvil cell technique. The phase transition from NaCl (B1)- to CsCl (B2)-type structure for CdO was observed at pressure about 90.6 GPa, which is uniquely observed in the transition-metal oxide family for the pressure-induced direct B1-B2 transition and in good agreement with the theoretical prediction. The bulk modulus of the B1 and B2 phases of CdO at zero pressure was estimated as 147 and 169 GPa, respectively. The 3d and 4d transition-metal monoxides ZnO and CdO were both predicted to undergo a B1-to-B2-type structural transformation under high pressure based on first-principles total energy calculations; the difference in the bulk moduli between the results of this experiment and simulations, and the possibility of a similar B1-to-B2 phase transition in ZnO are discussed.