Transition path to a dense efficient-packed post-delafossite phase. Crystal structure and evolution of the chemical bonding

Abstract

[EN] A(I)B(III)O(2) delafossite-type oxides are important technological compounds characterized by the linear coordination of the monovalent A metal by oxygen atoms. Based on results of in situ synchrotron X-ray diffraction measurements and ab initio calculations, we herein report on the high-pressure behavior of AgGaO2, to the best of our knowledge the first compound showing step-wise transitions of Ag coordination from linear (2) to octahedral (6), through a leaning delafossite structure. These transformations take place at similar to 10.5 and similar to 16.5 GPa, respectively. Our structural analysis evidences that the initial rhombohedral delafossite structure first becomes dynamically unstable, and distorts continuously via a gliding motion of the [GaO2] octahedral layers within the ab plane, and subsequently transform into another rhombohedral phase 8% denser. This structural sequence is associated with a simultaneous decrease in the bond order of the Ag-O bonds and an increase in the ionicity of the crystal. These results may help to unveil the high-pressure phases of several delafossite compounds which were reported to undergo phase transitions under compression that could not be identified.