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

•Two previously unknown pressure-induced phase transitions have been found in delafossite AgGaO2.•The initial delafossite phase transforms into another rhombohedral structure 8% denser.•Ag coordination changes from linear to octahedral, through a leaning delafossite structure.•The structural sequence is associated with an increase in the ionicity of the crystal. [Display omitted] AIBIIIO2 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 ~10.5 and ~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.