High-pressure structural stability in pseudo spin ladder compound CaCu₂O₃: a computational and experimental study

High-pressure synchrotron x-ray diffraction experiments combined with first-principles density functional theory (DFT) calculations on CaCu₂O₃compound were employed to investigate the structure up to ∼12 GPa. Rietveld refinement on ambient data indicates a weakly off stoichiometric composition Ca_1.068Cu_1.932O_2.676, which indicates the presence of antisite Ca-Cu defects. There was no indication of structural phase transition, but a strong anisotropic elastic compression was observed, notably more significant along thec-axis than thea-andb-axes. While the experimental bulk modulus ∼54 GPa obtained matches well with the DFT calculated value (∼56 GPa), an anomalous stiffening of the bulk structure beyond ∼3-5 GPa was observed yielding the bulk modulus of ∼103 GPa. A simultaneous microstructural rearrangement was also observed experimentally in the form of bond lengths and bond angles exhibiting a change in trend in their pressure evolution at this range. The observed changes in the local coordination environments clearly suggesting a second order isosymmetric phase transition.