Hole-doping-induced melting of spin-state ordering in PrBaCo2O5.5+x

The layered perovskite cobaltite RBaCo2O5.5 (R: rare-earth elements) exhibits an abrupt temperature-induced metal-insulator transition (MIT) that has been attributed to spin-state ordering (SSO) of Co3+ ions. Here we investigate the hole-doping member of PrBaCo2O5.5+x (0≤x≤0.24) with multiple techniques. The analysis of crystal and magnetic structures by electron and neutron diffraction confirm the SSO in the insulating phase of undoped PrBaCo2O5.5, which is melted by increasing the temperature across the MIT. In addition, we discover that hole doping to PrBaCo2O5.5 also melts the SSO in conjunction with an insulator-metal transition. The experimental results from electron and neutron diffraction and soft-x-ray absorption spectroscopy (XAS) all lead to the conclusion that hole-doping-induced MIT occurs in the same manner as the temperature-induced MIT. Therefore, we propose a unified mechanism that dominates the temperature- and hole-doping-induced MIT in the PrBaCo2O5.5+x system. Specifically, this mechanism involves symmetry breaking coupled with a SSO in the paramagnetic phase.