In situ structural determination of 3d and 5d perovskite oxides under high pressure by synchrotron x-ray diffraction

In contrast to the Mott transition found in RNiO3 (R= rare earths), the metal-insulator transition temperature in the perovskite NaOsO3 is not sensitive to pressure. The peculiarity may be correlated to how the crystal structure of NaOsO3 responds to high pressure, which has been rarely studied so far. The pressure-induced bond-length shrinking can increase the orbital overlap integral and therefore the electron bandwidth. However, in the orthorhombic perovskite structure, the pressure-induced bending in the bond angle Os-O-Os may compensate for the bandwidth broadening due to the bond-length shrinking in some circumstances. A recent structural study on polycrystalline NaOsO3 indicated that orthorhombic distortion is enlarged under high pressure. But, how the local structure changes under pressure remains unknown. Moreover, a highly unusual phase transition from the orthorhombic phase (Pbnm) to a polar phase (Pbn21) occurs at around 18 GPa [Sereika et al., npj Quantum Mater. 5, 66 (2020)]. Motivated by these concerns, we have done a more comprehensive structural study on NaOsO3 using single-crystal diffraction with synchrotron radiation at high pressures up to 41 GPa. Diffraction patterns over the entire pressure range can be refined well with the Pbnm structural model. Moreover, the refinement results reveal in detail how the local structures change under pressure corresponding to the enhanced orthorhombic distortion from the lattice parameters. We have carried out a systematic study for understanding the pressure effect on the orthorhombic perovskites in the context of the influences of the charge distributions in the ABO3 formula, i.e., A3+B3+O3, A2+B4+O3, and A1+B5+O3 and the B-site cations from the 3d to the 4d and 5d row of elements. To fulfill this purpose, we have revisited two families of 3d perovskites: RCrO3 and RFeO3.