Pressure/temperature/substitution-induced melting of A -site charge disproportionation in Bi 1 − x La x NiO 3 ( 0 ⩽ x ⩽ 0.5 )

Metal-insulator transitions strongly coupled with lattice were found in Bi{sub 1-x}La{sub x}NiO{sub 3}. Synchrotron x-ray powder diffraction revealed that pressure (P{approx}3 GPa, T=300 K), temperature (T{approx}340 K, x=0.05), and La substitution (x{approx}0.075, T=300 K) caused the similar structural change from a triclinic (insulating) to an orthorhombic (metallic) symmetry, suggesting melting of the A-site charge disproportionation. Comparing crystal structure and physical properties with the other ANiO{sub 3} series, an electronic state of the metallic phase can be described as [A{sup 3+}L lowbar {sup {delta}}, Ni{sup 2+}L lowbar {sup 1-{delta}}], where a ligand-hole L lowbar contributes to a conductivity. We depicted a schematic P-T phase diagram of BiNiO{sub 3} including a critical point (3 GPa, 300 K) and an inhomogeneous region, which implies universality of ligand-hole dynamics in ANiO{sub 3} (A=Bi, Pr, Nd... )