Intermetallic charge-transfer transition in Bi sub(1-x) La sub(x) NiO sub(3) as the origin of the colossal negative thermal expansion

The nature of triclinic to orthorhombic phase transition, at which colossal negative thermal expansion is observed, and the magnetic ordering of Bi sub(1-x) La sub(x) NiO sub(3) have been investigated with neutron powder diffraction (NPD) and x-ray absorption spectroscopy techniques. The presence of a charge-transfer transition from (Bi/La) super(3+) sub(0.5) Bi super(5+) sub(0.5) Ni super(2+) O sub(3) to (Bi/La) super(3+) Ni super(3+) O sub(3), accompanied by the simultaneous structural distortion, was confirmed. The NPD data also revealed that magnetic ordering is present only in the insulating triclinic phase. The metallic orthorhombic phase was found to be nonmagnetic down to 10 K.