抄録
BiNiO3 is a perovskite compound with a triclinic crystal structure and an unusual oxidation state of Bi3+0.5Bi5+0.5Ni2+O3 at ambient condition. Partial substitution of La3+ for Bi destabilizes the charge disproportionation of Bi into Bi3+ and Bi5+ and orthorhombic (Bi, La)3+Ni3+O3 state appears at high temperature. Because of the shrinkage of the Ni – O bond length, the high-temperature phase has smaller volume. The coexistence of both phases changing their fractions in Bi0.95La0.05NiO3 leads to a negative thermal expansion (NTE) with a dilatometric thermal expansion coefficient of − 82 × 10−6 /K, three times as large as the existing compound. This phenomenon is generally observed in Bi1−xLnx NiO3 (Ln: lanthanide ions). NTE is also expected for BiNi1−xMxO3 (M: trivalent ions) because Ni substitution with other trivalent ion stabilizes Bi3+(Ni, M)3+O3 state. The presence of 50 K hysteresis between the heating and cooling cycles of Bi1−xLnx NiO3 owing to the first order nature of the transition might be a problem for the practical application. From an analogy to relaxer ferroelectrics, suppression of the temperature hysteresis is also expected for BiNi1−xMxO3. BiNi1−xFexO3 samples showed NTE with reduced hysteresis compared with Ln substituted samples. This result strongly suggests that the phase transition is approaching to the second-order like.
