Two Types of Negative Thermal Expansion Observed in PbCr_1–xTi_xO₃

Two negative thermal expansions (NTEs) with different mechanisms were observed in solid solutions of perovskite-type oxides PbCrO 3 and PbTiO 3 . PbCr 1– x Ti x O 3 was found to adopt a cubic structure the same as that of PbCrO 3 for x ≤ 0.6 and a PbTiO 3 -type tetragonal structure for x ≥ 0.7. The NTE observed at x ≤ 0.6 was accompanied by a cubic-to-cubic phase transition originating from the rearrangement of Pb 2+ /Pb 4+ in a complex local structure called a charge glass. The volume shrinkage of −2.5% observed in PbCrO 3 is sufficiently large despite the absence of intermetallic charge transfer, which is the origin of pressure-induced cubic-to-cubic phase transition and 9.8% volume collapse. The NTE in the tetragonal phase was caused by the ferroelectric-to-paraelectric phase transition, the same as in PbTiO 3 . We succeeded in significantly lowering the working temperature of PbTiO 3 as an NTE material by Cr substitution while retaining a large volume shrinkage of 0.6%.