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 PbCrO₃ and PbTiO₃. PbCr_1-xTi_xO₃ was found to adopt a cubic structure the same as that of PbCrO₃ for x ≤ 0.6 and a PbTiO₃-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 Pb²⁺/Pb⁴⁺ in a complex local structure called a charge glass. The volume shrinkage of −2.5% observed in PbCrO₃ 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 PbTiO₃. We succeeded in significantly lowering the working temperature of PbTiO₃ as an NTE material by Cr substitution while retaining a large volume shrinkage of 0.6%.