Unusually Small Thermal Expansion of Ordered Perovskite Oxide CaCu₃Ru₄O

We measured the coefficient of thermal expansion (CTE) of conducting composite ceramics 30 vol.% CuO-mixed CaCu₃Ru₄O12 together with CaCu₃Ru₄O12 and CuO. Although conducting ceramics tend to show higher CTE values than insulators, and its CTE value does not match with other ceramic materials, the CTE of CaCu₃Ru₄O12 (7⁻9 × 10-6/K) was as small as those of insulators such as CuO (9 × 10-6/K), alumina (8 × 10-6/K), and other insulating perovskite oxides. We propose that the thermal expansion of CaCu₃Ru₄O12 was suppressed by the Cu-O bond at the A-site due to the Jahn⁻Teller effect. This unusually small CTE of CaCu₃Ru₄O12 compared to other conducting oxides plays a vital role enabling successful coating of 30 vol.% CuO-mixed CaCu₃Ru₄O12 thick films on alumina substrates, as demonstrated in our previous study.