The effects of iron doping on the giant negative thermal expansion (NTE) of Ca2RuO4−y ceramics are investigated. Fe-doped ruthenate ceramics exhibit a large NTE even though the crystallographic unit-cell volume shows no NTE. The anisotropic thermal strain of crystal grains consumes open spaces in the sintered body and causes giant bulk volume contraction on heating. This giant NTE is reproducible against repeated thermal cycling, indicating a tough microstructure. The thermal expansion of epoxy resin is fully suppressed below 400 K by the 56 vol % loading of the Fe-doped ruthenate ceramic powder, demonstrating tough microstructures and the capability of thermal expansion compensation of the ruthenate sintered body.

Colossal negative thermal expansion (NTE) recently discovered in layered ruthenate Ca2RuO4 is achieved by microstructural effects because of the crystal grains showing highly anisotropic thermal distortion and the pores in the sintered body. To improve the NTE properties, we have investigated element-substitution effects. We discovered that Sn doping expanded the operating-temperature window toward higher temperatures up to 700 K and improved the temperature linearity of NTE without decreasing the total volume change related to NTE. We discuss changes in NTE properties caused by Sn doping in terms of the electronic states and the structural transformation.