Fig. 1. (a) XPS analysis for the Ru 4d orbitals of the samples prepared with O₂/Ar ratios of 0.5%, 1.0%, 1.5%, and 2.0% and in ambient atmosphere conditions, red: Ru⁺⁵, yellow: Ru⁺⁴, green: Ru⁺³. (b)–(d) Rietveld analysis of the SXRD patterns for (b) Ca₂RO_3.87, (c) Ca₂RO_3.73 and (d) Ca₂RO_4.09.

Fig. 2. Thermal expansion property: (a) relative length change of ceramic cylinders measured by dilatometry; (b)–(e) changes of lattice constants, lattice volume, and (f) the Ca–Ru distance for (b) Ca₂RuO_4.09, (c) Ca₂RuO_3.87, and (d) Ca₂RuO_3.73 with temperature. The insert shows the octahedron rotation about the c-axis and tilt about an axis in the Ru–O(1) plane.

Fig. 3. (a)–(c) Temperature dependent changes of (a) orthorhombic strain, (b)–(d) Ru–O(1) bond lengths in (010) and (100) directions, and Ru–O(2) bond length of Ca₂RuO_4.09, Ca₂RuO_3.87, and Ca₂RuO_3.73, (f) RuO₆ octahedra.

Fig. 4. (a) Temperature dependence of (a) resistivity and (b) the field-cooled and zero-field-cooled susceptibility of Ca₂RuO_3.87, Ca₂RuO_3.73, and Ca₂RuO_4.09. Insertion: the magnetization curves at T = 10 K.

Table 1. Coefficients of axial and lattice volume expansion.