Thermodynamics of Solid‐Solution Formation in NiO‐CuO

Samples in the (Ni_xCu_1‐x)O system, 0.5 ≤x≤ 1.0, have been examined using powder X‐ray diffraction and high‐temperature Solution calorimetry. According to the X‐ray characterization samples form a one‐phase rock salt solid solution up to x= 0.30, and crystallize with a tetragonal structure at x= 0.35. The enthalpies of mixing in the rock salt region of the system show large negative deviations from ideal mixing. The tetragonal compound does not show a large increase in enthalpic stability relative to the rock salt solid solutions. The variation of stability with composition is complex and suggests that substantial stabilization occurs at Ni‐rich compositions. The thermodynamic mixing properties have been interpreted in terms of cation order between the Ni and Cu ions. The degree of cation order appears to increase up to the composition x= 0.65 at which a compound with substantial cation order crystallizes with a tetragonally distorted structure. The calorimetric results correlate well with TEM observations of a complex microstrueture.