Composition modulation, strain minimization and oriented growth of phases in equimolar CaCoFeMgNi multicomponent oxide

Stability, microstructure and oriented intergrowth of phases has been studied in quinary equimolar (CaCoFeMgNi)-oxide and its ternary and quaternary derivatives. Green compacted precursor oxides mixed in equimolar proportions were sintered at 1523 K for different lengths of time followed by water quenching. In the ternary (CoMgNi)-oxide, and quaternary (CaCoMgNi)-oxide; a solid solution (SS) phase with rock salt structure (a∼ 4.21 Å) and two SS phases one same as (CoMgNi)-oxide and another based on CaO with rock salt structure (a∼4.78 Å) are observed respectively. Lattice strain introduced by large size of Ca ion leads to the exsolution of Ca ions and formation of two SS phases. In the quinary oxide, two chemically segregated regions; one rich in Co, Mg and Ni ions and another in Ca and Fe ions are observed. The SS phase (a∼4.21 Å), in the Co, Mg, Ni ion rich regions, forms an intergrown helical structure with mutually rotated cubic domains after long term sintering at 1523 K in order to reduce lattice and interface strain. In the Ca and Fe rich regions, strongly oriented growth of a rock salt SS phase (a∼4.78 Å), a cubic spinel phase (a∼ 8.41 Å) and a 2/3rd octahedral void filled ordered hexagonal phase (a ∼2.94 Å, c ∼5.28 Å) with coherent and semicoherent interfaces are observed. Compound deformation twins and anti-phase boundaries in the hexagonal phase reduce the lattice strain further. Minimization of free energy through a trade-off between configurational entropy and strain energy leads to the stabilization of phases.