Comparison of the crystal chemistry and electrical properties of La2−xAxNiO4 (A = Ca, Sr, and Ba)

Abstract With an objective to understand the influence of the size of the A cations, the crystal chemistry and electrical properties of La2−xAxNiO4 (A = Ca, Sr, and Ba) are compared. The solubility limits of the A cations in La2−xAxNiO4 are 0.6, 1.5, and 1.1, respectively, for A = Ca, Sr, and Ba. At a given value of x, the Ca and Ba systems have lower oxygen content than that of the Sr system. The lower solubility limit and oxygen content for the Ca and Ba systems are due to a preference of Ca2+ for eightfold coordination and a larger degree of tensile stress in the Ni–O bonds of La2−xBaxNiO4 at higher doping levels. For x 0.6, the a parameter increases in the Sr and Ba systems, while the c parameter decreases in the Sr system and remains almost constant in the Ba system with increasing x. The increase in a parameter and the decrease or constancy of c parameter for x > 0.6 is due to a transfer of electron density from the dz2 band to the dx2−y2 band. While the Sr system exhibits a semiconductor-to-metal transition with increasing x, both the Ca and Ba systems are semiconducting for all values of x.