Comparison of the crystal chemistry and electrical properties of La(2-x)A(x)NiO(4) (A = Ca, Sr, and Ba)

With an objective to understand the influence of the size of the A cations, the crystal chemistry and electrical properties of La(2-x)A(x)NiO(4) (A = Ca, Sr, and Ba) are compared. The solubility limits of the A cations in La( 2-x)A(x)NiO(4) 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 tha n 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 coordin ation and a larger degree of tensile stress in the Ni-O bonds of La2-xBaxNi O4 at higher doping levels. For x < 0.6, the a parameter decreases in all t he three systems, while the c parameter increases in the Sr and Ba systems and decreases in the Ca system with increasing x. For x > 0.6, the a parame ter 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 d(z)(2) band to the d(x2-y2) band. While the Sr system exhibits a semiconductor-to-meta l transition with increasing x, both the Ca and Ba systems are semiconducti ng for all values of x. (C) 2000 Elsevier Science Ltd. All rights reserved.