Metal-insulator transition and crystal structure of La1-xSrxCoO3 as functions of Sr-content, temperature, and oxygen partial pressure

Electrical properties of La1-xSrxCoO3 (LSC) change with variations in Sr co ntent, temperature, and oxygen partial pressure. With a change in each vari able, LSC showed metal-insulator transition (MIT). The changes in electrica l properties of LSC mere correlated here with those of its crystal structur e. The variations of crystallographic parameters such as the length of a-ax is, the rhombohedral angle, the Co-O distance, and the Co-O-Co angle (theta ) were precisely determined by powder X-ray Rietveld analysis. Of these, th e variation of theta described most consistently the variation of conductio n state, metallic or insulating, of LSC, The Co-O-Co angle increased whenev er the conduction state changed from insulating to metallic by changing eac h variable, Sr content, temperature, and oxygen partial pressure, In additi on, MIT took place at a critical Co-O-Co angle of ca. 165 degrees in each c ase. It was concluded that the transition from insulator to metal is caused by the closing of the charge transfer band gap, induced by broadening of t he electronic bandwidths of the Co-3d and O-2p bands with an expansion of t he Co-O-Co angle. (C) 1999 Academic Press.