Dopant and Temperature Induced Structural Phase Transitions in La_2-xSr_xCuO₄

The local structure about La atoms in oriented La_2-xSr_xCuO₄ samples is investigated by means of polarized x-ray absorption fine structure (XAFS) measurements. Diffraction results indicate that the dopant and temperature induced structural transformations involve decreases, on the average, of the tilt angle of the CuO₆ octahedra to zero. The XAFS results show that locally the tilts do not disappear, the x induced phase transition has displacive and disorder components, and the T induced phase transition is purely of disorder character. It is also shown that the CuO₆ octahedra tilt locally only in the 100 direction.