Dopant and Temperature Induced Structural Phase Transitions inLa2−xSrxCuO4

The local structure about La atoms in oriented ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ 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 ${\mathrm{CuO}}_6$ 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 ${\mathrm{CuO}}_6$ octahedra tilt locally only in the $〈〉\left(100\right)$ direction.