Low temperature X-ray diffraction study of La 2−xM xCuO 4, M [dbnd], Sr

Abstract

The low temperature phase diagrams for La 2−xM xCuO 4, M Ba and Sr, were redetermined using high resolution synchroton x-ray diffractionm by monitoring the low temperature orthorhombic (LTo1, standard space group Cmca indexed Bmab) 020/200 and low temperature tetragonal (LTT, P4 2/ncm) 200 peaks. For M Ba, the LTT phase exists as a minority phase at the 10–15% level in the nominally LTO1 phase well above the bulk LTT transformation temperature. For M Sr there is no bulk LTT transformation, but up to 10% LTT phase is observed at low temperatures. For unsubstituted La 2CuO 4 no LTT contribution to diffraction is observed, down to an estimated sensitivity of 3%. For x>0.12, both for M Sr and Ba, the LTO1 200/020 diffraction peak splitting at low temperatures is inadequate to allow the observation of a possible minority LTT phase. Based on low temperature transmission electron microscopy studies of La 2−xBa xCuO 4, we propose a two step transformation from the LTO1 to the LTT phase. The LTT phase may be nucleated at relatively high temperatures at the LTO1 twin boundaries. For M Ba, near T=60K, the LTT phase also develops within the LTO1 twins, and the sample essentially completely transforms to LTT.