High-resolution x-ray scattering studies of structural phase transitions in underdoped La2−xBaxCuO4

We have studied structural phase transitions in high quality underdoped La2−xBaxCuO4 single crystals using high-resolution x-ray scattering techniques. Critical properties associated with the continuous high temperature tetragonal (HTT) (I4∕mmm) to middle temperature orthorhombic (MTO) (Bmab) phase transition were investigated in single crystal samples with x=0.125, 0.095, and 0.08, and we find that all behavior is consistent with three-dimensional XY criticality, as expected from theory. Power law behavior in the orthorhombic strain, 2(a−b)∕(a+b), is observed over a remarkably wide temperature range, spanning most of the MTO regime in the phase diagram. Low temperature measurements investigating the low temperature tetragonal (LTT) (P42∕ncm) phase, below the strongly discontinuous MTO→LTT phase transition, in x=0.125 and x=0.095 samples show that the LTT phase is characterized by relatively broad Bragg scattering, compared with that observed at related wave vectors in the HTT phase. This shows that the LTT phase is either an admixture of tetragonal and orthorhombic phases or that it is orthorhombic with very small orthorhombic strain, consistent with the “less orthorhombic” low temperature structure previously reported in mixed La2−xSrx−yBayCuO4 single crystals. We compare the complex temperature-composition phase diagram for the location of structural and superconducting phase transitions in underdoped La2−xBaxCuO4 and find good agreement with results obtained on polycrystalline samples.