Structural Studies of La2−xBaxCuO4 Between 11–293 K

Abstract

ABSTRACTNeutron powder diffraction data have been collected from samples of La1.9Ba0.1CuO4 and La1.85Ba0.15CuO4 in the temperature range 11–293 K. The first material undergoes a phase transition from the tetragonal K2NiF4-type structure (space group 14/mmm) to the orthorhombic La2CuO4-type structure (space group Bmab) at about 280 K, and then a second transition to another type of tetragonal (or near-tetragonal) structure with probable space group P42/ncm. The latter transition occurs predominantly between 50–40 K, but about 10% of the orthorhombic phase persists down to 15 K. Rietveld refinement was carried out at selected temperatures in the different structural regions. The differences among the low temperature structures can be cb acterized by tilting of the CuO6 octahedra about different rotation axes on the basis of the simple X-Y model previously proposed by Axe and coworkers. The tilt angle, which is a measure of the long-range order parameter, is about 3.5° at 15 K.Data sets were also collected from La1.85Ba0.15CuO4 at 293, 90 and 11K. At the two latter temperatures there was some peak broadening consistent with the distortions observed in previous high resolution neutron [Paul and co-workers] and x-ray [Axe and co-workers] powder studies. Rietveld analysis of the data indicated the presence of local octahedral displacements, but it was not possible to determine the nature of the correlations.