Low temperature microstructure and phase transitions in La 2−xSr xCuO 4 and La 2−xBa xCuO 4

Abstract

We have studied microstructural phenomena and structural phase transitions of La 2− x Sr x CuO 4 (with 0≤ x≤0.15) and La 2− x Ba x CuO 4 (with x=0.125) at low temperatures by convergent beam electron diffraction and electron microscopy. Microtwin density in the orthorhombic phase is found to increase with increasing dopant concentration. At the lowest temperature (∼10 K) in the present experiment, La 2− x Sr x CuO 4 remains orthorhombic and La 2− x Ba x CuO 4 exhibits a fine mixture of orthorhombic and tetragonal phases with a typical domain size ≤300 Å. The new phase transition does not alter the twin domain configurations that already exists in the orthorhombic phase. A new strain-relieving mechanism for the new phase transition with small variations of lattice parameters among the orthorhombic and tetragonal domains is observed. Convergent beam electron diffraction studies indicate that the samples are relatively homogeneous and free of distinct second phases, and variation of short range (∼ lattice parameter) lattice ordering, however, is commonly observed on a scale of ∼ 100 Å.