Pressure effects on the structural phase transitions in La 2−rBa xCuO 4

Abstract

Triple axis neutron scattering experiments with improved momentum transfer resolution were used to study the sequence of the structural phase transitions in La 2−xBa xCuO 4 under high pressure. For the composition x=0.12 and ordinary pressure we confirm the previously reported sequence of a high temperature tetragonal (HTT) phase (14/mmm) which, around 220 K, continuously changes into the low temperature orthorhombic (LTO) phase (Abma) and finally transforms almost completely into a low temperature tetragonal (LTT) phase (P4 3/ncm). Scanning the hydrostatic pressure up to 5 Kbars at given temperature chosen in the intermediate LTO phase we show that the orthorhombicity (a−b) (a+b) is continuously reduced at the rate of 1.310 −4/ Kbar compatible with a decrease of the HTT-LTO transition temperature at the rate of −6.5 K/ kbar. At low temperature we were able to study the pressure dependence of particular Bragg reflections and find that the LTO-LTT phase transition temperature is also depressed at the rate of −3 K/ kbar. Moreover we have determined the compressibility in the LTT phase. At 10 K we have found a compressibility of the order of 1.36 10 −3/ kbar while in the LTO we have measured a significantly smaller compressibility 4 10 −4/ kbar at about 80K. The evolution of the transition lines will be discussed within the framework of the electron-lattice coupling.