Theory on the structure of twin boundaries in La{sub 2{minus}x}Ba{sub x}CuO{sub 4}

The structure of the twin boundary in the low-temperature-orthorhombic phase of La{sub 2{minus}x}Ba{sub x}CuO{sub 4} is studied using a Landau-type free energy model which reproduces the x {minus} T phase diagram. The La{sub 2 {minus}x}Ba{sub x}CuO{sub 4} compound has a body-centered tetragonal (HTT) structure at high temperatures. Upon cooling it undergoes a structural phase transition to a low-temperature orthorhombic structure (LTO structure) which is caused by the tilting of the CuO{sub 6} octahedra about the (110) and ({bar 1}10) directions. Depending on the doping level x, the LTO structure may go through another phase transition at an even lower temperature to a low-temperature tetragonal phase (LTT phase). The authors find that the existence of LTT phase greatly changes the characteristics of the twin boundary in the LTO phase. LTT phase exists at twin boundaries of the LTO phase at temperatures well above the LTT to LTO transition temperature.