Fig. 9-9 Schematic crystal structures of La(2-x) BaxCuO4 with x around 0.125: (a) HTT, (b) LTO, and (c) LTT. The upper patterns are the top-views of the crystal structures, and depict how to tilt CuO6 octahedra to make either LTO or LTT out of HTT respectively. : the oxygen ions in CuO octahedra on the basal plane of the HTT structure form a two-dimensional square lattice, from which we can derive the LTO structure by shifting each slanting axis of the square lattice perpendicular to the page either up or down alternately, from which we can derive the LTT structure by shifting each lattice point either up or down alternately.

We have investigated, by using neutron scattering, how small changes in crystal structure affect superconductivity itself, in the course of understanding the mechanism of high-temperature superconductivity. We are able to distinguish between subtle modifications of the crystal structure of a material by impinging a high-intensity flux of monochromatic thermal neutrons from a nuclear reactor on the material and by measuring accurately the angular distribution of the scattered neutrons. We applied this method to La(2-x)Bax CuO4. This material is considered as a key substance for understanding the mechanism of high-Tc superconductivity, since the substance almost loses its superconductivity in a narrow range of x, the composition ratio of Ba, around x = 0.125. The La(2-x) Bax CuO4 with x = 0.125 has three crystallographic phases at ambient pressure in the following three ranges of temperature; the high-temperature tetragonal phase (HTT) above 200 K, the low-temperature octahedral phase (LTO) from 200 K down to 80 K, and the low-temperature tetragonal phase (LTT) below 80 K. The figure schematically shows the crystal structures of these three phases. The patterns above them are the top-views of the crystal structures, respectively HTT, LTO, and LTT from left to right, and they clearly show how to tilt the CuO6 octahedra to make either LTO or LTT out of HTT. We discovered, by means of the neutron scattering experiments mentioned above, that if hydrostatic pressures up to as high as 0.6 GPa are applied to the LTT, it transforms to the LTO which has a simpler way of tilting the octahedra than the LTT. We have also confirmed that the superconducting transition temperature Tc increases rapidly with pressure in coincidence with the transformation. From these facts we concluded that the crystal structure with the complexity of the tilted CuO6 octahedra in the LTT phase markedly hinders the development of superconductivity in La(2-x) Bax CuO4 with x = 0.125.

Reference S.Katano et al., Pressure effects on the structural phase transitions and superconductivity of La(2-x)BaxCuO4(x=0.125), Phys. Rev. B 48 6569 (1993).

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