PRESSURE-INDUCED STRUCTURAL-CHANGES AND CHARGE-TRANSFER IN TL2BA2CUO6+Z

Tl2Ba2CuO6+z has negative pressure coefficients of T(c), dT(c)/dp, that vary appreciably with oxygen content. We refined the structure parameters for three samples of Tl2Ba2CuO6+z with T(c)s of 0, 46 and 79 K from time-of-flight neutron powder diffraction data measured at 0-0.622 GPa and room temperature. The lattice parameters of the tetragonal unit cells and the unit-cell volume decreased linearly with increasing pressure. The three samples have similar bulk moduli, K = 82.2-87.6 GPa, which are 29-43% smaller than those of other hole-doped superconductors (118-147 GPa). An axial Cu-O(2) bond, and to a lesser extent an equatorial Cu-O (1) bond, within a [CuO6] octahedron are compressed pronouncedly under high pressure in Tl2Ba2CuO6+z. We propose that this marked shrinkage of the two Cu-O bonds at high pressure is caused by the transfer of electrons from the CuO2 conduction sheets to the charge reservoirs (BaO-TlO-TlO-BaO sheets), lowering the T(c) of this superconductor that is overdoped with holes. This charge-transfer model is consistent with the results of recent Hall-coefficient measurements under high pressure.