Crystal structure of Hg,Tl-1223 HTSC compound under high pressure and low temperature

Powder diffraction data for high temperature superconducting compound Hg 0.8 Tl 0.2 Ba 2 Ca 2 Cu 3 O 8+δ at pressure range 0–20 GPa and temperature interval 100–300 K are presented. Unit cell parameters and atomic coordinates are calculated. It is found, that Ba-atoms do not shift towards HgO x -planes and splitting of BaO-planes do not decrease with external pressure. This result contradicts with evident charge transfer between HgO x - and CuO 2 -planes. State of buckling of CuO 2 planes changes with pressure from slightly positive (for (zO2-zCu)value) at ambient pressure to strongly negative at P≥1.56 GPa. A growth of superconducting transition temperature as a function of the external pressure even for overdoped Hg,Tl-compounds up to P ≈ 15 GPa we explain as a result of an interplay between a sequence of pressure induced charge transfer and observed change of crystal structure.