Structure of ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$ $(0.06<\delta <0.19)$ at ambient and high pressure

The results of a neutron-diffraction study of the ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\mathrm{\delta }\mathrm{}}$ structure at ambient pressure and under external pressure at different extra oxygen concentrations are presented. The results have been analyzed together with the data of previous investigations. It is shown that in the cation-stoichiometric samples the ${\mathrm{O}}_{\delta }$ oxygen is only present in the center of the mercury layer, $T_c$ is parabolically dependent on $\delta ,$ and $T_{c,\max }$ is obtained at ${\delta }_{\mathrm{opt}}=0.13\mathrm{}\pm \mathrm{0.01.}$ The influence of pressure on the structure strongly depends on the doping level. At low oxygen content $(\delta \approx 0.06),$ the compression of the structure is practically uniform. An increase of the extra oxygen content to $0.19$ (overdoped state) results in the larger compression of the apical $\mathrm{C}\mathrm{u}-\mathrm{O}\left(2\right)\mathrm{}$ and ${\mathrm{B}\mathrm{a}-\mathrm{O}}_{\mathrm{\delta }}$ distances, while the ${\mathrm{HgO}}_2$ dumb-bell as well as the distance between $\mathrm{Ba}$ and $\mathrm{O}$ belonging to the $\left({\mathrm{CuO}}_2\right)$ layer become practically pressure independent. These results are in agreement with models, in which the effect of the charge transfer from the reservoir to the $\left({\mathrm{CuO}}_2\right)$ layers does not play a dominant role in the $T_c$ increase with pressure at low and optimal $\delta$ values, while in the overdoped state the charge transfer is enhanced under pressure, thus inducing the $T_c$ decrease.