Structure of the high-$T_c$ superconductor ${\mathrm{Ba}}_2\mathrm{Y}{\mathrm{Cu}}_3{\mathrm{O}}_{7+x}$ above (124 K) and below (71 K) $T_c$ by neutron diffraction

The crystal structure of the high-$T_c$ superconductor ${\mathrm{Ba}}_2\mathrm{Y}{\mathrm{Cu}}_3{\mathrm{O}}_{7+x}$ at 124 and 71 K was refined by Rietveld analysis of neutron-diffraction data. We found that at 124 and 71 K the structure is of the same space group Pmmm, but when the temperature is decreased and passes the critical temperature $T_c$, 0.12 of the oxygen population on the $1e$ site transfers to the $1b$ site. This remarkable change in the structure can be related to its superconductivity.