Electric field gradient at the Eu nucleus in EuBa₂Cu₃O_7-x around the superconducting transition temperature

In Mössbauer effect studies of the superconductor EuBa₂Cu₃O_7-x an appreciable anomaly around the superconducting transition temperature in the variation of both the electric quadrupole splitting of the ¹⁵¹Eu nucleus and the asymmetry parameter of the electric field gradient has been observed. The change in the electric field gradient components and the possible structural and electronic rearrangement in the Cu-O layers in the crystal in the process of superconducting transition is discussed.