Fig. 4.
Pressure-induced Eu valence transition from the isomer shift measurements. (A) The time domain 151Eu Synchrotron Mössbauer spectroscopy spectra (SMS) of EuPd3S4 at 50 K under various pressures taken using a single crystal sample with helium as the pressure transmitting medium. The pressure was measured at 50 K. Pressure was first increased from 0.7 GPa to 29.7 GPa, and then decreased to 25 GPa, in order to assess the reversibility of the pressure and the valence transition. The red dotted curves are fit to the SMS spectra using CONUSS (21). (B) Simulated energy-domain Mössbauer spectra based on the fitting results from each corresponding SMS spectrum. For these simulated patterns, the isomer shift of the Eu3+ peak was fixed to zero and only changes in the isomer shift and weight of the Eu2+ peaks are apparent. In (B), the blue and red dashed lines serve as visual aids to illustrate the evolution of Eu2+ isomer shift while Eu3+ remains fixed. The calibrated (using the EuS standard) pressure dependence of the absolute isomer shifts for the Eu2+ and Eu3+ peaks are shown for both samples in (C), while (D) shows the pressure dependence of the Eu2+ area fraction. The absolute value of isomer shift is calibrated by moving a reference sample (EuS, at room temperature and ambient pressure, with known isomer shift of Eu2+ at −11.496 mm/s) on the beam line. The detailed calibration process is explained in SI Appendix, Figs. S8 and S9.