Eu valency and recoil-free fraction in EuCo2Si2-x Ge x

Abstract

The valency of Eu in EuCo2Si2-xGe x withx=0, 0.5, 1.0, and 2 was determined by151Eu-Mossbauer, Eu-LIII X-ray absorption as well as magnetization studies in the temperature range from 4.1 to 650 K and at external pressures up to 66 kbar. Eu in EuCo2Si2 is found to be predominantly trivalent, while in EuCo2Ge2 it is divalent. In EuCo2Si2-xGe x (0≦x≦2), two separate narrow Mossbauer absorption lines are observed, corresponding to stable divalent and trivalent Eu ions. In EuCo2Si1.5Ge0.5, the relative spectral areas of the Eu2+ and Eu3+ Mossbauer lines are found to be strongly temperature and pressure dependent. A decrease in temperature or an application of pressure leads to small changes in the positions of the two absorption lines and to a pronounced increase in the relative spectral area of the Eu2+ component. On the other hand, X-ray absorption near-edge structure (XANES) measurements at theLIII threshold of Eu reveal no change in the relative intensity of the Eu2+ and Eu3+ subspectra as a function of temperature. From these observations it is concluded that the recoil-free fractions of the Eu2+ and Eu3+ Mossbauer resonances in EuCo2Si1.5Ge0.5 depend in a substantially different way on temperature. With the Debye approximation, Debye temperatures of θD(Eu2+)=175 K and ΘD(Eu3+)=250 K are derived. These results clearly show that relative abundances of Eu2+ and Eu3+ ions may only be derived from Mossbauer linie intensities, if appropriate corrections for differences in recoil-free fraction are applied; this applies even to mixed-valent systems, where Eu ions with different valencies occupy crystallographically equivalent sites.