Eu valency and recoil-free fraction in EuCo₂Si_2-xGe_x

The valency of Eu in EuCo₂Si_2-xGe_x with x=0, 0.5, 1.0, and 2 was determined by¹⁵¹Eu-Mössbauer, Eu-L_III 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 EuCo₂Si_2-xGe_x (0≦x≦2), two separate narrow Mössbauer absorption lines are observed, corresponding to stable divalent and trivalent Eu ions. In EuCo₂Si_1.5Ge_0.5, the relative spectral areas of the Eu²⁺ and Eu³⁺ Mössbauer 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 the L_III 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⁺ Mössbauer resonances in EuCo₂Si_1.5Ge_0.5 depend in a substantially different way on temperature. With the Debye approximation, Debye temperatures of θ_D(Eu²⁺)=175 K and Θ_D(Eu³⁺)=250 K are derived. These results clearly show that relative abundances of Eu²⁺ and Eu³⁺ ions may only be derived from Mössbauer 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.