Intermediate valence of europium in Eu_xLa_1-xBe₁₃

Mössbauer studies of the 21.6 keV transition of¹⁵¹Eu in cubic EuBe₁₃ and Eu_1/2La_1/2Be₁₃ in the temperature range 100 K to 600 K have been performed. The position of the absorption line in EuBe₁₃ moves from 0.60 mm/s at 100 K to 0.04 mm/s at 575 K. In Eu_1/2La_1/2B₁₃ the line moves from 0.50 mm/s at 100 K to -0.03 mm/s at 500 K. We conclude that Eu in Eu_xLa_1-xB₁₃ is in an intermediate valence state and we analyze the temperature dependence of the isomer shift in terms of an interconfigurational fluctuation model. The model contains temperature independent parameters E_exc, the interconfiguration excitation energy, T_f, the valence fluctuation temperature, and S₃, the isomer shift of pure Eu³⁺ in EuBe₁₃. Assuming S₃-S₂= 13 mm/s the analysis yields S₃=0.70 mm/s, T_f=400 K and E_exc=3000 K for EuBe₁₃, whereas T_f=460 K and E_exc=2750 K for Eu_1/2La_1/2B₁₃.