Anomalous thermal expansion of CeAl3 due to valence mixing

The lattice parameters of the compounds CeAl₃, LaAl₃, PrAl₃, and GdAl₃ have been measured with x-rays in the range $\text{5}\text{K}{}^{\mathrm{\le }}\text{T}{}^{\mathrm{\le }}\text{300}\text{K}$. In spite of their similarity, a significant difference between CeAl₃ and the other compounds is found in the relative volume change $\text{Δ}\text{V}\text{V}\text{(}\text{T}\text{)}$. This is attributed to an increasing admixture of Ce⁴⁺ with decreasing temperature. Furthermore, cristalline-field effects are observed in the case of PrAl₃.