Anomalous thermal expansion in substituted Nd2Fe17-xSix and Nd2Fe17-xAlx compounds

The temperature dependencies of the lattice parameters were measured from room temperature to 400°C for powders of the substituted compounds Nd₂Fe_17-xAl_x and Nd₂Fe_17-xSi_x, x=0,1,2,3, using high-temperature X-ray diffraction. All compounds crystallize in the rhombohedral Th₂Zn₁₇-type structure (space group R3¯ m). Invar-type anomalies are found in the temperature dependence of a and c below the Curie temperatures, T_c, but featuring different substitution effects of Fe by Al and Si atoms, respectively. The Nd₂Fe_17-xAl_x and Nd ₂Fe_17-xSi_x compounds retain easy-plane anisotropies upon substitution up to x=3. Therefore, the observed spontaneous volume magnetostrictions are due mainly to the isotropic exchange contribution of the iron sublattice. According to the localized model of exchange interactions in R-Fe compounds, the different substitution effects can be ascribed to the different preferential occupation tendencies of Al and Si atoms on the sites of the Th₂ Zn₁₇-type structure