Invar properties of the interstitial carbides of Nd2Fe 17

High-temperature X-ray diffraction experiments from 50°C to 400°C have been carried out to measure the thermal variations of the lattice parameters of Nd₂Fe₁₇C_x (x=0.5, 1.0), which has the rhombohedral structure of the Th₂Zn₁₇ type, space group R3¯m. The interstitial carbides Nd₂Fe₁₇C_x, together with their parent Nd₂Fe₁₇, exhibit Invar-like thermal expansion anomalies below their Curie temperatures, T _c. Although the introduction of carbon into Nd₂Fe ₁₇ leads to lattice expansion and enhancement of T_c, the observed positive spontaneous magnetostriction of the carbides is comparable with that of Nd₂Fe₁₇. It is proposed that such a spontaneous magnetostriction can be ascribed to the occurrence of some short Fe-Fe distances and the strong distance dependence of the positive and negative interactions