Materials with negative thermal expansion have many practical applications. However, these materials are known in only several oxide systems, and when the negative thermal expansion occurs, the contraction is usually small and limited to a narrow temperature range beyond room temperature. For obtaining a compound with negative thermal expansion in broad temperature range, the structural and magnetic properties of Gd2Fe17 compound were investigated by means of X-ray diffraction and magnetization measurements. The Gd2Fe17 compound annealed at 1050 oC had a Th2Zn17-type structure. There existed an anisotropic strong spontaneous magnetostriction and a negative thermal expansion in Gd2Fe17 compound. The average thermal expansion coefficients was =–7.40×10–6/K in the temperature range of 294–453 K and =–1.80×10–5/K in 453–534 K, respectively. The spontaneous magnetostrictive deformation ωS decreased from 4.34×10–3 to near zero with temperature increasing from 294 to 572 K. The spontaneous linear deformation λc was much larger than λa at the same temperature below about 500 K.
The structure and magnetic properties of Gd2Fe15.5Cr1.5 compound were investigated by means of X-ray diffraction and magnetization measurements.The Gd2Fe15.5Cr1.5 compound had a rhombohedral Th2Zn17-type structure.The Curie temperature of Gd2Fe15.5Cr1.5 compound was about 570 K.This value was about 60 K higher than that of the mother compound Gd2Fe17.Negative thermal expansion was found in Gd2Fe15.5Cr1.5 compound in a broad temperature range 294-572 K by X-ray dilatometry.The coefficient of the average thermal expansion was =-3.87×10-6 K-1 in 294-512 K,and-1.58×10-5 in 512-572 K.The magnetostriction deformations from 294 to 532 K were calculated by means of the differences between the experimental values of the lattice parameters and corresponding values extrapolated from the paramagnetic range.The result showed that the spontaneous volume magnetostrictive deformation ωS decreased linearly from 4.73×10-3 to 0.11×10-3 with the temperature increasing from 294 to 532 K.The analysis showed that the ωS mainly came from the contribution of the spontaneous linear magnetostrictive λc along the c axis.
The structural and the magnetic properties of Gd2Fe16Cr compound are investigated by x-ray diffraction and magnetization measurements. The Gd2Fe16Cr compound has a rhombohedral Th2Zn17-type structure. There exist an anisotropic strong spontaneous magnetostriction and a negative thermal expansion in the magnetic state of Gd2Fe16Cr compound. The average thermal expansion coefficient a = -7.03×10^-6/K in a temperature range of 294-454 K and a = -1.31×10^-5/K in 454-572 K are obtained. The spontaneous magnetostrictive deformation and the Curie temperature are discussed.
