Magnetovolume effect and magnetic transition in Gd_5Si_4

摘要

Magnetocaloric compound Gd_5Si_4 has a higher Curie temperature T_C=336 K than that of Gd pure metal (T_C=294 K). High temperature synchrotron x-ray powder diffraction experiments have been performed for Gd_5Si_4 from 300 to 400 K. Rietveld fitting of the results indicate that the compound retains the orthorhombic Gd_5Si_4-type structure with a space group of Pnma in the above temperature range, where the unit cell is composed of two well defined Gd-Si slabs connected to each other by Si-Si bonds along the b axis. The unit cell volume expands almost linearly with increasing temperature below and above T_C. However, the lattice thermal expansion is anisotropic and discontinuous around T_C. The lattice constants change from a=7.4834(l) A, b = 14.7476(3) A, and c=7.7493(2) to a=7.4846(2) A, b= 14.7452(4) A, and c=7.7560(l) as the temperature is increased from 325 to 345 K. b contracts by about 0.02% while c expands by about 0.08%. Electronic structure calculations show that the anisotropic lattice expansion near T_C changes the valence electron distribution between Gd and Si and thus to the Ruderman-Kittel-Kasuya-Yosida interaction, which affects the magnetic ordering and the magnetic transition in Gd_5Si_4.