Pressure effects in the giant magnetocaloric compounds Gd5(SixGe1-x)4

We report a study of the effect of hydrostatic pressure up to 9 kbar on selected compounds of the Gd-5(SixGe1-x)(4) series (x = 0.8, 0.45, 0.1) by means of ac magnetic susceptibility, compressibility, and linear thermal expansion measurements. The pressure-induced increase of the transition temperatures at the second-order boundaries of the phase diagram is rather moderate: dT(C)/dP congruent to 0.3 K kbar(-1) (x = 0.8)and dT(N)/dP congruent to +0.7 K kbar(-1) (x = 0.1). This effect is stronger in the 0 < x less than or equal to 0.5 range, where dT(C)/dP congruent to +3 K kbar(-1) (x = 0.45, 0.1), indicating that the ferromagnetic ordering can be simultaneously driven through a pressure-induced structural transformation. The values of d ln T-C/d ln V calculated with the use of the measured value of compressibility (K congruent to 1.8 Mbar(-1)) are significantly lower than those estimated from the concentration dependence of the lattice cell volume, thus demonstrating that the dependence of the transition temperatures upon changing the Si/Ge ratio across the series cannot be explained by a pure volume effect.