Structural and magnetic properties of Ho5(SixGe1-x)4

The crystal structure, phase relationship, and magnetic properties of the system Ho-5(SixGe1-x)(4) have been studied using polycrystalline samples with compositions x=0, 0.25, 0.5, 0.75, 0.875, and 1. At room temperature, the concentration range 0 <= x <= 0.5 presents an orthorhombic Sm5Ge4-type O(II) structure, whereas the monoclinic M phase sets in the compounds with concentrations x=0.75 and 0.875. The orthorhombic Gd5Si4-type O(I) structure is present only for x=1. Magnetic characterization has shown that the Ho-5(SixGe1-x)(4) system is paramagnetic at 300 K. The samples with an O(II) structure undergo a second order antiferromagnetic transition with T-N between 25 and 30 K. The compounds with x > 0.5 present a low temperature ferromagnetic phase with T-C rapidly increasing with Si content (dT(C)/dx=95 K). A second magnetic transition has been detected in the whole composition range at T-SR similar to 15 K, which might correspond to a reorientation of the easy magnetization axis. Linear thermal expansion experiments evidence no spontaneous structural transition with the exception of Ho5Ge4, where a small anomaly has been observed, which is probably due to the structural change of a small fraction of the sample volume.