Structural chemistry and magnetic behaviour of the ternary silicides UCu xSi 2− x (0.28≤ x≤0.96)

Abstract

The ternary silicides UCu x Si 2− x with 0.28≤ x≤0.96 have been prepared and investigated by X-ray powder diffraction, electron diffraction, electrical resistivity and magnetization measurements. A structural change occurs with the sequence α-ThSi 2 (tetragonal)→AlB 2 (hexagonal)→Ni 2In (hexagonal) as x increases, respectively, for x≅0.5 and x>0.84. The existence of Ni 2In-type, evidenced by transmission electron microscopy, is explained by a crystallographic order between copper and silicon atoms. This system exhibits an interesting magnetic phase diagram: (i) for x<0.50 the compounds show a spin-fluctuation behaviour; (ii) a ferromagnetic ordering appears for 0.50≤ x≤0.80 where the Curie temperature goes through a maximum at 91(1) K for x=0.70; (iii) finally the richest compositions in copper 0.92≤ x≤0.96 order antiferromagnetically with T N around 43–40 K.