Neutron diffraction studies of ThNi2 Co3 and ThNi3 Co2 at 5–297°K

Abstract

Neutron powder diffraction measurements of ThNi₂ Co₃ and ThNi₃ Co₂ show that, in the 5°–297°K range covering both the ordered and disordered magnetic phases, the Ni and Co atoms are distributed in (c); (1/3, 2/3, 0) and (g); (1/2, 0, 1/2), (0, 1/2, 1/2), (1/2, 1/2, 1/2) of $D_{\text{6h}}^1$ −P(6/m) mm with the following occupancy distribution: In (c), 1.10 Ni + 0.90 Co and 1.50 Ni + 0.50 Co; in (g), 0.90 Ni + 2.10 Co and 1.50 Ni + 1.50 Co, in ThNi₂ Co₃ and ThNi₃ Co₂, respectively. Hence, the numbers of the Co atoms in (c) and (g) in ${\mathrm{ThNi}}_{\text{5−5x}}$ ${\mathrm{Co}}_{\text{5x}}$ (0≤ × ≤1) are likely given by the interpolation formulas, (5x²+3x)/4 and (−5x² + 17x)/4, respectively. This implies that the atomic concentration of the larger moment carrier, e.g., Co, is higher in (g) than in (c) at any given x for 0<×<1. A reversed relation was postulated by van Diepen et al. for the isostructural ternaries ${\mathrm{ThCo}}_{\text{5−5x}}$ ${\mathrm{Fe}}_{\text{5x}}$ and ${\mathrm{ThNi}}_{\text{5−5x}}$ ${\mathrm{Fe}}_{\text{5x}}$ ; thus in these materials, Co and Ni preferentially occupy the 3g sites. The spontaneous ordered ferromagnetic moments of our systems are too small to give rise to the detectable magnetic intensity. Nevertheless, the diffraction effect due to the torque‐induced preferential alignment of the powder particles in the external magnetic field verified that the moment direction is the c axis in both ThNi₂ Co₃ and ThNi₃ Co₂. The field‐induced ferromagnetic moment above the Curie temperature aligns also in the c direction and is particularly large in ThNi₂ Co₃.