Neutron-Diffraction Study of the Ferromagnetic Mixed-Anion Spinel CuCr2Se3Br

A new class of mixed‐anion spinels has been prepared. These compounds have the general formula $\mathrm{Cu}[{\mathrm{Cr}}_2{\mathrm{]X}}_{3}Y$ where $\mathrm{X\hspace{0.17em}=\hspace{0.17em}}{\mathrm{S}}^{\text{2−}}\mathrm{,\hspace{0.17em}}{\mathrm{Se}}^{\text{2−}}\mathrm{,\hspace{0.17em}}{\mathrm{Te}}^{\text{2−}}$ and $\mathrm{Y\hspace{0.17em}=\hspace{0.17em}}{\mathrm{Cl}}^{-}\mathrm{,\hspace{0.17em}}{\mathrm{Br}}^{-}\mathrm{,\hspace{0.17em}}{\mathrm{I}}^{-}$ . The materials are ferromagnetic with $T_{\mathrm{C}}$ (Curie temperature) close to room temperature. A neutron‐diffraction study was made of ${\mathrm{CuCr}}_2{\mathrm{Se}}_3\mathrm{Br}$ which is a cubic spinel with $a_0\text{\hspace{0.17em}=\hspace{0.17em}10.416\hspace{0.17em}Å}$ and $T_{\mathrm{C}}\text{\hspace{0.17em}=\hspace{0.17em}274°}\mathrm{K}$ . The observed magnetic moment was ${\text{5.25\hspace{0.17em}μ}}_{\mathrm{B}}$ at 4°K and a field of 10 kOe. Neutron‐diffraction measurements were made at room temperature and at liquid nitrogen temperature ${\mathrm{(T/T}}_{\mathrm{C}}\text{\hspace{0.17em}=\hspace{0.17em}0.285)}$ . From the room‐temperature data the material was found to be a normal spinel with $\text{u\hspace{0.17em}=\hspace{0.17em}0.389±0.002}$ . The low‐temperature data showed magnetic scattering which was evaluated in terms of simple models: (a) a spin arrangement corresponding to ${\mathrm{Cu}}^{\text{1+}}[{\mathrm{Cr}}_2^{\text{3+}}]{\mathrm{Se}}_3\mathrm{Br}$ having zero moment on the A sites and a moment of ${\text{6\hspace{0.17em}μ}}_{\mathrm{B}}$ on the B sites, (b) a ferrimagnetic arrangement corresponding to ${\mathrm{Cu}}^{\text{2+}}[{\mathrm{Cr}}^{\text{3+}}{\mathrm{Cr}}^{\text{2+}}]{\mathrm{Se}}_3\mathrm{Br}$ with ${\text{1\hspace{0.17em}μ}}_{\mathrm{B}}$ on the A sites and ${\text{7\hspace{0.17em}μ}}_{\mathrm{B}}$ on the B sites. The first model gives very good agreement with the experimental data while model (b) is quite incompatible with the data.