Magnetic and crystallographic properties of ${\text{Gd}}_5{\text{Sb}}_{0.5}{\text{Ge}}_{3.5}$ were investigated using dc magnetization, ac magnetic susceptibility, and heat capacity of an oriented single crystal, combined with temperature and magnetic field dependent x-ray powder diffraction. The compound undergoes an unusual magnetostructural transition at 40 K and a nonmagnetic second-order transition around 63 K. The detailed crystallographic study of ${\text{Gd}}_5{\text{Sb}}_{0.5}{\text{Ge}}_{3.5}$ shows that contrary to the $R_5{\left({\text{Si}}_x{\text{Ge}}_{1-x}\right)}_4$ systems ($R$ is a rare-earth metal), the structural transition occurs without shear displacements of the ${}_{\infty }{}^2\left[R_5{T}_4\right]$ slabs ($T=\text{Si}$, Ge, and Sb), and a substantial volume change $\left(-0.5\%\right)$ does not lead to a change in crystallographic symmetry. The first-principles electronic structure calculations show higher interslab than intraslab ferromagnetic exchange interaction indicating that ${\text{Sm}}_5{\text{Ge}}_4$ type of structure supports a ferromagnetic ground state in ${\text{Gd}}_5{\text{Sb}}_{0.5}{\text{Ge}}_{3.5}$.

• Received 26 August 2009

DOI:https://doi.org/10.1103/PhysRevB.80.184416