The high-energy synchrotron x-ray powder diffraction experiments of LaRu${}_2$Al${}_{10}$ were carried out to clarify the charge-density distribution, which is derived by the maximum entropy method/Rietveld technique. The results indicate that the charge density between Ru and surrounding Al atoms is large, but that between La and surrounding Al atoms is small. We propose that the Ru-Al${}_{10}$ polyhedron is the fundamental component of the crystal and the two-dimensional layer is constructed by these polyhedra in the $ac$ plane and is stacked along the $b$ axis by way of the Al5 atom. The Ln ion is located inside the crevice formed by the surrounding Ru-Al${}_{10}$ polyhedra and behaves as a nearly free ion. We studied the thermal and magnetic properties of Nd$T$${}_2$Al${}_{10}$ ($T$ = Ru, Os), which exhibits the antiferromagnetic order at $T_{\mathrm{N}}=2.4$ and 2.2 K for $T$ = Ru and Os, respectively. We observed a small lattice distortion below $T_{\mathrm{N}}$, which indicates the small coupling between the orbital moment of the Nd ion and the lattice. The results of the magnetic susceptibility indicate the small crystalline electric field effect. These results mean that the Nd ion behaves as a nearly free ion, which is supported by the results of the charge-density distribution of LaRu${}_2$Al${}_{10}$.

• Received 29 June 2011

DOI:https://doi.org/10.1103/PhysRevB.84.115128