Crystal structures, physical properties and NMR experiments on the ternary rare-earth metal silicide boride compounds RE5Si2B8 (RE=Y, Sm, Gd, Tb, Dy, Ho)

The ternary rare-earth metal silicide borides RE₅Si₂B₈ (RE=Y, Sm, Gd, Tb, Dy Ho) were prepared by arc melting the elemental components and subsequent annealing up to . The crystal structure was determined for each term of the series from single-crystal X-ray data: tetragonal symmetry, space group P4/mbm, Z=2; unit cell parameters a=7.2616(3), and a=7.1830(2), for Sm₅Si₂B₈ and Ho₅Si₂B₈, respectively. The structure is a new type and can be structurally described as an intergrowth of ThB₄-like and U₃Si₂-like slabs of composition REB₄ and RE₃Si₂, respectively, alternating along the c direction. The boron and silicon substructures are wholly independent and well ordered. The magnetic properties are as follows: Y₅Si₂B₈ is a Pauli-type paramagnet above 1.8 K, Gd₅Si₂B₈ undergoes a weak (canted) ferromagnetic-like order at 70 K followed by a colinear antiferromagnetic spin alignment at 44 K. Tb₅Si₂B₈ and Dy₅Si₂B₈ order antiferromagnetically at a Néel temperature of T_N=45 and 28 K, respectively. In the paramagnetic regime, the effective moments are in good accord with the theoretical RE³⁺ free ion moments. The temperature dependence of the electrical resistivities for the Y, Gd, Tb, and Dy containing samples corroborates with the metallic state of the nonmagnetic (Y) and the magnetically ordered compounds. ¹¹B, ²⁹Si and ⁸⁹Y nuclear magnetic resonance (NMR) spectroscopy on nonmagnetic Y₅Si₂B₈ shows different signals, which correspond to the expected number of distinct crystallographic sites in the structure. ¹¹B NMR on Y₅Si₂B₈ indicates that the local magnetic susceptibilities are substantially different from the ones observed in the related compound YB₄.