Crystal structures, physical properties and NMR experiments on the ternary rare-earth metal silicide boride compounds RE5Si 2B 8 ( RE=Y, Sm, Gd, Tb, Dy, Ho)

Abstract

The ternary rare-earth metal silicide borides RE 5Si 2B 8 ( RE=Y, Sm, Gd, Tb, Dy Ho) were prepared by arc melting the elemental components and subsequent annealing up to T = 1850 K . 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 ) , c = 8.2260 ( 3 ) Å and a = 7.1830 ( 2 ) , c = 7.9900 ( 3 ) Å for Sm 5Si 2B 8 and Ho 5Si 2B 8, respectively. The structure is a new type and can be structurally described as an intergrowth of ThB 4-like and U 3Si 2-like slabs of composition REB 4 and RE 3Si 2, respectively, alternating along the c direction. The boron and silicon substructures are wholly independent and well ordered. The magnetic properties are as follows: Y 5Si 2B 8 is a Pauli-type paramagnet above 1.8 K, Gd 5Si 2B 8 undergoes a weak (canted) ferromagnetic-like order at 70 K followed by a colinear antiferromagnetic spin alignment at 44 K. Tb 5Si 2B 8 and Dy 5Si 2B 8 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 RE 3+ 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 Y 5Si 2B 8 shows different signals, which correspond to the expected number of distinct crystallographic sites in the structure. 11B NMR on Y 5Si 2B 8 indicates that the local magnetic susceptibilities are substantially different from the ones observed in the related compound YB 4.