The crystal structure of Gd5Si4

Experimental and Theoretical Study on the Nonmagnetic Li/Mg Cosubstitution in the Gd5-x(Li/Mg)xGe4 (x = 1.04, 1.17, 1.53) System

Three Li- and Mg-cosubstituted compounds in the Gd_5-x(Li/Mg)_xGe₄ (x = 1.04(2), 1.17(2), 1.53(2)) system have been successfully prepared by conventional high-temperature reactions. According to powder and single-crystal X-ray diffraction analyses, all three compounds adopt a Gd₅Si₄-type phase with the orthorhombic Pnma space group (Pearson code oP16, Z = 4) and six crystallographically independent atomic sites. The crystal structure can be described as a combination of two-dimensional Mo₂FeB₂-type _∞²[Gd₂(Li/Mg)Ge₂] layers and [Ge₂] dimers. Interestingly, as 64% of Li and 26% of Gd at the RE3 and RE2 sites, respectively, were exclusively substituted by Mg in Gd_3.47(1)Li_0.36(2)Mg_1.17(3)Ge₄, the lattice parameter b was selectively shortened as a result of the RE3-Ge1 bond shrinkage in comparison to that in Gd₄LiGe₄, while lattice parameters a and c remained nearly intact. A series of theoretical calculations using the tight-binding linear muffin-tin orbital (TB-LMTO) method indicated that the reduction of the particular RE3-Ge1 bond distance in the title compounds could also be explained by an optimization of bonding based on the corresponding RE3-Ge1 crystal orbital Hamilton population (COHP) curve. Moreover, the specific site preference of Mg for the RE3 site was supported by both size-factor as well as electronic-factor criteria on the basis of the smallest atomic size and the highest electronegativity of Mg among the three cations. Therefore, the overall electronic structure was further interrogated by a density of states (DOS) analysis. The influence of nonmagnetic Li/Mg cosubstitution for the magnetic Gd atoms in the title Gd_5-x(Li/Mg)_xGe₄ system on the magnetic characteristics was also thoroughly studied by isofield magnetization at 100 Oe and 10 kOe and isothermal magnetization measurements at 4 K using two of the title compounds: Gd_3.83(1)Li_0.48Mg_0.69(3)Ge₄ and Gd_3.47(1)Li_0.36(2)Mg_1.17(3)Ge₄.

Structural variations in Gd5Si4-xSnx: size vs. electronic effects

X-ray single crystal diffraction studies on the Gd5Si4xSnx series with 0 ≤ x ≤ 3 revealed an increase in the interslab T—T (T = Si, Sn) dimer distances and transition from Gd5Si4-type structures for 0 ≤ x ≤ 1.5 (d T—T = 2.49–2.71 Å) to the Pu5Rh4-type structure for x = 2 (d T—T = 3.04 Å). The Ti5Ga4-type structure, which is a stuffed version of the hexagonal Mn5Si3 structure, was observed for the Gd5SiSn3 sample (x = 3). Tight-binding linear-muffin-tin-orbital (TB-LMTO) calculations on Gd5Si2Sn2 show that the interslab (T1—T1) dimer stretching and the structural transition from the Gd5Si4-type to the Pu5Rh4-type results from the influence of the 5p atomic orbital energy and the larger size of the Sn atoms on the electronic structure, as compared to the Si and Ge atoms in Gd5Si4 and Gd5Si2Ge2.