Gd₅Si_4−xP_x: Targeted Structural Changes through Increase in Valence Electron Count

Abstract

Phase transformations in the Gd(5)Si(4-x)P(x) system (0 <or= x <or= 2), studied through X-ray diffraction techniques, reveal an intimate coupling between the crystal structure and valence electron count. An increase in the valence electron count through P substitution results in breaking the interslab T-T dimers (d(T-T) = 3.74 A; T is a mixture of Si and P) and shear movement of the ( proportional, variant)(2)[Gd(5)T(4)] slabs in Gd(5)Si(2.75)P(1.25). The Gd(5)Si(2.75)P(1.25) phase extends the existence of the orthorhombic Sm(5)Ge(4)-type structures to the valence electron count larger than 31 e(-)/formula unit. Tight-binding linear-muffin-tin-orbital calculations trace the origin of the T-T dimer cleavage in Gd(5)Si(2.75)P(1.25) to a larger population of antibonding states within the dimers.

Structural, magnetic and magnetocaloric properties of the Gd5Si4−xSbx (x=0.5–3.5) phases

“…In order to populate the T-T dimers with the Sb/Si atoms, the symmetry was lowered from Pnma to Pn2 1 a. Gd f-electrons were treated as core ones during the calculations. Structural parameters for the Gd 5 Si 4 phase were taken from the single crystal refinement results [16].…”

“…They display an intimate relation between a crystal structure, physical properties, valence electron count (VEC) and size effect [14][15][16] and thus afford a rational modification of their physico-chemical properties. The structure-VEC relationship was successfully utilized to control formation/breaking of the interslab T-T dimers and to induce desired structural transitions in the Gd 5 Ga x Ge 4 À x [17], Gd 5 Si 4 À x P x [16], La 5 À x Ca x Ge 4 and Ce 5 À x Ca x Ge 4 systems [18]. The size effect associated with a larger p-element was used to stretch dimers in the Gd 5 Si 4 À x Sn x system [19].…”

Crystal structure and magnetism of Gd5−xEuxGe4

“…A partial substitution of triel Ga atoms for tetral Ge ones reduced the VEC and reformed the T1-T1 dimers, resulting in a Sm 5 Ge 4 -to-Gd 5 Si 4 structural transformation. The intimate relationship between the VEC and crystal structure has been also demonstrated in other R 5 T 4 systems through the T substitution, e.g., Gd 5 Si 4Àx Pn x (Pn = P, Sb, Bi) [9][10][11][12], or through the R substitution, such as R 5Àx Ca x Ge 4 (R = La, Ce) [13], R 5Àx Mg x Ge 4 (R = Gd-Lu, Y) [14,15], and Yb 4 LiGe 4 [16].…”

“…3). Such behavior reflects the structural rigidity of the / 2 ½R 5 Ge 4 slabs and flexibility of the interslab bonds, and this was observed in the other R 5 T 4 systems [6,10]. For the samples with x P 0.50, d Ge1-Ge1 becomes shorter than 2.7 Å, suggesting formation of the Ge1-Ge1 dimers and structural transitions.…”

“…We believe that preferential site occupancy should also occur in Gd 5Àx Eu x Ge 4 . Distribution of different atoms over two or more independent sites is known as a coloring problem [44], which can be rationalized in terms of the electronic and geometric effects [10,[43][44][45].…”