Rare earth–germanium and –silicon compounds at 5:4 and 5:3 compositions

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.

Single-crystal investigation of Ce5Ag x Ge4−x (x = 0.1−1.08) with Sm5Ge4 type

The crystal structure of the phase Ce5Ag x Ge4−x (x = 0.1−1.08) has been determined using single-crystal X-ray diffraction data for Ce5Ag0.1Ge3.9. This phase is isotypic with Sm5Ge4: space group Pnma (No. 62), Pearson code oP36, Z = 4, a = 7.9632(2), b = 15.2693(5), c = 8.0803(2) Å; R1 = 0.0261, wR2 = 0.0460, 1428 F 2 values and 48 variables. The two crystallographic positions 8d and 4c show Ge/Ag mixing, leading to a slight increase in the lattice parameters as compared to those of the pure binary compound Ce5Ge4.

Redetermination of the crystal structure of R 5Si4 (R = Pr, Nd) from single-crystal X-ray diffraction data

The crystal structures of praseodymium silicide (5/4), Pr5Si4, and neodymium silicide (5/4), Nd5Si4, were redetermined using high-quality single-crystal X-ray diffraction data. The previous structure reports of Pr5Si4 were only based on powder X-ray diffraction data [Smith et al. (1967 ▸). Acta Cryst. 22 940-943; Yang et al. (2002b ▸). J. Alloys Compd. 339, 189-194; Yang et al., (2003 ▸). J. Alloys Compd. 263, 146-153]. On the other hand, the structure of Nd5Si4 has been determined from powder data [neutron; Cadogan et al., (2002 ▸). J. Phys. Condens. Matter, 14, 7191-7200] and X-ray [Smith et al. (1967 ▸). Acta Cryst. 22 940-943; Yang et al. (2002b ▸). J. Alloys Compd. 339, 189-194; Yang et al., (2003 ▸). J. Alloys Compd. 263, 146-153] and single-crystal data with isotropic atomic displacement parameters [Roger et al., (2006 ▸). J. Alloys Compd. 415, 73-84]. In addition, the anisotropic atomic displacement parameters for all atomic sites have been determined for the first time. These compounds are confirmed to have the tetra-gonal Zr5Si4-type structure (space group: P41212), as reported previously (Smith et al., 1967 ▸). The structure is built up by distorted body-centered cubes consisting of Pr(Nd) atoms, which are linked to each other by edge-sharing to form a three-dimensional framework. This framework delimits zigzag channels in which the silicon dimers are situated.

The structural and magnetic properties of the compound Tm5Ge4

The structural and magnetic properties of the compound Tm₅Ge₄ have been studied in detail as functions of temperature and magnetic field.

Structure of the clean Gd5Ge4(010) surface

We have characterized the (010) surface of Gd5Ge4 using scanning tunneling microscopy (STM) and x-ray photoelectron spectroscopy. Data from different samples have the following features in common: (1) the surface composition equals the bulk composition to within 5 at.%, both after ion etching and after annealing at temperatures of 400-1200 K; and (2) the surface exhibits terraces of two types. The height of the steps between similar terraces corresponds well to the separation between equivalent layers along the <010> direction in the bulk structure. Density functional theory (DFT) shows that the surface energy of the (0001) plane of hexagonal close-packed Gd is lower than that of the (111) plane of diamond-type Ge, suggesting that surfaces of Gd5Ge4 (for comparable density) should be rich in Gd. Indeed, DFT shows that among the bulk terminations of Gd5Ge4, a pure Ge termination is not favored. Each of the three remaining terminations (two pure Gd and one mixed, Gd-Ge) has its minimum surface energy in a different range of the possible Gd chemical potentials, indicating that different terminations may be stable under different conditions. DFT shows that the heights of the steps between dissimilar terraces, measured in STM, are consistent with the two pure Gd terminations.

Electron-deficient Eu(6.5)Gd(0.5)Ge6 intermetallic: a layered intergrowth phase of the Gd5Si4- and FeB-type structures

A novel electron-poor Eu(6.5)Gd(0.5)Ge₆ compound adopts the Ca₇Sn₆-type structure (space group Pnma, Z = 4, a = 7.5943(5) Å, b = 22.905(1) Å, c = 8.3610(4) Å, and V = 1454.4(1) ų). The compound can be seen as an intergrowth of the Gd₅Si₄-type (Pnma) R₅Ge₄ (R = rare earth) and FeB-type (Pnma) RGe compounds. The phase analysis suggests that the Eu(7-x)Gd(x)Ge₆ series displays a narrow homogneity range of stabilizing the Ca₇Sn₆ structure at x ≈ 0.5. The structural results illustrate the structural rigidity of the ²(∝)[R₅X₄] slabs (X = p-element) and a possibility for discovering new intermetallics by combining the ²(∝)[R₅X₄] slabs with other symmetry-approximate building blocks. Electronic structure analysis suggests that the stability and composition of Eu(6.5)Gd(0.5)Ge₆ represents a compromise between the valence electron concentration, bonding, and existence of the neighboring EuGe and (Eu,Gd)₅Ge₄ phases.

Remarkable rare-earth metal silicide oxides with planar Si6 rings

New rare-earth silicide oxides, La10Si8O3 (1) and Ce10Si8O3 (2), were synthesized through high-temperature reactions of the pure elements under controlled oxygen atmosphere conditions. The remarkable silicides crystallize in a unique crystal structure (space group P6/mmm; a = 10.975(3) A (La) and 10.844(1) A (Ce); c = 4.680(1) A (La) and 4.561(1) A (Ce)) that features a 3-D framework of corner-shared O-centered (La/Ce)6 octahedra, reminiscent of hexagonal tungsten bronzes, with planar Si6 rings enclosed within its hexagonal channels. Band structure calculations indicate the compounds are metallic, with optimized La-Si bonds, and a benzene-like [Si6]6- anion. Compound 1 exhibits temperature independent paramagnetism. Compound 2 exhibits Curie-Weiss paramagnetism, and an antiferromagnetic ordering below 7 K.

Structure, magnetism, and thermodynamics of the novel rare earth-based R5T4 intermetallics

After approximately 30 years of dormancy, the binary, ternary, and multicomponent intermetallic compounds of rare earth metals (R) with the group 14 elements (T) at the R5T4 stoichiometry have become a goldmine for materials science, condensed matter physics, and solid-state chemistry. In addition to providing numerous opportunities to clarify elusive structure-property relationships, the R5T4 compounds may soon be developed into practical materials by exploiting their unique sensitivity toward a variety of chemical and physical triggers. The distinctiveness of this series is in the remarkable flexibility of the chemical bonding between well-defined, self-assembled, subnanometer-thick slabs and the resultant magnetic, transport, and thermodynamic properties of the R5T4 compounds that can be controlled by varying either or both R and T, including mixed rare earth elements on the R-sites and different group 14 (and 13 or 15) elements occupying the T-sites. In addition to chemical means, the interslab interactions are tunable by temperature, pressure, and magnetic field. Presently, a substantial, yet far from complete, body of knowledge exists about the Gd compounds with T = Si and Ge. In contrast, only a little is known about the physics and chemistry of R5T4 alloys with other lanthanides, while compounds with T = Sn and Pb remain virtually unexplored.

Electron-Precise/Deficient La5-xCaxGe4 (3.4 ≤ x ≤ 3.8) and Ce5-xCaxGe4 (3.0 ≤ x ≤ 3.3): Probing Low-Valence Electron Concentrations in Metal-Rich Gd5Si4-type Germanides

We report for the first time the syntheses of electron-precise/deficient alloys, Ln5-xCaxGe4 (Ln = La, Ce; x = 3.37, 3.66, 3.82 for La; x = 3.00, 3.20, 3.26 for Ce), in the metal-rich R5Tt4 Zintl system (R = rare earth metal; Tt = Si, Ge). The new alloys extend the phase width from electron-rich to open-shell electron-deficient region in the metal-rich Zintl system and demonstrate possible occurrence of varied electron deficiencies in Zintl phases without structural changes, as a result of other existing structure-forming factors.

Making and breaking covalent bonds across the magnetic transition in the giant magnetocaloric material Gd5(Si2Ge2)

A temperature-dependent, single crystal x-ray diffraction study of the giant magnetocaloric material, Gd5(Si2Ge2), across its Curie temperature (276 K) reveals that the simultaneous orthorhombic to monoclinic transition occurs by a shear mechanism in which the (Si, Ge)-(Si,Ge) dimers that are richer in Ge increase their distances by 0.859(3) A and lead to twinning. The structural transition changes the electronic structure, and provides an atomic-level model for the change in magnetic behavior with temperature in the Gd5(SixGe1-x)(4).