Electron-Deficient Eu_6.5Gd_0.5Ge₆ Intermetallic: A Layered Intergrowth Phase of the Gd₅Si₄- and FeB-Type Structures

A novel electron-poor Eu_6.5Gd_0.5Ge₆ compound adopts the Ca₇Sn₆-type structure (space group <i>Pnma</i>, <i>Z</i> = 4, <i>a</i> = 7.5943(5) Å, <i>b</i> = 22.905(1) Å, <i>c</i> = 8.3610(4) Å, and <i>V</i> = 1454.4(1) ų). The compound can be seen as an intergrowth of the Gd₅Si₄-type (<i>Pnma</i>) R₅Ge₄ (R = rare earth) and FeB-type (<i>Pnma</i>) RGe compounds. The phase analysis suggests that the Eu_7–<i>x</i>Gd_<i>x</i>Ge₆ series displays a narrow homogneity range of stabilizing the Ca₇Sn₆ structure at <i>x</i> ≈ 0.5. The structural results illustrate the structural rigidity of the _∝²[R₅X₄] slabs (X = <i>p</i>-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_6.5Gd_0.5Ge₆ represents a compromise between the valence electron concentration, bonding, and existence of the neighboring EuGe and (Eu,Gd)₅Ge₄ phases

Tuning Magnetic and Structural Transitions through Valence Electron Concentration in the Giant Magnetocaloric Gd_5–<i>x</i>Eu_<i>x</i>Ge₄ Phases

Valence electron concentration is a viable chemical tool to control the crystal structure and magnetism of Gd₅Ge₄. A decrease in the valence electron concentration achieved through the substitution of Eu²⁺ for Gd³⁺ leads to the formation of the interslab Ge–Ge dimers, phase transitions to the Gd₅Si₂Ge₂- and Gd₅Si₄-type structures, and a ferromagnetic ordering in the Gd_5–<i>x</i>Eu_<i>x</i>Ge₄ system. Gd_4.75Eu_0.25Ge₄ and Gd_4.50Eu_0.50Ge₄ undergo temperature-induced magnetostructural transformations accompanied by giant magnetocaloric effects

Tuning Magnetic and Structural Transitions through Valence Electron Concentration in the Giant Magnetocaloric Gd_5–<i>x</i>Eu_<i>x</i>Ge₄ Phases

Valence electron concentration is a viable chemical tool to control the crystal structure and magnetism of Gd₅Ge₄. A decrease in the valence electron concentration achieved through the substitution of Eu²⁺ for Gd³⁺ leads to the formation of the interslab Ge–Ge dimers, phase transitions to the Gd₅Si₂Ge₂- and Gd₅Si₄-type structures, and a ferromagnetic ordering in the Gd_5–<i>x</i>Eu_<i>x</i>Ge₄ system. Gd_4.75Eu_0.25Ge₄ and Gd_4.50Eu_0.50Ge₄ undergo temperature-induced magnetostructural transformations accompanied by giant magnetocaloric effects

Decoupling the Electrical Conductivity and Seebeck Coefficient in the <i>RE</i>₂SbO₂ Compounds through Local Structural Perturbations

Compromise between the electrical conductivity and Seebeck coefficient limits the efficiency of chemical doping in the thermoelectric research. An alternative strategy, involving the control of a local crystal structure, is demonstrated to improve the thermoelectric performance in the <i>RE</i>₂SbO₂ system. The <i>RE</i>₂SbO₂ phases, adopting a disordered <i>anti</i>-ThCr₂Si₂-type structure (<i>I</i>4/<i>mmm</i>), were prepared for <i>RE</i> = La, Nd, Sm, Gd, Ho, and Er. By traversing the rare earth series, the lattice parameters of the <i>RE</i>₂SbO₂ phases are gradually reduced, thus increasing chemical pressure on the Sb environment. As the Sb displacements are perturbed, different charge carrier activation mechanisms dominate the transport properties of these compounds. As a result, the electrical conductivity and Seebeck coefficient are improved simultaneously, while the number of charge carriers in the series remains constant

Disorder-Controlled Electrical Properties in the Ho₂Sb_1–<i>x</i>Bi_<i>x</i>O₂ Systems

High-purity bulk samples of the Ho₂­Sb_1–<i>x</i>­Bi_<i>x</i>O₂ phases (<i>x</i> = 0, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared and subjected to structural and elemental analysis as well as physical property measurements. The Sb/Bi ratio in the Ho₂­Sb_1–<i>x</i>­Bi_<i>x</i>O₂ system could be fully traversed without disturbing the overall <i>anti</i>-Th­Cr₂Si₂ type structure (<i>I</i>4/<i>mmm</i>). The single-crystal X-ray diffraction studies revealed that the local atomic displacement on the Sb/Bi site is reduced with the increasing Bi content. Such local structural perturbations lead to a gradual semiconductor-to-metal transition in the bulk materials. The significant variations in the electrical properties without a change in the charge carrier concentration are explained within the frame of the disorder-induced Anderson localization. These experimental observations demonstrated an alternative strategy for electrical properties manipulations through the control of the local atomic disorder

Synthesis, Crystal Structure, and Electronic Properties of the Tetragonal (RE^IRE^II)₃SbO₃ Phases (RE^I = La, Ce; RE^II = Dy, Ho)

In our efforts to tune the charge transport properties of the recently discovered RE₃SbO₃ phases (RE is a rare earth), we have prepared mixed (RE^IRE^II)₃SbO₃ phases (RE^I = La, Ce; RE^II = Dy, Ho) via high-temperature reactions at 1550 °C or greater. In contrast to monoclinic RE₃SbO₃, the new phases adopt the <i>P</i>4₂/<i>mnm</i> symmetry but have a structural framework similar to that of RE₃SbO₃. The formation of the tetragonal (RE^IRE^II)₃SbO₃ phases is driven by the ordering of the large and small RE atoms on different atomic sites. The La_1.5Dy_1.5SbO₃, La_1.5Ho_1.5SbO₃, and Ce_1.5Ho_1.5SbO₃ samples were subjected to elemental microprobe analysis to verify their compositions and to electrical resistivity measurements to evaluate their thermoelectric potential. The electrical resistivity data indicate the presence of a band gap, which is supported by electronic structure calculations

Field-Induced Spin-Flop in Antiferromagnetic Semiconductors with Commensurate and Incommensurate Magnetic Structures: Li₂FeGeS₄ (LIGS) and Li₂FeSnS₄ (LITS)

Li₂FeGeS₄ (LIGS) and Li₂FeSnS₄ (LITS), which are among the first magnetic semiconductors with the wurtz-kesterite structure, exhibit antiferromagnetism with <i>T</i>_N ≈ 6 and 4 K, respectively. Both compounds undergo a conventional metamagnetic transition that is accompanied by a hysteresis; a reversible spin-flop transition is dominant. On the basis of constant-wavelength neutron powder diffraction data, we propose that LIGS and LITS exhibit collinear magnetic structures that are commensurate and incommensurate with propagation vectors <b>k</b>_m = [¹/₂, ¹/₂, ¹/₂] and [0, 0, 0.546(1)], respectively. The two compounds exhibit similar magnetic phase diagrams, as the critical fields are temperature-dependent. The nuclear structures of the bulk powder samples were verified using time-of-flight neutron powder diffraction along with synchrotron X-ray powder diffraction. ⁵⁷Fe and ¹¹⁹Sn Mössbauer spectroscopy confirmed the presence of Fe²⁺ and Sn⁴⁺ as well as the number of crystallographically unique positions. LIGS and LITS are semiconductors with indirect and direct bandgaps of 1.42 and 1.86 eV, respectively, according to optical diffuse-reflectance UV–vis–NIR spectroscopy