7 research outputs found
Electron-Deficient Eu<sub>6.5</sub>Gd<sub>0.5</sub>Ge<sub>6</sub> Intermetallic: A Layered Intergrowth Phase of the Gd<sub>5</sub>Si<sub>4</sub>- and FeB-Type Structures
A novel electron-poor Eu<sub>6.5</sub>Gd<sub>0.5</sub>Ge<sub>6</sub> compound adopts the Ca<sub>7</sub>Sn<sub>6</sub>-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)
Å<sup>3</sup>). The compound can be seen as an intergrowth of
the Gd<sub>5</sub>Si<sub>4</sub>-type (<i>Pnma</i>) R<sub>5</sub>Ge<sub>4</sub> (R = rare earth) and FeB-type (<i>Pnma</i>) RGe compounds. The phase analysis suggests that the Eu<sub>7–<i>x</i></sub>Gd<sub><i>x</i></sub>Ge<sub>6</sub> series
displays a narrow homogneity range of stabilizing the Ca<sub>7</sub>Sn<sub>6</sub> structure at <i>x</i> ≈ 0.5. The
structural results illustrate the structural rigidity of the <sub>∝</sub><sup>2</sup>[R<sub>5</sub>X<sub>4</sub>] slabs (X = <i>p</i>-element) and a possibility
for discovering new intermetallics by combining the <sub>∝</sub><sup>2</sup>[R<sub>5</sub>X<sub>4</sub>] slabs with other symmetry-approximate building blocks. Electronic
structure analysis suggests that the stability and composition of
Eu<sub>6.5</sub>Gd<sub>0.5</sub>Ge<sub>6</sub> represents a compromise
between the valence electron concentration, bonding, and existence
of the neighboring EuGe and (Eu,Gd)<sub>5</sub>Ge<sub>4</sub> phases
Tuning Magnetic and Structural Transitions through Valence Electron Concentration in the Giant Magnetocaloric Gd<sub>5–<i>x</i></sub>Eu<sub><i>x</i></sub>Ge<sub>4</sub> Phases
Valence electron concentration is a viable chemical tool
to control
the crystal structure and magnetism of Gd<sub>5</sub>Ge<sub>4</sub>. A decrease in the valence electron concentration achieved through
the substitution of Eu<sup>2+</sup> for Gd<sup>3+</sup> leads to the
formation of the interslab Ge–Ge dimers, phase transitions
to the Gd<sub>5</sub>Si<sub>2</sub>Ge<sub>2</sub>- and Gd<sub>5</sub>Si<sub>4</sub>-type structures, and a ferromagnetic ordering in the
Gd<sub>5–<i>x</i></sub>Eu<sub><i>x</i></sub>Ge<sub>4</sub> system. Gd<sub>4.75</sub>Eu<sub>0.25</sub>Ge<sub>4</sub> and Gd<sub>4.50</sub>Eu<sub>0.50</sub>Ge<sub>4</sub> undergo temperature-induced
magnetostructural transformations accompanied by giant magnetocaloric
effects
Tuning Magnetic and Structural Transitions through Valence Electron Concentration in the Giant Magnetocaloric Gd<sub>5–<i>x</i></sub>Eu<sub><i>x</i></sub>Ge<sub>4</sub> Phases
Valence electron concentration is a viable chemical tool
to control
the crystal structure and magnetism of Gd<sub>5</sub>Ge<sub>4</sub>. A decrease in the valence electron concentration achieved through
the substitution of Eu<sup>2+</sup> for Gd<sup>3+</sup> leads to the
formation of the interslab Ge–Ge dimers, phase transitions
to the Gd<sub>5</sub>Si<sub>2</sub>Ge<sub>2</sub>- and Gd<sub>5</sub>Si<sub>4</sub>-type structures, and a ferromagnetic ordering in the
Gd<sub>5–<i>x</i></sub>Eu<sub><i>x</i></sub>Ge<sub>4</sub> system. Gd<sub>4.75</sub>Eu<sub>0.25</sub>Ge<sub>4</sub> and Gd<sub>4.50</sub>Eu<sub>0.50</sub>Ge<sub>4</sub> undergo temperature-induced
magnetostructural transformations accompanied by giant magnetocaloric
effects
Decoupling the Electrical Conductivity and Seebeck Coefficient in the <i>RE</i><sub>2</sub>SbO<sub>2</sub> 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><sub>2</sub>SbO<sub>2</sub> system. The <i>RE</i><sub>2</sub>SbO<sub>2</sub> phases, adopting a disordered <i>anti</i>-ThCr<sub>2</sub>Si<sub>2</sub>-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><sub>2</sub>SbO<sub>2</sub> 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 Ho<sub>2</sub>Sb<sub>1–<i>x</i></sub>Bi<sub><i>x</i></sub>O<sub>2</sub> Systems
High-purity bulk samples of the Ho<sub>2</sub>Sb<sub>1–<i>x</i></sub>Bi<sub><i>x</i></sub>O<sub>2</sub> 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 Ho<sub>2</sub>Sb<sub>1–<i>x</i></sub>Bi<sub><i>x</i></sub>O<sub>2</sub> system could be fully traversed without disturbing
the overall <i>anti</i>-ThCr<sub>2</sub>Si<sub>2</sub> 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<sup>I</sup>RE<sup>II</sup>)<sub>3</sub>SbO<sub>3</sub> Phases (RE<sup>I</sup> = La, Ce; RE<sup>II</sup> = Dy, Ho)
In our efforts to tune the charge transport properties
of the recently discovered RE<sub>3</sub>SbO<sub>3</sub> phases (RE
is a rare earth), we have prepared mixed (RE<sup>I</sup>RE<sup>II</sup>)<sub>3</sub>SbO<sub>3</sub> phases (RE<sup>I</sup> = La, Ce; RE<sup>II</sup> = Dy, Ho) via high-temperature reactions at 1550 °C
or greater. In contrast to monoclinic RE<sub>3</sub>SbO<sub>3</sub>, the new phases adopt the <i>P</i>4<sub>2</sub>/<i>mnm</i> symmetry but have a structural framework similar to
that of RE<sub>3</sub>SbO<sub>3</sub>. The formation of the tetragonal
(RE<sup>I</sup>RE<sup>II</sup>)<sub>3</sub>SbO<sub>3</sub> phases
is driven by the ordering of the large and small RE atoms on different
atomic sites. The La<sub>1.5</sub>Dy<sub>1.5</sub>SbO<sub>3</sub>,
La<sub>1.5</sub>Ho<sub>1.5</sub>SbO<sub>3</sub>, and Ce<sub>1.5</sub>Ho<sub>1.5</sub>SbO<sub>3</sub> 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: Li<sub>2</sub>FeGeS<sub>4</sub> (LIGS) and Li<sub>2</sub>FeSnS<sub>4</sub> (LITS)
Li<sub>2</sub>FeGeS<sub>4</sub> (LIGS) and Li<sub>2</sub>FeSnS<sub>4</sub> (LITS), which are among the first magnetic semiconductors with the
wurtz-kesterite structure, exhibit antiferromagnetism with <i>T</i><sub>N</sub> ≈ 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><sub>m</sub> = [<sup>1</sup>/<sub>2</sub>, <sup>1</sup>/<sub>2</sub>, <sup>1</sup>/<sub>2</sub>] 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. <sup>57</sup>Fe and <sup>119</sup>Sn Mössbauer spectroscopy confirmed
the presence of Fe<sup>2+</sup> and Sn<sup>4+</sup> 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