10 research outputs found
Synthesis, Structural Characterization, and Physical Properties of the New Transition Metal Oxyselenide Ce<sub>2</sub>O<sub>2</sub>ZnSe<sub>2</sub>
The
quaternary transition metal oxyselenide Ce<sub>2</sub>O<sub>2</sub>ZnSe<sub>2</sub> has been shown to adopt a ZrCuSiAs-related
structure with Zn<sup>2+</sup> cations in a new ordered arrangement
within [ZnSe<sub>2</sub>]<sup>2–</sup> layers. The color of
the compound changes as a function of cell volume, which can vary
by ∼0.4% under different synthetic conditions. At the highest,
intermediate, and lowest cell volumes, the color is yellow-ochre,
brown, and black, respectively. The decreased volume is attributed
to oxidation of Ce from 3+ to 4+, the extent of which can be controlled
by synthetic conditions. Ce<sub>2</sub>O<sub>2</sub>ZnSe<sub>2</sub> is a semiconductor at all cell volumes with experimental optical
band gaps of 2.2, 1.4, and 1.3 eV for high, intermediate, and low
cell volume samples, respectively. SQUID measurements show Ce<sub>2</sub>O<sub>2</sub>ZnSe<sub>2</sub> to be paramagnetic from 2 to
300 K with a negative Weiss temperature of θ = −10 K,
suggesting weak antiferromagnetic interactions
Synthesis, Structural Characterization, and Physical Properties of the New Transition Metal Oxyselenide Ce<sub>2</sub>O<sub>2</sub>ZnSe<sub>2</sub>
The
quaternary transition metal oxyselenide Ce<sub>2</sub>O<sub>2</sub>ZnSe<sub>2</sub> has been shown to adopt a ZrCuSiAs-related
structure with Zn<sup>2+</sup> cations in a new ordered arrangement
within [ZnSe<sub>2</sub>]<sup>2–</sup> layers. The color of
the compound changes as a function of cell volume, which can vary
by ∼0.4% under different synthetic conditions. At the highest,
intermediate, and lowest cell volumes, the color is yellow-ochre,
brown, and black, respectively. The decreased volume is attributed
to oxidation of Ce from 3+ to 4+, the extent of which can be controlled
by synthetic conditions. Ce<sub>2</sub>O<sub>2</sub>ZnSe<sub>2</sub> is a semiconductor at all cell volumes with experimental optical
band gaps of 2.2, 1.4, and 1.3 eV for high, intermediate, and low
cell volume samples, respectively. SQUID measurements show Ce<sub>2</sub>O<sub>2</sub>ZnSe<sub>2</sub> to be paramagnetic from 2 to
300 K with a negative Weiss temperature of θ = −10 K,
suggesting weak antiferromagnetic interactions
On Sr<sub>1–<i>x</i></sub>Na<sub><i>x</i></sub>SiO<sub>3–0.5<i>x</i></sub> New Superior Fast Ion Conductors
On Sr<sub>1–<i>x</i></sub>Na<sub><i>x</i></sub>SiO<sub>3–0.5<i>x</i></sub> New Superior Fast Ion Conductor
Structural Characterization and Physical Properties of the New Transition Metal Oxyselenide La<sub>2</sub>O<sub>2</sub>ZnSe<sub>2</sub>
The quaternary transition metal oxyselenide La<sub>2</sub>O<sub>2</sub>ZnSe<sub>2</sub> has been shown to adopt a ZrCuSiAs-related
structure with Zn<sup>2+</sup> cations in a new ordered arrangement
within the [ZnSe<sub>2</sub>]<sup>2–</sup> layers. This cation-ordered
structure can be derived and described using the symmetry-adapted
distortion mode approach. La<sub>2</sub>O<sub>2</sub>ZnSe<sub>2</sub> is an direct gap semiconductor with an experimental optical band
gap of 3.4(2) eV, consistent with electronic structure calculations
Structural Characterization and Physical Properties of the New Transition Metal Oxyselenide La<sub>2</sub>O<sub>2</sub>ZnSe<sub>2</sub>
The quaternary transition metal oxyselenide La<sub>2</sub>O<sub>2</sub>ZnSe<sub>2</sub> has been shown to adopt a ZrCuSiAs-related
structure with Zn<sup>2+</sup> cations in a new ordered arrangement
within the [ZnSe<sub>2</sub>]<sup>2–</sup> layers. This cation-ordered
structure can be derived and described using the symmetry-adapted
distortion mode approach. La<sub>2</sub>O<sub>2</sub>ZnSe<sub>2</sub> is an direct gap semiconductor with an experimental optical band
gap of 3.4(2) eV, consistent with electronic structure calculations
An Exhaustive Symmetry Approach to Structure Determination: Phase Transitions in Bi<sub>2</sub>Sn<sub>2</sub>O<sub>7</sub>
The exploitable properties of many
materials are intimately linked
to symmetry-lowering structural phase transitions. We present an automated
and exhaustive symmetry-mode method for systematically exploring and
solving such structures which will be widely applicable to a range
of functional materials. We exemplify the method with an investigation
of the Bi<sub>2</sub>Sn<sub>2</sub>O<sub>7</sub> pyrochlore, which
has been shown to undergo transitions from a parent γ cubic
phase to β and α structures on cooling. The results include
the first reliable structural model for β-Bi<sub>2</sub>Sn<sub>2</sub>O<sub>7</sub> (orthorhombic <i>Aba</i>2, <i>a</i> = 7.571833(8), <i>b</i> = 21.41262(2), and <i>c</i> = 15.132459(14) Å) and a much simpler description
of α-Bi<sub>2</sub>Sn<sub>2</sub>O<sub>7</sub> (monoclinic <i>Cc</i>, <i>a</i> = 13.15493(6), <i>b</i> = 7.54118(4), and <i>c</i> = 15.07672(7) Å, β
= 125.0120(3)°) than has been presented previously. We use the
symmetry-mode basis to describe the phase transition in terms of coupled
rotations of the Bi<sub>2</sub>O′ anti-cristobalite framework,
which allow Bi atoms to adopt low-symmetry coordination environments
favored by lone-pair cations
Infinitely Adaptive Transition Metal Oxychalcogenides: The Modulated Structures of Ce<sub>2</sub>O<sub>2</sub>MnSe<sub>2</sub> and (Ce<sub>0.78</sub>La<sub>0.22</sub>)<sub>2</sub>O<sub>2</sub>MnSe<sub>2</sub>
This
article reports the syntheses, structures, and physical properties
of the oxychalcogenides (Ce<sub>1–<i>x</i></sub>La<sub><i>x</i></sub>)<sub>2</sub>O<sub>2</sub>MnSe<sub>2</sub> with <i>x</i> = 0–0.7. These materials have a layered
structure related to that of the LaOFeAs-derived superconductors but
with the transition metal sites 50% occupied. Ce<sub>2</sub>O<sub>2</sub>MnSe<sub>2</sub> contains alternating layers of composition:
[Ce<sub>2</sub>O<sub>2</sub>]<sup>2+</sup> and [MnSe<sub>2</sub>]<sup>2–</sup>. The size mismatch between the layers leads to an
incommensurate structure with a modulation vector of <b>q</b> = α<b>a</b>*<i>+ 0</i><b>b</b>*<i>+</i>0.5<b>c</b>* with α = 0.158(1), which can be
described with a (3 + 1)D superspace structural model in superspace
group <i>Cmme</i>(α,0,<sup>1</sup>/<sub>2</sub>)0<i>s</i>0 [67.12]. There is a strong modulation of Mn site occupancies,
leading to a mixture of corner- and edge-sharing MnSe<sub>4/2</sub> tetrahedra in the [MnSe<sub>2</sub>]<sup>2–</sup> layers.
The modulation vector can be controlled by partial substitution of
Ce<sup>3+</sup> for larger La<sup>3+</sup>, and a simple commensurate
case was obtained for (Ce<sub>0.78</sub>La<sub>0.22</sub>)<sub>2</sub>O<sub>2</sub>MnSe<sub>2</sub> with α = <sup>1</sup>/<sub>6</sub>. The materials respond to the change in relative size of the oxide
and chalcogenide blocks by varying the ratio of corner- to edge-sharing
tetrahedra. The superspace model lets us unify the structural description
of the five different ordering patterns reported to date for different
Ln<sub>2</sub>O<sub>2</sub>MSe<sub>2</sub> (Ln = lanthanide) materials.
Mn moments in Ce<sub>2</sub>O<sub>2</sub>MnSe<sub>2</sub> and (Ce<sub>0.78</sub>La<sub>0.22</sub>)<sub>2</sub>O<sub>2</sub>MnSe<sub>2</sub> order antiferromagnetically below <i>T</i><sub>N</sub> = 150 K, and Ce moments order below ∼70 K. The magnetic structures
of both materials have been determined using neutron diffraction.
Both materials are semiconductors; Ce<sub>2</sub>O<sub>2</sub>MnSe<sub>2</sub> has σ = 9 × 10<sup>–6</sup> Ω<sup>–1</sup> cm<sup>–1</sup> at room temperature and an
activation energy for charge carrier mobility from RT to 170 °C
of ∼0.4 eV
Systematic and Controllable Negative, Zero, and Positive Thermal Expansion in Cubic Zr<sub>1–<i>x</i></sub>Sn<sub><i>x</i></sub>Mo<sub>2</sub>O<sub>8</sub>
We
describe the synthesis and characterization of a family of materials,
Zr<sub>1–<i>x</i></sub>Sn<sub><i>x</i></sub>Mo<sub>2</sub>O<sub>8</sub> (0 < <i>x</i> < 1), whose
isotropic thermal expansion coefficient can be systematically varied
from negative to zero to positive values. These materials allow tunable
expansion in a single phase as opposed to using a composite system.
Linear thermal expansion coefficients, α<sub>l</sub>, ranging
from −7.9(2) × 10<sup>–6</sup> to +5.9(2) ×
10<sup>–6</sup> K<sup>–1</sup> (12–500 K) can
be achieved across the series; contraction and expansion limits are
of the same order of magnitude as the expansion of typical ceramics.
We also report the various structures and thermal expansion of “cubic”
SnMo<sub>2</sub>O<sub>8</sub>, and we use time- and temperature-dependent
diffraction studies to describe a series of phase transitions between
different ordered and disordered states of this material
Infinitely Adaptive Transition-Metal Ordering in Ln<sub>2</sub>O<sub>2</sub>MSe<sub>2</sub>‑Type Oxychalcogenides
A number of Ln<sub>2</sub>O<sub>2</sub>MSe<sub>2</sub> (Ln = La and Ce; M = Fe, Zn, Mn, and Cd) compounds,
built from alternating layers of fluorite-like [Ln<sub>2</sub>O<sub>2</sub>]<sup>2+</sup> sheets and antifluorite-like [MSe<sub>2</sub>]<sup>2–</sup> sheets, have recently been reported in the
literatures. The available MSe<sub>4/2</sub> tetrahedral sites are
half-occupied, and different compositions display different ordering
patterns: [MSe<sub>2</sub>]<sup>2–</sup> layers contain MSe<sub>4/2</sub> tetrahedra that are exclusively edge-sharing (stripe-like),
exclusively corner-sharing (checkerboard-like), or mixtures of both.
This paper reports 60 new compositions in this family. We reveal that
the transition-metal arrangement can be systematically controlled
by either Ln or M doping, leading to an “infinitely adaptive”
structural family. We show how this is achieved in La<sub>2</sub>O<sub>2</sub>Fe<sub>1–<i>x</i></sub>Zn<sub><i>x</i></sub>Se<sub>2</sub>, La<sub>2</sub>O<sub>2</sub>Zn<sub>1–<i>x</i></sub>Mn<sub><i>x</i></sub>Se<sub>2</sub>, La<sub>2</sub>O<sub>2</sub>Mn<sub>1–<i>x</i></sub>Cd<sub><i>x</i></sub>Se<sub>2</sub>, Ce<sub>2</sub>O<sub>2</sub>Fe<sub>1–<i>x</i></sub>Zn<sub><i>x</i></sub>Se<sub>2</sub>, Ce<sub>2</sub>O<sub>2</sub>Zn<sub>1–<i>x</i></sub>Mn<sub><i>x</i></sub>Se<sub>2</sub>, Ce<sub>2</sub>O<sub>2</sub>Mn<sub>1–<i>x</i></sub>Cd<sub><i>x</i></sub>Se<sub>2</sub>, La<sub>2–<i>y</i></sub>Ce<sub><i>y</i></sub>O<sub>2</sub>FeSe<sub>2</sub>, La<sub>2–<i>y</i></sub>Ce<sub><i>y</i></sub>O<sub>2</sub>ZnSe<sub>2</sub>, La<sub>2–<i>y</i></sub>Ce<sub><i>y</i></sub>O<sub>2</sub>MnSe<sub>2</sub>, and La<sub>2–<i>y</i></sub>Ce<sub><i>y</i></sub>O<sub>2</sub>CdSe<sub>2</sub> solid solutions
3D Transition Metal Ordering and Rietveld Stacking Fault Quantification in the New Oxychalcogenides La<sub>2</sub>O<sub>2</sub>Cu<sub>2–4<i>x</i></sub>Cd<sub>2<i>x</i></sub>Se<sub>2</sub>
A number of LnOCuCh (Ln = La–Nd,
Bi; Ch = S, Se, Te) compounds
have been reported in the literature built from alternating layers
of fluorite-like [Ln<sub>2</sub>O<sub>2</sub>]<sup>2+</sup> sheets
and antifluorite-like [M<sub>2</sub>Se<sub>2</sub>]<sup>2–</sup> sheets, where M is in the +1 oxidation state leading to full occupancy
of available MSe<sub>4/2</sub> tetrahedral sites. There is also a
family of related LnOM<sub>0.5</sub>Se (Ln = La & Ce, M = Fe,
Zn, Mn & Cd) compounds built from alternating layers of [Ln<sub>2</sub>O<sub>2</sub>]<sup>2+</sup> sheets and [MSe<sub>2</sub>]<sup>2–</sup> sheets, where M is in the +2 oxidation state with
half occupancy of available tetrahedral sites and complex ordering
schemes in two dimensions. This paper reports a new family of compounds
containing both +1 and +2 metal ions in the La<sub>2</sub>O<sub>2</sub>Cu<sub>2–4<i>x</i></sub>Cd<sub>2<i>x</i></sub>Se<sub>2</sub> family. We show how Cu<sup>1+</sup> and Cd<sup>2+</sup> ions segregate into distinct fully occupied and half occupied
checkerboard-like layers respectively, leading to complex long-range
superstructures in the third (stacking) dimension. To understand the
structure and microstructure of these new materials we have developed
and implemented a new methodology for studying low and high probability
stacking faults using a Rietveld-compatible supercell approach capable
of analyzing systems with thousands of layers. We believe this method
will be widely applicable