Synthesis, Structural Characterization, and Physical Properties of the New Transition Metal Oxyselenide Ce₂O₂ZnSe₂

The quaternary transition metal oxyselenide Ce₂O₂ZnSe₂ has been shown to adopt a ZrCuSiAs-related structure with Zn²⁺ cations in a new ordered arrangement within [ZnSe₂]^2– 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. Ce₂O₂ZnSe₂ 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 Ce₂O₂ZnSe₂ 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 Ce₂O₂ZnSe₂

The quaternary transition metal oxyselenide Ce₂O₂ZnSe₂ has been shown to adopt a ZrCuSiAs-related structure with Zn²⁺ cations in a new ordered arrangement within [ZnSe₂]^2– 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. Ce₂O₂ZnSe₂ 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 Ce₂O₂ZnSe₂ to be paramagnetic from 2 to 300 K with a negative Weiss temperature of θ = −10 K, suggesting weak antiferromagnetic interactions

On Sr_1–<i>x</i>Na_<i>x</i>SiO_{3–0.5<i>x</i>} New Superior Fast Ion Conductors

On Sr_1–<i>x</i>Na_<i>x</i>SiO_{3–0.5<i>x</i>} New Superior Fast Ion Conductor

Structural Characterization and Physical Properties of the New Transition Metal Oxyselenide La₂O₂ZnSe₂

The quaternary transition metal oxyselenide La₂O₂ZnSe₂ has been shown to adopt a ZrCuSiAs-related structure with Zn²⁺ cations in a new ordered arrangement within the [ZnSe₂]^2– layers. This cation-ordered structure can be derived and described using the symmetry-adapted distortion mode approach. La₂O₂ZnSe₂ 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 La₂O₂ZnSe₂

The quaternary transition metal oxyselenide La₂O₂ZnSe₂ has been shown to adopt a ZrCuSiAs-related structure with Zn²⁺ cations in a new ordered arrangement within the [ZnSe₂]^2– layers. This cation-ordered structure can be derived and described using the symmetry-adapted distortion mode approach. La₂O₂ZnSe₂ 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 Bi₂Sn₂O₇

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 Bi₂Sn₂O₇ 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-Bi₂Sn₂O₇ (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-Bi₂Sn₂O₇ (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 Bi₂O′ 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 Ce₂O₂MnSe₂ and (Ce_0.78La_0.22)₂O₂MnSe₂

This article reports the syntheses, structures, and physical properties of the oxychalcogenides (Ce_1–<i>x</i>La_<i>x</i>)₂O₂MnSe₂ 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. Ce₂O₂MnSe₂ contains alternating layers of composition: [Ce₂O₂]²⁺ and [MnSe₂]^2–. 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,0,¹/₂)­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_4/2 tetrahedra in the [MnSe₂]^2– layers. The modulation vector can be controlled by partial substitution of Ce³⁺ for larger La³⁺, and a simple commensurate case was obtained for (Ce_0.78La_0.22)₂O₂MnSe₂ with α = ¹/₆. 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 Ln₂O₂MSe₂ (Ln = lanthanide) materials. Mn moments in Ce₂O₂MnSe₂ and (Ce_0.78La_0.22)₂O₂MnSe₂ order antiferromagnetically below <i>T</i>_N = 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; Ce₂O₂MnSe₂ has σ = 9 × 10^–6 Ω^–1 cm^–1 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_1–<i>x</i>Sn_<i>x</i>Mo₂O₈

We describe the synthesis and characterization of a family of materials, Zr_1–<i>x</i>Sn_<i>x</i>Mo₂O₈ (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_l, ranging from −7.9(2) × 10^–6 to +5.9(2) × 10^–6 K^–1 (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” SnMo₂O₈, 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 Ln₂O₂MSe₂‑Type Oxychalcogenides

A number of Ln₂O₂MSe₂ (Ln = La and Ce; M = Fe, Zn, Mn, and Cd) compounds, built from alternating layers of fluorite-like [Ln₂O₂]²⁺ sheets and antifluorite-like [MSe₂]^2– sheets, have recently been reported in the literatures. The available MSe_4/2 tetrahedral sites are half-occupied, and different compositions display different ordering patterns: [MSe₂]^2– layers contain MSe_4/2 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 La₂O₂Fe_1–<i>x</i>Zn_<i>x</i>Se₂, La₂O₂Zn_1–<i>x</i>Mn_<i>x</i>Se₂, La₂O₂Mn_1–<i>x</i>Cd_<i>x</i>Se₂, Ce₂O₂Fe_1–<i>x</i>Zn_<i>x</i>Se₂, Ce₂O₂Zn_1–<i>x</i>Mn_<i>x</i>Se₂, Ce₂O₂Mn_1–<i>x</i>Cd_<i>x</i>Se₂, La_2–<i>y</i>Ce_<i>y</i>O₂FeSe₂, La_2–<i>y</i>Ce_<i>y</i>O₂ZnSe₂, La_2–<i>y</i>Ce_<i>y</i>O₂MnSe₂, and La_2–<i>y</i>Ce_<i>y</i>O₂CdSe₂ solid solutions

3D Transition Metal Ordering and Rietveld Stacking Fault Quantification in the New Oxychalcogenides La₂O₂Cu_{2–4<i>x</i>}Cd_2<i>x</i>Se₂

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 [Ln₂O₂]²⁺ sheets and antifluorite-like [M₂Se₂]^2– sheets, where M is in the +1 oxidation state leading to full occupancy of available MSe_4/2 tetrahedral sites. There is also a family of related LnOM_0.5Se (Ln = La & Ce, M = Fe, Zn, Mn & Cd) compounds built from alternating layers of [Ln₂O₂]²⁺ sheets and [MSe₂]^2– 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 La₂O₂Cu_{2–4<i>x</i>}Cd_2<i>x</i>Se₂ family. We show how Cu¹⁺ and Cd²⁺ 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