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

Rational Design of Materials with Extreme Negative Compressibility: Selective Soft-Mode Frustration in KMn[Ag(CN)₂]₃

We show that KMn[Ag(CN)₂]₃ exhibits the strongest negative linear compressibility (NLC) effect over the largest pressure range yet observed. Variable pressure neutron powder diffraction measurements reveal that its crystal lattice expands along the <i>c</i> axis of its trigonal cell under increasing hydrostatic pressure, while contracting along the <i>a</i> axis. This corresponds to a “wine-rack”-like mechanism for NLC that we find also results in anisotropic negative thermal expansion (NTE) in the same material. Inclusion of extra-framework K⁺ counterions has minimal effect on framework flexibility (and hence the magnitude of NTE/NLC) but selectively frustrates the soft phonon modes responsible for destroying NLC in the related material Ag₃[Co(CN)₆]

Local Structure of a Pure Bi <i>A</i> Site Polar Perovskite Revealed by Pair Distribution Function Analysis and Reverse Monte Carlo Modeling: Correlated Off-Axis Displacements in a Rhombohedral Material

Perovskite oxides with Bi³⁺ on the <i>A</i> site are of interest as candidate replacements for lead-based piezoelectric ceramics. Current understanding of the chemical factors permitting the synthesis of ambient-pressure-stable perovskite oxides with Bi³⁺ on the <i>A</i> site is limited to information derived from average structures. The local structure of the lead-free ferroelectric perovskite Bi­(Ti_3/8Fe_2/8Mg_3/8)­O₃ is studied by reverse Monte Carlo (RMC) modeling of neutron scattering data. The resultant model is consistent with the structure derived from diffraction but reveals key extra structural features due to correlated local displacements that are inaccessible from the average unit cell. The resulting structural picture emphasizes the need to combine symmetry-averaged long-range and local analysis of the structures of compositionally complex, substitutionally disordered functional materials. Local correlation of the off-axis displacements of the <i>A</i> site cation produces monoclinic domains consistent with the existence of displacement directions other than R (⟨111⟩_p) or T (⟨100⟩_p). The Bi displacements are correlated ferroelectrically both in the polar direction and orthogonal to it, providing evidence of the presence of monoclinic domains. The octahedral cation environments reveal distinct differences in the coordination geometry of the different <i>B</i> site metal ions. The local nature of these deviations and correlations makes them inaccessible to long-range averaged techniques. The resulting local structure information provides a new understanding of the stability of pure Bi ³⁺ <i>A</i> site perovskite oxides

High Pressure Crystal and Magnetic Phase Transitions in Multiferroic Bi_0.9La_0.1FeO₃

The crystal and magnetic structures of multiferroic Bi_0.9La_0.1FeO₃ have been studied using high resolution neutron powder diffraction in the pressure range 0–8 GPa. Two structural phase transitions are observed. The first, at ∼1 GPa, transforms the polar <i>R</i>3<i>c</i> structure to an antipolar PbZrO₃-like √2a_p × 2√2a_p × 2a_p perovskite superstructure; the second, at ∼5 GPa, results in a smaller, √2a_p × √2a_p × 2a_p unit cell and a structure described with <i>Ibmm</i> (nonstandard setting of <i>Imma</i>) symmetry, in which the a^–a^–b⁰ octahedral tilt system is retained and the antipolar cation displacements lost. Accompanying the changes in the nuclear structure, the antiferromagnetic spin structure evolves from a cycloid, with a modulation length, λ ≈ 770 Å, to collinear arrangements with the moments aligned along the <i>b</i>-axis (<i>Pbam</i>) and the <i>a</i>-axis (<i>Ibmm</i>) of the orthorhombic unit cells. In comparison with BiFeO₃ the transition from a rhombohedral to an orthorhombic structure is suppressed by ∼3 GPa, reflecting the dilution of the stereochemically active bismuth lone pair by lanthanum. A correlation between the cell contraction of Bi_1–<i>x</i>La_<i>x</i>FeO₃ (0.0 ≤ <i>x</i> ≤ 0.3) induced by chemical pressure and hydrostatic pressure on BiFeO₃ is determined, with substitution of 1 mol % of La approximately equivalent to application of 0.05 GPa. Bi_0.9La_0.1FeO₃ is found to have a higher bulk modulus than BiFeO₃

Local Crystal Structure of Antiferroelectric Bi₂Mn_4/3Ni_2/3O₆ in Commensurate and Incommensurate Phases Described by Pair Distribution Function (PDF) and Reverse Monte Carlo (RMC) Modeling

The functional properties of materials can arise from local structural features that are not well determined or described by crystallographic methods based on long-range average structural models. The room temperature (RT) structure of the Bi perovskite Bi₂Mn_4/3Ni_2/3O₆ has previously been modeled as a locally polar structure where polarization is suppressed by a long-range incommensurate antiferroelectric modulation. In this study we investigate the short-range local structure of Bi₂Mn_4/3Ni_2/3O₆, determined through reverse Monte Carlo (RMC) modeling of neutron total scattering data, and compare the results with the long-range incommensurate structure description. While the incommensurate structure has equivalent B site environments for Mn and Ni, the local structure displays a significantly Jahn–Teller distorted environment for Mn³⁺. The local structure displays the rock-salt-type Mn/Ni ordering of the related Bi₂MnNiO₆ high pressure phase, as opposed to Mn/Ni clustering observed in the long-range average incommensurate model. RMC modeling reveals short-range ferroelectric correlations between Bi³⁺ cations, giving rise to polar regions that are quantified for the first time as existing within a distance of approximately 12 Å. These local correlations persist in the commensurate high temperature (HT) phase, where the long-range average structure is nonpolar. The local structure thus provides information about cation ordering and B site structural flexibility that may stabilize Bi³⁺ on the A site of the perovskite structure and reveals the extent of the local polar regions created by this cation

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

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

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

Homologous Critical Behavior in the Molecular Frameworks Zn(CN)₂ and Cd(imidazolate)₂

Using a combination of single-crystal and powder X-ray diffraction measurements, we study temperature- and pressure-driven structural distortions in zinc­(II) cyanide (Zn­(CN)₂) and cadmium­(II) imidazolate (Cd­(<i>im</i>)₂), two molecular frameworks with the anticuprite topology. Under a hydrostatic pressure of 1.52 GPa, Zn­(CN)₂ undergoes a first-order displacive phase transition to an orthorhombic phase, with the corresponding atomic displacements characterized by correlated collective tilts of pairs of Zn-centered tetrahedra. This displacement pattern sheds light on the mechanism of negative thermal expansion in ambient-pressure Zn­(CN)₂. We find that the fundamental mechanical response exhibited by Zn­(CN)₂ is mirrored in the temperature-dependent behavior of Cd­(<i>im</i>)₂. Our results suggest that the thermodynamics of molecular frameworks may be governed by considerations of packing efficiency while also depending on dynamic instabilities of the underlying framework topology