Coordination Environment Of Cations Research Articles

Cation rearrangement at tetrahedral sites in the Cu/ZnAl2O4 spinel enhancing CO2 hydrogenation to methanol

Cu/ZnAl2O4 spinel is a promising catalyst for CO2-to-methanol due to its dual-site H2 activation. However, controlling surface properties and metal-support interactions (MSI) through the coordination environment of spinel cations remains underexplored. Herein, the occupancy of Zn2+ and Al3+ cations in Cu/ZnAl2O4 spinel is fine-tuned by manipulating the calcination atmosphere. More disordered structures (AlO4 and ZnO6) observed on the CZA-Ar catalyst benefit the creation of oxygen vacancies and surface hydroxyl groups, which facilitate the formation of highly dispersed small-sized Cu species. This enhances MSI and hydrogen spillover effects, thereby boosting CO2 conversion and the space-time yield (STY) of methanol. High-pressure in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results reveal that the crucial formate intermediates form at tetrahedral sites in ZnAl2O4 spinel, with more Al-HCOO- species observed on the CZA-Ar catalyst due to the formation of AlO4. This work advances the understanding of spinel catalysts in CO2 hydrogenation to methanol.

Superionic lithium transport via multiple coordination environments defined by two-anion packing.

Fast cation transport in solids underpins energy storage. Materials design has focused on structures that can define transport pathways with minimal cation coordination change, restricting attention to a small part of chemical space. Motivated by the greater structural diversity of binary intermetallics than that of the metallic elements, we used two anions to build a pathway for three-dimensional superionic lithium ion conductivity that exploits multiple cation coordination environments. Li7Si2S7I is a pure lithium ion conductor created by an ordering of sulphide and iodide that combines elements of hexagonal and cubic close-packing analogously to the structure of NiZr. The resulting diverse network of lithium positions with distinct geometries and anion coordination chemistries affords low barriers to transport, opening a large structural space for high cation conductivity.

Ni modulates the coordination environment of cations in Fe3O4 to efficiently catalyze lignin depolymerization

Lignin shows great potential for sustainable production of high-quality fuels and value-added chemicals. The development of efficient and highly stable multifunctional catalysts for the depolymerization of lignin into aromatic chemicals remains a great challenge. In this work, environmentally friendly NiFe2O4 spinel catalyst characterizing with rich oxygen vacancies and porous structures was constructed by introducing Ni to modulate the coordination environment of cations in Fe3O4. Under optimal conditions, the conversion of alkali lignin catalyzed by NiFe2O4 reached 91.0%, the yield of liquid product reached 83.6% and the yield of aromatic monomer products was 31.7%. Combined results from catalyst characterization, product analysis and density functional theory calculations showed that the Lewis acidic center of the catalyst was significantly improved by the introduction of Ni, which promoted the C-O bond breaking in lignin. The Ni-O-Fe structure facilitated the adsorption of lignin substrates and reaction intermediates, which resulted in the improved depolymerization efficiency of lignin. The present work provides valuable insights into the depolymerization of lignin by spinel catalysts, and offers ideas for the future design and development of binary or even ternary spinel catalysts for the depolymerization of lignin.

Pressure-driven phase transformations on Mg3Ca(CO3)4 huntite carbonate.

Magnesium and calcium carbonate minerals are significant reservoirs of Earth's carbon and understanding their behavior under different conditions is crucial for elucidating the mechanisms of deep carbon storage. Huntite, Mg3Ca(CO3)4, is one of the two stable calcium magnesium carbonate phases, together with dolomite. The distinctive cation coordination environment of Ca atoms compared to calcite-type and dolomite structures makes huntite a comparatively less dense phase. Here we examine the behavior of a polycrystalline natural huntite sample under room-temperature compression up to 38 GPa. Synchrotron X-ray diffraction and Raman spectroscopy experiments were carried out in a diamond-anvil cell using He as a highly hydrostatic pressure transmitting medium. XRD results suggest that the initial R32 huntite structure persists up to 21 GPa. The Raman experiment agrees with this result but also suggests the appearance of structural defects from 10 GPa on. Birch-Murnaghan equation of state parameters were fit to the pressure-volume huntite data resulting in zero-pressure volume V0 of 611.7(2) Å3, a bulk modulus B0 of 99.5(11) GPa and a pressure derivative of the bulk modulus of . At 21 GPa, huntite transforms to another trigonal phase (R3), designated here as huntite II. This phase persists up to at least 38 GPa, the maximum pressure reached in this study. The major structural differences between huntite and the huntite-II phase involve the tilting of the [CO3] units with respect to the basal plane and a rotation, which cause a progressive change in the coordination number of the Ca atoms, from 6 to 9. DFT calculations complement the experimental data, providing new insights into the structural response to high-pressure conditions of this magnesium-calcium double carbonate mineral.

Toggling Stereochemical Activity through Interstitial Positioning of Cations between 2D V2O5 Double Layers.

The 5/6s2 lone-pair electrons of p-block cations in their lower oxidation states are a versatile electronic and geometric structure motif that can underpin lattice anharmonicity and often engender electronic and structural instabilities that underpin the function of active elements in nonlinear optics, thermochromics, thermoelectrics, neuromorphic computing, and photocatalysis. In contrast to periodic solids where lone-pair-bearing cations are part of the structural framework, installing lone-pair-bearing cations in the interstitial sites of intercalation hosts provides a means of a systematically modulating electronic structure through the choice of the group and the period of the inserted cation while preserving the overall framework connectivity. The extent of stereochemical activity and the energy positioning of lone-pair-derived mid-gap states depend on the cation identity, stoichiometry, and strength of anion hybridization. V2O5 polymorphs are versatile insertion hosts that can accommodate a broad range of s-, p-, and d-block cations. However, the insertion of lone-pair-bearing cations remains largely underexplored. In this article, we examine the implications of varying the 6s2 cations situated in interlayer sites between condensed [V4O10]n double layers. Systematic modulations of lattice distortions, electronic structure, and magnetic ordering are observed with increasing strength of stereochemical activity from group 12 to group 14 cations. We compare and contrast p-block-layered MxV2O5 (M = Hg, Tl, and Pb) compounds and map the significance of local off-centering arising from the stereochemical activity of lone-pair cations to the emergence of filled antibonding lone-pair 6s2-O 2p-hybridized mid-gap states mediated by second-order Jahn-Teller distortions. Crystallographic studies of cation coordination environments and the resulting modulation of V-V interactions have been used in conjunction with variable-energy hard X-ray photoelectron spectroscopy measurements, first-principles electronic structure calculations, and crystal orbital Hamilton population analyses to decipher the origins of stereochemical activity. Magnetic susceptibility measurements reveal antiferromagnetic signatures for all the three compounds. However, the differences in V-V interactions significantly affect the energy balance of the superexchange interactions, resulting in an ordering temperature of 160 and 260 K for Hg0.5V2O5 and δ-Tl0.5V2O5, respectively, as compared to 7 K for δ-Pb0.5V2O5. In δ-Pb0.5V2O5, the strong stereochemical activity of electron lone pairs and the resulting electrostatic repulsions enforce superlattice ordering, which strongly modifies the electronic localization patterns along the [V4O10] slabs, resulting in disrupted magnetic ordering and an anomalously low ordering temperature. The results demonstrate a versatile strategy for toggling the stereochemical activity of electron lone pairs to modify the electronic structure near the Fermi level and to mediate superexchange interactions.

Na2Mg2P2O7F2: A new diphosphate of the cuspidine family without disorder designed by targeted structural modulation

A new alkali/alkali-earth metal pyrophosphate fluoride Na2Mg2P2O7F2 without the structural disorder in the cuspidine family has been successfully prepared in the sealed system. Na2Mg2P2O7F2 crystallizes in the space group P21/c (No. 14) of the monoclinic crystal system. The structure of Na2Mg2P2O7F2 is shown to be an intricate three-dimensional (3D) framework that consists of the 2D layers [Mg4O12F2]∞ and the 1D chain [Na2O8F3]∞ interlinked by isolated P2O7 groups, which demonstrates the precise coordination environment of cations and the framework in inorganic alkali/alkaline earth metal pyrophosphate fluoride. By comparing with other phosphate structures, we found that the introduction of F is conducive to the consistent arrangement of anionic group P2O7 and enriches the diversity of cationic polyhedra. In addition, first-principle theoretical studies were also carried out to aid the understanding of microstructures, and Na2Mg2P2O7F2 possesses a large band gap (7.1 eV) and has potential applications in the deep ultraviolet region.

Coordination environment dependent stability of Cu-based MOFs towards selective adsorption desulfurization

Cu-based metal organic frameworks (MOFs) are promising adsorbents for the selective adsorption desulfurization (SADS) towards mercaptans in fossil fuels, while the structural collapse problem restricts their application due to the strong interaction between the copper cations and mercaptans. To find a solution, the factors mattering to the SADS ability of MOFs are comprehensively discussed by comparing the structure–activity relationship of 16 MOFs. Especially, the influence of topology, structure of second building unit (SBU) and coordination environment of metal cations of these MOFs are investigated. The experimental results indicate that the binding strength between copper cations and mercaptan is mainly dependent by the coordination environment of the copper cations. And a MOF, ADODAA with copper cations squarely coordinated with two oxygen and two nitrogen atoms exhibits significantly higher stability and renewability over other Cu-based MOFs whose copper cations only coordinates with four oxygen or nitrogen atoms, due to its moderate binding strength towards mercaptan. This work unravels the strong dependence of the stability of MOFs on the coordination environment of its metal cations and provides a platform for constructing highly stable SADS adsorbents.

Structural transformation and anomalous diffusion in network forming liquid GeO2

In this study, we simulated the structural transformation and self-diffusion mechanism in liquid GeO2 oxide system. Under compression, structure of liquid GeO2 model tends to transform gradually from low-density phase to high-density phase. Concentration of basic structural units can be determined via density of material model. The average Ge–O bond distance in GeO4 tetrahedra is smaller than the ones in GeO5 pentahedra and GeO6 octahedra. Each GeOx polyhedron always exists a Ge–O bond with the longest length (the weakest bond). The diffusion mechanism in liquid GeO2 oxide system is via breaking the weakest bonds that accompany the change local coordination environment of Ge cations. The longest Ge–O bond in a GeO5 pentahedron is very weak in comparison to the one in other polyhedra. The diffusivity is significantly dependent on the number of GeO5 pentahedra. The increase of GeO5 under compression is the origin of anomalous diffusion in liquid GeO2 oxide. The increase of average Ge–O bond distance under compression is also clarified in this work.

Synthesis and structural characterization of U-phase, [3Ca2Al(OH)6][Na(H2O)6(SO4)2·6H2O] layered double hydroxide

We have determined the crystal structure of U-phase, which is one of the calcium-aluminate layered double hydroxides (Ca–Al LDHs). Based on X-ray powder diffraction data, we derived the hydrogen-free structural model with space group R3 and hexagonal unit-cell dimensions a ​= ​0.99404(1) and c ​= ​3.00791(4) nm. The location of hydrogen sites was determined by calculation methods. The final structural model, with structural formula [3Ca2Al(OH)6][Na(H2O)6(SO4)2‧6H2O] (Z ​= ​3, a ​= ​0.99372 and c ​= ​2.82658 ​nm), was composed of [Ca2Al(OH)6]+ layers and [Na(H2O)6(SO4)2‧6H2O]3– interlayers. They were alternately stacked in c-axis direction and connected by hydrogen bonds to build the overall structure. The positions of Ca2+ in Ca(OH)64− octahedra were significantly shifted from the centroids of octahedra. Considering the coordination environment of divalent cations in Mn2+-Al LDH and Fe2+-Al LDH, the remarkable displacements of Ca2+ sites in the relevant octahedra would be one of the common structural characteristics of Ca–Al LDHs.

Design of Materials for Energy Storage and Conversion in High Temperature Environment

The kinetics and thermodynamics of electrochemical energy storage and conversion technologies such as fuel cells and electrolyzers often times benefit from high temperature operation. For example, the thermodynamic electrical energy requirements for water electrolysis decrease substantially from 1.23 to below 1 volt by increasing the process temperature from ambient conditions to 850 °C. Unfortunately, materials degradation via sintering, crystal structure disproportion to thermodynamically more stable phases, and interfacial reactions also increases exponentially with increasing temperature. Lifetime requirements for energy conversion technologies often times exceed 10 years of usage with no more than 20% degradation.[1] The requirement of high gas/electrode/solid electrolyte interfacial area for large areal current densities repeatedly necessitates the usage of nano to micro structured materials, further acerbating materials degradation. Oftentimes it is much easier to synthesize a promising new electrode or electrolyte material than it is to characterize the long-term stability of a material in the extremely reducing or oxidizing high temperature environments.[2] We have developed experimental methods in conjunction with thermochemical modeling to evaluate the stability of important classes of perovskite and fluorite oxide materials.[3] We have performed thermogravimetric, FT-IR and electrochemical linear sweep voltammetry methods to rapidly determine oxide materials stability under operationally relevant conditions and these results are compared to stability calculations. Generalizations can be made to predict expected materials redox stability based on relatively straightforward chemical bonding principles, known cation coordination environments and more sophisticated computational chemistry methods. Stability determinations of perovskite oxide electrocatalysts for electrochemical oxidative coupling of methane offer an excellent examples of our approach towards evaluation of materials durability under challenging temperature and reducing conditions.[1] A. Hauch, S. D. Ebbesen, S. H. Jensen, and M. Mogensen, “Highly efficient high temperature electrolysis,” J. Mater. Chem., vol. 18, no. 20, pp. 2331–2340, 2008, doi: 10.1039/B718822F.[2] C. Zhu, S. Hou, X. Hu, J. Lu, F. Chen, and K. Xie, “Electrochemical conversion of methane to ethylene in a solid oxide electrolyzer,” Nat. Commun., vol. 10, no. 1, p. 1173, 2019, doi: 10.1038/s41467-019-09083-3.[3] F. H. G. Kannan P. Ramaiyan, Luke H. Denoyer, Angelica Benavidez, “Electrochemical oxidative coupling of methane to produce higher hydrocarbons using Sr2Fe1.5Mo0.5O6-d electrocatalysts,” Submitt. Commun. Chem., 2021.

Spectroscopic and Computational Study of Boronium Ionic Liquids and Electrolytes.

Boronium cation-based ionic liquids (ILs) have demonstrated high thermal stability and a >5.8 V electrochemical stability window. Additionally, IL-based electrolytes containing the salt LiTFSI have shown stable cycling against the Li metal anode, the "Holy grail" of rechargeable lithium batteries. However, the basic spectroscopic characterisation needed for further development and effective application is missing for these promising ILs and electrolytes. In this work, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and density functional theory (DFT) calculations are used in combination to characterise four ILs and electrolytes based on the [NNBH2 ]+ and [(TMEDA)BH2 ]+ boronium cations and the [FSI]- and [TFSI]- anions. By using this combined experimental and computational approach, proper understanding of the role of different ion-ion interactions for the Li cation coordination environment in the electrolytes was achieved. Furthermore, the calculated vibrational frequencies assisted in the proper mode assignments for the ILs and in providing insights into the spectroscopic features expected at the interface created when they are adsorbed on a Li(001) surface. A reproducible synthesis procedure for [(TMEDA)BH2 ]+ is also reported. The fundamental findings presented in this work are beneficial for any future studies that utilise IL based electrolytes in next generation Li metal batteries.

Li3La2(BO3)3 and Li1.75Na1.25La2(BO3)3: A Great Enhancement in Birefringence Induced by Optimal Arrangement of π-Conjugated [BO3] Units.

In virtue of the essential role in controlling polarized light, outstanding ultraviolet (UV) and deep-ultraviolet (DUV) birefringent crystals are imperative in many advanced optical instruments. To design the UV and DUV crystals with large birefringence, we paid more attention to combining the excellent gene, π-conjugated [BO3] unit and metal cations beneficial to blue-shift the cutoff edge; finally, two rare-earth borates Li3La2(BO3)3 and Li1.75Na1.25La2(BO3)3 have been synthesized using the high-temperature solution method. Compared with Na3La2(BO3)3, Li3La2(BO3)3 and Li1.75Na1.25La2(BO3)3 are isostructural, and the isolated [BO3] units are arranged nearly parallel to each other in the structure, which is conducive to generating a larger birefringence. The structural comparison between the two crystals and Na3La2(BO3)3 indicates that the various coordination environments of alkali metal cations play an important role in the evolution of the crystal structure from Li3La2(BO3)3 and Li1.75Na1.25La2(BO3)3 to Na3La2(BO3)3. This work can contribute to a better understanding of the enhancement in birefringence from Na3La2(BO3)3 (0.023 @ 1064 nm) to Li3La2(BO3)3 (0.078 @ 1064 nm) with the perspective of structure-property relationships. Meanwhile, the two title crystals possess the DUV cutoff edge (<190 nm), suggesting that they can be applied as the DUV birefringent crystals.

Ion Clusters and Networks in Water-in-Salt Electrolytes

Water-in-salt electrolytes (WiSEs) are a class of super-concentrated electrolytes that have shown much promise in replacing organic electrolytes in lithium-ion batteries. At the extremely high salt concentrations of WiSEs, ionic association is more complicated than the simple ion pair description. In fact, large branched clusters can be present in WiSEs, and past a critical salt concentration, an infinite percolating ionic network can form spontaneously. In this work, we simplify our recently developed thermodynamic model of reversible ionic aggregation and gelation, tailoring it specifically for WiSEs. Our simplified theory only has a handful of parameters, all of which can be readily determined from simulations. Our model is able to quantitatively reproduce the populations of ionic clusters of different sizes as a function of salt concentration, the critical salt concentration for ionic gelation, and the fraction of ions incorporated into the ionic gel, as observed from molecular simulations of three different lithium-based WiSEs. The extent of ionic association and gelation greatly affects the effective ionic strength of solution, the coordination environment of active cations that is known to govern the chemistry of the solid-electrolyte interface, and the thermodynamic activity of all species in the electrolyte.

Crystal Structure of BaCa(CO3)2 Alstonite Carbonate and Its Phase Stability upon Compression.

New single-crystal X-ray diffraction experiments and density functional theory (DFT) calculations reveal that the crystal chemistry of the CaO–BaO–CO2 system is more complex than previously thought. We characterized the BaCa(CO3)2 alstonite structure at ambient conditions, which differs from the recently reported crystal structure of this mineral in the stacking of the carbonate groups. This structural change entails the existence of different cation coordination environments. The structural behavior of alstonite at high pressures was studied using synchrotron powder X-ray diffraction data and ab initio calculations up to 19 and 50 GPa, respectively. According to the experiments, above 9 GPa, the alstonite structure distorts into a monoclinic C2 phase derived from the initial trigonal structure. This is consistent with the appearance of imaginary frequencies and geometry relaxation in DFT calculations. Moreover, calculations predict a second phase transition at 24 GPa, which would cause the increase in the coordination number of Ba atoms from 10 to 11 and 12. We determined the equation of state of alstonite (V0 = 1608(2) Å3, B0 = 60(3) GPa, B′0 = 4.4(8) from experimental data) and analyzed the evolution of the polyhedral units under compression. The crystal chemistry of alstonite was compared to that of other carbonates and the relative stability of all known BaCa(CO3)2 polymorphs was investigated.

Investigating the ranges of (meta)stable phase formation in (InxGa1−x)2O3 : Impact of the cation coordination

We investigate the phase diagram of the heterostructural solid solution (InxGa1-x)2O3 both computationally, by combining cluster expansion and density functional theory, and experimentally, by means of TEM measurements of pulsed laser deposited (PLD) heteroepitaxial thin films. The shapes of the Gibbs free energy curves for the monoclinic, hexagonal and cubic bixbyite alloy as a function of composition can be explained in terms of the preferred cation coordination environments of indium and gallium. We show by atomically resolved STEM that the strong preference of indium for six-fold coordination results in ordered monoclinic and hexagonal lattices. This ordering impacts the configurational entropy in the solid solution and thereby the (InxGa1-x)2O3 phase diagram. The resulting phase diagram is characterized by very limited solubilities of gallium and indium in the monoclinic, hexagonal and cubic ground state phases respectively but exhibits wide metastable ranges at realistic growth temperatures. On the indium rich side of the phase diagram a wide miscibility gap is found, which results in phase separated layers. The experimentally observed indium solubilities in the PLD samples are in the range of x=0.45 and x=0.55 for monoclinic and hexagonal single-phase films, while for phase separated films we find x=0.5 for the monoclinic phase, x=0.65-0.7 for the hexagonal phase and x>0.9 for the cubic phase. These values are consistent with the computed metastable ranges for each phase.

The Importance of Competitive Solvent and Anion Coordination for Multivalent Battery Electrolytes

Multivalent battery concepts based on magnesium or calcium metal anodes have the potential to provide higher energy density than traditional lithium ion batteries. However, current multivalent electrolytes possess less than optimal compatibility with these reactive metals and with desirable high voltage cathode materials. Understanding the origin of these issues requires an understanding of multivalent cation coordination environments in both the bulk and at the relevant interfaces. In this presentation, elucidation of the relative coordination strengths of “state-of-the-art” multivalent battery salts and solvents will be described. Moreover, the impact of these competitive coordination tendencies on metal deposition efficiency will be discussed along with the efficacy of tuning such interactions to improve magnesium and calcium electrolyte performance.This work was supported by the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

SYSTEM Ag2S – SnS2 – P2S5

The quasiternary system Ag2S – SnS2 – P2S5 is characterized by quasi-binary sections between the binary and ternary phases. The existence of solid solutions Ag7+xP1-xSnxS6 (x =0÷ 0,31) and Ag8-xSn1-xPxS6 (x = 0÷0,21) was established on the quasi-binary section Ag7PS6 – Ag8SnS6. Replacement of P atoms by a mixture of [Ag + Sn] atoms in α-solid solution leads to an increase in the lattice parameters. The replacement of Sn atoms by P atoms in the β-solid solution is accompanied by a decrease in the lattice parameters. Moreover, the character is not linear, but is described by a positive deviation from Vegard’s rule. The crystal structure of two phases Ag7.31P0.69Sn0.31S6 ( а = 10.526(1) Å, SG (No 198) P213) та Ag7.8Sn0.8P0.2S6 (а = 15.205(3) Å, b = 7.527(1) Å, c = 10.663(2) Å; SG (No 33) Pna21) which are the final compositions of solid solutions. The tetra phase Ag7.31P0.69Sn0.31S6 is formed by replacing P atoms with Sn atoms. As a result, we obtain a defective, relative to the position of the atoms P, position 4a, which is inhabited by a mixture of atoms [Ag + Sn]. The coordination environment of cations is represented by tetrahedra, monohedra and dumbbells: Ag1 has a coordinating environment of sulfur atoms in the form of a dumbbellhas [S3-Ag1-S1], Ag2 is centered in the monohedron of the triangle [2S4- Ag2-S3] ; Ag3 is coordinated into a tetrahedron [Ag3S1S2S3S4], the atoms [Ag / Sn + P] have a tetrahedral environment of sulfur atoms: {[Ag/Sn+P]S23S4}. The tetra phase Ag7.8Sn0.8P0.2S6 is formed by replacing Sn atoms with P. atoms. As a result, we obtain position 4a, in which a mixture of atoms [0.8Sn + 0.2P] is localized. The coordination environment of cations is represented by tetrahedra, monohedra and dumbbells: Ag1 has a coordination environment of four sulfur atoms [Ag1S2S4S5S6], Ag2 is centered in a tetrahedron [Ag2S1S3S5S6]; Ag3 is coordinated in a tetrahedron [Ag3S1S2S5S6], Ag4 – in a monohedron [Ag4S1S4S6], Ag5 – in a monohedron [Ag5S1S2S5], Ag6 has a tetrahedral environment of sulfur atoms [Ag6S3S4S5S6]], Ag7 is coordinated in the center of the monohedron [Ag7S3S3S6], Ag8 forms a dumbbell shape with Sulfur atoms [S3-Ag8-S5]. [Sn + P] atoms have a tetrahedral environment of Sulfur atoms {[Sn+P]S1S2S3S4}.

Influence of Alkaline-Earth-Metal Substitutions on the Bismuth Ruthenate Structure: Bi2-xA'xRu2O6O'1-y (A' = Mg, Ca, Sr).

Solid solutions with the formula of Bi2-xA'xRu2O7-y (A' = Mg, Ca, Sr; 0 ≤ x ≤ 0.2 for Mg, 0 ≤ x ≤ 1 for Ca, and 0 ≤ x ≤ 0.5 for Sr) have been synthesized and characterized. The crystal structures for these phases are found to be in the pyrochlore family, crystallizing in the cubic space group Fd3̅m with complex A/A' cation coordination environments. The Bi cation is found to be off-center from the ideal position because of a lone-pair distortion, while the positions of the substituted A' cations vary based on the size and ionicity. The neutron structure refinements reveal a similar propensity to off-center regarding Ca and Sr, while Mg features the largest static displacement of up to 0.48 Å. Interestingly, this is one of only two known pyrochlores with Mg2+ located in an 8-coordinated site. The average Ru oxidation state for each substitution is found to increase, and charge compensates for the lower divalent A' substitution. The solid solutions show temperature-independent resistance across the series with small changes in magnitude that scale with the amount of substitution, while displaying Pauli paramagnetic behavior throughout.

Inverse Design of Ultralow Lattice Thermal Conductivity Materials via Materials Database Screening of Lone Pair Cation Coordination Environment.

The presence of lone pair (LP) electrons is strongly associated with the disruption of lattice heat transport, which is a critical component of strategies to achieve efficient thermoelectric energy conversion. By exploiting an empirical relationship between lattice thermal conductivity, κL, and the bond angles of pnictogen group LP cation coordination environments, we develop an inverse design strategy based on a materials database screening to identify chalcogenide materials with ultralow κL for thermoelectrics. Screening the ∼635 000 real and hypothetical inorganic crystals of the Open Quantum Materials Database based on the constituent elements, nominal electron counting, LP cation coordination environment, and synthesizability, we identify 189 compounds expected to exhibit ultralow κL. As a validation, we explicitly compute the lattice dynamical properties of two of the compounds (Cu2AgBiPbS4 and MnTl2As2S5) using first-principles calculations and successfully find both achieve ultralow κL values at room temperature of ∼0.3-0.4 W/(m·K) corresponding to the amorphous limit. Our data-driven approach provides promising candidates for thermoelectric materials and opens new avenues for the design of phononic properties of materials.

Synthesis of Samarium OxysulfateSm2O2SO4 in the High-Temperature Oxidation Reaction and Its Structural, Thermal and Luminescent Properties.

The oxidation process of samariumoxysulfide was studied in the temperature range of 500–1000 °C. Our DTA investigation allowed for establishing the main thermodynamic (∆Hºexp = −654.6 kJ/mol) and kinetic characteristics of the process (Ea = 244 kJ/mol, A = 2 × 1010). The enthalpy value of samarium oxysulfate (ΔHºf (Sm2O2SO4(monocl)) = −2294.0 kJ/mol) formation was calculated. The calculated process enthalpy value coincides with the value determined in the experiment. It was established that samarium oxysulfate crystallizes in the monoclinic symmetry class and its crystal structure belongs to space group C2/c with unit cell parameters a = 13.7442 (2), b = 4.20178 (4) and c = 8.16711 (8)Å, β = 107.224 (1)°, V = 450.498 (9)Å3, Z = 4. The main elements of the crystalline structure are obtained and the cation coordination environment is analyzed in detail. Vibrational spectroscopy methods confirmed the structural model adequacy. The Sm2O2SO4luminescence spectra exhibit three main bands easily assignable to the transitions from 4G5/2 state to 6H5/2, 6H7/2, and 6H9/2 multiplets.