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Controlling and predicting the morphology of lanthanide sesquioxides in thin film form is vital to their use in current applications. In the present study, single and codeposited Sm2O3, Er2O3, and Lu2O3 thin films were grown on yttria-stabilized zirconia (8%) substrates by radio frequency magnetron sputtering at room temperature and 500 °C. The effect of two different substrate temperatures and altering the oxide cation on the structural and morphological properties of the films was analyzed. The thin films were characterized by profilometry, scanning electron microscopy, transmission electron microscopy, and x-ray diffraction. The single-component Lu2O3 and Sm2O3 films obtained were of the cubic phase, and the Er2O3 was a mix of cubic and monoclinic phases. It was observed for both the Er2O3 and Lu2O3 films that increasing the substrate temperature to 500 °C resulted in larger grained polycrystalline films. In contrast, large grained polycrystalline films were obtained at both room temperature and 500 °C for Sm2O3 and uneven granularity increased as temperature increased. Codeposition of Lu2O3 and Sm2O3, and Lu2O3 and Er2O3 resulted in a cubic bixbyite phase (the C phase of the lanthanide sesquioxide) solid solution. It was observed that the structure and morphology of the films can be controlled by manipulating deposition parameters. Both substrate temperature and altering the oxide cation contributed to changes in crystallinity and grain structure, which can modify the chemical and physical properties of the films for their applications.

Two series of new lanthanide thiophosphates with partial Ca occupancy of the Ln sites, Cs0.3(Ca0.3Ln0.7)PS4 (Ln = Ce, Pr, Nd, Sm, Gd, Tb, and Dy) and Cs0.5(Ca0.5Ln0.5)PS4 (Ln = Pr, Nd, Sm, Gd, and Tb), were synthesized using a CsI flux and structurally characterized. The first series with an idealized formula of Cs0.3(Ca0.3Ln0.7)PS4 crystallizes in the Rm space group and belongs to a new structure type that consists of a channel containing [(Ca0.3Ln0.7)PS4]0.3− framework, where the channels are occupied by severely disordered Cs+ cations. A second new series with formula Cs0.5(Ca0.5Ln0.5)PS4 crystallizes in the monoclinic C2/c space group and exhibits a layered structure consisting of [(Ca0.5Ln0.5)PS4]0.5− layers with Cs+ cations located between the layers for charge balance. Together with the parent structure type, LnPS4, these three structure types illustrate how the LnPS4 structure changes with Cs+ cation incorporation, reducing its dimensionality from 3D to 2D. The magnetic properties of Cs0.3[(Ca0.3Ce0.7)PS4] and Cs0.3[(Ca0.3Pr0.7)PS4] were studied and revealed no magnetic transition down to 2 K.

Spinels represent an important class of technologically relevant materials, used in diverse applications ranging from dielectrics, sensors and energy materials. While solid solutions combining two “single spinels” have been explored in a number of past studies, no ordered “double” spinels have been reported. Based on our first principles computations, here we predict the existence of such a double spinel compound MgAlGaO4, formed by an equimolar mixing of MgAl2O4 normal and MgGa2O4 inverse spinels. After studying the details of its atomic and electronic structure, we use a cluster expansion based effective Hamiltonian approach with Monte Carlo simulations to study the thermodynamic behavior and cation distribution as a function of temperature. Our simulations provide strong evidence for short-ranged cation order in the double spinel structure, even at significantly elevated temperatures. Finally, an attempt was made to synthesize the predicted double spinel compound. Energy Dispersive X-ray Spectrometry and X-ray diffraction Rietveld refinements were performed to characterize the single-phase chemical composition and local configurational environments, which showed a favorable agreement with the theoretical predictions. These findings suggest that a much larger number of compounds can potentially be realized within this chemical space, opening new avenues for the design of spinel-structured materials with tailored functionality. Materials with a spinel structure are used in various applications, including in the nuclear industry and as dielectrics. Here, first principle calculations and Monte Carlo simulations predict that an ordered double spinel structure is stable, supported by preliminary experimental data.

Two polymorphs of a new cesium ferrogermanate zeotype, CsFeGeO4, were synthesized using the molten CsCl-CsF flux growth approach at 900 °C. The orthorhombic polymorph, referred to as (1), crystalli...

Most ternary sulfides belonging to the MGaS2 structure-type have been known for many years and are well-characterized. Surprisingly, there have been no reports of the NaGaS2 composition, which contains Na, a monovalent cation slightly larger in size than Li, found in LiGaS2 , a compound known for its non-linear optical properties. Now it is demonstrated for the first time that the unique reversible water absorption in NaGaS2 has resulted in its absence from previous reports owing to difficulties encountered when characterizing this compound by SC XRD. The layered structure of this compound coupled with uniquely easy migration of water molecules between the layers allows for ion exchange with 3d and 5f metal cations. Some cations, for example, Ni2+ , facilitate exfoliation of the layers, providing a facile synthetic route to a new class of 2D chalcogenide materials and furthermore demonstrating that NaGaS2 can readily uptake uranyl species from aqueous solutions.

A family of rubidium containing mixed-metal titanates, RbxMyTi8-yO16 (M = Mg, Mn, Fe, Ni, and Cu) was prepared as high quality single crystals employing a molten RbCl-RbF flux at 850 °C. This is th...

The effect of lanthanide contraction often results in topological and symmetry changes in compounds with the same compositions as a function of lanthanide cation size. Here we report on the first example of a lanthanide thiophosphate exhibiting a change in the lanthanide cation environment without any topological or symmetry change. A series of new lanthanide thiophosphates with mixed alkali cations were obtained via a flux crystal growth technique using a CsI flux. The obtained compounds Cs2NaLn(PS4)2 (Ln = La-Nd, Sm, and Gd-Ho) were grown as large single crystals (∼0.1-1 mm3) and characterized using single-crystal X-ray diffraction and magnetic susceptibility measurements. As we moved across the series, the structural studies revealed a change in the lanthanide coordination environment depending on the identity of the lanthanide. Although all compounds in the Cs2NaLn(PS4)2 series crystallize in the same space group and have the same Wyckoff atom positions, a slight change in size between Sm3+ and Gd3+ causes a subtle change in coordination number from 9 (for Ln = La-Sm) to 8 (for Ln = Gd-Ho), resulting in two distinct but virtually identical structure types. Ab initio calculations were performed, and the observed experimental trend was corroborated computationally. Magnetic measurements performed on the Cs2NaLn(PS4)2 (Ln = Ce, Pr, Nd, Gd, and Tb) compounds revealed paramagnetic behavior.

Four new cesium-containing cobalt oxide complexes were reported. Cs(Co0.5Si0.5)SiO4 (1), Cs1.29(5)Co0.69(5)Ge1.81(5)O5 (2), and its ordered analogue Cs2CoGe4O10 (3) were synthesized using a mixed CsCl–CsF flux at 850 °C or 900 °C. The structure of (1) closely resembles that of known zeolite and feldspar structures, and (1) crystallizes in the noncentrosymmetric monoclinic space group Im with lattice parameters of a = 8.9926(4) A, b = 5.4599(2) A, c = 9.3958(6) A, and β = 91.5928(18)°. Complexes (2) and (3) crystallize in the same new structure with a highly porous three-dimensional framework in the tetragonal space group I with lattice parameters of a = 7.4239(14) A and c = 13.169(3) A for (2) and a = 7.3540(6) A and c = 13.1122(11) A for (3). The formation of (2) vs. (3) can be controlled based on slight variations in the quantities of the starting materials. Single-crystal-to-single-crystal ion exchange of (1) in a molten RbNO3 bath resulted in 14% Cs exchange with Rb, affording the composition Cs0.86Rb0.14(Co0.5Si0.5)SiO4 (4). First-principles density functional theory calculations were performed to elucidate the electronic and magnetic properties and stabilities of (1) and (3) at 0 K.

Single crystals of two new germanates, [(Rb6F)(Rb4F)][Ge14O32] and [(Rb6F)(Rb3.1Co0.9F0.96)][Co3.8Ge10.2O30F2], were synthesized via high temperature RbCl/RbF flux growth. Both compounds crystallize in the cubic space group F3m and possess the germanium framework of the previously reported salt inclusion material (SIM), [(Cs6F)(Cs3AgF)][Ge14O32], related to the Ge7O16 zeolitic family. These materials demonstrate the ability to accommodate a variety of salt-inclusions, and exhibit chemical flexibility enabling modifications of the framework through incorporation of Co. Alteration of the salt-inclusion led to intrinsic luminescence of [(Rb6F)(Rb4F)][Ge14O32] while modification of the framework resulted in an unanticipated Rb/Co salt/inclusion in [(Rb6F)(Rb3.1Co0.9F0.96)][Co3.8Ge10.2O30F2]. Fluorescence measurements were performed on [(Rb6F)(Rb4F)][Ge14O32]. First-principles calculations in the form of density functional theory (DFT) were performed for [(Rb6F)(Rb3.1Co0.9F0.96)][Co3.8Ge10.2O30F2] to elucidate its electronic and magnetic properties, and stability at 0 K.

Spinel compounds, with the general chemical formula AB2O4, are a wide class of materials, where A and B can be a variety of cations, providing this structure with a great deal of functional flexibility and giving rise to its considerable scientific interest. Recently, a spinel with the general formula ABB′O4 has been predicted, increasing the possible usability of the spinels due to the higher cation diversity in the so-called double spinel structure. Here, we use density functional theory calculations to predict if double spinels can be formed between experimentally synthesized normal and inverse single spinels. Our computations reveal that 49 double spinels have negative mixing enthalpies and are thus thermodynamically stable, with most of the stable compounds being formed from one of two distinct cation orderings. We show that the 17 different cations that form the different double spinels have a preferred site, tetrahedral or octahedral, except for Mn, Fe and Co which can occupy both sites interchangeably. We also study the relation between mixing enthalpies and cation-specific properties, as well as ways to classify the double spinels into distinct types and spinel groups depending on the cation ordering and cation oxidation states, respectively. By applying the Sure Independence Screening and Sparsifying Operator (SISSO) approach on the coordination-dependent ionic radii of the elemental constituents, we show that an interplay of local strain and electrostatic dominated terms can be used to separate the double spinels into distinct structural types depending on the cation order and their oxidation states.

Their very flexible chemistry gives oxide materials a richness in functionality and wide technological application. A specific group of oxides that have a structure related to fluorite but with less oxygen, termed anion-deficient fluorite structural derivatives and with pyrochlores being the most notable example, has been shown to exhibit a diversity of useful properties. For example, the possibility to undergo a transition from an ordered to disordered state allows these oxides to have high radiation tolerance. Atomistic-scale calculations in the form of molecular dynamics (MD) and density functional theory (DFT) have been extensively used to understand what drives this order/disorder transition. Here we give a brief overview of how atomistic-scale calculations are utilized in modeling disorder in pyrochlores and other anion-deficient fluorite structural derivatives. We discuss the modeling process from simple point defects to completely disordered structures, the dynamics during the disordering process, and the use of mathematical models to generate ordered solid-solution configurations. We also attempt to identify the challenges in modeling short range order and discuss future directions to more comprehensive models of the disordered structures.

The development and qualification of silicide-based nuclear fuel would significantly benefit from an adequate understanding of the U–Si phase space, yet issues remain with respect to confidence in the phase diagram, especially in the USi–USi 2 composition range. Experimental investigations have shown the existence of hexagonal and tetragonal phases in the USi x 1.5 x 2 phase space. We subsequently performed a detailed analysis of the dependence of the U–Si convex hull on values of the on-site Coulomb correction, U eff , (DFT + U ) using the U -ramping method. This information allowed us to study the influence of electron correlation, and to find U eff values that best represents the U–Si convex hull. Lastly, the temperature dependence was calculated using determined Gibbs energies of the phases. In the USi–USi 2 phase space, two stable phases were observed up to 1200 °C, with a possible hexagonal to tetragonal phase transition at 580 °C for one of the phases.

Single crystals of Cs6Ln2Ge11O28 (Ln = La, Pr, and Nd) and Cs12Bi3Ge24O58F were grown from a CsCl/CsF flux. These compounds crystallize in new structure types and are members of a new structural family related to the Wadeite structure by polyhedral substitution. Elucidation of this structural family, dubbed the superwadeites, allows for the prediction of the existence and structure of Cs2Ge5O11. Density functional theory calculations support that Cs2Ge5O11 is a stable compound. Ultimately, this compound is grown via a solid-state reaction, and its structure is refined via Rietveld refinement.

To determine the influence of the lanthanide size on the structures and properties of thiophosphates, a thiophosphate series containing different lanthanides was synthesized via high temperature flux crystal growth and their structures and physical properties analyzed and compared. Layered thiohypophosphates NaLnP2S6 (Ln = La, Ce, Pr) and thiopyrophosphates CsLnP2S7 (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Yb, Y) were grown out of an iodide flux using consistent reaction conditions across both series. Under the mildly reducing iodide flux reaction conditions, a rather rare example of phosphorus reduction from the +5 to the +4 oxidation state was observed. Both resultant structure types are based on lanthanide thiophosphate sheets with the alkali cations located between them. Magnetic susceptibility measurements were conducted and revealed Curie-Weiss behavior of the samples, with a Van Vleck contribution in the CsSmP2S7 sample. UV-vis data was found to be in good agreement with the literature, indicating little influence of the sulfide environment on the localized 4f orbitals.

Uranium nitride-silicide composites are being considered as a high-density and high thermal conductivity fuel option for light water reactors. During development, chemical interactions were observed near the silicide melting point which resulted in formation of an unknown U Si N ternary phase. In the present work, U Si N composite samples were produced by arc-melting U3Si2 under an argon-nitrogen atmosphere to form the ternary phase. The resulting samples were characterized by SEM/EDS-EPMA and XRD, and demonstrated an equilibrium between U3Si2, UN, USi and a U Si N phase with a distinct crystallographic structure. Rietveld refinement of the ternary structure was performed, considering the ternary structures existent in the analogue U Si C system, and a good fit was obtained for the hexagonal U20Si16N3 phase. DFT + U calculations were performed in parallel to evaluate the thermodynamic and dynamic stability of the ternaries U20Si16N3 and U3Si2N2. The calculated enthalpy of formation and phonon dispersion support the existence of stable U20Si16N3 and U3Si2N2, although some soft modes in the U20Si16N3 phase phonons are observed. The results presented here thus demonstrate the occurrence of at least one ternary phase in the U Si N system.

The flux synthesis, solid state synthesis, and characterization of a new aluminate, Cs2(UO2)Al2O5, are reported. Cs2(UO2)Al2O5 crystallizes in the tetragonal space group I41/amd with lattice parame...

Experiments show that zeolites, such as Linde Type A (LTA), are promising materials for radioactive decontamination processes. In this work, the thermodynamic properties associated with alkaline earth ion (Ca2+, Sr2+ and Ba2+) exchange in pure silica LTA was studied using periodic first-principles calculations. The adsorption energies of alkaline earth ions compared with that of Na+ were investigated. The driving forces for alkaline earth exchange and related isotherms were calculated, and analyzed as a function of the electron chemical potential. The results demonstrate that Na+ in a pure silica LTA can completely be removed by Ba2+ and almost completely be removed by Sr2+ from a stream, but cannot be effectively exchanged by Ca2+. We also showed that electron-donating dopants should suppress Sr2+ and Ba2+ ion exchange, and that there is a substantial preference for incorporating Ba2+ over Sr2+. Lastly, the substantial difference between the adsorption energies of the ions in an assumed vacuum and those computed in water suggests that a solvation model is needed for accurate representation of ion adsorption in an LTA zeolite.

The novel phases Rb4NaU3O12−xFx (1), K4NaU3O12−xFx (2), and Rb2.1K1.9KU3O12−xFx (3) were synthesized by molten flux methods using mixed alkali fluoride melts. The oxyfluorides crystallize in the cubic space group Imm with a lattice parameters of 8.7472(2) A, 8.6264(2) A, and 8.8390(3) A, respectively. All three structures crystallize in a cubic perovskite structure, ABO3 (A4BB′3O12), where the A site is fully occupied by an alkali cation, and the B site is shared by the remaining smaller alkali cation and uranium in an ordered fashion such that the alkali cation on the B site is surrounded by square uranyl bipyramids. The structures were characterized by single crystal X-ray diffraction, energy dispersive spectroscopy, X-ray absorption near edge structure spectroscopy, X-ray photoelectron spectroscopy, magnetic susceptibility measurements, DFT calculations, thermogravimetric analysis, and UV-vis spectroscopy, all of which support the presence of U(V) in the three new materials.

In this study we combine experimental synthesis and density functional theory (DFT) calculations to gain insight into the polymorphism of A4[(UO2)3(PO4)2O2] (A = Na, K, Rb, Cs) uranyl phosphate structures. Single crystals of a new 3D uranyl phosphate, Cs4[(UO2)3(PO4)2O2], were grown by molten flux methods using a CsCl flux. DFT calculations, using the DFT+U method, were carried out to study the difference between this new 3D uranyl phosphate and a family of recently described layered uranyl phosphates. Variation of the computed properties with changes in Ueff values are also studied. The DFT results agree with the experimental observations, showing that the Cs-containing 3D polymorph and the K-containing layered polymorphs are more stable than their respective layered and 3D polymorph. We show an increase in the difference between the total energies of the layered and 3D polymorphs and an increase in the band gaps with increasing Ueff value. Volume-based thermodynamics was also applied to calculate the to...

U-Si systems have recently received considerable attention due to the potential application of U3Si2 as a high-density fuel under an accident tolerant fuel initiative. However, the thermodynamic stability of the more recently reported adjacent U5Si4 phase is uncertain and could play a significant role in fuel performance. In this work, the enthalpy of formation of the phase predicted by density functional theory (DFT) using the DFT + U formalism is used with an evolutionary algorithm (USPEX) to evaluate stability and possible atomic structures for U5Si4. The structure of U-Si convex hull phases and the confirmed U3Si2 structure were predicted providing confidence in the reliability of the evolutionary algorithm, as well as to obtain a convex hull with comparable enthalpies of formation. Subsequently, the code was applied to the U5Si4 composition for cells with 18 and 36 atoms, predicting a 36-atom hexagonal symmetrized unit cell with space group P6/mmm as the lowest-energy configuration, agreeing with that experimentally reported for U5Si4. Yet, phonon calculations using the density functional perturbation theory formalism, demonstrated that the predicted structure is dynamically unstable, exhibiting negative vibrational modes for the uranium. These indicated that generated shears are directed toward the formation of potential uranium octahedral sites, analogous to those occupied by carbon atoms in U20Si16C3. It was thus concluded that omitting the U5Si4 phase from assessed U-Si phase equilibria is currently justified.

Two polymorphs of a new cesium ferrogermanate zeotype, CsFeGeO

Triuranium disilicide (U3Si2) fuel with silicon carbide (SiC) composite cladding is being considered as an advanced concept/accident tolerant fuel for light water reactors thus, understanding their chemical compatibility under operational and accident conditions is paramount. Here we provide a comprehensive view of the interaction between U3Si2 and SiC by utilizing density functional theory calculations supported by diffusion couple experiments. From the calculated reaction energies, we demonstrate that triuranium pentasilicide (U3Si5), uranium carbide (UC), U20Si16C3, and uranium silicide (USi) phases can form at the interface. A detailed study of U3Si2 and SiC defect formation energies of the equilibrated materials yielding the interfacial phases U20Si16C3, U3Si5 and UC reveal a thermodynamic driving force for generating defects in both fuel and cladding. The absence of either the U3Si2 or SiC phase, however, causes the defect formation energies in the other phase to be positive, removing the driving force for additional interfacial reactions. The diffusion couple experiments confirm the conclusion with demonstrated restricted formation of U3Si5, UC, and U20Si16C3/USi phases at the interface. The resulting lack of continuous interaction between the U3Si2 and SiC, reflects the diminishing driving force for defect formation, demonstrating the substantial stability of this fuel-cladding system., Triuranium disilicide fuel and silicon carbide cladding system is of importance for accident tolerance fuel initiative. Here the authors discuss the role of interface interaction between the U3Si2 fuel and SiC cladding in their use as an advanced concept in light water reactors.

The effect of lanthanide contraction often results in topological and symmetry changes in compounds with the same compositions as a function of lanthanide cation size. Here we report on the first example of a lanthanide thiophosphate exhibiting a change in the lanthanide cation environment without any topological or symmetry change. A series of new lanthanide thiophosphates with mixed alkali cations were obtained via a flux crystal growth technique using a CsI flux. The obtained compounds Cs

Most ternary sulfides belonging to the MGaS

U 3 Si 2 is of interest to the nuclear industry as a candidate fuel material due to its high uranium density and high thermal conductivity. However, it has been observed to react with hydrogen, resulting in material decrepitation. As a result, it is important to understand the thermodynamics of the U 3 Si 2 -H system. In this study, the thermodynamics of the hydrogen absorption reaction of U 3 Si 2 were determined experimentally using Sievert’s gas absorption and related to crystallographic evolution with hydrogen content using X-ray diffraction. Experimentally-determined thermodynamic parameters were compared with results from density functional theory modeling. Results from this study were also compared with those determined in previous work. Sievert’s gas absorption results were used to develop the pressure-composition-temperature (PCT) curves of the U 3 Si 2 -H system. It was found that the hydride phase exhibited a maximum stoichiometry between U 3 Si 2 H 1.8 and U 3 Si 2 H 2 . The two-phase region for hydride formation from U 3 Si 2 exhibited a miscibility gap with a critical temperature between 623 and 673 K, as calculated from the PCT curves. Analysis of the PCT curves also showed that both the enthalpy and entropy of the hydrogen absorption reaction increased with hydrogen content but were lower than the values for uranium trihydride formation from uranium metal. The enthalpy of reaction for hydrogen absorption was calculated to range between -86.9 and -94.8 kJ mol − 1 , while the entropy of reaction was calculated to range between 101.9 and 138.8 J mol − 1 K − 1 . DFT modeling of the thermoydnamic stability of the U 3 Si 2 hydride phases yielded a decomposition temperature of U 3 Si 2 H 2 of approximately 600 K, which was consistent with the experimental results. Similarly, the DFT-calculated enthalpy and entropy of reaction to form U 3 Si 2 H 1.5 were determined to be -106.5kJ mol − 1 and 121.8J mol − 1 K − 1 , respectively, which were both in close agreement with the experimentally-determined values.

As part of Accident Tolerant Fuel initiative for light water reactors, uranium silicide and silicide-nitride are being considered as fuels that can be combined with a more robust cladding such as a...

Single crystals of eight new layered uranyl phosphates were grown from alkali chloride fluxes: Cs1.4K2.6[(UO2)3O2(PO4)2], Cs0.7K3.3[(UO2)3O2(PO4)2], Rb1.4K2.6[(UO2)3O2(PO4)2], K4[(UO2)3O2(PO4)2], K2.9Na0.9Rb0.2[(UO2)3O2(PO4)2], K2.1Na0.7Rb1.2[(UO2)3O2(PO4)2], Cs1.7K4.3[(UO2)5O5(PO4)2], and Rb1.6K4.4[(UO2)5O5(PO4)2]. All structures crystallize in the monoclinic space group, P21/c and contain uranyl phosphate layers with alkali metals located between the layers for charge balance. Ion exchange experiments on Cs0.7K3.3[(UO2)3O2(PO4)2], Rb1.4K2.6[(UO2)3O2(PO4)2], and K4[(UO2)3O2(PO4)2] demonstrated that Cs and Rb cations cannot be exchanged for K cations; however, K cations can be readily exchanged for Na, Rb, and Cs. Enthalpies of formation were calculated from density functional theory (DFT) and volume-based thermodynamics (VBT) for all six structures. A value for the enthalpy of formation of the phosphuranylite sheet, [(UO2)3O2(PO4)2]4–, was derived using single-ion additive methods coupled with VBT. DFT a...

The first entirely pentavalent uranium borate, Na2(UO2)(BO3), was synthesized under mild hydrothermal conditions. The single-crystal structure was solved in the orthorhombic space group Cmcm with a = 10.0472(3) A, b = 6.5942(2) A, and c = 6.9569(2) A. Magnetic susceptibility measurements revealed an antiferromagnetic transition at 12 K and an effective magnetic moment of 2.33 μB. Density functional theory calculations indicated dynamic stability of the structure above 0 K.

Using first-principles DFT we systematically investigate the thermodynamic stability and off-stoichiometry in 108 nominal 19-electron half-Heusler (hH) compounds. We demonstrate unambiguously that considering a cation deficiency towards the off-stoichiometric valence balanced, VEC = 18 composition is necessary for explaining the stability of all previously reported nominal VEC = 19 compounds. This is understandable in terms of an energy benefit from valence balance considering the valence of each atom using Zintl chemistry that offsets the energy penalty of forming defects in nearly all cases. Thus, we propose a valence balanced rule to understand the ground state stability of half-Heuslers irrespective of stoichiometry and nominal electron count (8, 18 or 19). Using this generalized rule we (a) predict 16 previously unreported nominal 19-electron XYZ half-Heuslers and (b) rationalize the reports of giant off-stoichiometries in compounds such as Ti(1−x)NiSb which has been known for over 50 years. Of the 16 new compounds predicted here, Ti(1−x)PtSb was synthesized and the half-Heusler phase confirmed through X-ray studies. The flexibility in stoichiometry of the half-Heusler systems to attain a stable valence balanced composition by accommodating large defect concentrations opens up multiple dimensions for discovery of multi-component defective half-Heuslers based on intrinsic and extrinsic defects which compensate for the nominally non-18-electron count of the structure.

Nanostructured systems formed by two distinct phases are particularly promising for high efficiency thermoelectrics due to the reduction in thermal conductivity afforded by the nanostructured phase. However, the choice of the matrix and nanostructured phases represents a challenging materials discovery problem due to the large compositional space involved. Heusler phase thermoelectrics are particularly promising candidates for nanostructuring since these compounds often possess favorable electronic thermoelectric properties but relatively high thermal conductivity. Here, we have developed a high-throughput screening strategy to predict promising candidates for nanostructuring systems based on two Heusler phases. Our search includes all two-phase systems involving full, half, and inverse Heusler in the Open Quantum Materials Database, in total a search space of ∼1011 possible combinations of two Heusler compounds. To reduce this space, our screening approach starts with a set of known thermoelectrics as ma...

The flux synthesis, solid state synthesis, and characterization of a new aluminate, Cs

Single crystals of Rb0.74Ga6.62Ti0·38O11 (RGTO) were grown from a mixed RbCl–RbF flux at 850 °C. The compound crystallizes in the RbGa7O11 structure type, which is reminiscent of the hollandite and β-Ga2O3 structure types. RGTO crystallizes in the monoclinic space group P2/m with lattice parameters a = 8.3355 (8) A, b = 3.0286 (3) A, c = 9.5028 (9) A, and β = 114.620 (3)°. The crystal structure of RGTO is comprised of GaO6 and mixed (Ga/Ti)O6 octahedra and GaO4 tetrahedra connected in a complex three-dimensional, anionic framework exhibiting eight-sided channels that are occupied by disordered Rb cations required for charge balance. First-principles calculations in the form of density functional theory were performed, which indicated the complex to be a charge transfer semiconductor.

The intermetallic compound U3Si2 has received interest in recent years for use as accident tolerant fuels in light water reactors due to its combination of high thermal conductivity, reasonably high melting point, and high uranium density compared to UO2. Although U3Si2 is well characterized there are ucertainties about the remaining phases in the U–Si system which could be potential fission products of the silicide fuel. In an effort to better detail the U–Si binary phase diagram, the 44–60 at.% Si region of the U–Si phase diagram was investigated experimentally by analyzing samples with nominal compositions U/Si = 5/4, U/Si = 1/1 and U/Si = 2/3 using SEM-EDS, XRD, and high temperature time-of-flight neutron diffraction, supported by DFT calculations. Both the U/Si = 5/4 and the U/Si = 2/3 samples were composed of two phases, U3Si2 + USi and USi + U3Si5 for U/Si = 5/4 and U/Si = 2/3, respectively. Both U5Si4 and U2Si3 were found from DFT calculations to have imaginary phonon frequencies and an enthalpy of formation above the U–Si convex hull, and thus likely to be thermodynamically unstable compared to the other phases, consistent with the experimental results. In the literature there are three different structures that are proposed for the USi phase, and for the first time, high temperature neutron diffraction measurements were collected on the compound synthesized with the U/Si = 1/1 composition to verify the correct structure. The structure of the USi phase was confirmed to be tetragonal with the I4/mmm space group from room temperature to 1100 °C. The USin crystal structure has 6 U atom sites and 8 Si atoms sites of which two are partially occupied.

Due to its high thermal conductivity and uranium density, U3Si2 has been considered as a candidate for use as an accident tolerant fuel (ATF). In order to fully assess its suitability and performance as a fuel, the impact of fission products (FPs) on the stability and performance of U3Si2 must be investigated. The interactions of FPs and U3Si2 have had relatively little study until now and require experimental and computational examination. U3Si2 was doped with individual FPs to explore U-Si-FP interactions and phase equilibria that may impact the performance of the ATF during irradiation. Elemental Ce, Mo, Y, or Zr were used to individually dope U3Si2 at a concentration of 5 wt% FP. A diffusion couple of a 1:1 Mo:Zr alloy and U3Si2 was heated to 1200 °C in order to consider the impacts of multiple FPs on the stability and structure of the fuel. Samples were characterized for FP solubility and secondary phase formation using electron microscopy, energy dispersive spectroscopy, and x-ray diffraction. First principles density functional theory calculations complemented the experimental effort to understand FP behavior. Experimental and computational findings were used in the development of a thermodynamic database containing 8 major FPs and their associated silicide phases. Fuel compositions generated from depletion calculations were used to thermodynamically model the equilibrium phases of the fuel undergoing burnup.

Using first-principles calculations, we studied the adsorption of alkali ions in pure silica Linde Type A (LTA) zeolite. The probability of adsorbing alkali ions from solution and the driving force for ion exchange between Na+ and other alkali ions at the different adsorption sites were analyzed. From the calculated ion exchange isotherms, we show that it is possible to exchange Na+ with K+ and Rb+ in water, but that is not the case for systems in a vacuum. We also demonstrate that a solvation model should be used for the accurate representation of ion exchange in an LTA and that dispersion interactions should be introduced with care.

Formation enthalpies and Gibbs energies of actinide and rare-earth containing SIMs with silicate and germanate frameworks are reported. Volume-based thermodynamics (VBT) techniques complemented by density functional theory (DFT) were adapted and applied to these complex structures. VBT and DFT results were in closest agreement for the smaller framework silicate structure, whereas DFT in general predicts less negative enthalpies across all SIMs, regardless of framework type. Both methods predict the rare-earth silicates to be the most stable of the comparable structures calculated, with VBT results being in good agreement with the limited experimental values available from drop solution calorimetry.