Combined neutron and X-ray total scattering with calorimetric measurements of the solid solution series Ho2Ti2−xZrxO7 reveals a complex order–disorder transition across short, intermediate, and long length scales induced by chemical substitution.
We describe the synthesis, crystal structures, and optical absorption spectra/colors of 3d-transition-metal-substituted α-LiZnBO derivatives: α-LiZn M BO (M =Co (0
State-of-the-art
high temperature oxide melt solution calorimetry
and density functional theory were employed to produce the first systematic
study of thermodynamic stability in a series of binary and ternary
Chevrel phases. Rapid microwave-assisted solid-state heating methods
facilitated the nucleation of pure-phase polycrystalline M
y
Mo6S8 (M = Fe, Ni, Cu; y = 0, 1, 2) Chevrel phases, and a stability trend was observed
wherein intercalation of M
y
species engenders
stability that depends on both the electropositivity and ionic radii
of the intercalant species. Ab initio calculations
indicate that this stability trend results from competing ionic and
covalent contributions, where transition metal intercalation stabilizes
the Chevrel structure through increased ionicity but destabilizes
the structure through reduced covalency of the Mo6S8 clusters. Our calculations predicted that over intercalation
of high-valent M
y
species leads to slight
destabilization of the Mo6 octahedral cores, which we confirm
using calorimetry and X-ray absorption spectroscopy. Our combined
computational and calorimetric analysis reveals the interplay of the
foundational principles of ionic and covalent bonding characteristics
that govern the thermodynamic stability of Chevrel and other inorganic
phases.
We describe the synthesis, crystal structures, and optical absorption spectra of transition metal-substituted spiroffite derivatives, Zn(2-x)M(x)Te3O8 (M(II) = Co, Ni, Cu; 0 < x ≤ 1.0). The oxides are readily synthesized by solid state reaction of stoichiometric mixtures of the constituent binaries at 620 °C. Reitveld refinement of the crystal structures from powder X-ray diffraction (XRD) data shows that the Zn/MO6 octahedra are strongly distorted, as in the parent Zn2Te3O8 structure, consisting of five relatively short Zn/M(II)-O bonds (1.898-2.236 Å) and one longer Zn/M(II)-O bond (2.356-2.519 Å). We have interpreted the unique colors and the optical absorption/diffuse reflectance spectra of Zn(2-x)M(x)Te3O8 in the visible, in terms of the observed/irregular coordination geometry of the Zn/M(II)-O chromophores. We could not however prepare the fully substituted M2Te3O8 (M(II) = Co, Ni, Cu) by the direct solid state reaction method. Density Functional Theory (DFT) modeling of the electronic structure of both the parent and the transition metal substituted derivatives provides new insights into the bonding and the role of transition metals toward the origin of color in these materials. We believe that transition metal substituted spiroffites Zn(2-x)M(x)Te3O8 reported here suggest new directions for the development of colored inorganic materials/pigments featuring irregular/distorted oxygen coordination polyhedra around transition metal ions.
Understanding composition-structure-property relationships of high-alumina nuclear waste glasses are important for vitrification of nuclear waste at the Hanford Site. Two series of glasses were designed, one with varying Al:Si ratios and the other with (Al + Na):Si ratios based on the international simple glass (ISG, a simplified nuclear waste model glass), with Al 2 O 3 ranging from 0 to 23 mol% (0 to 32 wt%). The glasses were synthesized and characterized using electron probe microanalysis, X-ray photoelectron spectroscopy, small angle X-ray scattering, high-temperature oxide melt solution calorimetry, and infrared spectroscopy. Glasses were crystal free, and the lowest Na 2 O and Al 2 O 3 glass formed an immiscible glass phase. Evolution of various properties-glass-transition temperature, percentage of 4-coordinated B, enthalpy of glass formation-and infrared spectroscopy results indicate that structural effects differ based on the glass series.
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The
recent finding of local weberite-like ordered domains in disordered
and radiation damaged pyrochlore oxides has sparked interest in studying
the structure, stability, and order–disorder in compounds that
form in the weberite structure. In order to understand the relationships
among the energetics, structure, and disordering, weberites of the
formula RE3TaO7 (RE = La, Nd, Sm–Yb)
were synthesized by conventional solid-state techniques. High temperature
oxide melt solution calorimetry was used to determine their enthalpies
of formation. Rietveld refinement of PXRD patterns shows that the
La compound forms in the weberite La3NbO7 (Cmcm) structure; the Nd compound has both Y3TaO7 (C222
1
)-type
and La3NbO7-type polymorphs; the Sm–Ho
compounds crystallize in the weberite Y3TaO7 (C222
1
) structure;
and the Ho–Yb compounds adopt the defect fluorite (Fm3̅m) disordered structure. Depending on the reaction
temperature, Ho3TaO7 crystallizes in ordered
Y3TaO7 (low temperature) or disordered defect
fluorite (high temperature) structures. The formation enthalpy of
weberites becomes more exothermic with increasing rare earth ionic
radius, implying an increase in stability, i.e., La3TaO7 is most stable and Yb3TaO7 is least
stable with respect to the component oxides. The calorimetric data
also show that ordered Ho3TaO7 (Y3TaO7 structure) is energetically more stable by 9.2 ±
1.1 kJ/mol than disordered Ho3TaO7 (defect fluorite
structure).
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