Intercalated Water Drives Anomalous Thermal Expansion in the Tetragonal Zircon Structured Bismuth Vanadate BiVO4 Photocatalyst.

The thermal transformation of the tetragonal-zircon (tz-) to tetragonal-scheelite (ts-)BiVO4 was studied by in situ synchrotron X-ray diffraction, thermogravimetric analysis, and Fourier-transformed infrared spectroscopy. Upon heating, the tetragonal zircon polymorph of BiVO4 (tz-BiVO4) transitioned to the ts-polymorph between 693-773 K. Above 773 K, single phase ts-BiVO4 was observed before transitioning to the monoclinic fergusonite (mf-) polymorph upon cooling. An anomaly in thermal expansion was observed between 400-500 K, associated with the loss of intercalated H2O/NH4 + from the coprecipitation procedure. Heating tz-BiVO4 resulted in contraction of the V-O bond distance and VO4 polyhedra volume, ascribed to rotation of the tetrahedra groups. Attempts to study this by neutron diffraction failed due to the large incoherent scatter from the hydrogenous species. Efforts to remove these species while maintaining the tz-BiVO4 structure were unsuccessful, suggesting they play a role in stabilizing the tz-polymorph. The local structure of both mf-BiVO4 and tz-BiVO4 were investigated by X-ray pair distribution function analysis, revealing local distortions.

Seeing the Unseen: The Structural Influence of the Lone Pair Electrons in PbWO4.

Pair distribution function (PDF) analysis of the scheelite-type material PbWO4 reveals previously unidentified short-range structural distortions in the PbO8 polyhedra and WO4 tetrahedra not observed in the similarly structured CaWO4. These local distortions are a result of the structural influence of the Pb2+ 6s2 lone pair electrons. These are not evident from the Rietveld analysis of synchrotron X-ray or neutron powder diffraction data, nor do they strongly influence the X-ray PDF (XPDF). This illustrates the importance of neutron PDF (NPDF) in the study of such materials. First-principles density function theory (DFT) calculations show that the Pb2+ 6s2 electrons are hybridized with the O2- 2p electrons near the Fermi level. The presence of local-scale distortions has previously been neglected in studies of structure-functionality relationships in PbWO4 and other scheelite-structured photocatalytic materials, including BiVO4, and this observation opens new avenues for their optimization.

Tetrahedra Rotational and Displacive Disorder in the Scheelite-Type Oxide CsReO4.

Scheelite-type metal oxides are a notable class of functional materials, with applications including ionic conductivity, photocatalysis, and the safe storage of radioactive waste. To further engineer these materials for specific applications, a detailed understanding of how their properties can change under different conditions is required─not just in the long-range average structure but also in the short-range local structure. This paper outlines a detailed investigation of the metal oxide CsReO4, which exhibits an uncommon orthorhombic Pnma pseudo-scheelite-type structure at room temperature. Using synchrotron X-ray diffraction, the average structure of CsReO4 is found to undergo a transformation from the orthorhombic Pnma pseudo-scheelite-type structure to the tetragonal I41/a scheelite-type structure at ∼440 K. In the X-ray pair distribution function analysis, lattice strain and rotations of the ReO4 tetrahedra are apparent above 440 K despite the increase in long-range average symmetry, revealing a disconnect between the structural models at different length scales. This study demonstrates how the bonding requirements and ionic radii of the A-site cation can induce disorder that is detectable at different length scales, affecting the physical properties of the material.

Resolving Fast Relative Kinetics in Inorganic Solid-State Synthesis.

Solid-state syntheses are generally regarded as being slow, limited by transport, and, as such, are often only stopped to check the products after many hours at high temperature. Here, using a custom-designed reactor to rapidly initiate solid-state syntheses, we are able to capture the earliest stages of a reaction using in situ X-ray scattering. For the reaction of TiO2 and Li2CO3 to form spinel lithium titanate (Li4Ti5O12)─an anode material for fast-charging applications─we capture two distinct kinetic regimes, including fast initial kinetics in the first seconds-minutes of the reaction that account for significant product formation. We use an Avrami model to compare the reaction at high temperatures (700-750 °C), which results in the rapid formation of Li4Ti5O12 within minutes, and lower temperatures (482 °C), consistent with conditions that might be chosen based on "Tamman's rule", a common heuristic. Our analysis reveals characteristic Avrami slopes (i.e., dimensionalities) for each step in the chemical transformation. We anticipate that the fast initial reaction kinetics found here are likely to be common in the synthesis of other materials used in battery electrodes, solid-state electrolytes, ion-conductive membranes, etc. where ion transport is a prerequisite for functionality.

Tetrahedral Displacive Disorder in the Scheelite-Type Oxide RbReO4.

Oxides exhibiting the scheelite-type structure are an important class of functional materials with notable applications in photocatalysis, luminescence, and ionic conductivity. Like all materials, understanding their atomic structure is fundamental to engineering their physical properties. This study outlines a detailed structural investigation of the scheelite-type oxide RbReO4, which exhibits a rare long-range phase transition from I41/a to I41/amd upon heating. Additionally, in the long-range I41/a model, the Re-O tetrahedral distance undergoes significant contraction upon warming. Recent studies of other scheelite oxides have attributed this apparent contraction to incoherent local-scale tetrahedral rotations. In this study, we use X-ray pair distribution function analysis to show that RbReO4 undergoes a unique symmetry-lowering process on the local scale, which involves incoherent tetrahedral displacements. The rare I41/a to I41/amd long-range phase transition was found to occur via a change from static to dynamic disorder on the local scale, which is due to the combination of the size of the A-site cation and lattice expansion. This demonstrates how careful manipulation of the ionic radius of the A-site in the scheelite structure can be used to induce local-scale disorder, which has valuable implications for tailoring the physical properties of related materials.

Understanding the Re-entrant Phase Transition in a Non-magnetic Scheelite.

The stereochemical activity of lone pair electrons plays a central role in determining the structural and electronic properties of both chemically simple materials such as H2O, as well as more complex condensed phases such as photocatalysts or thermoelectrics. TlReO4 is a rare example of a non-magnetic material exhibiting a re-entrant phase transition and emphanitic behavior in the long-range structure. Here, we describe the role of the Tl+ 6s2 lone pair electrons in these unusual phase transitions and illustrate its tunability by chemical doping, which has broad implications for functional materials containing lone pair bearing cations. First-principles density functional calculations clearly show the contribution of the Tl+ 6s2 in the valence band region. Local structure analysis, via neutron total scattering, revealed that changes in the long-range structure of TlReO4 occur due to changes in the correlation length of the Tl+ lone pairs. This has a significant effect on the anion interactions, with long-range ordered lone pairs creating a more densely packed structure. This resulted in a trade-off between anionic repulsions and lone pair correlations that lead to symmetry lowering upon heating in the long-range structure, whereby lattice expansion was necessary for the Tl+ lone pairs to become highly correlated. Similarly, introducing lattice expansion through chemical pressure allowed long-range lone pair correlations to occur over a wider temperature range, demonstrating a method for tuning the energy landscape of lone pair containing functional materials.

Neutron diffraction study of the monoclinic - tetragonal phase transition in NdNbO4 and NdTaO4.

Phase transition and high-temperature properties of NdNbO4 and NdTaO4 were studied in situ using powder neutron diffraction methods. Both oxides undergo a reversible phase transition from a monoclinic I2/a phase at low temperatures to a tetragonal I41/a phase at high temperatures. The phase transition has been investigated through analysis of the spontaneous strains and symmetry distortion modes. Well below the transition temperature, Tc, the thermal evolution of the lattice parameters and symmetry modes suggest the transition is continuous, although a small discontinuity in both the spontaneous strains and symmetry distortion modes shows the transition is strictly first order. Analysis of the refined structures reveals that the Nb and Ta cations are best described as having a distorted 6-coordinate arrangement in the monoclinic structure, with four short and two long bonds. Breaking of the two long bonds at high temperatures, resulting in a transformation of the Nb(Ta) coordination to a regular tetrahedron, is believed to be responsible for the first order nature of the transition.

Insights into the structural variations in SmNb1-xTaxO4 and HoNb1-xTaxO4 combined experimental and computational studies.

The impact of Ta doping on two orthoniobates SmNbO4 and HoNbO4 has been studied using a combination of high-resolution powder diffraction and Density-Functional Theory calculations. In both ANb1-xTaxO4 (A = Sm, Ho) series the unit cell volume decreases as the Ta content increased demonstrating that the effective ionic radii of Ta is smaller than that of Nb in this structure. The average Sm-O distance and volume of the SmO8 polyhedra were invariant of the Ta content across the SmNb1-xTaxO4 solid solution whereas the average M-O (M = Nb or Ta) distance and MO6 polyhedral volume decrease with Ta doping. The analogous Ho oxides HoNb1-xTaxO4 do not form a complete solid solution when the samples were prepared at 1400 °C, rather there is a miscibility gap around x = 0.95, with HoTaO4 exhibiting the M'-type P2/c structure rather than the M-type I2/a structure of HoNbO4. Increasing the synthesis temperature to 1450 °C eliminates the miscibility gap. The energy difference between the P2/c and I2/a structures of HoTaO4 is found to be nearly 30 meV per f.u. with the total energy of the P2/c phase of HoTaO4 being more negative. First-principles calculations, carried out using Density-Functional Theory, reveal significant covalent character in the Nb-O bonds, which is reduced in the corresponding tantalates. Anisotropy in the Born Effective Charge tensors demonstrates the impact of the long M-O bond identified in the structural studies showing that the Nb and Ta cations are effectively six-coordinate. The similarity in the frequency of the intense Raman peak near 800 cm-1 due to the symmetric stretching of the Ta-O bonds is consistent with the description of that both polymorphs of HoTaO4 contain TaO6 octahedra.

Effect of Long- and Short-Range Disorder on the Oxygen Ionic Conductivity of Tm2(Ti2-xTmx)O7-x/2 "Stuffed" Pyrochlores.

The long-range average and short-range local structures in the Tm2(Ti2-xTmx)O7-x/2 (x = 0.00-0.67) series were studied using a combination of diffraction and spectroscopic techniques. The long-range average structure, established from synchrotron X-ray and neutron powder diffraction data, shows the development of multiphase regions from x = 0.134 and the formation of antisite cation disorder from x = 0.402. The crystal field splitting of the Ti4+ ions, as derived from the Ti L3-edge X-ray absorption near-edge structure (XANES) spectroscopy, decreases gradually from 2.17 to 1.92 eV with increasing Tm3+ content (x), reflecting the increase in coordination number from 6 to predominantly 7. This is consistent with a gradual evolution of the short-range local disorder from x = 0.00 to 0.67. These results suggest that local disorder develops gradually throughout the entire composition range, whereas changes in the long-range disorder occur more suddenly. Electrochemical impedance spectroscopic results show an increase in oxygen ionic conductivity at 1000 °C, by a factor of 4 upon doping at x = 0.268. This suggests that inducing small amounts of disorder into the pyrochlore structure, by stuffing, may lead to applications of this material as a solid electrolyte in solid-oxide fuel cells.