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 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

Long-Range A-Site Cation Disorder in NaA(MO4)2 (M = Mo, W) Double Scheelite Oxides

Synchrotron X-ray and neutron powder diffraction methods have been used to obtain accurate long-range average structures of some double scheelite compounds of the type NaA(BO4)2 (A = La, Pr, Nd, Sm, Lu, and Bi; B = Mo, W) at room temperature. Phase pure samples were synthesized using standard solid-state methods. Rietveld refinements using combined synchrotron X-ray diffraction (SXRD) and neutron diffraction (NPD) revealed a random distribution of the Na and A-type cations regardless of the presence of 6s2 lone pairs (such as Bi3+) and the difference in oxidation states and ionic radii between the cations. The NaA(BO4)2 (A = La, Pr, Nd, Sm, Lu, and Bi) series displayed linear trends in lattice parameters and AO8 polyhedra volume with the ionic radius of the A-type cation for the lanthanoids, but a deviation from the trend was observed for A = Bi3+. The NaBi(BO4)2 structure has a smaller than expected unit cell volume than based on extrapolation from the corresponding NaLn(BO4)2 series, possibly due to short-range ordering of the 6s2 lone pair electrons

Beyond the Ionic Radii: A Multifaceted Approach to Understand Differences between the Structures of LnNbO4 and LnTaO4 Fergusonites

Synchrotron X-ray powder diffraction methods have been used to obtain accurate structures of the lanthanoid tantalates, LnTaO4, at room temperature. Three different structures are observed, depending on the size of the Ln cation: P21/c (Ln = La, Pr), I2/a (Ln = Nd-Ho), and P2/c (Ln = Tb-Lu). BVS analysis indicated that TaV is six-coordinate in these structures, with four short bonds and two longer bonds. Synchrotron X-ray powder diffraction methods were also used to observe the impact of Ta doping on the orthoniobates, Ln(Nb1-xTax)O4 (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Yb and Lu). Where both the niobate and tantalate oxide were isostructural (fergusonite structure, space group I2/a), complete solid solutions were prepared. In these solid solutions, the unit cell volume decreases as the Ta content increases. The subtle interaction evident between the LnO8 and BO6 sublattices in the fergusonite-type oxides was not observed in the related pyrochlore oxides. A combined synchrotron X-ray and neutron powder diffraction study of the series Ho(Nb1-xTax)O4 was used to determine accurate atomic positions of the anions, and hence, bond lengths. This revealed a change in the (Nb/Ta)-O bond lengths, reflective of the difference in the valence orbitals of Nb(4d) and Ta(5d). Examination of the partial density of states demonstrates differences in the electronics between Nb and Ta, leading to a difference in the bandgap. This study highlights the importance of the long B-O contacts in the fergusonite structures, and its potential impact on the I2/a to I41/a phase transition

Cation and lone pair order-disorder in the polymorphic mixed metal bismuth scheelite Bi3FeMo2O12

The Bi3FeMo2O12 system is examined as a rare example of a transition metal oxide which, upon heating, undergoes a symmetry lowering and 2:1 ordering of the transition metal cations. The compound was synthesised in the tetragonal scheelite structure (S.G. #88: I41/a) by a sol-gel method and converted into the monoclinic polymorph (S.G. #15: C2/c) by calcination above 500 °C. The structure of both polymorphs was analysed using a combination of X-ray and neutron diffraction data, and the temperature-dependent phase transition between these was investigated in situ using variable temperature neutron powder diffraction and scanning transmission electron microscopy. The results show that the structural phase transition takes place at low temperature (~500 °C) and is 1st order in nature, as evident from the coexistence of both structures. The transition from tetragonal to monoclinic results in reduction of the equivalent unit cell volume. The role of the Bi3+ 6s lone pairs in the temperature-driven phase transition has been studied using neutron pair distribution function analysis. Local structure analysis via neutron total scattering revealed the Bi3+ 6s lone pairs to be stereochemically active in both structures, with short correlation lengths in the tetragonal structure and long correlation lengths in the monoclinic structure, leading to the facile phase conversion and to a more efficient packing density with highly correlated lone pairs in the monoclinic structure. Magnetization isotherms of the tetragonal structure collected at 1.8 K exhibit ferromagnetic behavior, suggesting that the interplay between the observed short-range monoclinic order, defects and surface-to-bulk effects alters the magnetic interaction, leading to short range ferromagnetic interactions, which is highly unexpected given the low temperature antiferromagnetic order observed in the monoclinic structure

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

Revealing phase boundaries by weighted parametric structural refinement

Parametric Rietveld refinement from powder diffraction data has been utilized in a variety of situations to understand structural phase transitions of materials in situ. However, when analysing data from lower-resolution two-dimensional detectors or from samples with overlapping Bragg peaks, such transitions become difficult to observe. In this study, a weighted parametric method is demonstrated whereby the scale factor is restrained via an inverse tan function, making the phase boundary composition a refinable parameter. This is demonstrated using compositionally graded samples within the lead-free piezoelectric (BiFeO3)x (Bi0.5K0.5TiO3)y (Bi0.5Na0.5TiO3)1-x-y and (Bi0.5Na0.5TiO3)x (BaTiO3) 1-x systems. This has proven to be an effective method for diffraction experiments with relatively low resolution, weak peak splitting or compositionally complex multiphase samples

Surface and structure engineering of MXenes for rechargeable batteries beyond lithium

With the rapid growth in renewable energy, researchers worldwide are trying to expand energy storage technologies. The development of beyond-lithium battery technologies has accelerated in recent years, amid concerns regarding the sustainability of battery materials. However, the absence of suitable high-performance materials has hampered the development of the next-generation battery systems. MXenes, a family of 2D transition metal carbides and/or nitrides, have drawn significant attention recently for electrochemical energy storage, owing to their unique physical and chemical properties. The extraordinary electronic conductivity, compositional diversity, expandable crystal structure, superior hydrophilicity, and rich surface chemistries make MXenes promising materials for electrode and other components in rechargeable batteries. This report especially focuses on the recent MXene applications as novel electrode materials and functional separator modifiers in rechargeable batteries beyond lithium. In particular, we highlight the recent advances of surface and structure engineering strategies for improving the electrochemical performance of the MXene-based materials, including surface termination modifications, heteroatom doping strategies, surface coating, interlayer space changes, nanostructure engineering, and heterostructures and secondary materials engineering. Finally, perspectives for building future sustainable rechargeable batteries with MXenes and MXene-based composite materials are presented based upon material design and a fundamental understanding of the reaction mechanisms

A Local Atomic Mechanism for Monoclinic-Tetragonal Phase Boundary Creation in Li-Doped Na0.5K0.5NbO3Ferroelectric Solid Solution

ABO3 perovskites display a wide range of phase transitions, which are driven by A/B-site centered polyhedral distortions and/or BO6 octahedral tilting. Since heterogeneous substitutions at the A/B-site can locally alter both polyhedral distortions and/or tilting, they are often used to create phase boundary regions in solid solutions of ABO3, where the functional properties are highly enhanced. However, the relationships between doping-induced atomistic structural changes and the creation of phase boundaries are not always clear. One prominent example of this is the Li-doped K0.5Na0.5NbO3 (KNNL), which is considered a promising alternative to traditional Pb-based ferroelectrics. Although the electromechanical properties of KNNL are enhanced for compositions near the morphotropic phase boundary (MPB), the atomistic mechanism for phase transitions is not well understood. Here, we combined neutron total scattering experiments and density functional theory to investigate the long-range average and short-range (∼10 Å) structural changes in KNNL. We show that the average monoclinic-to-tetragonal (M-T) transition across the MPB in KNNL can be described as an order-disorder-type change, which is driven by competition between a longer-range polarization field of monoclinic structural units and local distortions of the disordered AO12 polyhedra. The current study demonstrates a way to clarify dopant-induced local distortions near phase boundaries in complex solid solution systems, which will be important for the rational design of new environmentally sustainable ferroelectrics

Synthesis and crystal structures of two polymorphs of Li4–2xMg1+xTeO6

Two polymorphs of lithium magnesium tellurate Li4–2xMg1+xTeO6 have been prepared by solid-state reactions and their crystal structures characterised by powder X-ray and neutron diffraction. For x 0, a monoclinic C2/m phase is obtained, structurally similar to other O3 type honeycomb layered tellurate and antimonate compounds. The basic structure consists of [Mg2TeO6]3− honeycomb layers alternating with Li layers, with some anti-site disorder of Li and Mg between layers, analogous to the structure of Li4ZnTeO6. For 0 < x < ~0.5 (specifically, x = 0.33) an orthorhombic Fddd phase is obtained, with a rock-salt superstructure containing disordered Li/Mg cation sites surrounding ordered TeO6 octahedra, analogous to the structure of Li3Co2TaO6