Browsing by Author "Mondal, S"

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    Beyond the ionic radii: A multifaceted approach to understand differences between the structures of LnNbO4 and LnTaO4 fergusonites
    (Elsevier, 2023-01-05) Mullens, BG; Saura-Múzquiz, M; Marlton, FP; Avdeev, M; Brand, HEA; Mondal, S; Vaitheeswaran, G; Kennedy, BJ
    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. © 2022 Elsevier B.V.
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    Insight into the structural variations of fergusonite-type structures: combined experimental and computational studies
    (International Union of Crystallography, 2021-08-14) Mullens, BG; Avdeev, M; Brand, HEA; Mondal, S; Vaitheeswaran, G; Kennedy, BJ
    The development of carbon-neutral energy-generation is critical to combatting climate change. One such technology is the development of next-generation ion conductors for solid-oxide fuel cells (SOFCs). SOFCs offer a much more efficient method to extract energy from hydrogen or hydrocarbon fuels than current combustion engines due to their one-step chemical process. However, a bottleneck to the large-scale uptake of SOFCs is the poor performance of the conducting electrolytes that separate the anode from the cathode. Various lanthanoid fergusonite structures (LnBO4) have recently been proposed as solid electrolyte candidates in solidoxide fuel cells, with increased high-temperature ionic conductivity being measured in chemically doped lanthanum orthoniobates (LaNbO4) [1]. However, a phase transition from I2/a to I41/a within the operational temperature of SOFCs makes these structures nonideal. To understand the effects of chemical doping on the structure and electrochemical properties of these fergusonite structures, several complex fergusonites have been investigated [2-3]. Of interest is the substitution of NbV for TaV on the B-site, which has shown a decrease in the unit cell volume of the structure [4]. This is particularly remarkable, given the two metal cations have the same ionic radius and Ta has an extra 5d valence shell compared to the 4d shell of Nb. Such substitution has further shown to increase the I2/a to I41/a first-order phase transition temperature, highlighting the potential of the properties of these structures to be specifically ‘tailored’ to be used for SOFCs. Various solid-solution series of Ln(Nb1-xTax)O4 (Ln = La-Lu) have been synthesised using conventional solid-state methods. Synchrotron X-ray and neutron powder diffraction methods have been used to investigate their structures, focusing on changes in both their unit cell volumes and the temperature of the I2/a to I41/a phase transitions. Whilst the fergusonite structure is a monoclinic structure derived of the tetragonal scheelite aristotype, its structure is based on BO6 polyhedra as opposed to BO4 scheelite polyhedra. These studies have revealed several anomalies, revealing that different structures can be isolated by controlling the size of the Ln ion and synthetic conditions, and that the volume of the BO6 polyhedra and length of the B–O bonds change depending on its surrounding Ln ion. This data surprisingly implies that the AO8 polyhedra act as a rigid framework in which the BO6 polyhedra respond. The experimental data has been further reinforced by ground state energy calculations performed using density functional theory. This is a landmark accomplishment that has not been previously used in similarly studied structures. These insights can be used in the development and engineering of novel and advanced electrolyte materials for SOFCs. © The Authors
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    Insights into the structural variations in SmNb1−xTaxO4 and HoNb1−xTaxO4 combined experimental and computational studies
    (Royal Society of Chemistry, 2021-06-01) Mullens, BG; Avdeev, M; Brand, HEA; Mondal, S; Vaitheeswaran, G; Kennedy, BJ
    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. © The Royal Society of Chemistry 2021
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    Structural transformation in inverse-perovskite REPt3B (RE = Sm and Gd–Tm) associated with large volume reduction
    (American Chemical Society, 2017-07-05) Mondal, S; Mazumdar, C; Ranganathan, R; Avdeev, M
    In this work, we report the structural phase transformation of tetragonal inverse-perovskite REPt3B (RE = Sm, and Gd–Tm) compounds to cubic perovskite structure, with a large volume reduction of about 9% (reduction of the c axis, ∼17%; increase in the a axis, ∼5%). The structural stability of the cubic phase, however, could only be maintained by lowering the lattice parameter of the off-stoichiometric REPt3Bx (x < 1), formed in the process of annealing. The combined effect of phase transformation and stoichiometric defects is argued to be responsible for the observed volume collapse. Unexpectedly, the application of a large hydrostatic pressure of ∼20 GPa does not have any significant effect on the crystal structure. Neutron diffraction studies and heat capacity measurements unambiguously confirm different magnetic transition temperatures in the tetragonal and cubic phases. The different physical properties of these two phases demonstrate the interrelationship between the crystal chemistry and the physics of the system. The synthetic route to cubic REPt3Bx identified in this work may be utilized to prepare new ternary rare-earth intermetallics in a cubic perovksite form, which was previously found to facilitate unconventional superconductivity. © 2017 American Chemical Society