Browsing by Author "Mullens, BG"

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    Average and local ordering of Yb2(Ti2-xYbx)O7-x/2 ‘stuffed’ pyrochlores: the development of a robust structural model
    (Elsevier, 2021-10-01) Mullens, BG; Zhang, Z; Avdeev, M; Brand, HEA; Cowie, BCC; D'Angelo, AM; Múzquiz, MS; Kennedy, BJ
    The long-range (average) and short-range (local) structures in the Yb2(Ti2-xYbx)O7-x/2 (x ​= ​0.00–0.67) series were studied using a combination of diffraction and spectroscopic techniques. The 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 anti-site disorder from x ​= ​0.335. The local structure, established from X-ray absorption near-edge structure (XANES), shows a gradual evolution of short-range disorder. The crystal field splitting energy of the Ti4+ ions decreases from 2.15 to 1.91 ​eV with increasing Yb3+ content, reflecting the increase in coordination number from 6 to predominantly 7. Electrochemical impedance spectroscopic studies show an increase in oxygen ionic conductivity by almost a factor of 3 upon doping with small amounts of Yb3+ (x ​= ​0.067). These results imply that the disordering across the anion and cation sublattices are different and inducing small amounts of disorder into the pyrochlore structure may lead to applications in solid-oxide fuel cells. © 2021 Elsevier Inc.
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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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    Cation order and magnetic behaviour in mixed metal bismuth scheelite Bi3FeMo2O12
    (International Union of Crystallography, 2021-08-14) Saura-Múzquiz, M; Mullens, BG; Liu, J; Vogt, T; Maynard-Casely, HE; Avdeev, M; Kennedy, BJ
    The scheelites are a family of compounds with chemical formula ABO4, and a characteristic crystal structure consisting of AO8 dodecahedra and BO4 tetrahedra. This structure is flexible and can accommodate a large variety of cations with a range of atomic radii and valence combinations. Scheelite-type oxides, such as CaWO4, BiVO4 and NaLa(MoO4)2 have been extensively studied due to their diverse range of physical and electronic properties [1]. In particular, Bi3+ containing molybdates have been found to be efficient photocatalysts due to the strong repulsive force of the 6s2 lone pair of Bi3+, resulting in distortion of the BO4 tetrahedra and alteration of the band gap [2, 3]. In 1974 Bi3FeMo2O12 (BFMO) was reported as the first scheelite-type compound containing trivalent cations on the tetrahedral sites [4]. Interestingly, two different polymorphs of BFMO can be isolated by varying the synthesis conditions [5]. The tetragonal scheelitetype polymorph, described by space group I41/a with a = 5.32106(13) Å and c = 11.656(4) Å, can be prepared by a sol-gel route from aqueous solution of the constituent ionic species and has a disordered arrangement of the Fe and Mo cations. When heated above 500 °C, a 2:1 ordering of the Mo and Fe cations occurs, which lowers the symmetry to monoclinic (C2/c). The corresponding superstructure has a tripling of the a axis (a = 16.9110 (3) Å, b = 11.6489(2) Å, c= 5.25630(9) Å, β = 107.1395(11)°). The two structures are illustrated in Figure 1. In the present study, both polymorphs of BFMO were synthesized and their structure and magnetic properties characterized using a combination of powder diffraction, microscopy and magnetometry techniques. In situ neutron powder diffraction (NPD) measurements of the structural evolution of disordered tetragonal BFMO with increasing temperature showed that no amorphization takes place prior to the formation of the ordered monoclinic phase. The lack of a structural break-down, despite the substantial cation movement required in such a transformation, suggests that a certain degree of local cation order exists in the “disordered” tetragonal phase, facilitating the direct conversion to the fully ordered monoclinic structure. Instead of the expected amorphization and recrystallization, the conversion takes place via a 1st order phase transition, with the tetragonal polymorph exhibiting negative thermal expansion prior to its conversion into the monoclinic structure. Zero-field-cooled/field-cooled and field-dependent magnetization curves of the monoclinic structure revealed the existence of a magnetic transition below 15 K. The long-range nature of the low temperature magnetic structure in the monoclinic polymorph was verified by high-resolution NPD data, which revealed the emergence of an incommensurate magnetic structure. There is no evidence for long-range magnetic order in the tetragonal polymorph. This is, to the best of our knowledge, the first study of the phase transition mechanism and magnetic properties of this complex system and represents a milestone in the structural understanding and targeted design of Bi3+ containing molybdates as efficient photocatalysts. © 2021 The Authors
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    Characterisation of Pb2Rh2O7 and Y2Rh2O7: an unusual case of pyrochlore stabilisation under high pressure, high temperature synthesis conditions
    (Royal Society of Chemistry (RSC), 2024-02-01) Injac, SD; Mullens, BG; Romero, FD; Avdeev, M; Barnett, C; Yuen, AKL; Patino, MA; Mukherjee, S; Vaitheeswaran, G; Singh, DJ; Kennedy, BJ; Shimakawa, Y
    Two novel oxides with Pb2Rh2O7 and Y2Rh2O7 compositions were synthesised using high pressure, high temperature techniques at 19 GPa and 8 GPa, respectively. Structurally, both compounds were determined to crystallise in the cubic pyrochlore structure, space group Fd[3 with combining macron]m, with no observed oxygen vacancies. Both oxides have effectively identical Rh–O bond lengths of 1.987 Å and a bond-valence sum (BVS) of 4.2 that confirm a Rh4+ oxidation state. Physical property measurements for Pb2Rh2O7 are consistent with a metallic ground state. This is similar to other Pb2M2O7 oxides where M = Ru, Ir, and Os. Y2Rh2O7 represents an unusual case of the lower density (6.356 g cm−3) pyrochlore structure being stabilised under high pressure conditions, while the analogous, higher density (7.031 g cm−3) perovskite YRhO3 is stabilised by synthesis under ambient pressure conditions. The Rh4+ state results in a S = ½ magnetic ground state. Magnetisation measurements suggest strong AFM coupling in Y2Rh2O7. However, long range AFM order is not observed down to 2 K presumably due to the geometric frustration of the pyrochlore lattice. Specific heat and resistivity measurements indicate a large electronic contribution to the heat capacity. The Wilson ratio of 4.78(11) is well above 2, indicating nearness to magnetism and the likely presence of Rh moments in the background of the conduction electrons. Catalytic activity indicated a greater correlation with other Rh pyrochlores as opposed to dependence on the Rh oxidation state. Facebook Twitter LinkedIn YouTube © Royal Society of Chemistry
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    Disorder by design: long- and short-range pyrochlore ordering
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Mullens, BG; Kennedy, BJ; Zhang, ZM; Saura-Múzquiz, M
    Carbon-neutral energy generation is being developed in order to combat climate change. Two technologies of current interest, which are related to renewable and nuclear energy respectively, are next-generation oxygenion conductors for fuel cells and materials suitable for long-term storage and disposal of radioactive nuclear wastes [1-2]. Pyrochlores of the structure A2B2O7 have found immense applications in each of the above areas. Ionic conductors for fuel cells require flexibility and movement in their anionic sublattice, whereas the storage of radioactive nuclear wastes needs a robust lattice from which ions cannot escape. This is a seemingly contradiction in requirements. It is believed that the oxygen vacancies present in the pyrochlore structure allow for the development of short-range disorder, whilst keeping the long-range order intact [3]. The pyrochlore structure can be viewed as a superstructure of the defect-fluorite structure. The defect-fluorite structure (A2BO5) consists of a random distribution of cations and oxygen vacancies. However, by choosing A and B with a particular ionic radii ratio, the ordered pyrochlore superstructure may form under ambient conditions. This ordering of oxygen vacancies may be analysed using neutron powder diffraction and used to reason the enhanced properties and applications of pyrochlores [4]. The current work aims to characterise oxygen-vacancy disorder in defect pyrochlores so to enable the rational design of defect pyrochlores that are optimised for specific applications. We have done this by looking at ‘stuffed’ pyrochlores of the form A2(B2−xAx)O7−x/2 where the smaller B-type cation, in this case Ti4+, is progressively replaced by a larger A-type cation (Tm3+). We wish to determine whether controlling the disorder in the cation sublattice will allow us to tailor-make stuffed pyrochlores targeting specific applications across ionic conductivity, magnetism, photocatalysis and the storage of long-term radioactive waste. Series of stuffed pyrochlores have been synthesised using conventional solid-state methods and their longrange average structures characterised by Rietveld refinement against combined neutron and synchrotron X-ray diffraction data. The local short-range order has been characterised by Raman spectroscopy and XANES. Other measurements have also been performed regarding their applications, demonstrating a vast improvement in their ionic conductivity at high temperatures. These results will be presented, along with a judgement as to whether inducing certain types of disorder within the pyrochlore structure can lead to them being purposely engineered for specific applications. © 2020 The Authors
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    Effect of long- and short-range disorder on the oxygen ionic conductivity of Tm2(Ti2–xTmx)O7–x/2 “stuffed” pyrochlores
    (American Chemical Society, 2021-03-10) Mullens, BG; Zhang, Z; Avdeev, M; Brand, HEA; Cowie, BCC; Múzquiz, MS; Kennedy, BJ
    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. © 2021 American Chemical Society
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    Experimental and computational insights into the anomalous thermal expansion of (Nh4)Reo4
    (Elsevier, 2022-11) Saura-Múzquiz, M; Mullens, BG; Avdeev, M; Jharapla, Prathap KJ; Vaitheeswaran, G; Gupta, MK; Mittal, R; Kennedy, BJ
    The temperature dependence of the structure and the ground state properties of scheelite-type NH4ReO4 have been studied using neutron powder diffraction (NPD) and Density Functional Theory (DFT), respectively. Despite the large incoherent background in the experimental NPD, associated with the presence of hydrogen, accurate and precise structural parameters were obtained. Comparison of the results of the NPD and DFT studies shows that the observed anomalous thermal contraction in NH4ReO4 is a consequence of thermally induced rotational disorder of the NH4 groups. Comparing the experimentally determined and optimized structures reveals deformation of the NH4 tetrahedra that is responsible for the unusual tetragonal distortion of this material. The Raman spectra of NH4ReO4 is presented and the modes are assigned based on the DFT calculations. © 2022 Elsevier Inc.
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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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    Insight into the variations of ABO4 structures: combined experimental and computational studies
    (Australian Nuclear Science and Technology Organisation, 2021-11-23) Mullens, BG; Saura-Múzquiz, M; Marlton, FP; Brand, HEA; Avdeev, M; 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 more efficient method of extracting 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 ABO4 structures have recently been proposed as solid electrolyte candidates in SOFCs, with increased hightemperature ionic conductivity being measured in chemically doped LaNbO4. However, the various phase transitions of these materials within the operational temperature of SOFCs makes them non-ideal. To understand the effects of chemical doping on the structure and electrochemical properties, several complex ABO4 structures have been investigated. In this work, we present the solid-solution series Ln(Nb1􀀀xTax)O4 (Ln = La-Lu). Using a combination of synchrotron X-ray and neutron powder diffraction methods, these studies have revealed several anomalies across the series. The structures appear to be sensitive to the size of the Ln cation and their synthesis conditions, with a difference in ionic conduction performance being observed. This 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