Browsing by Author "Saura-Múzquiz, M"
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- ItemBeyond 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, BJSynchrotron 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.
- ItemCation 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, BJThe 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
- ItemCation order in mixed metal bismuth scheelite Bi3FeMo2O12(Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Saura-Múzquiz, M; Gaza, M; Maynard-Casely, HE; Kennedy, BJThe scheelites are a family of compounds with chemical formula AB O4 where A and B can represent a variety of different cations. The highly versatile scheelite crystal structure consists of A O8 dodecahedra and B O4 tetrahedra and gives rise to a variety of interesting properties depending on the combination of cations.1 Scheelite-type oxides including CaWO4, BiVO4 and NaLa(MoO4)2 have been extensively studied for applications exploiting some of these properties including luminescence, ferroelectricity, ionic conductivity and photocatalytic activity. In particular, Bi3+ containing molybdates are efficient photocatalysts2, 3 due to the strong repulsive force of the 6s2 lone pair of Bi3+, resulting in distortion of the B O4 tetrahedra and alteration of the band gap. The compound of interest in the present study, Bi3FeMo2O12 (BFMO), was reported by Sleight et al. in 1974 as the first scheelite type compound containing trivalent cations on the tetrahedral site.4 Notably, two different polymorphs of BFMO can be isolated.5 The ideal tetragonal scheelite-type structure in space group I 41/ a (#88) can be prepared by a wet chemical route from aqueous solution of the constituent elements. Jeitschko et al. reported in 1975 that, when the tetragonal scheelite structure is heated above 600 C° for ~10 h, a 2:1 ordering of the Mo and Fe cations occurs, which lowers the symmetry to monoclinic in space group C 2/ c (#15), and gives rise to a tripling of the a axis. Here, phase pure BFMO in the disordered tetragonal structure was synthesized by a wet chemical route. The conversion from the disordered tetragonal to the ordered monoclinic structure was examined by in situ neutron powder diffraction in order to understand the temperature dependence of the phase transition and cation order in the mixed metal bismuth scheelite. The study shows no amorphization prior to the formation of the ordered monoclinic phase. Given the substantial cation movement involved in such a transformation, the lack of structural break-down suggests that a certain degree of local cation order may already exist in the tetragonal phase, facilitating the conversion into a fully ordered monoclinic structure. This hypothesis is further supported by an opening in the field-dependent magnetization curve of the tetragonal phase at 1.8 K. © The authors.
- ItemDisorder 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, MCarbon-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
- ItemElucidating the relationship between nanoparticle morphology, nuclear/magnetic texture and magnetic performance of sintered SrFe12O19 magnets(Royal Society of Chemistry, 2020-04-22) Saura-Múzquiz, M; Eikeland, AZ; Stingaciu, M; Andersen, HL; Granados-Miralles, C; Avdeev, M; Luzin, V; Christensen, MSeveral M-type SrFe12O19 nanoparticle samples with different morphologies have been synthesized by different hydrothermal and sol–gel synthesis methods. Combined Rietveld refinements of neutron and X-ray powder diffraction data with a constrained structural model reveal a clear correlation between crystallite size and long-range magnetic order, which influences the macroscopic magnetic properties of the sample. The tailor-made powder samples were compacted into dense bulk magnets (>90% of the theoretical density) by spark plasma sintering (SPS). Powder diffraction as well as X-ray and neutron pole figure measurements and analyses have been carried out on the compacted specimens in order to characterize the nuclear (structural) and magnetic alignment of the crystallites within the dense magnets. The obtained results, combined with macroscopic magnetic measurements, reveal a direct influence of the nanoparticle morphology on the self-induced texture, crystallite growth during compaction and macroscopic magnetic performance. An increasing diameter-to-thickness aspect ratio of the platelet-like nanoparticles leads to increasing degree of crystallite alignment achieved by SPS. Consequently, magnetically aligned, highly dense magnets with excellent magnetic performance (30(3) kJ m−3) are obtained solely by nanostructuring means, without application of an external magnetic field before or during compaction. The demonstrated control over nanoparticle morphology and, in turn, crystal and magnetic texture is a key step on the way to designing nanostructured hexaferrite magnets with optimized performance. © Royal Society of Chemistry 2020
- ItemExperimental 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, BJThe 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.
- ItemInsight 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, BJThe 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(Nb1xTax)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
- ItemNanoengineered high-performance hexaferrite magnets by morphology-induced alignment of tailored nanoplatelets(American Chemical Society, 2018-11-15) Saura-Múzquiz, M; Granados-Miralles, C; Andersen, HL; Stingaciu, M; Avdeev, M; Christensen, MMagnetic materials are ubiquitous in electric devices and motors making them indispensable for modern-day society. The hexaferrites currently constitute the most widely used permanent magnets (PMs), accounting for 85% (by weight) of the global sales of PMs. This work presents a complete bottom-up nanostructuring protocol for preparation of magnetically aligned, high-performance hexaferrite PMs with a record-high (BH)max for dry-processed ferrites. The procedure includes the supercritical hydrothermal flow synthesis of anisotropic magnetic-single-domain strontium hexaferrite (SrFe12O19) nanocrystallites of various sizes, and their subsequent compaction into bulk magnets by spark plasma sintering (SPS). Interestingly, Rietveld modeling of neutron powder diffraction data reveals a significant difference between the magnetic structure of the thinnest nanoplatelets and the bulk compound, indicating the Sr-containing atomic layer to be the termination layer. Subsequently, high-density SrFe12O19 magnets (>95% of the theoretical density) are produced by SPS of the flow-synthesized nanoplatelets. Texture analysis by X-ray pole figure measurements demonstrates how the anisotropic shape of the nanoplatelets causes a self-induced alignment during SPS, without application of an external magnetic field. The self-induced texture is accompanied by crystallite growth along the magnetic easy-axis, i.e., the thickness of the platelets, resulting in high-performance PMs with square hysteresis curves and (BH)max of 30 kJ/m3. The (BH)max is further enhanced by annealing, reaching 36 kJ/m3 after 4 h at 850 °C, which exceeds the (BH)max of the highest grade of dry-processed commercial ferrites worldwide. © 2018 American Chemical Society
- ItemNeutron diffraction studies of nanostructured SrFe12O19 magnets(International Conference on Neutron Scattering, 2017-07-12) Saura-Múzquiz, M; Stingaciu, M; Eikeland, AZ; Andersen, HL; Granados-Miralles, C; Lucin, V; Avdeev, M; Christensen, MPhase pure, highly crystalline SrFe12 19 nanoparticles have been synthesized by hydrothermal and sol-gel synthesis methods. By varying synthesis parameters and method, SrFe12 19 nanoplatelets of various sizes and morphologies can be obtained. The nuclear and magnetic structure of the samples have been studied by neutron and X-ray diffraction, revealing a clear size dependency on the long range magnetic order. Subsequent compaction of the tailor-made powder samples into bulk magnets is carried out by Spark Plasma Sintering. Powder diffraction as well as X-ray and neutron pole figure analyses were performed on the compacted magnets. The obtained results, together with macroscopic magnetic measurements, reveal a direct influence between nanoparticle morphology, texture and magnetic performance. The platelet-like morphology of the nanoparticles leads to highly aligned magnets without the need of an externally applied magnetic field. Therefore, by varying the morphology of the platelets prior to compaction, the final magnetic properties of the sample can be tuned. Meticulous characterization based on neutron and X-ray diffraction techniques reveals the relationship between synthesis conditions, crystal-, nano- and magnetic structure, and macroscopic magnetic performance. Extensive control over each step of the nanostructuring process is essential in the design of materials with tailored physical properties.
- ItemOxygen deficient lead-technetium pyrochlore, the first example of a stable valence V technetium oxide?(European Association of Geochemistry and the Geochemical Society (Goldschmidt), 2021-07-05) Thorogood, GJ; Avdeev, M; Carter, ML; Losurdo, L; Saura-Múzquiz, M; Thorogood, KJ; Ting, J; Wallwork, KS; Zhang, Z; Kennedy, BJDespite the fact that Technetium V oxides are possible there are very few reports of their existence. Most recently Lawler et.al. [1] have reported the structure of Tc2O5 “tech red” and have noted that it is indeed volatile. It is apparent from this study that there is no stable form and they draw parallels with a well-studied analogue of Tc2O5, Re2O5 that disprortioniates into Re(4+) and Re(7+) species. Given these parallels we investigated PbTcO3 as reported by Muller et.al [2] to be a pyrochlore in an attempt to determine if there were parallels with Pb2Re2O7-d. The structure of lead-technetium pyrochlore has been refined in space group with a = 10.36584(2) Å using a combination of synchrotron X-ray and neutron powder diffraction data and confirmed via Electron Diffraction. The oxide is found to be oxygen deficient with a stoichiometry of Pb2Tc2O6.86. The displacive disorder of the Pb cations is evident from the refinements as has been observed Bi2Tc2O7-d. X-ray absorption measurements at the Tc K-edge demonstrate the valence of the Tc is greater than 4.0 as anticipated from the refined oxygen stoichiometry. Raman spectroscopy confirms the local coordination of the Technetium leading us to conclude that this pyrochlore is the first example of a stable valence V Technetium oxide. [1] Lawler, K. V. et al. Unraveling the mystery of ‘tech red’-a volatile technetium oxide. Chem. Commun. 54, 1261–1264 (2018). [2] Muller, O., White, W. B. & Roy, R. Crystal chemistry of some technetium-containing oxides. J. Inorg. Nucl. Chem. 26, 2075–2086 (1964).
- ItemSynthesis and structure of oxygen deficient lead-technetium pyrochlore, the first example of a valence V technetium oxide(Frontiers Media, 2021-07-01) Kennedy, BJ; Ablott, TA; Avdeev, M; Carter, ML; Losurdo, L; Saura-Múzquiz, M; Thorogood, KJ; Ting, J; Wallwork, KS; Zhang, ZM; Zhu, HL; Thorogood, GJThe structure of lead-technetium pyrochlore has been refined in space group F d 3 ¯ m with a = 10.36584(2) Å using a combination of synchrotron X-ray and neutron powder diffraction data and confirmed via Electron Diffraction. The oxide is found to be oxygen deficient with a stoichiometry of Pb2Tc2O7-d. Displacive disorder of the Pb cations is evident from the refinements, as has been observed in Bi2Tc2O7-d. X-ray absorption spectroscopic measurements at the Tc K-edge demonstrate the valence of the Tc is greater than 4.0 as anticipated from the refined oxygen stoichiometry. Raman spectroscopy confirms the presence of disorder leading us to conclude that this pyrochlore is the first example of a valence V technetium oxide. © 2021 Kennedy, Ablott, Avdeev, Carter, Losurdo, Saura-Muzquiz, Thorogood, Ting, Wallwork, Zhang, Zhu and Thorogood. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY).