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Browsing by Author "Avdeev, M"

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100-position robotic sample changer for powder diffraction with low-background vacuum chamber
(Wiley-Blackwell, 2010-04) Olsen, SR; Pullen, SA; Avdeev, M
At the new Australian OPAL research reactor, experiments carried out at room temperature use a substantial fraction of beam time on the high-resolution powder diffractometer, Echidna. With an average data collection time of 2 h and a complicated safety interlock system to protect users, the need for a fully automated and remotely controlled system was quickly realized. This report presents a solution based on a commercial four-axis robot capable of loading samples from two 50-position sample trays, in any order, into an automatically evacuated chamber. This chamber significantly reduced background signal arising from air scattering, with the effect being especially pronounced at low and high 2 angles. In the case of textured or inhomogeneous samples, the system may be re-configured so that the robot rotates the sample in the beam or translates it vertically through the beam. © 2010, Wiley-Blackwell.
“114”-Type nitrides LnAl(Si4−xAlx)N7Oδ with unusual [AlN6] octahedral coordination
(Wiley, 2017-02-28) Huang, S; Huang, Z; Cao, P; Zujovic, Z; Price, JR; Avdeev, M; Que, M; Suzuki, F; Kido, T; Ouyang, X; Kaji, H; Fang, M; Liu, YG; Gao, W; Söhnel, T
Aluminum–nitrogen six-fold octahedral coordination, [AlN6], is unusual and has only been seen in the high-pressure rocksalt-type aluminum nitride or some complex compounds. Herein we report novel nitrides LnAl(Si4−xAlx)N7Oδ (Ln=La, Sm), the first inorganic compounds with [AlN6] coordination prepared via non-high-pressure synthesis. Structure refinements of neutron powder diffraction and single-crystal X-ray diffraction data show that these compounds crystallize in the hexagonal Swedenborgite structure type with P63mc symmetry where Ln and Al atoms locate in anticuboctahedral and octahedral interstitials, respectively, between the triangular and Kagomé layers of [SiN4] tetrahedra. Solid-state NMR data of high-purity La-114 powders confirm the unusual [AlN6] coordination. These compounds are the first examples of the “33-114” sub-type in the “114” family. The additional site for over-stoichiometric oxygen in the structure of 114-type compounds was also identified. © 1999-2021 John Wiley & Sons, Inc.
3d-electron Heisenberg pyrochlore Mn2Sb2O7
(American Physical Society, 2016-11-21) Peets, DC; Sim, H; Avdeev, M; Park, JG
In frustrated magnetic systems, geometric constraints or the competition among interactions introduce extre-mely high degeneracy and prevent the system from readily selecting a low-temperature ground state. The most frustrated known spin arrangement is on the pyrochlore lattice, but nearly all magnetic pyrochlores have unquenched orbital angular momenta, constraining the spin directions through spin-orbit coupling. Pyrochlore Mn2Sb2O7 is an extremely rare Heisenberg pyrochlore system with directionally unconstrained spins and low chemical disorder. We show that it undergoes a spin-glass transition at 5.5 K, which is suppressed by disorder arising from Mn vacancies, indicating this ground state to be a direct consequence of the spins' interactions. The striking similarities to 3d transition-metal pyrochlores with unquenched angular momenta suggests that the low spin-orbit coupling in the 3d block makes Heisenberg pyrochlores far more accessible than previously imagined. ©2016 American Physical Society
3DBVSMAPPER : a program for automatically generating bond-valence sum landscapes. Corrigendum
(International Union of Crystallography, 2017-12-01) Sale, M; Avdeev, M
An equation in the article by Sale & Avdeev [J. Appl. Cryst. (2012), 45, 1054–1056] is corrected. © International Union of Crystallography.
3DBVSMAPPER: a program for automatically generating bond-valence sum landscapes
(International Union of Crystallography, 2012-10-01) Sale, M; Avdeev, M
A computer program, 3DBVSMAPPER, was developed to generate bond-valence sum maps and bond-valence energy landscapes with minimal user intervention. The program is designed to calculate the spatial distributions of bond-valence values on three-dimensional grids, and to identify infinitely connected isosurfaces in these spatial distributions for a given bond-valence mismatch or energy threshold and extract their volume and surface area characteristics. It is implemented in the Perl scripting language embedded in Accelrys Materials Studio and has the capacity to process automatically an unlimited number of materials using crystallographic information files as input. © International Union of Crystallography
A3A′3Zn6Te4O24 (A = Na, A′ = rare earth) garnets: A-site ordered noncentrosymmetric structure, photoluminescence, and Na-ion conductivity
(American Chemical Society, 2021-11-16) Fang, Z; Jiang, P; Avdeev, M; Wei, H; Wang, R; Jiang, X; Yang, T
A large number of oxides that adopt the centrosymmetric (CS) garnet-type structure (space group Ia3̅d) have been widely studied as promising magnetic and host materials. Hitherto, no noncentrosymmetric (NCS) garnet has been reported yet, and a strategy to NCS garnet design is therefore significant for expanding the application scope. Herein, for the series A3A′3Zn6Te4O24 (A = Na, A′ = La, Eu, Nd, Y, and Lu), we demonstrated that the structural symmetry evolution from CS Ia3̅d (A′ = La) to NCS I4122 (A′ = Eu, Nd, Y, and Lu) could be achieved due to the A-site cationic ordering-driven inversion symmetry breaking. Na3A′3Zn6Te4O24 (A′ = rare earth) are the first garnets that possess NCS structures with A-site cationic ordering. Diffuse reflectance spectra and theoretic calculations demonstrated that all these NCS garnets are indirect semiconductors. Moreover, their potential applications as host materials for red phosphors and Na-ion conductors were also investigated in detail, which firmly confirmed the NCS structure and A-site cationic ordering. Our findings have paved the way to design NCS or even polar garnets that show intriguing functional properties, such as ferroelectricity, multiferroicity, and second harmonic generation. © 2021 American Chemical Society
Ab initio thermodynamic optimization of Ni-rich Ni–Co–Mn oxide cathode coatings
(Elsevier, 2020-02-29) Liu, B; Liu, JH; Yang, J; Wang, D; Ye, CC; Wang, DY; Avdeev, M; Shi, S; Yang, JH; Zhang, WQ
The effectiveness of surface coatings in improving the stability and cycling performance of cathodes has been demonstrated since they are first proposed in the 1990's. However, the progress since then is made mostly using the trial-and-error method. Herein, an automated electrochemical-chemical stability design scheme based on first-principles thermodynamics calculations of reaction models is presented to optimize coatings for Ni-rich nickel–cobalt–manganese oxide (NCM) cathodes. Given that the coating must possess a wider electrochemical window than the cathode without the occurrence of Li-ion redistribution at the cathode/coating interface, the reaction energies of both lithium insertion/extraction and decomposition process associated with the coating are used as one of the two screening criteria. As the coating is also required to be chemically stable in Li residues and hydrofluoric-acid containing liquid environment, the positive reaction energy achieved by adjusting molar ratio of the components is used as another criterion. Using these two screening criteria, we demonstrate that lithium-containing metal phosphates, rather than previously suggested Li-containing metal oxides, are the optimal coatings for Ni-rich NCM cathodes, which is confirmed experimentally. The proposed approach is general and can be used to find optimal coating materials for any other cathodes. © 2020 Elsevier B.V.
Aliovalent M site substitutions of [PO4]3− and [HfO4]4− for [AlO4]5− and [GaO4]5− in the Sr3MO4F-type anti-perovskite oxyfluoride
(IOP Publishing, 2019-01-17) Keil, ST; Chhoeun, J; Avdeev, M; Sullivan, E
Rare earth-free phosphor materials have been synthesized via aliovalent doping of [PO4]3− and [HfO4]4− for [AlO4]5− and [GaO4]5− in the Sr3MO4F host lattice (space group I4/mcm). High temperature synthesis was used to synthesize the novel products Sr2.5-xBa0.5Al1-xPxO4F (0.05 ≤ x ≤ 0.15), Sr2.5-xBa0.5Ga1-xPxO4F (0.05 ≤ x ≤ 0.15) and (0.025 ≤ x ≤ 0.075). The structures of these oxyfluorides were analyzed via powder X-ray diffraction (PXRD) and neutron powder diffraction (NPD), and their photoluminescent properties were analyzed through spectrofluorimetry before and after being treated under reducing conditions. Optimal doping was found at a concentration of x = 0.10 for Sr2.5-xBa0.5Al1-xPxO4F (a = 6.8762(4) Å; c = 11.1273(6) Å) resulting in a maximum emission at 454 nm when excited at 214 nm and x = 0.05 for the Ga analog (a = 6.87591(7) Å; c = 11.3722(2) Å) corresponding to an emission centered around 529 nm from excitation with 227 nm. When doping with Hf4+ the optimal concentration for was x = 0.025 (a = 6.8607(1) Å; c = 11.1682(3) Å), resulting in a broad emission between 410–600 nm when excited at 251 nm. © 2019 The Electrochemical Society
Alkali metal cation and proton conductors: relationships between composition, crystal structure, and properties
(Wiley, 2009-07-15) Avdeev, M; Nalbandyan, VB; Shukaev, IL
This chapter contains sections titled: Principles of Classification, and General Comments; Crystal‐Chemistry Factors Affecting Cationic Conductivity; Crystal Structural Screening and Studies of Conduction Paths; Conductors with Large Alkali Ions; Lithium Ion Conductors; Proton Conductors; References. © 2009 Wiley‐VCH Verlag GmbH & Co. KGaA
Alkali metal-modified P2 NaxMnO2: crystal structure and application in sodium-ion batteries
(American Chemical Society, 2020-08-18) Sehrawat, D; Rawal, A; Cheong, S; Avdeev, M; Ling, CD; Kimpton, JA; Sharma, N
Sodium-ion batteries (NIBs) are an emerging alternative to lithium-ion batteries because of the abundance of sodium resources and their potentially lower cost. Here we report the Na0.7MnO2 solid state synthesized at 1000 °C that shows two distinct phases; one adopts hexagonal P2-type P63/mmc space group symmetry, and the other adopts orthorhombic Pbma space group symmetry. The phase ratio of P2 to the orthorhombic phase is 55.0(5):45.0(4). A single-phase P2 structure is found to form at 1000 °C after modification with alkali metals Rb and Cs, while the K-modified form produces an additional minor impurity. The modification is the addition of the alkali elements during synthesis that do not appear to be doped into the crystal structure. As a cathode for NIBs, parent Na0.7MnO2 shows a second charge/discharge capacity of 143/134 mAh g–1, K-modified Na0.7MnO2 a capacity of 184/178 mAh g–1, Rb-modified Na0.9MnO2 a capacity of 159/150 mAh g–1, and Cs-modified Na0.7MnO2 a capacity of 171/163 mAh g–1 between 1.5 and 4.2 V at a current density of 15 mA g–1. The parent Na0.7MnO2 is compared with alkali metal (K, Rb, and Cs)-modified NaxMnO2 in terms of surface morphology using scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy, scanning electron microscopy, 23Na solid-state nuclear magnetic resonance, and X-ray photoelectron spectroscopy and in terms of electrochemical performance and structural electrochemical evolution using in situ or operando synchrotron X-ray diffraction. © 2020 American Chemical Society
Analysis of neutron-scattering data using atomistic modelling methods
(Australian Nuclear Science and Technology Organisation, 2009) Kearley, GJ; Avdeev, M
In many cases the scientific value of neutron-scattering data can be increased considerably by using atomistic or molecular modelling methods as an aid to understanding the experimental data. We give examples of how modelling helps to choose between ambiguous analyses, provide starting models for analytical methods and give understanding of how the structure and dynamics of a material relate to its function. © Australian Nuclear Science and Technology Organisation
Anion Disorder in Lanthanoid Zirconates Gd2-xTbxZr2O7
(American Chemical Society., 2013-08-05) Reynolds, EM; Blanchard, PER; Kennedy, BJ; Ling, CD; Liu, S; Avdeev, M; Zhang, Z; Cuello, GJ; Tadich, A; Jang, LY
The pyrochlore?defect fluorite order?disorder transition has been studied for a series of oxides of the type Gd2?xTbxZr2O7 by a combination of diffraction and spectroscopy techniques. Synchrotron X-ray diffraction data suggest an abrupt transition from the coexistence of pyrochlore and defect fluorite phases to a single defect fluorite phase with increasing Tb content. However neutron diffraction data, obtained at ? ≈ 0.497 Å for all Gd-containing samples to minimize absorption, not only provide evidence for independent ordering of the anion and cation sublattices but also suggest that the disorder transition across the pyrochlore?defect fluorite boundary of Ln2Zr2O7 is rather gradual. Such disorder was also evident in X-ray absorption measurements at the Zr L3-edge, which showed a gradual increase in the effective coordination number of the Zr from near 6-coordinate in the pyrochlore rich samples to near 7-coordinate in the Tb rich defect fluorites. These results indicate the presence of ordered domains throughout the defect fluorite region, and demonstrate the gradual nature of the order?disorder transition across the Gd2?xTbxZr2O7 series. © 2013, American Chemical Society.
Anomalous thermal expansion in orthorhombic perovskite SrIrO3: Interplay between spin-orbit coupling and the crystal lattice
(Americal Physical Society, 2014-06-16) Blanchard, PER; Reynolds, EM; Kennedy, BJ; Kimpton, JA; Avdeev, M; Belik, AA
The structure of the orthorhombic (Pbnm) polytype of SrIrO3 has been investigated between 3 and 1100 K using a combination of synchrotron and neutron diffraction methods. The orthorhombic structure persists to 1100 K, the highest temperature available in this work. This is a consequence of the larger than expected octahedral tilting estimated from the neutron diffraction studies. We postulate that the strong spin-orbit coupling of the Ir4+ cation, which splits the t2g band, introduces additional strain on the lattice. This introduces unusual thermal expansion of the cell. SrIrO3 was characterized by resistivity, magnetization, and specific heat measurements. Metallic conductivity was observed between 2 and 300 K without indication of the previously reported metal-insulator transition. The Sommerfeld constant γ was 3.12(2) mJ mol−1 K−2, and a Fermi-liquid behavior was observed between 2 and 30 K with positive magnetoresistence of up to 2% (at 70 kOe and between 2 and 50 K).© 2014, American Physical Society.
Antiferromagnetic exchange weakening in the TbRhIn5 intermetallic system with Y-substitution
(Elsevier, 2018-07-01) Amaral, RP; Lora-Serrano, R; Garcia, DJ; Iwamoto, WA; Betancourth, D; Cadogan, JM; Muñoz-Pérez, S; Avdeev, M; Cobas-Acosta, R; Bittar, EM; Duque, JGS; Pagliuso, PG
We report measurements of the temperature dependent specific heat and magnetic susceptibility in single crystals of the series of intermetallic compounds Tb1−xYxRhIn5 (nominal concentrations x = 0.0, 0.15, 0.3, 0.4, 0.5, 0.7). A mean field approximation to simulate the macroscopic properties along the series has been used. Neutron diffraction data in powdered samples of Tb0.6Y0.4RhIn5 and Tb0.6La0.4RhIn5 reveal antiferromagnetic (AFM) propagation vector k→=[12012] with the magnetic moments oriented along the tetragonal c axis or canted from this axis, respectively for Y and La-substitutions. Considering both the simulations of the magnetic exchange and neutron diffraction data, we discuss the role of combined effects of crystalline electric field (CEF) perturbations and dilution in the evolution of magnetic properties with Y and La contents. In particular, we found negligible variations of the Bnm parameters along the Y series. The decrease of TN with x is fully dominated by magnetic dilution effects. © 2018 Elsevier Ltd.
Antiferromagnetic order of ferromagnetically coupled dimers in the double pyrovanadate CaCoV2O7
(Cornell University, 2021-08-02) Murasaki, R; Nawa, K; Okuyama, D; Avdeev, M; Sato, TJ
Magnetic properties of the pyrovanadate CaCoV2O7 have been studied by means of the bulk magnetization and neutron powder diffraction measurements. Magnetic susceptibility in the paramagnetic phase shows Curie-Weiss behavior with negative Weiss temperature ≃ −22.5 K, indicating dominant antiferromagnetic interactions. At TN = 3.44 K, CaCoV2O7 shows antiferromagnetic order, accompanied by a weak net ferromagnetic moment of ∼ 0.05 μB/Co2+. Neutron powder diffraction confirms the formation of antiferromagnetic order below TN. It was further confirmed from the magnetic structure determination that the two Co2+ ions in the adjacent edge-sharing octahedra have almost parallel (ferromagnetic) spin arrangement, indicative of a formation of a ferromagnetic spin dimer. The antiferromagnetic order is, in turn, stabilized by sizable inter-dimer antiferromagnetic interactions.
Antiferromagnetic spin structure and negative thermal expansion of Li2Ni(WO4)2
(American Physical Society, 2015-07-10) Karna, SK; Wang, CW; Sankar, R; Avdeev, M; Singh, A; Panneer Muthuselvam, I; Singh, VN; Guo, GY; Chou, FC
We report the results of a study on the crystal and magnetic structure of Li2Ni(WO4)2 with a neutron diffraction technique. The Ni2+ spins of S = 1 for NiO6 octahedra are coupled via corner-sharing, nonmagnetic double tungstate groups in a super-superexchange route. Two magnetic anomalies at TN1∼ 18 K and TN2∼ 13 K are revealed from the measured magnetic susceptibility χ(T), and TN2 is confirmed to be the onset of a commensurate long-range antiferromagnetic (AF) ordering through neutron diffraction. A negative thermal expansion phenomenon is observed below TN2, which has been interpreted as a result of competing normal thermal contraction and long-range AF spin ordering through counterbalanced WO4 and NiO6 polyhedral local distortion. The AF spin structure has been modeled and used to show that Ni spins with a saturated magnetic moment of ∼1.90(27)μB that lies in the a−c plane approximately 46∘(±10∘) off the a axis. The experimental results are compared and found to be consistent with theoretical calculations using density-functional theory with a generalized gradient approximation plus on-site Coulomb interaction. ©2015 American Physical Society
Antiferromagnetism and metamagnetism in ErFeCuGe4O12
(Elsevier, 2018-09-15) Xu, D; Avdeev, M; Battle, PD
Polycrystalline ErFeCuGe4O12 has been prepared in a solid-state reaction. It adopts a tetragonal crystal structure; space group P4/nbm with a = 9.6416(1), c = 4.7532(1) at room temperature. The Er3+ cations are in square-antiprismatic coordination and the Fe3+ and Cu2+ cations are disordered over one six-coordinate site. The magnetic moments of the three cations adopt an antiferromagnetic arrangement on cooling below 20 K in H = 0 kOe. The magnetic structure consists of ferromagnetic (001) sheets with the spin direction in neighbouring sheets alternating between [001] and [00 ̅At 5 K the ordered moment of Er3+ was determined by neutron diffraction to be 7.90(3) µB and the mean moment of Fe3+ and Cu2+ was 2.43(2) µB. The magnetic structure is unchanged in an applied field of 10 kOe but in fields ≥ 20 kOe the compound begins a metamagnetic transition to a ferromagnetic structure with all atomic moments aligned along [001]. © 2018 Elsevier Inc.
Antiferromagnetism in a technetium oxide. Structure of CaTcO3
(American Chemical Society, 2011-01-16) Avdeev, M; Thorogood, GJ; Carter, ML; Kennedy, BJ; Ting, J; Singh, DJ; Wallwork, KS
The technetium perovskite CaTcO3 has been synthesized. Combining synchrotron X-ray and neutron diffraction, we found that CaTcO3 is an antiferromagnetic with a surprisingly high Neel temperature of ~800 K. The transition to the magnetic state does not involve a structural change, but there is obvious magnetostriction. Electronic structure calculations confirm the experimental results. © 2011, American Chemical Society
Antiferromagnetism weakening with Y-substitution in the TbRhIn5 intermetallic system
(arXivLabs, 2017-08-31) Amaral, RP; Lora-Serrano, R; Garcia, DJ; Betancourth, D; Cadogan, JM; Muñoz-Pérez, S; Cobas-Acosta, R; Avdeev, M; Bittar, EM; Duque, JGS; Pagliuso, PG
We report measurements of the temperature dependence specific heat, magnetic susceptibility in single crystals of the series of intermetallic compounds Tb1−xYxRhIn5 (nominal concentrations x=0.15,0.30,0.40,0.50 and 0.70). A mean field approximation to simulate the macroscopic properties along the series has been used. Neutron diffraction data in powdered sample of nominal concentration Tb0.6Y0.4RhIn5 reveal AFM propagation vector k=[12 0 12] with the magnetic moments oriented close to the tetragonal \textit{c} axis. We discuss the role of combined effects of crystalline electric field (CEF) perturbations and dilution in the magnetic properties evolution with Y content. In particular, we suggest that changes in the Tb-In first neighbors distances, i.e. the TbIn3 cuboctahedra distortion, are responsible for changes in the Tb crystalline potential and the possible reorientation of Tb magnetic moments for x>0.4. This reflects non negligible variations of the Bmn crystal field parameters and the energy levels splitting with \textit{x}. CC BY: Creative Commons Attribution
Apparent critical phenomena in the superionic phase transition of Cu2-xSe
(IOP Science, 2016-01-11) Kang, SD; Danilkin, SA; Aydemir, U; Avdeev, M; Studer, AJ; Snyder, GJ
The superionic phase transition of Cu2-xSe accompanies drastic changes in transport properties. The Seebeck coefficient increases sharply while the electrical conductivity and thermal diffusivity drops. Such behavior has previously been attributed to critical phenomena under the assumption of a continuous phase transition. However, applying Landau's criteria suggests that the transition should be first order. Using the phase diagram that is consistent with a first order transition, we show that the observed transport properties and heat capacity curves can be accounted for and modeled with good agreement. The apparent critical phenomena is shown to be a result of compositional degree-of-freedom. Understanding of the phase transition allows to explain the enhancement in the thermoelectric figure-of-merit that is accompanied with the transition. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft

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