Engineering topics

Avdeev, Maxim

Publications and source records attributed to Avdeev, Maxim.

At least 73 records · Page 4

DOE OSTI2016

Structural chemistry and magnetic properties of the perovskite Sr{sub 3}Fe{sub 2}TeO{sub 9}

A polycrystalline sample of perovskite-like Sr{sub 3}Fe{sub 2}TeO{sub 9} has been prepared in a solid-state reaction and studied by a combination of electron microscopy, Mössbauer spectroscopy, magnetometry, X-ray diffraction and neutron diffraction. The majority of the reaction product is shown to be a trigonal phase with a 2:1 ordered arrangement of Fe{sup 3+} and Te{sup 6+} cations. However, the sample is prone to nano-twinning and tetragonal domains with a different pattern of cation ordering exist within many crystallites. Antiferromagnetic ordering exists in the trigonal phase at 300 K and Sr{sub 3}Fe{sub 2}TeO{sub 9} is thus the first example of a perovskite with 2:1 trigonal cation ordering to show long-range magnetic order. At 300 K the antiferromagnetic phase coexists with two paramagnetic phases which show spin-glass behaviour below ~80 K. - Graphical abstract: Sr{sub 3}Fe{sub 2}TeO{sub 9} has a 2:1 ordered arrangement of Fe{sup 3+} and Te{sup 6+} cations over the octahedral sites of a perovskite structure and is antiferromagnetic at room temperature. - Highlights: • 2:1 Cation ordering in a trigonal perovskite. • Magnetically ordered trigonal perovskite. • Intergrowth of nanodomains in perovskite microstructure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2016

Synthesis, crystal structure, electrical properties, and sodium transport pathways of the new arsenate Na{sub 4}Co{sub 7}(AsO{sub 4}){sub 6}

A new sodium cobalt (II) arsenate Na{sub 4}Co{sub 7}(AsO{sub 4}){sub 6} has been synthesized by a solid-state reaction and its crystal structure determined from single crystal X-ray diffraction data. It crystallizes in the monoclinic system, space group C2/m, with a=10.7098(9) Å, b=14.7837(9) Å, c=6.6845(7) Å, and β=105.545(9)°. The structure is described as a three-dimensional framework built up of corner-edge sharing CoO{sub 6}, CoO{sub 4} and AsO{sub 4} polyhedra, with interconnecting channels along [100] in which the Na{sup +} cations are located. The densest ceramics with relative density of 94% was obtained by ball milling and optimization of sintering temperature, and its microstructure characterized by scanning electron microscopy. The electrical properties of the ceramics were studied over a temperature interval from 280 °C to 560 °C using the complex impedance spectroscopy over the range of 13 MHz–5 Hz. The ionic bulk conductivity value of the sample at 360 °C is 2.51 10{sup −5} S cm{sup −1} and the measured activation energy is Ea=1 eV. The sodium migration pathways in the crystal structure were investigated computationally using the bond valence site energy (BVSE) model and classical molecular dynamics (MD) simulations. - Graphical abstract: Correlation between crystal structure, microstructure and ionic conductivity . Display Omitted - Highlights: • A new arsenate Na{sub 4}Co{sub 7}(AsO{sub 4}){sub 6} was prepared by solid state reaction. • Its crystal structure was determined by powder X-ray diffraction. • Na{sup +} ionic conductivity was probed by complex impedance spectroscopy. • Na{sup +} conduction pathways were modeled by bond-valence method and molecular dynamics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2016

Structural, Magnetic, and Optical Properties of A 3 V 4 (PO 4 ) 6 (A = Mg, Mn, Fe, Co, Ni)

Combined synchrotron and neutron powder diffraction indicates that A 3 V 4 (PO 4 ) 6 (A = Mg, Mn, Fe, Co, Ni) compounds crystallize with triclinic $P\bar{1}$ symmetry. Lattice parameters expand as expected with successive increases in the ionic radius of the A 2+ ion. Cation disorder on the octahedral sites increases as the ionic radii of A 2+ ion decreases. Direct-current magnetic susceptibility measurements indicate that all compounds with magnetic A 2+ ions order anti-ferromagnetically with transition temperatures ranging from 12 to 15 K. Effective magnetic moments for A 3 V 4 (PO 4 ) 6 (A = Mg, Mn, Fe, Co, Ni) are 5.16, 11.04, 10.08, 9.76, and 7.96 μ B per formula unit, respectively, in line with calculated values for high-spin transition metal ions. With the exception of Co 3 V 4 (PO 4 ) 6 the ultraviolet–visible spectra are dominated by d–d transitions of the V 3+ ions. Here, the striking emerald green color of Co 3 V 4 (PO 4 ) 6 arises from the combined effects of d–d transitions involving both V 3+ and Co 2+ .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2016

Structural studies of the rhombohedral and orthorhombic monouranates: CaUO{sub 4}, α-SrUO{sub 4}, β-SrUO{sub 4} and BaUO{sub 4}

The structures of some AUO{sub 4} (A=Ca, Sr, or Ba) oxides have been determined using a combination of neutron and synchrotron X-ray diffraction, supported by X-ray absorption spectroscopic measurements at the U L{sub 3}-edge. The smaller Ca cation favours a rhombohedral AUO{sub 4} structure with 8-coordinate UO{sub 8} moieties whilst an orthorhombic structure based on UO{sub 6} groups is found for BaUO{sub 4}. Both the rhombohedral and orthorhombic structures can be stabilised for SrUO{sub 4}. The structural studies suggest that the bonding requirements of the A site cation play a significant role in determining which structure is favoured. In the rhombohedral structure, Bond Valence Sums demonstrate the A site is invariably overbonded, which, in the case of rhombohedral α-SrUO{sub 4}, is compensated for by the formation of vacancies in the oxygen sub-lattice. The uranium cation, with its flexible oxidation state, is able to accommodate this by inducing vacancies along its equatorial coordination site as demonstrated by neutron powder diffraction. - Graphical abstract: Diffraction studies of AUO{sub 4} (A = Ca, Sr, or Ba) oxides reveal the importance of the bonding requirements of the A site cation in determining whether the structure is rhombohedral or orthorhombic. - Highlights: • Structures of AUO{sub 4} ( A = Ca Sr, Ba) refined against X-ray and Neutron diffraction. • The alkali cations size has a dramatic effect on the crystal structure. • Smaller cations favouring a rhombohedral structure. • Oxygen vacancies to stabilise the rhombohedral structure in SrUO{sub 4}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2016

Apparent critical phenomena in the superionic phase transition of Cu 2- x Se

The superionic phase transition of ${\mathrm{Cu}}_{2-x}\mathrm{Se}$ 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. In conclusion, understanding of the phase transition allows to explain the enhancement in the thermoelectric figure-of-merit that is accompanied with the transition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2015

Structure, stability, and photoluminescence in the anti-perovskites Na{sub 3}W{sub 1−x}Mo{sub x}O{sub 4}F (0≤x≤1)

Single-phase ordered oxyfluorides Na{sub 3}WO{sub 4}F, Na{sub 3}MoO{sub 4}F and their mixed members Na{sub 3}W{sub 1−x}Mo{sub x}O{sub 4}F can be prepared via facile solid state reaction of Na{sub 2}MO{sub 4}·2H{sub 2}O (M=W, Mo) and NaF. Phases produced from incongruent melts are metastable, but lower temperatures allow for a facile one-step synthesis. In polycrystalline samples of Na{sub 3}W{sub 1−x}Mo{sub x}O{sub 4}F, the presence of Mo stabilizes the structure against decomposition to spinel phases. Photoluminescence studies show that upon excitation with λ=254 nm and λ=365 nm, Na{sub 3}WO{sub 4}F and Na{sub 3}MoO{sub 4}F exhibit broad emission maxima centered around 485 nm. These materials constitute new members of the family of self-activating ordered oxyfluoride phosphors with anti-perovskite structures which are amenable to doping with emitters such as Eu{sup 3+}. - Graphical abstract: Directed synthesis of the ordered oxyfluorides Na{sub 3}W{sub 1−x}Mo{sub x}O{sub 4}F (0≤x≤1) has shown that a complete solid solution is attainable and provides the first example of photoluminescence in these materials. - Highlights: • Na{sub 3}W{sub 1−x}Mo{sub x}O{sub 4}F is a complete solid solution with hexagonal anti-perovskite structure. • The presence of even small amounts of Mo stabilizes the structure against decomposition. • Na{sub 3}W{sub 1−x}Mo{sub x}O{sub 4}F has broad emissions centered ≈485 nm (λ{sub ex}=254 nm and λ{sub ex}=365 nm). • These materials constitute a new family of self-activated oxyfluoride phosphors. • Na{sub 3}W{sub 1−x}Mo{sub x}O{sub 4}F materials are amenable to doping with emitters such as Eu{sup 3+}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2015

Cooling field tuned magnetic phase transition and exchange bias-like effect in Y{sub 0.9}Pr{sub 0.1}CrO{sub 3}

Cooling magnetic field dependence of magnetic phase transition has been observed in Y{sub 0.9}Pr{sub 0.1}CrO{sub 3}. G{sub z}F{sub x} order (spin structure of PrCrO{sub 3}) is dominant after zero field cooling (ZFC), whereas G{sub x}F{sub z} order (spin structure of YCrO{sub 3}) is dominant after cooling under a field higher than 100 Oe. Positive/negative exchange bias-like effect, with large vertical shift and small horizontal shift, has been observed after FC/ZFC process. The vertical shift can be attributed to the frozen ordered Pr{sup 3+} and Cr{sup 3+} spins in magnetic domains, because of the strong coupling between Pr{sup 3+} and Cr{sup 3+} sublattices; while the horizontal shift is a result of the pinning of spins at the interfaces. The frozen structure is generated by the field used for the measurement of the initial magnetization curve of M(H) for the ZFC cooled sample, while it is generated by the cooling field for the sample cooled under a cooling field higher than 100 Oe.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2015

Structural changes and self-activated photoluminescence in reductively annealed Sr{sub 3}AlO{sub 4}F

White light emission of self-activated photoluminescence (PL) in Sr{sub 3}AlO{sub 4}F under 254 nm light is only observed after annealing in a reducing atmosphere of 5%H{sub 2}/95%Ar. High-resolution neutron powder diffraction reveals that the FSr{sub 6} octahedrons and AlO{sub 4} tetrahedrons in this anti-perovskite structure are closer packed in reduced than in air-annealed samples which show no PL. Careful analysis of temperature-dependent neutron powder diffraction data establishes smaller isotropic displacement parameters for Sr(1) and O in Sr{sub 3}AlO{sub 4}F annealed in a reducing atmosphere indicating that the denser packing of the polyhedral sub-units leads to a slightly deeper potential for the Sr(1) and O atoms. Both the air- and reductively-annealed samples have identical thermal expansion within the temperature range between 3 and 350 K. The Debye temperatures were calculated using the atomic displacement parameters and show no significant differences between the air and reductively annealed samples making the Debye temperature a bad proxy for self-activated PL. - Graphical abstract: Annealing Sr{sub 3}AlO{sub 4}F under reducing conditions results in an intense self-activated photoluminescence which is correlated with a denser packing of FSr{sub 6} and AlO{sub 4} polyhedra. - Highlights: • Sr{sub 3}AlO{sub 4}F made in air does not show self-activated photoluminescence. • Only when annealing Sr{sub 3}AlO{sub 4}F in a reducing gas is photoluminescence observed. • FSr{sub 6} and AlO{sub 4} polyhedra in reduced Sr{sub 3}AlO{sub 4}F structure are packed more efficient. • Smaller displacement parameters are found for under-bonded Sr(1) and O sites.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2015

Temperature and composition phase diagram in the iron-based ladder compounds Ba 1 - x Cs x Fe 2 Se 3

We investigated the iron-based ladder compounds (Ba,Cs)Fe₂Se₃. Their parent compounds BaFe₂Se₃ and CsFe₂Se₃ have different space groups, formal valences of Fe, and magnetic structures. Electrical resistivity, specific heat, magnetic susceptibility, x-ray diffraction, and powder neutron diffraction measurements were conducted to obtain a temperature and composition phase diagram of this system. Block magnetism observed in BaFe₂Se₃ is drastically suppressed with Cs doping. In contrast, stripe magnetism observed in CsFe₂Se₃ is not so fragile against Ba doping. A new type of magnetic structure appears in intermediate compositions, which is similar to stripe magnetism of CsFe₂Se₃, but interladder spin configuration is different. Intermediate compounds show insulating behavior, nevertheless a finite T-linear contribution in specific heat was obtained at low temperatures.

Hawai, Takafumi↗

DOE OSTI2015

The magnetic structures and transitions of a potential multiferroic orthoferrite ErFeO{sub 3}

Rare-earth orthoferrites are very interesting due to their appealing optical and multiferroic properties. In this study, the magnetic structures and transitions of a typical rare-earth orthoferrite, ErFeO{sub 3}, have been reinvestigated in detail. The spin-reorientation transition of the Fe{sup 3+} magnetic phase and the low-temperature magnetic ordering of Er{sup 3+} were observed by neutron powder diffraction. The corresponding magnetic structures have been solved anew by symmetry analysis and refinement of the diffraction results. The magnetic moments of Fe{sup 3+} align in an antiferromagnetic way along the c axis with a weak ferromagnetic component along the b axis below the Néel temperature and above the spin-reorientation transition. Below the spin-reorientation transition, the Fe{sup 3+} moments rotate into an antiferromagnetic ordering state along the b axis with weak ferromagnetic alignment along the c axis. The spin-reorientation takes place in the bc plane. The Er{sup 3+} moments align antiferromagnetically with a C{sub y} mode below 4.5 K. For the Fe{sup 3+} moments, an additional C{sub x} mode is induced by the ordering of the Er{sup 3+} moments. Namely, they change from G{sub y}F{sub z} mode into C{sub x}G{sub y}F{sub z} mode in the Pnma space-group setting. This study resolves the long-lasting dispute about the magnetic structure of ErFeO{sub 3} at low temperature.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2015

Structural and magnetic properties of RTiNO{sub 2} (R=Ce, Pr, Nd) perovskite nitride oxides

Neutron powder diffraction indicates that CeTiNO{sub 2} and PrTiNO{sub 2} crystallize with orthorhombic Pnma symmetry (Ce: a=5.5580(5), b=7.8369(7), and c=5.5830(4) Å; Pr: a=5.5468(5), b=7.8142(5), and c=5.5514(5) Å) as a result of a{sup –}b{sup +}a{sup –} tilting of the titanium-centered octahedra. Careful examination of the NPD data, confirms the absence of long range anion order in both compounds, while apparent superstructure reflections seen in electron diffraction patterns provide evidence for short range anion order. Inverse magnetic susceptibility plots reveal that the RTiNO{sub 2} (R=Ce, Pr, Nd) compounds are paramagnetic with Weiss constants that vary from −28 to −42 K. Effective magnetic moments for RTiNO{sub 2} (R=Ce, Pr, Nd) are 2.43 μ{sub B}, 3.63 μ{sub B}, and 3.47 μ{sub B}, respectively, in line with values expected for free rare-earth ions. Deviations from Curie–Weiss behavior that occur below 150 K for CeTiNO{sub 2} and below 30 K for NdTiNO{sub 2} are driven by magnetic anisotropy, spin–orbit coupling, and crystal field effects. - Graphical abstract: The structure and magnetism of the oxide nitride perovskites RTiNO{sub 2} (R=Ce, Pr, Nd) have been explored. The average symmetry is shown to be Pnma with a random distribution of oxide and nitride ions and a{sup −}b{sup +}a{sup −} tilting of the titanium-centered octahedra, but electron diffraction shows evidence for short range anion order. All three compounds are paramagnetic but deviations from the Curie Weiss law are seen below 150 K for R=Ce and below 30 K for R=Nd. - Highlights: • The oxide nitride perovskites RTiNO{sub 2} (R=Ce, Pr) have been prepared and their structures determined. • Diffraction measurements indicate short range cis-order of O and N, but no long range order. • Compounds are paramagnetic with Weiss constants that vary from −28 to −42 K. • CeTiO{sub 2}N and NdTiO{sub 2}N deviate from Curie–Weiss behavior below 150 and 30 K, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2015

Single crystal and magnetic structures of maricite-type AgMnVO{sub 4}

Single crystals of the ternary manganese vanadate AgMnVO{sub 4}, were grown using AgVO{sub 3} flux. The structure was determined from single crystal X-ray diffraction data. The magnetic structure and properties of AgMnVO{sub 4} were characterized by magnetic susceptibility, specific heat, and low-temperature neutron powder diffraction measurements. AgMnVO{sub 4} crystallizes in the maricite-type structure with space group Pnma, a=9.5393(12), b=6.8132(9), c=5.3315(7) Å and Z=4. AgMnVO{sub 4} contains MnO{sub 4} chains made up of edge-sharing MnO{sub 6} octahedra, and these chains are interlinked by the VO{sub 4} and AgO{sub 4} tetrahedra. The specific heat measurements indicate a 3D-antiferromagnetic ordering at ∼12.1 K and the neutron powder diffraction measurements at 5 K show that the Mn{sup 2+}magnetic moments are antiferromagnetically coupled within the chains which are antiferromagnetically coupled to each other. - Graphical abstract: The AgMnVO{sub 4} crystals could be grown in AgVO{sub 3} flux and the magnetic structure was determined from neutron powder diffraction data. The spins in each MnO{sub 4} chain along [0 1 0] are antiferromagnetically coupled, while these antiferromagnetic chains are antiferromagnetically coupled along [0 0 1] but have a non-collinear arrangement along [1 0 1]. - Highlights: • We have been able to grow AgMnVO{sub 4} single crystals, using AgVO{sub 3} flux. • We solved its crystal structure using single crystal data. • We carried out magnetic susceptibility and specific heat measurements. • We solved the magnetic structure from low-temperature neutron powder diffraction data. • We compared the magnetic structure to that of NaFePO{sub 4}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2014

The interplay of microstructure and magnetism in La{sub 3}Ni{sub 2}SbO{sub 9}

La{sub 3}Ni{sub 2}SbO{sub 9} adopts a perovskite-related structure in which the six-coordinate cation sites are occupied alternately by Ni{sup 2+} and a disordered arrangement of Ni{sup 2+}/Sb{sup 5+}. A polycrystalline sample has been studied by neutron diffraction in applied magnetic fields of 0≤H/kOe≤50 at 5 K. In 0 kOe, weak magnetic Bragg scattering consistent with the adoption of a G-type ferrimagnetic structure is observed; the ordered component of the magnetic moment was found to be 0.89(7) µ{sub B} per Ni{sup 2+} cation. This increased to 1.60(3) µ{sub B} in a field of 50 kOe. Transmission electron microscopy revealed variations in the Ni:Sb ratio across crystallites of the sample. It is proposed that these composition variations disrupt the magnetic superexchange interactions within the compound, leading to domain formation and a reduced average moment. The application of a magnetic field aligns the magnetisation vectors across the crystal and the average moment measured by neutron diffraction increases accordingly. The role played by variations in the local chemical composition in determining the magnetic properties invites comparison with the behaviour of relaxor ferroelectrics. - Graphical abstract: Composition variations across a crystal of La3Ni2SbO9 result in anomalous magnetic behavior. - Highlights: • Ordered magnetic moment of Ni{sup 2+} increases in an applied magnetic field. • Composition variations detected by electron microscopy. • Increase in magnetic moment can be explained by presence of composition variations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2014

Magnetic structures of β{sub I}-Li{sub 2}CoSiO{sub 4} and γ{sub 0}-Li{sub 2}MnSiO{sub 4}: Crystal structure type vs. magnetic topology

The magnetic structure and properties of the candidate lithium-ion battery cathode materials Pbn2{sub 1}(≡Pna2{sub 1}) Li{sub 2}CoSiO{sub 4} and P2{sub 1}/n Li{sub 2}MnSiO{sub 4} have been studied experimentally using low-temperature neutron powder diffraction and magnetometry. Both materials undergo long-range antiferromagnetic ordering, at 14 K and 12 K respectively, due to super–super-exchange mediated by bridging silicate groups. Despite having different crystal structures (wurtzite- vs. “dipolar”-type), Li{sub 2}CoSiO{sub 4} and Li{sub 2}MnSiO{sub 4} have the same topology in terms of magnetic interactions, and adopt collinear magnetic structures of the same type with the propagation vectors (0, 1/2, 1/2) and (1/2, 0, 1/2), respectively. The magnetic moments in the two materials are aligned in parallel and obliquely to the distorted closed-packed layers of oxygen atoms. The experimentally observed values of the ordered magnetic moments, 2.9 μ{sub B} and 4.6 μ{sub B}, are close to those expected for d{sup 7} Co{sup 2+} and d{sup 5} Mn{sup 2+}, respectively. - Graphical abstract: Despite the different crystal structures β{sub I}-Li{sub 2}CoSiO{sub 4} and γ{sub 0}-Li{sub 2}MnSiO{sub 4} have similar magnetic topology and as a result adopt magnetic structure of the same type. - Highlights: • Magnetic structures of Li{sub 2}CoSiO{sub 4} and Li{sub 2}MnSiO{sub 4} were studied for the first time. • Both materials antiferromagnetically order around 12–14 K. • Despite different crystal structure magnetic structures are of the same type. • The fact is attributed to similar topology of magnetic interactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2014

Crystal structures of orthorhombic, hexagonal, and cubic compounds of the Sm{sub (x)}Yb{sub (2−x)}TiO{sub 5} series

A series of single phase compounds with nominal stoichiometry Sm{sub (x)}Yb{sub (2−x)}TiO{sub 5} (x=2, 1.4, 1, 0.6, and 0) have been successfully fabricated to generate a range of crystal structures covering the most common polymorphs previously discovered in the Ln{sub 2}TiO{sub 5} series (Ln=lanthanides and yttrium). Four of the five samples have not been previously fabricated in bulk, single phase form so their crystal structures are refined and detailed using powder synchrotron and single crystal x-ray diffraction, neutron diffraction and transmission electron microscopy. Based on the phase information from diffraction data, there are four crystal structure types in this series; orthorhombic Pnma, hexagonal P6{sub 3}/mmc, cubic (pyrochlore-like) Fd-3m and cubic (fluorite-like) Fm-3m. The cubic materials show modulated structures with variation between long and short range ordering and the variety of diffraction techniques were used to describe these complex crystal structure types. - Graphical abstract: A high resolution image of the compound Sm{sub 0.6}Yb{sub 1.4}TiO{sub 5} showing contrast from lattice fringes and the corresponding fast Fourier transform (FFT) of the HREM image with pyrochlore related diffraction spots marked “P” and fluorite marked “F”. The crystal is oriented down the [1 1 0] zone axis based on the Fd-3m structure. The ideal crystal structure (no vacancies) of the cubic, pyrochlore-like (Sm{sub 0.6}Yb{sub 1.4}TiO{sub 5}). - Highlights: • First fabrication of bulk single-phase material with stoichiometry Sm{sub 2}TiO{sub 5}. • Systematic study of crystal structure types within Ln{sub 2}TiO{sub 5} series (Ln=lanthanides). • A novel technique using IFFT of HREM images to study cubic structures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2013

Crystal and magnetic structure of (1−x)BiFeO{sub 3}–xSrTiO{sub 3} (x=0.2, 0.3, 0.4 and 0.8)

The effect of doping SrTiO{sub 3} into BiFeO{sub 3} on the magnetic and crystal structure has been explored using powder neutron diffraction for (1−x)BiFeO{sub 3}–xSrTiO{sub 3} where x=0.2, 0.3, 0.4, 0.6 and 0.8. While the data are not sensitive to the cycloidal component of the magnetic ordering, the evolution of the collinear antiferromagnetic moment of the G-type antiferromagnetic component with T and x has been explored, as have structural parameters. It is found that for x≤0.4 pure phase samples form in the R3c cell, and for x=0.8 a non-magnetic Pm3{sup ¯}m phase is obtained. The x=0.6 sample gives a mixed phase. Through the R3c phase the magnetic structure does not change appreciably apart from the reduction of magnetic moment magnitude with the increasing T and/or x. - Graphical abstract: A simple phase diagram for (1−x)BiFeO{sub 3}–xSrTiO{sub 3} , where hexagons indicate the rhombohedral R3c phase and boxes the cubic phase. Filled symbols indicate magnetic ordering. The black arrow indicates T{sub N} for BiFeO{sub 3}. Display Omitted - Highlights: • We have established part of the phase diagram for doping SrTiO{sub 3} into BiFeO{sub 3}. • (1−x)BiFeO{sub 3}–xSrTiO{sub 3} is isostructural with BiFeO{sub 3} up to x<0.4, and cubic for x>0.85. • We have examined the evolution of ordered magnetic moment with T and x.

36 MATERIALS SCIENCE↗

DOE OSTI2013

Ordered vs. disordered perovskites; structural studies of Fe-doped SrIrO{sub 3} and SrRuO{sub 3}

The structures of the two Fe containing perovskites Sr{sub 2}IrFeO{sub 6} and SrRu{sub 0.5}Fe{sub 0.5}O{sub 3} have been established using a combination of synchrotron and neutron diffraction methods. Sr{sub 2}IrFeO{sub 6} and SrRu{sub 0.5}Fe{sub 0.5}O{sub 3} are shown to be monoclinic I2/m and tetragonal I4/mcm respectively The former exhibits a rock-salt like ordering of the Fe and Ir cations and displays a sequence of phase transitions associated with the loss of the octahedral tilts upon heating; 12/m→I4/m→Fm3{sup ¯}m. The Fe and Ru cations are disordered in SrRu{sub 0.5}Fe{sub 0.5}O{sub 3} and this shows a single structural phase transition upon heating due to the loss of the in-phase tilts, viz. I4/mcm→Pm3{sup ¯}m. In both cases XANES measurements show partial oxidation of the Fe{sup 3+} to Fe{sup 4+}. The difference in the structures between the two is remarkable given the similar size of Ir{sup 5+} and Ru{sup 5+}, and this is reflected in their magnetic properties. - Graphical abstract: Sr{sub 2}IrFeO{sub 6} and SrRu{sub 0.5}Fe{sub 0.5}O{sub 3} are shown, using a combination of synchrotron and neutron diffraction, to be monoclinic I2/m with cation ordering and tetragonal I4/mcm with disordered Fe and Ir, respectively. Both undergo phase transitions upon heating due to the loss of the octahedral tilts. Display Omitted - Highlights: • Sr{sub 2}IrFeO{sub 6} shown to be monoclinic and shows the transitions upon heating I2/m→I4/m→ Fm3{sup ¯}m. • SrRu{sub 0.5}Fe{sub 0.5}O{sub 3} is tetragonal and shows a single I4/mcm→Pm3m transition upon heating. • The Fe and Ru cations are disordered in SrRu{sub 0.5}Fe{sub 0.5}O{sub 3} but ordered in Sr{sub 2}FeIrO{sub 6}. • XANES measurements show partial oxidation of the Fe{sup 3+} to Fe{sup 4+}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2013

Designing new n=2 Sillen–Aurivillius phases by lattice-matched substitutions in the halide and [Bi{sub 2}O{sub 2}]{sup 2+} layers

A new n=2 Sillen–Aurivillius compound Bi{sub 3}Sr{sub 2}Nb{sub 2}O{sub 11}Br has been synthesised based on Bi{sub 3}Pb{sub 2}Nb{sub 2}O{sub 11}Cl by simultaneously replacing Pb{sup 2+} with Sr{sup 2+} and Cl{sup −} with Br{sup −}. Rietveld refinements against X-ray and neutron powder diffraction data revealed a significant relative compression in the stacking axis (c-axis) of the new compound. Sr{sup 2+} doping reduces the impact of the stereochemically active 6s{sup 2} lone pair found on Pb{sup 2+} and Bi{sup 3+}, resulting in a contraction of the c-axis by 1.22% and an expansion of the ab plane by 0.25%. This improves the inter-layer compatibility with the larger halide Br{sup −}. Analysis of X-ray absorption near-edge spectroscopy data show that the ferroelectric distortion of the B-site cation is less apparent in Bi{sub 3}Sr{sub 2}Nb{sub 2}O{sub 11}Br compared to Bi{sub 3}Pb{sub 2}Nb{sub 2}O{sub 11}Cl, and variable-temperature neutron diffraction data show no evidence for a ferroelectric distortion. - Graphical abstract: The structural expansion and contractions in the stacking axis of co-doped Sillen–Aurivillius phases. Display Omitted - Highlights: • A new Sillen–Aurivillius phase Bi{sub 3}Sr{sub 2}Nb{sub 2}O{sub 11}Br is synthesised based on Bi{sub 3}Pb{sub 2}Nb{sub 2}O{sub 11}Cl. • The stacking axis contracts although local expansion accommodates the larger halide Br. • Majority of the contraction is observed in the [Bi{sub 2}O{sub 2}]{sup 2+} layers where we replaced Pb{sup 2+} for Sr{sup 2+}. • Removing the Pb{sup 2+} lowers the lone pair effect and improves inter-layer compatibility. • XANES and NPD suggest that the ferroelectric properties are minimised or suppressed.

36 MATERIALS SCIENCE↗