Alexander Ignatov

Polar and Magnetic Mn2FeMO6 (M = Nb, Ta) with LiNbO3-type Structure - High Pressure Synthesis

Polar oxides are of much interest in materials science and engineering. Their symmetry-dependent properties such as ferroelectricity/multiferroics, piezoelectricity, pyroelectricity, and second-order harmonic generation (SHG) effect are important for technological applications. [1] However, polar crystal design and synthesis is challenging, because multiple effects, such as steric or dipole-dipole interactions, typically combine to form non-polar structures; so the number of known polar materials, especially polar magnetoelectric materials, is still severely restricted. [2] Therefore, it is necessary for the material science community to develop new strategies to create these materials.

Energy dispersive X-ray diffraction of lithium–silver vanadium phosphorous oxide cells: in situ cathode depth profiling of an electrochemical reduction–displacement reaction

Li/Ag2VO2PO4 cells exhibit high power output and a 15 000 fold decrease in impedance upon initial discharge. Energy dispersive X-ray diffraction (EDXRD) allows dimensional resolution of the reaction progress in situ, revealing that silver metal (Ag0) initially forms at the electrode–electrolyte interface. This report contains the first description of an in situ EDXRD analysis of a cathode located within an intact Li-anode cell.

Polar and Magnetic Layered A-Site and Rock Salt B-Site-Ordered NaLnFeWO6 (Ln = La, Nd) Perovskites

We have expanded the double perovskite family of materials with the unusual combination of layered order in the A sublattice and rock salt order over the B sublattice to compounds NaLaFeWO6 and NaNdFeWO6. The materials have been synthesized and studied by powder X-ray diffraction, neutron diffraction, electron diffraction, magnetic measurements, X-ray absorption spectroscopy, dielectric measurements, and second harmonic generation. At room temperature, the crystal structures of both compounds can be defined in the noncentrosymmetric monoclinic P2(1) space group resulting from the combination of ordering both in the A and B sublattices, the distortion of the cell due to tilting of the octahedra, and the displacement of certain cations. The magnetic studies show that both compounds are ordered antiferromagnetically below T(N) ≈ 25 K for NaLaFeWO6 and at ∼21 K for NaNdFeWO6. The magnetic structure of NaNdFeWO6 has been solved with a propagation vector k = ((1/2) 0 (1/2)) as an antiferromagnetic arrangement of Fe and Nd moments. Although the samples are potential multiferroics, the dielectric measurements do not show a ferroelectric response.

In situ strain profiling of elastoplastic bending in Ti–6Al–4V alloy by synchrotron energy dispersive x-ray diffraction

Elastic and plastic strain evolution under four-point bending has been studied by synchrotron energy dispersive x-ray diffraction. Measured strain profiles across the specimen thickness showed an increasing linear elastic strain gradient under increasing four-point bending load up to ∼2 kN. The bulk elastic modulus of Ti–6Al–4V was determined as 118 GPa. The onset of plastic deformation was found to set in at a total in-plane strain of ∼0.008, both under tension and compression. Plastic deformation under bending is initiated in the vicinity of the surface and at a stress of 1100 MPa, and propagates inward, while a finite core region remains elastically deformed up to 3.67 kN loading. The onset of the plastic regime and the plastic regime itself has been verified by monitoring the line broadening of the (100) peak of α-Ti. The effective compression/tension stress-strain curve has been obtained from the scaling collapse of strain profile data taken at seven external load levels. A similar multiple load scal...

Magnetic and Structural Studies of the Multifunctional Material SrFe0.75Mo0.25O3−δ

SrFe0.75Mo0.25O3-δ has been recently discovered as an extremely efficient electrode for intermediate temperature solid oxide fuel cells (IT-SOFCs). We have performed structural and magnetic studies to fully characterize this multifunctional material. We have observed by powder neutron diffraction (PND) and transmission electron microscopy (TEM) that its crystal symmetry is better explained with a tetragonal symmetry (I4/mcm space group) than with the previously reported orthorhombic symmetry (Pnma space group). The temperature dependent magnetic properties indicate an exceptionally high magnetic ordering temperature (TN ∼ 750 K), well above room temperature. The ordered magnetic structure at low temperature was determined by PND to be an antiferromagnetic coupling of the Fe cations. Mössbauer spectroscopy corroborated the PND results. A detailed study, with X-ray absorption spectroscopy (XAS), in agreement with the Mössbauer results, confirmed the formal oxidation states of the cations to be mixed valence Fe(3+/4+) and Mo(6+).

Pb2MnTeO6 Double Perovskite: An Antipolar Anti-ferromagnet

Pb2MnTeO6, a new double perovskite, was synthesized. Its crystal structure was determined by synchrotron X-ray and powder neutron diffraction. Pb2MnTeO6 is monoclinic (I2/m) at room temperature with a regular arrangement of all the cations in their polyhedra. However, when the temperature is lowered to ∼120 K it undergoes a phase transition from I2/m to C2/c structure. This transition is accompanied by a displacement of the Pb atoms from the center of their polyhedra due to the 6s(2) lone-pair electrons, together with a surprising off-centering of Mn(2+) (d(5)) magnetic cations. This strong first-order phase transition is also evidenced by specific heat, dielectric, Raman, and infrared spectroscopy measurements. The magnetic characterizations indicate an anti-ferromagnetic (AFM) order below TN ≈ 20 K; analysis of powder neutron diffraction data confirms the magnetic structure with propagation vector k = (0 1 0) and collinear AFM spins. The observed jump in dielectric permittivity near ∼150 K implies possible anti-ferroelectric behavior; however, the absence of switching suggests that Pb2MnTeO6 can only be antipolar. First-principle calculations confirmed that the crystal and magnetic structures determined are locally stable and that anti-ferroelectric switching is unlikely to be observed in Pb2MnTeO6.

Fatigue crack growth “overload effect”: mechanistic insights from in-situ synchrotron measurements:

Synchrotron-based, high-energy X-ray diffraction measurements are used to study the local strain fields underlying the transient fatigue crack growth rate retardation produced by a single overload cycle known as the overload effect. Specifically, 4140 steel compact tension specimens fatigued for varying levels of crack growth after an overload cycle have been studied with in-situ diffraction under varying external loads. The load responses of the strain at the overload-position, versus at the crack tip, are focused upon in detail. The large compressive residual strain at the overload-point is observed to remain essentially unchanged even after the overload-point is left in the wake of the propagating crack tip. The differential strain-load response at the crack-tip/overload position before and immediately after the overload is seen to be unchanged. Once the overload point is behind the crack tip, a highly nonlinear behavior is observed in which the load response of the strain field transfers from the overload -point to the crack tip when the load exceeds a critical value. The results are discussed in terms of plasticity-induced crack face contact at the overload point as an important local mechanism contributing to the “overload effect” in this specific system.

Synthesis and structure determination of ferromagnetic semiconductors LaAMnSnO6 (A = Sr, Ba)

LaAMnSnO6 (A = Sr, Ba) have been synthesized by high temperature solid-state reactions under dynamic 1% H2/Ar flow. Rietveld refinements on room temperature powder X-ray diffraction data indicate that LaSrMnSnO6 crystallizes in the GdFeO3-structure, with space groupPnma and, combined with transmission electron microscopy, LaBaMnSnO6 in Imma. Both space groups are common in disordered double-perovskites. The Mn3+ and Sn4+ ions whose valence states were confirmed by X-ray absorption spectroscopy, are completely disordered over the B-sites and the BO6 octahedra are slightly distorted. LaAMnSnO6 are ferromagnetic semiconductors with a TC = 83 K for the Sr- and 66 K for the Ba-compound. The title compounds, together with the previously reported LaCaMnSnO6 provide an interesting example of progression from Pnma to Imma as the tolerance factor increases. An analysis of the relationship between space group and tolerance factor for the series LaAMnMO6 (A = Ca, Sr, Ba; M = Sn, Ru) provides a better understanding of the symmetry determination for double perovskites.

Hole doping and structural transformation in CsTl1-xHgxCl3.

CsTlCl(3) and CsTlF(3) perovskites have been theoretically predicted to be superconductors when properly hole-doped. Both compounds have been previously prepared as pure compounds: CsTlCl(3) in a tetragonal (I4/m) and a cubic (Fm3̅m) perovskite polymorph and CsTlF(3) as a cubic perovskite (Fm3̅m). In this work, substitution of Tl in CsTlCl(3) with Hg is reported, in an attempt to hole-dope the system and induce superconductivity. The whole series CsTl(1-x)HgxCl(3) (x = 0.0, 0.1, 0.2, 0.4, 0.6, and 0.8) was prepared. CsTl(0.9)Hg(0.1)Cl(3) is tetragonal as the more stable phase of CsTlCl(3). However, CsTl(0.8)Hg(0.2)Cl(3) is already cubic with the space group Fm3̅m and with two different positions for Tl(+) and Tl(3+). For x = 0.4 and 0.5, solid solutions could not be formed. For x ≥ 0.6, the samples are primitive cubic perovskites with one crystallographic position for Tl(+), Tl(3+), and Hg(2+). All of the samples formed are insulating, and there is no signature of superconductivity. X-ray absorption spectroscopy indicates that all of the samples have a mixed-valence state of Tl(+) and Tl(3+). Raman spectroscopy shows the presence of the active Tl-Cl-Tl stretching mode over the whole series and the intensity of the Tl-Cl-Hg mode increases with increasing Hg content. First-principle calculations confirmed that the phases are insulators in their ground state and that Hg is not a good dopant in the search for superconductivity in this system.

Crystallographic and magnetic properties of Pb2−xBixIr2O7−δ (0 ≤ x ≤ 2)

The title series of compounds have been synthesized by solid state reaction and characterized by x-ray diffraction (PXD), neutron powder diffraction (PND), second harmonic generation (SHG), x-ray absorption spectroscopy (XAS) and magnetization measurements. The crystal structures have been refined from PND data in the centrosymmetric space group with pyrochlore structure. No sign of a break of the centrosymmetry has been observed by SHG and no change of the structure to a polar space group in any component of the series, as it was previously reported for Pb2Ir2O7−δ with non-centrosymmetric space group. We have determined by PND that the oxidation state of Ir is slightly decreasing from 5+-to-4+ in the Pb-rich to almost 4+ in the Bi-rich samples. This evolution is confirmed by XAS and also it explains the progression of the crystallographic parameters. All the samples are paramagnetic in the temperature range measured and the magnitude of the effective magnetic moment is enhanced with Bi content, correlated with the enhancement of Ir4+ compared to Ir5+; suggesting Ir5+ to be present in a trigonal antiprism crystal field splitting and therefore Ir4+ as the only magnetic cation.