Vladimir Pomjakushin

Synthesis, crystal structure, and chemical stability of the superconductor FeSe1−x

We report on a comparative study of the crystal structure and the magnetic properties of FeSe1−x (x=0.0–0.15) superconducting samples by neutron powder-diffraction and magnetization measurements. The samples were synthesized by two different methods: a “low-temperature” one using powders as a starting material at T≃700 °C and a “high-temperature” method using solid pieces of Fe and Se at T≃1075 °C. The effect of a starting (nominal) stoichiometry on the phase purity of the obtained samples, the superconducting transition temperature Tc, as well as the chemical stability of FeSe1−x at ambient conditions were investigated. It was found that in the Fe-Se system, a stable phase exhibiting superconductivity at Tc≃8 K exists in a narrow range of selenium concentration (FeSe0.974±0.005).

Simultaneous antiferromagnetic Fe3+ and Nd3+ ordering in NdFe3(11BO3)4

By means of magnetic susceptibility and specific heat measurements, x-ray and unpolarised neutron diffraction investigations on powder and single-crystal samples, simultaneous long-range antiferromagnetic Fe and Nd ordering in NdFe3(11BO3)4 with R 3 2 chemical structure has been found at temperatures below TN = 30.5(5) K down to 1.6 K. At temperatures down to 20 K to the propagation vector is khex = [0,0,3/2] and becomes slightly incommensurate at lower temperatures. Symmetry analysis yields magnetic spiral configurations with the magnetic moments oriented parallel to hexagonal basal plane according to the irreducible representations tau_3 in the commensurate case. This is in agreement with the easy directions of magnetisation perpendicular to the c-axis as determined by magnetic susceptibility measurements. At 1.6 K the magnetic Fe moment amounts to 4.9 muB close to the free ion moment of Fe3+. The magnetic Nd3+ moment saturates presumably due to crystal-field effects at 2.7 muB.

Angle-dispersive neutron diffraction under high pressure to 10GPa

We present a method which allows high-quality powder neutron diffraction patterns to be obtained under pressure by angle-dispersive diffraction to at least 10GPa. This technique uses a new type of Paris–Edinburgh press in conjunction with sintered boron nitride anvils. As an example, we show NiO diffraction patterns obtained under purely hydrostatic pressures up to 10GPa. These data were collected within a few hours, and are free from any contaminating signal from the pressure cell. High-resolution nuclear and magnetic structural information can be readily extracted by Rietveld refinements, without additional data correction. This technique will allow powder neutron diffraction at elevated pressures to become a standard tool on continuous neutron facilities.

Single magnetic chirality in the magnetoelectric NdFe3(11BO3)4

We have performed an extensive study of single-crystals of the magneto-electric NdFe

An original polymorph sequence in the high-temperature evolution of the perovskite Pb2TmSbO6.

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Atom order and thermodynamic properties of the ternary Laves phase Ti(TiyNixAl1–x–y)2

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Neutron Powder Diffraction Study of the Phase Transition in BaTi2O5

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Evidence for SrHo2O4 and SrDy2O4 as model J1-J2 zigzag chain materials

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Spiral magnetic structure in spin- 5 frustrated trimerized chains in SrMn3P4O14

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Magnetic excitations in the spin-trimer compounds Ca3Cu3- xNix(PO4)4 (x=0,1,2)

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