D. V. Sheptyakov

DN-12 time-of-flight high-pressure neutron spectrometer for investigation of microsamples

Abstract A few years ago a high-pressure neutron spectrometer DN-12 was created at the IBR-2 pulsed reactor. Neutron diffraction and inelastic incoherent neutron scattering (with samples having large incoherent neutron scattering cross section) experiments at pressures up to 5xa0GPa can be performed with the DN-12 using sapphire anvil cells. After first successful experiments, the development of the DN-12 spectrometer was done during last year. The new parameters and recent experiments performed with the DN-12 both before and after modernization are reviewed.

Iron-vacancy superstructure and possible room-temperature antiferromagnetic order in superconducting CsyFe2! xSe2

A). The propagation vector star corresponds to the 5 times bigger unit cell given by transformation A = 2a + b, B =! a + 2b, C = c. A solution for the atomic structure is found in the space group I4/m with an ordered pattern of iron vacancies corresponding to the iron deficiency x = 0.29 and Cs stoichiometry y = 0.83. The superstructure satellites are more pronounced in the neutron diffraction patterns suggesting that they can have some magnetic contribution. We have sorted out all possible symmetry adapted magnetic configurations and found that the presence of antiferromagnetic ordering with the ordered magnetic moment of Fe with # 2µB does not contradict the experimental data. However, the solutions space is highly degenerate and we cannot choose a specific solution. Instead we propose possible magnetic configurations with the Fe magnetic moments in (ab )p lane or alongc axis. The superstructure is destroyed above Ts # 500 K by a first-order-like transition.

Slicing the Perovskite Structure with Crystallographic Shear Planes: The AnBnO3n-2 Homologous Series

A new A(n)B(n)O(3n-2) homologous series of anion-deficient perovskites has been evidenced by preparation of the members with n = 5 (Pb(2.9)Ba(2.1)Fe(4)TiO(13)) and n = 6 (Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16)) in a single phase form. The crystal structures of these compounds were determined using a combination of transmission electron microscopy and X-ray and neutron powder diffraction (S.G. Ammm, a = 5.74313(7), b = 3.98402(4), c = 26.8378(4) Å, R(I) = 0.035, R(P) = 0.042 for Pb(2.9)Ba(2.1)Fe(4)TiO(13) and S.G. Imma, a = 5.7199(1), b = 3.97066(7), c = 32.5245(8) Å, R(I) = 0.032, R(P) = 0.037 for Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16)). The crystal structures of the A(n)B(n)O(3n-2) homologues are formed by slicing the perovskite structure with (101)(p) crystallographic shear (CS) planes. The shear planes remove a layer of oxygen atoms and displace the perovskite blocks with respect to each other by the 1/2[110](p) vector. The CS planes introduce edge-sharing connections of the transition metal-oxygen polyhedra at the interface between the perovskite blocks. This results in intrinsically frustrated magnetic couplings between the perovskite blocks due to a competition of the exchange interactions between the edge- and the corner-sharing metal-oxygen polyhedra. Despite the magnetic frustration, neutron powder diffraction and Mössbauer spectroscopy reveal that Pb(2.9)Ba(2.1)Fe(4)TiO(13) and Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16) are antiferromagnetically ordered below T(N) = 407 and 343 K, respectively. The Pb(2.9)Ba(2.1)Fe(4)TiO(13) and Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16) compounds are in a paraelectric state in the 5-300 K temperature range.

Investigation of the HgBa2CuO4+δ structure under external pressures up to 5 GPa by neutron powder diffraction

Abstract Neutron powder diffraction investigation of the changes in the structure of the mercury superconductor HgBa 2 CuO 4+δ has been carried out in the pressure range 0–5.07 GPa at room temperature. The compressibility values of the unit cell parameters and several bond distances in the structure have been determined. The relative reductions of the unit cell parameters are close to each other and are approximately 1.7% in the base plane and 2.3% along the c -axis. At the same time, the interatomic distance modulations are strongly inhomogeneous: approximately 4% for the apical Cuue5f8O bond and only about 0.4% for the apical Hgue5f8O bond.

Very large magnetoresistance and spin state transition in Ba-doped cobaltites

Structural, magnetization, and magnetotransport measurements have been performed for anion deficient La0.5Ba0.5Co1− x Fe x O3−δ ( x ≤ 0.4 ) and La1− y Ba y CoO3−δ ( 0.5 ≤ y ≤ 0.6 ) perovskites. It has been found that the iron doped compositions with x ≤ 0.23 are predominantly ferromagnetic. The Curie point and magnetization are slightly larger in insulating phase (xu2009=u20090.1) than in metallic one (xu2009=u20090.05). Magnetoresistance ratio rises with lowering temperature and increasing iron content reaching three orders of magnitude in predominantly antiferromagnetic state ( x ≥ 0.25 ). The rise of the barium content in La1− y Ba y CoO3−δ series above yu2009=u20090.5 leads to stabilization of antiferromagnetic phase and strong enhancement of the magnetoresistance. Antiferromagnetic ordering is accompanied by increase in resistivity. The lack of the low field intergrain magnetoresistance is in agreement with a weak spin polarization of the charge carriers. Magnetoresistance is associated with antiferromagnet–ferromagnet (from mixed high/low into intermediate spin state) transition induced by magnetic field. It is suggested that ferromagnetism is originated from superexchange interaction via oxygen.

Real disordered crystal structure and Curie temperature of intermetallic compounds Y2Fe17−xMx (M=Si or Al)

Abstract The crystal structure of intermetallic compounds Y 2 Fe 17 , Y 2 Fe 15.3 Al 1.7 and Y 2 Fe 15.3 Si 1.7 was studied by powder X-ray and neutron diffraction. The real disordered structure of the compounds was found to be a disordered variant of the Th 2 Ni 17 -type structure, in which the Y atoms from the 2b sites substitute in part the Fe “dumbbells” on 4f sites and the Fe atoms occupy part of vacant 4e sites. Besides, the Y atoms implant in 2c sites in the iron plane of the compounds, which causes plane distortions and splitting of the 12j Fe sites into two subsites. The substitution of Fe by Si or Al was found to have a preferential character for sites in the order: 12k, 4f, 4e, 6g and 12j. Partial substitution of Al or Si for Fe is established as resulting in an increase in the atomic Fe–Fe spacing in the 4f “dumbbell” site. The Curie temperature of the compounds Y 2 Fe 17 , Y 2 Fe 15.3 Al 1.7 and Y 2 Fe 15.3 Si 1.7 is proportional to the atomic Fe–Fe spacing in the “dumbbell” site.

Effect of fluorination and high pressure on the structure and properties of the Hg-bearing superconducting Cu mixed oxides

Abstract The T c variation of HgBa 2 CuO 4+ δ (Hg-1201) and HgBa 2 CuO 4 F δ can be achieved by a change in the carrier concentration and by a compression of the structure under high pressure. Oxygenated and fluorinated series exhibit a cupola-shaped behavior for the T c vs. δ dependence, but the curves are shifted away from each other along the δ axis. NPD showed double amount of extra fluorine in comparison with extra oxygen for the oxygenated Hg-1201 phases with close T c s. An exchange of the extra oxygen by a double amount of fluorine causes a significant compression of the apical Cuue5f8O bond distances, while the in-plane ones, as well as T c , do not vary. Fluorination of Hg-1223 resulted in a slight increase in T c in comparison with oxygenated material. The influence of the external pressure on the structure and T c of Hg-1201 strongly depends on the doping level. An increase in the extra oxygen content from underdoped to overdoped state results in the larger compression of the apical Cuue5f8O and Baue5f8O Hg distances while the HgO 2 dumbbell as well as a distance between Ba and O from the (CuO 2 ) layers become practically pressure independent.

Low-temperature structural anomalies in Pr0.5Sr0.5CoO3

The magnetic and crystal structures of the Pr0.5Sr0.5CoO3 metallic ferromagnet have been studied by the neutron diffraction technique. It is demonstrated that below 150 K, the compound is mesoscopically separated into two crystalline phases with different spatial symmetries and with different directions of the magnetic anisotropy. The phase separation exists down to 1.5 K, and at temperatures below 90 K, the low-symmetry phase occupies about 80% of the sample volume. The main structural difference between the phases is the configuration of oxygen atoms around praseodymium and, to a certain extent, around cobalt. The ferromagnetic structure with the magnetic moment lying in the basal plane of the structure (μCo ≈ 1.7 μB at 1.5 K) arises at 234 K, whereas the component directed along the long axis of the unit cell appears at 130 K. The formation of the new structural phase and change in the orientation of the magnetic moment give rise to the anomalies of the physical and magnetic characteristics of this compound observed earlier at temperatures about 120 K.

Frustrated pentagonal Cairo lattice in the non-collinear antiferromagnet Bi4Fe5O13F

The crystal and magnetic structures and underlying magnetic interactions of Bi4Fe5O13F, a model system for studying the physics of the Cairo pentagonal spin lattice, are investigated by transmission electron microscopy, low-temperature synchrotron x-ray and neutron powder diffraction, thermodynamic measurements, and density functional band-structure calculations. The crystal structure of Bi4Fe5O13F contains infinite rutile-like chains of edge-sharing FeO6 octahedra interconnected by the Fe2O7 groups of two corner-sharing FeO4 tetrahedra. The cavities between the chains are filled with the fluorine-centered Bi4F tetrahedra. The Fe3+ cations form pentagonal units that give rise to an unusual topology of frustrated exchange couplings and underlie a sequence of the magnetic transitions at T1= 62 K, T2 = 71 K, and TN = 178 K. Below T1, Bi4Fe5O13F forms a fully ordered non-collinear antiferromagnetic structure, whereas the magnetic state between T1 and TN may be partially disordered according to the sizable increase in the magnetic entropy at T1 and T2. Therefore, Bi4Fe5O13F shows the evidence of intricate magnetic transitions that were never anticipated for the pentagonal Cairo spin lattice. Additionally, it manifests a sillimanite (Al2SiO5)-based homologous series of compounds that feature the pentagonal magnetic lattice spaced by a variable number of octahedral units along the rutile-type chains.

Structural and Magnetic Phase Transitions in the AnBnO3n-2 Anion-Deficient Perovskites Pb2Ba2BiFe5O13 and Pb1.5Ba2.5Bi2Fe6O16

Novel anion-deficient perovskite-based ferrites Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 were synthesized by solid-state reaction in air. Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 belong to the perovskite-based A(n)B(n)O(3n-2) homologous series with n = 5 and 6, respectively, with a unit cell related to the perovskite subcell a(p) as a(p)√2 × a(p) × na(p)√2. Their structures are derived from the perovskite one by slicing it with 1/2[110]p(101)p crystallographic shear (CS) planes. The CS operation results in (101)p-shaped perovskite blocks with a thickness of (n - 2) FeO6 octahedra connected to each other through double chains of edge-sharing FeO5 distorted tetragonal pyramids which can adopt two distinct mirror-related configurations. Ordering of chains with a different configuration provides an extra level of structure complexity. Above T ≈ 750 K for Pb2Ba2BiFe5O13 and T ≈ 400 K for Pb(1.5)Ba(2.5)Bi2Fe6O16 the chains have a disordered arrangement. On cooling, a second-order structural phase transition to the ordered state occurs in both compounds. Symmetry changes upon phase transition are analyzed using a combination of superspace crystallography and group theory approach. Correlations between the chain ordering pattern and octahedral tilting in the perovskite blocks are discussed. Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 undergo a transition into an antiferromagnetically (AFM) ordered state, which is characterized by a G-type AFM ordering of the Fe magnetic moments within the perovskite blocks. The AFM perovskite blocks are stacked along the CS planes producing alternating FM and AFM-aligned Fe-Fe pairs. In spite of the apparent frustration of the magnetic coupling between the perovskite blocks, all n = 4, 5, 6 A(n)Fe(n)O(3n-2) (A = Pb, Bi, Ba) feature robust antiferromagnetism with similar Néel temperatures of 623-632 K.