Crystal and magnetic structure of the double perovskite Sr2CoUO6: a neutron diffraction study

Sr2CoUO6 double perovskite has been prepared as a polycrystalline powder by solid-state reaction, in air. This material has been studied by X-ray, neutron powder diffraction (NPD) and magnetic measurements. At room temperature, the crystal structure is monoclinic, space group P2(1)/n, Z= 2, with a= 5.7916(2), b= 5.8034(2), c= 8.1790(3) A, beta= 90.1455(6)degrees. The perovskite lattice consists of a completely ordered array of CoO6 and UO6 octahedra, which exhibit an average tilting angle phi= 11.4 degrees. Magnetic and neutron diffraction measurements indicate an antiferromagnetic ordering below TN = 10 K. The low-temperature magnetic structure was determined by NPD, selected among the possible magnetic solutions compatible with the P2(1)/n space group, according with the group theory representation. The propagation vector is k= 0. A canted antiferromagnetic structure is observed below TN = 10 K, which remains stable down to 3 K, with an ordered magnetic moment of 2.44(7)mu(B) for Co2+ cations. The magnetic moment calculated from the Curie-Weiss law at high temperatures (5.22 mu(B)/f.u.) indicates that the orbital contribution is unquenched at high temperatures, which is consistent with high-spin Co2+((4)T(1g) ground state) in a quasi-regular octahedral environment. Magnetic and structural features are consistent with an electronic configuration Co2+[3d(7)]-U6+[Rn].     

High-pressure neutron diffraction study of superhydrated natrolite

Neutron powder diffraction data were collected on a sample of natrolite and a 1:1 (v/v) mixture of perdeuterated methanol and water at a pressure of 1.87(11) GPa. The natrolite sample was superhydrated, with a water content double that observed at ambient pressure. All of the water deuterium atoms were located and the nature and extent of the hydrogen bonding elucidated for the first time. This has allowed the calculation of bond valence sums for the water oxygen atoms, and from this, it can be deduced that the key energetic factor leading to loss of the additional water molecule upon pressure release is the poor coordination to sodium cations within the pores.     

High-pressure synthesis and study of the crystal and magnetic structure of the distorted SeNiO3 and SeMnO3 perovskites

We describe the preparation of SeMO(3) (M = Ni, Mn) under high pressure conditions (3.5 GPa), starting from reactive H(2)SeO(3) and MO mixtures, contained in sealed gold capsules under the reaction conditions 850 degrees C for 1 h. The polycrystalline samples have been studied by neutron powder diffraction (NPD) data and magnetization measurements. SeMO(3) (M = Ni, Mn) are orthorhombically distorted perovskites (space group Pnma). Below T(N) approximately 104 K (M = Ni) and T(N) approximately 53.5 K (M = Mn) these oxides experience an antiferromagnetic ordering, as demonstrated by susceptibility and NPD measurements. The magnetic reflections observed in the neutron patterns can be indexed with a propagation vector k = 0. Both compounds present the same magnetic structure, which is given by the basis vector (0, 0, A(z)). It can be described as antiferromagnetic (010) layers of magnetic moments lying along the c direction, which are antiferromagnetically coupled along the b direction. For the Ni(2+) ions, the ordered magnetic moment at T = 2.3 K is 2.11(3) micro(B), whereas for Mn(2+) at T = 2.6 K, |m| = 4.64(2) micro(B), consistent with the electronic configurations te (Ni(2+)) and te (Mn(2+)).     

A neutron diffraction and 170Yb Mössbauer investigation of the perovskite ytterbium titanium oxide

A phase with the perovskite structure (Pbnm) and a composition YbTiO_2.95 has been prepared by a high-temperature carbothermic method. Neutron diffraction shows a colinear ferrimagnetic structure at 7 K with Yb and Ti moments antiparallel along the c-axis of the orthorhombic cell and an Yb moment of 1.8(3)μ_B. ¹⁷⁰Yb Mössbauer measurements find a more precise and accurate value of 2.0(1)μ_B from the maximum hyperfine field. From the temperature dependence of the hyperfine field a T_c = 42(1) K is found. The Yb sublattice magnetization below T_c follows a Brillouin function. At low temperature a distribution of hyperfine fields is observed which is attributed to a random distribution of defects surrounding the Yb sites. The magnetic structure is discussed in relation to possible values for the crystal field parameters, especially B²₀.     

The crystallographic and magnetic structure of Ni2O3H

A nickel oxide hydroxide, nominally Ni₂O₃H, has been synthesized using hydrothermal techniques. The crystallographic and magnetic structures of the material have been determined at 4.5 K from powder neutron diffraction data. The material, which is nonstoichiometric with respect to Ni and H, contains Ni²⁺ and Ni⁴⁺ rather than Ni³⁺ and is orthorhombic (space group Pnmn; a = 5.084(1) Å, b = 2.9103(6) Å, c = 13.954(3) Å). Magnetic moments are aligned along the c axis with antiferromagnetic order. Powder neutron diffraction data collected above the Néel temperature, at 298 and 473 K, revealed no significant changes to the structure or localized electron model.     

High-pressure synthesis, crystal and electronic structures, and transport properties of a novel perovskite HgSnO3

We synthesized a novel perovskite-type oxide, HgSnO3, under high pressure and high temperature, and investigated the crystal and electronic structures as well as the transport properties. It was found that HgSnO3 possesses a trigonal-hexagonal lattice with space group R3c. The band gap of HgSnO3 estimated by diffuse reflectance spectrum measurement is relatively small (1.6 eV), irrespective of the large octahedral tilting distortion. The small band gap is caused by the increase in the bandwidth of the conduction and valence bands due to mixing between the empty Hg 6s orbitals and the antibonding Sn 5s-O 2p states and the mixing between the filled Hg 5d orbitals and the O 2p states, respectively. The electronic resistivity, Seebeck coefficient, and Hall coefficient measurements indicate that as-synthesized HgSnO3 is an n-type semiconductor.     

DFT study of electronic and magnetic structure of perovskite and post-perovskite CaRhO3

Within density functional theory the equations of state for perovskite (PV) and post-perovskite (PPV) forms of CaRhO₃ are obtained with equilibrium values of volume in agreement with experiment. Energy magnitudes point to a stabilization of PPV versus PV. This is interpreted by analyses of the charge density and the chemical bonding plots, showing that the Rh–O interactions within two oxygen sublattices are selectively differentiated and reinforced for one of the two oxygen sublattices within PPV variety with respect to PV one. Investigation of the magnetic properties shows no magnetic order and a metallic character for PV while ferromagnetism occurs in PPV with a tendency to insulating behavior. This long range order is favored by direct t_2g–t_2g interactions through edge sharing octahedra in PPV CaRhO₃, stronger than indirect t_2g–p_π–t_2g ones in PV variety.     

High-pressure synthesis, structure, and characterization of a post-perovskite CaPtO3 with CaIrO3-type structure

A new ternary platinum oxide, CaPtO3 was synthesized under a pressure of 7 GPa and a temperature of 1000 degrees C. The crystal structure of CaPtO3 was determined by Rietveld analysis of the X-ray powder diffraction data. CaPtO3 has a layered CaIrO3-type structure (orthorhombic, space group: Cmcm), which is the same as that of a post-perovskite MgSiO3 in the Earth's lower mantle. The magnetic susceptibility data indicate that the Pt ion in CaPtO3 is tetravalent in the low spin state with an electron configuration of t2g(6)eg(0)(S = 0). This finding is consistent with the insulating behavior.     

Powder neutron diffraction and magnetic susceptibility of 248CmO2

We report the first neutron diffraction experiments on a sample of approximately 55 mg of CmO₂, using the rare isotope ²⁴⁸Cm. Preparative techniques have assured that the sample is close to stoichiometry and X-ray patterns show only the expected f.c.c. lines. The neutron diffraction experiments confirm that the sample is nearly stoichiometric and rule out the possibility that the oxygen atoms form a superlattice to accommodate a mixture of Cm₃₊ and Cm⁴⁺ ions. Magnetic susceptibility measurements give a μ_eff value of (3.36 ± 0.06)μ_B for the effective paramagnetic moment, consistent with many other determinations of this property in Cm(IV) compounds, which have been interpreted in the past as indicating substantial Cm³⁺ impurity. In view of the contradiction between bulk susceptibility on the one hand and thermogravimetric and diffraction results on the other hand, we suggest that a re-evaluation of the electronic ground states in the actinide dioxides may be needed. Some other contradictory experimental results are given.     

The structure of calcium-ammonia solutions by neutron diffraction

The microscopic structures of calcium-ammonia solutions have been established by using neutron diffraction. Total structure factors measured at 230 K reveal immediately the evolution of an uncommonly intense diffraction prepeak in the metallic solutions. As concentration is increased from 4 mole percent metal to 10 mole percent metal (i.e., saturation), this feature intensifies and shifts from 0.6 to 0.9 A(-1). It is therefore evidence of well developed intermediate-range ordering among the solvated cations, and is a microstructural signature of the observed strong phase separation of metallic (concentrated) and nonmetallic (dilute) solutions. The technique of isotopic labelling of *N by 15N was then used in conjunction with difference analysis to focus on the solvent structure in metallic solutions at 4 and 10 mole percent metal. These nitrogen-centered functions are analyzed in conjunction with classical Monte Carlo computer simulation techniques, to provide us with detailed insight into the calcium solvation and the extent of hydrogen bonding. We find that calcium is solvated by approximately 6-7 ammonia molecules, with a Ca-N distance of around 2.45 A. There is evidence of hydrogen bonding among the solvent molecules, even in the saturated 10 mole percent metal solution.     

Electron diffraction investigation of the structure of the 3,3-dimethyl-3-silathiophane molecule

Chemistry Faculty, M. V. Lomonosov Moscow State University. Irkutsk Institute of Organic Chemistry. Translated from Zhurnal Strukturnoi Khimii, Vol. 32, No. 2, pp. 148–151, March–April, 1991.     

An investigation of the structure of 12HBaCoO2.6 by electron microscopy and powder neutron diffraction

A combination of high-resolution electron microscopy and profile analysis of powder neutron data has been used to determine the structure of the perovskite-related phase 12HBaCoO_2.6. The structure is based on a 12-layer stacking sequence (ccchhh)₂ (space group $\text{P6}{}_3\text{mmc}$). The oxygen vacancies were found to be nonrandom and are introduced by the replacement of some BaO₃ layers by BaO₂ layers of the type found in Ba₃V₂O₈.     

Modeling the structure of amorphous MoS3: a neutron diffraction and reverse Monte Carlo study

A model for the structure of amorphous molybdenum trisulfide, a-MoS3, has been created using reverse Monte Carlo methods. This model, which consists of chains of MoS6 units sharing three sulfurs with each of its two neighbors and forming alternate long, nonbonded, and short, bonded, Mo-Mo separations, is a good fit to the neutron diffraction data and is chemically and physically realistic. The paper identifies the limitations of previous models based on Mo3 triangular clusters in accounting for the available experimental data.     

High-pressure hydrothermal crystal growth and multiferroic properties of a perovskite YMnO3

Orthorhombic perovskites RMnO(3) are representative of spin-driven ferroelectrics. When the radius of the rare-earth ion R is smaller than that of Dy, for instance in YMnO(3), the orthorhombic phase becomes metastable at ambient pressure, which impedes the crystal growth; thus, the detailed magnetic and multiferroic properties of the metastable phase have not been characterized. In this work, we successfully obtained single crystals of orthorhombic YMnO(3) using quasi-hydrothermal conditions under a high pressure of 5.5 GPa. Magnetic and dielectric measurements under magnetic fields revealed that the magnetic ground state is the commensurate E-type antiferromagnetic, while a cycloidal spin phase likely coexists in the intermediate temperature range, which enhances the magnetoelectric response to external fields.