Molecular Constants of MnF6 −2 Anion

Abstract

The normal coordinate analysis of XY₆ type of anions and molecules employing different force fields is well established.^1,2 Recently, the vibrational spectra and complete frequency assignment of MnF₆ ^?2 has been reported by Flint.³ Using the available vibrational data the force constants have been evaluated employing three potential models viz. general valence force field (GVFF), Urey-Bradley force field (UBFF) and orbital valence force field (OVFF). The mean amplitudes of vibration and Bastiansen-Morino shrinkages have also been computed at two temperatures.     

Antiferromagnetic ordering in the double perovskites La2−xSrxCoRuO6

The electric transport and magnetic susceptibility of double perovskites La_2?xSr_xCoRuO₆ have been studied over a temperature range up to 800–1000 K. The crystal and magnetic structure has been determined by neutron diffraction on two samples of the series, x=0.6 and 1.4, which represent the electron- and hole-doped systems with respect to “ideal” single-valent insulator x=1. The study shows that spins in both the Co and Ru f. c. c. like sublattices exhibit a long-range ordering of the antiferromagnetic type II (T_N=60 K for x=0.6 and T_N=60–80 K for x=1.4).     

Magnetic properties of CuSb2−xTaxO6 with tri-rutile structure

We report magnetic properties of CuSb_2−xTa_xO₆ with tri-rutile type structure. Solubility limit was determined to be x=1.0 by the solid-state reaction and X-ray diffraction measurements. The value of J, the magnetic interaction obtained by Bonner–Fisher fitting, was found to decrease gradually with Ta content x. The value of T_N also decreases with x and seems to be zero at x=0.6. As a result of our structural analysis, this fact shows the crossover from one-dimensional to two-dimensional magnetic system with a local distortion caused by Ta substitution. Although the ground state of the compound with x>0.6 is not clear, it can be considered that the long-range antiferromagnetic order disappears and that the quantum critical regime such as a spin-singlet state is realized in this system.     

Crystal structure and photoluminescence of (Ba1−x−ySryEux)9Sc2Si6O24

Divalent europium-doped alkaline earth metal silicate phosphors, (Ba_1?x?ySr_yEu_x)₉Sc₂Si₆O₂₄ (x=0.005–0.1, y=0–0.95), have been successfully prepared by solid-state reaction at 1350 °C. The analysis of X-ray diffraction shows that the compounds are in a single phase at the proper concentration of Sr²⁺. At room temperature, the Eu²⁺-activated Ba₉Sc₂Si₆O₂₄ phosphor exhibits a single emission band peaking at about 506 nm. With the increasing content of Sr²⁺, the luminescent intensity of (Ba_1?x?ySr_yEu_x)₉Sc₂Si₆O₂₄ weakens, and the emission peak shifts towards red. Luminescence concentration quenching occurs when Eu²⁺ content x is more than 1 mol% in (Ba_1?x?ySr_yEu_x)₉Sc₂Si₆O₂₄ (y=0/0.2). At low temperatures (Ba_0.9?ySr_yEu_0.1)₉Sc₂Si₆O₂₄ (y=0/0.2) phosphors have two emission bands corresponding to different Eu²⁺ crystallographic sites. The high energy peak (P₁) is quenched at room temperature, while the low energy peak (P₂) weakens much more slowly owing to the energy transfer from P₁ to P₂.     

Phase transition studies in CoSiF6·6H2O and Co1−xZnxSiF6·6H2O by Mn(II) EPR

Phase transition studies on single crystals of CoSiF₆·6H₂O and Co_1−xZn_xSiF₆·6H₂O (x = 0.082, 0.182, 0.248) using Mn(II) EPR at X-band were carried out in the temperature range 10–300 K. Phase transitions with considerable thermal hysteresis have been detected in CoSiF₆·6H₂O and Co_1−xZn_xSiF₆·OH₂O (x = 0.082, 0.182). In CoSiF₆·6H₂O, the transition temperatures during cooling (T^c_c) and during heating (T^h_c) were found to be considerably less than those obtained previously from magnetic anisotropy experiments, i.e. 230 and 246 K as against 246 and 259 K. These characteristic temperatures for the dilute crystals having x = 0.082 and 0.182 were 205, 218 K and 175, 185 K, respectively. It is thus evident that magnetic dilution with zinc causes a progressive reduction in the transition temperatures as well as in the thermal hysteresis. The phase transition manifests itself in a large discontinuous increase in the Mn(II) zero-field splitting parameter (D), i.e. from −187 ± 1 G to −290 ± 1 G, in these crystals. The hyperfine coefficient (A) also increases discontinuously from 99 ± 1 G to 102 ± 1 G at T_c. In the dilute crystal with x = 0.248 a very slow continuous decrease in D and practically no change in A have been detected with a lowering of the temperature. A small temperature independent positive g-shift (g = 2.020 ± 0.005) has been observed in all the crystals studied. The spin-lattice relaxation time (T₁) of the Co(II) ion in CoSiF₆·6H₂O at different temperatures, both above and below T_c, has been estimated from the observed Mn(II) hyperfine linewidth. A discontinuous change in T₁ at T_c has been observed. Analysis of the temperature dependence of the linewidth has further revealed that a two-level Orbach process describes well the thermal behaviour of the spin-lattice relaxation of the Co(II) ions in these crystals. The location of the first excited level in CoSiF₆·6H₂O determined from such studies at temperatures both above and below T_c has been examined in the light of the ligand field energy scheme of the Co(II) ion in an octahedral field. It is concluded that the phase transition in this crystal is probably accompanied by a change in sign of the axial field parameter (Δ).     

Tunneling conductance of a Bi2−xPbxSr2Ca2Cu3O10−y-SnO2 junction

Fine structures were observed in the tunneling conductance of a sintered Bi 2223-SnO₂ junction. The structures are weaker than those of a single crystalline Bi2212. They correspond to the Raman spectrum of Bi2223 and approximately to the phonon density of states of Bi2212. The structures must therefore be phonon structures, and phonons may contribute substantially to highT _c superconductivity. Multiphonon structures are scarcely discernible. Hence we propose a new model, in place of the prior multiphonon model, to explain the rapid increase inT _c with the CuO₂ layer number. 2 (21 K)=68±4 meV inT _c=98±5 K. 2 (0)/k _B T _c is 8.1±0.9. The temperature dependence of the gap was also observed and discussed.     

Electrical conductivity of LiNb1−xTaxWO6

Polycrystalline LiNb_1−xTa_xWO₆, which is a lithium ionic conductor, is fabricated. This system forms a solid solution throughout the range of 0⩽x⩽1. The electrical conductivity of the LiNb_1−xTa_xWO₆ system is studied by the ac and the dc methods. LiNb_0.75Ta_0.25WO₆ shows the highest conductivity in the system, 1.0 × 10^-3 (S/m) at 150°C. The electronic transference number of this solid solution is generally less than 10^-3, and LiNb_0.75Ta_0.25WO₆ shows the smallest electronic transference number, 6 × 10^-5. In addition, the activation energy of lithium ionic conductivity is found to become higher as the Ta content increases.     

Magnetoelastic effects in La1−xCexB6

The elastic constants of La_1−xCe_xB₆ (x = 0.00, 0.01, 0.03) single crystals were determined by ultrasonic measurements. The temperature dependence of the elastic symmetry modes c₄₄ and (c₁₁ − c₁₂)/2 is explained on the basis of the crystalline-electric-field level scheme of CeB₆ by taking the magnetoelastic interaction into account. An influence of the Kondo effect on the elastic constants could not be detected.     

Elastic properties of (Pb y Sn1−y )2P2S6 solid solutions

Elastic properties of (Pb _y Sn_1?y )₂P₂S₆ solid solutions were studied using Brillouin scattering technique. Different scattering geometries were used for sound velocities determination that make it possible to find all components of the stiffness tensor. The concentration dependencies of volume compressibility, the Grüneisen parameter and Debye temperature were investigated. The results obtained were used to analyze chemical bonding with substitution of tin by lead at room temperature in the crystals under consideration.     

Crystal structure of the superconducting infinite-chain cuprate Ba2Cu2.89O6−y

The orthorhombic structure of Ba₂Cu_2.89O_6−y (sp. gr. Pccm, a=13.065(15), b=20.654(21) and c=11.431(8) Å) contains two nonintersecting symmetrically equivalent sets of parallel CuO₂-chains running along [110] and [1̄10] directions. Along the c-axis the Cu–O sheets are separated by identical layers of Ba-atoms. The Cu–O chains are distorted within the xy plane producing zigzag. Three of 14 positions of copper atoms were found to be partly vacant that caused the significant shifts of the neighbouring Ba atoms in the plane. The determined structure of Ba₂Cu_2.89O_6−y is compared to that of the monoclinic Ba₂Cu₃O₆ modification, of another infinite-chain compound Sr_0.73CuO₂, and to one of the substructures of ladder compounds (Sr,Ca)₁₄Cu₂₄O_41+y. The superconductivity in Ba₂Cu_2.89O_6−y is associated with the CuO₂-chains consisting of CuO₄-squares sharing edges. The attention is paid to the coincidence of the temperatures of the superconducting transitions in Ba₂Cu_2.89O_6−y and in ladder compound with the temperature of the sharp magnetic transition in another infinite-chain compound Sr_0.73CuO₂.     

Magnetic and magneto-transport properties of (Ba0.8Sr0.2)2−xNdxFeMoO6

The variation in structural, magnetic and magneto-transport properties of the double perovskite system (Ba_0.8Sr_0.2)_2?xNd_xFeMoO₆ {0.0<X<0.5} induced by Nd³⁺ doping (electron doping) has been studied and compared. The samples were prepared by standard solid state reaction method in a reducing atmosphere. The parent compound showed a saturation magnetic moment value of 3.75 μ_B/f.u. at an applied field of 0.5 T and a change in magnetoresistance value up to 26% (77 K, 0.8 T). The Rietveld refinement of the X-ray diffraction data showed a continuous decrease in lattice parameters and Fe–Mo ordering with increasing Nd³⁺ doping. The Curie temperature was found to increase with Nd³⁺ doping (3 K per % of Nd) while the saturation magnetic moment values and magnetoresistance values were found to decrease. The observed variations in magnetic and magneto-transport properties of the system are explained on the basis of increasing antisite disorder defects and band filling effects induced by electron doping. We have observed the dominant role of band filling in determining the low field magnetoresistance of these systems.     

Thermophoresis-assisted vapour phase synthesis of CeO2 and CexY1−xO2−δ nanoparticles

Thermophoresis-assisted chemical vapour deposition (CVD) was used to produce nano-sized particles of ceria and yttria-doped ceria. The solid precursors cerium trichloride and yttrium trichloride were used with oxygen. After homogeneous nucleation, condensation and coagulation, thermophoresis-assisted particle collection was achieved in two different ways. Particle dimensions were studied by transmission electron microscopy (TEM) and X-ray diffraction (XRD)-line broadening. Crystallite and particle dimensions varied from 9 to 30 nm and from 30 to 80 nm, respectively. The influence of process conditions is discussed.     

Structure and dielectric properties of Bi6 − x Sr x Ti2 − x Nb2 + x O18 (x = 0–2) solid solutions

The structural and electrophysical characteristics of a series of solid solutions of layered perovs-kite-like oxides Bi_{6 ? x} Sr _x Ti_{2 ? x} Nb_{2 + x} O₁₈ (x = 0, 0.25, 0.5, 1.0, 1.5, 2.0) have been studied. The temperature dependences of the relative permittivity ?/?₀(T) and dielectric loss tangent tanδ have been measured. The dependences of the maximum of the permittivity ?/?₀, Curie temperature T _C, lattice parameters, and the unit cell volume on x have been obtained. The structural parameter a, which corresponds to the polar direction, and the value of the orthorhombic distortion of the unit cell have been found to demonstrate noticeable negative deviations from the Vegard’s law. It has been established that the variations of the orthorhombic distortion correlate with the variations of the permittivity maximum; however, they do not markedly influence the Curie temperature that varies linearly over entire range of changes in x.     

Magnetic phase transitions in PrMn2−xCrxGe2

The structural and magnetic properties of PrMn_2−xCr_xGe₂ (0⩽x⩽1.0) were studied by X-ray diffraction and magnetization measurements. The powder samples crystallize in the ThCr₂Si₂-type structure, and the lattice constants at room temperature show almost no variation as Cr substitutes Mn. The observed phase transitions are summarized in a proposed magnetic x−T phase diagram and compared with previous Moessbauer spectroscopy and neutron diffraction results for x=0.     

Preparation and properties of substances in the series Cd2−xAg2xP2Se6, 0⩽x⩽2

Cd₂P₂Se₆ - Ag₄P₂Se₆ system was investigated and Cd_2−xAg_2xP₂Se₆ mixed crystals have been found for 0<x<0.3. Their conductivity has been found to have ionic character. Silver ion conductivity values are reported. From diffuse reflectance data, the bandgaps for Cd₂P₂Se₆ and Ag₄P₂Se₆ have been estimated as equal to 2.4 eV and 1.8 eV, respectively.     

Observation of the decaya 2 − (1320)→π − η′

The reaction ^– C ^– C has been studied at 36 GeV/c. A clear signal for the decaya ₂ ^– (1320) ^– · is observed in the ^– mass spectrum. The measured ratio of branchings is BR (a ₂ ^{– –} )/BR(a ₂ ^{– –} );(3.4±0.8±0.5)·10^–2.     

The hole concentration on oxygen sites in the highT c superconductor Y1−Ba2−Cu3−O7−x

From XPS core level spectroscopy the average copper charge on the Cu sites in the high temperature superconductor Y₁Ba₂Cu₃O_7–x is determined as function of the oxygen vacancy concentrationx. Analysis of these data leads to the suggestion that there are holes on the oxygen sites in the basal plane of the crystal structure. The probability for holes on these oxygen ions is rather constant for 0x0.3 with a value of 0.64 and decreases to zero forx=0.5. The dependence of the superconducting transition temperature on the hole concentration is discussed. An energy level diagram for Cu²⁺ and Cu³⁺ in YBa₂Cu₃O_7–x is constructed.