Magnetic susceptibility of Co4+(d5) in octahedral and tetrahedral environments

Measurements of magnetic susceptibility on compounds containing stoichiometric Co⁴⁺ are reported. The compound Ba₂CoO₄ has the Co⁴⁺(d⁵) ion at a tetrahedral site and displays a susceptibility of the expected magnitude for $\text{S =}\text{5}\text{2}$. The compounds Ba₃Co₂CO₉ and BaCoO₃ have the Co⁴⁺ at an octahedral site and show a susceptibility expected for low spin, $\text{S =}\text{1}\text{2}$. For the low spin case significant deviations from Kotani's calculated susceptibility were observed. Improvement of the theory was made through incorporation of the effects of distortion from perfect octahedral symmetry and the inclusion of higher electronic configurations above t⁵₂ in the ²T₂ ground state. A case of low spin Ni in octahedral environment is also reported.     

Phase transitions in a Cs2−xK1+xBiCl6 solid solution

A solid solution with a Cs_2?xK_1+xBiCl₆ (0 ≤ x ≤ 1) formulation and an elpasolite-related structure was prepared. At room temperature the symmetry is cubic (Fm3m) for x = 0 and triclinic $\text{(P}\text{I}\text{)}$ for x ≠ 0. For 0 ≤ x ≤ 1, various techniques enabled us to detect a phase transition of the ferroelastic-paraelastic type at t_c (°C) temperature. The t_c and ΔH_tc values are correlated to the size of the alkali ions.     

Infrared chemiluminescence from the reactions F + HBr,F + H2Se,and Cl + H2Se

Infrared chemiluminescence from HF and HCl has been observed and yielded vibrational and rotational population distributions for the reactions F + HBr, F + H₂Se, and Cl + H₂Se. Evaluation of the spectra recorded by a commercial Fourier-transform spectrometer under low-flow conditions gave the following relative vibrational populations: for F ? HBr. N_{υ = 1} : N_{υ = 2} : N_{υ = 3} : N_{υ = 4} = 0.45 : 0.31 : 0.13 : 0.11: for F + H₂Se, N_{υ = 1} : N_{υ = 2} : N_{υ = 3} : N_{υ = 4} : N_{υ = 5} = 0.29 : 0.35 : 0.24 : 0.09 : 0.03: for Cl + H₂Se, N_{υ = 1} : N_{υ = 2} : N_{υ = 3} = 0.40 : 0.51 : 0.09. All three vibrational surprisal plots show a significant deviation from linearity. Neglecting the contributions from N_{υ = 0}, the total energy is partitioned into vibration and rotation as follows: 〈f_V〉 = 0.49 and 〈f_R〉 = 0.09 for F + HBr, 〈f_V〉 = 0.41 and 〈f_R〉 = 0.07 for F + H₂Se, 〈f_V〉 = 0.53 and 〈f_R〉 = 0.10 for Cl + H₂Se. Inclusion of estimates for N_{υ = 0} gives the more realistic values 〈f_V〉 = 0.24, 0.34, and 0.49 respectively. Whereas 9 ± 3% of the collisions between F + HBr yield Br in the excited ²P_{$\text{1}\text{2}$} state, no rovibrationally excited HSe fragments were detected in the two other systems. Consistent values for the bond dissociation energy D₀⁰(HSeH) = 329 ± 5 kJ/mol and the enthalpy of formation ΔH₁₀⁰ (HSe) = 137 ± 5 kJ/mol are derived from the highest observed HCl and HF levels.     

Observation of nonlinearity and asymmetry in energies of EPR transitions of 160Gd3+ in La(C2H5SO4)3 · 9D2O at low magnetic fields

The energies of EPR transitions of ¹⁶⁰Gd³⁺ in La(C₂H₅SO₄₃ · 9D₂O at 77.2 K are observed to be nonlinear functions of field at low fields. The + $\text{3}\text{2}$, + $\text{1}\text{2}$ and ?$\text{3}\text{2}$, ?$\text{1}\text{2}$ transition energies converge asymmetrically below 10 G and differ by only ≈ MHz at the lowest fields employed.     

New defect vanadium dioxide phases

Two new monoclinic V₂O₄ phases were prepared at high pressure from the regular monoclinic (M₁) form of V₂O₄. The unit cell dimensions for the unmodified monoclinic (M₂) phase are: a = 9.083, b = 5.763, c = 4.532 Å, and β = 91.30°. The space group $\text{C 2}\text{m}$ is consistent with the crystallographic data. The new vanadium dioxide exhibited a structural transition and an abrupt, reversible change in resistivity (approx. 4 orders of magnitude) at 66°C similar to that observed in M₁-type V₂O₄. This new form of V₂O₄ is believed to be stabilized by chemical and structural defects. Controlled substitution of V⁵⁺ for V⁴⁺ in the structure led to yet another monoclinic (M₃) phase. This phase is closely related to the M₂ phase. The M₃ unit cell dimensions are: a = 4.506, b = 2.899, c = 4.617 Å, and β = 91.79°, having the space group $\text{P 2}\text{m}$. The substitution of V³⁺ yielded only monoclinic (M₁) derivatives. The modified products have varied semiconductor to metal transition temperatures which depend on the type and amount of substitution and defect structure.     

The crystal structure of Sc2Ru5B4

The crystal structure of Sc₂Ru₅B₄ has been determined by single-crystal X-ray analysis. Sc₂Ru₅B₄ crystallizes in the primitive monoclinic space group $\text{P2}\text{m}$ with a = 9.983(6), b = 8.486(4), c = 3.0001(3)Å, γ = 90.01(7)°, Z = 2. Deviations from the orthorhombic space group Pbam-D⁹_2h are small but significant. Intensity measurements were obtained from a four-circle diffractometer. The structure was solved by Patterson methods and refined by full matrix least-squares calculation. $\text{R =}\text{∑|ΔF|}\text{∑|F}{}_0\text{|}\text{= 0.036}$ for an asymmetric set of 863 independent reflections (|F₀|>2σ(F₀)). The crystal structure is characterized by two different types of boron atoms: (a) isolated borons B(1) and B(3) in distorted trigonal Ru-prisms with tetrakaidekahedral metal coordination: 6Ru + 3Sc, and (b) boron atoms B(2) and B(4) with a pronounced tendency to form boron pairs (B(2)-B(2) = 1.86 Å, B(4)-B(4) = 1.89 Å); the metal coordination of these boron atoms is 6Ru + 2Sc. Sc atoms have a coordination number of 17 consisting of 10Ru + 2Sc + 5B. The crystal structure of Sc₂Ru₅B₄ is a pentagon layer structure (Ru, B atoms) with a 4.3.4.3²-secondary layer of Sc atoms. The structure is furthermore related to the structure types of Ti₃Co₅B₂ and CeCo₃B₂. From powder photographs Sc₂Os₅B₄ is isotypic. No superconductivity was observed for Sc₂(Ru, Os)₅B₄ down to 1.5 K.     

Collinear collisions in a well potential

Collinear collisions on the H₃⁺ potential energy surface for different mass combinations m, M, m are investigated by means of classical trajectory computations. Both, the probability for reaction, P_t, and for complex formation, P_e, depend periodically on the mass parameter (1 + 2m/M)^{$\text{1}\text{2}$}. This can easily be understood in terms of vibrational excitation of the symmetric and asymmetric modes of the reaction complex. If the collinear constraint is lifted, none of these features survive.     

Electrical and magnetic properties and homogeneity ranges of mixed-valence cesium-vanadium oxides

Single crystals of CsV₂O₅, Cs₂V₅O₁₃, Cs_xV₂O₅, and CsV₃O₇, grown from melts, have been subjected to electrical conductivity and magnetic susceptibility measurements. CsV₂O₅ and Cs₂V₅O₁₃ are insulators exhibiting Curie-Weiss' law behavior with p_eff close to $\text{1.73}\text{V}{}^{\mathrm{4+}}\text{ion}$. They are stoichiometric according to the X-ray studies. CsV₂O₅, which has a very limited composition range (x ≈ 0.30?;0.33), is a semiconductor with temperature-dependent paramagnetism ($\text{p}{}_{\mathrm{<mtext>eff</mtext>}}\text{=}\text{1.83}\text{V}{}^{\mathrm{4+}}\text{ion}$). Cs_xV₃O₇ (0.30 ? x ? 0.40) is antiferromagnetic with a Néel temperature of ≈230 K. The temperature variation of the lattice parameters of Cs_0.33V₃O₇ has been determined with a Bond-type diffractometer. The same crystal was used for the conductivity measurements, and together these studies indicate that the transition from the semiconducting antiferromagnetic state to the semiconducting paramagnetic state takes place stepwise. The observed properties are discussed with reference to the crystal structures of the compounds.     

The enthalpies of formation of CH3Mn(CO)5 and of CH3Re(CO)5, and the strengths of the CH3Mn and CH3Re bonds

From measurements of the heats of iodination of CH₃Mn(CO)₅ and CH₃Re(CO)₅ at elevated temperatures using the ‘drop’ microcalorimeter method, values were determined for the standard enthalpies of formation at 25° of the crystalline compounds: ΔH^o_f[CH₃Mn(CO)₅, c] = ?189.0 ± 2 kcal mol^?1 (?790.8 ± 8 kJ mol^?1), ΔH^o_f[Ch₃Re(CO)₅,c] = ?198.0 ± kcal mol^?1 (?828.4 ± 8 kJ mo^?1). In conjunction with available enthalpies of sublimation, and with literature values for the dissociation energies of MnMn and ReRe bonds in Mn₂(CO)₁₀ and Re₂(CO)₁₀, values are derived for the dissociation energies: D(CH₃Mn(CO)₅) = 27.9 ± 2.3 or 30.9 ± 2.3 kcal mol^?1 and D(CH₃Re(CO)₅) = 53.2 ± 2.5 kcal mol^?1. In general, irrespective of the value accepted for D(MM) in M₂(CO)₁₀, the present results require that, D(CH₃Mn) = $\text{1}\text{2}$D(MnMn) + 18.5 kcal mol^?1 and D(CH₃Re) = $\text{1}\text{2}$D(ReRe) + 30.8 kcal mol^?1.     

A study of the Li1+xTi2−xO4 spinel system by diffuse-reflectance spectroscopy

The room-temperature diffuse-reflectance spectra of compositions within the Li_1+xTi_2?xO₄ spinel system ($\text{0 ≤ x ≤}\text{1}\text{3}$) show three absorption bands in the range 4000 to 48,000 cm^?1. Two high-energy absorption bands correspond to charge-transfer transitions from the oxygen-2p valence band to the titanium t_2g and σ1 conduction bands, where the σ1 band of e_g character has hybridized titanium-3d and titanium-4s parentage. The absorption band arising from promotion of electrons to the empty σ1 band does not alter with composition whereas the absorption band arising from promotion of electrons to the partially filled t_2g band narrows as the concentration of conduction electrons in the t_2g band decreases. These two high-energy absorption bands fall entirely within the ultraviolet spectral region, and the absorption edge in $\text{Li}{}_{\mathrm{<mtext>4</mtext><mtext>3</mtext>}}\text{Ti}{}_{\mathrm{<mtext>5</mtext><mtext>3</mtext>}}\text{O}{}_4\text{(x =}\text{1}\text{3}\text{)}$ occurs at 24,300 cm^?1 (3.02 eV). A low-energy absorption band is observed in compositions with $\text{x <}\text{1}\text{3}$ and in samples of $\text{Li}{}_{\mathrm{<mtext>4</mtext><mtext>3</mtext>}}\text{Ti}{}_{\mathrm{<mtext>5</mtext><mtext>3</mtext>}}\text{O}{}_4$ reduced in hydrogen at elevated temperatures. This band straddles the boundary between the visible and infrared spectral regions and shifts toward lower energy as the concentration of conduction electrons in the t_2g band decreases. The possible origins of the band are discussed; the argument is in favor of a d-d interband transition from states in the partially filled t_2g band to states in the empty σ1 band.     

Characterization of poly(styrene-co-divinylbenzene) microgels

Tightly cross-linked poly(styrene-co-divinylbenzene) microgels with molecular weights in the range 10⁷ to 10⁸ g mole^?1 were studied in solution in dimethylformamide containing 7 g dm^?3 LiBr. Laser light scattering photon correlation spectroscopy (PCS) was used to measure z-average translational diffusion coefficients D?_z and dilute solution viscometry to measure intrinsic viscosities [η]; values of the equivalent hydrodynamic radii 〈R〉${}_{\text{η}}^{\text{?}}$ and 〈R_D^?1〉 were derived. Weight-average molecular weights M?_w and z-average mean-square radii of gyration 〈S²〉_z were determined by conventional light scattering. The microgels were investigated by gel permeation chromatography, and molecular weight data obtained using the [η]M calibration procedure were in satisfactory agreement with results obtained by electron microsocopy and conventional light scattering.     

The rotational spectrum and molecular properties of a hydrogen-bonded dimer of phosphine and hydrogen fluoride

⁵⁷Fe Mössbauer spectra have been obtained for Fe(p-CH₃C₆H₄SO₃)₂ between 2.3 and 300 K in zero field, and at 2.3 and 4.2 K in longitudinal applied magnetic fields ranging from 1.1 to 5.6 T. The complex is a fast-relaxing paramagnet under all conditions studied and there is no evidence of antiferromagnetic exchange coupling. The FeO₆ chromophore is distorted by a trigonal elongation and the orbital ground state is the [($\text{2}\text{3}$)^{$\text{1}\text{2}$}|±2〉 ? ($\text{1}\text{3}$)^{$\text{1}\text{2}$}|?1〉] doublet. The temperature dependence of the quadrupole splitting has been analysed via a crystal-field model to provide estimates of the axial field splitting parameter Ds = -93 cm^-1, spin-orbit coupling constant λ = -70 cm^-1, and fine structure constant Dσ = -28 cm^-1. The magnetic properties of the complex are described by treating the ground state as a non-Kramers doublet with fictitious spin ? = $\text{1}\text{2}$. Five separate Mössbauer-Zeeman spectra can be fitted in this spin-hamiltonian approximation with identical values of the g- and A-tensor components, viz. g_⊥ = 1.0, g_u = 9.0; A_⊥ ≈ 2.0 mm s^-1. A_u = -1.79 mm s^-1. The trigonal z axis, the z axis of the electric field gradient tensor, and the easy axis of magnetisation are collinear, and the saturation value of the internal hyperfine field along this axis is +13.0 T.     

Observation of a possible pure electric quadrupole transition in solid Cs2NaSmCl6

A no-phonon transition has been observed in Cs₂NaSmCl₆ at 6355 cm^?1. This transition is assigned, in octahedral symmetry, as E″_u(⁶H_{$\text{5}\text{2}$}) → E'_u(⁶F_{$\text{1}\text{2}$}) and is proposed to be of pure electric quadrupole origin. A comparison between the experimental and calculated intensity and the orientation-dependent intensity of an associated vibronic transition lend support to this assignment.     

The importance of spin relaxation in the delayed fluorescence of molecular crystals

Deactivation rate constants of spin-orbital excited Br atoms in the reactions Br(²P_{$\text{1}\text{2}$}) + O₂ → Br(²P_{$\text{3}\text{2}$}) + O₂ (k₁), and Br(²P_{$\text{1}\text{2}$}) + NO → Br(²P_{$\text{3}\text{2}$}) + NO (k₄) have been measured with a photodissociative IBr laser on the electronic transition ²P_{$\text{1}\text{2}$}?²P_{$\text{3}\text{2}$} in the Br atom (λ = 2.7 μm). The values obtained are (6.4 ± 1.8) × 10^?14 cm³ s^?1 and (1.9 ± 0.6) × 10^?12 cm³ s^?1, respectively. Comparison with published data leads to the conclusion that, contrary to a widely accepted point of view, the high rate constants for the quenching of excited halogen atoms are due to resonant energy transfer processes and not to the paramagnetic nature of the quencher.     

Composition-dependent microhardness and its relationship to bonding in sodium tungsten bronzes

The technique of microhardness measurements using diamond indenters is outlined and assessed for its potential use in quantifying bonding changes and studying reactions in nonstoichiometric crystals. Results are presented for both Vickers and Knoop hardness values on {001} and {011} crystal planes of cubic sodium tungsten bronzes, Na_xWO₃, with x in the range 0.5 to 0.75. The Knoop data show that in only one direction, 〈110〉 on {001}, is the hardness sensitive to changes in composition. Hardness in the 〈110〉 directions and the degree of anisotropy increase as the sodium content of the bronze increases. All the crystal faces examined showed marked anisotropic behavior, with 〈110〉 being about 50% harder than 〈100〉 on {001} faces, while on {011} planes hardness increases in the sequence $\text{〈100〉:〈21}\text{1}\text{〉:〈11}\text{1}\text{〉 ≈ 〈01}\text{1}\text{〉}$. Hardness results from isomorphous and isoelectronic ReO₃ are considered with the Na_xWO₃ data to show the dominant role played by Na⁺WO₃ matrix interactions in determining the properties of these materials. The results are discussed in terms of current bonding theories for bronzes.     

Contribution à l'étude structurale de la wüstite solide de haute température

The structure of wüstite Fe_1?zO is studied by neutron diffraction on one polycrystalline sample under equilibrium conditions at high temperature. The mean isotrope temperature factor B is expressed as a sum of two parts, B_Th and B_St, which vary linearly with a single parameter, respectively temperature and z. A classification is established for clusters ($\text{m}\text{n}$) settled from m vacancies in octahedral sites and n Fe^III ions in interstitial sites. Sixteen values have been experimentally determined for the vacancies to interstitials ratio $\text{? =}\text{(z + t)}\text{t}\text{=}\text{m}\text{n}$. A constant value of ?, which is lower than 3, is observed. This result characterizes the short-range order. It eliminates several possibilities of clusters like those obeying the relation $\text{? =}\text{(1 + 3n)}\text{n}$. Other clusters, namely ($\text{16}\text{6}$) or ($\text{40}\text{14}$), might agree. The ($\text{8}\text{3}$) and ($\text{9}\text{4}$) clusters obtained from ($\text{4}\text{1}$) clusters joined by an edge would be the more likely. An analysis of diffuse scattering eliminates the hypothesis of large domains with inverse spinel structure. The structural differences between the three varieties W₁, W₂ and W₃ would not be found in a structural change of clusters.     

Über Ordnungs-Unordnungsphänomene bei Sauerstoff perowskiten vom Typ A2+3B2+M5+2O9

Perovskites of the type A²⁺₃B²⁺M⁵⁺₂O₉, where A²⁺ = Ba, Sr; B²⁺ = Mn, Co, Ni, Zn; M⁵⁺ = Nb, Ta, show order-disorder phenomena. At lower temperatures a thermodynamically unstable disordered cubic perovskite is formed ($\text{1}\text{3}$ formula unit—$\text{AB}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{M}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{O}{}_3$—in the cell), which transforms irreversibly into a 1: 2 ordered high-temperature form with 3L structure (sequence (c)₃). For A²⁺ = Ba this lattice is hexagonal (space group $\text{P}\text{3}\text{m1}$; one formula unit in the cell); with A²⁺ = Sr a triclinic distortion is observed. For Ba₃CoNb₂O₉ a second transformation into a cubic disordered perovskite takes place at 1500°C. This transition is reversible and of the order-disorder type. The vibrational and diffuse reflectance spectra are discussed.     

Crystal structures and magnetic properties of BaTiF5 and CaTiF5

Single crystals of BaTiF₅ and CaTiF₅ were obtained by the Czochralski and Bridgman techniques, respectively. The crystal structures were determined by X-ray diffraction; BaTiF₅: $\text{14}\text{m}\text{, a = 15.091(5)}$Å, c = 7.670(3)Å; CaTiF₅: $\text{I2}\text{c}\text{, a = 9.080(4)}$Å, b = 6.614Å, c = 7.696(3)Å, β = 115.16(3)°. Both structures are characterized by the presence of either branched or straight chains of TiF₆ octahedra. BaTiF₅ contains the unusual dimeric unit (Ti₂F₁₀)^4?. Magnetic susceptibility measurements were performed on both compounds in the temperature range 4.2 to 300 K, however, no evidence for magnetic interactions between the Ti³⁺ moments were observed.     

The crystal structure of vanadium ditelluride,V1+xTe2

Vanadium ditelluride, V_1.04Te₂, has a Cd(OH)₂-type structure with unit cell dimensions a_h = 3.638 Å and c_h = 6.582 Å above the transition temperature T_t of 482 K. Below T_t the structure is monoclinic, space group $\text{C2}\text{m}$, with cell dimensions a_m = 18.984 Å(≈3a_h√3), b_m = 3.5947 Å (≈a_h), c_m = 9.069 Å (≈√(3a²_h + c²_h)), β = 134.62°. This low-temperature form is isostructural with NbTe₂ and TaTe₂ (which do not show a phase transition); the vanadium atoms form double zigzag chains with VV distances of 3.316 Å, which distort the Te lattice. Complex diffraction patterns were observed due to the simultaneous occurrence of the distortion of the Cd(OH)₂-type structure of vanadium ditelluride in three equivalent directions. Similar patterns were found for the Nb and Ta ditellurides.