New chloro and triphenylsiloxy derivatives of dioxomolybdenum(VI) chelated with pyrazolylpyridine ligands: Catalytic applications in olefin epoxidation

Dioxomolybdenum(VI) complexes of general formula [MoO₂X₂L₂] (X = Cl, OSiPh₃; L₂ = 2-(1-butyl-3-pyrazolyl)pyridine, ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate) were prepared and characterised by ¹H NMR, IR and Raman spectroscopy. The assignment of the vibrational spectra was supported by ab initio calculations. A single crystal X-ray diffraction study of the complex [MoO₂Cl₂{ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate}] showed that the compound is monomeric and crystallises in the tetragonal system with space group P4₁. The four complexes are active and selective catalysts for the liquid-phase epoxidation of olefins by tert-butylhydroperoxide. Selectivities to the corresponding epoxides were mostly 100% (for conversions of at least 34%) for the substrates cyclooctene, cyclododecene, 1-octene, trans-2-octene and (R)-(+)-limonene. For styrene epoxidation, the corresponding diol was also formed in significant quantities. The turnover frequencies for cyclooctene epoxidation at 55 °C were around 340 mol mol_Mo⁻¹ h⁻¹ for the chloro complexes and 160 mol mol_Mo⁻¹ h⁻¹ for the triphenylsiloxy complexes. The addition of co-solvents (1,2-dichloroethane or n-hexane) had a detrimental effect on catalytic activities. Kinetic studies for the two complexes bearing the ligand ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate revealed an apparent first order dependence of the initial rate of cyclooctene conversion with respect to cyclooctene or oxidant concentration.     

Vibrational interactions in dimethylgold(III) halides and carboxylates

The far-infrared and Raman spectra of binuclear molecules [Me₂AuX]₂ (X = Cl, Br, I) and [Me₂Au(OOCR)]₂ (R = Me, CF₃, Bu^t, Ph) in the 600–70 cm⁻¹ region are reported. The experimentally measured vibrational frequencies of [Me₂AuX]₂ are in a good agreement with density functional theory predictions. The Au…Au vibrational interactions predicted to be in the 270–60 cm⁻¹ region of [Me₂AuX]₂ far-IR and Raman spectra have been observed. The Raman-active Au…Au vibrations of the [Me₂Au(OOCR)]₂ molecules were found to be in the same region as those of [Me₂AuX]₂. The Au–X stretching modes were observed between 100 and 250 cm⁻¹ in accordance with the DFT predictions. Their frequencies in the IR spectra of [Me₂AuX]₂ increase in the sequence I < Br < Cl while the AuC₂ stretching frequencies decrease in the same order. This fact might be an evidence of the decreasing covalent character of the gold-halogen bridges. The Au–O stretching bands of dimethylgold(III) carboxylates have been observed in the 500–250 cm⁻¹ region, and Au–C stretching frequencies of both [Me₂AuX]₂ and [Me₂Au(OOCR)]₂ compounds have been found between 600 and 500 cm⁻¹.     

A new class of proton-conducting ionic plastic crystals based on organic cations and dihydrogen phosphate

Choline dihydrogen phosphate ([N_1.1.1.2OH]DHP) and 1-butyl-3-methylimidazolium dihydrogen phosphate ([C₄mim]DHP) were synthesized as a new class of proton-conducting ionic plastic crystals. Both [N_1.1.1.2OH]DHP and [C₄mim]DHP showed solid–solid phase transition(s) and showed a final entropy of fusion lower than 20 J K⁻¹ mol⁻¹ which is consistent with Timmerman’s criterion for molecular plastic crystals. The ionic conductivity of [N_1.1.1.2OH]DHP was in the range of 10⁻⁶ S cm⁻¹–10⁻³ S cm⁻¹ in the plastic crystalline phase. On the other hand, the ionic conductivity of [C₄mim]DHP showed about 10⁻⁵ S cm⁻¹ in the plastic crystalline phase. [N_1.1.1.2OH]DHP showed one order of magnitude higher ionic conductivity than [C₄mim]DHP in the temperature range where the plastic phase is stable.     

Coupled chemoenzymatic transfer hydrogenation catalysis for enantioselective reduction and oxidation reactions

Stereoselective reductions of prochiral ketones were performed using a new thermophilic, NAD-dependent alcohol dehydrogenase from Thermus sp. (TADH). The enzyme was produced on 2L-scale from recombinant Escherichia coli and purified by a simple, one-step heat treatment procedure yielding 220 mg of pure enzyme. Regeneration of NADH was catalyzed by the organometallic complex [Cp*Rh(bpy)(H₂O)]²⁺ using formate as a reducing agent. The catalytic performance of [Cp*Rh(bpy)(H₂O)]²⁺ in terms of total number of catalytic cycles and number of catalytic cycles per hour achieved herein (up to 1500 and more than 400 h⁻¹, respectively), are the highest reported for a non-enzymatic nicotinamide regeneration system so far. Chemoenzymatic reduction reactions in a two liquid phase setup were performed on a gramme-scale, for example, 1.3 g of enantiopure (1S,3S)-3-methylcyclohexanol was obtained after purification. The volumetric productivity reached up to 3.9 mM h⁻¹ with an average of 2.6 mM h⁻¹ (5.3 g L⁻¹ d⁻¹) over 10 h. In addition, chemoenzymatic oxidations utilizing the same catalyst set and molecular oxygen as a terminal electron acceptor were performed. Thus, the preparative value of chemoenzymatic transfer hydrogenations with [Cp*Rh(bpy)(H₂O)]²⁺ as a regeneration catalyst coupled especially to thermophilic ADHs was demonstrated.     

Vibrational spectroscopy of phthalocyanine and naphthalocyanine in sandwich-type (na)phthalocyaninato and porphyrinato rare earth complexes: Part 14. The infrared and Raman characteristics of phthalocyanine in “pinwheel-like” homoleptic bis[1,8,15,22-tetrakis(3-pentyloxy)phthalocyaninato] rare earth(III) double-decker complexes

The infrared (IR) spectroscopic data and Raman spectroscopic properties for a series of 13 “pinwheel-like” homoleptic bis(phthalocyaninato) rare earth complexes M[Pc(α-OC₅H₁₁)₄]₂ [M = Y and Pr–Lu except Pm; H₂Pc(α-OC₅H₁₁)₄ = 1,8,15,22-tetrakis(3-pentyloxy)phthalocyanine] have been collected and comparatively studied. Both the IR and Raman spectra for M[Pc(α-OC₅H₁₁)₄]₂ are more complicated than those of homoleptic bis(phthalocyaninato) rare earth analogues, namely M(Pc)₂ and M[Pc(OC₈H₁₇)₈]₂, but resemble (for IR) or are a bit more complicated (for Raman) than those of heteroleptic counterparts M(Pc)[Pc(α-OC₅H₁₁)₄], revealing the decreased molecular symmetry of these double-decker compounds, namely S₈. Except for the obvious splitting of the isoindole breathing band at 1110–1123 cm⁻¹, the IR spectra of M[Pc(α-OC₅H₁₁)₄]₂ are quite similar to those of corresponding M(Pc)[Pc(α-OC₅H₁₁)₄] and therefore are similarly assigned. With laser excitation at 633 nm, Raman bands derived from isoindole ring and aza stretchings in the range of 1300–1600 cm⁻¹ are selectively intensified. The IR spectra reveal that the frequencies of pyrrole stretching and pyrrole stretching coupled with the symmetrical CH bending of –CH₃ groups are sensitive to the rare earth ionic size, while the Raman technique shows that the bands due to the isoindole stretchings and the coupled pyrrole and aza stretchings are similarly affected. Nevertheless, the phthalocyanine monoanion radical Pc′⁻ IR marker band of bis(phthalocyaninato) complexes involving the same rare earth ion is found to shift to lower energy in the order M(Pc)₂ > M(Pc)[Pc(α-OC₅H₁₁)₄] > M[Pc(α-OC₅H₁₁)₄]₂, revealing the weakened π–π interaction between the two phthalocyanine rings in the same order.     

Functionalized N-heterocyclic carbene iridium complexes: Synthesis,structure and addition polymerization of norbornene

Picolyl, pyridine, and methyl functionalized N-heterocyclic carbene iridium complexes [Cp¹Ir(C^N)Cl]Cl (4, C^N = 3-Methyl-1-picolyimidazol-2-ylidene), [Cp¹Ir(C^N)Cl][Cp¹IrCl₃] (5), [Cp¹Ir(C-N)Cl]Cl (6, C-N = 3-Methyl-1-pyridylimidazol-2-ylidene) and [Cp¹Ir(L)Cl₂] (7, L = 1,3-dimethylimidazol-2-ylidene) have been synthesized by transmetallation from Ag(I) carbene species, and characterized by ¹H NMR, ¹³C NMR spectra and elemental analyses. The molecular structures of 5–7 have been confirmed by X-ray single-crystal analyses. The iridium carbene complexes 4 and 6 show moderate catalytic activities (3.03 × 10⁵ g PNB (mol Ir)^?1 h^?1 and 1.70 × 10⁶ g PNB (mol Ir)^?1 h^?1) for the addition polymerization of norbornene in the presence of methylaluminoxane (MAO) as co-catalyst. The produced polynorbornene have been characterized by IR, ¹H NMR and ¹³C NMR spectra, showing it follows the vinyl-addition-type of polymerization.     

New thioether–dithiolate complexes of Cp1Ir and some reactivity features

The reaction of [Cp1IrCl₂]₂ (Cp* = η⁵ − C₅Me₅) with the tridentate 3-thiapentane-1,5-dithiolate ligand, S(CH₂CH₂S⁻)₂ (tpdt), led to the formation of [Cp1Ir(η³ − tpdt)] (1) in 81% isolated yield. Subsequent reactions of 1 with [Cp1IrCl₂]₂ in 2:1 and 1:1 molar equiv ratios resulted in the formation of [Cp1Ir(μ − η²:η³ − tpdt)Cp1IrCl][PF₆] (2) and [Cp1Irμ − η²:η³ − tpdt)Cp1IrCl][Cp1IrCl₃] (3) in 86 and 79% yields, respectively, based on 1, whereas the reactions of 1 with [(COD)IrCl]₂ (COD = 1,5-cyclooctadiene) in 2:1 and 1:1 molar equiv ratios resulted in the formation of the homo-bimetallic derivatives Cp1Ir(μ − η¹:η³ − tpdt)(COD)IrCl (4) (92% yield) and [Cp1Ir(μ − η²:η³ − tpdt)(COD)Ir] [(COD)IrCl₂] (5) (82% yield). Reactions between 1 and [(COD)RhCl]₂, yielded the hetero-bimetallic derivatives Cp1Ir(μ − η¹:η³ − tpdt)(COD)RhCl (6) and [Cp1Ir(μ − η²:η³ − tpdt)(COD)Rh][(COD)RhCl₂] (7), in 92 and 93% yields, respectively. The reaction of 1 with methyl iodide gave mono-methylated derivative [Cp1Ir(η³-C₄H₈S₃Me)]I (8) (93% yield). All these compounds have been comprehensively characterized.     

Study of CuZnMOx oxides (M = Al,Zr, Ce,CeZr) for the catalytic hydrogenation of CO2 into methanol

CuO–ZnO–Al₂O₃ catalysts were synthesized by two methods, sol–gel and co-precipitation syntheses. Al₂O₃ was then substituted with other supports, such as ZrO₂, CeO₂ and CeO₂–ZrO₂ in order to have a better understanding of the support's effect. These catalysts containing 30 wt% of Cu were then tested for CO₂ hydrogenation into methanol. The effect of reaction temperature and GHSV on the catalytic behaviour was also investigated. The best results were obtained with a 30 CuO–ZnO–ZrO₂ catalyst synthesized by co-precipitation and calcined at 400 °C. This catalyst presents a good CO₂ conversion rate (23%) with 33% of methanol selectivity, leading to a methanol productivity of 331 g_MeOH.kg_cata⁻¹·h⁻¹ at 280 °C under 50 bar and a GHSV of 10,000 h⁻¹.     

Highly selective thiocyanate electrode based on bis-bebzoin-semitriethylenetetraamine binuclear copper(II) complex as neutral carrier

A novel selective thiocyanate PVC membrane electrode based on bis-bebzoin-semitriethylenetetraamine binuclear copper(II) [Cu(II)₂–BBSTA] as neutral carrier is reported, which displays an anti-Hofmeister selectivity sequence in following order: SCN⁻ > ClO₄⁻ > I⁻ >Sal⁻ >SO₃²⁻ >NO₃⁻ > H₂PO₄⁻ > Cl⁻ >NO₂⁻ > SO₄²⁻. The electrode exhibits Nernstian potential linear range to thiocyanate from 1.0 × 10⁻¹ to 9.0 × 10⁻⁷ mol/l with a detection limit 7.0 × 10⁻⁷ mol/l and a slope of −57.0 mV/decade in pH 5.0 of phosphorate buffer solution at 25 °C. The response mechanism is discussed in view of the AC impedance technique and the UV spectroscopy technique. From comparison of potentiometric response characteristics between the binuclear metallic complex copper(II) [Cu(II)₂–BBSTA] and mononuclear copper(II) metallic complex [Cu(II)–BBSDA], an enhanced response towards thiocyanate from the electrode based on binuclear metallic complex copper (II) [Cu(II)₂–BBSTA] was observed. The electrode based on binuclear copper(II) compound was used to determine the thiocyanate content in waste water with satisfactory results.     

Heptanuclear high-spin complex compounds based on S-donors

The precursors [Fe(III)(^SYL)Cl] (^SYLH₂) = N,N′-bis(1-hydroxy-Y-2-benzyliden)-1,6-diamino-3-thiohexane, (Y = H, 3EtO, 5Me) are high-spin (S = 5/2) complexes. The precursors are combined with [Fe(II)(CN)₆]⁴⁻ and [Co(III)(CN)₆]³⁻ to yield star-shaped heptanuclear clusters, [Fe(II)(CN–Fe(III)^SYL)₆]Cl₂ and [Co(III)(CN–Fe(III)^SYL)₆]Cl₃. The star-shaped compounds are high-spin (HS) systems at room temperature. On cooling to 20 K some of the iron(III) centers perform some HS–HS transition.     

Synthesis,structure and magnetic properties of pyrazolate-bridged dinuclear complexes [{M(NCS)(4-Phpy)}2(μ-bpypz)2] (M = Co2+ and Fe2+)

The synthesis and magnetic characterization of pyrazolato-bridged dinuclear complexes [{M(NCS)(4-Phpy)}₂(μ-bpypz)₂] (Hbpypz = 3,5-bis(2-pyridyl)-pyrazole; 4-Phpy = 4-phenylpyridine; M = Co²⁺ (1) and Fe²⁺ (2)) are described together with the X-ray crystal analysis of the cobalt complex. The structure of 1 shows that the desired coordination has been achieved with the cobalt atoms being coordinated to two bpypz to form the dimer. The X-ray diffraction patterns show 1 and 2 to be isomorphous at room temperature. 2 displays a single spin-crossover transition between the [HS–HS] and [LS–LS] states with T_c = 150 K.     

Mesoporous Fe2O3 nanoparticles as high performance anode materials for lithium-ion batteries

This work introduces an effective, inexpensive, and large-scale production approach to the synthesis of Fe₂O₃ nanoparticles with a favorable configuration that 5 nm iron oxide domains in diameter assembled into a mesoporous network. The phase structure, morphology, and pore nature were characterized systematically. When used as anode materials for lithium-ion batteries, the mesoporous Fe₂O₃ nanoparticles exhibit excellent cycling performance (1009 mA h g^− 1 at 100 mA g^− 1 up to 230 cycles) and rate capability (reversible charging capacity of 420 mA h g^− 1 at 1000 mA g^− 1 during 230 cycles). This research suggests that the mesoporous Fe₂O₃ nanoparticles could be suitable as a high rate performance anode material for lithium-ion batteries.     

Preface

The anisotropic rototranslational scattering spectra of nitrogen gas at high frequency up to 700 cm⁻¹ for several temperatures and from low densities are analyzed in terms of new site–site (M3SV) intermolecular potential and interaction-induced pair polarizability models, using quantum spectral shapes computations. Our theoretical calculations take into account multipole contributions from the mean value and anisotropy of the dipole–dipole polarizability tensor α, two independent components of the dipole–octopole polarizability tensor E and dipole–dipole–quadrupole hyperpolarizability tensor B. The high-frequency wings are discussed in terms of the collision-induced rotational Rayleigh effect and estimates for the dipole–octopole polarizability |E₄| are obtained and checked with recent ab initio theoretical value. Good comparison is found in the frequency range 0–400 cm⁻¹ between the theoretical and experimental spectra. When an exponential contribution [exp(−ν/ν₀)] with ν₀ = 425 cm⁻¹ is considered to model very short-range light scattering mechanisms at room temperature, good agreement is found over the whole frequency range.     

Electronic and vibrational circular dichroism spectroscopic study of non-covalent interactions of meso-5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin with (dG-dC)10 and (dA-dT)10

The non-covalent interactions of (dG-dC)₁₀ and (dA-dT)₁₀ with 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin (TMPyP) were studied using the combination of electronic circular dichroism (ECD), vibrational circular dichroism (VCD) spectroscopy, and UV–vis and IR absorption spectroscopy at different ratios of both components r = [oligonucleotide]/[TMPyP] = 2/1–10/1 where [oligonucleotide] and [TMPyP] are the amount concentrations of oligonucleotide per base-pair and TMPyP, respectively. It was shown that TMPyP with (dG-dC)₁₀ provided hemiintercalative binding mode for r = 4/1 that is manifested in vibrational spectra: The absorption band assigned to the C6O6 stretching vibration of guanine is shifted from 1683 to 1672 cm⁻¹, the corresponding VCD couplet from 1694(−)/1674(+) to 1684(−)/1663(+) cm⁻¹ and its intensity decreases. The absorption band assigned to the C2O2 stretching vibration of cytosine is shifted from 1652 to 1644 cm⁻¹ and its intensity increases. TMPyP with (dA-dT)₁₀ provided three binding modes: (i) external binding to the phosphate backbone, (ii) external minor groove binding for the ratios >6/1 and (iii) external major groove binging associated with the partial B- to Z-transition for the ratios <4/1. The major groove binding is manifested in VCD spectra by the intensity decrease of the bands 1655 and 1638 cm⁻¹ assigned to the thymine vibrations while the bands assigned to the adenine vibrations are unchanged. In the (dA-dT)₁₀–TMPyP complexes, the external binding to the phosphate backbone accompanied by self-stacking of porphyrins along the phosphate backbone chain is preferred at temperatures higher than 40 °C.     

P, ρ, and T measurements of the (limonene + β-pinene) mixtures

The density, ρ, and two derived properties, isothermal compressibility, κ_T, and the coefficient of cubic expansion, α_P, were obtained for the mixtures of 1-methyl-4-(1-methylethenyl)-cyclohexene, known as limonene, and (1S,5S)-6,6-dimethyl-2-methylenebibyclo[3.1.1]heptane, known as β-pinene, for nine different compositions and the pure components at five pressures from 20 MPa to 40 MPa and six temperatures from 283.15 K to 358.15 K. The experimental uncertainty for ρ, κ_T, and α_P were respectively ±0.5 kg · m⁻³, ±14 TPa⁻¹, and ±0.005k K⁻¹, with k = 2 for all of them. Density behaviour with temperature and pressure was as expected. The values of α_P and κ_T increase with temperature and decrease with increasing pressure. Two different equations of state, conventional SAFT and PC-SAFT, were applied to predict the densities of the mixture. The best predictions were achieved with PC-SAFT.     

A N,N′-diacetate benzodioxotetraazamacrocycle and its transition metal complexes

Reaction of 2,9-dioxo-1,4,7,10-tetraazabicyclo[1.10.1]hexadeca-1(11),13,15-triene-4,7-diacetic acid (H₂L1) with CuCl₂ · 2H₂O in ethanol at pH 6 led to the monomeric benzodioxochlorocomplex [Cu(L′1)Cl] (1) (HL′1 = monoethylesther of H₂L1). X-ray structural analysis has shown that in complex 1 the Cu is five-coordinated by two nitrogen and two oxygen atoms of the macrocycle and by a chloride, displaying a square pyramidal coordination geometry. One of the acetate arms does not coordinate to the Cu and has suffered an in situ ethanolic esterification reaction. The protonation constants of H₂L1 and the stability constants of its complexes with Cu²⁺, Ni²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ were determined by potentiometric methods and in some cases by ¹H NMR spectroscopy. The stability constants of the complexes follow the trend [Ni(H₋₁L1)]⁻ > [Cu(H₋₁L1)]⁻ ≫ [Pb(H₋₁L1)]⁻ > [Zn(H₋₁L1)]⁻ > [Cd(H₋₁L1)]⁻, probably due to steric requirements. Spectroscopic measurements (absorption and EPR) at different pH values have shown the effect of the pH on the coordination sphere of the Cu complexes.     

Electronic structure and Raman spectroscopy study of dibarium magnesium orthoborate,Ba2Mg(BO3)2

The samples of dibarium magnesium orthoborate Ba₂Mg(BO₃)₂ were synthesized by solid-state reaction. The X-ray diffraction (XRD) patterns and Raman spectra of the samples were collected. Electronic structure and vibrational spectroscopy of Ba₂Mg(BO₃)₂ were systematically investigated by first principle calculation. A direct band gap of 4.4 eV was obtained from the calculated electronic structure results. The top valence band is constructed from O 2p states and the low conduction band mainly consists of Ba 5d states. Raman spectra for Ba₂Mg(BO₃)₂ polycrystalline were obtained at ambient temperature. The factor group analysis results show the total lattice modes are 5E_u + 4A_2u + 5E_g + 4A_1g + 1A_2g + 1A_1u, of which 5E_g + 4A_1g are Raman-active. Furthermore, we obtained the Raman active vibrational modes as well as their eigenfrequencies using first-principle calculation. With the assistance of the first-principle calculation and factor group analysis results, Raman bands of Ba₂Mg(BO₃)₂ were assigned as E_g (42 cm⁻¹), A_1g (85 cm⁻¹), E_g (156 cm⁻¹), E_g (237 cm⁻¹), A_1g (286 cm⁻¹), E_g (564 cm⁻¹), A_1g (761 cm⁻¹), A_1g (909 cm⁻¹), E_g (1165 cm⁻¹). The strongest band at 928 cm⁻¹ in the experimental spectrum is assigned to totally symmetric stretching mode of the BO₃ units.     

Liquid range temperature of ionic liquids as potential working fluids for absorption heat pumps

The liquid range temperature of six ionic liquids (ILs) was determined in this work with the aim to propose suitable absorbents for heat pump systems. The selected ILs have three different cations, imidazolium, pyridinium and choline and each was combined with four different anions [NTf₂]⁻, [OTf]⁻, [MeSO₃]⁻ and [BETI]⁻. The lower limit, given by solid − liquid transitions, was determined using differential scanning calorimetry (DSC). The upper limit is given by the degradation temperature. This temperature is determined using thermogravimetric technique (TGA). Dynamic and isothermal methods have been combined to estimate the maximum operation temperature. ILs ageing effect was also analysed in this work.     

The thermodynamic properties of DyBr3(s) and DyI3(s) from T=5 K to their melting temperatures

Low-temperature calorimetric measurements have been performed on DyBr₃(s) in the temperature range (5.5 to 420 K ) and on DyI₃(s) from T=4 K to T=420 K. The data reveal enhanced heat capacities below T=10 K, consisting of a magnetic and an electronic contribution. From the experimental data on DyBr₃(s) a C⁰_p,m (298.15 K) of (102.2±0.2) J·K⁻¹·mol⁻¹ and a value for {S⁰_m (298.15 K) − S⁰_m (5.5 K)} of (205.5±0.5) J·K⁻¹·mol⁻¹, have been obtained. For DyI₃(s), {S⁰_m (298.15 K) − S⁰_m (4 K)} and C⁰_p,m (298.15 K) have been determined as (226.9±0.5) J·K⁻¹·mol⁻¹ and (103.4±0.2) J·K⁻¹·mol⁻¹, respectively. The values for {S⁰_m (5.5 K) − S⁰_m (0)} for DyBr₃(s) and {S⁰_m (4 K) − S⁰_m (0)} for DyI₃(s) have been calculated, giving S⁰_m (298.15 K)=(212.3±0.9) J·K⁻¹·mol⁻¹ in case of DyBr₃(s) and S⁰_m (298.15 K) =(233.1±0.7) J·K⁻¹·mol⁻¹ for DyI₃(s). The high-temperature enthalpy increment has been measured for DyBr₃(s) in the temperature range (525 to 799 K) and for DyI₃(s) in the temperature range (525 to 627 K). From the results obtained and enthalpies of formation from the literature, thermodynamic functions for DyBr₃(s) and DyI₃(s) have been calculated from T→0 to their melting temperatures at 1151.0 K and 1251.5 K, respectively.     

Raman scattering study of calcium hexaboride

We report significant difference in the Raman spectra of two different kinds of CaB₆ single crystals grown from boron purity 99.9% (3N) or 99.9999% (6N), respectively. Our Raman spectra of CaB₆ (3N), which are similar to those of previous measurement [N. Ogita, S. Nagai, N. Okamoto, M. Udagawa, F. Iga, M. Sera, J. Akimitsu, S. Kunii, Phys. Rev. B 68 (2003) 224305], show peaks at 781.3 cm⁻¹ (T_2g), 1140.1 cm⁻¹ (E_g), and 1283.5 cm⁻¹ (A_1g). The E_g mode shows a characteristic double-peak feature due to an additional weak broad peak centered at 1156.0 cm⁻¹. However, the Raman spectra of CaB₆ (6N) show sharp peaks at 772.5 cm⁻¹ (T_2g), 1137.9 cm⁻¹ (E_g), and 1266.6 cm⁻¹ (A_1g). The peak frequencies are down shifted as much as 17 cm⁻¹. In addition, no additional peak feature is observed for the E_g mode so that the mode is symmetric in the case of CaB₆ (6N). The X-ray powder diffraction patterns for both CaB₆ (3N) and CaB₆ (6N) show that the lattice parameters are essentially the same. The majority of the impurity in the 99.9% (3N) boron is assessed to be C. Thus we prepared Ca(B_0.995C_0.005)₆, CaB₆ (6N) doped with C, and looked for the difference in the Raman spectra. The Raman spectra of Ca(B_0.995C_0.005)₆ are nearly identical to those of CaB₆ (6N), indicating that the difference in the Raman spectra of CaB₆ (3N) and CaB₆ (6N) is not due to C impurity. However, presence of impurity, even if small amount, seems to be enough to trigger local-structure changes to lower symmetry inducing the difference in Raman spectra of CaB₆ (3N) and CaB₆ (6N).