Magnetic ordering in HoRu2Si2, HoRh2Si2, TbRh2Si2 and TbIr2Si2 by neutron diffraction

Neutron diffraction study of polycrystalline HoRu₂Si₂, HoRh₂Si₂, TbRh₂Si₂, and TbIr₂Si₂ was performed in the temperature range between 4.2 and 300 K. For HoRu₂Si₂ the magnetic spin alignment of a linear transverse wave mode below the Néel temperature 19 K is observed. This static moment wave is propagating along the b-axis with k=(0, 0.2, 0) and is polarized in the c-axis. The root-mean-square and maximum saturation moments per Ho atom are 9.26 and 13.09μ_B, respectively. HoRh₂Si₂, TbRh₂Si₂ an TbIr₂Si₂ are simple collinear antiferromagnets of +-+- type with Néel temperatures of (27±1), (98±2) and (72±3) K, respectively. For TbRh₂Si₂ and TbIr₂Si₂ magnetic moments are localized on RE ions only and are aligned along the tetragonal axis, while for HoRh₂Si₂ they form an angle ø = (28±3)°.     

13C-selective IRMPD of CBr2F2/Cl2 and CCl2F2/Br2 systems

The CO₂ TEA laser irradiation of CBr₂F₂ in the presence of Cl₂ yielded ¹³C-enriched CBrClF₂ and ¹³C-enriched CCl₂F₂ under selected experimental conditions. As the photolysis proceeded, the ¹³C concentration of CBrClF₂ decreased gradually and that of CCl₂F₂ increased up to 90% or higher. These results can be explained by the mechanism involving the secondary ¹³C-selective IRMPD of the primary product CBrClF₂. On the other hand, the carbon-containing product for a CCl₂F₂/Br₂ system was only CBrClF₂; the further IRMPD of which probably regenerated CBrClF₂ in the presence of Br₂. The decomposition probabilities of ¹²C- and ¹³C-containing molecules in both systems were measured as functions of laser line, laser fluence, and reactant pressures.     

Spin fluctuation effects in substituted CeRu2Si2 and YbPd2Si2 alloys

Effects of chemical substitution in CeRu₂Si₂, a well-studied heavy fermion system and YbPd₂Si₂ have been investigated through magnetic susceptibility and x-ray diffraction in the systems CeRu _x Si₂, CeRu_2−x Os _x Si₂, CeRu₂Si_2−x Ge _x and YbPd₂Si_2−x Ge _x . Replacing silicon by germanium generates normal chemical pressure effect, namely, Ce and Yb atoms in CeRu₂Si₂ and YbPd₂Si₂ became more and less magnetic respectively. With increasing Ge concentration, CeRu₂Si_2−x Ge _x exhibits larger susceptibility at low temperature, goes to an antiferromagnetic state and finally becomes ferromagnetic. In YbPd₂Si_2−x Ge _x , increasing Ge concentration drives Yb atoms to more divalent state. Electronic effects are more pronounced in CeRu_2−x Os _x Si₂ though CeRu₂Si₂ and CeOs₂Si₂ have very nearly the same lattice parameters. It is conjectured that CeRu₂Si_2−x Ge _x may be the first Ce-based heavy fermion having a magnetic ground state. The authors felicitate Prof. D S Kothari on his eightieth birthday and dedicate this paper to him on this occasion.     

Conversion of nitrogen oxides in N2:O2:CO2 and N2:O2:CO2:NO2 mixtures subjected to a dc corona discharge

This paper concerns the influence of a direct current (dc) corona discharge on production and reduction of NO, NO₂ and N₂O in N₂:O₂:CO₂ and N₂:O₂:CO₂:NO₂ mixtures. The corona discharge was generated in a needle-to-plate reactor. The positively polarized electrode consisted of 7 needles. The grounded electrode was a stainless steel plate. The gas flow rate through the reactor was varied from 28 to 110 cm³/s. The time-averaged discharge current ranged from 0 to 6 mA. It was found that in the N₂:O₂:CO₂ mixture the corona discharge produced NO, NO₂ and N₂O. In the N₂:O₂:CO₂:NO₂ mixture the reduction of NO₂ was between 6–56%, depending on the concentration of O₂, gas flow rate and corona discharge current. The NO₂ reduction was accompanied by production of NO and N₂O. The results show that efficient reduction of nitrogen oxides by a corona discharge cannot be expected in the mixtures containing N₂ and O₂ if reducing additives are not employed.     

IR-Spectra of M2UO2F4.H2O Complexes

This work is devoted to the study of and Rb₂UO₂F₄.H₂O and Cs₂UO₂F₄. H₂O IR-spectra with the aim of obtaining a set of vibration frequencies characterising M₂U0₂F₄.H₂O complexe elucidating the role and nature of water bonds in the structures of the above-mentioned compounds; and receiving preliminary information on the structure of M₂UO₂F₄.H₂O. The investigated compounds were synthesized in accord with our previous paper¹. M₂UO₂D₂O and M₂UO₂F₄.HDO were obtained by recrystallizing M₂UO₂F₄.H₂O from D₂O and HDO respectively     

μ+SR study of LaNi2As2, URh2Si2 and CeRh2Si2

Muon spin relaxation experiments have been carried out in the paramagnetic and magnetically ordered states of URh₂Si₂ and CeRh₂Si₂. As the magnetic structure of these compounds is well known, these measurements can help to characterise their magnetic properties probed by μSR and to understand the μSR results of the heavy fermion compounds of the same crystallographic family. Our measurements show that the muons occupy two different crystallographic sites. The spectra of URh₂Si₂ and CeRh₂Si₂ in the magnetically ordered states are very different, probably reflecting their different magnetic structures. The spectra obtained on CeRh₂Si₂ are similar to the published spectra of the heavy fermion compound CeCu_2.1 Si₂. Muon spin rotation measurements on LaNi₂As₂ indicate that the muon is diffusing at 150 K.     

Polarized X-ray absorption study of Bi2Sr2CuO6 and Bi2Sr2CaCu2O8

Polarization dependent X-ray absorption measurements on single crystal Bi₂Sr₂CuO₆ (T _c =9 K) and Bi₂Sr₂CaCu₂O₈ (T _c =80 K) with one and two CuO₂ layers, respectively, show no energy shift of the Cu 2p main peak, and no relation between the amount of 3d(m=0, ±1) character and the critical temperatureT _c . At grazing incidence a structure in Bi₂Sr₂CaCu₂O₈ is found at 937 eV, which can be ascribed to a composite state of Cu 3d(z ²), Cu 4s ando–2p(z).     

Europium valency in EuCu2−x Fe x Si2, EuCu2Si2−x Ge x,EuCu2−x Si2+x and EuNi2−x Si2+x

Mössbauer spectroscopy, X-ray photoemission spectroscopy and magnetization studies of EuCu_2?x Fe _x Si₂, EuCu₂Si_2?x Ge _x , EuCu_2?x Si_2+x , and EuNi_2?x Si_2+x have been performed. In EuFe₂Si₂ and EuNi₂Si₂ the Eu ion is trivalent, in EuCu₂Si₂ it is of intermediate valency. In all systems withx=0, many inequivalent Eu sites of intermediate valency are formed. In EuCu_2?x Fe _x Si₂, the average valency of Eu moves nonmonotonically toward Eu³⁺. In EuCu₂Si_2?x Ge _x , EuCu_2?x Si_2+x and EuNi_2?x Si_2+x , the Eu average valency moves quickly towards Eu²⁺. In all three systems, whenx=1, all Eu ions are already stable divalent. The systems EuCuSi₃ and EuNiSi₃ order magnetically at 40 K and 34 K, and exhibit hyperfine fields of 323 kOe and 458 kOe, respectively. XPS and Mössbauer isomer shift determinations of the average Eu valence as a function of temperature in EuCu_1.5Fe_0.5Si₂ are in good agreement. The experimental observations concerning the Eu valencies can not be explained solely in terms of the local volume available to the Eu ion; the nature of chemical environment plays a dominant role.     

Infrared observation of OC-C2H2, OC-(C2H2)2 and their isotopologues

ABSTRACT

The fundamental band for the OC-C₂H₂ dimer and two combination bands involving the intermolecular bending modes ν₉ and ν₈ in the carbon monoxide CO stretch region are re-examined. Spectra are obtained using a pulsed supersonic slit jet expansion probed with a mode-hop free tuneable infrared quantum cascade laser. Analogous bands for OC-C₂D₂ and the fundamental for OC–DCCH as an impurity are also observed and analysed. A much weaker band in the same spectral region is assigned to a new mixed trimer, CO-(C₂H₂)₂. The trimer band is composed uniquely of a-type transitions, establishing that the CO monomer is nearly aligned with the a-inertial axis. The observed rotational constants agree well with ab initio calculations and a small inertial defect value indicates that the trimer is planar. The structure is a compromise between the T-shaped structure of free acetylene dimer and the linear geometry of free OC-C₂H_2. A similar band for the fully deuterated isotopologue CO-(C₂D₂)₂ confirms our assignment.     

Infrared spectra and structural properties of Na2O-B2O9-ZrO2-SiO2 glasses

The structure of glasses of the Na₂O-B₂O₃-ZrO₂-SiO₂ system, which form the basic ingredients of opacified zirconium-type glazes, was studied by infrared spectroscopy. The problems of boron ion coordination and the role of Zr⁴⁺ in glass structure are considered.The present paper is a continuation of [1], which deals with glasses of the Na₂O-B₂O₃-ZrO₂-SiO₂ system.     

Glass forming regions and dielectric properties of Na2O-Bi2O3-TiO2-P2O5 glasses

The glass forming regions have been determined in the quaternary systems Na₂O-Bi₂O₃-0.05TiO₂-P₂O₅ and Na₂O-0.05Bi₂O₃-TiO₂-P₂O₅. The largest vitreous do main has been found in the diagram with 5mol% of titanium oxide TiO₂- The variation of dielectric constant has been followed along two lines inside the glass regions. e'_r increases with increasing amount of sodium oxide Na₂O and diminishes with increasing percentage of either Bi₂O₃ or TiO₂.     

Sonocatalytic degradation of organic dyes and comparison of catalytic activities of CeO2/TiO2, SnO2/TiO2 and ZrO2/TiO2 composites under ultrasonic irradiation

The CeO₂/TiO₂, SnO₂/TiO₂ and ZrO₂/TiO₂ composites were prepared by dispersing various nano-sized oxides (CeO₂, SnO₂, ZrO₂ and TiO₂) with ultrasound and mixing TiO₂ with CeO₂, SnO₂ and ZrO₂, respectively, in boiling water in a molar ratio of 4:1, followed by calcining temperature 500 °C for 60 min. Then a series of sonocatalytic degradation experiments were carried out under ultrasonic irradiation in the presence of CeO₂/TiO₂, SnO₂/TiO₂ and ZrO₂/TiO₂ composites and nano-sized TiO₂ powder. Also, the influences of heat-treatment temperature and heat-treatment time on the sonocatalytic activities of CeO₂/TiO₂, SnO₂/TiO₂ and ZrO₂/TiO₂ composites, and of irradiation time and solution acidity on the sonocatalytic degradation of Acid Red B were investigated by UV–vis spectra. It was found that the sonocatalytic degradation of Acid Red B shows significant variation in rate and ratio that decreases in order: CeO₂/TiO₂ > SnO₂/TiO₂ > TiO₂ > ZrO₂/TiO₂ > SnO₂ > CeO₂ > ZrO₂, and the corresponding ratios of Acid Red B in aqueous solution are 91.32%, 67.41%, 65.26%, 41.67%, 28.34%, 26.75% and 23.33%, respectively. And that the degradation ratio is only 16.67% under onefold ultrasonic irradiation. Because of the good degradation efficiency, this method may be an advisable choice for the treatment of non- or low-transparent wastewaters in the future.     

Peak effect studies in single crystals CeRu2 and 2H-NbS2

We have studied the peak effect (PE) phenomenon in single crystals of weakly pinned superconductors CeRu₂ and 2H-NbS₂. 2H-NbS₂ is iso-structural and iso-electronic to 2H-NbSe₂, whose similarity with CeRu₂ as regards the PE representing the order-to-disorder transformation of the flux line lattice was claimed some time ago. We report on the step change in equilibrium magnetization across the peak effect in CeRu₂. We also present the vortex phase diagram of 2H-NbS₂ obtained from the magnetization data, and compare the PE phenomenon in 2H-NbS₂ and 2H-NbSe₂.     

Theoretical study of the reaction of H2 with a Cu2Pt2 cluster

The study of the interaction of a pyramidal tetramer of Cu₂Pt₂ with the H₂ is reported here through ab initio multiconfigurational self-consistent field (MC-SCF) calculations, plus extensive multireference configuration interaction (MR-CI), variational and perturbative calculations. The lowest three electronic states X ¹A′, a ³A′ and a ¹A′ of the bare cluster were considered in order to study this interaction. For the H₂ C_s approaching a Pt vertex, results show that the Cu₂Pt₂ pyramid cluster in its X ¹A′ and a ¹A′ states can spontaneously capture and dissociate the H₂. For the H₂ C_s approaching a Cu vertex, where H₂ is located in the C_s reflecting plane, the Cu₂Pt₂ cluster in its X ¹A′ electronic state shows capture of the hydrogen molecule after surmounting an energy barrier; moreover, in this approach the Cu₂Pt₂ cluster in its a ¹A′ electronic state shows spontaneous capture of the hydrogen molecule. For the H₂ approaching a Cu vertex, where the C_s reflecting plane bisects the H₂ molecule, the Cu₂Pt₂ cluster in its three lowest-lying states is able to capture the hydrogen molecule after surmounting a small barrier. The Cu₂Pt₂+H₂ C_s face-on interactions show a lower H₂ activation than that which was obtained in the equivalent Pt₄+H₂ interactions.     

Infrared absorption of iodomethylperoxy (syn-ICH2OO) radical generated upon photolysis of CH2I2 and O2 in solid para-H2

Irradiation of a para-hydrogen (p-H₂) matrix containing diiodomethane (CH₂I₂) and O₂ at 3.2 K with light at 280 ± 20 nm, followed by annealing of the matrix at 4.0 K, yielded infrared (IR) absorption lines at 2982.4, 1408.9, 1231.8, 1226.5/1225.6, 1085.6, 917.7, 841.6/841.1, 550.5, and 490.2 cm^?1 that are assigned to the syn-iodomethylperoxy (syn-ICH₂OO) radical. Further irradiation of the matrix at 365 nm diminished these features. Experiments with CH₂I₂ and ¹⁸O₂ yielded lines of syn-ICH₂¹⁸O¹⁸O at 1407.3, 1228.1/1227.7, 1217.7/1217.0, 1031.5, 899.9/899.4, 836.7/836.0, and 473.6 cm^?1. The assignments are based on the photolytic behaviour and comparisons of observed vibrational wavenumbers, IR intensities, and ¹⁸O-isotopic shifts with those predicted with the B3LYP/aug-cc-pVTZ-pp method. The observation is consistent with a mechanism that, upon photolysis of CH₂I₂ at 280 nm, CH₂I was formed and subsequently reacted with O₂ to yield syn-ICH₂OO. Compared with the gaseous reaction CH₂I + O₂ → CH₂OO + I at low pressure, observation of ICH₂OO in a p-H₂ matrix instead of CH₂OO in the gaseous phase indicates that the excess energy of internally excited ICH₂OO, produced upon reaction of CH₂I + O₂, was rapidly quenched in the matrix so that ICH₂OO became readily stabilised without further decomposition to form CH₂OO + I.     

Local and itinerant Magnetism AND Crystal structure of RRh2Ge2 and RRu2Ge2 (r = rare earth)

The compounds RRh₂Ge₂; and RRu₂Ge₂ were synthesized X-ray studies show that they have the expected ThCr₂Si₂; tetragonal-type structure Magnetization studies at 1 8–300 K, ¹⁵¹Eu Mossbauer studies at 4 l, 77 and 300 K and the crystallography studies show the following- All RRh₂Ge₂; like RRh₂Si₂; exhibit two magnetic phase transitions, one corresponding to the antiferromagnetic ordering of the local rare earth moments. T_N = 8–90 K, the other corresponding to the itinerant electron ordering of the Rh sublattice. T_M = 3–9 K The heavy rare earths in RRu₂G₂; order antiferromagnetically and undergo a spin-flop transition in a relatively low magnetic field, <10 kOe The light elements in RRu₂Ge₂; order in a ferromagnetic, somewhat unclear structure NdRu₂Ge₂, like NdRu₂Si₂, exhibits two peaks in the magnetization curves Again, the lower may correspond to itinerant electron ordenng or, alternatively, to spin reorientation of the rare earth sublattice Eu in both EuRh₂Ge₂ and EuRu₂Ge₂ is divalent, whereas Ce in both CeRh₂Ge₂ and CeRu₂Ge₂ is trivalent For all rare earths the ordenng transition in RRh₂Ge₂ is higher than in RRu₂Ge₂. This fact can be associated with the smaller R-R distances in RRh₂Ge₂ and/ or due to the stronger magnetic character of the Rh 4d conduction electrons Companson of the magnetic properties and ¹⁵¹Eu hyperfine interactions of Eu²⁺Rh₂Ge₂, Eu²⁺Ru₂Ge₂, Eu²⁺Rh₂Si₂ and Eu³⁺Ru₂Si₂ with all the other systems leads to the conclusion that the conduction electrons play the dominant role in determining the magnetic properties of these systems Crystal-field effects are also of considerable importance, since the Mossbauer studies yield for the second-order crystal-field parameter A⁰₂〈r²_4f〉 the huge values +385 and +282 K for EuRu₂Ge₂; and EuRh₂Ge₂, respectively The easy axis of magnetization in the Eu compounds is in the basal plane The large second-order crystal field predicts well the direction of the easy axis for all other rare earths No superconductivity has been observed in any of the compounds, down to 1 8 K A companson of the magnetic properties of the germanides with those of the silicides shows great similanties, the differences being accounted for by the different unit cell sizes and c/a ratios.     

Vibrationally inelastic scattering of H∼ ions from H2, N2, O2 and CO2

State-resolved measurements are presented for vibrational excitation of H₂, N₂, O₂ and CO₂ by H^? impact in the collision energy rangeE _cm=20–180 eV and for scattering in the forward direction (0±0.5°). The data obtained from the measurements are the relative intensities and differential cross sections for vibrational excitation up toν′=4, the transition probabilitiesP _0→ν′ and the vibrational energy transferΔE _vib. For the systems H^?-H₂, N₂, O₂ the vibrational inelasticity increases in the order H₂-N₂-O₂. The mechanism for vibrational excitation in these systems is due to transient charge transfer from the H^? ion into antibonding orbitals of the target molecule which provides a bond stretching force during the collision. For H₂ and N₂, the results are compared with corresponding measurements for H⁺ scattering where the interaction mechanism is quite different. In the case of CO₂, vibrational excitation in forward scattering is caused primarily by the long-range dipole interaction. The spectra are very similar for H^?-CO₂ and H⁺-CO₂. Finer details are attributed to the influence of transient charge transfer and valence interactions.     

Magnetic properties of CaNi2P2 and the corresponding lanthanoid nickel phosphides with ThCr2Si2 type structure

CaNi₂P₂ is Pauli paramagnetic with slightly increasing susceptibility at lower temperatures. The same temperature dependence is observed for the susceptibility of LaNi₂P₂. The absolute values of this metallic conductor, however, are shifted to the diamagnetic region. At temperatures above 100 K the compounds LnNi₂P₂ with Ln  Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm show normal Curie-Weiss behavior with magnetic moments slightly lower than those of the free Ln³⁺ ions. The paramagnetic Curie temperatures of these compounds vary between −5 and +12 K. The well established intermediate valence compound EuNi₂P₂ also shows a liner reciprocal susceptibility vs temperature dependence, however, the corresponding paramagnetic Curie temperature is much lower: Θ = −124 K. Ferromagnetic ordering is inferred from the positive paramagnetic Curie temperatures for PrNi₂P₂ (Θ = 12 K) and NdNi₂P₂ (Θ = 10 K). GdNi₂P₂ is antiferromagnetic with a Néel temperature of T_N = 10.5 K. SmNi₂P₂ shows Van Vleck paramagnetism. CeNi₂P₂ and YbNi₂P₂ exhibit temperature dependent paramagnetism, which may be rationalized with a mixed valence of cerium and ytterbium. Down to 1.8 K no superconducting transitions could be observed for CaNi₂P₂, LaNi₂P₂ or CeNi₂P₂. The magnetic properties of these compounds are discussed in accordance with a simple band structure model derived from Zintl's concept.     

Electrical and Thermal Transport Properties of n-type Bi6Cu2Se4O6 (2BiCuSeO + 2Bi2O2Se)

New thermoelectric materials, n-type Bi₆Cu₂Se₄O₆ oxyselenides, composed of well-known BiCuSeO and Bi₂O₂Se oxyselenides, are synthesized with a simple solid-state reaction. Electrical transport properties, microstructures, and elastic properties are investigated with an emphasis on thermal transport properties. Similar to Bi₂O₂Se, it is found that the halogen-doped Bi₆Cu₂Se₄O₆ possesses n-type conducting transports, which can be improved via Br/Cl doping. Compared with BiCuSeO and Bi₂O₂Se, an extremely low thermal conductivity can be observed in Bi₆Cu₂Se₄O₆. To reveal the origin of low thermal conductivity, elastic properties, sound velocity, Grüneisen parameter, and Debye temperature are evaluated. Importantly, the calculated phonon mean free path of Bi₆Cu₂Se₄O₆ is comparable to the interlayer distance for BiO─CuSe and BiO─Se layers, which is ascribed to the strong interlayer phonon scattering. Contributing from the outstanding low thermal conductivity and improved electrical transport properties, the maximum ZT ≈0.15 at 823 K and ≈0.11 at 873K are realized in n-type Bi₆Cu₂Se_3.2Br_0.8O₆ and Bi₆Cu₂Se_3.6Cl_0.4O₆, respectively, indicating the promising thermoelectric performance in n-type Bi₆Cu₂Se₄O₆ oxyselenides.     

Production of highly concentrated 13C by continuous two-stage IRMPD. CBr2F2/HI,CCl2F2/HI,and CBrClF2/HI mixtures

Difluoromethane CH₂F₂ containing 90–98% ¹³C was obtained in the selective IRMPD of mixtures of CBr₂F₂/HI, CCl₂F₂/HI, and CBrClF₂/HI. In CBr₂F₂/HI mixtures, the intermediate product CHBrF₂ resulting from the reaction between the initial decomposition fragment CBrF₂ and HI underwent secondary selective IRMPD to form highly ¹³C-enriched CH₂F₂ in continuous laser irradiation. The intermediate product in the mixtures of CCl₂F₂/HI and CBrClF₂/HI was found to be CHClF₂, but no significant secondary photodecomposition in CBrClF₂/HI mixtures occurred owing to the low absorption cross section of CHClF₂ at the adopted laser frequencies and fluences. The observed decomposition probabilities and selectivities under different conditions with respect to laser frequency, fluence, and partial pressures of halogenated difluoromethanes and HI suggest that CBr₂F₂ is one of the better candidates for practical ¹³C separation by IRMPD.