The vapor-phase infrared and Raman spectra of p-difluorobenzene (h4) and (d4)

This new study of the vapor-phase infrared and Raman spectra of p-difluorobenzene (h₄) and (d₄) includes assignments from the high-resolution electronic spectrum, from fluorescence spectra, and one assignment from the two photon electronic spectrum. A new set of frequencies and assignments has been made for the fundamentals of this important intermediate-sized molecule. The most important changes concern the assignments previously made for the lowfrequency a_u mode (ν₈), two members of the b_2u class (ν₁₉ and ν₂₀), and one very important change for the lowest frequency fundamental of the b_3u (out-of-plane) class (ν₃₀), which has been observed directly in the far infrared. The results are all corroborated by the assignments of the perdeuterated analog where changes in previous assignments of Fermi resonance pairs have also been made. Some interesting differences between the Raman spectra of the vapor and the liquid perproto compounds have not been adequately explained in this study.     

Magnetic phase transition in (Mn0.95Ni0.05)3B4

It was found that the metallic compound (Mn_0.95Ni_0.05)₃B₄ was ferromagnetic below 195 K and antiferromagnetic between 195 and 354 K. The transition temperature from ferromagnetic to antiferromagnetic increases with increasing external magnetic field. On the other hand, the transition temperature from antiferromagnetic to paramagnetic decreases with increasing magnetic field. It is expected that the present results might be explained by the theoretical results on the coexistence of ferro- and antiferromagnetism in the itinerant electron system reported by Moriya and Usami.     

X-ray diffraction,differential scanning calorimetric and spectroscopic studies of phase transitions in the bidimensional compound (C12H25NH3)2CdCl4

The existence of three main crystalline phases (called III, II and I) in (C₁₂H₂₅NH₃)₂CdCl₄ has been revealed by differential scanning calorimetry. X-ray diffraction and spectroscopic studies. The crystal- lographic evolution with increasing temperature appears to be monoclinic (III) → orthorhombic (II) → tetragonal (I). The low temperature phase III is the only ordered structure. The phase transition (III-II), which is of first order type, corresponds to an order-disorder mechanism involving the organic part of the structure (alkylammonium chains) whereas the phase transition (II-I), which is of second-order type, is related to the arrangement of the mineral matrix (octahedra of perovskite layers). An intermediate disordered form II', stable in a very narrow temperature range and structurally similar to the form II, has also been observed, so that the transformation (III-II) proceeds, in fact, in two steps (III-II'-II). The variation enthalpies observed at the transitions (III-II'-II) and analyzed through an order-disorder mechanism demonstrate the high disorder of the alkylammonium chains in form II, in agreement with spectroscopic results. No thermal anomaly or spectroscopic modification is observed for the high temperature transition (II-I).     

Detection of a second Br site in K2Pt(CN)4Br0.3 3.2 H2O (KCP) by 18Br and 39K NMR

By nuclear magnetic resonance we have detected between 20 and 300 K two bromine signals with a temperature-independent intensity ratio of 2.5 ± 0.3 proving the existence of a second Br site in KCP. The result is supported by the observation of three potassium signals. Thus, three types of unit cells exist: 40% contain defect water, 43% have Br in site I and 17% Br in site II.     

Vibrational study of H3OUO2PO4 · 3 H2O (HUP) and related compounds. phase transitions and conductivity mechanisms: part I,KUO2PO4 · 3 H2O (KUP)

Infrared and Raman spectra of polycrystalline KUO₂PO₄ · 3 H₂O (KUP) and its isotopic derivatives KUO₂P¹⁸O₄ · 3 H₂O and KUO₂PO₄ · 3 D₂O have been investigated in the 4000-10-cm^?1 range at different temperatures. An assignment of the bands in terms of UO₂, PO₄ and H₂O vibrations has been proposed. Combined differential scanning calorimetry and spectroscopic data show two diffuse phase transitions near 130 and 230 K. Comparison of the vibrational spectra of phase I at 300 K and phase IV at 100 K indicates that ordering of the water molecules with subsequent ordering of PO₄ tetrahedra on a site with lower symmetry appears to be the main mechanism responsible for the phase transformation. All the six O-H distances of water molecules in phase IV are found to be crystallographically nonequivalent. Conducting ion frequencies and the corresponding force constants have been determined for the analogous compounds MUP with M = K⁺, Na⁺, Ag⁺, NH⁺₄, Tl⁺ and H₃O⁺ and compared with other properties of these ionic conductors. Conductivity mechanisms in these materials are discussed.     

Orientational order in CO and N2 monolayers on graphite studied by X-ray diffraction

X-ray diffraction has been used to study the structure and orientational phase transitions of CO and N₂ adsorbed on graphite (Papyex). Both form orientationally ordered 2√3 × 2√3 R30° commensurate phases on graphite at low temperatures (10 K). The in-plane herringbone structure of N₂ has been confirmed but CO has more orientational disorder than N₂, which may be associated either with tilting, random static or systematic, of the molecules away from the surface and/or with orientational order of shorter range than the centre of mass order. In the first case the average tilt would have to be about 26° and in the second case the orientational correlation length would have to be 200 Å compared with 450 Å for the translational order. The orientational phase transition is sharp for N₂, occurring over the range 27–30 K, in agreement with previous work. For CO the transition is broad and starts at lower temperatures. This and the structural data indicate that a point quadrupolar interaction is not a suitable model for comparing the properties of N₂ and CO layers. The orientational phase transition in the incommensurate phase of N₂ is found to be broad and occurs below 20 K. For CO it is sharper than for the commensurate phase and occurs at a higher temperature. The lattice parameter changes by 0.75% across the orientational phase transition. For both N₂ and CO there is evidence of translational disorder in the commensurate phases but it cannot be interpreted quantitatively.     

Structure,vibrational study and conductivity of the trihydrated uranyl bis(dihydrogenophosphate): UO2(H2PO4)2·3H2O

The X-ray structure (293 K) of UO₂(H₂PO₄)₂·3H₂O has been refined (R = 0.062): M_r = 518g, space group: P2₁/c (Z = 4); $\text{a = 10.816(1)}\text{A}\text{?}$, $\text{b = 13.896(2)}\text{A}\text{?}$, $\text{c = 7.481(1)}\text{A}\text{?}$, β = 105.65(1)^°, $\text{V = 1082.7(2)}\text{A}\text{?}{}^3$; D_c = 3.17 Mg m^?3. The structure consists of infinite chains along the (101) axis with U atoms bridged by two H₂PO₄ groups. The U atom is surrounded by a pentagonal bipyramid of oxygen atoms, one of them being an equatorial water molecule. The cohesion between the chains is ensured by hydrogen bonds involving the two last water molecules. An assignment of IR and Raman bands with isotopic substitution spectra is proposed. A phase transition at 128 K was made evident by DSC and spectroscopy. The room-temperature phase is characterized by a high disorder of the OH bond orientation while in the low-temperature phase H₂O and POH species appear well oriented. The conductivity seems to occur by proton transfer and protonic-species rotation at the POH-water molecular interface between the chains. ac conductivity has been determined by means of the complex-impedance method ($\text{σ}{}_{\mathrm{<mtext>RT</mtext>}}\text{～ (3?12) × 10}{}^{\mathrm{?5}}\text{Ω}{}^{\mathrm{?1}}\text{cm}{}^{\mathrm{?1}}$; E ～ 0.20 eV).     

Thermal evolution of C2H2 and C4H2 on Pd(111) studied by high-resolution electron energy loss spectroscopy

The thermal evolution of acetylene and ethylene on a palladium (111) surface has been studied by high-resolution electron energy loss spectroscopy in the temperature range 150K–500K. Formation of ethylidyne ( CCH₃) near room temperature is important for both molecules, whereas CH is the major surfaces hydrocarbon species formed at high temperatures.     

Pressure-induced antiferromagnetism in ferromagnetic Fe51.5Rh48.5 alloy

The pressure-temperature magnetic phase diagram based on electrical resistivity measurements was determined for Fe-Rh alloys, ferromagnetic down to 4.2 K, from room temperature to the Curie point (750 K) and for pressure up to 100 kbar. A pressure-induced first order ferromagnetic-antiferromagnetic phase transition line was found with an inhomogeneous, mixed phase existing at pressures lower than 50 kbar. A new, qualitative model is proposed to explain the phase transitions, the absence of magnetic moment on Rh atoms in the AF state and the shape of the p-T diagram. The model is based on the excitonic antiferromagnetism of semimetallic Fe-Rh and it is connected with the pecularities of the electroni structure and the shape of the Fermi surface.     

Spectral distribution of steady-state photoconductivity in amorphous As2Se3

The spectral distribution of steady-state photoconductivity has been examined for a series of bulk and evaporated samples of amorphous As₂Se₃. All bulk samples, irrespective of preparation technique or sample treatment, show a distinct shoulder in the photocurrent spectral distribution at energies near 1.4 eV. This feature is interpreted as spectroscopic evidence for the existence of a well-defined defect level in the gap of amorphous As₂Se₃. Evaporated As₂Se₃ films do not show any structure in the spectral response.     

Interaction of CO,CO2 and O2 with nonpolar,stepped and polar Zn surfaces of ZnO

The nonpolar (10$\text{1}$0), stepped (40$\text{4}$1) and (50$\text{5}$1), and the polar (0001) surfaces of ZnO were prepared. Stable unreconstructed nonpolar and stepped surfaces were obtained. LEED analyses showed that the step height and the step width of the stepped surfaces were similar to the theoretical values. The polar surface showed a 1 × 1 LEED pattern of six-fold symmetry after annealing at 500°C, and evidence of a more complicated pattern at 300–400°C. Temperature programmed desorption of CO resulted in the desorption of CO from the stepped and the polar surfaces. However, desorption of CO₂ was observed from the stoichiometric nonpolar surface, and no desorption from the reduced nonpolar surface. CO₂ was also observed by interacting CO with all surfaces at elevated temperatures. A total of four temperature programmed desorption peaks of CO₂, α, β, γ, and δ were observed. The α and β peaks were observed on the nonpolar and the stepped surfaces, and the γ peak was observed on the polar surface. The α peak was assigned to adsorption on a surface ZnO pair, and the β peak was assigned to adsorption on an anion vacancy or a step. While adsorbed water enhanced the β, preadsorbed methanol reduced it. O₂ adsorption was similar on the nonpolar and the stepped surfaces, but was weak on the polar surface.     

Anisotropic photocorrosion of n-type MoS2 MoSe2, and WSe2 single crystal surfaces: the role 0f cleavage steps,line and screw dislocations

The photocorrosive behavior of the semiconducting transition metal dichalcogenides in contact with aqueous electrolytes is extremely anisotropic. Enhanced reactivity was observed at cleavage steps and on either line or screw dislocation cores emerging at the surface exposing mainly (101&#x0304;1) faces lying in 〈112&#x0304;0〉 directions. Corrosion patterns, however, could never be observed on smooth (0001) faces. The corrosion products of the molybdenum compounds are soluble, whereas WSe₂ formed WO₃ precipitates on the surface.     

Emission spectra of polyatomic radical cations in the gas phase: Ã2Πu → X̃2Πg band systems of Cl(CC)2 Cl+, Br(

Emission spectra of the dihalodiacetylene radical cations, X(CC)₂X⁺ with X = Cl, Br and I, excited in the gaseous phase by low energy e     

A neutron scattering study of the low temperature modulated phases of the perovskite-type layer structure PAMC [= (C3H7NH3)2MnCl4]

Elastic neutron scattering experiments between 100 and 180 K show clearly that ?-PAMC is incommensurately modulated with the modulation vector being temperature-dependent and approaching $\text{1}\text{3}\text{b}{}^1$ at the ? → ζ phase transition. This displacive phase transition appears to be of the lock-in type. It is triggered by the freezing of dynamically disordered propylammonium ions.     

The preparation and characterization of the cobalt skutterudites CoP3, CoAs3 and CoSb3

Single crystals of the cobalt skutterudites CoP₃, CoAs₃ and CoSb₃ have been prepared by the chemical vapor transport technique using chlorine as the transport agent. Chemical analysis and density measurements were used to determine accurately the stoichiometry of these crystals. Physical properties of chemically pure single crystals were obtained by X-ray diffraction, magnetic susceptibility and electrical measurements. CoP₃ is a diamagnetic semiconductor with an optical band gap of 0.45 eV. CoAs₃ and CoSb₃ are also diamagnetic and show an increase in their resistivity as a function of increasing temperature. In addition, they do not give any measurable absorption edge but absorb radiation at all frequencies from 0.4 to 4.3 eV.     

Local order and structural transitions in the molecule-based layer ferrimagnet (n-C4H9)4N FeFe(C2O4)3

Local structural order and temperature-dependent structural variation have been studied in the molecular-based layer ferrimagnet (n-C₄H₉)₄N Fe^IIFe^III(C₂O₄)₃ by EXAFS and high resolution X-ray powder diffraction. The EXAFS spectra measured at the Fe K-edge are successfully modelled by successive O, C, O and metal shells, showing that even when there is extensive structural disorder due to stacking faults, the local structural order in this class of ferrimagnets is fully retained. In this salt, which shows remarkable negative magnetisation at low temperature (Néel class Q), the EXAFS Debye-Waller factor has a discontinuity at 40 K, corresponding to one found in the magnetisation. At the same temperature there is also a change in the expansion of the lattice as evidenced by the high resolution X-ray powder diffraction.     

Birefringence,X-ray and neutron diffraction measurements on the structural phase transitions of (CH3NH3)2 MnCl4 and (CH3NH3)2FeCl4

We used optical birefringence, X-ray and neutron diffraction methods with single crystals to study the structural phase transitions of the perowskite-type layer structures of (CH₃NH₃)₂MeCl₄ with Me=Mn, Fe. The Mn-compound shows the following structural transitions at 394 K — a continuous order-disorder phase transition from tetragonal symmetry $\text{I4}\text{mmm}$ to orthorhombic space group Abma (Cmca in reference 10); at 257 K — a discontinuous transition to a second tetragonal modification; at 95 K — a discontinuous transition to a monoclinic phase. For the Fe-compound the corresponding transition temperatures are 328 K and 231 K, respectively. A low temperature monoclinic phase could not be observed. The lattice parameters of the different modifications were determined as a function of temperature. The temperature dependent course of the order parameter has been investigated for the order—disorder transition. For both compounds, all the methods used gave the same value for the critical exponent of β = 0.315.     

Magnetic properties of the rare-earth diborodicarbides (RB2C2)

The magnetic susceptibility of RB₂C₂ has been measured in the temperature range of 3–300 K. Curie-Weiss fits to the susceptibilities led to effective moments in agreement with those expected for R³⁺ ions. The RB₂C₂ (R = Ce, Nd, Sm, Gd, Tb, Er, and Tm) compounds are antiferromagnetic. Metamagnetic transitions at low fields were observed for CeB₂C₂ and TbB₂C₂. The compounds, DyB₂C₂ and HoB₂C₂, are ferromagnets with complex magnetic structures. Praseodymium borocarbide becomes a Van Vleck paramagnet at low temperature. The magnetic ordering temperatures of these compounds are discussed in terms of their crystal structure and the RKKY theory.     

Vibrational study of H3OUO2PO4·3 H2O (HUP) and related compounds. Phase transitions and conductivity mechanism: Part II,H3OUO2PO4·3 H2O

Infrared and Raman spectra of polycrystalline H₃OUO₂PO₄·3 H₂O (HUP) and its D and P¹⁸O₄ derivatives, in the form of dense transparent disks and wet powder, have been investigated at various temperatures in the 100–300 K region. The bands due to framework vibrations are similar to those of KUP, whereas those for the protonic species are different. OH stretching and bending bands of the oxonium ion have been identified at 2920, 1740 and 1160 cm^?1 in the low-temperature spectrum of HUP. Differential scanning calorimetry (DSC) and infrared (IR) intensity investigations show a phase transition between 274 and 260 K. The mechanism of the phase transition consists, as in the case of KUP, of ordering of the protonic species, which induces ordering of PO₄ tetrahedra. The ordering can be influenced by excess water content, stacking faults and stress (ferroelastic behaviour is evidenced). The conductivity mechanism in HUP is discussed.