Raman active lattice spectrum of single crystal 2,4,6-triphenyltriazine

The Raman spectra of single crystal 2,4,6-triphenyltriazine have been recorded in the lattice region at 300 and 30°K. The interpretation of the results was based on assuming that the molecular structure is D_3h (i.e. complete planarity) instead of the most probable non-planar conformation of C₁ symmetry. Assignment of the lattice vibrations was based on a rigid-body approximation. Considerable temperature effect was observed in the spectrum when the crystal was cooled to 30°K. This seemed to indicate large anisotropic thermal motions in the crystalline solid.     

The clean thermally induced W {001} (1 × 1) → (√2 × √2) R 45° surface structure transition and its crystallography

A (√2 × √2)R45° surface structure on W {001} produced only by cooling below ~370 K, first reported by Yonehara and Schmidt, has been investigated by LEED, AES, work function change, characteristic loss and low energy Auger fine structure measurements. No significant changes at any energy up to 520 eV occur in the standard Auger spectrum upon cooling to 220 K for as long as 30 min after a flash to >2 500 K. The work function of the (√2 × √2) R45° at 210 K is 20 ± 10 mV below that of the (1 × 1) surface, and a sensitive feature in the fine structure of the N₇VV AES transition shows approximately 60% attenuation. Unlike for H₂ adsorption, the “surface plasmon” loss peak exhibits little if any measurable attenuation and no measurable shift in energy as the crystal cools to form the (√2 × √2)R45°. The rate of intensity buildup in the $\text{1}\text{2}$-order LEED beams is strictly temperature dependent, and significant differences exist between the $\text{1}\text{2}$-order LEED spectra produced by cooling and those produced by H₂ adsorption. Only 2-fold symmetry was observed in the LEED beam intensities at exactly normal incidence, rather than 4-fold as expected for statistically equal numbers of rotationally equivalent domains. The LEED I-V spectra for 24 fractional order beams and 12 integral order beams, taken over large energy ranges at normal incidence, clearly establish that the beam intensities display 2 mm point group symmetry, and hence a preference of one domain orientation over the other. No beam broadening or splitting effects were apparent, implying only incoherent scattering from the various domains. The half-order beam spectra (±h/2, ±h/2) are identical in relative intensity to the (±h/2, ±h/2) spectra but different in absolute intensity by a constant factor, which can be explained only by domains with p2mg space group symmetry rather than just p2mm. Adsorption of H₂ onto the cooled (√2 × √2)R45° structure restores the 4-fold symmetry in the LEED beam intensities at normal incidence, giving a c(2 × 2) hydrogen structure, the same as when adsorbing H₂ onto the above room temperature (1 × 1) crystal. This strongly supports the observed p2mg symmetry as being a true property of the cooled (√2 × √2)R45° surface structure. These results show that the (1 × 1) → (√2 × √2) R45° transition produced by cooling is a transition involving displacement of surface W atoms, and that it apparently can be characterized as an order-order, second degree, homogeneous nucleation process, which is strongly prohibited by the presence of impurities or defects.     

Optical spectra of Cu porphin and VO porphin in single crystal triphenylene

Polarized optical absorption spectra of copper porphin and vanadyl porphin in single crystals of triphenylene have been obtained at room temperature, 77°K and 4.2°K. Luminescence spectra of these samples are also reported. The gross features of the spectra of these two metalloporphins are similar but the spectral lines in vanadyl porphin are much sharper than those in copper porphin, revealing much more satellite structure. It is shown that the central metal can substantially affect metalloporphyrin absorption spectra, contrary to the case of broad band solution spectra. The data is consistent with porphin molecules in substitutional sites of triphenylene and a crystal field reduction of the usual D_4h metalloporphyrin symmetry to D_2h.     

Intensity and half-width measurements in the 1·525 μm band of acetylene

Line intensities at 150°K and 295°K, self-broadened half-widths at 171°K, 200°K, 250°K and 295°K, and hydrogen-broadened half-widths at 171°K, 200°K and 295°K have been measured in the ν₁+v₃ band of C₂H₂ at 1·525 μm. The absolute intensity of the band has been determined independently by employing the Wilson-Wells-Penner-Weber technique. Our best estimate for the absolute intensity of the band is S_v=7·82 ± 0·07 cm^?2 atm^?1 at 295°K. Line intensities calculated using this value of S_v are in good agreement with the measured intensities at the two extreme temperatures of 150°K and 295°K considered in the present study, thereby not suggesting any significant intensity anomalies. Line positions have been measured for the first time for this band for R(29)?P(25).     

Spin-lattice relaxation of protons and deuterons in NH4NO3 and ND4NO3

The spin-lattice relaxation timesT ₁ were measured for protons and deuterons in polycrystalline NH₄NO₃ and ND₄NO₃. The investigation was carried out at temperatures between about 80°K and 430°K using the NMR pulse method. From the measured values ofT ₁ the activation energies for the reorientation motions of NH ₄ ⁺ and ND ₄ ⁺ ions below 200°K were calculated to be 2.07±0.11 kcal/mole and 2.56±0.23 kcal/mole, respectively. The quadrupole coupling constante ² Qq/h of the deuteron in ND ₄ ⁺ ion was found to be 194±30 kc/s.     

Variation du champ seuil avec une tension uniaxiale dans FeCl2 A 4,2°K

In FeCl₂, at low temperature, the Fe²⁺ ions present an antiferromagnetic order characterised by an easy magnetization axis perpendicular to the ion-containing planes. There exists a value h_s of a magnetic field parallel to the axis, which transforms the system from an antiferromagnetic order into a saturated paramagnetic order. There we present the variations of h_s at 4·2°K as well as the deformations of the sample, when we impose upon it a strain parallel to the axis without allowing its lateral dilatation. These results, when added to those already obtained with χ_i in similar experiments enable us to determine the variations of the crystalline anisotropy and anti-ferromagnetic exchange (between Fe²⁺ ions belonging to two consecutive planes) in terms of the deformations. Our results concerning h_s are different from the ones obtained when exerting a hydrostatic strain. We compare the two cases by using the relations between elastic constants of FeCl₂ obtained at 300°K and recorded in the Appendix.     

Measurement of intensities of multiplets in the 2ν3-band of methane at low temperatures

The integrated intensities of the multiplets P(1)–P(10), R(0)–R(9), and of the Q-branch in the 2ν₃-band of ¹²CH₄ have been measured at 102°K, 152°K, 202°K, 251°K, and 300°K. Comparison of our data with theoretical line strengths confirms, at all of the temperatures mentioned, the intensity anomalies observed by Margolis⁽⁵⁾ for lines in this band. The integrated intensity of the 2ν₃-band is found to be S_v = (1·76±-0·04)(300/T (°K)) cm^?2 atm^?1.     

Self-consistent phonon treatment of second-order displacive ferroelectric

The high-temperature Curie-Weiss law of electric susceptibilityχ can be explained by quartic interactions between ak～ 0 optic mode, whose normal mode co-ordinateq ₀ is proportional to the polarization, and acoustic vibrations of the lattice. These interactions will either stabilize q₀ at low temperature, preventing a ferroelectric transition, or else such a transition will occur. To obtain coexisting paraelectric and ferroelectric phases at 0 °K, with distinctχ vs.T curves for each phase as observed bySaifi andCross in SrTiO₃, one must invoke anharmonic interactions between q₀ and low-lying, temperature-dependent optic modes. These interactions are treated by the self consistent phonon method, which introduces an effective harmonic HamiltonianH _h and calculates the Helmholtz functionF to first order inH-H_h. The approximation is optimized by minimizingF with respect to the renormalized optical mode frequencies appearing inH _h, for which a system of integral equations is obtained by the minimization procedure. The condition that there are enough low-lying optic modes to produce coexisting phases leads to a prediction of weak optic dispersion at 0 °K. Under these circumstances, the self-consistent phonon equations can be approximated by a pair of coupled cubic equations. From the solution of these, we conclude thatF has three minima at 0 °K in SrTiO₃, corresponding to paraelectric and two oppositely polarized ferroelectric states. 180° domain walls must be few in number and immobile under a field, except in the presence of a high dislocation concentration.     

Air-broadened line widths of methane at atmospheric temperatures

Air-broadened half-widths of lines R(0), R(1) and R(2) in the ν₃-fundamental of ¹²CH₄ have been measured at 200°K, 250°K and 300°K.     

Neutronh 11/2 alignments in the triaxial nucleus133La

High spin states have been studied in¹³³La via the¹²²Sn (¹⁵N, 4ny) fusion evaporation reaction. Bands build on low lying h_11/2,g_7/2 and d_5/2 proton states have been identified. At higher spin a h_11/2 neutron alignment is observed. The softness with respect to the triaxial deformation makes the nuclear shape sensitive to the quasiparticle configurations and coexistence between states withy ≈ + 30°,y ≈ ? 30° andy ≦ ? 60° was found. The results have been interpreted using total routhian surface (TRS) model calculations.     

Optical investigation of ErFeO3

The absorption spectrum of single crystals of ErFeO₃ has been investigated in the red and near infrared spectral region in the temperature range between 1.2 °K and 4.2 °K and at 20 °K and 77 °K. Between 77 °K and 4.2 °K a constant splitting of the absorption lines is observed. Below the Néel-temperature of the erbium sublattice at 4.5 °K the splitting of the absorption lines increases; the saturation value extrapolated to 0 °K of the splitting of the lowest crystal field level of the⁴ I _15/2 groundterm is (6.08±0.30) cm^?1. By measuring the Zeeman effect the groundstate magnetic moment is determined asμ=(6.6±0.2)μ _B. The measured temperature dependence of the splitting of the lowest crystalfield level of the⁴I_15/2 groundterm is compared with that calculated by a Monte Carlo method.     

Electronic structure of pure and doped orthorhombic La2CuO4

The electronic structure of orthorhombic La₂CuO₄ has been investigated by first principles pseudofunction band calculations and group theoretical analysis. We find that pure as well as doped compounds remain metallic at all finite temperatures as a conseqence of the Cmca (D¹⁸_2h) space group symmetry. The experimentally observed rapid rise in resistivity below 30°K suggests a structural transition to a lower symmetry space group that could be driven electronically or magnetically. One possible candidate is monoclinic C2/m (C³_2h) which is a subgroup of Cmca and can be obtained by distorting the CuO bonds or rotating the CuO₆ octahedra. Implications for superconductivity are discussed.     

On the apparent absence of triplet-triplet absorption in pure organic molecular crystals

The triplet-triplet absorption spectra of naphthalene-h ₈ in a 3-methylpentane glass at 77 K and naphthalene-h ₈ in a naphthalene-d ₈ crystal at 4·2 K are reported. A broad continuum absorption unique to the mixed crystal spectrum is believed to be due to a guest-induced transition into the host conduction band. Absence of the absorption in pure crystals of naphthalene at temperatures ranging from 1·8 K to 300 K is also discussed.     

Temperature-dependent light scattering from magnetic excitons in TlCoF3

Raman spectra of antiferromagnetic thallium cobaltous fluoride have been obtained with 4579A argon ion laser excitation at temperatures from 4°K to T_N = 94 ± 2°K. The features observed consist of six Co²⁺ excitons ranging in energy from 325 to 1070 cm^-1, at two-magnon peak with low-temperature energy of 315 cm^-1, and a one-magnon feature whose 4°K energy is 37 cm^-1. The energy and linewidth of the one-magnon scattering has been measured from 4°K to about 0.8 T_N; it is found that the magnon becomes critically damped at about 0.8 T_N, in good agreement with our previous observations on RbCoF₃. The Co²⁺ excitons observed at 325, 380, 410, 730 (weak), 960, and 1070 cm^-1 agree in energy quite well with the KCoF₃ levels calculated by Buyers, Holden et al. as 340, 400, 467, 767, 967 and 1050 cm^-1.     

Raman study of (NH4)2CuCl4·2H2O—I: Dynamics and structure in low temperature phase

We report the results of a Raman scattering study of (NH₄)₂CuCl₄2H₂O at 300, 205 and 100°K, in order to elucidate the dynamics and phase transition in this double salt. A group theoretical calculation of the symmetry vectors, in the high temperature phase (D_4h¹⁴), is made and the various modes are identified. The deuterated compound (ND₄)₂CuCl₄·2D₂O has also been investigated to help in identifying the modes involving motion of the ammonium ions and water molecules. Through a careful analysis of the spectra at 100°K, the space group in the low temperature phase has been established as D_2d³. The important consequence of this result is that this leads to parallel spatial ordering of ammonium tetrahedra in this compound in the low temperature phase.     

Raman spectra of α-GaTe single crystals

Raman-active lattice vibrational modes of GaTe have been investigated at 300°K in the frequency range 13–300 cm^-1. The spectra of the Bridgman grown crystals were excited with the 1.06 μm line of the continuously operated YAG : Nd³⁺ laser. Fourteen Raman bands were observed. All phonon modes have been assigned on the basis of their observed polarisation dependence to an irreducible representation of the appropriate point group. The results are consistent with the existence of GaTe in monoclinic α-polytype (C³_2h).     

Anomalous behaviour of the Urbach edge and phase transitions in TlGaSe2

The long-wavelength tail of the optical absorption in TlGaSe₂, crystals at α=30−150cm^-1 is shown to obey Urbach's rule in the temperature range 4.2–294°K. The anomalous behaviour of the parameters of this rule suggests the presence of two phase transitions in TlGaSe₂ at 246°K and 101°K beside the known phase transitions at 120°K and 107°K. The presence of phase transitions in TlGaSe₂ at 246°K and 101°K is also confirmed by means of the heat capacity measurement.     

Spin waves in MnO; from 4°K to temperatures close to TN

Spin waves have been measured in MnO by inelastic scattering of neutrons from 4 °K to a temperature 0·25 °K below T_N. The 4 °K spectrum is interpreted in the frame of linear spin wave theory with effective exchange integrals J₁^? = 0·321 meV, J₁⁺ = 0·424 meV, J₂¹ = 0·446 meV and a phenomenological parameter for anisotropy D₁ = 0·059 meV. The effect of the actual dipole-dipole Hamiltonian is shown to give a priori a very good account of the lifting of the degeneracy of spin waves near the Brillouin zone center. The Bloch model for interacting spin waves and theories based on Green function approximations have been adapted to the MnO case, in order to compute the main properties (mean magnetization of a sublattice, anisotropic deformation, extra isotropic contraction and magnon spectrum) at many temperatures. Comparison of these with experimental results tends to favour a generalized Callen renormalization model, which gives an overall fit from 4 °K, to temperalures close to T_N.     

Influence of the sample orientation in Sn<Subscript>2</Subscript>P<Subscript>2</Subscript>S<Subscript>6</Subscript> crystals on the hydrostatic piezoelectric coefficients

The influence of the sample orientation on the effective value of the hydrostatic piezoelectric coefficients d _h ⁽ⁱ⁾ of Sn₂P₂S₆ crystals has been studied. The hydrostatic piezoelectric coefficients d _h ⁽¹⁾ and d′ _h ⁽³⁾ , were measured, d _h ⁽¹⁾ =(244±3) pC/N and d′ _h ⁽³⁾ =(92±1) pC/N. The hydrostatic piezoelectric coefficient d _h ⁽³⁾ for orthogonal axis system was calculated to be d _h ⁽³⁾ =(87±2) pC/N. The, optimal orientation of the sample has been found as (Xy l)−20°-cut. Maximal value of the effective hydrostatic piezoelectric coefficient d _h ⁽¹⁾ equals 260 pC/N. Double rotated samples were also studied. The orientation of the samples insensitive to the pressure has been found. The theoretical mean value of hydrostatic piezoelectric coefficient (d _h ) _mean corresponding to randomly oriented Sn₂P₂S₆ grains in a poled composite has been calculated to be (d _h ) _mean =136 pC/N.     

Measurements of intensities and nitrogen-broadened linewidths in the CO fundamental at low temperatures

Intensities and nitrogen-broadened half-widths of lines R(0), R(8) and R(16) in the fundamental band of ¹²C¹⁶O have been measured at 83°K, 100°K, 150°K, 200°K and 298°K. The intensities of several other lines in the P- and R-branches of the band have also been measured at 298°K. The absolute intensity derived from the line intensity data using the Herman-Wallis formula is S^°_v = 273 ± 10 cm^-2atm^-1 at S.T.P. A separate measurement employing the Wilson-Wells-Penner-Weber method has yielded S^°_v = 277 ± 4 cm^-2 atm^-1 at S.T.P. Both of these values are within 6 per cent of most of the previously published direct measurements of this parameter. The values for the line intensities reported earlier by other authors are lower by nearly 16 per cent.