Optical phonon modes and structure of ZnGa2Se4 and ZnGa2S4

We present the infrared and Raman study of the optical phonon modes of the defective compounds ZnGa₂Se₄ and ZnGa₂S₄. Most of the compounds have been found to crystallize in the thiogallate structure (defect chalcopyrite) with space group where all cations and vacancies are ordered. For some Zinc compounds a partially disordered cationic sublattice with various degrees of cation and vacancy statistical distribution, which lead to the higher symmetry (defect stannite), has been reported. For ZnGa₂Se₄ we have found three modes of A symmetry, showing Raman activity only. In addition, we have observed each five modes of B and E symmetry, showing infrared as well as Raman activity. The number of modes and their symmetry assignment, based on polarized measurements, clearly indicate space group for the investigated crystals of ZnGa₂Se₄.Regarding ZnGa₂S₄ we have found three modes exclusively showing Raman activity (2A⊕1B₁), and only eight modes showing infrared as well as Raman activity (3B₂⊕5E). The assignment of the modes has been derived by analyzing the spectral positions of the vibrational modes in comparison to a number of compounds. From the number and symmetry assignment of the optical phonon modes we confirm that ZnGa₂S₄ most likely crystallizes in space group .     

Vibrational spectroscopy studies of a pressure-induced disproportionation reaction of LaH2

The pressure-induced disproportionation reaction of LaH₂ was investigated by infrared reflection and Raman measurements at ambient temperature. The relative reflection intensity in the 4000-6000 cm⁻¹ region began to decrease significantly at a pressure of about 12 GPa and fell to 10% of the initial value at 20 GPa. Absorption peaks, which appeared around 1200 and 700 cm⁻¹ at pressures above 14 GPa, were assigned to the hydrogen vibrations at the tetrahedral and octahedral sites of the fcc metal lattice, respectively. The peak frequencies measured in the 14-30 GPa range were similar to those observed in LaH₃. These infrared results indicated insulating LaH₃ precipitated from metallic LaH₂. Above 20 GPa, a Raman peak related to the hydrogen vibration in the octahedral sites appeared around 700 cm⁻¹, and was likely due to lattice distortion. The disproportionation reaction of LaH₂ into and solid solution LaH_x (x<1) was confirmed.     

Effects of lattice and local dynamics in EPR spectra and electron spin relaxation of vibronic Cu(imidazole)6 complexes in Zn(imidazole)6Cl2·4H2O crystals

Cu(im)₆ complexes in Zn(im)₆Cl₂·4H₂O exhibit a strong Jahn-Teller effect which is static below 100 K and the complex in localized in the two low-energy potential wells. We have reinvestigated electron paramagnetic resonance (EPR) spectra in the temperature range 4.2-300 K and determined the deformation directions produced by the Jahn-Teller effect, energy difference 11 cm⁻¹ between the wells and energy 300 cm⁻¹ of the third potential well. The electron spin relaxation was measured by electron spin echo (ESE) method in the temperature range of 4.2-45 K for single crystal and powder samples. The spin-lattice relaxation is dominated by a local mode of vibration with energy 11 cm⁻¹ at low temperatures. We suppose that this mode is due to reorientations (jumps) of the Cu(im)₆ complex between the two lowest energy potential wells. At intermediate temperatures (15-35 K), the T₁ relaxation is determined by the two-phonon Raman processes in acoustic phonon spectrum with Debye temperature Θ_D=167 K, whereas at higher temperatures the relaxation is governed by the optical phonon of energy 266 cm⁻¹. The ESE dephasing is produced by an instantaneous diffusion below 15 K with the temperature-independent phase memory time , then it grows exponentially with temperature with an activation energy of 97 cm⁻¹. This is the energy of the first excited vibronic level. The thermal population of this level leads to a transition from anisotropic to isotropic EPR spectrum observed around 90 K. FT-ESE gives ESEEM spectrum dominated by quadrupole peaks from non-coordinating ¹⁴N atom of the imidazole rings and the peak from double quantum transition ν_dq. We show that the amplitude of the ν_dq transition can be used to determine the number of non-coordinating nitrogen atoms.     

Room temperature stable structures and phase transition of Na2Al2B2O7

By Rietveld refinement of the X-ray diffraction (XRD) data of powdered Na₂Al₂B₂O₇ samples aged for over 3 months, we found that Na₂Al₂B₂O₇ at room temperature is a mixture of two phases with space group and P6₃/m, respectively. The structures of the two phases can be refined with identical cell parameters of a=4.80760(11) Å, c=15.2684(5) Å and are composed by [Al₂B₂O₇]²⁻_∝ double layers stacking alternatively with Na⁺ ions along the c-direction, but differ at in-plane bond orientations of the BO₃/AlO₄ groups within the double layers: in P6₃/m phase B-O₁/Al-O₁ bonds of the two layers are perfectly aligned, whereas in phase they are twisted by 46.4/41.6° around c-axis against each other. It is also found that a freshly prepared sample contains only the phase, but part of the phase will transfer to P6₃/m phase slowly at room temperature and the transition can be reversed by heating the aged sample above 220 °C.     

Spectroscopic properties of Tm -doped Ba3Gd2(BO3)4 crystal

crystal with the size up to Φ 13 mm×44 mm was grown successfully by the Czochralski technique and its optical properties were presented. The absorption cross-section and emission cross-section were presented. Also, the potential laser gain near 1.9 μm was investigated. In the framework of the Judd-Ofelt (J-O) theory, the intensity parameters were calculated to be: Ω₂=11.375×10⁻²⁰ cm², Ω₄=5.077×10⁻²⁰ cm² and Ω₆=6.524×10⁻²⁰ cm². The spectroscopic parameters of this crystal such as the oscillator strengths, radiative transition probabilities, radiative lifetime as well as the branching ratios were calculated, too. This crystal is promising as a tunable infrared laser crystal.     

Crystal field effect and geometric frustration in Er2Ti2O7 —an XY antiferromagnetic pyrochlore

Magnetic susceptibility of powder Er₂Ti₂O₇ (ErT) is measured between 300 K and 80 K. shows a Curie-Weiss (CW) type behaviour with   ErTiO_3.5 and . A crystal field (CF) analysis of our experimental data, g-values (g_∥=0.27 and g_⊥=7.8) and the positions of two CF levels (reported earlier from an inelastic neutron scattering study) provide CF parameters and CF levels of the ground ⁴I_15/2 and excited multiplets of ErT. The theoretical follows a CW-type behaviour, with . Single-ion magnetic anisotropy (χ_⊥−χ_∥) is 9500×10⁻⁶ emu/mol ErTiO_3.5 at 300 K, which increases by ∼54 times at 10 K and ErT resembles an XY planar system. It can be inferred from CF analysis that the earlier observed change of from −13 K to −22 K below 50 K is not due to the CF effect. Nuclear hyperfine (HF) levels of ¹⁶⁷ErT and ¹⁶⁶ErT are calculated and the theoretical curve of vs. T (K) for T<T_N matches the observed results. Mössbauer lines expected for ¹⁶⁶ErT are also predicted.     

Gas phase electronic spectrum of propadienylidene C3H2

The rotationally resolved vibronic bands in the forbidden electronic transition of the cumulene carbene C₃H₂ have been observed in the gas phase by cavity ring down absorption spectroscopy through a supersonic planar plasma with allene as precursor. The band detected in the 16 223 cm⁻¹ region is a result of vibronic interaction and is assigned to a combination of a₁ and b₂ vibrations with a frequency around 2250 cm⁻¹. Another vibronic band near 15 810 cm⁻¹ has an unusual rotational structure because the K_a = 0-1 subband is absent. It is assigned to a combination of a₁ and b₁ vibrations, ∼1850 cm⁻¹, which borrow intensity from the near lying state due to a-type Coriolis coupling. A rotational analysis using a conventional Hamiltonian for an asymmetric top molecule yields molecular constants for the vibrational excited levels of the ùA₂ state, which were used for the determination of the geometry. The stronger transition of C₃H₂, measured in a neon matrix in the 16 161-24 802 cm⁻¹ range, was not detected. The reason for this is a short lifetime of the state, leading to line broadening.     

Magnetic study of Mg0.95Mn0.05Fe2O4 ferrite nanoparticles

The magnetic properties of Mg_0.95Mn_0.05Fe₂O₄ ferrite samples with an average particle size of ∼6.0±0.6 nm have been studied using X-ray diffraction, Mössbauer spectroscopy, dc magnetization and frequency dependent real χ^′(T) and imaginary χ^″(T) parts of ac susceptibility measurements. A magnetic transition to an ordered state is observed at about 195 K from Mössbauer measurements. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization have been recorded at low field and show the typical behavior of a small particle system. The ZFC curve displays a broad maximum at , a temperature which depends upon the distribution of particle volumes in the sample. The FC curve was nearly flat below , as compared with monotonically increasing characteristics of non-interacting superparamagnetic systems indicating the existence of strong interactions among the nanoparticles. A frequency-dependent peak observed in χ^′(T) is well described by Vogel-Fulcher law, yielding a relaxation time and an interaction parameter . Such values show the strong interactions and rule out the possibility of spin-glass (SG) features among the nanoparticle system. On the other hand fitting with the Néel-Brown model and the power law yields an unphysical large value of τ₀ (∼6×10⁻⁶⁹ and 1.2×10⁻²² s respectively).     

Study of CeNi4Mn by neutron diffraction

We report neutron diffraction measurements on CeNi₄Mn, which has recently been identified as a soft ferromagnet with a sizeable spin-transport polarization. Our data show conclusively that the Mn atoms occupy a unique site (4c) in the unit cell, which has the symmetry of the cubic MgCu₄Sn-type structure. We infer a moment of 4.6 μ_B on Mn at 17 K, which is oriented ferromagnetically along the {101} plane. The amplitude of the Mn vibrational motion is found to be larger than that of Ce and Ni atoms at all temperatures, thereby lending support to theoretical prediction of rattling phonon modes in this compound.     

Physical properties of the cubic spinel LiMn2O4

We have performed an ab initio study of structural, electronic, magnetic, vibrational and thermal properties of the cubic spinel LiMn₂O₄ by employing the density functional theory, the linear-response formalism, and the plane-wave pseudopotential method. An analysis of the electronic structure with the help of electronic density of states shows that the density of states at the Fermi level (N (E_F)) is found to be governed by the Mn 3d electrons with some contributions from the 2p states of O atoms. It is important to note that the contribution of Mn 3d states to N(_EF)$N\left(E_F\right)$ is as much as 85%. From our phonon calculations, we have obtained that the main contribution to phonon density of states (below 250 cm⁻¹) comes from the coupled motion of Mn and O atoms while phonon modes between 250 cm⁻¹ and 375 cm⁻¹ are characterized by the vibrations of all the three types of atoms. The contribution from Li increases rapidly at higher frequency (above 375 cm⁻¹) due to the light mass of this atom. Finally, the specific heat and the Debye temperature at 300 K are calculated to be 249.29 J/mol K and 820.80 K respectively.     

4-benzyl pyridinium dihydrogenmonophosphate C6H5CH2C5H4NHH2PO4 : Single-crystal structure and its conductivity in polycrystalline form

The salt 4-benzyl pyridinium dihydrogenmonophosphate is monoclinic P2₁/c with the following unit cell dimensions: ; ; ; and β=97.328(11). Also, , D_x=1.403, , F(000)=560; ; and R=0.0495 and R_w=0.0964 for 3733 independent reflections. The structure consists of infinite parallel two-dimensional planes built of H₂PO₄⁻ anions and C₆H₅CH₂C₅H₄NH⁺ cations mutually connected by strong O-H ?O and N-H ?O hydrogen bonding. There are no contacts other than the normal Van der Waals interactions between the layers. The conductivity relaxation parameters associated with some H⁺ conduction have been determined from an analysis of the spectrum measured in a wide temperature range.     

Low-temperature measurements of magnetic susceptibility and specific heat of Eu2Ti2O7—An XY pyrochlore

Magnetic susceptibility obtained from magnetization measurement (for fields H=0.1 and 1.0 T) of polycrystalline Eu₂Ti₂O₇ shows two distinct features. Firstly, increases on cooling below 300 K and attains a temperature-independent constant value at 68 K (T_max). Secondly, shows an antiferromagnetic increase below 4.9±0.1 K. The former behavior is explained by crystal field (CF) theory. CF levels and wave functions of ground and excited states are determined accurately from analyses of and earlier reported Mössbauer and optical spectra. Analysis of vs. 1/T curve at low temperatures gives the classical nearest-neighbor exchange interaction J^cl=−0.76 K and a weak dipolar interaction D_nn=0.0056 K. C_P of polycrystalline sample of Eu₂Ti₂O₇ and Y₂Ti₂O₇ are measured between 1.8-35 and 1.8-120 K respectively and θ_D vs. T (K) curves are calculated. At 4 K, θ_D of Eu₂Ti₂O₇ shows a kink and dC_P/dT curve show a maximum. Optical results show energy exchange between Eu³⁺ ions at intrinsic and extrinsic (defect) sites via super-exchange interaction at low temperature which may account for the observed anomalous behavior of and C_P.     

Stretch-bend combination polyads in the ÃAu state of acetylene, C2H2

Rotational analyses are reported for a number of newly-discovered vibrational levels of the S₁-trans (ùA_u) state of C₂H₂. These levels are combinations where the Franck-Condon active and vibrational modes are excited together with the low-lying bending vibrations, and . The structures of the bands are complicated by strong a- and b-axis Coriolis coupling, as well as Darling-Dennison resonance for those bands that involve overtones of the bending vibrations. The most interesting result is the strong anharmonicity in the combinations of (trans bend, a_g) and (in-plane cis bend, b_u). This anharmonicity presumably represents the approach of the molecule to the trans-cis isomerization barrier, where ab initio results have predicted the transition state to be half-linear, corresponding to simultaneous excitation of and . The anharmonicity also causes difficulty in the least squares fitting of some of the polyads, because the simple model of Coriolis coupling and Darling-Dennison resonance starts to break down. The effective Darling-Dennison parameter, K₄₄₆₆, is found to increase rapidly with excitation of , while many small centrifugal distortion terms have had to be included in the least squares fits in order to reproduce the rotational structure correctly. Fermi resonances become important where the K-structures of different polyads overlap, as happens with the 2¹3¹B¹ and 3¹B³ polyads (B = 4 or 6). The aim of this work is to establish the detailed vibrational level structure of the S₁-trans state in order to search for possible S₁-cis (¹A₂) levels. This work, along with results from other workers, identifies at least one K sub-level of every single vibrational level expected up to a vibrational energy of 3500 cm⁻¹.     

New analysis of the Coriolis-interacting v2 and v5 bands of CH3Br and CH3Br

The and fundamental bands of CH₃⁷⁹Br and CH₃⁸¹Br have been studied by Fourier transform infrared spectroscopy with an unapodized resolution of 0.004 cm⁻¹, corresponding to an improvement of one order of magnitude compared to previous studies. For both isotopomers, some 2427 (2239) lines were newly assigned for the parallel and the perpendicular bands and, in addition, 80 perturbation-allowed transitions were also added. The ground-state axial rotational constants A₀ were redetermined from allowed and perturbation-allowed infrared transitions observed in the v₂ and v₅ bands around the local crossing. The A₀ values obtained for both isotopomers are more accurate but fully compatible with those obtained previously. Using those results, and the variation of the rotational constants with vibration, new accurate equilibrium constants A_e and B_e have been also determined for CH₃⁷⁹Br and CH₃⁸¹Br. The excited states v₂=1 and v₅=1 are coupled by Coriolis-type interactions (Δl=±1,ΔK=±1) and (Δl=?1,ΔK=±2), while the l₅=±1 levels of v₅ interact also through “l(2,2)”-type interaction (Δl=±2,ΔK=±2). The Coriolis coupling term was determined to be for CH₃⁷⁹Br and for CH₃⁸¹Br. All interaction parameters have been determined with higher accuracy, compared to previous studies. A total of 4213 (3704) line positions with J?68(64) and K?16(11) including all available data was fitted using 20 (18) parameters with a root-mean-square deviation of 0.0007 (0.0006) cm⁻¹ for CH₃⁷⁹Br and CH₃⁸¹Br, respectively. Two different but equivalent forms of reduced Hamiltonians with two different sets of constrained constants were successfully applied according to Lobodenko's reduction [J. Mol. Spectrosc. 126 (1987) 159]. The ratio of the transition moments, |d₂/d₅|=1.65, and a positive sign of the Coriolis intensity perturbation d₂×ζ₂₅×d₅ were determined. Therefore, it has been possible to generate an accurate prediction of the whole spectrum between 1200 and 1650 cm⁻¹, including Q branches.     

Quantum critical behaviour in doped Y bB12 studied by ab initio calculations

The Kondo insulator Y bB₁₂ is known to undergo a transition to the metallic state with doping or under an external magnetic field. Within the virtual crystal approximation (VCA), we calculated the occupation of the Yb 4f and 5d shells, and , as a function of doping of Y bB₁₂ with the rare earths Tm and Lu. We found that exhibits an anomalous change at the critical concentration of the dopant, in agreement with experiment ( for Y b_1−xLu_xB₁₂ and for Y b_1−xTm_xB₁₂). We suggest that the critical behaviour seems to be strictly connected with the change of and in consequence the change of the Yb valency.     

The high-resolution infrared spectrum of thiophene between 600 and 1200 cm: A spectroscopic and theoretical study of the fundamental bands ν6, ν7, ν13, and the c-Coriolis interacting dyad ν5, ν19

The Fourier transform infrared spectrum of gaseous thiophene, C₄H₄S, has been recorded in the 600-1200 cm⁻¹ spectral region with a resolution of ca. 0.0030 cm⁻¹. Five fundamental bands ν₁₃ (B₁, 712.1 cm⁻¹), ν₇ (A₁; 840.0 cm⁻¹), ν₆ (A₁; 1036.4 cm⁻¹), ν₅ (A₁; 1081.5 cm⁻¹) and ν₁₉ (B₂; 1084.0 cm⁻¹) have been analysed by the standard Watson model (A-reduction). Ground state rotational and quartic centrifugal distortion constants have been obtained from a simultaneous fit of ground state combination differences from four of these bands and previous microwave transitions. Upper state spectroscopic constants have been obtained for all five bands from single band fits using the Watson model. A strong c-Coriolis resonance perturbs the close lying ν₅ and ν₁₉ bands. We have analysed this dyad system by a model including first and second order Coriolis resonance using the theoretically predicted Coriolis coupling constant . From this analysis we locate the previously unobserved ν₁₉ band at 1083.969 cm⁻¹. The rotational constants, ground state quartic centrifugal distortion constants, anharmonic frequencies, and vibration-rotational constants (α-constants) predicted by quantum chemical calculations using a cc-pVTZ basis with B3LYP methodology, are compared with the present experimental data, where there is generally good agreement. A complete set of anharmonic frequencies and α-constants for all fundamental levels of the molecule is given.     

The v6 = 1 and v6 = 2 vibrational states of DCF3

The high-resolution infrared spectra of DCF₃ were reinvestigated in the ν₆ fundamental band region near 500 cm⁻¹ and around 1000 cm⁻¹ with the aim to assign and analyze the overtone level of the asymmetric CF₃ bending vibration v₆ = 2.The present paper reports on the first study of both its sublevels (A₁ and E corresponding to l = 0 and ±2, respectively) through the high-resolution analysis of the overtone band and the hot and bands.The well-known “loop method”, applied to and , yielded ground state energy differences Δ(K, J) = E₀(K, J) − E₀(K − 3,J) for the range of K = 6 to 30.In the final fitting of molecular parameters, we used the strategy of fitting all upper state data together with the ground state rotational transitions.This is equivalent to that calculating separately the and coefficients of the K-dependent part of the ground state energy terms from the combination loops.All rotational constants of the ground state up to sextic order could be refined in the calculation.This led to a very accurate determination of C₀ = 0.18924413(25) cm⁻¹, , and also .In the course of analyzing simultaneously the overtone band together with the and ν₆ bands, the original assignment of the fundamental ν₆ band [Bürger et al., J. Mol. Spectrosc. 182 (1997) 34-49] was found to be incompatible with the present one. Assignments of the (k + 1, l₆ = +1)/(k − 1,l₆ = −1) levels had to be interchanged, which changed the value of Cζ₆ = −0.14198768(26) cm⁻¹ and the sign of the combination of constants C − B − Cζ in the v₆ = 1 level to a negative value.     

High-resolution infrared and theoretical study of the fundamental bands ν2, ν4 and ν9 and the c-Coriolis interacting dyad ν5, ν14 of 1,3,4-thiadiazole

The Fourier transform gas-phase IR spectrum of 1,3,4-thiadiazole, C₂H₂N₂S, has been recorded with a resolution of ca. 0.003 cm⁻¹ in the 800-1500 cm⁻¹ spectral region. Five fundamental bands ν₂(A₁; 1391.9 cm⁻¹), ν₄(A₁; 964.4 cm⁻¹), ν₅(A₁; 894.6 cm⁻¹), ν₉(B₁; 821.5 cm⁻¹), and ν₁₄(B₂; 898.4 cm⁻¹) have been analysed using the Watson model. Ground state rotational and quartic centrifugal distortion constants as well as upper state spectroscopic constants have been obtained from fits. The ν₄ and ν₉ bands are unperturbed while a strong c-Coriolis resonance perturbs the close-lying ν₅ and ν₁₄ bands. This dyad system has been analysed by a model including first and second order c-Coriolis resonance using the theoretically predicted Coriolis coupling constant . The ν₂ band is strongly perturbed by a local resonance, and we obtain a set of spectroscopic parameters using a model including second order a-Coriolis resonance with the inactive ν₁₀ + ν₁₄ band. Ground state rotational and quartic centrifugal distortion constants, anharmonic frequencies, and vibration-rotational α-constants predicted by quantum chemical calculations using a cc-pVTZ basis and B3LYP methodology, have been compared with the present experimental data, where there is generally good agreement.