Investigation of the Kerr effect and third-order susceptibility constants in a nematic liquid crystal

In this paper, the static Kerr effect was investigated for the W-1680 at temperatures above the nematic-isotropic transition temperature and compared with 9CHBT. We experimentally measured the electro-optical Kerr constant and the pre-transitional behavior. The Kerr law has been confirmed and the variation of the inverse Kerr constant, with a temperature above the smectic-isotropic transition temperatures, were determined. The linear dependence of (T − T*)^− 1 on the Kerr constant is found to be in good agreement with the predications of the Landau-De Gennes model. Additionally, the temperature dependence of third order susceptibility has been studied. This value for these samples will reduce when the temperature increases.     

Superconducting thin films of MxMo6S8 type

Thin films of M_xMo₆S₈, where M=Pb, Sn, Sn-Al and Cu, known as the Chevrel phases have been prepared by d.c. getter sputtering method and the optimal conditions of their preparation have been determined. The transition temperatures reached: 10.16, 13.66, 11.74 and 12.86 K for thin films with M=Cu, Sn, Sn-Al and Pb respectively. The highest critical fields H_c²(0) of 428 kG were obtained for Pb compounds.     

PMRD investigations of molecular dynamics and phase transitions in the liquid crystal 6CHBT

Proton Magnetic Relaxation Dispersion (PMRD) measurements were carried out over a wide Larmor frequency range near the isotropic-nematic transition temperature (T _NI) and in the mid nematic phase of a low viscous liquid crystal 4-(trans-4′-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT), with a view to examining the pre-transitional effects just above isotropic-nematic transition (T > T _NI) and the role of director fluctuations (DF) on the nematic phase stability of this system. The results near the transition indicate critical slowing down of the short-range nematic order modes just above T _NI with a mean field critical exponent characterizing the coherence length (ξ). The observed temperature independence of the correlation time associated with the reorientations (τ_R) and the extension of DF mode spectrum, in the mid-nematic phase, to relatively higher resonance frequencies seem to suggest fairly low hindering barriers for the tumbling of the molecules about their short axes.     

Influence of electric field on photoluminescence of lanthanide-doped nematic liquid crystal

Highly resolved luminescence and luminescence excitation spectra of recently synthesized LnL₃bpy complexes, where Ln=Eu³⁺, Tb³⁺, bpy=2,2′- bipyridyne and L=phosphoroazo derivative of β-diketone: CCl₃C(O)NP(O)(OCH₃)₂, in solid state and dissolved in nematic liquid crystal 6CHBT [4-(isothiocyanatophenyl)-1-(trans-4-hexyl)cyclohexane] were measured. The photoluminescence intensity of the complexes dissolved in 6CHBT was found to exhibit a strong dependence on the electric field. The mechanism of the effect and reason of its asymmetry with respect to the sign of electric field are discussed in the paper. Luminescence decay time in various environments and experimental quantum yields of the luminescence of these compounds in CDCl₃ and CHCl₃ solutions were measured.     

Crystal → nematic phase transition in the liquid crystalline system 1‐isothiocyanato‐4‐(trans‐4‐propylcyclohexyl) benzene (3CHBT) probed by temperature‐dependent micro‐Raman study and DFT calculations

Raman spectra of 3CHBT in unoriented form were recorded at 14 different temperature measurements in the range 25–55 °C, which covers the crystal → nematic (N) phase transition, and the Raman signatures of the phase transition were identified. The wavenumber shifts and linewidth changes of Raman marker bands with varying temperature were determined. The assignments of important vibrational modes of 3CHBT were also made using the experimentally observed Raman and infrared spectra, calculated wavenumbers, and potential energy distribution. The DFT calculations using the B3LYP method employing 6‐31G functional were performed for geometry optimization and vibrational spectra of monomer and dimer of 3CHBT. The analysis of the vibrational bands, especially the variation of their peak position as a function of temperature in two different spectral regions, 1150–1275 cm⁻¹ and 1950–2300 cm⁻¹, is discussed in detail. Both the linewidth and peak position of the ( C H ) in‐plane bending and ν(NCS) modes, which give Raman signatures of the crystal → N phase transition, are discussed in detail. The molecular dynamics of this transition has also been discussed. We propose the co‐existence of two types of dimers, one in parallel and the other in antiparallel arrangement, while going to the nematic phase. The structure of the nematic phase in bulk has also been proposed in terms of these dimers. The red shift of the ν(NCS) band and blue shift of almost all other ring modes show increased intermolecular interaction between the aromatic rings and decreased intermolecular interaction between two  NCS groups in the nematic phase. Copyright © 2009 John Wiley & Sons, Ltd.     

GREAT MAGNETIC ENTROPY CHANGE IN La(Fe, M)13 (M=Si, Al) WITH Co DOPING

Very large magnetic entropy change Δ S_M, which originates from a fully reversible second-order transition at Curie temperature T_C, has been discovered in compounds La(Fe, Si)₁₃, La(Fe, Al)₁₃ and those with Co doping. The maximum change ΔS_M\approx19 J·kg^-1·K^-1, achieved in LaFe_11.4Si_1.6 at 209K upon a 5T magnetic field change, exceeds that of Gd by more than a factor of 2. The T_C of the Co-doped compounds shifts to higher temperatures. ΔS_M still has a considerable large magnitude near room temperature. The phenomena of very large ΔS_M, convenience of adjustment of T_C, and also thesuperiority of low cost, strongly suggest that the compounds La(Fe, M)₁₃ (M=Si, Al) with Co doping are suitable candidates for magnetic refrigerants at high temperatures.     

Photoluminescence properties of Li6CaB3O8.5: M3+ (M3+: Dy and Sm)

Li₆CaB₃O_8.5: M³⁺ (M³⁺: Dy and Sm) phosphors were synthesized by a solution combustion synthesis. The synthesized materials were characterized by using the powder XRD. The emission and excitation spectra of these materials were measured at room temperature with a spectrofluorometer. Both Li₆CaB₃O_8.5: Dy³⁺ and Li₆CaB₃O_8.5: Sm³⁺ phosphors emit red, yellow and green light. Consequently, these materials are promising phosphors for white LEDs.     

Reinvestigation of the thermoelectric properties of Ag8GeTe6

High‐density polycrystalline samples (above 98% of the theoretical density) of Ag₈GeTe₆ were prepared by solid‐state reactions of Ag₂Te, GeTe, and Te, followed by hot‐pressing. The thermoelectric properties were measured at temperatures ranging from room temperature to around 700 K. The thermal conductivity values were extremely low (0.25 Wm^–1 K^–1 at room temperature), and consequently Ag₈GeTe₆ exhibited a relatively high thermoelectric figure of merit, ZT = 0.48 at 703 K. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

EPR of Mn2+ in single crystals of [Zn(H2O)6]SnF6 and [Co(H2O)6]SnF6; Co2+ spin-lattice relaxation time

EPR studies have been carried out in Mn²⁺-doped single crystals of [M(H₂O)₆]SnF₆ (M  Zn, Co) at different temperatures using X-band microwave frequency. Mn²⁺ has been found to substitute for Zn²⁺ or Co²⁺ exhibiting a unique magnetic complex with z-axis directed long the c-axis of the crystals. Observation of resolved Mn²⁺ spectrum in [Co(H₂O)₆]SnF₆ and broadening of the resonance lines on cooling the crystals have been explained on the basis of host spin-lattice relaxation narrowing. The T₁ of Co²⁺ has been estimated to be ≈ 1.8 × 10⁻¹² s at 293 K.     

The absorption and magnetic circular dichroism of spectra of Cs2NaPrCl6

The room temperature absorption and magnetic circular dichroism spectra and the absorption spectrum at liquid helium (liquid He) temperature have been measured for Cs₂NaPrCl₆. At room temperature the crystal is cubic and the Pr³⁺ sites have O _h symmetry. All terms above 15 000 cm^-1, except ¹S₀, have been assigned and a previous assignment in PrCl₆ ^3- has been shown to be incorrect. The transition at 20 631 cm^-1 is assigned to ³H₄(A _1g ) →³P₁(T _1g ), in contradiction to previous assignments of Pr³⁺ spectra in other systems. A rich vibrational structure was observed in every transition. Vibrations have been assigned using the site group approximation and there is substantial agreement with the vibrational assignments in Cs₂NaEuCl₆. A crystal phase transition takes place between room temperature and liquid nitrogen temperature and the O _h forbidden transitions, A _1g →E_g and T _2g , are observed. At lower temperatures many additional lines are observed but it is unclear presently whether they are due to lower symmetry or a breakdown of the site group approximation.     

Structure and magnetic entropy change of suck-cast Gd60Co26Al6Ge8 alloy

The Gd₆₀Co₂₆Al₆Ge₈ alloy has been prepared by the copper-mold suck-casting and its phase component has been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). It is shown that this alloy consists of primary crystalline Gd₅Ge₃ phase and amorphous matrix. The glass transition temperature (T_g) and crystallization temperatures (T_x) occur at 292 and 320 °C, respectively. The maximal magnetic entropy change (ΔS_M) under 0-5 T field is about 7.6 J (kg⁻¹ K⁻¹) at 155 K and the refrigeration capacity (RC) is about 768 J kg⁻¹, which makes Gd₆₀Co₂₆Al₆Ge₈ bulk metallic glass matrix composite a promising candidate for magnetic refrigerant.     

Magnetic behaviour and DCEMS study of SnO2 films implanted with 57Fe

Fe implanted SnO₂ films (5 × 10¹⁶ and 1 × 10^{17 57}Fe ions/cm²) characterized by conversion electron Mossbauer spectroscopy (CEMS) are reviewed. The substrate temperatures affect the growth of precipitated iron oxides. The Fe ion implanted film at room temperature (RT) shows no Kerr effect and no magnetic sextet in CEM spectra. The SnO₂ film implanted with ⁵⁷Fe at the substrate temperature of 300 °C show a small Kerr effect although the magnetic sextet is not observed, but post-annealing results in the disappearance of the Kerr effect. This magnetism is considered to be due to defect induced magnetism. Some samples were measured by CEMS at 15 K. SnO₂ (0.1 at %Sb and 3 at %Sb) films, implanted at 500 °C and the post-annealed samples, show RT ferromagnetism due to formation of clusters of magnetite and maghemite, respectively. The layer by layer analysis of these films within 100 nm in thickness has been done by depth sensitive CEMS (DCEMS) using a He + 5 % CH₄ gas counter. The structures and compositions of Fe implanted SnO₂ films, and the effects due to post-annealing were investigated.     

One-magnon light scattering and spin-reorientation transition in epitaxial BiFeO3 thin films

Raman scattering from one-magnon excitation has been observed for the first time in epitaxial BiFeO₃ thin films grown on (1 1 1) SrTiO₃ substrates. The intensities and the frequency of the magnon mode at 18.9 cm⁻¹ (M₁) showed a discrepancy at the characteristic temperatures of ∼140 and 200 K and the magnon mode at 27.9 cm⁻¹ (M₂) disappeared at ∼200 K suggesting spin-reorientation (SR) transition in the epitaxial BFO film. The dc susceptibility measurement showed a large discrepancy near these two temperatures evidently elucidating the spin-reorientation transition mechanism. The partial spectral weight of the magnon modes is believed to be transferred to the lowest phonon mode appearing at 72.8 cm⁻¹ and higher magnon mode M₂ disappearing near 200 K reveal magnon-phonon coupling near to SR transition.     

Photoluminescence and Optical Absorption of Cs2NaScF6:Cr3+

The main objective of this paper is the characterization of the spectroscopic properties of new materials that are prospective laser media. This approach allows for the comparison of the properties of the Cr³⁺ in different environments. Here, we have studied the photoluminescence and optical absorption of Cs₂NaScF₆:Cr³⁺ single crystals. On the basis of near-infrared luminescence measurements at 2, 77, and 300 K the observed lines originated from the Cr³⁺-centres were associated with the transition and the lifetimes were obtained. In spite of the quenching observed as a function of temperature at least 10% of the 2 K emission intensity for Cs₂NaScF₆ doped with 1% of Cr³⁺ remains at room temperature. Besides, the 2 K emission broad band could be well described in terms of normal modes of the octahedral complex [CrF₆]³⁻, and the Racah and crystal-field parameters calculated.     

Double magnetic entropy change peaks and high refrigerant capacity in Gd1-xHoxNi compounds in the melt-spun form

Gd_1-xHo_xNi melt-spun ribbons were fabricated by a single-roller melt spinning method. All the compounds crystallize in an orthorhombic CrB-type structure. The Curie temperature (T_C) was tuned between 46 and 99 K by varying the concentration of Gd and Ho. A spin reorientation (SRO) transition is observed around 13 K. Different from T_C, the SRO transition temperature is almost invariable for all compounds. Two peaks of magnetic entropy change (ΔS_M) were found. One at the higher temperature range was originated from the paramagnet-ferromagnet phase transition and the other at the lower temperature range was caused by the SRO transition. The maximum of ΔS_M around T_C is almost same. The other maximum of ΔS_M around SRO transition, however, had significantly positive relationship with x. It reached a maximum about 8.2 J kg⁻¹ K⁻¹ for x = 0.8. Thus double large ΔS_M peaks were obtained in Gd_1-xHo_xNi melt-spun ribbons with the high Ho concentration. And the refrigerant capacity power reached a maximum of 622 J kg⁻¹ for x = 0.6. Gd_1-xHo_xNi ribbons could be good candidate for magnetic refrigerant working in the low temperature especially near the liquid nitrogen temperature range.     

Magnetic behaviour of Cu2FeGeSe4

Measurements of magnetic susceptibility χ as a function of temperature T and of magnetisation M as a function of applied magnetic field H at a number of fixed temperatures were made on polycrystalline samples of Cu₂FeGeSe₄. The χ versus T data show that an antiferromagnetic transition occurs at 20 K and that a second transition occurs at 8 K, indicating a transition to weak ferromagnetic form. The M versus H curves indicated that at all temperatures below 70 K bound magnetic polarons (BMP) occur, in the paramagnetic, antiferromagnetic and weak ferromagnetic ranges. Below 8 K, the M versus H curves exhibited magnetic hysteresis, and this is attributed to the interaction of the BMPs with tetragonally anisotropic matrix. The B versus H curves were well fitted by a Langevin-type of equation, and the variation of the fitting parameters determined as a function of temperature. These showed that above 20 K the total BMP magnetisation fell almost linearly with increasing temperature and effectively disappeared at 70 K. The number of BMPs remained practically constant with temperature having a mean value of 6.55×10¹⁸/cm³. The analysis gave a value of 213 μ_B for the average magnetic moment of a BMP, corresponding to 42.4 Fe atoms. Using a simple spherical model, this gives the radius of a BMP as 12.0 Å.     

The effect of Ag substitution in the mixed conductor Cu7PSe6

Cu₇PSe₆ is a mixed conductor that crystallizes in the simple cubic structure at room temperature. Structural transitions above and below room temperature are accompanied by step-like changes in electrical conductivity. The substitution of Ag⁺ for Cu⁺ in Cu₇PSe₆ stabilized the simple cubic structure over a wider range of temperatures than is observed for the pure compound. A disproportionate decrease in electrical conductivity accompanies modest levels of silver substitution. The prominent step in electrical conductivity associated with the low-temperature crystallographic phase transition disappears in (Ag_xCu_1−x)₇PSe₆ solid solutions for a composition parameter x=0.20, replaced by two distinct changes in the slope of conductivity below room temperature.     

Structure-dependent magnetic susceptibility of sharp poly(ethylene oxide) fractions

Nine sharp fractions of poly(ethylene oxide) (PEO) glycol with number-average molar masses (M _n) in the range from 0.6 × 10³ to 20 × 10³ (PEO-0.6 to PEO-20) were characterized by magnetic susceptibility χ measured in the temperature interval 293 K to 378 K. In contrast to the liquidlike PEO-0.6 with temperature-invariant χ, the values of χ for each of the remaining solid samples, after the initial increase, exhibited two plateaus separated by a relatively narrow temperature interval of their second increase. The jumps of χ at lower and higher temperatures were attributed to a solid-state transition of unspecific nature and to the melting of the crystal fraction, respectively.

The temperature-invariant values of χ_n in the melt state above T _m pass through a minimum for the sample PEO-2.0 and then increase again with (M_n) to a limiting value χ_∞ = ?0.622 × 10^?6. It is concluded that a considerable contribution of the molar-mass-dependent “paramagnetism” χ_P = χ ^? χ_d (where χ_d is the diamagnetic contribution estimated by Kirkwood's equation) to the total magnetic susceptibility of PEO fractions reflects distortions of the spherical symmetry of the electron shells around chain atoms resulting from the discontinuous change of both inter- and intrachain interactions as the (M_n) increases through and above the critical crossover molar mass (M_cr ) = 2 × 10³.     

Mössbauer study of the modified pyrochlores CsMFeF6 (M = Mn,Ni)

⁵⁷Fe Mössbauer spectra of the modified pyrochlores CsMFeF₆ (M = Mn, Ni) were obtained between 1.5 K and room temperature. The spectra for single crystal and powder absorbers are very similar, and demonstrate the randomness associated with the iron sites. Nevertheless, the presence of two distinct iron sites, recently reported by Varret and Courbion for powder samples, is confirmed. Both CsMnFeF₆ and CsNiFeF₆ are antiferromagnets with Néel temperatures, T_N, of 25 and 5.2 K, respectively. The hyperfine fields lie on the Brillouin function with $\text{S =}\text{5}\text{2}$. The hyperfine parameters show that the iron ions are ferric in the high-spin state.