Showing papers in "Physica B-condensed Matter in 2005"

Model for thermal conductivity of carbon nanotube-based composites

Abstract: Considering the carbon nanotubes’ (CNTs) orientation distribution a new model of effective thermal conductivity of CNTs-based composites is presented. Based on Maxwell theory, the formulae of calculating effective thermal conductivity of CNTs-based composites are given. The theoretical results on the effective thermal conductivity of CNTs/oil and CNTs/decene suspensions are in good agreement with the experimental data. The model is valid for the transport properties of the CNTs-based composites.

Pressure-induced α → ω transition in titanium metal: a systematic study of the effects of uniaxial stress

Abstract: The effects of uniaxial stress on the pressure-induced α → ω transition in pure titanium (Ti) are investigated by means of angle dispersive X-ray diffraction in a diamond-anvil cell. Experiments under four different pressure environments reveal that: (1) the onset of the transition depends on the pressure medium used, going from 4.9 GPa (no pressure medium) to 10.5 GPa (argon pressure medium); (2) the α and ω phases coexist over a rather large pressure range, which depends on the pressure medium employed; (3) the hysteresis and quenchability of the ω phase is affected by differences in the sample pressure environment; and (4) a short-term laser heating of Ti lowers the α → ω transition pressure. Possible transition mechanisms are discussed in the light of the present results, which clearly demonstrate the influence of uniaxial stress in the α → ω transition.

Thermoelectric properties of Bi-doped Mg2Si semiconductors

Abstract: The thermoelectric properties of Bi-doped Mg2Si (Mg2Si:Bi=1:x) fabricated by spark plasma sintering process have been characterized by Hall effect measurements at 300 K and by measurements of electrical resistivity ( ρ ) , Seebeck coefficient (S), and thermal conductivity ( κ ) between 300 and 900 K. Bi-doped Mg2Si samples are n-type in the measured temperature range. The electron concentration of Bi-doped Mg2Si at 300 K ranges from 1.8×1019 cm−3 for the Bi concentration x = 0.001 to 1.1×1020 cm−3 for x = 0.02 . The solubility limit of Bi in Mg2Si is estimated to be about 1.3 at% and first-principles calculation revealed that Bi atoms are expected to be primarily located at the Si sites in Mg2Si. The electrical resistivity, Seebeck coefficient, and thermal conductivity are strongly affected by the Bi concentration. The sample of x = 0.02 shows a maximum value of the figure of merit, ZT, is 0.86 at 862 K.

High performance n-type PbTe-based materials for thermoelectric applications

Abstract: Lead telluride-based compounds are known for their favorable thermoelectric properties in the 50–600 °C temperature range. The transport properties of homogeneous, cold-compacted and sintered PbI 2 -doped PbTe samples were measured and compared to those of cast samples with similar dopant concentrations. Such a comparison is mandatory in order to determine the potential of graded samples, prepared by powder metallurgy, for thermoelectric applications. The present work focuses on improving the thermoelectric efficiency of PbTe-based materials by functional grading and by taking advantage of the specific features of indium as dopant element. The potential for achieving high thermoelectric efficiency in In-doped n-type PbTe-based thermoelectric compounds is described, and is compared to that of PbI 2 -doped PbTe. Design, synthesis and characterization procedures are reported for fabricating In- and PbI 2 -doped PbTe compounds with desired composition profiles.

Size effect on melting temperature of nanosolids

Abstract: By considering the surface effects, the melting temperature of nanosolids (nanoparticles, nanowires and nanofilms) has been predicted based on size-dependent cohesive energy. It is shown that the melting temperature of free standing nanosolids decreases with decrease in the solid size, and the melting temperature variation for spherical nanoparticle, nanowire and nanofilm of a material in the same size is 3:2:1. The present theoretical results on Sn and Pb nanoparticles, In nanowires and nanofilms are consistent with the previous experimental values.

AC impedance analysis of LaLiMo2O8 electroceramics

Abstract: Complex impedance analysis of a new rare earth-based ceramic oxide, LaLiMo 2 O 8 , prepared by a standard solid-state reaction technique has been carried out. Material formation under the reported conditions has been confirmed by X- ray diffraction studies. A preliminary structural analysis indicates the crystal structure to be orthorhombic. Electrical properties of the material sample have been studied using AC impedance spectroscopy technique. Impedance spectrum results indicate that the electrical properties of the material are strongly dependent on temperature and it bears a good correlation with the sample microstructure (i.e. the presence of bulk, grain boundary, etc.) in different temperature ranges. Evidences of temperature-dependent electrical relaxation phenomena in the material have also been observed. The bulk resistance, evaluated from complex impedance spectrum has been observed to decrease with rise in temperature showing a typical negative temperature coefficient of resistance (NTCR)-type behavior like that of semiconductors. The DC conductivity shows typical Arrhenius behavior when observed as a function of temperature. The AC conductivity spectrum has provided typical signature of an ionically conducting system and is found to obey Jonscher's universal power law. Modulus analysis has indicated the possibility of hopping mechanism for electrical transport processes in the system with non-exponential-type conductivity relaxation.

Structural and optical properties of pure PMMA and metal chloride-doped PMMA films

Abstract: Films of PMMA and PMMA doped with CrCl3 or CoCl2 with different contents were prepared using the casting technique. The absorption spectral analysis showed that the Cr3+ are present in octahedral symmetrical form in the PMMA matrix, while Co2+ are present in mixed octahedral and tetrahedral form. The calculations of the ligand field parameters revealed the formation of metal–polymer complex. The dispersion studies of pure PMMA film and PMMA films doped with CrCl3 or CoCl2 were investigated using spectrophotometric measurements of transmittance and reflectance in the wavelength range 250–1500 nm. Analysis of the refractive index and absorption coefficient was discussed. The change of the calculated values of the optical energy gaps with increasing CrCl3 or CoCl2 content has been interpreted in terms of the structural modifications of the PMMA matrix. The Urbach tail in the mobility gap is discussed.

Proton-conducting polymer electrolyte films based on chitosan acetate complexed with NH4NO3 salt

Abstract: Chitosan acetate-ammonium nitrate (NH4NO3) films have been prepared by the solution-cast technique. Fourier transform infrared spectroscopy (FTIR) showed that complexation has occurred. FTIR exhibited shifts in amine and carbonyl bands from 1553 to 1520 cm−1 and 1636 to 1617 cm−1. A new peak was also observed at 1746 cm−1. XRD shows that all complexes are amorphous. The highest conductivity at room temperature is 2.53×10−5 S cm−1 for the film containing 45 wt% NH4NO3. The conductivity of the samples is dependent on the number of mobile ions and mobility.

Electrical and magnetic properties of polyaniline/Fe3O4 nanostructures

Abstract: We report on electrical and magnetic properties of polyaniline (PANI) nanotubes (∼150 nm in diameter) and PANI/Fe 3 O 4 nanowires (∼140 nm in diameter) containing Fe 3 O 4 nanoparticles with a typical size of 12 nm. These systems were prepared by a template-free method. The conductivity of the nanostructures is 10 −1 –10 −2 S/cm; and the temperature dependent resistivity follows a ln ρ∼T −1/2 law. The composites (6 and 20 wt% of Fe 3 O 4 ) show a large negative magnetoresistance compared with that of pure PANI nanotubes and a considerably lower saturated magnetization ( M s =3.45 emu/g at 300 K and 4.21 emu/g at 4 K) compared with the values measured from bulk magnetite ( M s =84 emu/g) and pure Fe 3 O 4 nanoparticles ( M s =65 emu/g). AC magnetic susceptibility was also measured. It is found that the peak position of the AC susceptibility of the nanocomposites shifts to a higher temperature (>245 K) compared with that of pure Fe 3 O 4 nanoparticles (190–200 K). These results suggest that interactions between the polymer matrix and nanoparticles take place in these nanocomposites.

Abrupt metal–insulator transition observed in VO2 thin films induced by a switching voltage pulse

Abstract: An abrupt metal–insulator transition (MIT) was observed in VO2 thin films during the application of a switching voltage pulse to two-terminal devices. Any switching pulse over a threshold voltage for the MIT of 7.1 V enabled the device material to transform efficiently from an insulator to a metal. The characteristics of the transformation were analyzed by considering both the delay time and rise time of the measured current response. The extrapolated switching time of the MIT decreased down to 9 ns as the external load resistance decreased to zero. Observation of the intrinsic switching time of the MIT in the correlated oxide films is impossible because of the inhomogeneity of the metallic and the insulating states. This indicates that the intrinsic switching time is in the order of less than a nanosecond.

Effect of sintering temperature on electrical transport properties of La0.67Ca0.33MnO3

Abstract: A systematic investigation of lanthanum-based manganite, La0.67Ca0.33MnO3, has been undertaken with a view to understand the influence of varying crystallite size, in the nanoscale, on various physical properties. The materials were prepared by the sol–gel route by sintering at four different temperatures starting from 800 to 1100 °C, with an interval of 100 °C. After the usual characterization of these materials structurally by XRD, their metal-insulator transition (TP) as well as magnetic transition (TC) temperatures were determined. Surprisingly these materials are found to exhibit two different types of behaviors, viz, while TC is found to decrease from 253 to 219 K, TP is increasing from 145 to 195 K with increasing sintering temperature. A systematic study of electrical conductivity of all four materials was undertaken not only as a function of temperature (80–300 K), but also as a function of magnetic field up to 7 T mainly to understand the detailed conduction mechanism in these materials. On analyzing the data by using several theoretical models, it has been concluded that the metallic (ferromagnetic) part of the resistivity (ρ) (below TP) fits well with the equation ρ ( T ) = ρ 0 + ρ 2.5 T 2.5 , indicating the importance of grain/domain boundary effects (ρ0) and electron–magnon scattering processes (∼T2.5). On the other hand, in the high temperature (T>TP) paramagnetic insulating regime, the adiabatic small polaron and VRH models fit well in different temperature regions, thereby indicating that polaron hopping might be responsible for the conduction mechanism.

A reversible pressure-induced phase transition in β-glycine at 0.76 GPa

Abstract: Single crystals of β-glycine were studied at pressures up to 7.6 GPa in a diamond anvil cell (DAC) in situ by Raman spectroscopy and polarized optical microscopy. A reversible phase transition was observed at 0.76 GPa. A boundary between the two phases could be observed in the microscope if the phase transition was slow enough. The phase transition was accompanied by cracking of the crystal. The birefringence in the high-pressure phase 2 was close to that in the starting phase 1, so that the phase transition can be supposed to preserve the monoclinic crystal system. Every vibrational band in the spectrum of the high-pressure phase can be related to a corresponding band in the starting phase, excluding lattice vibrations. The frequencies of most vibrations changed by a jump at the transition point. Low-frequency lattice modes showed linear pressure dependence when pressure increased from 0.76 to 7.6 GPa. This can be a manifestation of the softening of the libration modes of glycine at the phase transition point. At high pressures (in the range of 6.5–7.6 GPa) the shifts of the frequencies of some vibrational bands were nonlinear. This can be related to the rotations and twisting of the zwitter-ions of glycine.

Half-metallic ferromagnetic semiconductors of V- and Cr-doped CdTe studied from first-principles pseudopotential calculations

Abstract: The electronic structure and the ferromagnetism of V- and Cr-doped zinc-blende semiconductor CdTe have been investigated by spin-polarized calculations with first-principles plane-wave pseudopotential method within the generalized gradient approximation for the exchange-correlation potential. We find that the V- and Cr-doped zinc-blende CdTe show half-metallic behavior with a total magnetic moment of 3.0 and 4.0 mu(B) per supercell, respectively. It may be useful in semiconductor spintronics and other applications. (c) 2005 Elsevier B.V. All rights reserved.

Current transport in Zn/p-Si(1 0 0) Schottky barrier diodes at high temperatures

Abstract: In this study, we have performed behavior of the non-ideal forward bias current–voltage (I–V) and the reverse bias capacitance–voltage (C–V) characteristics of Zn/p-Si (metal–semiconductor) Schottky barrier diode (SBDs) with thin interfacial insulator layer. The forward bias I–V and reverse bias C–V characteristics of SBDs have been studied at the temperatures range of 300–400 K. SBD parameters such as ideality factor n, the series resistance (RS) determined Cheung's functions and Schottky barrier height, Φ b , are investigated as functions of temperature. The ideality factor n and RS were strongly temperature dependent and changed linearly with temperature and inverse temperature, respectively. The zero-bias barrier heights Φ b 0 ( I – V ) calculated from I–V measurements show an unusual behavior that it was found to increase linearly with the increasing temperature. However, the barrier height Φ b ( C – V ) calculated from C–V measurements at 500 kHz frequency decreased linearly with the increasing temperature. The correlation between Φ b 0 ( I – V ) and Φ b ( C – V ) barrier heights have been explained by taking into account ideality factors n and the tunneling factor ( α χ 1 / 2 δ ) in the current transport mechanism. Also, the temperature dependence of energy distribution of interface state density ( N SS ) was determined from the forward I–V measurements by taking into account the bias dependence of the effective barrier height. The higher values of n and RS were attributed to the presence of a native insulator on Si surface and to high density of interface states localized at semiconductor–native oxide layer (Si/SiO2) interface.

Structural, ferroelectric and optical properties of PZT thin films

Abstract: We report on the structural, ferroelectric and optical properties of lead zirconate titanate (PZT) thin films (with a molar ratio of Zr:Ti::65:35) deposited by sol–gel technique on ITO-coated corning 7059 glass substrates. A seed layer of PbTiO3 (0.1mm) was coated by sol–gel on the substrates before depositing PZT. A metal/ferroelectric/metal (MFM) structure was used for electrical property measurements, formed by depositing gold electrodes on top of the film. The films were characterized for C–V, I–V, P–E and optical transmission. Relatively low remnant polarization (Pr ¼ 3:6mC=cm 2 ) was observed for the films. Value of optical band gap was found to be 3.4 eV. The results are discussed.

SnS films for photovoltaic applications: Physical investigations on sprayed SnxSy films

Abstract: Thin films of tin sulphide $(Sn_xS_y)$ have been deposited on antimony-doped tin oxide-coated glass substrates using spray pyrolysis. The depositions were made using 0.1 M equimolar solutions of tin chloride and thiourea at different substrate temperatures varied in the range $100-450^0C$. The physical properties of the films were studied using EDAX, XRD, SEM, AFM, van der Pauw method and spectrophotometry. The obtained results were discussed in the view of testing the suitability of SnS films as an absorber for the fabrication of photovoltaic devices. The films formed for temperatures of $300-375^0C$ were nearly stoichiometric (Sn/S = 1.03), single phase (SnS) and showed a strong (1 1 1) preferred orientation with an average grain size of 0.37 $\mu$m. These single-phase films exhibited p-type conductivity with an average electrical resistivity of $30\Omega$cm and a net carrier concentration of $2 \times 10^1^5 cm^-^3$. These layers had a direct energy band gap of $\sim1.32eV$ with an absorption coefficient of $\sim10^5 cm^-^2$ above the fundamental absorption edge. The films deposited at temperatures $ 375^0C$ deviated from stoichiometry and additional phases were found to be present.

Co–Zn ferrite nanoparticles for ferrofluid preparation: Study on magnetic properties

Abstract: Co–Zn substituted nanoferrites having stoichiometric composition Co1−xZnxFe2O4 with x ranging from 0.1 to 0.5 were prepared by chemical coprecipitation method. The precipitated particles were used for the preparation of ferrofluid. Ferrofluids having Co0.5Zn0.5Fe2O4 particles could be used for the energy conversion application utilizing the magnetically induced convection for thermal dissipation. The final estimated cation contents, agreed with the initial degree of substitution. The powder samples were characterized by XRD, TEM, VSM and Mossbauer studies. The precipitated particles showed single-phase fcc spinel structure for all compositions of zinc. The magnetic parameters such as Ms, Hc, Mr, Tc and particle size were found to decrease with the increase in zinc substitution. In the case of particles with higher zinc concentration, both ferrimagnetic nanoparticles and particles exhibiting superparamagnetic behavior were present. The fine particles were suitably dispersed in heptane using oleic acid as the surfactant. Volatile nature of the carrier chosen helped in altering the number concentration of the magnetic particles in a ferrofluid.

AC conductivity and dielectric properties of Co-doped TiO2

Abstract: The alternating current (AC) conductivity and dielectric properties of the Co-doped TiO 2 were investigated. The temperature dependence of AC conductivity and the parameter s , is reasonably well interpreted by the correlated barrier hopping (CBH) model. The activation energy ( E ), and the density of states at Fermi level, N ( E F ) were determined. The dielectric constant decreases with frequency at low frequencies and increases at high frequencies.

Improving the understanding of the melting behaviour of Mo, Ta, and W at extreme pressures

Abstract: We discuss the existing conflicts between experimentally measured and theoretically calculated melting curves of Mo, Ta, and W. By assuming that vacancy formation plays a fundamental role in the melting process, an explanation for the measured melting curves is provided. Furthermore, we show that the Lindemann law fits well all the measured melting curves of BCC transition metals if the Gruneisen parameter is written as a power series of the interatomic distance. For completeness, we examine possible reasons for current disagreements between shock-wave and DAC experiments. To solve them, we propose the existence of an extra high P–T phase for Mo, Ta, and W.

Optical constants of Zn1−xLixO films prepared by chemical bath deposition technique

Abstract: Zn1−xLixO films of different Li concentrations x = 0.0 – 0.5 were successfully deposited on glass substrates using chemical bath deposition (CBD) technique. Optical properties of the films were studied extensively in the wavelength range 190–800 nm from the measurements of the optical transmittance (T) and optical reflectance (R). The mechanism of the optical absorption follows the rule of direct allowed transition. Both the optical energy gap Eg and absorption coefficient ( α ) were found to be Li concentration-dependent. The refractive index values have been fitted to the single-oscillator models. The values obtained for the single-oscillator energy Eo are consistent with the optical gap results. The dependence of the optical constants of ZnO on the photon energy ( h ν ) at selected concentrations of Li were discussed. The exponential dependence of the absorption coefficient as a function of the incident photon energy suggests that the Urbach rule is obeyed and indicated the formation of a band tail. It was found that the optical energy gap decreases with increasing Li content; on the other hand the band tail width exhibits the opposite behaviour.

The role of V2O5 in the modification of structural, optical and electrical properties of vanadium barium borate glasses

Abstract: Vanadium barium borate glasses were prepared by a normal melt quench technique. The infrared spectra of these V2O5·BaO·B2O3 glasses were recorded over a continuous spectral range (400–4000 cm−1) in an attempt to study their structure systematically. The conversion from three- to four-fold coordinated boron took place. The fundamental absorption edge for all the glasses was analyzed in terms of the theory proposed by Davis and Mott. The position of the absorption edge and hence the value of the optical band gap was found to depend on the semiconducting glass composition. The absorption in these glasses is believed to be associated with indirect transitions. The origin of the Urbach energy is associated with phonon-assisted indirect transitions. The theoretical optical basicity has been calculated and is correlated with a change in the optical band gap. The variation in density and molar volume with composition has been investigated in terms of the structural modifications that take place in the glass matrix on addition of V2O5. The DC electrical conductivity as function of the V2O5:BaO and the V2O5:B2O3 ratio has been measured. The change in conductivity and activation energy with composition indicates that the conduction process varies from ionic to polaronic.

Structure, optical and electrical properties of ZnSe thin films

Abstract: ZnSe thin films were prepared by the vacuum evaporation technique. The influence of substrate temperature on compositional, structural, optical and electrical properties of polycrystalline ZnSe films was investigated using Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD), Scanning electron microscopy (SEM), optical transmission and DC conduction studies. The composition analysis shows the nearly stoichiometric nature of the deposited film. The X-ray diffractograms reveals the cubic structure of the film oriented along the (1 1 1) direction. The structural parameters such as particle size [28.41–50.24 nm], strain [1.381–0.785×10−3 lin−2 m−4] and dislocation density [1.285–0.412×1015 lin m−2] were evaluated. Optical transmittance measurements indicate the existence of direct allowed optical transition with a corresponding energy gap in the range of 2.72–2.60 eV. In the DC conduction studies the conduction mechanism is found to be exponential trap distribution and the results are discussed.

Complex AC impedance, transference number and vibrational spectroscopy studies of proton conducting PVAc–NH4SCN polymer electrolytes

Abstract: The polymer electrolytes composed of poly (vinyl acetate) (PVAc) with various stoichiometric ratios of ammonium thiocyanate (NH4SCN) salt have been prepared by solution casting method. The polymer–salt complex formation and the polymer–proton interactions have been analysed by FT-IR spectroscopy. The conductivity and dielectric measurements are carried out on these films as a function of frequency at various temperatures. The complex impedance spectroscopy results reveal that the high-frequency semicircle is due to the bulk effect of the material. The conductivity is found to increase in the order of 10−8–10−4 S cm−1 at 303 K with the increase in salt concentration. The ionic transference number of mobile ions has been estimated by Wagner's polarization method and the results reveal that the conducting species are predominantly due to ions. The transient ionic current (TIC) measurement technique has been used to detect the type of mobile species and to evaluate their mobilities. The dielectric spectra show the low-frequency dispersion, which is due to the space charge effects arising from the electrodes.

AC conductivity and frequency dependence of the dielectric properties for copper doped magnetite

Abstract: A series of polycrystalline spinel ferrites with composition CuxFe3−xO4+δ ( x = 0.2 , 0.4, 0.6, 0.8 and 1) were prepared by the standard ceramic method. The effect of Cu-ion substitution on the AC electrical conductivity and dielectric properties at different frequencies from 50 Hz up to 5 MHz was studied. The AC conductivity results were discussed in terms of the electron hopping model. The dispersion of the dielectric constant was discussed in the light of Koops’ phenomenological theory and the Rezlescu model. The dielectric loss tangent tan δ curves exhibit dielectric relaxation peaks which are attributed to the coincidence of the hopping frequency of the charge carriers with that of the external fields. The frequency exponent factor (S) was estimated and it was found to vary between 0.4 for x = 1 (CuFe2O4) and 0.83 for x = 0.2 .

Detection of humidity based on quartz crystal microbalance coated with ZnO nanostructure films

Abstract: The possibilities and properties of ZnO nanorod and nanowire films-coated quartz crystal microbalance (QCM) as a humidity sensor have been investigated. The morphology and crystal structures of ZnO nanorods and nanowires were characterized with X-ray diffraction (XRD) and scanning electron microscopy (SEM). It can be found that the frequency shift of the ZnO nanostructures-coated QCM linearly decreases with increasing relative humidity over the range of 5–97% RH. The experimental results demonstrated that ZnO nanostructures-coated QCM are usable as a humidity sensor and as an analytical device. It appears that the ZnO nanomaterial films can be used as efficient humidity sensors.

Synthesis of nanocrystalline spinel CoFe2O4 via a polymer-pyrolysis route

Abstract: Nanocrystalline CoFe2O4 spinel ferrites were synthesized via the pyrolysis of polyacrylate salt precursors prepared by in situ polymerization of metal salts and acrylic acid. The pyrolytic behaviors of the polymeric precursors were analyzed by use of simultaneous thermogravimetric and differential thermal analysis (TG-DTA). The structural characteristics of the calcined products were obtained by powder X-ray diffraction (XRD), infrared spectroscopy (IR) and transmission electron microscope (TEM). The results revealed that cobalt ferrites had nano-sized morphology and good crystallinity even if calcined at moderate temperature like 500 °C for 3 h. The average size of nanocrystalline cobalt ferrites ranged from 20 to 30 nm with a narrow size distribution, while the particle size increased with the increase of the calcination temperature. Magnetic properties were obtained at room temperature using a vibrating sample magnetometer. The samples exhibited hysteresis loop typical of magnetic behaviors, indicating that the presence of an ordered magnetic structure could exist in the mixed spinel system. The as-calcined cobalt ferrites at 500 °C exhibited the highest magnetization value of 77.4 emu/g at 10 kOe, while the highest remanence and coercivity of 35.6 emu/g and 1445 Oe, respectively, for those calcined at 700 °C were obtained.

Photoelectrochemical properties of undoped and Ti-doped WO3

Abstract: Undoped and Ti-doped WO 3 (up to 0.65 at% Ti) polycrystalline specimens were investigated. The flat band potential was determined based on the photocurrent experiments; the effect of Ti concentration on solar energy conversion efficiency was discussed. It was found that incorporation of titanium ions into WO 3 lattice leads to the decrease of the recombination process and shifts the flat band potential towards positive values. The incident photon-to-current efficiency (IPCE) assumes highest values for WO 3 +0.4 at% Ti. The maximum solar energy conversion efficiency has been observed at voltage bias 0.45–0.65 V.

Mössbauer study of chromium-substituted nickel ferrites

Abstract: The structural and magnetic properties of the Cr substituted NiCrxFe1−xO4 ( 0 ⩽ x ⩽ 14 ) spinel ferrites have been investigated by means of X-ray diffraction and Mossbauer spectroscopy techniques Their crystal structures are found to be pure cubic phases The Mossbauer spectra at 295 and 78 K of all samples showed two well-resolved magnetic patterns corresponding to the tetrahedral A-sites and octahedral B-sites The cation distributions driven from the area under resonance curve of each site revealed that the compounds are gradually transferred from perfect inverse spinel to partially normal spinel structure The behavior of the magnetic properties as a function of Cr3+ concentration has been explained on the basis of the driven cation distribution and it showed that the chrome-rich compositions can be explained in terms of the non-collinear spin model

Very long physical ageing in inorganic polymers exemplified by the GexSe1−x vitreous system

Abstract: The effects of very long physical ageing up to 13 yr on vitreous chalcogenide glasses have been investigated for GexSe1−x glasses, x = 4 , 8 and 12 at%, by means of calorimetric methods. We show that the rejuvenation of such very old materials can be performed without any problem. More importantly, we find that a long ageing leads to a phase separation process. This could explain why the configurational (or conformal entropy) does not apparently reach its equilibrium value as already observed by Gomez Ribelles et al. (Macromolecules 28 (1995) 5867) on other polymeric materials.

Ferromagnetic V-doped SnO2 thin films

Abstract: V-doped SnO2 thin films have been fabricated by pulsed laser deposition technique on various types of substrates. The distribution of V atoms over the whole thickness of the films is rather uniform. While V:SnO2 films on R-cut Al2O3 substrate are paramagnetic, V:SnO2 films on either LaAlO3 or SrTiO3 substrates are ferromagnetic above room temperature. V:SnO2 films on LaAlO3 substrates could reach a large magnetic moment of about 3 μ B /impurity atom.