S. Taha

Physical properties and phase transitions in sodium tartrate dihydrate

We report here on the electrical properties (resistance, dielectric constant and pyroelectric current behaviour), magnetic susceptibilityXM and differential thermal analysis (DTA) for the polycrystalline powdered samples of sodium tartrate dihydrate (STD), Na2C4H4O6·2H2O. These mentioned properties are used as a probe for the detection and the studying of the different transitions that have been found in this salt. It has been observed that the various results (electric, magnetic and thermal) strongly support each other and confirm the actual presence of the phase transformations in this compound.

The effect of temperature and phase state on the infrared spectrum of Ba(NO3)2

Abstract The temperature dependence of the infrared spectra of Ba(NO 3 ) 2 is reported in the whole ordered-disordered phase, from about 25°C up to 400°C. The study includes the temperature dependence of the infrared-active lattice modes. The frequency ν 1 denotes the totally symmetric NO stretching mode, ν 2 the bending mode, ν 3 the asymmetric stretching mode, ν 4 the asymmetric in-plane bending mode, 2ν 1 , 2ν 3 the first overtones, and (ν 1 + ν 4 ) and (ν 1 + ν 3 ) the combination modes. We have measured and interpreted the IR spectral band shapes, intensities and frequencies of the internal modes as a function of temperature. The results reveal that the transformation from the ordered state (phase II) to the disordered state (phase I) of Ba(NO 3 ) 2 is indicated by clear variations in the spectral parameters for (ν 1 + ν 4 ), 2ν 1 , (ν 1 + ν 3 ) and 2ν 3 . The temperature dependence of the relaxation parameters confirms the gradual transformation due to the growing of the disorder in the crystal structure which appears at about 300°C and is associated with a thermal energy of about 0.012 eV.

Temperature dependence of IR analysis of Sr(NO3)2

Abstract The infrared spectra of Sr(NO3)2 have been thoroughly investigated under normal pressures and at different temperatures during its phase transition II → I at ~320 ° C. The study includes measurements and interpretations of the IR spectral band shape, intensities and frequencies of the internal modes as functions of temperature. Special attention is paid to the bending mode v2, the asymmetric stretching mode v3, the first overtones (2v1, 2v3) and the combination mode (v1 + v4). The results reveal that the transformation from the ordered state (phase II) to the disordered state (phase I) of Sr(NO3)2 can be monitored by clear variations in the spectral parameters for v2, (2v1, 2v3), v3 and (v1 + v4) modes. According to the IR analysis reported here, the energy barrier of the reorientation of the NO3−1 ions is found to be 0.29 eV in the ordered phase II and 0.28 eV in the disordered phase I.

Electrical and calorimetric properties of sodium hydrogen tartrate monohydrate

This report considers some electrical parameters (dielectric constant and resistivity) and some calorimetric properties (thermal heat capacity, thermal conductivity and thermal diffusivity) as well as the DTA thermogram in the temperature range 30 < T < 140°C of sodium hydrogen tartrate monohydrate, NaHC4H4O6·H2O (SHTMHD). The accurate measurements of these parameters have revealed the presence of a structural phase transition at about 83°C, in addition to the phase transformation that results from the loss of water (dehydration). The data are correlated to the restricted rotation of the tartrate ions as well as to the hydrogen bonding scheme.

A study of the phase transformation in anthranilic acid single crystals

The electrical properties (resistance, dielectric constant and pyroelectric current) as a function of temperature in the range 30°C < T < 120°C of anthranilic acid single crystals (o-aminobenzoic add, C6H4NH2COOH), were accurately measured and correlated to the structural phase change that has been observed at 81 °C. The activation energies of the two phases were determined. The recorded DTA thermogram supports this phase change. In addition, the mean molar susceptibility χM of the powdered sample was accurately determined in the same temperature range; the result indicates that the structural phase change takes place at 81°C.

Measurement of thermophysical properhes of KNO3

Abstract The thermal properties, specific heat capacity Cp, thermal conductivity λ and thermal diffusivity a of KNO3 were measured in the temperature range 300–500 K. Specific heat capacity measurements indicate the presence of a structural phase transition from orthorhombic to rhombohedral at 408 K. The mechanism of heat transfer is due to phonons, whereas the effects due to electrons and bipolars are negligible in the measured temperature range.

Magneto-electrical studies of p-nitrophenol

Abstract The mean molar susceptibility ( X M ) as a function of temperature in the range 30 T p -nitrophenol, NO 2 C 6 H 4 OH, (PNP). The electrical parameters of PNP, such as d.c. resistance, dielectric constant and pyroelectric current behaviour, have been measured accurately in the same temperature range for single crystals and polycrystalline powder. All these properties are interpreted in the light of the arrangement of the molecules in the unit cell as well as the structural phase change observed at 63°C.

Electrical properties of carbon-doped KNO3 thin layers

A series of carbon-doped KNO3 thin layers was grown from the melt by a special technique. The changes of the dielectric constant, d.c. resistivity, energy of vacancy formation, pyroelectric current and ferroelectric hysteresis loops with carbon concentration were investigated. The present study aims to clarify the effect of doping carbon as a conducting material on the properties of the dielectric KNO3 thin layers by extensive use of experimental data from isomorphous sets of solid solutions of carbon and KNO3. These systems were studied in the vicinity of the order-disorder phase transition region for various values of carbon concentrationx betweenx=0 andx=0.08. For the carbon doped layers, large changes in the electrical properties were found depending on the doped carbon concentration. The dielectric constant and d.c. resistivity were reduced appreciably, while the pyroelectric current showed improved behaviour in the presence of carbon, as compared with pure KNO3 thin layers. Anomalous ferroelectric hysteresis loops were obtained in carbon-doped layers with a reduction of the ferroelectric loop temperature region by ca. 10° C at carbon concentrationx=0.07. The ferroelectric loops of the carbon-doped layers could be obtained <x = 0.07 with little loop deformation. The results showed that the carbon doping process delayed the phase transition point by 5 to 6° C forx=0.06. Also, the energy of vacancy formation showed its minimum value of 0.45 eV atx=0.03 to 0.04. The marked change in the detailed character of the different electric properties of the doped layers during phase transition in the presence of carbon was interesting.

Ferroelectricity associated with the metastable phase III of AgNO3

The dielectric constant, dc resistance, D-E ferroelectric hysteresis loop and dilatometric analysis of the three phases I, II, and III of AgNO3 single crystals has been studied over the temperature range 100–200° C. A ferroelectric behaviour of the metastable phase III was detected here for the first time similar to what happened in KNO3. The ferroelectric is attributed here to Ag+-ion vacancy formation in the unit cell of AgNO3. The energy activating the process of vacancy formation was found to beEv=2.6 eV. It was found that an ionic shift from one lattice point to another requires an amount of energy to overcome a potential barrierEm=0.1 eV. A model is suggested to explain such behaviour. Dilatometric analysis indicated that this metastable phase transition I→II is accompanied by an expansion of the unit cell.

Electrical and thermal properties of NH4BrxCl1-x, single crystal

Abstract The effect of partial replacement of Cl − ions by Br − ions in the mixed system of ammonium chloride and ammonium bromide during a set of thermal heating cycles is studied. The study includes electrical measurements (d.c. resistivity, dielectric constant and the spontaneous thermal current), differential thermal analysis and dilatometric thermal analysis in the temperature range from −60 to 30 °C. The results show a large change in those measured parameters which are sensitive to the thermal agitation process and to the presence of the bromine ions in the mixed crystal system close to the morphotropic boundary, x = 0.5. At this boundary, the temperature region of the ordered phase is extended by about 28° C and the transformation enthalpy was reduced to 41.3 cal mol −1 . According to the different experimental data of the system NH 4 Br x Cl 1- x , a phase diagram predicted that the order-disorder phase transition point changed widely according to the value of x , and reached −2° C when the mixed crystal system contained 50% Br − ions.