Mohga F. Mostafa

Phase transition and electric properties of long chain Cd(II) layered perovskites

The new layered perovskite [(CH2)12(NH3)2]CdCl4 material exhibits a second order phase transition at T 1p = 376.8 K (ΔS 1 ≃ 12.5 J K−1 mol−1) of the order–disorder type. The dielectric permittivity has been studied in the temperature range 300–400 K and frequency range 0.1–100 kHz, on heating and cooling. The ac conductivity is thermally activated with different activation energies in the range of ionic conduction. The variation of ac conductivity with frequency follows the power law: Proton hopping and localised reorientational motion, ionic hopping of chlorine vacancies and large polarons are possible conduction mechanisms in the different temperature ranges. Comparison with other isomorphous transition metal perovskites is discussed.

Structural Phase Transition and the Dielectric Permittivity of the Model Lipid Bilayer [(CH2)12(NH3)2]CuCl4

Structural phase transitions in the lipid-like bilayer material [(CH2)12(NH3)2]CuCl4 have been observed using differential thermal scanning. The compound shows an irreversible thermochromic transition at ˜ 465 K and three reversible transitions at T 1 = 433 ± 4 K and T 2 = 411 ± 2 K and T 3 = 358 K. The transition at 350 K is ascribed to chain melting. The other two correspond to crystalline phase transformation. Phase (IV) T3 = 358 ± 2K Phase (III) T2 = 411 ± 2K Phase (II) T1 = 433 ± 4K Phase (I) Dielectric permittivity is studied as a function of temperature in the range 300-440 K and frequency, range (60 Hz-100 kHz). It confirms the observed transitions. The dielectric permittivity reflects rotational and conformational transitions for the compound. The variation of the real part of the conductivity with temperature is thermally activated in the temperature range above 350 K, with frequency-dependent activation energy, the values of activation energy lie in the range of ionic hopping. The dependence of the conductivity on frequency follows the universal power law σ = σ0 + A(T) ω s ( T ) with 0

Preparation and characterization of a new series of perovskite-like structures showing evidence of structural transitions: (Methyltriphenylphosphonium)2 BX4, B = Mn, Co, Cu, and Hg, and × = CI/I

Abstract A new series of perovskite-like compounds with the general chemical formula A2BX4 where A = methyltriphenylphosphonium, B = Mn, Co, Cu, and Hg, and × = CI/I is synthesised. IR spectra and magnetic susceptibility measurements were used to confirm the formation of the desired compounds. Differential thermal analysis reveals the presence of three structural phase transitions for the Mn compound at T= 79°, 89.09°C and 98.18°C. The latter two are first-order transitions. The Cu compound shows two transitions at 105°C and 138.6°C, the latter being of first order. The Hg compound shows a change in slope at 80°C. The dielectric permittivity of the Mn and Cu compounds as function of temperature and at different frequencies is presented. The transitions are related to the motion of the CuCI4 ∼2 and [CH3(C6H5)3P] +.

The frequency dependence of the conductivity and dielectric relaxation of [(CH2)3(NH3)2]Cu(II)Cl4

The structural, dielectric and conductive properties of [(CH2)3(NH3)2]Cu(II)Cl4 have been studied. The material shows an order-disorder transition at T1 = (333±2) K and a ferroelectric phase transitions at T2 = (434±3) K. Results of differential thermal analysis, infrared spectroscopy, X-ray diffraction, AC conductivity and permittivity measurements are reported and discussed. The conductivity results are interpreted in terms of barrier hopping at low and intermediate temperatures, and band-type conduction at high temperatures. - PACS: 81.05.-t, 77.22.-d

Ferroelectricity in a New Luminescent Organic-Inorganic Hybrid: [CH3(C6H5)3P]2MnBr4

The material was synthesized, investigated by DSC, X-ray powder diffraction and impedance measurement. Ferroelectric activity was found below 310 K. The material crystallizes in a monoclinic system, space group P21 at T= 298 K, a =9.623 Å, b =12.565 Å, c =16.45 Å and β = 105.6°. Spontaneous polarization was estimated by Sawyer-Tower Bridge and measured by pyrocharge shows ferroelectricity. Permittivity shows a sharp frequency dependent peak with Curie temperature at 310 K.

Preparation, Characterization and Crystal Structure of the Room Temperature Phase of [(CH3)(C6H5)3P]2[ZnBr4]: A Member of the A2BX4 Family

The compound bis(methyltriphenylphosphonium) tetrabromozincate(II), [C19H18P]2[ZnBr4], Mr = 939.640, has a monoclinic unit cell, space group P21. The lattice parameters are a = 9.7693(4) Å , b = 12.5508(4) Å , c = 16.5372(6) Å , α = 90.00°, β = 105.2670(11)°, γ = 90.00°, V = 1956.11(11) Å3, Z = 2, Dx = 1.595 mgm−3 at T = 298 K. The structure consists of one distorted [ZnBr4]2− tetrahedron and two [(CH3)(C6H5)3P]+ cations. Differential scanning calorimetry indicates a continuous second-order transition at (276±2) K that may be classified as a commensurate to incommensurate transformation. A first-order transition to a higher symmetry is associated with a four-fold rotation of the [ZnBr4]2− ion and a change of entropy ΔS = 22.92 J/(K· mol) at T = (362±3) K. Dilatometric measurements showed a decrease of the lattice parameters in the temperature range 230 - 260 K, confirmed the transition at (276±2) K, and indicated the presence of a third transition at 282 K.

Magnetic and Electric Studies of a New Cu(II) Perovskite-like Material

Thermal analysis of a lipid-like bilayer of [(CH2)9 (NH3)2]CuCl4 indicates one minor transition at 303 K and the following five major ones (with entropies in J/K-mole): The initial magnetic susceptibility at 320 Hz and a magnetic field of 160 A/m in the temperature range 75 - 290 K revealed a Curie-Weiss behavior of phase (V) and possible ferromagnetic ordering of phases (VI) and (IV) at low temperatures. The variation of the dielectric permittivity with temperature at 60.0 Hz - 100 kHz indicates large changes of the dipole moment at the transition temperatures. The conductivity is thermally activated and frequency dependent, following the universal power law: σ (ω) =σdc +A(T)ωs(T). Values of s being phase dependent. For phase (I), T > 383 K, band type conduction prevails. Hopping conduction is found in phases(II) and (III). Comparison with other Cuand Cd-containing materials is made. -PACS: 76, 77

Conduction mechanisms in the magnetic insulator [(CH3)2NH2]2CoCl4

Magnetic susceptibility in the temperature range 170–370 K and ac permittivity and conductivity in the frequency range 5 Hz–1 kHz over the same temperature range for the compound [(CH3)2NH2]2CoCl4 were carried out. The results indicate three phase transitions at Tc 1 = 245 ± 3 K, Tc 2 = 290 ± 5 K and Tc 3 = 320 ± 6 K. At T ≥ 290 K the paramagnetic compound conducts via band type mechanism with band gap = 2.42 eV. At lower temperatures correlated barrier hopping mechanism prevails, with an activation energy = 0.52 ± 0.04 eV, an effective barrier height = 0.73 eV and a hopping distance = 36.5 Å.

Dielectric Permittivity and AC Conductivity Investigation for the New Model Lipid Bilayer Material: (CH2)10(NH3)2CdCl4

Abstract Differential thermal scanning of the new lipid-like bilayer material (CH2)10(NH3)2CdCl4 showed two structural phase transitions, with onset temperatures at T2 = (359 ± 2) K and T1 = (415± 1) K. Permittivity measurements were performed between room temperature and 450 K at 60-100 kHz. A step-like rise in permittivity at T2, associated with an order-disorder transition, is attributed to chain melting. Two anomalies at (413 ± 1) K and (430 ± 3) K, showing thermal hysteresis of -8 and 10 K, respectively, in dicate first order transitions which are associated with crystalline phase change. The AC conductivity follows an Arrhenius-type relation with the activation energy ΔE varying with the frequency / according to the relation ΔE = ΔE0 [1 -exp (f0/f)] where ΔE0,f0 and a are 0.95 eV, 52 Hz and 0.11, respectively. The frequency dependent conductivity follows the power law σ= σdc +Bωx with 0.3 <s< 1.5 for hopping conduction of hydrogen and/or chloride ions in the high temperature range, and localized hopping and/or orientational motion predominating temperatures lower than 413 K. Variations of B and s with temperature are discussed. PACS No. 76, 77

Electric and magnetic study of the new biodegradable complexes bis-(cyclopentanonyl-2,5-DI(p-aminobenzylidine)MCl4, M = Mn+2, Co+2 and Cu+2

Abstract The permittivity and AC conductivity in the temperature range 290 K- 460 K, in the frequency range 5.0 Hz-4.0 kHz, magnetic susceptibility over the temperature range 80K-460K, at different magnetic fields as well as x-ray diffraction and IR spectra have been measured for the biodegradable materials [A]2MCl4, in which A is cyclopentan-onyl-2,5-di(p-aminobenzylidine) and M = Mn+2, Co +2 and Cu +2. X-ray analysis indicated that the materials crystallize in orthorhombic unit cell of dimensions a = 15.41(1) Å, b= 16.77(1) Å, c= 19.38(1) Å, a= 15.75(1) Å, b= 16.95(1) Å, c= 19.56(1) Å and a = 15.89(l)Å, b = 17.53(l)Å, c = 20.01(2)Å for the Mn +2, Co+2 and Cu +2 materials, respectively. Electric and magnetic results indicate the following structural phase transitions: The conductivity follows Arrhenius relation. Near room temperature the activation energies are small and have nearly the same value (ΔEa∼170meV) for the three materials. At the high temperature phase, ΔE1 is frequency dependent and follows the relationship ΔE = ΔE0 [1 - exp - (f0/f)]α with Eo varies from 1.2eV (Mn) to 2.88 eV (Cu). The frequency dependence of conductivity follows the universal power law [sgrave]∼ B(T)ωs(T) Variation of s(T) and In B(T) with temperature shows discontinuities at the phase transitions. At high temperatures, phase (I), band type conduction prevails