M.F. Mostafa

Permittivity and ac conductivity study of the layered perovskite [(CH3)(C6H5)3P]2MnCl4 showing evidence of phase transition

Abstract The permittivity and ac conductivity of the compound bis(methyltriphenylphosphonium)2tetrachloro-manganate(II) have been measured in the temperature range 300–400 K and in the frequency range 0.1–20 kHz . Differential thermal analysis reveals structural phase transition in the temperature range 365 K 1 kHz) and the conductivity results. The conductivity follows the Arrhenius relation with different activation energies and conduction mechanisms: region (I) → 345±3 K region (II) → 365±3 K region (III) → 372±2 K region (IV) with activation energies ΔE1∼10 meV, ΔE2∼1–0.2 eV and ΔE4∼2–1 eV for temperature regions (I), (II) and (IV), respectively. In region (I) extrinsic type conduction prevails. Analysis of the experimental data based on the jump relaxation model, showed that translation motion of the proton and reorientation hopping between equivalent sites of the metal chloride anion and the phenyl groups are responsible for the observed ac conductivity in the temperature region (II). In region (IV) overlap large polarons predominates .

Field dependence of the susceptibility maximum for two-dimensional antiferromagnet

Abstract The magnetic susceptibility measurements on layered type structure ( CH 3 NH 3 ) 2 FeCl 2 Br 2 revealed a transition temperature T N ( H = 0) ≈ 100 K . The transition temperature of ( CH 3 NH 3 ) 2 FeCl 4 was previously found to be T N ( H = 0) ≈ 95 K . The effect of magnetic field on the transition temperature and peak intensity for both compounds has been investigated.

Magnetic susceptibility of (CH3NH3)2FeCl3Br: An example of a canted spin system

Abstract Magnetic susceptibility of (CH3NH3)2FeCl3Br is measured in the temperature range 80–200°K. A sharp peak characteristic of a spin canted system is found at TN(H=0)=98° K. Magnetic field dependence of the susceptibility and the effect of the halide ion size on the canted spin is discussed.

Mößbauer study of magnetic and structural phase transitions of a layered system

Abstract Mosauer measurements of hexane-diammonium-tetrachloro-ferrate over the temperature range 78–320 K for powdered and single-crystal absorbers show a transition to the antiferromagnetically ordered state at (105.0 ± 0.2) K. The center shift indicates an electronic ground state of 3d 6 4s 0.2 . The quadrupole splitting implies that Fe(II) exists in a distorted octahedral field and that the ground state is a doublet, with negative electric field gradient. The recoilless fraction, the center shift and the quadrupole splitting reveal the existence of a structural phase change at (235 ± 5) K. The Debye temperature of the lower temperature phase is (166 ± 5) K aand (155 ± 3) K for the higher. Transition to the ordered state takes place between 105 and 110 K where an increase of the spin correlation time is found. A three-dimensional transition associated with a weak ferromagnetic interaction as a result of canting of the spins is observed at about 102 K. At 77 K the effective magnetic field strength was found to be (225 ± 3) kOe and to make an angle of 80° ± 3° with the principle axis of the electric field gradient.

Magnetic history of hexanediammonium tetrachloroferrate(II)

Abstract The magnetic susceptibility of the two-dimensional antiferromagnet (CH 2 ) 6 (NH 3 ) 2 FeCl 4 is reported. The compound has unusual dependence on its magnetic history below its Neel temperature T N . Though the magnetic ordering is associated with a structural distortion, this distortion does not seem to explain the magnetic behaviour of the compound. The results have been interpreted in terms of mixed magnetic phases which are likely to exist when the sample is cooled in a magnetic field.

Mossbauer investigation of some layered Fe(II)Cl compounds

Mossbauer effect studies (ME) for members of the alkylene-diammonium series, (CH2)n(NH3)2Fe(II)Cl4, where n=3, 4, 5, and 6 are presented. At 78 K the ME spectra reveal similar general features showing an 8 line well split hyperfine spectra; the effective magnetic field is in the range 18.5–20 T. Fitting the magnetization curves to the theoretical models showed that all compounds are best fitted to a 2d Ising system with β values in the range of 0.124–0.151, D=1.05–1.12, and TN=102.2–105.2 K for n=3 to n=6, respectively. Structural phase transitions have been found for the n=3 and 6 compounds at Ts=230 and 242 K, respectively. Electric permittivity of two members of the series is presented and related to the ME results.

Magnetic studies of the layered compounds (CH2)n(NH3)2FeCl4, n = 2, 3 and 6

Abstract The magnetic susceptibility as a function of temperature and field strength of the series of compounds (CH2)n(NH3)2FeCl4, n = 2, 3 and 6 have been investigated. The three compounds undergo magnetic transitions at TN(H = 0) = 105.0±1, 94.5+0.5 and 98.0+0.5 K. From the susceptibility and DTA measurements no evidence for structural phase changes is found in the investigated temperature range. The results are interpreted in terms of a canted antiferromagnetic arrangement. With increasing n the interlayer spacing increases and the magnetic structure becomes more and more two dimensional.

Electric and magnetic investigations of the new layered compound bis(DL-alaninium) tetrachlorocuperate

Abstract Powder X-ray diffraction at room temperature, magnetic and electric properties in the temperature range between liquid nitrogen and room temperature have been investigated for the compound (DL-alaninium)2CuCl4. At room temperature it has a tetragonal unit cell with lattice constants a = b = 0.556 nm and c = 1.665 nm. The compound is found to be thermochromic, changing color from a low temperature green phase to a high temperature yellow phase at Ttr = 285 K. The dielectric constant reflects this order-disorder phase transition. The conductivity shows three thermally activated regions with different activation energies. Possible conduction mechanisms are discussed.

Magnetic susceptibility studies of (CH2)3(NH3)2FeCl2Br2 and (CH2)6(NH3)2FeCl2Br2

Abstract The magnetic susceptibility of the layered compounds (CH 2 ) 3 (NH 3 ) 2 FeCl 2 Br 2 and (CH 2 ) 6 (NH 3 ) 2 FeCl 2 Br 2 has been measured in the range 80 T T T

High-temperature susceptibility of copper compounds containing CuCl2-4 or Cu2Cl2-6 ions

We have measured the magnetic susceptibility of [(C2H5)2NH2]2 CuCl4, (iso-C3H7NH3)2CuCl4, and (iso-C3H7NH3)CuCl3 in the temperature range 80 < T < 400 K. These compounds show thermochrimic first order phase transitions at 315.5, 329.5 and 324.5 K, respectively, which is related to the change in geometry of the chlorine coordination of Cu2+. From the susceptibility data, estimates of the super-exchange interaction can be inferred. For [(C2H5)2NH2]2CuCl4 and (iso-C3H7NH3)2CuCl4 our results evidenced the presence of ferromagnetic interactions between the Cu ions within the network formed by CuCl2-4 ions. For the (iso-C3H7NH3)CuCl3 the interaction between two Cu2+ ions forming the dimer can be estimated. The interaction between the dimers is found to be much weaker.