Mohamed El Omari

Electronic and ionic conductivity of glasses inside the Li2O–MoO3–P2O5 system

Abstract A wide range of coloured electronic or mixed electronic–ionic glasses has been shown in the alkali oxide poor-region of the Li 2 O–MoO 3 –P 2 O 5 system, next to the colourless ionic glasses located in the alkali oxide rich-region. These coloured glasses have been studied by electron paramagnetic resonance (EPR) spectroscopy. The Mo 5+ and Mo 6+ cation percentages have been evaluated for several series of glasses corresponding to either a fixed Li 2 O rate or a constant Mo/P ratio. The electronic and the ionic contributions to the total conductivity of these glasses have been determined. The existence of a polaronic hopping conduction mechanism between Mo 5+ and Mo 6+ cations is suggested in the purely electronic conductor glasses. An ion–polaron soft coupling is proposed in the ionic–electronic conductor glasses.

Ionic conductivity properties and 19F NMR investigation in Ln1−yCdyF3−y (Ln=Ce, Nd) solid solutions with tysonite-type structure: Part I: Ionic conductivity properties

Abstract New fast F − ion conductors with tysonite-type structure have been obtained inside anion-deficient non-stoichiometric solid solutions of Ln 1− y Cd y F 3− y formulation (Ln=Ce, Nd). An exchange mechanism between fluoride sublattices has been identified for the ( y ≠0) compositions. A second exchange mechanism at lower temperature has been shown for CeF 3 and NdF 3 . The presence of both electrical anomalies in the temperature dependence of LnF 3 (Ln=Ce, Nd) is confirmed by C p variation and X-ray diffraction analysis as a function of temperature.

Short-range order and diffusion processes in the Na1-xBixF1+2x anion-excess solid solution

Abstract The F − diffusion processes within the anion-excess solid solution Na 1− x Bi x F 1+2 x with fluorite-type structure have been studied by 19 F-NMR investigation. Two types of fluoride ions, F n and F′, are distinguished in the 19 F-NMR spectra of rigid lattice at low temperature ( T =175 K) and the distribution of fluoride ions among the two sites is determined by deconvolution of the experimental spectra. The short-range order in these materials is analogous to the clustering process observed in the K 1− x Bi x F 1+2 x solid solution. It corresponds to an order between cubic entities and cubooctaedral clusters. Two diffusion mechanisms are identified on both sides of a T r temperature. The first one at T T r corresponds to an F − ions exchange inside cubic or cubooctaedral clusters independently, and the second at T > T r to an F − exchange between cubic and cubooctaedral clusters.

Ionic conductivity properties and 19F NMR investigation of Ln1-yCdyF3-y (Ln = Ce, Nd) solid solutions with tysonite-type structure : Part II : 19F NMR investigation

Abstract A 19 F NMR investigation of anion deficient solid solutions of Ln 1− y Cd y F 3− y (Ln=Ce, Nd) with the tysonite-type structure has been carried out as a function of temperature. Exchange processes, at first between the F 1 and F 2 sublattices, then between the F 1 and F 3 ones, have been identified at increasing temperature between three initial fluoride sublattices. The F 1 sublattice of the tysonite-structure appears at the same time as the first sublattice that is mobile at low temperatures and the sublattice where guest-induced vacancies are preferentially located. Correlations have been established between the F − ion diffusion and the transport properties (Part I) determined by impedance spectroscopy.

Short-range order and F− ion diffusion inside the Pb1 − xAlxF2 + x solid solution Part II: 27Al NMR investigation and proposal of clustering process

Abstract A 27 Al nuclear magnetic resonance (NMR) investigation at room temperature of the Pb 1 − x Al x F 2 + x (0 ≤ x ≤ 0.12) solid solution and of the ordered Pb 9 Al 2 F 24 phase is carried out. Two different types of aluminium ions are identified and their ratio is determined as a function of x . A clustering process based on the formation of column clusters, more and more extended when x increases, is proposed on the basis of results issued from the 19 F (Part I) and 27 Al NMR investigations.

Short-range order and F− ion diffusion inside the Pb1 − xAlxF2 + x solid solution Part I: 19F-NMR investigation

Abstract A 19 F-NMR investigation of the Pb 1 − x Al x F 2 + x (0 ≤ x ≤ 0.12) solid solution and of the ordered Pb 9 Al 2 F 24 phase is undertaken as a function of temperature. Several F − ion groups become progressively mobile at the NMR time scale at increasing temperature, the first, (F i ) pb and (F n ) pb , are located in the surroundings of Pb 2+ cations, the second ones, (F rd ) AI and (F ra ) Al , in that of Al 3+ cations. Four types of fluoride ions are distinguished in the 19 F-NMR spectra of rigid lattice at low temperature ( T = 175 K) and the distribution of fluoride ions among the four sites is determined by deconvolution of the spectra registered. The sites occupied by the (F n ) pb anions are identified with the normal anionic sites of the fluorite network, the others with different interstitial sites.

Characterization of four phase transitions in Pb5Al3F19, including a new transition at 670 K, by impedance and NMR spectroscopy

The ac (alternating current) conductivity of Pb5Al3F19 between 293 and 743 K has been analyzed within a combined complex impedance, modulus, and permittivity formalism. The antiferroelectric phase IV to ferroelastic phase III, and the phase III to paraelastic phase II, transitions have been characterized; the latter is shown to be accompanied by higher F− ion mobility above T=368±5 K. A new dielectric anomaly observed isochronously in e′(f,T) at 670 K corresponds to an expected, but hitherto unobserved, transition from phase II to the paraelectric prototype phase I of Pb5Al3F19. A 19F NMR investigation between 125 and 430 K of F− ion diffusion properties in Pb5Al3F19 shows that half the anions have become mobile at the ferroelectric phase V to phase IV transition temperature and that almost all the anions are mobile at the phase IV to phase III transition temperature. These motions are shown to be short range, and no correlation is found between phase formation and F− ion diffusion properties in Pb5Al3F19...

Effect of oxygen on short-range order in the Ba1−xBixF2+x solid solution

Abstract Various compositions within the range of the fluorite-type solid solution in the BaF 2 –BiF 3 –BiO l.5 system are investigated by impedance spectroscopy. This study is carried out along two lines of compositions: δ 1 , corresponding to the Ba l−2 z /3 Bi 2 z /3 F 2+4 z /3 O z solid solution, where z is the oxygen rate; and δ 2 , corresponding to the Ba l− x Bi x F 2+ x −0.30 O 0.15 solid solution, where x is the bismuth rate. An approach to the nature of the fluorine–oxygen order is proposed which derives from the clustering process in the Ba l− x Bi x F 2+ x fluoride solid solution.

19F NMR investigation of the Pb1−xMgxF2 solid solution

Abstract An 19 F NMR investigation of the Pb 1− x Mg x F 2 solid solution is undertaken as a function of temperature. Several F − ion sublattices are identified, they become progressively mobile on the NMR time scale at increasing temperature. Clusters are proposed from the composition dependence of the anionic distribution determined by deconvolution of the low temperature 19 F NMR spectra. Temperature dependence of the jump frequency shows the existence of several F − ion motions corresponding to exchange motions inside and between fluoride sublattices.

Neutron diffraction study of the Pb1−xAlxF2+x solid solution

Abstract Powder neutron diffraction studies have been carried out on the fluorite-type Pb 1− x Al x F 2+ x (0≤ x ≤0.12) solid solution. Three interstitial sites, F′, F′′ and F′′′, were observed, in addition to the normal F n sites of the fluorite structure at (1/4,1/4,1/4). Four types of fluoride sites, F n , F′, F′′ and F′′′, were identified with the four sublattices (F n ) Pb , (F ra ) Al , (F i ) Pb and (F rd ) Al suggested from 19 F- and 27 Al-NMR investigations. The clustering process proposed in Pb 1− x Al x F 2+ x on the basis of NMR results is confirmed and the cluster characteristics are determined.