Jean Senegas

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.

7Li NMR in electrochemically intercalated γLixV2O5Bronzes (0.95 ⩽x ⩽1.9)

Abstract MAS spectra (recorded at 5 kHz) of γ ue5f8 Li x V 2 O 5 (0.95 ⩽5 x ⩽ 1.9) allow the observation of an important narrowing of the NMR linewidth. The interpretation of the results, in agreement with previous X-ray and neutron diffraction studies of Li 0.95 V 2 O 5 , shows that the lithium atoms occupy various sites in the γ-phase network for x 1. Static Q-echo spectra reveal broader linewidths which may be attributed to strong magnetic interactions of the lithium nuclei with localized V 4+ magnetic moments. The quadrupolar effect of Li 0.95 V 2 O 5 calculated on the base of an ionic charge model ( v m calc = 17 kHz ) is in good agreement with the experimental value v m exp = 18 kHz .

NMR study of the cation distribution in the systems Li1+5xTa1-xO3 and Li1+xTa1-xTixO3

A /sup 7/Li NMR study of members of the solid-solution systems Li /sub 1+5x/ Ta /sub 1-x/ O/sub 3/ and Li /sub 1+x/ Ta /sub 1-x/ Ti /sub x/ O/sub 3/ has indicated that the excess Li/sup +/ ions occupy interstitial tetrahedral sites in the former. In the latter system, the Li/sup +/ ions appear to occupy interstitial tetrahedral sites for small values of x, but mostly octahedral sites for x > 0.1. Defect-cluster models are proposed that rationalize these findings as well as the evolution with x of the ferroelectric Curie temperature.

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.

Crystal structure and NMR studies of a cubic perovskite: The fluoride NaBaLiNiF6

The crystal structure of NaBaLiNiF6 has been determined from X-ray single crystal data. The crystal symmetry is cubic, space group Fm3m, with an unit-cell constant of a = 7.992(1)A. Barium and sodium atoms occupy the A-sites (CN = 12) with a small disordering of 10% between the two independent positions. The Li and Ni atoms are statistically disordered in the B-sites. These crystallographic results have been confirmed by magnetic susceptibility measurements and by NMR experiments on 19F, 23Na and 7Li nuclei.

Phase transitions and electrical properties of CsH(SO4)0.76(SeO4)0.24 mixed crystals

The study by X-ray diffraction, calorimetry, vibrational and impedance spectroscopy of CsH(SO4)0.76(SeO4)0.24 new solid solution is presented. Crystals of this composition undergo two phase transitions at T = 333 and 408 K. The last one at 408 K is a superionic-protonic transition (SPT) related to a rapid [HS(Se)O4−] reorientation and fast H+ diffusion. A sudden jump in the conductivity plot confirms the presence of this transition. Above 408 K, this high temperature phase is characterized by high electrical conductivity (7 × 10t-3 Ω− cm−1) and low activation energy (Ea < 0.3 eV).

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.

NMR study of the proton mobility in RbH(SO4)0.81(SeO4)0.19 protonic conductor

Abstract Crystals of RbH(SO4)0.81(SeO4)0.19 formulation are studied by 1H NMR spectroscopy. The 1H line-shape, the line width and the T1 and T2 relaxation times are determined as a function of temperature. The activation energies deduced from the temperature dependence of line width and relaxation times are compared with the activation energy issued from conductivity measurements by impedance spectroscopy. The results obtained are discussed and supported by the help of the Ngai model.

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±5u2009K. 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...