M. Barj

Domaines d'existence, distorsions structurales et modes de vibration des ions conducteurs dans les reseaux hotes de type nasicon

Abstract The NASICON structure domain of stability has been investigated in the ZrO 2 -P 2 O 5 -M 2 O-SiO 2 (M = Li, Na, K, Ag) system (Hf, Ti and partial Ta substitutions of Zr ions have also been studied) using X-Ray diffraction and vibrational spectroscopy. Metastability phenomena have been pointed out for some compositions. The different compounds have been synthetized in the form of powder using a sol-gel process and in the form of single crystals using various techniques. IR and Raman spectra are interpreted by using a neso-silicate (phosphate) structural description: a framework of XO 4 tetrahedra (X = P, Si) surrounded by octahedral cavities filled with M and M″ cations M″ = Sc, Ti, Zr, Hf). As different kinds of structural distortions are observed as a function of composition, distortions also occur against temperature. Local disorder and long range correlations are simultaneously observed : for some compositions e.g. Na 5 Zr(PO 4 ) 3 , a NASICON single crystal can be described as a “glass” with respect to the tetrahedra sublattice and as quasi a perfect crystal with respect to the cations one. Assignment of internal modes (tetrahedra modes), of M″ cations modes and of conducting cations (M) modes and of tetrahedra external modes is proposed on the basis of chemical substitution shift. Phase transitions are pointed out for the two kinds of sublattice (tetrahedra, cations). The influency of the “history” of samples is evidenced by DTA and vibrational spectroscopy. Correlations between local disorder, phase transitions and variations of conductivity activation energy are discussed.

Submicronic MgAl2O4 powder synthesis in supercritical ethanol

Submicronic MgAl2O4 powders have been synthesized by decomposition of the double alkoxide Mg[Al(O-secBu)4]2 in supercritical ethanol. The influence was studied of various experimental parameters (reaction time and temperature, reactional medium concentration and density) on the main characteristics of the obtained materials. Particularly, sample crystallinity and chemical composition, particle size distribution and mean diameter were investigated by X-ray diffraction, IR spectroscopy, laser scattering and electronic microscopy.

Raman and infrared spectra of some chromium Nasicon-type materials: Short-range disorder characterization

Abstract The infrared and Raman spectra of the Nasicon-type chromium systems are found to be very sensitive to the composition and to the nature of the metallic ions. The frequency shift of the infrared active modes involving the metallic ions is correlated to the modification of the crystal field around these ions. The static short-range disorder, particularly characterized from the Raman spectra, is essentially interpreted in terms of Na+ ion site occupation change vs the composition and the temperature.

Infrared studies of lithium intercalation in the FePS3 and NiPS3 layer-type compounds

Infrared spectra (700-30 cm-1) of several lithium intercalates (chemically prepared) LixMPS3, with M=Fe, Ni and 0<x<1.5, have been recorded and compared with those known for the corresponding host lattices. These lithium intercalates are mainly characterized by new absorption bands at 336 cm-1 and 310 cm-1 for the iron and nickel compounds, respectively. These bands assigned to lithium vibrations increase progressively with lithium content : it is concluded that Li+ ions are more likely to occupy the 2d and 4h “octahedral” sites in the gaps. In addition, the spectra of the nickel derivatives reveal some geometric distortion within the layers and a progressive strengthening of the Ni-S interactions. These results are correlated to the best energy yields obtained in NiPS3/lithium batteries.

Reactions in supercritical fluids, a new route for oxide ceramic powder elaboration, synthesis of the spinel MgAl2O4

Abstract We have performed the decomposition of the alkoxide Mg(Al(OR) 4 ) 2 , (R ≡ sec-butyl group), in supercritical fluid ethanol. The partly crystallized submicronic powder formed around 350°C and 15MPa leads to the spinel MgAl 2 O 4 after heating at 1200°C. The dried solid particle size ranges from about 0.04 to 0.2μm. The main characteristics of these powders have been compared with those of samples elaborated by sol-gel process. From IR spectroscopic study, a first attempt has been made in order to determine the nature of reactions taking place in the supercritical fluid medium.

Heat capacity and Na+ ion disorder in Nasicon-type solid electrolytes Na3M2P3O12 (M2= Fe2, Cr2, ZrMg) in the temperature range 10 to 300 K

Heat-capacity measurements on Nasicon-type solid electrolytes have been performed by adiabatic calorimetry. The lattice contribution is estimated from the usual model using spectroscopic data and the excess heat capacities are discussed in terms of low-temperature magnetic transitions in iron and chromium derivatives and of Na+ ion disorder in the three compounds at higher temperatures. The entropy excess at 300 K is related to the relative ionic conductivity and the occupancy factors of Na+-available sites.

Low temperature thermodynamic study on nasicon type solid electrolytes Na3Cr2P3O12 and Na3ZrMgP3O12

We have determined the low temperature heat capacity of NASICON type compounds Na3Cr2P3O12 and Na3ZrMgP3O12 by adiabatic calorimetry from 8 to 300 K. The lattice contributions are estimated from a combination of Debye and Einstein functions. The excess entropy due to Na+ ion disorder is found to be lower in the chromium compound in agreement with the relative ionic conductivities at 300 K. A disorder model in Na3M2P3O12 (M=Fe, Cr) is proposed that takes into account the occupancy factors of Na+ on different available sites.

Hydrothermal synthesis and recrystallization of compounds belonging to the NASICON family: Synthesis and crystallization of Na4Zr2Si3O12

Abstract Compounds of the Nasicon family have been prepared by a hydrothermal method in order to obtain single crystals adapted to polarized Raman studies. Among the hydrothermal synthesis of Na 4 Zr 2 Si 3 O 12 reported in the literature, we consider that the method proposed by Alyamovskaya and Chuklantsev is the most suitable to and we have adapted it to recrystallization in a temperature gradient. Reactant products may be oxides as well as silicates. The conditions for obtaining pure Na 4 Zr 2 Si 3 O 12 are the followings: (i) the ZrO 2 /SiO 2 molar ratio must be equal to 2/3, (ii) Na 2 O which must be in large excess, has to be introduced via NaOH solutions containing more than 30 % of sodium hydroxide in weight, (iii) for pressures (P) higher than 400 bar the temperatures (T°) must be higher than 330°C. Single crystals of (0.5 mm) have been obtained within ten days, for T=450°C and P higher than 400 bar.

Raman spectra of lamellar CdPS3 intercalated with alkali ions

Abstract Raman (10–700 cm −1 ) spectra of lamellar CdPS 3 intercalated with alkali (Na, K, Rb, Cs) cations corresponding to general formula Cd 0.75 PS 3 A 0.5 have been investigated in the 500-80 K temperature range for different rates of hydration. Some Raman bands of the host lattice are sensitive towards intercalation and hydration. Potential barriers associated with the mobility of the intercalated species have been estimated from the low frequency Raman bands. Different structural modifications have been observed, they are likely due to the population of Cd ++ vacant sites by alkaline cations.

POTENTIAL-ENERGY CALCULATION AND CONDUCTIVITY MECHANISM IN NA3CR2P3O12 AND NA1+XZR2-XCRXP3O12

Potential energy calculations using Coulomb, short-range repulsive and polarization terms have been performed using Wangs method, combined with the Ewalds procedure in order to analyze different individual and collective in-phase motions of Na + ions in NASICON homologs. It is concluded that direct Na(2)−Na(2) jumps take place at 300°C on a time scale as long that they do not contribute to ionic conductivity. The Na(1)−Na(2) jumps occur on a much shorter time scale. At 300°C a frequency jump of 3×10 +8 s −1 is derived. The corresponding potential barrier V o ≈9 Kcal mol −1 is somewhat higher than the activation energy obtained from conductivity measurements. Correlated motions of cation pairs (caterpillar-like mechanism) do not lower the barrier height. Finally, tilting motions of PO 4 tetrahedra, able to open specific bottlenecks, are investigated.