G. Lucazeau

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.

Conduction mechanism in Na+ β-Al2O3

Abstract Potential calculations using Coulomb, short-range repulsive and polarization terms have been performed using Wangs method combined with the Busing WMIN version in order to analyze different collective in-phase modes of conductivity in Na+ β-Al2O3. It is concluded that among the various modes, the interstitialcy like caterpillar mechanism is the most favourable for conductivity and that the formation of pairs of Na+ is favoured by the polarization energy. A critical review of some conductivity mechanisms is made. Among the three categories of Na+ ions which are reported, at 300 K, two have a residence time shorter than 5 × 10−10 s. These results are compared with Raman and quasi elastic neutron results.

Infrared, Raman and neutron scattering studies of β- and β″-alumina: a static and dynamical structure analysis

Abstract Infrared and Raman data on β-aluminates with different compositions ( x )=0.0, 0.25, 0.66) containing Na + , Ag + , K + , Tl + , H 3 O + , NH + 4 ions are reviewed. β-gallates and β″-aluminates are also considered. Assignments of the cation modes are discussed in terms of oscillations of single cations and of normal modes of clusters (pairs and triangles formed by the conducting cations). The assignment of the spinel block spectra is proposed in the light of recent normal mode calculation and comparison of different experimental results in the literature. Potential barriers and activation energies obtained from these techniques are compared with calculation results and with data obtained from different sources. Inelastic and recent quasi elastic neutron scattering results are reviewed and compared with those deduced from the vibrational study.

Vibrational and normal-mode analysis of stoichiometric β-Al2O3

Abstract A complete normal-coordinate analysis of stoichiometric Na + , K + , Ag + , Tl + β-aluminates and of Na + β-gallates is presented. The overall agreement between calculated and experimental frequencies is excellent. Polarized Raman spectra at 300 and 16 K of stoichiometric β-aluminates, on which our assignments are based, are reported. A complete description of the normal modes is given in terms of potential energy distribution and of cartesian atomic displacements. It is shown that the in-plane and out-of-plane motions of the conducting Na + cations are only coupled to a small extent with the spinel blocks. This is less true for heavier conducting cations. Seven spinel block modes have been found to be slightly cation dependent. A general observation is that on one hand, the largest atomic displacements are along the structural axes, on the other hand the high-frequency spectrum is dominated by the normal modes of AlO 4 and Al 2 O 7 entities, indicating some covalent character of Al-O bonds. Finally some specific modes are discussed, in particular those related to spinel-block motions as a whole, and those expected to be sensitive towards the compensating charges.

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.

Study of Na+ motions in βAl2O3 by quasi-elastic neutron scattering

Abstract The commom non stoichiometric Al 11 O 17+x/2 Na 1+x (x = 0.25) Alumina in the powder state has been studied by quasi elastic neutron scattering at -196, 44, 200, 300 and 400°C using incident wave lengths of 4.1 and 5.1 A, the time of flight spectra have been corrected in S(Q,W) quasi elastic spectra and inelastic frequency distribution P(Q,W)(Q→O) in the energy range 1–80 meV (8–650 cm −1 ). Quasi elastic contribution is clearly evidenced above room temperature; it has been fitted with different jump models based on structural data. It is shown that even at 400°C, only 15 % Na cations are mobile on the time scale of IN6 (τ −10 s). Two kinds of motions have been characterized : localized jumps (τ ∼ 10 −12 s, activation energy ∼ 0.050 eV) and long range diffusion (D ∼ 0.005 eV, activation energy 0.25 eV).

Neutron scattering in Ag+ and Na+ β-aluminas

Abstract Na + and Ag + β-alumina for various stoichiometries and for various hydration states have been studied by neutron scattering using incident energy of 0.8 meV with a resolution of 20 μeV in the 200–325° C temperature range. From the weak quasielastic signal (QES) observed for the non- stoichiometric (NS) Ag + β-alumina a translational diffusion constant D t has been estimated at about 10 -5 cm 2 s -1 at 200°C. No such QES has been observed for stoichiometric Ag + and Na + compounds. The dehydration of Na + β-alumina has been evidenced by the disappearance of the broad QES above 300°C; correlatively the low frequency inelastic spectrum of Na + of β-alumina is changed.

ß″-and ion-rich ß-alumina: Comparison of vibrational spectra and conductivity parameters

Abstract Single crystals of s″-alumina containing Na + , K + , Ag + and Tl + ions are prepared and studied by Raman spectroscopy between 2 and 1000 cm −1 in the 20–400 K temperature range. Far-infrared measurements are performed between 10 and 250 cm −1 . The Raman bands assigned to the spinel block vibrational modes are broad ( Δv = 20 cm −1 ); this reflects a high degree of disorder and can be related to stabilizing Mg 2+ ions randomly distributed. In-plane cation vibrations are identified below 150 cm −1 in infrared and Raman spectra. Potential barriers associated with this type of motion are discussed. The temperature dependence of the relevant low-frequency Raman spectra for K + ue5f8s″-alumina may be interpreted in terms of a small proportion of very mobile K + ions. Tl + spectra are discussed in terms of clusters. Finally, a comparison with literature results shows that the s″ phase can be differentiated from t he ion-rich s-alumina phase of the same composition.

Vibrational spectra of different phases belonging to the β and β″ alumina and to the nasicon family

Abstract Vibrational spectra of β and β″ alumina for different concentrations in conducting cations are presented. In particular the authenticity of the stoichiometric form ( x = 0) is underlined and the distinction between the ion rich β alumina x = 0.66 and the β″ x = 0.66 structure is emphasized. Vibrational couplings between Ag + and Tl + cations are evidenced and discussed in terms of clusters. Potential barriers, estimated from low frequencies increase slightly when the cation density increases. Finally we have investigated the Raman and IR spectra of Nasicon and some terms of its family : Na 3 Sc 2 (PO 4 ) 3 , K 3 Zr 2 Si 2 PO 12 , Na 3 Zr 2 Si 2 PO 12 .