Beevers-Ross Site Research Articles

Far-infrared absorption of sodium β alumina

As-grown sodium beta alumina is non-stoichiometric and shows strong absorption in the region 20-200 cm-1 with partly resolved structure. The authors show that in material close to stoichiometry sharp absorption lines are observed in the same spectral range. They conclude from these studies that a sharp intense peak at 59 cm-1 is due to vibrations of Na+ ions near singly occupied Beevers-Ross sites. They suggest that other absorption peaks in the far infrared are due to complexes involving sodium ions and charge-compensating interstitial oxygen ions, and propose that complexes responsible for absorption at 102 cm-1 are particularly effective in contributing to the ionic conductivity.

Neutron scattering study of the proton dynamics in NH+4 and OH+3 β alumina

Quasielastic neutron scattering experiments have been performed at different resolutions on powdered samples of β alumina containing either NH 4+ or more or less hydrated OH3+ cations. The existence of reorientations with a proton jump rate of 1.4 x 1012 s-1 at 395 K is confirmed for the ammonium derivative. Furthermore, it is shown that only the cations occupying Beevers-Ross sites (∼ 75 %) are concerned by this fast motion, in agreement with the predictions of previous X-Ray and spectroscopic studies. No reorientational motion is detected for the fully hydrated β alumina but a dehydrated sample obtained by heating the latter above 670 K presents a quasielastic broadening which can be associated with jump rotations of OH3+ cations around their threefold axis. The frequency of this motion is 4.1 x 10 11 s-1 at 395 K and 4.9 x 1011 s-1 at 485 K. At these two temperatures the percentage of rotating protons, 44 %, is constant and corresponds rather well to the expected non stoichiometric composition 1.33(OH3+)2O, 11 Al1O 3 in which one third of the unit cell planes are occupied by two hydrogen bonded cations in middle-oxygen sites, the other two planes being occupied by only one cation relatively free to rotate. The same sample undergoes a partial rehydration at room temperature. Only 24 % of the protons are then found to be involved in a reorientational motion with a frequency of 2 x 1011 s-1. With a resolution of the order of 1 μeV, another kind of broadening seems to occur in the case of the ammonium derivative. It could be due to translational jump diffusion characterized by a self-diffusion constant of the order of 10-8 cm2 s-1. This result is consistent with the conductivity measurements.

Raman scattering from 6Li + and 7Li + ions in lithium beta-alumina

Raman bands due to translational modes of 6Li + and 7Li + ions in beta-alumina were found in the frequency range of 340–410 cm −1, which is much higher than expected from the corresponding values reported for other alkali-metal ions (20–100 cm −1). Based on a model calculation, it is believed that the large shifts to higher frequencies are due to deviations of the Li + ions from the mirror plane into “pocket sites” formed by oxygen ions above or below the Beevers-Ross sites

Vibrational spectra and structure of H+(H2O)nβ- alumina

Single crystals and polycrystalline samples of β-alumina containing hydrated proton H+(H2O)n have been studied by infrared and, to some extent, by Raman spectroscopy. The spectra of fully hydrated and more of less deyhdrated samples and their deuterated derivatives have been compared to those of Na+ β-alumina and of mixed crystals with different M+/H+ ratios (M=Na, K, Ag). It can be shown that the samples heated above 400 °C contain H3O+ ions occupying Beevers–Ross sites as dominant species; midoxygen sites with single and double occupancy are less populated. The spectroscopic data are consistent with nearly nonhydrogen-bonded H3O+ ions having a slightly distorted C3v pyramidal structure. The fully hydrated β-alumina, on the other hand, appears to contain also higher species such as H5O2+ and H7O3+. The former is believed to have an unsymmetrical structure, H3O+H2O, with terminal groups hydrogen bonded to the oxygen atoms of spinel blocks. The latter is characterized by OH stretching bands similar to those of H7O3+ClO4−.

On the ionic conduction in β-alumina: Potential energy curves and conduction mechanism

Potential energy curves for various carrier ions in β-alumina have been calculated. All ions except the carrier ions (Li+, Na+, Ag+, K+, Rb+, Cs+) were assumed fixed in their equilibrium positions. These ionic positions were taken from Roth’s work. In our calculation the motion of carrier ions is restricted on lines of the two dimensional hexagonal network. We show that the site energy difference between the anti-Beevers–Ross site and the Beevers–Ross site is approximately 2 eV in stoichiometric crystals. In nonstoichiometric crystals the extra carrier ions may be paired to form interstitialcy pairs. The potential energy barriers for the in-phase motion of the interstitialcy pairs are found to be comparable to the experimental activation energies. Attempt frequencies have been calculated, and a simple random walk model is used for the electrical conductivities.