Raman study of the formation of transition alumina single crystal from protonic ?/?? aluminas

Abstract

Transition aluminas are usually obtained by thermal treatment of aluminium hydroxides between 450 and 1000 ° C. X-ray diffraction [1] but also IR spectroscopy [2] have been used to characterize the various phases (7, 0, &, x . . . ) occurring before crystallization in corundum (e-alumina). However, samples prepared this way are in the form of small crystallites and, aluminium oxides being poor scatterers, the use of Raman scattering was not successful. In previous work [3, 4], we have shown by X-ray diffraction and electron paramagnetic resonance how a protonic /3 (or /3") alumina single crystal can be transformed by heating into an e-alumina oriented polycrystal through the formation of a transition alumina single crystal. To our knowledge, this is the unique method to prepare easily large single crystals (a few tens of mm 2) of transition aluminas. These crystals are suitable for special studies, such as those of surface properties. The structure of/?-alumina consists in spinel blocks separated by a loosely packed plane where conducting iotas diffuse rapidly. The bridging between spinel blocks is made by two tetrahedra sharing the same oxygen ion. The difference between/3 and/?" phases resides in the nature of the conducting plane: only in the /3 phase is there a mirror plane from a crystallographic point of view. A protonic/?-alumina single crystal with composition equal to 1.25M20 • 11A1203 (M + = NH4 + or H+(H20),) is transformed above 400°C into a stoichiometric/?-alumina single crystal (composition close to (I-I30)20 • 1 IA1203). This transformation takes place from the reaction of interstitial oxygens ensuring the charge compensation mechanism, with the protons liberated by the desolvation of protonic ions [5-7]. Further heating above 800°C involves the diffusion of the remaining protons in the spinel blocks where they are trapped, Stacking faults are created by the collapse of some loosely packed conducting planes in between the spinel blocks. The samples remain in the form of single crystals; the /3-alumina unit cell is multiplied by three or even six along the a direction of the hexagonal plane (Fig. 1), whereas complex superstructures and diffusions are observed along the c direction [7]. Finally a topotactic growth of e-alumina crystallites is observed above 1100 ° C [4, 7, 8]. Similar transformations are observed for ion-rich /?and fl"-alumina single crystals with composition close to Allo.33Mgo.66017M1.66 (M = NH2, H+(H20)~). However, due to the new charge compensation mechanism (Mg2+/A13+ ion substitution) the stoichiometric form is not obtained and stacking faults occur at lower temperature with a drastic collapse of spinel blocks, above 320 and 400°C for ion-rich/3 and/3" phase, respectively [9]. A transition alumina phase with spinel-like structure is formed. Above 670°C syntactic nucleation of nonstoichiometric spinel occurs [9, 10]. Finally, separation into e-alumina and MgAI204 spinel is observed above 1000°C [10]. This quasi-continuous evolution of relatively well-ordered spinel-like structures into disordered ones provides a good opportunity to state the vibrational fingerprinting evolution of spinel-like materials. However, such structures are found for many solid-state reactions in ceramics (e.g. mullite