Protons and deuterons in stoichiometric and nonstoichiometric MgAl 2 O 4
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I. INTRODUCTION Spinel (MgAl 2 O 4 ) is a high-temperature oxide whose melting point, ~ 2400 K, lies between those of its chemical constituents of MgO and A12O3. Its structure is cubic, with the Al 3 + and Mg2 + ions occupying different symmetry sites: octahedral and tetrahedral, respectively. Even though it is chemically stable at high temperatures, its high-temperature properties have received relatively little attention compared to MgO and A12O3 crystals. At elevated temperatures, in-diffusion and out-diffusion of hydrogenic species (protons and deuterons) in spinel is inevitable. This paper reports an investigation on the diffusion characteristics of these species. The presence of hydrogen in spinel can be monitored using infrared absorption of OH~ ions. These ions are present in the as-received materials and are characterized by absorption bands, which lie in the 3000-4000 c m " ' region. Therefore deuterons were used in order to distinguish the diffusing specie from protons inherently present in the material.
A typical sample used for optical and diffusion studies had an area of about 0.5 cm2 and a thickness of about 0.05 cm. Infrared absorption measurements were performed with a Perkin-Elmer model 580 spectrophotometer. Visible and ultraviolet measurements were made with a Cary 17 D. Samples were heated in flowing D 2 O vapor inside an alumina tube inserted in the horizontal axial hole of an Astro furnace (model No. 10002560 FP) with a graphite heating element. The diffusion coefficients for deuterons at a given temperature can be obtained from the growth rate of the absorbance, A, of the OD^ bands. The mathematical formalism has been given elsewhere1 and the procedure for analyzing our diffusion data has been described previously.2 In brief it requires knowledge of the initial absorbance of the diffusing specie, the absorbance after anneals of time t, and the absorbance at saturation (after long anneals), to be referred to as Ao, A,, and Aa , respectively. The diffusion coefficient D is related to these quantities by the formula At — Ao
(1)
/
II. EXPERIMENTAL DETAILS AND FORMALISM The spinel samples used in this investigation were obtained from Union Carbide Corporation (UC) and Adolph Meller (AM). The former were grown by the Czochralski method and the latter by the Verneuil techniques. Both crystals were characterized chemically and crystallographically (see Sec. III). The UC crystals were indeed single crystals and essentially stoichiometric. The AM materials were nonstoichiometric and found to be strongly oriented polycrystalline.
where 2/ is the thickness. We neglected diffusion through the edges of the samples and treated the problem as one dimensional. This is justified because the samples used in our experiments were thin (thickness
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