Hydrogen bonding and diffusion in mullite

Abstract

Traces of OH groups have been identified in the infrared absorption spectra of a nominally anhydrous synthetic mullite single crystal. The OH absorption profiles were resolved with four peaks for polarizations E// a, E// b and E// c, respectively. The integrated absorption intensities correspond to an H 2O content of about 10 ppm (wt) using an average extinction coefficient according to the mean wavenumber relation. The length of O 1–H⋯O 2 bonds (O 1–H hydroxyl groups with hydrogen bridging towards a neighboring oxygen, O 2) range between 276 and 314 pm following empirical relations for hydrogen bonding in aluminosilicates. According to lattice energy calculations the infrared peak positions can be associated with two different classes of hydrogen positions assuming a substitution Si 4+ ⇔ 4H +: one type of H atoms is bound on tetrahedral faces of substituted Si-sites involving intense O 1–H⋯O 2 hydrogen bridging. A second type of H atoms form more isolated O–H groups directed into the structural channels of mullite running along the crystallographic c axis. These OH dipoles show polarizations perpendicular to the c axis. A third type of OH dipole is oriented parallel to the c axis and could be assigned to appropriate pairs of oxygen in two neighboring unit cells, e.g. O c1 –H⋯O c2 . Heating experiments for 12 h at 1200 °C, 6 h at 1300 °C and 4 h at 1400 °C reveal a significant decrease of OH concentration on ppm level. OH absorption profiles measured on cross-sections by infrared microscope technique yielded for example at 1300 °C diffusion coefficients of D a ≈ D b ≈ 8 × 10 −9 (parallel to the a and b axis) and D c ≈ 1.5 × 10 −8 cm 2/s (parallel to the c axis). The observation D c > D a ≈ D b corresponds to a preferred diffusion parallel to the structural channels in c direction. The temperature dependence of the diffusion coefficient D c of hydrogen outward diffusion is described with an Arrhenius activated behavior (190 kJ/mol). It includes the diffusion coefficient of hydrogen inward diffusion, which was obtained by submitting originally H-free mullite at 1670 °C to a water-rich atmosphere.