Structural Control of Elastic Constants of Mullite in Comparison to Sillimanite
B Hildmann, H M. Ledbetter, Sung Kim, H A. Schneider
The nine elastic stiffness coefficients C, of a mulitte single crystal (2Al2O3.SiO2) were measured using Acoustic Resonance Spectroscopy (ARS). The obtained values are similar to those of the structurally related alumino-silicate phase sillimanite (Al2O3.SiO2). Characteristic elastic properties of the two minerals are interpreted with the help of their crystal structures and atomic-force constants for silimanite. The high longitudinal stiffness coefficients C33 of mulite (difference}352 GPa) and silimanite (difference}388 GPa) are due to continuous stiff load-bearing tetrahedral chains parallel to c, while the soft octahedral chains have minor direct influence. They stabilize the tetrahedal chains against tilting. The lower C33 value of mulite in comparison to silimanite may be caused by a weakening of the load-bearing tetrahedral chains parallel to c due to partial replacement of Si by the weaker bonded Al. The longitudinal stiffness coefficients perpendicular to c are significantly lower, due to the sequences of alternating soft octahedral and stiff tetahedral units. Within the plane (001), the compliant octahedra exhibit stiffness-controlling influence with coefficinets parallel to b (c22 difference} 233 GPa) being somewhat lower than parallel to a (c11 difference} 291 GPa), due to the most compliant octahedral al(1)-O(D) bonds, which are more effective parallel to b rather than to a. Because octahedra are unaffected by the Al to Si substitution, C11 and C22 coefficients of mulite and sillimanite are very similar. Shear stiffness coefficients of mulite increase from C55 (difference}77 GPa) to C66 (difference}80 GPa) to C44 (difference}110 GPa), indicating increasing resistance against shear deformation within the planes (010), (001), and (100). The lattice plane of the highest shear stiffness (100) is built up of an oxygen-oxygen network, diagonally braced along <011> (Jagerzaun effect). This network with short oxygen-oxygen distances can be sheared by compression and elongation along oxygen-oxygen interaction lines only, which is ragher unlikely. Due to the lack of such networks in the planes (010) and (001) bending and deformation of structural units become easier, and consequently C55 and C66 are lower than C44. All three shear stiffness coefficients of mulite are slightly lower than those of sillimanite due to the reduction of the mean tetrahedral bond strength in mulite caused by partial substitution of Si by Al.
, Ledbetter, H.
, Kim, S.
and Schneider, H.
Structural Control of Elastic Constants of Mullite in Comparison to Sillimanite, Journal of the American Ceramic Society
(Accessed June 3, 2023)