Structural mechanisms underlying near-zero thermal expansion in β-eucryptite: A combined synchrotron x-ray and neutron Rietveld analysis

• Published in: Journal of materials research Vol. 14; no. 7; pp. 3138 - 3151
• Main Authors: Xu, Hongwu, Heaney, Peter J., Yates, Douglas M., Von Dreele, Robert B., Bourke, Mark A.
• Format: Journal Article
• Language: English
• Published: New York, USA Cambridge University Press 01.07.1999
• ISSN: 0884-2914; 2044-5326
• DOI: 10.1557/JMR.1999.0421

The structures of ordered and disordered β-eucryptite have been determined from Rietveld analysis of powder synchrotron x-ray and neutron diffraction data over a temperature range of 20 to 873 K. On heating, both materials show an expansion within the (001) plane and a contraction along the c axis. However, the anisotropic character of the thermal behavior of ordered β-eucryptite is much more pronounced than that of the disordered compound; the linear expansion coefficients of the ordered and disordered phases are αa = 7.26 × 10−6 K−1; αc = −16.35 × 10−6 K−1, and αa = 5.98 × 10−6 K−1; αc = −3.82 × 10−6 K−1, respectively. The thermal behavior of β-eucryptite can be attributed to three interdependent processes that all cause an increase in a but a decrease in c with increasing temperature: (i) Si/Al tetrahedral deformation, (ii) Li positional disordering, and (iii) tetrahedral tilting. Because disordered β-eucryptite does not exhibit tetrahedral tilting, the absolute values of its axial thermal coefficients are smaller than those for the ordered sample. At low temperatures, both ordered and disordered β-eucryptite exhibit a continuous expansion parallel to the c axis with decreasing temperature, whereas a remains approximately unchanged. Our difference Fourier synthesis reveals localization of Li ions below room temperature, and we suggest that repulsion between Li and Al/Si inhibits contraction along the a axes.