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

摘要

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 (00l) 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 α_a = 7.26 x 10~(-6) K~(-1), α_c = -16.35 x 10~(-6) K~(-1), and α_a = 5.98 x 10~(-6) K~(-1); α_c = -3.82 x 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.