Thermal expansion and structural transformations of stuffed derivatives ofquartz along the LiAlSiO4-SiO2 join: a variable-temperature powder synchrotron XRD study

The structural behavior of stuffed derivatives of quartz within the Li1-xAl 1-xSi1+xO4 system (0 less than or equal to x less than or equal to 1)has be en studied in the temperature range 20 to 873 K using high-resolution powde r synchrotron X-ray diffraction (XRD). Rietveld analysis reveals three dist inct regimes whose boundaries are defined by an Al/Si order-disorder transi tion at x = similar to0.3 and a beta-alpha displacive transformation at x = similar to0.65. Compounds that are topologically identical to P-quartz (0 less than or equal to x < similar to0.65) expand within the (0 0 1) plane a nd contract along c with increasing temperature: however, this thermal anis otropy is significantly higher for structures within the regime 0 less than or equal to x < similar to0.3 than for those with compositions similar to0 .3 less than or equal to x < <similar to>0.65. We attribute this disparity to a tetrahedral tilting mechanism that occurs only in the ordered structur es (0 less than or equal to x < similar to0.3). The phases with similar to0 .65 less than or equal to x < 1 adopt the a-quartz structure at room temper ature, and they display positive thermal expansion along both a and c from 20 K to their alpha-beta transition temperatures. This behavior arises main ly from a rotation of rigid Si(Al)-tetrahedra about the (100) axes. Landau analysis provides quantitative evidence that the charge-coupled substitutio n of Li + Al for Si in quartz dampens the alpha-beta transition. With incre asing Li + Al content, the low-temperature modifications exhibit a marked d ecrease in spontaneous strain; this behavior reflects a weakening of the fi rst-order character of the transition. In addition, we observe a linear dec rease in the alpha-beta critical temperature from 846 K to near 0 K as the Li + Al content increases from x = 0 to x = similar to0.5.