THERMAL EXPANSION AND STRUCTURAL TRANSFORMATIONS OF STUFFED DERIVATIVES OF QUARTZ ALONG THE LIALSIO4 - SIO2 JOIN: A VARIABLE-TEMPERATURE POWDER SYNCHROTRON XRD STUDY

Abstract

The structural behavior of stuffed derivatives of quartz within the Li1−x Al1−x Si1+x O4 system (0 ≤ x ≤ 1) has been studied in the temperature range 20 to 873 K using high-resolution powder synchrotron X-ray diffraction (XRD). Rietveld analysis reveals three distinct regimes whose boundaries are defined by an Al/Si order-disorder transition at x=~0.3 and a β–α displacive transformation at x=~0.65. Compounds that are topologically identical to β-quartz (0 ≤ x < ~0.65) expand within the (0 0 1) plane and contract along c with increasing temperature; however, this thermal anisotropy is significantly higher for structures within the regime 0 ≤ x < ~0.3 than for those with compositions ~0.3 ≤ x < ~0.65. We attribute this disparity to a tetrahedral tilting mechanism that occurs only in the ordered structures (0 ≤ x < ~0.3). The phases with ~0.65 ≤ x ≤ 1 adopt the α-quartz structure at room temperature, and they display positive thermal expansion along both a and c from 20 K to their α–β transition temperatures. This behavior arises mainly from a rotation of rigid Si(Al)-tetrahedra about the <100> axes. Landau analysis provides quantitative evidence that the charge-coupled substitution of Li+Al for Si in quartz dampens the α–β transition. With increasing Li+Al content, the low-temperature modifications exhibit a marked decrease in spontaneous strain; this behavior reflects a weakening of the first-order character of the transition. In addition, we observe a linear decrease in the α–β critical temperature from 846 K to near 0 K as the Li+Al content increases from x=0 to x=~0.5.