Thermal expansion and structural transformations of stuffed derivatives of quartz along the LiAISiO₄-SiO₂ join: A variable-temperature powder synchrontron XRD study

The structural behavior of stuffed derivatives of quartz within the Li_1-xAl_1-xSi₁ + _xO₄ system (O≤ × ≤ 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 A1/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 ≤ × < ∼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 ≤ × < ∼0.3 than for those with compositions ∼0.3 ≤ × < ∼0.65. We attribute this disparity to a tetrahedral tilting mechanism that occurs only in the ordered structures (0 ≤ × < ∼0.3). The phases with ∼0.65 ≤ × ≤ 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(A1)-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.