Inversion of quartz solid solutions at cryogenic temperatures

Abstract

Quartz solid solution crystals of six different compositions were obtained from crystallization of glass powders belonging to the Li2O–Al2O3‐SiO2 (LAS) system. They were analyzed in situ by laboratory‐based X‐ray diffraction down to cryogenic temperatures (−190°C). Temperature‐resolved analysis of their lattice parameters allowed determination of the critical inversion temperature Tc in these materials, marking the displacive phase transition from a high‐quartz‐ to a low‐quartz‐like lattice. Integrating available data from other literature sources, an updated phase diagram for the occurrence of high and low quartz solid solution phases is provided for the LAS system; these data are expected to support future development of functional materials relying on these crystalline phases.

Mg‐bearing quartz solid solutions as structural intermediates between low and high quartz

“…The mutual variation of the a and c lattice parameters obtained in the samples at room temperature is compiled in Table 1 and evaluated in Figure 3: the data exhibit a coherent trend, in good agreement with the values from Xu et al (2015) 24 and within the scatter range of other available references for Mg‐bearing Qss 5,14,26 . An increase in a is interestingly accompanied by a decrease in c in the MAS system, whereas a contemporary overall growth of both parameters is known from the LAS system and here reported for comparison 7,18,20 . In addition, the MAS data does not seem to directly converge toward the values of pure low or high quartz 31 .…”

“…Although an increase in the amount of dopants produces in both cases an expansion of the unit cell, the mutual variations of a and c show strikingly different behaviors. From a structural point of view, Li‐bearing Qss are typically characterized in terms of either fully low‐quartz‐ or high‐quartz‐like structures, respectively, at high and low SiO 2 content, 18,20 whereas recent results by other authors have already pointed out that this views are not applicable to Mg‐bearing Qss 24 . In fact, according to the results of the present work, the average structure of these phases clearly resembles low quartz only at very low Mg + Al content and approaches a “pseudo‐hexagonal” character with increasing doping, though never actually accomplishing the fully hexagonal symmetry typical of high quartz (at least at room temperature).…”

“…A, Plot of the room‐temperature lattice parameters obtained for Mg‐bearing Qss within this work and from some literature references, 5,14,24,26 as well as for pure quartz 31 (* labels the value pertaining to high quartz at 600°C). The plot contains also the values obtained from literature sources for Li‐bearing Qss, 10,18,20 for the purposes of comparison. Lines are only meant as a guide to the eye.…”

“…At a lower extent of structural stuffing, however, the room‐temperature framework collapses into that of low quartz (hereafter LQss), as often indirectly identified through the positive thermal expansion of the phases 6 . These structural changes are relatively abrupt and they are mirrored by the linear compositional dependence of the quartz inversion temperature ( T c in the following), from the conventional value of 573°C in pure quartz 21 down to cryogenic temperatures 19,20 …”

“…Li‐bearing Qss are by far the most investigated members of this family: it is known that they display a high‐quartz‐like structure (hereafter HQss) at room temperature, from the composition of the endmember β‐eucryptite (LiAlSiO 4 ) 16 down to approximately 82.5 mol% SiO 2 content 17‐20 . At a lower extent of structural stuffing, however, the room‐temperature framework collapses into that of low quartz (hereafter LQss), as often indirectly identified through the positive thermal expansion of the phases 6 .…”