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

Abstract

Structural analogies between the crystalline polymorphs of SiO 2 and other natural or synthetic compounds of different composition were already identified in the early days of X-ray crystallography, such as cristobalite-like carnegieite 1 and trydimite-like nepheline 2 (both with stoichiometry NaAlSiO 4). These structures have been specifically termed stuffed derivatives if the partial substitution of Si 4+ by Al 3+ (or by other lower valence cations) in the tetrahedral sites leads to the incorporation of additional extra-framework cations 3 to maintain overall charge neutrality. 4 Prime examples of the above-described category of compounds are the quartz-like solid solutions obtainable in the magnesium aluminosilicate (MAS), 5,6 lithium aluminosilicate (LAS), 6,7 and zinc aluminosilicate (ZAS) 6 compositional systems, as well as in their mutual mixtures. 6-10 These quartz solid solutions (Qss) are of particular interest in materials science for representing the major component of LAS glass-ceramics with near-zero thermal expansion and high thermal shock resistance. 11 Moreover they act as crystalline precursors during the crystallization of cordierite from glass 5,12,13 ; the production of high-strength materials based on the formation of Si-rich Qss in MAS glasses has also been suggested. 14,15 Li-bearing Qss are by far the most investigated members of this family: it is known that they display a high-quartzlike 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 These structural changes

A threshold heating rate for single‐stage heat treatments in glass‐ceramics containing seed formers

“…The curves pertaining to LMSi‐T plotted lower than those of LMInt‐T, whereas LMAl‐T reached the highest values. These observations were in line with the known expansion of the Qss unit cell volume with a higher degree of structural stuffing, 14,17,36‐38 dependent on the availability of Al, Mg, and Li in the glass. At a comparable ceramization stage, the Qss unit cell volumes were interestingly higher in LMInt‐Tu and LMAl‐Tu than in their seeded counterparts; at LMSi‐T composition, again, no clear differences were visible.…”

“…Finally, LMAl‐Ts and LMInt‐Ts developed Qss crystals with noticeably lower unit cell volumes than their untreated counterparts. These differences, interpreted in terms of a different degree of structural stuffing of the crystals, 14,17,36‐38 may be again explained through the modification of the glass matrix induced by the TiO 2 ‐related seed formation. In fact, STEM characterization (Figure 2) revealed non‐negligible incorporation of Mg and Al into the TiO 2 ‐bearing nuclei after the preliminary heat treatments.…”

Towards new zero-thermal-expansion materials: Li-free quartz solid solutions stuffed with transition metal cations

“…1) by trial and error to maximize the amount and unit cell volume of Qss (i.e. their degree of structural stuffing) while minimizing the appearance of secondary phases, also based on previous works on Li-and Mg-bearing Qss [21,23]. FeQss proved more sensitive to the redox conditions and could therefore only be transformed into Qss when the heat treatments took place in vacuum: we particularly used an Anton Paar's HTK1200N heating chamber attached to a diffractometer to monitor the process, quickly cooling the sample after the formation of Qss to avoid its decomposition.…”

“…Between 0 °C and 50 °C, commercially available materials based on Qss such as Zerodur ® can achieve a coefficient of thermal expansion (CTE) as low as (0 ± 0.007) x 10 -6 K -1 , with spatial variations that are less than 0.005 x 10 -6 K -1 over the whole volume of a telescope mirror blank with a diameter of 4 meters [13,14]. Indeed, the thermal expansion of Qss can be finely tuned, taking advantage of their variable Al/Si ratio and of the full solid solution [8,9,15,16] existing between: (i) Mgbearing Qss, exhibiting trigonal symmetry and positive thermal expansion [17][18][19][20][21]; (ii) Li-and Znbearing Qss, which were reported to be fully hexagonal (at a sufficiently high level of chemical stuffing) and to contract upon heating [8,12,[22][23][24][25][26][27].…”

Spray‐dried sol‐gel glass‐ceramic powders based on the tunable thermal expansion of quartz and keatite solid solutions

“…Quartz solid solutions (Qss) and keatite solid solutions (Kss) have been therefore appreciated for decades as the main crystalline constituents of zero thermal expansion glass‐ceramics, in which they typically precipitate from a multicomponent parent glass 12 . Negative thermal expansion is instead generally not attainable in the compositionally analogous magnesium aluminosilicate (MAS) system, since the available silicate phases, that is, Mg‐bearing Qss at low temperatures and indialite/cordierite at high temperatures, exhibit CTEs going from slightly to noticeably positive values in the absence of additional dopants 13,14 …”