Alpha-quartz 1. Crystallography and crystal defects

Abstract

Crystallography of alpha-quartz is discussed with special referenoe to the existing ambiguities regarding handedness of its enantiomorphic forms and a mnemonic has been suggested. Previous x-ray diffraction topographic ,studies of synthetic quartz are critically reviewed and analysed to understand the origin, nature and location of dislocations. It is suggested that dislocations associated with cell boundaries, characteristic of the Z-zone grown portions of synthetic quartz, are pure a-type edge dislocations but possibly with an alternating non-conservative climb component associated with the predominating glide component.

Alpha-Quartz 2. Crystal chemistry, and nature and distribution of impurities

“…These defects would again, as in the case of scheme (2), lower the mechanical Q of the synthetic crystals. The areal variation of mechanical Q of synthetic quartz, increasing with increase in dislocation density, as detected by Auvray and Regreny (1973) supports this conclusion (Saha et al 1979).…”

“…Obviously this scheme would be unlikely for the ideal structure, since there is not enough space to accommodate an excess oxygen atom in either of the two channels. However, it appears to be a plausible alternative for the opened-up structure with (i) planar defects like the faults detected by Lang (1967) in natural quartz, and (ii) with dislocations, and with planar defects coplanar with the dislocations like the cell-wall stacking fault type of fault fringes of synthetic quartz (Saha et al 1979). In this case hydrogen will be strongly attached to oxygen framework, and depending upon the extent of incorporation of hydrogen impurity (which will be independent of concentration of aluminium impurity; see Kats 1962) planar or line defects may occur which will adversely affect the acoustic loss property of the crystals.…”

0D chiral hybrid indium(iii) halides for second harmonic generation

“…SCXRD analysis suggests that ( R -MPEA) 6 InCl 9 crystallizes in the P 3 2 21 space group and ( S -MPEA) 6 InCl 9 crystallizes in the P 3 1 21 space group, which are enantiomers belonging to the nonpolar 32 point group of the trigonal system. 72 They have the same cell parameters ( a = 17.05 Å, b = 17.05 Å, c = 39.52 Å, and β = 120° for ( R -MPEA) 6 InCl 9 , and a = 17.01 Å, b = 17.01 Å, c = 39.21 Å, and β = 120° for ( S -MPEA) 6 InCl 9 ). More detailed data for crystal diffraction refinements are summarized in Table S1 †.…”