Hydrothermal single crystal growth and structural investigation of the stuffed tridymite family as NLO materials

The structures, crystal growth and preliminary NLO properties of a broad class of compounds known as the stuffed tridymites are investigated. The stuffed tridymites of the formula ABCO4, where A are the alkali metal ions and B and C are Group 13 and 14 metal ions, respectively, form an extensive series of structures, all of which are based on symmetry breaking of the parent tridymite (SiO2) structure. In all cases the alkali ion resides in channels running parallel to the c-axis formed from six-membered rings of tetrahedra of the metal ions. A large array of structures can be formed though combinations of ordering of the B/C sites, staggered and eclipsed tetrahedra, up/down orientations of the tetrahedra relative to the c-axis, and distortions of the six-membered rings. These symmetry breaking steps can be mixed and matched to form many different structures, and nearly all are in polar acentric space groups. The primary cause of centrosymmetric crystal formation is disorder of the B/C sites. When the sites are well ordered most of the structures are acentric with polar axes. High quality single crystals of most products can be grown using a high pressure hydrothermal method. Preliminary NLO experiments indicate that the compounds are acentric, but have low NLO conversions, which is expected due to the low polarizability of the building blocks. The extremely high percentage of polar acentric structure types, and the ability to correlate the structures to systematic variations of the very large and flexible tridymite structural class, suggests that this can serve as a starting point for designing crystals with many other useful physical properties.