The structure of "orthorhombic" KAlSiO (sub 4) -Ol; evidence for Al-Si order from MAS NMR data combined with Rietveld refinement and electron microscopy

Dry synthesis of KAlSiO (sub 4) at temperatures from 900 to 1500 degrees C yielded products with slightly different powder X-ray-diffraction patterns. "Orthorhombic" K (sub x) Al (sub x) Si (sub 2-x) O (sub 4) -O1 with x nearly equal 1 was obtained as a substantially single phase after heating at 1000 degrees C for one day; we refined its crystal structure from powder X-ray-diffraction data in space group P12 (sub 1) 1 [MM = 158.17 g/mol, a 15.669(2), b 9.057(1), c 8.621(1) Aa, beta 90.16(1) degrees , V 1223.5 Aa (super 3) , Z = 12, D (sub x) = 2.57 g cm (super -3) , R (sub B) = 0.080]. It is composed of a relatively open [AlSiO (sub 4) ] framework that is a topological variant of tridymite (t) having the supercell (s) metric a (sub s) nearly equal 3a (sub t) , b (sub s) nearly equal a (sub t) +2b (sub t) , c (sub s) nearly equal c (sub t) . The space group P12 (sub 1) 1 allows for Al-Si ordering, and refinement of distance-least-squares restrained models, although problematic owing to the pronounced pseudosymmetry, indicates preference for an ordered pattern where Al and Si are distributed on alternating tetrahedra [dSi-O/dAl-O = 1.628(1)/1.719(1) Aa], so that every SiO (sub 4) tetrahedron is coordinated to four AlO (sub 4) tetrahedra and vice versa. The alternating distribution was independently inferred from (super 29) Si and (super 27) Al MAS NMR spectroscopic data, and the framework model obtained from Rietveld refinement with Si on tetrahedron T1 could be used to successfully simulate the observed Si(Al (sub 4) ) doublet peak in the (super 29) Si spectrum. Electron diffraction showed that triple twinning with a rotation of 120 degrees around c of the metrically almost hexagonal P12 (sub 1) 1 cell is ubiquitous and enhances, in the diffraction experiment, the pseudosymmetry inherited from the tridymite subcell. Furthermore, the diffraction aspect of single individuals (P (super *) 2 (sub 1) (super *) ) confirms that the screw axes 2 (sub 1) --and--2 (sub 1) of the orthorhombic supergroup P2 (sub 1) 2 (sub 1) 2 (sub 1) are only approximated.