The structure of "orthorhombic" KAlSiO4-O1: evidence for Al-Si order from MAS-NMR data combined with Rietveld refinement and electron microscopy

Dry synthesis of KAlSiO4 at temperatures from 900 to 1500°C yielded products with slightly different powder X-ray-diffraction patterns. “Orthorhombic” KxAlxSi2–xO4-O1 with x ≈ 1 was obtained as a substantially single phase after heating at 1000°C for one day; we refined its crystal structure from powder X-ray-diffraction data in space group P1211 [MM = 158.17 g/mol, a 15.669(2), b 9.057(1), c 8.621(1) Å, b 90.16(1)°, V 1223.5 Å3, Z = 12, Dx = 2.57 g cm–3, RB = 0.080]. It is composed of a relatively open [AlSiO4] framework that is a topological variant of tridymite (t) having the supercell (s) metric as ≈ 3at, bs ≈ at + 2bt, cs ≈ ct. The space group P1211 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) Å], so that every SiO4 tetrahedron is coordinated to four AlO4 tetrahedra and vice versa. The alternating distribution was independently inferred from 29Si and 27Al 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(Al4) doublet peak in the 29Si spectrum. Electron diffraction showed that triple twinning with a rotation of 120° around c of the metrically almost hexagonal P1211 cell is ubiquitous and enhances, in the diffraction experiment, the pseudosymmetry inherited from the tridymite subcell. Furthermore, the diffraction aspect of single individuals (P*21*) confirms that the screw axes 21-- and --21 of the orthorhombic supergroup P212121 are only approximated.