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" K_xAl_xSi_2-x O₄-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, β 90.16(1)°, V 1223.5 ų, Z = 12, D_x = 2.57 g cm^-3, R_B = 0.080]. It is composed of a relatively open [AlSiO₄] framework that is a topological variant of tridymite (t) having the supercell (s) metric a_s ≈ 3a_t, b_s ≈ a_t + 2b_t, c_s ≈ c_t. 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.