Crystal structures of Na and K aluminate mullites

Mullite-type alkali aluminates (KyNa1-y)(0.67)Al6O9.33 were synthesized. fr om amorphous Al and alkali nitrates by sol-gel techniques. Rietveld refinem ents of six members of the solid solution series (y = 0.0, 0.2,..., 1.0), t ogether with Fourier syntheses and grid search analyses show that the Na an d K atoms reside in the vacant Oc sites, with K at 1/2, 0, 1/2 and Na on a split site off the special position. The number of alkali atoms is restrict ed to 2/3 atoms per unit cell due to crystal chemical constraints. Conseque ntly, unlike the aluminosilicate mullites, alkali mullites do not form a so lid solution series with varying oxygen composition. All compounds studied here crystallize in space group Pbam with lattice constants ranging from a = 7.6819(4) Angstrom, b = 7.6810(4) Angstrom, c = 2.91842(8) Angstrom for t he Na aluminate to a = 7.6934(3) Angstrom, b = 7.6727(3) Angstrom, c = 2.93 231(7) Angstrom for the K aluminate mullite.