Computational studies of Si/Al ordering in aluminosilicate tetrahedral framework structures

An extensive computational research programme on Si/Al ordering in tetrahedral framework structures is surveyed. Earlier results on the enthalpy of ordering are briefly reviewed, and their reliability discussed in the light of ab initio calculations. The main reason is discussed why the ordering temperature often lies well below that expected from Bragg Williams theory, with new quantitative results. The actual pathway of Si/Al ordering appears complex from some inconclusive ab initio calculations, but the presence of hydrogen clearly lowers the barrier substantially.