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In the high-temperature polymorph Na2SO4(I) up to 30% cation vacancies can be generated by substitution of Na + by bi-and trivalent ions. Phase diagrams and lattice parameters of these solid solutions are presented. The hexagonal high-temperature form and its monoclinic distortions can be quenched to room temperature. A structure determination was carried out. Structural and experimental data are: P63/mmc, a=5.326(2), c=7.126(3) A, V=175.06/~ 3, Z=2, Dx = 2.695 Mg m -3, h(Ag Ka) = 0"55936 fk, R = 0.09 for 165 reflections. The crystal structure is characterized by strong orientational disorder of the SOn tetrahedra. The disorder, aliovalent solid solutions, thermal expansion and ionic conductivity are discussed.