Similarities and peculiarities between the crystal structures of the hydrates of sodium sulfate and selenate

The crystal structures of the two hydrates Na2SeO4 center dot 10H(2)O and Na2SeO4 center dot 7.5H(2)O are studied for the first time. The structures of Na2SeO4 center dot 10H(2)O and Na2SeO4 center dot 7H(2)O are reinvestigated as a function of temperature with respect to the degree of disorder of the O atoms of SO42- in the decahydrate and the O atom of water in the heptahydrate. For Na2SeO4 center dot 10H(2)O, the unit site occupancy factor (SOF) of O atoms of SO42- was determined at 120 K. After the temperature dependence of the lattice parameters was studied from 120 to 260 K, it was shown that SOF decreased from 1.0 at 120 K to 0.247 at room temperature. The interesting fact that two salts with different chemical compositions and different crystal structures (Na2SeO4 center dot 7H(2)O, tetragonal, space group P4(1)2(1)2 and Na2SeO4 center dot 7.5H(2)O, monoclinic, space group C2/c) can act mutually as a crystal nucleus is accounted for by similarities in certain fragments of their crystal structures. This phenomenon is attributed to similarities between particular elements of their structures.