Dependence of the crystal structure parameters on the size of the structural units in some isomorphous isostructural series

Previous studies of kieserite analogues and also other systems (Tutton's sa lts, alums and beta-K2SO4 isomorphs) have shown that in a series of isostru ctural/isomorphous compounds the unit-cell parameters and volumes vary line arly with the effective ionic radii (the 'size') of the structural units. T his is in line with the results of Shannon (Acta Cryst. A32 (1976) 751-767) . Somewhat unexpectedly, other important parameters of the crystal structur e, such as fractional atomic co-ordinates, exhibit systematic variations as well. An attempt is made to reveal the reasons that are at the origin of t his finding.