Distances in divalent metal hydroxides that have the CdI2-type structure. (a) Experimentally determined values of d(M—M) against d(M—O). (b) d(M—O) determined by experiment against the value calculated using ionic radii. (c) h(oct) against r(M). The data are for Mg(OH)2 (Catti et al., 1995; Černý et al., 1995; Chakoumakos et al., 1997; Desgranges et al., 1996; Isetti, 1965; Zigan & Rothbauer, 1967), Ni(OH)2 (Kazimirov et al., 2010), Co(OH)2 (Pertlik, 1999), Fe(OH)2 (Parise et al., 2000), Mn(OH)2 (Christensen & Ollivier, 1972), Cd(OH)2 (Bertrand & Dusausoy, 1970), Ca(OH)2 (Busing & Levy, 1957) and Zn(OH)2 (Baneyeva & Popova, 1969). There are two data points for β-Zn(OH)2: one calculated from Baneyeva & Popova's structure that is labelled `Zn cBan' and a second from this work that is labelled `Zn c1'.