Plot of the a parameter against x for a range of Zn—Al LDH phases reported in the literature. The data involve a variety of interlayer anions, which are represented by different symbols (that are identified in the figure legend). The full line is the result of the linear regression analysis of the data. The filled diamonds represent the β polymorph of Zn(OH)2; the upper filled diamond is from Baneyeva and Popova's structure (Baneyeva & Popova, 1969) and the lower one is from Richardson's (2013) structure. The dashed line represents the values of a calculated from theory [using equation (1)]; the open diamond can be taken to represent a theoretical α form of Zn hydroxide [Zn(OH)2·H2O]. The data are from: Aimoz et al. (2012), Barriga et al. (1998), Benito et al. (2008), Rojas Delgado et al. (2008), Johnson & Glasser (2003), Kooli et al. (1997), Lakraimi et al. (2006), Leroux, Adachi-Pagano et al. (2001), Lozano et al. (2012) (zaccagnaite), Merlino & Orlandi (2001) (zaccagnaite), Miyata (1975), Radha et al. (2007a), Radha et al. (2011), Seftel et al. (2008) (the a parameter for these data was calculated from the XRD patterns because the values given in their Table 1 are not consistent with the patterns); Thevenot et al. (1989), Troutier-Thuilliez et al. (2009), Vieira et al. (2009), Witzke & Raade (2000) (*zincwoodwardite).