The Crystal Structures of two Anhydrous Magnesium Hydroxychloride Phases from in situ Synchrotron Powder Diffraction Data

Abstract The crystal structures of two members of the solid solution series Mg(OH) x Cl y x + y = 2, Mg(OH) 1.7 Cl 0.3 ( P $\tilde {3}$ m 1, a = 3.169(2) Å, c = 5.530(12), V = 48.1(1) Å 3 at T = 365 °C) and MgOHCl ( R $\tilde {3}$ m , a = 3.3877(4) Å, c = 17.534(4) Å, V = 174.27(6) Å 3 at T = 625 °C) were determined from in situ synchrotron powder diffraction data at high temperature upon dehydration of 3Mg(OH) 2 ·MgCl 2 ·8H 2 O (F3) and 5Mg(OH) 2 ·MgCl 2 ·8H 2 O (F5) phases. The crystal structures of Mg(OH) 1.7 Cl 0.3 (example of ss‐type‐OH) and MgOHCl (example of ss‐type‐Cl) can be related to the C19 (CdCl 2 ) and C6 (CdI 2 ) structure type, respectively, with the disordered chloride and hydroxide anions occupying the same crystallographic site in layers.