Monoclinic superstructure of SrMgF4 with perovskite‐type slabs

Crystals of Ce‐doped SrMgF₄, strontium magnesium tetrafluoride, have been found to have a monoclinic P2₁ structure with doubled a and tripled c cell lengths compared with the orthorhombic Cmcm structure previously reported in the literature. The perovskite‐type slabs, composed of corner‐sharing MgF₆ octahedra and Sr atoms, are stacked along the b axis. The six crystallographically independent MgF₆ octahedra are rotated so as to provide long periodicities along a and c . The coordination numbers and bond distances around the six crystallographically independent Sr atoms are slightly different in each case. In the superstructure, the Sr atoms lie on local mirror planes which are thought to originate at the high‐temperature phase transition.