Monoclinic superstructure of SrMgF4 with perovskite-type slabs

Crystals of Ce-doped SrMgF4, strontium magnesium tetrafluoride, have been found to have a monoclinic P2(1) 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.

Bonding without ionisation

Charges are calculated for diatomic molecules by partitioning the promolecular density with projection operators derived from free atom potentials. The promolecular charges thus obtained have physically reasonable magnitudes, which are moderately sensitive to interatomic distances and thus to bond type. Their signs are negative for cations and positive for anions. Trends in these charges correlate with subshell structure. Due to the long range of their electrostatic potentials, the cations compete successfully for electrons at the expense of the anions in the density partitioning. This implies that the cation states become overfilled when the atoms overlap, favouring a flow of electrons away from the cation towards the vacant anion states. That accounts for the positive cations and negative anions observed when molecular densities are partitioned with projection operators based on free atom electron densities.