High-pressure x-ray- and neutron-diffraction studies ofBaF2: An example of a coordination number of 11 inAX2compounds

Abstract

${\mathrm{BaF}}_2$ was studied by angle-dispersive x-ray and neutron time-of-flight powder diffraction as a function of pressure. The normal pressure phase which has the fluorite structure transforms at about 3 GPa to an orthorhombic structure. The atomic positions have been refined in situ for this phase from the neutron data by the Rietveld method. These refined positions are typical of those observed in other cotunnite-type structured compounds. Above 12 GPa, a second crystallographic transformation occurs. The structure is of the ${\mathrm{Ni}}_2$In type as confirmed by Rietveld refinements of the x-ray diffraction data. At these phase transformations, the coordination number of barium increases from 8 to 9 and then to 11, to our knowledge, the highest value observed in ${\mathit{AX}}_2$ compounds. This last value indicates a new structure type is to be added in their structure map. The molar volume of the ${\mathit{AX}}_2$ compounds under normal conditions varies linearly with the cube of the mean cation-anion distance. The slopes of these straight lines are approximately inversely proportional to the coordination number. This demonstrates the improving of the packing efficiency and explains quantitatively the effect of pressure on volume at phase transitions.