The compound Er2BaF8 is prepared by heating appropriate amounts of BaF2 and ErF3 at 900 °C for 18 h in a dry argon filled quartz ampoule. The polycrystalline sample obtained was characterized by a Rietveld refinement of the observed powder diffraction data. This compound crystallizes in a monoclinic lattice with unit cell parameters: a=6.9620(2), b=10.4960(3), c=4.2541(1) Å and β=99.685(1)°, V=306.14(1) Å3, Z=2, (space group C 2/m, No. 12). The structure analysis reveals the presence of one kind of Ba and Er atoms in the unit cell. There are three different kinds of fluorine atoms, namely F1, F2, and F3 in the ratio of 8:4:4. The Er atoms form a square-antiprism polyhedron with eight fluorine atoms, which share three of its edges to form a ring of six ErF8 polyhedra. The rings of ErF8 polyhedra are connected via two opposite corners along the c-axis of the unit cell. The voids formed within the rings are occupied by Ba atoms, which maintain a twelve-coordination polyhedron with the fluorine atoms. As per the Rietveld analysis, the F3 site may exist as a disordered state by shifting the fluorine atom from the two-fold axes. © 2004 International Centre for Diffraction Data.