Europium arsenic oxide Eu₄As₂O: a filled La₂Sb structure and its relation to the K₂NiF₄ and GeTeU types

Eu4As20, tetragonal, 14/mmm (D~]); Z = 2, FW 773.68, a = 4.7924 (4), c = 16.1933 (9) A. c/a = 3.379. U = 371.9 A3; F(000) = 649, D, = 6.71 g cm-3;/1 = 439 cm -1 (Mo K¢0. Diffraction data from a crystal --0.03 × 0.07 × 0.03 mm with Mo KCt radiation and a graphite monochromator gave R, = 3.8% for 529 observed hkl when corrected for absorption and refined by full-matrix least squares with allowance for anomalous dispersion. The structure is a filled version of the La,Sb type with O atoms occupying the octahedral holes at 2(b) (0,0,1⁄2), As atoms in 4(e)Iz = 0.1354 (2)1 and Eu atoms in 4(e)with z = 0.3261 (1) and 4(c) (0,1⁄2,0). The average O E u distance is 2.54 A but the individual distances O-Eu(1) = 2.40 (×4) and O-Eu(2) = 2.82 A (× 2) may indicate the different nature of the two independent Eu atoms. The As atom has nine Eu atoms as nearest neighbors with an average As-Eu of 3.35 A; this arrangement is usually described in terms of a tricapped trigonal prism but the description in terms of a monocapped square antiprism is more convenient in this case because there is an As contact of 4.39 A opposite the uncapped square face. Both Eu atoms can be described as 14-coordinated with Eu(l) clearly occupying less volume: average distances for CN = 14 are Eu(1) = 3.30 and Eu(2) = 3.66 A. This structure is closely related to the K2NiF 4 and GeTeU types.