BA11KX7O2 (X = P, AS) - 2 NOVEL ZINTL PHASES WITH INFINITE CHAINS OF OXYGEN-CENTERED BA-6 OCTAHEDRA, ISOLATED X3- AND DIMERIC X-2(4-) ANIONS

Reactions of ''BaX'' (X = P, As) with Ba, K and BaO in tantalum tubes at 900-1000 degrees C yielded black, very air- and moisture-sensitive crystals of Ba11KP7O2 and isotypic Ba11KAs7O2 which were characterized by EDX and X-ray diffraction (orthorhombic, Fddd, Z=8; a=1069.9(1), b =1514.3(2), c=3164.6(4) pm and a=1087.8(2), b=1542.3(2), c=3232.4(4) p m, respectively). The structure contains infinite zigzag chains, (1)(i nfinity)[Ba4Ba2/2O], of oxygen-centered, corner-sharing Ba-6 octahedra along [100]. They are connected by linear strings built of alternatin g isolated X atoms and X-2 dimers to form layers parallel to (001). Wh ile the isolated X atoms are surrounded by eight Ba forming a distorte d cube, the X-2 dimers center a Ba-12 polyhedron which is comprised of a pair of face-sharing Ba square antiprisms. This results in a cube-a ntiprism-antiprism-cube sequence of face-sharing Ba polyhedra. Additio nal X atoms function as spacers between the layers and connect them al ong [001]. Two atom positions are statistically occupied by Ba and K, and the formula may be written as Ba112+K+X53-(X-2)O-4-(2)2- according to the Zintl-Klemm concept.