New arsenides with ThCr2Si2-type or related structures:: The compounds ARh2As2 (A: Eu, Sr, Ba) and BaZn2As2

Four new arsenides of rhodium and zinc were prepared by heating mixtures of the elements at high temperatures (1000-1200 degrees C) and investigated by single crystal X-ray methods. EuRh2As2 (a = 4.067(1), c = 11.319(2) angstrom) and BaRh2As2 (a = 4.053(1), c = 12.770(3) angstrom) crystallize with the well-known ThCr2Si2-type (I4/mmm; Z = 2). Due to the rigid layers of RhAs4 tetrahedra, and to the atomic size of europium and barium, the As-As distances between the layers with values of 2.97 and 3.66 angstrom, respectively, are very long. SrRh2As2 is polymorphic and undergoes two phase transitions at about 190 and 282 degrees C. Main features of the three crystal structures are also layers of RhAs4 tetrahedra. At room temperature alpha-SrRh2As2 (a = 5.676(1), b = 6.178(2), c = 11.052(2) angstrom) probably crystallizes with the BaNi2Si2-type (Cmcm; Z = 4), whereas beta-SrRh2As2 (a = 5.760(3), b = 6.067(4). c = 11.264(5) angstrom, Fmmm, Z = 4) forms a new orthorhombically distorted variant of the ThCr2Si2-type. Single crystals grown in a flux of lead and quenched at high temperature show that the gamma-phase (a = 4.112(1), c = 11.431(6) angstrom) crystallizes with the ThCr2Si2-type. The same is true for the high temperature modification of BaZn2As2 (beta-phase; a = 4.120(1), c = 13.578(1) angstrom), whereas the already known alpha-BaZn2As2 forms the alpha-BaCu2S2-type (Pnma; Z = 4) consisting of a 3D-network of edge- and vertex-sharing ZnAs4 tetrahedra with Ba atoms in the voids of this network.