Rare-Earth Manganese Copper Pnictides RE2Mn3Cu9Pn7 (Pn = P, As): Quaternary Ordered Variants of the Zr2Fe12P7-Type Structure.

The pnictides RE2Mn3Cu9Pn7 (Pn = P, As) have been prepared by stoichiometric reaction of the elements at 800 °C. They are quaternary ordered variants of the hexagonal Zr2Fe12P7-type structure (Pearson symbol hP21, space group P6..., Z = 1; a = 9.6444(3)-9.5970(7) Å, c = 3.9027(1)-3.776l(3) Á for RE2Mn3Cu9P7 (RE = La-Nd, Sm, Gd-Dy); a = 9.9376(6)-9.9130(3) Â, c = 4.0194(2)-3.961l(1) Å for RE2Mn3Cu9Pn7 (RE = La--Nd)). Of the four possible sites available for the transition metal, the square pyramidal site (CN5) is occupied preferentially by Mn atoms, whereas the three tetrahedral sites (CN4) are occupied by Cu atoms. On proceeding to smaller RE members in the RE2Mn3Cu9Pn7 series, one of the transition-metal-centered polyhedra (Cu1) tends to become less distorted, while die remaining three (Cu2, Cu3, Mn4) become more distorted. Band structure calculations on La2Mn3Cu9P7 confirm that Mn--P and Cu--P contacts provide the strongest bonding interactions. Electrical resistivity measurements on Ce2Mn3Cu9P7 reveal metallic behavior with transitions at 165 and 18 K, probably of magnetic origin. [ABSTRACT FROM AUTHOR]