Ba2Cu18-xAs10: A new mixed-valent ternary copper-rich arsenide with metallic properties

The new barium copper pnictide Ba2Cu18-xAs10 (x approximate to 1.67) was ob tained from a direct element combination reaction in a sealed graphite tube at 700 degreesC, and its structure was determined by single-crystal X-ray diffraction methods. It crystallizes in the trigonal space group P (3) over bar 1c (No. 163) with a = 6.7329(1) Angstrom, c = 11.6747(2) Angstrom, and Z = 1. Ba2Cu18-xAs10 has a three-dimensional structure with straight bottl e-shape parallel tunnels running along the c-direction. The Ba atoms reside in the tunnels. The [Cu18-xAs10](4-) framework features Cu atom vacancies with x = 1.67. The compound is metallic with a room-temperature resistivity along the c-axis of 1.7 x 10(-4) Ohm .cm and a Seebeck coefficient of simi lar to +2.5 muV/K. Band structure calculations in the extended Huckel appro ximation suggest that the defects on the Cu sites enhance its metallic char acter.