Nonstoichiometric rare-earth copper arsenides RECu1+xAs2 (RE = La, Ce, Pr)

Single crystals of the ternary rare-earth copper arsenides RECu1+xAs2 (RE = La, Ce, Pr) have been prepared by reactions of the elements in the presenc e of I-2, and their structures have been determined by X-ray diffraction. T hey crystallize in tetragonal space groups, with the La member in I4/mmn (L aCu1.23(1)As-2, a = 3.0901(4) Angstrom, c = 20.243(3) Angstrom, Z= 4) and t he Ce and Pr members in P4/nmm (CeCu1.10(1)As2, a = 4.0265(4) Angstrom, 10. 071(2) Angstrom, PrCu1.09(1)As2, a = 4.0086(2)Angstrom, c = 10.0496(9) Angs trom; Z= 2 for both). LaCu1.23(1)As2 represents a stuffed variant of the Sr ZnBi2 structure type, while CeCu1.10(1)As2 and PrCu1.09(1)As2 are stuffed v ariants of the HfCuSi2 structure type. The nonstoichiometry of these compou nds arises from partial occupation of a square pyramidal site by Cu, The st ructural chemistry of these ternary arsenides is thus more complicated than previously assumed. Magnetic susceptibility measurements indicate diamagne tism for LaCu1.23(1)As2, paramagnetism for CeCu1.10(1)As2, and antiferromag netism (T(N)approximate to 4K) for PrCu1.09(1)As2. (C) 1999 Academic Press.