Crystal structures and variable magnetism of PbCu2(XO3)2Cl2 with X = Se, Te.

Novel Cu(2+)-based compounds PbCu2(SeO3)2Cl2 (space group C2/c; a = 13.056(1) Å; b = 9.5567(9) Å; c = 6.9006(6) Å; β = 90.529(7)°; RI = 0.0371) and PbCu2(TeO3)2Cl2 (space group P2(1); a = 7.2401(2) Å; b = 7.2688(2) Å; c = 8.2846(2) Å; β = 96.416(2)°; R(I) = 0.0570) have been obtained by solid-state synthesis. Their crystal structures are remarkably dissimilar and underlie a very different magnetic behavior. While PbCu2(SeO3)2Cl2 can be well described by a spin-chain model with an exchange coupling of J1 ≃ 160 K, PbCu2(TeO3)2Cl2 is a spin-dimer system that, however, features a comparable magnetic nearest-neighbor coupling of J ≃ 213 K. PbCu2(SeO3)2Cl2 orders antiferromagnetically below 12 K, whereas PbCu2(TeO3)2Cl2 lacks long-range magnetic order down to at least 2 K, owing to the strong dimerization of the Cu(2+) spins. Crystal structures of both compounds are rationalized in terms of relevant magnetic exchange pathways, and the implications for a broader range of Cu(2+) compounds are discussed.