Abstract
Magnetic and caloric measurements on potassium lead hexanitrocuprate, K2PbCu(NO2)6, agree well with the theory for an antiferromagnetic Heisenberg linear S=1/2 chain. Results for a related compound, in which lead is replaced by barium, can be analyzed in terms of an antiferromagnetic Heisenberg S=1/2 simple quadratic lattice. These classifications are supported by existing data on the crystal structures of these compounds at low temperatures.