Random Exchange Antiferromagnetic Heisenberg Chains*

Copper \textit{bis}pyridine dichloride (CPC) and copper \textit{bis}pyridine dibromide (CPB) are isostructural Heisenberg S = $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ linear chains in which the copper atoms are bridged by two halides with the pyridine molecules in the axial sites. The exchange strengths are 27 K (CPC) and 45 K (CPB). We have prepared mixed halide versions Cu(py)$_{2}$[Cl$_{1-x}$Br$_{x}$]$_{2}$ for values of x from 0 to 1. We will report on the effect of the exchange randomness on the susceptibilities and N\'{e}el temperatures of this family of compounds, as determined by dc-magnetometry, muon spin relaxation, and low-temperature calorimetry.