LOW-TEMPERATURE SUSCEPTIBILITIES OF THE QUASI-ONE-DIMENSIONAL SPIN-1 2 HEISENBERG ANTIFERROMAGNETS (6MAP)CUCL3 AND (3MAP)CUCL3 - SPIN-PEIERLS TRANSITION VERSUS BROKEN-CHAIN EFFECTS/

The magnetic susceptibilities of the two spin-1/2 Heisenberg antiferro magnetic chain systems, 6-methyl-2-aminopyridinium trichlorocuprate(II ) [(6MAP)CuCl3] and 3-methyl-2-aminopyridinium trichlorocuprate(II) [( 3MAP)Cu%l(3)], have been reinvestigated. Their behavior cannot be desc ribed by a simple uniform Heisenberg antiferromagnetic chain model at low temperature, but rather by either a spin-Peierls model with a para magnetic impurity contribution or a broken-chain model. The spin-Peier ls model assumes a uniform antiferromagnetic chain above the transitio n temperature and a weakly dimerized antiferromagnetic chain below tha t temperature and yields uniform-chain exchange constants J/k of -54.5 and -59 K for (6MAP)CuCl3 and (3MAP)CuCl3, respectively, with the alt ernating exchange ratio 0.98<alpha<1 for both. The possible spin-Peier ls transition temperatures are about 2 K. It is also possible to expla in the data by a broken-chain model which gives exchanges of -50.5 and -55.6 K for (6MAP)CuCl3 and (3MAP)CuCl3, respectively. In this model the average chain lengths of two compounds are estimated.