THE HALDANE-STATE REALIZED IN (CH3)(2)CHNH3CUCL3 FERROMAGNETIC-ANTIFERROMAGNETIC ALTERNATING HEISENBERG-CHAINS WITH S=1 2/

We report, on the basis of magnetic susceptibility measurements, that the S = 1/2 compound (CH3)(2)CHNH3CuCl3 (IPACuCl())3 forms ferromagnet ic and antiferromagnetic alternating linear chains with J(1)/k = -23.5 K and J(2)/k = +54.1 K, which indicate that a pair of ferromagnetical ly coupled spins behaves as a spin of S = 1. This compound can, theref ore, be discussed in terms of the Haldane conjecture for integer spins . The energy gap is estimated to be 17.1-18.1 K. On the contrary, the isomorphous compound IPACuBr(3) is found to form antiferromagnetic and antiferromagnetic alternating chains with J(1)/k = -61 K and J(2)/k = -33 K; the energy gap is estimated as 98 K. (C) 1998 Elsevier Science B.V. All rights reserved.