Formation of alternating antiferromagnetic Heisenberg chains in (CH3)2CHNH(3)CuBr(3)

We report that antiferromagnetic and antiferromagnetic (AF-AF) alternating Heisenberg chains with S = 1/2 are formed in (CH3)(2)CHNH3CuBr3, which is in contrast to the almost isomorphous compound (CH3)(2)CHNH3CuCl3, because the Cl compound forms ferromagnetic-dominant ferromagnetic and antiferromagnetic (F-AF) alternating Heisenberg chains [H. Manaka et al.: 3, Phys. Soc. Jpn. 66 (1997) 564]. From the analysis of the dependence of magnetic susceptibility on temperature, chi(T), the two different antiferromagnetic exchange interactions defined in the Hamiltonian H = -2J Sigma S-2i . S2i+1- 2J(2) Sigma S-2i . S2i-1 are determined to he J1/k = -61 K and J(2)k = -33 K. Applying the low temperature experimental data to the formula chi(T) alpha T-1/2 exp(-Delta/kT) we estimate the energy gap Delta to be 98 K. We discuss why AF-AF and F-AF alternating chains are formed in the Br and the CI compounds, respectively in spite of their almost isomorphous crystal structures.