Pressure-induced crossover from alternating to uniform interaction in a S=1/2 one-dimensional Heisenberg antiferromagnet

We have confirmed the pressure-induced crossover from the alternating to the uniform interaction in a one-dimensional (1D) organic antiferromagnet by the measurement of heat capacity. The pentafluorophenyl nitroxide radical (F5PNN), which is a S = 1/2 1D Heisenberg antiferromagnet with the alternating interaction ratio alpha = \J(2)/J(1)\ similar or equal to 0.4(J(1)/k(B) = -3.1 K) at ambient pressure, has transformed into the uniform one (alpha = 1; J(1)/k(B) = -4.5 K) under the pressure of 6.5 kbar. This pressure-induced crossover was made clear by observing the change of low-temperature gradient of heat capacity, which gradually transformed from exponential to linear dependence on temperature, by the pressurization. (C) 1999 Elsevier Science Ltd. All rights reserved.