Effects of Hydrostatic Pressure and Uniaxial Strain on Spin-Peierls Transition in an Organic Radical Magnet, BBDTA$\cdot$InCl4

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We investigated the effects of hydrostatic pressure and uniaxial strain on the spin-Peierls (SP) transition of an organic radical magnet, benzo[1,2-d:4,5-d$'$]bis[1,3,2]dithiazole(BBDTA)$\cdot$InCl4. It has a one-dimensional coordination polymer structure along its $c$-axis and its SP transition occurs at 108 K. The SP transition temperature $T_{\text{SP}}$ decreased to 99 K at a hydrostatic pressure of 10 kbar, while it increased to 132 K at a uniaxial strain along the $c$-axis of 8 kbar. The pressure dependences of $T_{\text{SP}}$ under these two conditions were discussed by evaluating two parameters, namely, the intrachain interaction $2J/k_{\text{B}}$ and the effective spin–lattice coupling parameter $\eta$, that are related to $T_{\text{SP}}$ by the equation $T_{\text{SP}}=1.6\eta J/k_{\text{B}}$. Under ambient pressure, the $a$- and $c$-axes of this material shortened monotonically with decreasing temperature, while the $b$-axis elongated below $T_{\text{SP}}$. In this study, we found the correlation between $\eta$ and the change in the lattice constant $b$. $2J/k_{\text{B}}$ increased with increasing hydrostatic pressure and uniaxial strain, suggesting that the contraction along the $c$-axis does not depend on the manner of pressurization. From the evaluation of $\eta$, the observed variation in $T_{\text{SP}}$ is explained by the difference between the changes in $b$ under the two pressurization conditions.
2009-12-15