The one‐dimensional bond‐alternating antiferromagnet F5PNN has a singlet ground state with a spin gap in zero field. To study the competition between the quantum magnetic state and the lattice energy, we measured a dielectric constant ε to detect the lattice distortion due to the magnetic properties of F5PNN. In addition to the monotonic change of ε above a few Kelvins, we observed an obvious change of ε at the lower temperatures. This change is likely related to the temperature dependence of the free energy of the magnetic chains whose exchange interactions are of the order of 5 K. These results suggest a continuous lattice distortion of the bond‐alternating chains.

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