Strong interplay between magnetic and structural properties in the spin-1/2 chain molecular compound D-F₅ PNN

We present elastic and inelastic neutron scattering results on spin-1/2 chain antiferromagnet D- F5 PNN. We show that a structural transition occurs in zero field below Tc ∼710 mK from C2/c space group (uniform chains) to Pc (bond-alternating chains). When applying a magnetic field at 50 mK, the space group gets back to C2/c above Hc1 ∼1 T. From the dispersion curves of the magnetic excitations measured at T=50 mK, we determine the values of the exchange parameters at 0 and 9 T, which evidence that the magnetic field makes the chains evolve from alternating to uniform. This change in behavior can be correlated unambiguously with the field-induced structural transition. These results evidence "mirror" field- and temperature-induced magnetocrystalline transitions.