Martensitic transformation of CsFeS2 driven by the singlet to néel state transition

Abstract

CsFeS 2 is regarded as an example of a quasi one-dimensional alternating antiferromagnetic Heisenberg chain above about 70 K. In accordance with this picture, an energy gap of 10 meV was observed for the singlet to triplet excitation at a zone center near the transition temperature by our neutron measurements. The large excitation width implies a strong coupling of excitons to phonons, and LA phonons along the (001) direction become ill-defined for q greater than 0.3. At about 70 K, a first order transition occurs, whereby the singlet ground state changes to a Neel state and a simultaneous structural transformation takes place, which is a martensitic transformation. A mechanism for such martensitic transformation is discussed based on the similarity of the magneic excitation and phonon properties of the present compound and the γ-Mn alloys with Fe and Cu, which are itinerant electron magnetic systems.