Two Ongoing Magnetic Transitions Originating from Spin-Peierls Dimerization and Cation Orientation Transformation in an S = 1/2 Spin Chain System

The phase transition materials, with abrupt change of electrical, magnetic, or optical responses in the phase transition vicinity, may have applications in data storage, signal processing, and switchable devices. Herein, we present the study of structural, magnetic, dielectric, and thermal properties for an S = 1/2 magnetic chain system, [DMPy][Ni(mnt)2] (1; DMPy+ = N-dimethyl pyrrolidinium and mnt2- = maleonitriledithiolate). Two ongoing magnetostructural transitions appear at ∼270 and ∼307 K, giving low- (LTP), intermediate- (ITP), and high-temperature (HTP) phases of 1. LTP crystallizes in P21/c, while both ITP and HTP belong to Pnma space group. The LTP-ITP phase transition is broken symmetry associated with one-dimensional (1D) regular magnetic chain dimerization, the spin gap opened in LTP, and no visible latent heat, showing the typical character of the second-order spin-Peierls transition. The ITP-HTP transformation is an isostructural phase transition, which is coupled with the sharp change of the cation orientation, accompanied by latent heat and dielectric anomaly, with the typical character of the first-order phase transition. This study demonstrates the coexistence of diverse mechanisms of magnetostructural phase transition in an 1D radical salt and opens a way for design and preparation of new switchable magnetic and electric materials.