Order-disorder structural phase transition and magnetocaloric effect in organic-inorganic halide hybrid (C₂H₅NH₃)₂CoCl₄

Highlights: • New Co-based halide hybrid is synthesized. • Order-disorder structural phase transition is investigated thoroughly. • Electronic absorption spectra show effects of geometric distortion and spin orbit coupling. • DFT calculations reveal a wide band gap and also presence of weak magnetic interactions. • Magnetic studies reveal field induced spin flipping at low temperature and sizeable change in associated magnetic entropy which is comparable with that of cluster magnets. - Abstract: We report a detailed experimental and theoretical investigation of structural, optical, magnetic and magnetothermal properties of single crystals of a new organic-inorganic hybrid (C₂H₅NH₃)₂CoCl₄. Grown by slow evaporation method at room temperature, the compound crystallizes in centrosymmetric orthorhombic structure (Pnma) which undergoes a reversible phase transition at 235/241 K (cooling/heating) to noncentrosymmetric P2₁2₁2₁ space group symmetry associated with order-disorder transformation of carbon atoms of the ammonium cations as well as molecular rearrangement. Electronic absorption spectra of the compound are typical of geometrically distorted [CoCl₄]^2- tetrahedra having spin-orbit coupling effect. The isolated nature of [CoCl₄]^2- tetrahedra in the crystal reflect in paramagnetic behaviour of the compound. Interestingly, field induced spin flipping behaviour is observed at low temperature. First principles density functional calculations reveal weak magnetic interaction among cobalt spins with ferromagnetic state being the ground state. The entropy change associated with the spin flipping has been experimentally estimated by magnetic and heat capacity measurements which has a maximum value of 16 J Kg⁻¹ K⁻¹ at 2.5 K under 7 T magnetic field. To the best of our knowledge, this is the first report on magnetocaloric effect observed in an organic-inorganic halide compound. The estimated value is sizable and is comparable to that of well-known transition metal molecular cluster magnets Mn₁₂ or Fe₁₄. The overall findings promise to enlighten new routes to design and constitute multifunctional organic-inorganic halide materials.