Invited Speaker
Georgiy Levchenko

Georgiy Levchenko

Professor, State Key Laboratory of Superhard Materials and International Centre of Future Science, Jilin University, China
Speech Title: Temperature and Pressure Induced Spin Transition in Fe(II) Hofmann-like Two Dimensional Coordination Polymer [Fe(Fpz)2M(CN)4] where M = Pt, Pd and Fpz = F-pyrazine: Experiment and Theory

Abstract: A comprehensive experimental and theoretical study of both thermal-induced spin transition (TIST) as a function of pressure and pressure-induced spin transition (PIST) at room temperature for the bi-functional two-dimensional Hofmann-like SCO polymers [Fe(Fpz)2MCN)4], where (M =Pt, Pd and Ni, Fpz = F-pyrazine) is reported. The TIST studies at different fixed pressures have been carried out by magnetic susceptibility measurements, while PIST studies have been performed by means of powder X-ray diffraction, Raman, infrared and visible spectroscopies. A combination of the theory of elastic interactions and numerical Monte Carlo simulations has been used for the analysis of the cooperative interactions in TIST and PIST studies. A complete (T, P) phase diagrams for the compounds [Fe(Fpz)2Pt(CN)4] has been constructed. The critical temperatures of the spin transition follow a near lineal dependence with pressure, meanwhile the hysteresis width shows a nonmonotonic behavior contrary to theoretical predictions. The analysis shows the exceptional role of the total entropy and phonon contribution in setting the temperature of the spin transition and the width of the hysteresis. The anomalous behavior of the thermal hysteresis width under pressure in [Fe(Fpz)2M(CN)4] is a direct consequence of a local distortion of the octahedral geometry of the Fe(II) centers for pressures at higher pressure. Interestingly, there is not a coexistence of the high- and low-spin (HS and LS, respectively) phases in TIST experiments, while in PIST experiments, the coexistence of the HS and LS phases in the metastable region of the phase transition induced by pressure is observed for a first time in a first-order gradual spin transition with hysteresis. It is shown that the appearance of mixed state under pressure is caused by the spin-elastic interactions dependent from average value of the order parameter. These interactions determine also the shape of spin hysteresis and its transformation from rectangular to parallelogram type.

Keywords: Pressure, spin transition, Monte Carlo, elastic interactions model, polymer

Biography: Georgiy Levchenko is a Professor of State Key Laboratory of Superhard Materials and International Centre of Future Science of Jilin University, China. He also is affiliated with Donetsk Institute of Physics and Engineering of National Academy of Science of Ukraine as Head of Physics of Phase Transformations Department. He collaborates with many scientific groups of Europe and America. He is a winner of the Shubnikov Prize (2013) and Correspondent мember of the National Academy of Science of Ukraine. His fields of interest are: material science; molecular materials; Superhard materials; Solid state physics; pressure, structure, magnetic, resistive and optical experiments. His works in the field of pressure influence on magnetic and optical properties of molecular magnets are widely known in the molecular science.