Theoretical Study On The Negative Thermal Expansion Mechanism Of Cyanide Coordination Compounds

Positive thermal expansion is considered to be one of the fundamental properties of matter.However,some materials have also been found to exhibit abnormal negative thermal expansion(NTE)in a certain temperature range,which has important applications in the field of precision instruments and thermal expansion composites.The NTE materials with a wide temperature range driven by the low frequency phonon are still very rare.In the dissertation,the cyanide coordination compounds were studied based on density functional theory(DFT)calculations to find new negative thermal expansion materials and reveal the underlying mechanisms.In recent years,Cu SCN as a new type of inorganic hole-transporting cyanide coordination semiconductor with a wide-bandgap(>3.4 e V),is attracting much attention in fabrication of perovskite solar cells.In the first part of the dissertation,by using first principles DFT and quasi-harmonic approximation(QHA)approach,we have stuided lattice dynamics and abnormal thermal expansion of the system,including?-and?-Cu SCN phases.The influence of the abnormal thermal expansion of the lattice on the electronic structure,especially on the band gap of the system were explored and discussed.We found that due to the shearing modes between Cu S layers and the torsion modes of the Cu-N?C-S atomic chains,the?-Cu SCN exhibits anisotropic negative thermal expansion(NTE),the calculated average negative expansion coefficient is-6.68×10^-6K^-1(0-400K).While the?-Cu SCN displays strong anisotropic thermal expansion,where the lattice parameter in the c-axis direction shows NTE due to the shearing modes between the Cu S layers and the transverse vibration of the cyanide CN,but the lattice parameters in a,b-axis directions show strong positive thermal expansion.The?-Cu SCN has a direct band gap,which increases slightly with increasing temperature.However,the?-Cu SCN has an indirect band gap at low temperature,which converts to a direct band gap near the temperature of 375 K due to the strong positive expansion in ab plane.Our work revealed mechanisms of the abnormal thermal expansion of the two phases and a strong coupling between the anisotropic thermal expansion and the electronic structures of the systems.In the second part of the dissertation,another the cyanide coordination compound Fe Fe(CN)₆ were studied at the atomic scale by first principles calculations to explore its thermal expansion property and to check the modulation of the NTE by Na insertion.It was found that the system exhibits NTE in a wide temperature range,the negative thermal expansion is mainly caused by the transverse vibrations of C and N atoms of cyanide group in the same or opposite directions.The calculated results show that the average thermal expansion coefficient of the system in the low temperature region(0-300K)is about-4.7×10^-6K^-1,which is in good agreement with the measured value(-4×10^-6K^-1).For the case of Na insertion,we found that the NTE of the system is significantly inhibited.By controlling the density level of the doped Na,near zero NTE of the system can be achieved in the low temperature region(0-300 K),which was also consistent with the experimental results.