Prediction And Mechanism Of Negative Thermal Expansion Properties Of Hexacyano Prussian Blue Analogues From Theoretical Study

In nature,most compounds will expand and shrink when the temperature rises and reduces respectively.However,there are also a small number of compounds that show the opposite phenomenon in a certain temperature range,that is,the phenomenon of negative thermal expansion（NTE）.The application of NTE plays an important role in the fields of high-precision instruments and equipment.But now the number and types of NTE materials are scarce,so in this work the recent developed concept of average atomic volume（AAV）has been used to predict and search for new NTE compounds.A more comprehensive exploration is carried out based on density functional theory（DFT）calculations to improve the understanding of the NTE mechanism.The main conclusions are as follows:Firstly,we use the first-principles calculations based on density functional theory（DFT）combined with the recent developed AAV to predict the NTE property of isotropic compounds of Prussian blue analogues（PBAs）AB（CN）₆（A=Al,Ga,In;B=Co,Fe,Mn,Cr,V,Ti）in a wide temperature range.Our results show clearly that the coefficient of the NTE of PBAs has a near linear relationship with the AAV and is also influenced by the element substitution at A or B site.Lattice dynamic simulations indicate that the main reason of the NTE is the low-frequency transverse vibration of the C≡N groups,especially the transverse vibration of the N atoms.Thus,the element substitution at the A site can more effectively tune the NTE behavior of the systems than that at the B site,proving the different roles of chemical bonds with different properties on the NTE.In addition to this,it is also found that in AB（CN）₆,the transverse acoustic phonon mode also contributes to NTE.Our present work not only expands the kinds of isotropic NTE compounds,but also gives some insight into the relationship between AAV and NTE.Secondly,the concept of AAV is also used to predict the anisotropic NTE compounds REFe（CN）₆（RE=La,Sm,Ho,Lu）family.The NTE of the systems have been studied by the first-principles calculations and compared to the results obtained by the in situ neutron powder diffraction（NPD）and X-ray absorption fine structure spectroscopy（EXAFS）.According to our results a good linear relationship between the thermal expansion coefficient and the average atomic volume is also found:for the systems with the larger AAV,the low-frequency phonon modes with the stronger transverse vibration of the C≡N groups and the larger negative Grüneisen parameters can be found.Then the greater coefficients of negative thermal expansion are achieved.Our finding is of great significance to the control and design of thermal expansion materials,and provides a new method for predicting and searching for NTE materials.