Synthesis And Properties Research Of Phase Transition Compounds Based On Benzyltrimethylammonium Bromide Ligands

Phase transition materials have many special physical properties,such as ferroelectricity,switchable dielectric,electro-optical effects and non-linear optical properties,and show great potential in areas such as data storage,environmental monitoring and signal processing.From the current research,it is still in the development stage how to obtain phase transition materials with ideal high phase transition temperature and rich physical properties.In order to find phase transition materials with excellent properties,this thesis synthesises a total of 10 compounds by molecular design and modification strategies using BTAB（BTAB=benzyltrimethylammonium bromide）and its derivatives as the organic ligand part,varying different inorganic anionic frameworks,and explores their phase transition properties,semiconductivity,dielectric properties,non-linear optics and ferroelectric properties through a molecular design and modification strategy.The main contents of this thesis are as follows:1.In Chapter 3,in order to explore new methods to regulate the band gap,four new organic-inorganic hybrid compounds（BTAB）₃Sb₂Cl₉（1）,（4-Br-BTAB）₃Sb₂Cl₉（2）,（4-Cl-BTAB）₃Sb₂Cl₉（3）and（4-F-BTAB）₃Sb₂Cl₉（4）were obtained by coordination reactions with Sb Cl₃ using benzyltrimethylammonium bromide（BTAB）and its derivatives as organic ligands.Compound 1 crystallised in the space group P2₁/c and compounds 2,3 and 4 all crystallised in the space group P-1.It was determined by differential scanning calorimetry（DSC）that compounds 1,3 and 4 underwent reversible high-temperature phase transitions at 470.8 K/461.8 K,469.3 K/401.8 K and 471.2 K/388.5 K,respectively,and that the generation of this phase transition behaviour was associated with enhanced electronegativity of the halogens（Br,Cl,F）.The UV-Vis diffuse reflectance spectroscopy of compounds 1-4 gave band gap values of 2.933 eV,2.898 eV,2.856 eV and 2.788 eV respectively,showing good semiconductor behaviour.The analysis of the crystal structures yielded bond angles of 82.76°,79.42°,79.28°and 78.41°for Sb-Cl-Sb of compounds 1-4,respectively.It was shown that the substitution of Br,Cl and F atoms in the organic cations led to a gradual decrease in the Sb-Cl-Sb bond angle of the inorganic anion[Sb₂Cl₉]^3-thereby achieving band gap modulation.2.In Chapter 4,in order to obtain high-temperature phase transition compounds,the BTAB organic ligand was used as the building block and the F atom was introduced at the meta-and para-position of the benzene ring to obtain 3-F-BTAB,4-F-BTAB ligands respectively.The three ligands were ligated with HBF₄ to give compounds（BTAB）BF₄（5）,（3-F-BTAB）BF₄（6）,and（4-F-BTAB）BF₄（7）.,respectively.Compounds 5,6 and 7crystallised in the P2₁/n,P2₁/m and P2₁/n space groups,respectively,and the phase transition temperatures were determined by differential scanning calorimetry（DSC）to be 410.4 K and366.0 K.The phase transition temperature of compound 7 was increased to 488.4 K due to the introduction of para-F atoms.UV-Vis diffuse reflectance spectroscopy tests demonstrated that compounds 5 and 7 had band gap values of 4.34 eV and 4.41 eV.In combination with the analysis of the crystal structure and Hirshfeld surface calculations,the domain-limiting effect induced by the substitution of para-F atoms in the organic cation increased the phase transition temperature by 78.0 K.The high-temperature phase transition compounds were successfully obtained.3.In Chapter 5,in order to obtain phase change materials with rich physical properties,the coordination reactions with HReO₄ using（4-X-BTAB）（X=F,Cl,Br）as the organic ligand yielded compounds（4-F-BTAB）ReO₄（8）,（4-Cl-BTAB）ReO₄（9）and（4-Br-BTAB）ReO₄（10）,and precise molecular design successfully induced compounds 8,9 and 10 to crystallize in the polar space group Pmn2₁,P2₁,P2₁,respectively.The second harmonic（SHG）response was enhanced from 1.08,1.47 to 3.23 times KDP due to the substitution of F,Cl and Br atoms corresponding to the increase in phase transition temperature of compounds 8,9 and 10 from 415.5 K,460.3 K to 474.4 K.The saturation polarization values P_s for compounds 8 and 9 were measured by electron hysteresis lines were 0.12μC cm^-2 and 0.34μC cm^-2,respectively,verifying the presence of ferroelectricity.The UV-Vis diffuse reflectance spectra measured band gap values of 3.304 eV,3.521 eV and 3.378 eV for compounds 8-10,respectively,exhibiting a wide band gap semiconductor behaviour.Combined with the analysis of the crystal structure and Hirshfeld surface calculations,the enhanced intermolecular C-H···O interactions caused by the substitution of F,Cl and Br atoms in the organic ligands are the main reason for the large increase in phase transition temperature and SHG signal.