New Materials with Switchable Magnetic and Electric Properties

Twenty one new Fe(III) complexes of the form [Fe(R-sal)2333]X are reported. Complexes of this type have shown an enhanced proportion of non-centrosymmetric crystallisation. The total block crystallises in non-centrosymmetric space groups 41% of the time, compared to 22% of structures in the Cambridge Structural Database. Magnetostructural studies were performed to study the magnetic switching behaviour, yielding eleven complexes stable in the low-spin state up to high temperatures, six complexes stable in the high-spin state and four undergoing thermal spin crossover. The relevance of a variation of amine N-H orientation is studied, with its effect being strongly correlated to the spin state of the material but not to the centrosymmetry of the crystalline product. Computational studies are reported which detail the calculation of polarisation magnitude and direction for 10 structures using the Modern Theory of Polarisation. A method for determining the true branch of the polarisation lattice for the calculation result was studied. This involved the use of a “sum of its parts” calculation on each unit of a unit cell and summing together to approximate the polarisation for the entire unit cell. Density Functional Perturbation Theory was used to predict the piezoelectric tensor of a compound, [Fe(3-OEt-sal)2333]ClO4·0.89MeCN. Piezoresponse Force Microscopy was employed to study the piezoelectric behaviour of a range of crystalline products. This involved vertical and lateral scans, along with measurements of the piezoelectric constants d33 and d15 by means of a Vac sweep. In-depth microscopy studies were undertaken on selected crystalline samples, chosen for their crystal morphology and facile polarisation direction. This included extensive lateral PFM on a high-spin Fe “232” complex [Fe(3-NO2-5-OMe-sal)2333]ClO4·MeOH, careful orientation of the crystals of high-spin Fe “333” complex [Fe(3-OEt-sal)2333]ClO4·0.89MeCN to measure directly both the d33 and d15 values, and LPFM on an Fe “333” complex [Fe(3-NO2-5-OMe-sal)2333]ClO4 that undergoes partial thermal SCO beginning at 300 K. Evidence for pyroelectricity was detected in both the latter samples. Thermal dependence of the d33 for [Fe(3-OEt-sal)2333]ClO4·0.89MeCN and the d15 value of [Fe(3-NO2-5-OMe-sal)2333]ClO4 was apparent, with the latter showing a stark diminution of the d15 value over the same temperature range as the SCO for this compound.