Exploration And Characterization Of Several New Ferroic Materials Based On Molecules

According to the types of order parameters and the responses to the external field,ferroicity can be divided into ferroelectricity,ferromagnetism,ferroelasticity,etc.,and is widely used in magnets,storage,sensing,etc.The ferroicity-generation and multiferroic-coupling mechanisms have always been frontier subjects in condensed matter physics.Recently,molecule-based ferroelectrics and ferromagnets with excellent properties have been found in all-organic and organic-inorganic hybrid materials,which expands the exploration scope of ferroic materials.Organic molecules have wide variety,flexible structure,and are soft,low-cost,environment-friendly.Molecule-based ferroicity is easy to be further optimized by replacement and modification to obtain better properties.In addition,there may be new physical mechanisms in molecule-based ferroic materials.In this dissertation,the following progresses have been made in new molecule-based ferroic materials with pyroelectricity,magnetoelastic coupling and superlattice domains:1.Discovery of all-organic pyroelectric with high performance:amantadine formate（AF）.AF is a ferroelectric with a Curie temperature of 327 K.Unlike most ferroelectrics,the ferro-paraelectric phase transition of AF belongs to the second-order,which results in large pyroelectric coefficients in a wide temperature range near room temperature.The pyroelectric coefficient is–170μC/m²·K at 298 K.At the molecular level,the pyroelectricity of AF comes from the rotation of amantadine ions and the change of disorder degree of formate ions when the temperature changes.The dielectric constant（13.5@1k Hz）of AF is relatively small compared with other pyroelectrics,resulting in an ultra-high pyroelectric voltage figure of merit（0.705 m²/C）,higher than those of commercial pyroelectric materials such as triglycine sulfate,Li Ta O₃,and etc.Meanwhile,AF has the advantages of low density,low hardness and low cost,and has potential applications in flexible and high sensitivity pyroelectric detectors.2.Discovery of above-room-temperature coupling between magnetism and ferroelastic domains（CFM）in organic-inorganic hybrids.Organic-inorganic hybrids are usually softer than inorganic materials,and therefore have advantages in magnetoelastic coupling devices.Traditional CFM relies on the coexistence of ferroelastic and ferromagnetic order.In this dissertation,we proposed that paramagnetic anisotropy can couple with ferroelastic domains.By introducing molecule-based magnetic groups with large paramagnetic anisotropy,we successfully synthesized a new organic-inorganic hybrid material Cs Cu（HCOO）₂Cl that exhibits CFM at room temperature.Its magnetism below 331 K is related to the distribution of ferroelastic domains.This finding provides the first example that CFM can occur above room temperature in organic-inorganic hybrid materials.3.Two-dimensional organic-inorganic hybrid perovskites with complex magnetism were synthesized through a coordination-design strategy.The three-dimensional organic-metal formates show rich ferroic and multiferroic-coupling properties,attracting lots of attention in recent years.By partially replacing the formate ions in the metal coordination octahedrons with chloride ions,we synthesized a series of new layered perovskites:A_nCu（HCOO）₂Cl_n,（n=1,2）,where A are organic cations distributed between the layers.These compounds have complex magnetic transition at low temperatures.When A is pyrrolidine ions and n=2,the orientation of pyrrolidine ions will change from disorder to order at both 267 K and 256 K,resulting in ferroelasticity.This work not only shows a new layered perovskite system,but also provides a new route for dimensional regulation in perovskites.4.Superlattice domains induced by ordering of organic ions were found in organic-inorganic hybrid perovskite.Ferroic domains in crystals not only generate new physical phenomena,but also have been widely used in data storage and other fields.In this dissertation,we find that the orientations of pyrrolidine ions in two-dimensional perovskite（pyrrolidine）Cu（HCOO）₂Cl undergo disorder-order transition at 240 K,making the lattice form a5^1/2×5^1/2×1 superstructure.Two superlattice domains with different orientations appear in the crystal below 240 K,resulting from the breaking of translational symmetry.This work shows a new way to realize superlattice domains.Meanwhile,（pyrrolidine）Cu（HCOO）₂Cl has an ultra-high and reversible pyroelectric output at 240 K,and its piezoelectric coefficient changes the sign at 240 K,which make it have great potential in thermal-mechanical-electrical coupling devices.