Phase Transitions And Dielectric Properties Of Small Molecular Phosphonium-And Ammonium-Based Compounds

Crystalline materials, such as dielectric, piezoelectric, pyroelectric, ferroelectric, and nonlinear optical crystals, have an extensive range of technological utilizations in capacitors, piezoelectric sensors, infrared detectors, laser frequency multipliers, and nonvolatile memories. According to the Curie symmetry principle and Neumann’s principle, electrical and optical properties of crystals are inseparable from symmetry. Only crystals belonging to the certain point groups could exhibit the particular electrical and optical properties. Taking ferroelectric property as an example, it exists only in the 10 polar point groups, that is,1 (C1), 2 (C2), m (C1h), mm2 (C2v),4 (C4),4mm (C4v),3 (C3),3m (C3v),6 (C6), and 6mm (C6v). Remarkably, phase transitions of crystals normally lead to changes in crystal structures and symmetries. Not only that, the physical properties including thermal properties, dielectric properties, second harmonic generation effects, and pyroelectric properties will also display significant anomalies in the vicinities of the phase transition temperatures. It is especially true for ferroelectric phase transitions. Accompanying the symmetry breaking from a paraelectric phase to a ferroelectric phase, the switchable spontaneous polarization appears and the temperature-dependent dielectric constant will more than double near the Curie temperature. In contrast to crystallographic analyses and ferroelectric characterizations, dielectric measurements are simpler and more convenient. Consequently, as a direct sign of phase transition, dielectric anomaly could be used in distinguishing phase transition compounds firstly, and then their ferroelectric properties should be further revealed. It is one of the most effective ways to obtain more novel ferroelectrics.In learning about how to design and construct molecular ferroelectrics, some simple organic ammonium cations, such as ammonium ([NH4]+), methylammonium ([MeNH3]+, Me =-CH3), dimethylammonium ([Me2NH2]+), trimethylammonium ([Me3NH]+), tetramethyl-ammonium ([Me4N]+), diisopropylammonium ([(Me2CH)2NH2]+), imidazolium, and pyridi-nium, attract our attention. This is because the small molecular moieties can easily experience motions or rotations triggered by temperature and thus result in structural phase transitions. Diverse compounds with zero-, one-, two-, or three-dimensional structures can be assembled by the simple ammonium cations and anionic building blocks, such as metal ions, ligands, and inorganic acids, benefiting from the structural tunability. Such compounds are also particularly useful in coupling ferroelectricity, antiferroelectricity, and ferromagnetism and thus become potential multifunctional materials. Inspired by this, since both P and N belong to Group V, phosphonium compounds also easily undergo structural phase transitions triggered by temperature similar to the ammonium analogues. However, in addition to the distinct valence shell electronic structures, the radius of P atom is also larger than that of N atom. Therefore, comparing with C-N bond length and C-N-C bond angle, C-P bond length is longer and C-P-C bond angle is smaller accordingly. These differences will affect the packing modes and phase transition behaviors of phosphonium compounds, being helpful to obtain new materials other than the ammonium compounds.On the basis of the above-mentioned analyses, several representative series of small molecular phosphonium- and ammonium-based compounds have been designed and synthesized. Herein, the crystal structures, phase transitions, and physical properties like dielectric properties of these compounds are studied and discussed. (Ⅰ) In Chapter Two, a series of switchable dielectrics were designed and constructed based on the N-heterocyclic ammonium cations, that is,1-propyl-l-methylpiperidinium perchlorate ([PMpip][C104],1), 1-cyanomethyl-l-methylpiperidinium perchlorate ([CMpip][C104],2), and 1-cyanomethyl-l-methylmorpholinium perchlorate ([CMmor][ClO4],3). Variations in the substituent group and ring structure of the cation result in distinct crystal structures and hydrogen-bonding conformations and thus cause different melt points, phase transition behaviors, and dielectric properties. (Ⅱ) In Chapter Three, tetraethylammonium halogenochromates [(CH3CH2)4N]-[ClCrO3] (4) and [(CH3CH2)4N][BrCrO3] (5), which exhibit reversible phase transitions and switchable dielectric properties above room temperature, and tetraalkylphosphonium chromates [(CH3)4P]2[Cr207] (6) and [(CH3CH2)4P]2[Cr3O10] (7), undergoing phase transitions below room temperature, were designed and presented to reveal the effects of ammonium and phosphonium cations on the crystal structures and physical properties. (Ⅲ) Chapter Four includes three tetraalkylphosphonium tetrahalogenoferrates:[(CH3)4P][FeCl4] (8), [(CH3)4P][FeBr4] (9), and [(CH3CH2)3PCH3][FeCl4] (10), which possess multiple switchable physical properties accompanying the sequential phase transitions above room temperature. Furthermore, all these compounds display weak antiferromagnetic interactions below room temperature, but only compound 9 undergoes a magnetic phase transition above room temperature. Modifications in the cation and anion have obvious influences on the phase transition temperatures as well physical properties. (IV) In Chapter Five, [(CH3CH2)4N]-[Cd(SCN)3] (11) and [(CH3CH2)4P][Cd(SCN)3] (12) having one-dimensional perovskite-like structures were designed and obtained. The two analogues, which exhibit switchable dielectric properties, undergo reversible phase transitions above room temperature. Notably, compound 11 crystallizes in a polar space group at room temperature and is preliminary identified as a ferroelectric by characterizing the ferroelectric domains. However, since the room temperature phase of compound 12 belongs to a nonpolar paraelectric phase, the possibility of high-temperature ferroelectricity can be excluded.