Mixed Trichlorocuprates

This dissertation details the synthesis and characterization of a new multiferroic family, (NH4)xK(1-x)CuCl3, where (0 <= x <= 1.00). The incentive for investigation of this family arose from the need for a set of unique multiferroic materials, and this avenue had not yet been explored. The novelty of this compound comes from two angles. First was the potential for ferroelectricity, which often comes from the ammonium ion. Second is the unique magnetic behavior of both NH4CuCl3 (ACC) and KCuCl3 (KCC), both of which are different despite the fact that they are isomorphous at room temperature. Given the subtleties that crystallographic structure hold in both the dielectric and magnetic properties, it seemed well worth investigating this family not only for the dielectric studies, but also to answer some longstanding structural questions as well. Answers provided by this dissertation can be closely related to the study of multiferroic materials. Thus, the focus of chapter one is on the need for new multiferroic materials and various approaches used to overcome physical limitations that have been set by nature. But, a careful and systematic study of this new family of potential multiferroics required the use of many techniques, which includes X-ray crystallography, mass spectroscopy, heat capacity, dielectric relaxation, pyroelectric current measurements, electron paramagnetic resonance (EPR), magnetic susceptibility and pulsed-field magnetization at millikelvin temperatures. The principles and operations for some of instruments will be shown in chapters two and three. Again, the project was initially undertaken with the goal of finding a ferroelectric transition in ACC. This was suspected because hydrogen bonding is often a mechanism for the formation of the net dipole moment necessary for a ferroelectric transition. As shown in chapter four, a lambda-type anomaly was indeed observed at 67 K, with subsequent pyroelectric measurements confirming our original conjecture. Heat capacity measurements also verified the presence of the transition with an anomaly at the same temperature. Afterwards, deuteration of the ammonium ion showed an increase in the transition temperature, suggesting that hydrogen bonding may play a role in the underlying mechanism. Finally, promising millikelvin capacitance measurements revealed the possibility of magnetoelectric coupling near the well known antiferromagnetic ordering temperature. All of these observations related to the phase transition of ACC are delineated in chapter four. Although not readily related to the dielectric phase transition, chapter five describes some EPR measurements taken at variable temperatures and orientations. Now that it was shown the ammonium ion plays a role in the ferroelectric transition, it seemed desirable to tune the transition temperature. In chapter six, it will be shown that this was achieved not only by deuteration, but through substitution of the ammonium ion with ce{K^+}, which in terms of ionic radius and charge was expected to freely substitute itself into the ammonium position. With a 10% doping level, the transition greatly broadened with a FWHM of 25 K and shifted down to 42 K. In addition, a dispersion peak developed at 28 K. All of these observations suggest that the transition is extremely sensitive to the doping concentration. The impetus of the final chapter arose from my secondary interest in the magnetic properties of the trichlorocuprate family, especially with regards to K+ and NH4+. Several literature studies had already been made by doping with thallium, i.e. KxTl(1-x)CuCl3, but none with Kx(NH4)(1-x)CuCl3. Study of the magnetic susceptibility and magnetization with variation of the ammonium concentration led to the finding of definite trends with the energy gaps. Random dimers and Bose Glasses may serve as a good models for the behavior of the mixed compounds.