UC Davis

This thesis describes how I used the molten-hydroxide flux method of single crystalsynthesis to synthesize some of the largest reported single crystals of KNi4S2 (142), KNi2S2 (122), and K2Ni3S4 (234). This molten-hydroxide flux method had previously never been implemented in our lab and, with my optimizations, could open up a previously inaccessible phase-space of meta-stable compounds for our group to explore. I also report the first-known magnetization, resistivity, and heat-capacity data for KNi4S2. Although the KNi4S2 phase is isostructural to cuprate superconductors, we do not observe signs of superconductivity nor antiferromagnetic order down to T = 2 K. Instead, our magnetization measurements suggest that KNi4S2 may be a weak itinerant ferromagnet (TC ≈ 25 K). Growing large single crystals allowed us to study magnetic anisotropy, and I found that with KNi4S2, the a and c appear to be isotropic while b seems to be the easy magnetic direction. Resistivity measurements from T = 1.8 − 300 K show the 142 phase has a resistivity similar to a bad metal. While I measure a relatively high residual resistivity ratio (RRR) of 44 demonstrating good sample quality, no phase transition is observed in either resistivity or heat capacity measurements. This may be consistent with the weak itinerant nature of the magnetism, although we cannot exclude the possibility of an impurity origin of the ferromagnetic transition. In addition to the 142 phase, I discuss the synthesis and present x-ray diffraction (XRD) patterns and magnetization data for the 122 and 234 phases.