Engineered materials form the foundation for the technologies used in modern society. Materials play crucial roles in microprocessors, energy conversion, displays, and high-temperature coatings, for example. The discovery of new materials is driven by fundamental research, with the goal of finding new phases exhibiting a desired set of properties or refining existing ones to create higher-efficiency solar cells or batteries with greater energy capacity. Materials synthesis and characterization stands as the foundation of this progress, and enhancing synthesis techniques represents a core endeavor that expands the realm of materials valuable for technological applications. This work examines the structure and properties of nitride materials and kagome superconductors using x-ray and neutron scattering and advances the floating zone crystal growth technique by developing a novel ultra high-pressure laser furnace.
MnSiN2 is a transition metal nitride where Mn and Si ions form an ordered cation distribution in a wurtzite-derived structure. Polycrystalline samples of the material were synthesized and examined using synchrotron x-ray and neutron diffraction. The nitride exhibits a Néel ordering temperature of TN = 443 K and a magnetic canting transition at Tcant = 433 K. The canted magnetic structure is resolved using refinements of the low temperature neutron diffraction data. Density functional theory calculations reveal that the nitrogen atoms act as σ− and π−donors, which enhances magnetic superexchange coupling and leads to the high Néel temperature in the compound.
The ternary nitride Ca3CrN3 hosts Cr3+ ions in a triangular nitrogen coordination. The S = 1/2 moments on the low-spin Cr-ions form a zig-zag spin chain and signatures of low-dimensional magnetism are observed in magnetic susceptibility measurements. The Bonner-Fisher model is used to fit the susceptibility feature and reveals a large intra-chain exchange constant of J = 337 K. Low temperature AC susceptibility measurements and powder neutron diffraction experiments exclude long-range magnetic order and identify the compound as a quasi-1D quantum spin chain. Floating zone crystal growth experiments were conducted in the zirconium and niobium nitride systems. Growth experiments of zirconium nitride under nitrogen pressures of pN2 = 35 bar produced samples of the cubic phase with a sodium chloridestructure and a superconducting critical temperature of Tc = 10 K. The crystal growth could not be stabilized for sufficient time to obtain a single grain. Further experiments focused on the niobium nitride system under pressures of pN2 = 300 bar. A single crystal of the nitrogen deficient tetragonal phase Nb4N3 with a Tc = 7 K was obtained. Higher nitrogen pressures could allow for crystal growth of stoichiometric niobium nitride and higher laser powers for longer growth durations in the zirconium nitride system. These results motivate further work advancing the floating zone technique. A novel ultra high-pressure laser floating zone furnace is presented. The Thermochemical High-pressure Optical Reactor (Thor) enables nitrogen gas pressures of up to p = 6900 bar combined with a maximum continuous-wave laser power of P = 3000 W. The furnace is commissioned by demonstrating the crystal growth of a Ruby gemstone. Thor is specifically designed to expand the floating zone crystal growth technique to nitride materials where decomposition previously prevented the growth of bulk single crystals.
The AV3Sb5 (A = K, Rb, Cs) family of kagome superconductors is studied using synchrotron x-ray diffraction. Refinements of the low temperature crystallographic structure are carried out to resolve the structural distortions caused by the charge density wave (CDW) transition. The compounds KV3Sb5 and RbV3Sb5 form a 2 × 2 × 2 supercell in space group Fmmm and host a staggered tri-hexagonal deformation of the vanadium layers. CsV3Sb5 displays a more complex structural evolution when cooled through the charge density wave transition. A staged progression of ordering from a 2 × 2 × 1 to a 2 × 2 × 2 into a final 2 × 2 × 4 supercell is found. The low temperature 2 × 2 × 4 supercell exhibits an average structure where vanadium layers display both tri-hexagonal and Star of David patterns of deformation. Finally, the competition between charge density wave order and superconductivity is studied in CsV3Sb5 by hole-doping the structure with Sn. The compounds in the CsV3Sb5−xSnx series show two ’domes’ in the superconducting transition temperature upon increasing Sn concentration. Single crystal x-ray diffraction measurements reveal a diminished periodicity of charge order along the interlayer direction upon low doping. Beyond the peak of the first superconducting dome, the parent charge density wave state vanishes and quasi-1D incommensurate charge correlations are stabilized. These unidirectional charge correlations demonstrate an inherent electronic rotational symmetry breaking in CsV3Sb5 and reveal a complex landscape of charge correlations within its electronic phase diagram.
