Single crystal growth of high entropy ternary phosphides and parent phases
Open Access
- Author:
- Iwabuchi, Yasuyuki
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- February 18, 2025
- Committee Members:
- Venkatraman Gopalan, Thesis Advisor/Co-Advisor
Jon-Paul Maria, Committee Member
John Mauro, Program Head/Chair
Zhiqiang Mao, Thesis Advisor/Co-Advisor - Keywords:
- High entropy materials
Nonlinear optics
Single crystal growth
Phosphides
Second harmonic generation - Abstract:
- This thesis focuses on the development of novel nonlinear optical (NLO) materials through high entropy engineering and the exploration of scalable crystal growth techniques for MgSiP2 and related systems. In Chapter 2, high entropy engineering was applied to ternary phosphides based on MgSiP2, including (Mg, Zn, Cd, Mn)SiP2 (A4SiP2) and (Mg, Ca, Sr, Ba, Zn)SiP2 (A5SiP2). These systems were successfully synthesized using the Sb-flux method, with A4SiP2 retaining space group I4̅2d, while A5SiP2 crystallized in a new space group I4̅2m with multiple high entropy sites. This structural transformation was driven by the inclusion of large atoms such as Sr and Ba, causing significant unit cell distortion. Flux material was also found to influence crystallization, as substituting Sb with Sn led to entirely different structures, demonstrating the critical role of flux in high entropy materials growth. Furthermore, starting compositions were shown to affect the resulting crystal compositions and optical bandgaps, with A4SiP2 systems displaying tunable bandgaps dependent on Mg content. Beyond these systems, exploratory work identified combinations of elements that could or could not form high entropy chalcopyrite phases. This chapter highlights high entropy engineering in ternary phosphides, aiming to stabilize unconventional phases, fine-tune physical properties, and explore novel strategies for advanced material design. In Chapter 3, the synthesizability of potential parent phases for high entropy systems was examined. SrSiP2 and EuSiP2 were successfully synthesized and exhibited a plate-like morphology, suggesting they do not have space group I4̅2d. However, CaSiP₂ and BaSiP₂ could not be stabilized in the Sb-flux system. We demonstrated the potential for EuSiP2 to exhibit ferromagnetic property. Both SrSiP2 and EuSiP2 were highly air-sensitive, which hindered detailed structural and property characterization. These findings indicate that the Sr, Ba, Ca, and Eu elements in high-entropy A5SiP₂ systems are stabilized by entropy. In Chapter 4, scalable crystal growth techniques for MgSiP2 were investigated to overcome the challenges of small crystal sizes. Thermal stability analysis revealed that MgSiP2 decomposes at 800 °C and melts near 1340 °C, rendering traditional melt growth impractical. Flux growth optimization, including adjustments to cooling rate and raw material quantity, showed limited success in growing larger crystals. Flux-vertical Bridgman growth produced polycrystalline MgSiP2 with improved homogeneity through modifications of sample preparation and setup, but challenges such as quartz tube rupture and polycrystallinity persisted. Although polycrystals were obtained in this study, the potential to achieve single crystals of MgSiP2 remains feasible with improved experimental conditions, making this approach the most promising method. Horizontal flux growth was also explored, but primarily resulted in SiP and only minimal MgSiP2. This method was deemed unsuitable for single-crystal growth of MgSiP2. In summary, we have synthesized the first high entropy ternary phosphides. This research highlights the effectiveness of high entropy engineering in stabilizing novel ternary phosphides. Additionally, this study advanced scalable crystal growth techniques for MgSiP2, with flux-vertical Bridgman growth emerging as the potential method for achieving large, high-quality MgSiP2 single crystals, contingent on further optimization and precise control of growth conditions. This work paves the way for the development of novel material synthesis methods utilizing high entropy engineering and broadening our insights into the synthesis of single crystals of phosphides.