Crystallography and Defect Structure of Alkaline-Earth Hexaborides─CaB₆, SrB₆, and BaB₆─Doped with Lithium

We explore the effects of lithium doping on the structure of three alkaline-earth hexaborides─CaB₆, SrB₆, and BaB₆. Lithium incorporation was characterized using inductively coupled plasma mass spectrometry and X-ray diffraction analyses, demonstrating a systematic expansion of the cubic Pm3̅m lattice with increasing lithium concentration. Scanning and transmission electron microscopy revealed that lithium doping induces distinct surface pitting and localized lattice distortions. Solid-state ⁷Li nuclear magnetic resonance (NMR) spectroscopy provided deeper insight into the lithium coordination environments. One-dimensional NMR confirmed the ionic character of lithium and demonstrated the presence of minor side products, which were largely eliminated by acid treatment of the powders. Two-dimensional exchange spectroscopy NMR identified two distinct ionic lithium environments within the BaB₆ lattice. The absence of cross-peaks with side-product signals confirmed spatial separation and chemical stability of lattice-confined lithium. Our analyses establish a foundational understanding of alkali metal doping behavior in boron-rich ceramics and highlight the structural role of lithium as a dopant in hexaboride systems, supporting future investigations into how lithium incorporation may influence electronic properties.