Path Less Traveled: A Contemporary Twist on Synthesis and Traditional Structure Solution of Metastable LiNi 12 B 8 .

Achieving kinetic control to synthesize metastable compounds is a challenging task, especially in solid-state reactions where the diffusion is slow. Another challenge is the unambiguous crystal structure determination for metastable compounds when high-quality single crystals suitable for single-crystal X-ray diffraction are inaccessible. In this work, we report an unconventional means of synthesis and an effective strategy to solve the crystal structure of an unprecedented metastable compound LiNi 12 B 8 . This compound can only be produced upon heating a metastable layered boride, HT -Li 0.4 NiB ( HT : high temperature), in a sealed niobium container. A conventional heating and annealing of elements do not yield the title compound, which is consistent with the metastable nature of LiNi 12 B 8 . The process to crystallize this compound is sensitive to the annealing temperature and dwelling time, a testament to the complex kinetics involved in the formation of the product. The unavailability of crystals suitable for single-crystal X-ray diffraction experiments prompted solving the crystal structure from high-resolution synchrotron powder X-ray diffraction data. This compound crystallizes in a new structure type with space group I 4 /mmm ( a = 10.55673(9) Å, c = 10.00982(8) Å, V = 1115.54(3) Å 3 , Z = 6). The resulting complex crystal structure of LiNi 12 B 8 is confirmed by scanning transmission electron microscopy and solid-state 11 B and 7 Li NMR spectroscopy analyses. The extended Ni framework with Li/Ni disorder in its crystal structure resulted in the spin-glass or cluster glass type magnetic ordering below 24 K. This report illustrates a "contemporary twist" to traditional methodologies toward synthesizing a metastable compound and provides a recipe for solving structures by combining the complementary characterization techniques in the cases where the traditionally used single-crystal X-ray diffraction method is nonapplicable.