6LiInP2Se6 Single Crystals: Growth, Defect Structure, α-Particle Spectroscopy, and Direct-Conversion X-ray Response.

Room-temperature semiconductor crystals combining high resistivity, efficient charge transport, and a well-defined defect landscape are central to direct-conversion radiation detection. Here, we report the first demonstration of direct-conversion X-ray response in the layered selenophosphate 6LiInP2Se6 (LIPSe). Two crystal growth routes are advanced in parallel: chemical vapor transport (CVT), optimized for thin, high-purity flakes, and Bridgman-Stockbarger growth with iterative purification, scaled to centimeter-sized ingots. Both yield single crystals having clean (00l) surfaces and indistinguishable powder X-ray diffraction, Raman, and microscopic signatures, as well as matched band-edge alignment from photoelectron yield spectroscopy. Thermally stimulated current spectroscopy resolves trap levels spanning ∼0.13-0.68 eV, with estimated total trap densities of ∼1.2 × 1013 cm-3 (CVT) and ∼9.8 × 1014 cm-3 (Bridgman), while both crystals retain the high bulk resistivity required for room-temperature operation. Under 241Am irradiation, CVT detectors deliver well-resolved α-particle pulse-height spectra (∼22% energy resolution) with polarization-limited hole collection, while Bridgman detectors yield lower-bound single-carrier Hecht μτ products of ∼10-6-10-5 cm2 V-1 for both carriers. Under modulated X-ray exposure, sensitivities of ∼2.1 × 103 µC Gyair-1 cm-2 (CVT, ∼33 kV cm-1) and ∼6.5 × 103 µC Gyair-1 cm-2 (Bridgman, ∼22 kV cm-1) are achieved, where the larger Bridgman absorption volume offsets its trap-limited transport, identifying thickness scaling under continued defect control as the most direct path forward.