Phase Transition across Anisotropic NbS₃ and Direct Gap Semiconductor TiS₃ at Nominal Titanium Alloying Limit

Alloying selected layered transitional metal trichalcogenides (TMTCs) with unique chain-like structures offers the opportunities for structural, optical, and electrical engineering thus expands the regime of this class of pseudo-one-dimensional materials. Here, the novel phase transition in anisotropic Nb_{(1- x )} Ti_x S₃ alloys is demonstrated for the first time. Results show that Nb_{(1- x )} Ti_x S₃ can be fully alloyed across the entire composition range from triclinic-phase NbS₃ to monoclinic-phase TiS₃ . Surprisingly, incorporation of a small concentration of Ti (x ≈ 0.05-0.18) into NbS₃ host matrix is sufficient to induce triclinic to monoclinic transition. Theoretical studies suggest that Ti atoms effectively introduce hole doping, thus rapidly decreases the total energy of monoclinic phase and induces the phase transition. When alloyed, crystalline and optical anisotropy are largely preserved as evidenced by high resolution transmission electron microscopy and angle-resolved Raman spectroscopy. Further Raman measurements identify Raman modes to determine crystalline anisotropy direction and offer insights into the degree of anisotropy. Overall results introduce Nb_{(1- x )} Ti_x S₃ as a new and easy phase change material and mark the first phase engineering in anisotropic van der Waals (vdW) trichalcogenide systems for their potential applications in two-dimensional superconductivity, electronics, photonics, and information technologies.