system*

We report a combined experimental and theoretical study of the PdSe_2−xTe_x system.  With increasing Te fraction, structural evolutions, first from an orthorhombic phase (space group Pbca) to a new monoclinic phase (space group C2/c) and then to a trigonal phase (space group P−3m1), are observed accompanied with clearly distinct electrical transport behaviors. This new monoclinic phase (C2/c) belongs to a very rare Verbeekite polymorphism and is discovered within a narrow range of Te composition (0.3≤x≤0.8). Interestingly, in the trigonal phase region, superconductivity with significantly enhanced critical temperature is observed within a narrow range of Te content (1.0≤x≤1.2) in comparison with that of 1T-PdTe₂ phase. The superconductivity has been unambiguously confirmed by the magnetization, resistivity, and specific heat measurements. The detailed electrical transport analysis and electronic calculations across the rich phase diagram will be presented. The unusual superconductivity properties, the new polymorphic structure, and the rich phase diagram in this system could further stimulate interest to explore the new superconducting mechanism and new types of polymorphs in the large family of transition metal dichalcogenides.