  We report a combined experimental and theoretical study of the PdSe2-xTex
system. With increasing Te fraction, structural evolutions, first from an
orthorhombic phase (space group Pbca) to a monoclinic phase (space group C2/c)
and then to a trigonal phase (space group P-3m1), are observed accompanied with
clearly distinct electrical transport behavior. The monoclinic phase (C2/c) is
a completely new polymorphic phase and is discovered within a narrow range of
Te composition (0.3 \leq x \leq 0.8). This phase has a different packing
sequence from all known transition metal dichalcogenides to date. Electronic
calculations and detailed transport analysis of the new polymorphic
PdSe1.3Te0.7 phase are presented. In the trigonal phase region,
superconductivity with enhanced critical temperature is also observed within a
narrow range of Te content (1.0 \leq x \leq 1.2). The rich phase diagram, new
polymorphic structure as well as abnormally enhanced superconductivity could
further stimulate more interest to explore new types of polymorphs and
investigate their transport and electronic properties in the transition metal
dichalcogenides family that are of significant interest.
