  Polymorphism in materials often leads to significantly different physical
properties - the rutile and anatase polymorphs of TiO2 are a prime example.
Polytypism is a special type of polymorphism, occurring in layered materials
when the geometry of a repeating structural layer is maintained but the layer
stacking sequence of the overall crystal structure can be varied; SiC is an
example of a material with many polytypes. Although polymorphs can have
radically different physical properties, it is much rarer for polytypism to
impact physical properties in a dramatic fashion. Here we study the effects of
polytypism and polymorphism on the superconductivity of TaSe2, one of the
archetypal members of the large family of layered dichalcogenides. We show that
it is possible to access 2 stable polytypes and 2 stable polymorphs in the
TaSe2-xTex solid solution, and find that the 3R polytype shows a
superconducting transition temperature that is nearly 17 times higher than that
of the much more commonly found 2H polytype. The reason for this dramatic
change is not apparent, but we propose that it arises either from a remarkable
dependence of Tc on subtle differences in the characteristics of the single
layers present, or from a surprising effect of the layer stacking sequence on
electronic properties that instead are expected to be dominated by the
properties of a single layer in materials of this kind.
