Polytypism, polymorphism, and superconductivity in TaSe₂₋ₓTeₓ

Polymorphism in materials often leads to significantly different physical properties—the rutile and anatase polymorphs of TiO ₂ 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 TaSe ₂, one of the archetypal members of the large family of layered dichalcogenides. We show that it is possible to access two stable polytypes and two stable polymorphs in the TaSe ₂₋ₓTe ₓ solid solution and find that the 3R polytype shows a superconducting transition temperature that is between 6 and 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 T c 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 are typically expected to be dominated by the properties of a single layer in materials of this kind. Significance Although polymorphs of a substance can often have dramatically different physical properties, polytypes, which occur when the geometry of a structural layer is maintained but the number of layers in the layer-stacking sequence is changed, rarely do. Here we find, using random substitution of Te for some of the Se to induce structural changes in TaSe ₂, a classic layered dichalcogenide, so that the transition temperature to superconductivity ( T c) is significantly different for different polytypes and polymorphs and especially differs when going from one polytype to another. This observation implies either a surprising sensitivity of T c to the layer-stacking sequence or a similarly surprising sensitivity of T c to the small changes in layer geometry that accompany the change in polytype.