Structure and superconductivity of the intercalation compounds of TiNCl with pyridine and alkali metals as intercalants

Abstract

Highly crystalline TiNCl with the FeOCl structure was prepared by chemical vapor transport, and intercalated with pyridine (Py) and alkali metals (A = Li, Na, K, and Rb). The structure of each intercalation compound was determined by the Rietveld refinement. Pyridine molecules were taken up to a composition Py_0.25TiNCl, and arranged with the molecular plane perpendicular to the TiNCl layers. The pyridine intercalated compound showed superconductivity at a transition temperature (T_c) of 8.6 K. The alkali metal intercalated compounds A_xTiNCl also became superconductors with much higher T_cs of ∼16.3 K. Upon intercalation the TiNCl crystalline layers are mutually shifted in such a way that the primitive cell was changed into different kinds of polytypes with centered cells, depending on the dimensions of the intercalants between the layers. The ab initio electrical band calculation suggested that the pyridine and alkali metal intercalated compounds have different density of states (DOS) profiles, and should have different characters in superconductivity.