Abstract
Layered β-HfNCl was electron-doped by the intercalation of alkaline earth metals AE (AE = Ca, Sr, Ba) using liquid ammonia solutions. The metals are cointercalated with ammonia, and the intercalation compounds AEx(NH3)yHfNCl have basal spacings almost constant at ∼12 Å, irrespective of the kind of AE used. The divalent AE concentration was widely varied over a range of x = 0.09–0.40, corresponding to an electron-doping concentration of twice as much as this value. The intercalation compounds become superconductors. The transition temperature Tc is almost constant at 23–24 K over the wide range of x with different kinds of AE. The superconductor is not yet over-doped at x = 0.4. The basal spacing d of the intercalated compound is further increased up to ∼19 Å by swelling with voluminous tetrahydrofuran (THF) and propylene carbonate (PC) molecules. The highest Tc of 26.0 K was obtained on the Ca and THF cointercalated compound Ca0.11(THF)yHfNCl (d = 15.0 Å), a new record of Tc in the electron-doped metal nitride chloride system.
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