The superconductivity of Li-intercalated beta-HfNCl in the light of the HTSC cuprates

The layered chloronitride semiconductor -HfNCl when partially intercalated with organosolvated Li shows superconductivity at 26 K, a value striking enough to demand some comparison with (La,Sr) 2 CuO 4 , etc. As with d 8 , d 9 , d 10 , so between d 0 , d 1 , d 2 a sufficiently rapid shift occurs in the state level energies to allow a double-loading fluctuation to become resonant with E F . This is effected (as 2 Hf IV 2- ) within the manifestly heterogeneous environment of the Li-intercalated system. However, unlike the cuprate circumstance, there now occurs no strong negative- U ordering of on-site state energies to create any close analogue. Nonetheless, the much lower T c of the corresponding Zr (4d) material would indicate some non-phononic `chemical' interpretation for the superconductivity. A scenario more like that advocated earlier for the Rb 3 C 60 family of intercalated fullerenes looks more appropriate. This rests on disordered local homopolar bond behaviour, arising within the Hf-Hf array of the bisandwich -structure on intercalation.