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

The layered chloronitride semiconductor β-HfNCl when partially intercalated with organosolvated Li shows superconductivity at 26 K, a value striking enough to demand some comparison with (La,Sr)2CuO4, etc. As with d8, d9, d10, so between d0, d1, d2 a sufficiently rapid shift occurs in the state level energies to allow a double-loading fluctuation to become resonant with EF. This is effected (as 2HfIV2-) 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 Tc 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 Rb3C60 family of intercalated fullerenes looks more appropriate. This rests on disordered local homopolar bond behaviour, arising within the Hf-Hf array of the bisandwich β-structure on intercalation. (author)