| dc.description |
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)<sub>2</sub>CuO<sub>4</sub>, etc. As with d<sup>8</sup>, d<sup>9</sup>, d<sup>10</sup>, so between d<sup>0</sup>, d<sup>1</sup>, d<sup>2</sup> a sufficiently rapid shift occurs in the state level energies to allow a double-loading fluctuation to become resonant with E<sub>F</sub>. This is effected (as <sup>2</sup>Hf<sub>IV</sub><sup>2-</sup>) 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<sub>c</sub> 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<sub>3</sub>C<sub>60</sub> 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. |
