A number of erroneous reports about new HTSCs dur-Spinel solid solutions of composition Mg 2؊x Ti 1؉x O 4 have been ing the past few years, the apparent instability of the reprepared and investigated in the range 0 Յ x Յ 1. With increasported resistivity anomalies, and the fact that no cross ing titanium content, the lattice parameter of the cubic unit checking for superconductivity by magnetic measurements cell increases from a ؍ 844.00(3) pm to a maximum value of (Meissner or shielding effect) was performed by these 850.66(5) pm. Simultaneously, electrical properties evolve from groups give reason to regard the mentioned reports with insulating to low resistive semiconducting behavior. The resisa certain amount of suspicion. On the other hand, there tivity of samples with compositions x Յ 0.68 is thermally actiare some aspects which let this spinel system indeed appear vated with activation energies above 70 meV. The magnetic to be a potential candidate for high temperature superconproperties of the solid solutions are mainly determined by a ductivity.
temperature dependent magnetic moment of Ti 3؉ in an octahe-First, a common basic feature of oxide superconductors dral crystal field. Reports about high-T c superconductivity in the solid solution series cannot be confirmed. © 1996 Academic is that all are members (sometimes end members) of solid Press, Inc.
solutions which exhibit a metal/insulator transition upon compositional variation. Superconductivity generally occurs in the metallic regime close to the transition (11, 12).