  In a recent experiment, Siegrist et al. [Nature Materials 10, 202 (2011)]
investigated the metal-insulator transition (MIT) of GeSb_2Te_4 on increasing
annealing temperature. The authors conclude that this material exhibits a
discontinuous MIT with a finite minimum metallic conductivity. The striking
contrast to reports on other disordered substances motivates the present
in-depth study of the influence of the MIT criterion used on the
characterization of the MIT. First, we discuss in detail the inherent biases of
the various available approaches to locating the MIT. Second, reanalyzing the
GeSb_2Te_4 measurements, we show that this material resembles other disordered
solids to a large extent: according to a widely-used approach, these data may
also be interpreted in terms of a continuous MIT. Checking the justification of
the respective fits, however, uncovers inconsistencies in the experimental
data. Third, comparing with previous experimental studies of crystalline Si:As,
Si:P, Si:B, Ge:Ga, CdSe:In, n-Cd_{0.95}Mn$_{0.05}Se,
Cd_{0.95}Mn_{0.05}Te_{0.97}Se_{0.03}:In, disordered Gd, and nano-granular Pt-C,
we show that such an inconclusive behavior occurs frequently: the analysis of
the logarithmic temperature derivative of the conductivity highlights serious
inconsistencies in the original interpretations in terms of a continuous MIT.
In part, they are common to all these studies and seem to be generic, in part,
they vary from experiment to experiment and may arise from measurement
problems. Thus, the question for the character of the MIT of these materials
has to be considered as yet open. The challenges now lie in improving the
measurement precision and in developing a microscopic theory capable of
explaining the seemingly generic features.
