We advert to the fact that presence of valence fluctuations (VFs) in semiconductors with in-gap impurity bands unconditionally leads to dynamical changes (fluctuations) of energies of localized impurity states. We provide arguments that in the impurity subnetwork consisting of centers having energy levels fluctuating around the Fermi energy there exist favorable conditions for hops from oc-

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We report about influence of external pressure on electrical resistivity of EuB 5.99 C 0.01 , the compound believed to be intrinsically inhomogeneous due to fluctuation of carbon content. Our results show that the low-temperature resistivity maximum shifts to lower temperature with applied pressure, opposite to the behavior reported for stoichiometric EuB 6. The origin of such qualitative difference we associate with the increasing volume fraction of the phase that is not compatible with ferromagnetic ordering (originating in regions with relatively higher carbon concentration) with enhancing pressure. Our results support a recent proposition [1] that carbon-rich regions strongly influence magnetotransport properties of carbon-doped EuB 6 , such as they play a role of "spacers", which prevent percolation of ferromagnetic phase.

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