ELECTROMIGRATION OF HYDROGEN IN V, NB AND TA - A NEW DESCRIPTION OF THE MEASURED SAMPLE-RESISTIVITY-DEPENDENT EFFECTIVE VALENCE

Resistivity-dependent expressions for the direct force valency Z(d) are presented, starting from a recently proposed new theory for the driving force in electromigration. Using these expressions a good description is found for the measured effective valency of hydrogen in the transition metals V, Nb and Ta. Fits based on a constant Z(d) which are at present available led to the values 1.1, 0.44 and 1.23 for Z(d) in the three hosts in question. These values are not understood yet. According to the new expressions, Z(d) ranges between the bare-charge value of unity and the completely screened value of zero, which are limiting values obtained in the high- and low-resistivity limits, respectively. While the available experimental data are not able to discriminate between the description with a constant Z(d) and the new one with a resistivity-dependent Z(d), the latter can be considered as much more satisfactory. Regarding a long-standing controversy concerning the magnitude of the direct valency, it is concluded that the new theory in fact reconciles the two extremes involved.