Resistivity and Hall measurements were carried out on crystals in the temperature range 300°–60°K. Single crystals were grown by a modified Bridgman technique from stoichiometric melts and from melts containing 1, 2, 10, and 18 at. % excess Sn and 3 at. % excess Mg. In the mixed conduction and extrinsic ranges, it was found possible to describe the calculated temperature dependence of the conduction mobility on the basis of a combination of lattice vibration (acoustic mode) and ionized impurity scattering. A stoichiometry effect is observed. The crystals appear to dissolve the excess constituent producing n‐type crystals (excess Sn) and p‐type crystals (excess Mg). The observed effect corresponds to a solubility of 10−5‐10−6 action excess constituent. This result is briefly discussed in terms of bonding in .