It was found that all of the group-IIa metal sulfides (figure 4) exhibited predominantly ionic conductivity. A comparison of the measured activation energies for these sulfides, with those for other alkaline-earth sulfides and oxides, indicated that the predominant charge carriers were metal cations in the ionically conductive sulfides. At higher pressures, the conductivities of MgS and BeS were linearly dependent upon the S partial pressure; leading to a slope of 1/6 in logarithmic plots. It was deduced that these sulfides were p-type conductors within that pressure range. A theoretical analysis showed that fully ionized metal vacancies were the predominant defects. In the case of Sc2S3 and Y2S3, this slope was equal to -3/16; thus indicating a predominance of Sc and Y interstitial ions. At higher pressures, Y2S3 also exhibited a 3/16 dependence; thus indicating a p-type conduction that was due to fully ionized Y vacancies.

H.Nakamura, Y.Ogawa, A.Kasahara, S.Iwasaki: Materials Transactions, 1995, 36[10], 1263-70