Anomalous thermal expansion of Pb-Te system semiconductors

The thermal expansion properties of sintered samples of PbTe doped with 0.5 mass% PbI 2 and those of Pb 0.5 Sn 0.5 Te were evaluated by two different methods such as: (1) by determining the apparent expansion coefficient obtained from dimensional changes and (2) by estimating the crystal lattice expansion coefficient calculated from the crystal lattice spacing. Abnormally large values of the apparent expansion coefficients were observed only for samples prepared from fine powders with a particle size of 25 μ m or less at high temperatures above 400 ° C , while such large values were not obtained for other samples in that temperature range. No significant differences were observed between the lattice expansion coefficients of the samples prepared from the fine powders and those of the other samples. Thermogravimetric analysis revealed that the abnormally large expansion developed in the crystal grain boundaries. The grain boundary effects were estimated by comparing the apparent expansion coefficients and the lattice expansion coefficients. Some substances were found to be deposited in the apparatus during the experiment, and they were identified as compounds that including tellurium by the x-ray diffraction analysis. It was concluded that this curious expansion phenomenon was caused by decomposition and sublimation of tellurium from the telluride samples at the high temperature range.