Structure-cell data and expansion coefficients of bismuth telluride

Accurate measurements of the structure-cell parameters of bismuth telluride, Bi 2 Te 3 , have been made using annealed powders and etched rods cut from single-crystal ingots. Previously reported anomalies in the X-ray powder intensity data are found to originate entirely in preferred orientation of the crystallites. A study of the thermal expansion of the unit cell over the temperature range -195 to 400° C has revealed a marked anisotropy between the axial expansion coefficients, that for the c axis (22.2 × 10 -6 per °C at 20° C) being almost twice that for the a axis (12.9 × 10 -6 ). Near 200° C changes occur in both the principal axial expansions, comparable with those observed at low temperatures in certain hexagonal metal structures.