Compressibility of Hexagonal Selenium by X-Ray and Neutron Diffraction

In order to resolve reported differences in the compressibility of hexagonal selenium, lattice parameters have been measured at pressures up to 140 kbar. Independent measurements have been made by neutron time-of-flight and by x-ray diffraction. The results from these methods are in good agreement. The a axis is found to contract, but the c axis expands with increasing pressure. Values obtained for the average volume, a-axis, and c-axis zero-pressure compressibilities are 49.7±15, 31.9±7, and −14.3±4×10−4 kbar−1, respectively. These results compare favorably with a correlation of bulk modulus and cohesive energy. The expansion of the c axis under pressure is comparable to the contraction of the c axis with increase of temperature.