Color of red mercuric iodide at cryogenic temperatures

We investigated a red to yellow color change observed in alpha -HgI2 when c ooled below 150 K. A phase transformation to beta -HgI2, which has a yellow color, was ruled out by a variable temperature x-ray diffraction study. In stead the color change at cryogenic temperatures is caused by a shift in th e transmission edge to shorter wavelengths which we attribute to a widening band gap at low temperatures. Using optical transmission spectroscopy the width of the band gap was measured between 10 and 330 K. At room temperatur e the gap was 2.11 +/-0.03 eV, which is significantly smaller than the most recently published values of similar to2.3 eV. This smaller band gap was f urther verified by measuring the thermoelectric current at elevated tempera tures. (C) 2001 American Institute of Physics.