10 January 2003 Color of red mercuric iodide at cryogenic temperatures
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We investigated a red to yellow color change observed in α-HgI2 when cooled below 150 K. A phase transformation to β-HgI2, which has a yellow color, was ruled out by a variable temperature x-ray diffraction study. Instead the color change at cryogenic temperatures is caused by a shift in the 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 temperature the gap was 2.11 ± 0.03 eV which is significantly smaller than the most recently published values of ~ 2.3 eV. This smaller band gap was further verified by measuring the thermoelectric current at elevated temperatures.
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Alan W. Hunt, Alan W. Hunt, Guido Campi, Guido Campi, Brendan Twamley, Brendan Twamley, Fred P. Vaccaro, Fred P. Vaccaro, Russell B. Tjossem, Russell B. Tjossem, Kelvin G. Lynn, Kelvin G. Lynn, } "Color of red mercuric iodide at cryogenic temperatures", Proc. SPIE 4784, X-Ray and Gamma-Ray Detectors and Applications IV, (10 January 2003); doi: 10.1117/12.450820; https://doi.org/10.1117/12.450820

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