Thermochromism of HgI<sub>2</sub> - METTLER TOLEDO

Thermochromism of HgI2

Introduction

Thermochromism is the term used to describe a reversible change in the color of a substance as its temperature is varied. It is a property often encountered in both inorganic and organic compounds. The phenomenon has to do with temperature-dependent structural phase transitions in the substance whereby the orbitals of electrons that absorb light in the visible wavelength region of the spectrum are deformed.

This causes the absorption and reflection properties of a substance (and hence its color) to change. Most inorganic compounds of the light elements up to calcium are white.

The reason for this is that atomic distances are short and the wavelength range in which absorption of light takes place (and hence “colors” occur) is in the ultraviolet region. This region is of course not accessible to the human eye. Colored compounds are therefore not encountered until they contain elements in which the d-orbitals are increasingly occupied; for example ZnS is white, CdS yellow and HgS black. Thermochromic compounds are often used as optical temperature indicators where the colors distinguish between different temperature ranges.

 

Experimental Details

DSC is an excellent method for detecting phase transitions resulting from structural changes in a substance.

To observe color changes occurring at the same time, the sample is viewed and recorded with a suitable camera and software. This is now very easy to do with the HP DSC827e : a microscope objective is mounted on a special quartz glass cover and the sample monitored with a CCD camera. The setup is shown schematically in Figure 1.

Schematic diagram of the HP DSC827e with the accessory for reflected light microscopy 

The experiments described in this article were performed with a JVC (KY-F55B) color CCD camera and the Soft Imaging Systems GmbH “GrabIt” frame capture card. AnalySIS software (also from Soft Imaging Systems GmbH) was used for image capture and analysis.

 

Results 

It is well known that mercury(II) iodide can exist in two different crystalline forms. The change in structure that occurs when the substance is heated is accompanied by a color change from red to yellow. Figure 2 shows images of the sample taken at four different temperatures during a DSC heating run. The powdered sample (22.325 mg) was evenly distributed in a 40-µL aluminum crucible.

The images show behavior typical for solid-solid transitions of powders: each crystalline particle undergoes the phase transition individually on heating and on cooling. The structural change associated with the change in color gives rise to an endothermic peak in the DSC curve (Figure 3). 

The color change can also be quantitatively evaluated from the mean image intensity, which corresponds to the mean value of the red, blue, and yellow color channels. The mean image intensity should be approximately proportional to the partial areas of the phase transition peak (i.e. proportional to the “conversion” of the phase transition peak). This comparison is made in Figure 4 and shows that the two curves are in excellent agreement.

Conclusions

A simple microscope accessory allows the appearance of a sample to be observed and recorded while it is measured in an HP DSC827e . The technique measures the calorimetric behavior of the sample (DSC) and at the same time provides information on changes in color, shrinkage behavior, and flow or crystallization processes. This is extremely helpful for the reliable interpretation of a DSC curve. Measurements of HgI2 as an example showed that a solid-solid transition occurs at about 140 °C with an accompanying change in color from red to yellow. These and other measurements (not described here) showed that the individual crystals undergo this transition both on heating and on cooling. 

Thermochromism of HgI2  | Thermal Analysis Application No. UC 266 | Application published in METTLER TOLEDO Thermal Analysis UserCom 26