18.7: Triglycine Sulphate
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This has the formula (NH2CH2COOH)3H2SO4, and variations on this have given some of the highest pyroelectric coefficients. The structure is shown below:
The glycine (NH2CH2COOH) groups are polar, but the most important is the glycine 1 group, as the reversal of the polarization of the material is associated with a rotation about the ‘a’ axis of this group. This changes the crystal into a mirror image of itself. In either state, below the Curie point, (~50oC) the crystal is point group 2, with a polar axis along the ‘b’ axis. Typically, the crystal is grown from aqueous solution, with whatever modifications are required. Modifications may include deuteration or more esoteric ideas such as substituting glycine for other amino acid style groups. These have altered properties such as Curie temp., or the thermal stability of the pyroelectric coefficient.
It has a pyroelectric coefficient of −5.5 × 10−4 C m−2 K−1, measured at 30oC. It is useful for the pyroelectric ‘vidicon’, a device used as a camera, for thermal imaging. This is used by disaster teams to find people trapped under rubble, etc.