Negative Thermal Expansion of Mercurous Halides

Thermal expansions of single crystals of mercurous halides grown by the physical vapor transport method have been investigated. The unit cell lattice parameters were measured by using x-ray diffraction, whereas, the bulk thermal expansion was investigated using the dilatometer instrument as a function of temperature. It was found by both techniques that mercurous halides exhibit positive linear thermal expansion along the a -axis and negative linear thermal expansion along the c -axis. However, the volume expansion in both methods was found to be positive. The coefficient of thermal expansion along the a -axis was significantly larger than the one along the c -axis giving a net positive volume expansion.