Optical birefringence in the solid isotopic methanes

A sensitive method of measuring optical birefringence in crystals has been developed. The technique is used to measure birefringence quantitatively in the lowest temperature phase (phase III) of the three solid phases of the isotopic methanes. For the experiments, the methanes were each condensed in a thin glass cell mounted in a cryostat, and their optical properties were examined in the temperature region from 6°K to the melting points. Gross cracking of the solids, due to contraction during cooling, was not troublesome because the area of specimen that needed to be viewed was no more than 0.035 mm². The phases III of CD₄, CHD₃, and CH₃D were observed to have stable birefringences of magnitudes ${\text{3.8(± 0.4) × 10}}^{\text{−4}}{\text{,3.8(± 0.5) × 10}}^{\text{−4}}$ , and ${\text{1.7(± 0.4) × 10}}^{\text{−4}}$ , respectively. In contrast to this, the birefringence of CH₄ was found to be both time and temperature dependent and is interpreted as resulting from conversion between spin species of the molecule. The birefringence of phase III of CD₄ is consistent with the tetragonal structure tentatively assigned on the basis of recent x‐ray and neutron diffraction studies.