Absorption measurements
in the region of 2200 A to 10,000 A were made on optically polished thoria crystals of 99.9 per cent purity. Firing the crystals in air and in vacuum at different temperatures gave the red, white and yellow forms described by Weinreich and Dzmforth.' The present improved instrumentation gave a cut-off at 3550 A for all forms.
Vacuum firing at high temperature (1.500") gives a yellowish form. The white form is produced by lower temperatures and/or short times. Results will be reported regarding the progressive change from white to yellow forms as the temperature and time of firing are increased. Thin films prepared by oxidizing films of metallic thorium show rapid increase of absorption at 3550 A.
Since the white state of thoria shows no absorption band, it seems to be the stoichiometric form of thorium oxide, with an optical intrinsic gap at 3.5 ev. The change to the red form seems attributable to thorium vacancies, since the oxygen ion is apparently too large to enter into interstitial sites. The absorption band at 4050 A (3.0) ev. in the yellow crystals is presumably due to oxygen vacancies or to interstitial thorium.