Anhydrous Zinc Borate as a Host Crystal in Luminescence*

Abstract

The best results for preparing Zinc Borate luminophors are obtained by thermal treatment from melts of ZnO: B2O3 in proper molar proportions. Compositions between 50% and 61% ZnO give transparent glasses, which yield separable cubic crystals by devitrification. A compound of molar relation 4 ZnO: 3 B2OS3 not reported in the literature is deduced from a re-examination of the chemical and structural constitution of these crystals. This compound—more properly expressed OZn4(BO2)6—possesses a crystalline structure of the Sodalite (ultramarine) type. Each unit cell contains two molecules of metaborate. This crystal shows a violet emission band under 2537 A and cathode ray excitation. When the molar ratio is 2: 1 (70% ZnO), or higher, the devitrification gives no single crystal, but a product of yellow luminescence under 2537 Å excitation consisting of a glassy phase and crystals which are not cubic. Preparations of intermediate composition show the presence of both phases (cubic and non-cubic) each one associated with its luminescent band. Upon heating a noncubic Zinc Borate in hydrogen part of the Zinc is lost by reduction and subsequent volatilization, thus incurring a change to the cubic structure as a consequence of the composition change. The Manganese has a valence of 2 +, when incorporated to the cubic crystals, and gives rise to a strong green emission whereas the resulting luminescence in the non-cubic phase is orange and the valence is greater than 2+. No direct influence of the firing atmosphere on the valency of the activator has been found in the preparation of such products. When the orange-emitting phosphor is heated in a reducing atmosphere, however, the structural change from non-cubic to cubic is always associated wit the change of the Mn emission band (orange→green) and a simultaneous valency conversion Mn> 2+→Mn2+. As, Sb, and Bi activated cubic Zinc Borates also create well defined emission bands. Arsenic incorporates as As5+, antimony as Sb3+ and Sb5+, whereas bismuth as Bi3+. A speculation is made for the explanation of these effects.