EFFECT OF TEMPERATURE ON CHROMATICITY COORDINATES OVER A 700-DEGREES RANGE - A STUDY OF ALKALI-METAL URANATES

The diffuse reflectance spectra of the thermochromic alkali metal (Li, Na, K) mono- and diuranates have been recorded from room temperature down to nearly -200 degrees C, and additionally for the potassium uran ates up to 500 degrees C. The colours, ranging from yellow to red, wer e characterised using the CIELAB and CIELUV systems. An essentially li near relationship was found between chromaticity co-ordinates and temp erature, provided no physical or chemical reaction occurred. Thermochr omaticity has previously only been examined between ambient and 130 de grees C, but it is here established that the colours of thermochromic materials can be predicted outside the experimentally measured range. The solid state reaction xM(2)UO(4) + (x-1) H2O double left right arro w M2O (UO3)x + (2x - 2) MOH, where x > 1 and M not equal Li, was also followed by recording their diffuse reflectance spectra, at controlled humidity levels at room temperature. The reaction proceeds more readi ly as the radius of M increases, and the beta form of Na2UO4 was found to be more hygroscopic than the alpha form. (C) 1998 Elsevier Science Ltd. All rights reserved.