Structural phase transformations in crystalline gallium orthophosphate

Structural changes proceeding in gallium orthophosphate crystals under ther mal treatment have been studied by means of thermoanalytical methods and X- ray diffraction. The samples were characterized with respect to their water content by means of IR absorption. The occurrence of structural changes ha s been investigated as a function of thermal history, annealing temperature , annealing duration, and cooling rate. Freshly crystallized material shows always the low-quartz analogue modification, During the first heating a tr ansformation into the high-cristobalite form proceeds at about 970 degrees C. This structural change is connected with a weight loss and the emanation of vapor phase species, both indicating the release of water from the samp les. Cooling down the high-cristobalite form can lead either to the metasta ble low-cristobalite or the thermodynamically stable low-quartz modificatio n. The cooling rate is the deciding factor for the occurrence of one or the other phase. The stability range of crystals originally in the low-cristob alite form has been studied by various annealing procedures. It is conclude d that the low-quartz structure is thermodynamically stable and can be deli berately obtained between room temperature and about 930 degrees C, while t he high-cristobalite modification is stable at higher temperatures. The low -cristobalite form, observed at room temperature nearly without any excepti on after a first high-temperature treatment, is only metastable. Its occurr ence can be suppressed by proper thermal processing. (C) 2000 Academic Pres s.