Preparation, phase transition and thermal expansion studies on low-cristobalite type Al1−xGaxPO4 (x=0.0, 0.20, 0.50, 0.80 and 1.00) |
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Authors: | SN Achary AK Tyagi SK Kulshresththa |
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Institution: | a Applied Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, Indiab Novel Materials and Structural Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India |
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Abstract: | The orthorhombic (α) low-cristobalite type AlPO4 and GaPO4 and their solid solutions are prepared by co-precipitation followed by high temperature annealing of the precipitate. The single phasic nature of the products is ascertained by powder XRD at room temperature. The high temperature behavior of these samples is studied by HT-XRD over the temperature range of 25-1000°C. All these compositions undergo an orthorhombic to cubic (β, high-cristobalite) phase transition at elevated temperature. The unit cell parameters at different temperatures are determined by refining the observed powder diffraction profiles. The phase transition is accompanied by a significant increase in the unit cell volume, leading to the formation of a low dense structure. The variation of unit cell volume with temperature for each composition shows that the orthorhombic phase has a significantly larger thermal expansion than the cubic (high temperature) phase. The high temperature behavior of all the compositions except the GaPO4 is similar. GaPO4 undergoes a phase separation to a more stable quartz type phase above 800°C. However, the quartz type phase again transforms to the high cristobalite (β) phase at 1000°C. Thermal expansions of all these phases are explained in term of the variation of M-O-P angle as a function of temperature. |
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Keywords: | Phase transition Thermal expansion Phosphates Cristobalite High temperature XRD |
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