Mixing Properties and Crystallization Behaviour of the Scheelite-Powellite Solid Solution

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• 作者：&Aacute ; ngeles Fern&aacute ; ndez-Gonz&aacute ; lez ; &Aacute ; ngel Andara ; Manuel Prieto
• 刊名：Crystal Growth & Design
• 出版年：2007
• 出版时间：March 2007
• 年：2007
• 卷：7
• 期：3
• 页码：545 - 552
• 全文大小：224K
• 年卷期：v.7,no.3(March 2007)
• ISSN：1528-7505

文摘

Different compositional members of the CaWO₄-CaMoO₄ (scheelite-powellite) solid solution were obtained usingprecipitation experiments and crystallization in a gel. The solids were characterized by X-ray diffraction (XRD), scanning electronmicroscopy-energy dispersive spectrometry (SEM-EDS), and electromagnetic pulse (EMP). XRD analyses of the crystals formedin the precipitation experiments show that the unit cell parameters (tetragonal, I4₁/a) vary with composition in a nonlinear way.Whereas the parameter a increases with the tungsten content, c decreases and the unit cell volume shows a minimum for intermediatecompositions. From these crystallographic data, a negative excess volume of mixing has been determined. The excess enthalpy ofmixing has been determined using solution calorimetry, resulting to be clearly negative for all compositions. The negative characterof both parameters points toward a nonideal solid-solution with a tendency to ordering. On the basis of these data, a Guggenheimexpansion series (a₀ = -1.422, a₁ = 0.213, and a₂ = -0.130) is proposed to describe the excess free energy of mixing. The solidphase activity coefficients obtained from this series have been used to calculate the Lippmann phase diagram and the equilibriumpartitioning for the system CaWO₄-CaMoO₄-H₂O. Finally, the crystallization behavior in gels and particularly the developmentof concentric compositional zoning of high gradient is explained on the basis of the proposed nonideal solid-solution model.