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首页> 外文期刊>Journal of Alloys and Compounds: An Interdisciplinary Journal of Materials Science and Solid-state Chemistry and Physics >On the thermodynamics of the rhombohedral-cubic phase transition of thulium(III)oxide fluoride
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On the thermodynamics of the rhombohedral-cubic phase transition of thulium(III)oxide fluoride

机译：氧化三氟化fluoride菱形-立方相变的热力学

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摘要

The rhombohedral(beta)-->cubic(alpha) phase transition of stoichiometric TmOF and the phase behaviour of samples of the analytical composition Tm(O,F), with 1.85 less than or equal to y less than or equal to 2.10 were studied by quantitative differential thermal analysis (DTA) and by X-ray powder diffraction analysis. It was found that the beta-->alpha transition occurs at 808+/-3 K with an increment of the formula volume of 0.5+/-0.05 Angstrom(3), and that alpha-TmOF decomposes in a peritectoid reaction at ca. 915 K into a fluorite-related, presumably orthorhombic phase of composition Tm(O,F)(1.93+/-0.02) and an orthorhombic vernier-type phase of composition Tm9O8F11 with a=5.3765(5) Angstrom, b=49.402(5) Angstrom and c=5.4607(4) Angstrom. Tm(O,F)(1.93) is formed by a eutectoid reaction of C-Tm2O3 with alpha-TmOF at ca. 900 K, An accurate determination of the enthalpy of the beta-->alpha transition of TmOF was found to be impossible because of the proximity of the temperatures of transition and decomposition. Therefore the enthalpy of transition was determined indirectly by measuring the temperatures and enthalpies of transition of solid solutions Tm1-xCaxO1-xF1+x with 0.017 less than or equal to x less than or equal to 0.062 by differential scanning calorimetry (DSC). It was found that both, T-tr and Delta(tr)H(T(tr))(0), depend linearily on the CaF2 content of the solid solutions, and that the decomposition of the cubic alpha-modification is suppressed by the incorporation of CaF2 into the TmOF lattice. The following thermodynamic data relating to the beta-alpha phase transition of stoichiometric TmOF could be derived by extrapolation to the formula TmOF: Delta(tr)H(808)(0)=5343+/-267 J mol(-1) and Delta(tr)S(808)(0)=6.84+/-0.34 J mol(-1) K-1 By correlating the entropies of the beta-->alpha transition with the temperatures of transition of ROF with R=Y, La-Tm it was found that the entropies for R=La-Eu and Tm are clustering around 6.6+/-0.4 J mol(-1) K-1 and those for R=Gd-Er and Y around 8.5+/-0.5 J mol(-1) K-1, independently of the cationic radii, and that TmOF occupies a special position within the ROF series. (C) 1998 Elsevier Science S.A. [References: 15]
机译：研究了化学计量TmOF的菱面体β->立方α相变和分析成分Tm（O，F）的样品的相行为（1.85小于或等于y小于或等于2.10）通过定量差热分析（​​DTA）和X射线粉末衍射分析。已发现β->α跃迁发生于808 +/- 3 K，分子式体积增加0.5 +/- 0.05埃（3），并且α-TmOF在约200摄氏度的腹膜反应中分解。 915 K转变为萤石相关的，可能是组成Tm（O，F）（1.93 +/- 0.02）的正交晶相和组成Tm9O8F11的正交晶游标型相，其a = 5.3765（5）埃，b = 49.402（5 ），c = 5.4607（4）埃。 Tm（O，F）（1.93）是由C-Tm2O3与α-TmOF的共析反应在约200℃形成。 900 K，由于过渡和分解温度的接近，不可能准确确定TmOF的β->α跃迁的焓。因此，通过差示扫描量热法（DSC）通过测量固溶体Tm1-xCaxO1-xF1 + x的温度和转变焓来间接确定转变焓，其中Tm1-xCaxO1-xF1 + x小于或等于x小于或等于0.062。发现T-tr和Delta（tr）H（T（tr））（0）都直接取决于固溶体中CaF2的含量，并且立方α-改性的分解受到将CaF2掺入TmOF晶格中。可以通过外推式TmOF得出与化学计量TmOF的β-α相变有关的以下热力学数据：Delta（tr）H（808）（0）= 5343 +/- 267 J mol（-1）和Delta （tr）S（808）（0）= 6.84 +/- 0.34 J mol（-1）K-1通过将β->α跃迁的熵与ROF跃迁的温度与R = Y，La相关联-Tm，发现R = La-Eu和Tm的熵聚集在6.6 +/- 0.4 J mol（-1）K-1附近，而R = Gd-Er和Y的熵聚集在8.5 +/- 0.5 J附近mol（-1）K-1，与阳离子半径无关，并且TmOF在ROF系列中占据特殊位置。 （C）1998 Elsevier Science S.A. [参考：15]

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