Synthesis, Structures, and Thermal Expansion of the La2W2−xMoxO9 Series
Synthesis, Structures, and Thermal Expansion of the La2W2−xMoxO9 Series
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DOI：
10.1006/jssc.2002.9622
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发表时间：
2002-08
影响因子：
5.4
通讯作者：
J. Collado;M. Aranda;A. Cabeza;P. Olivera-Pastor;S. Bruque
中科院分区：
文献类型：
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作者：
J. Collado;M. Aranda;A. Cabeza;P. Olivera-Pastor;S. Bruque
摘要采用陶瓷法合成了La 2 W2 − xMoxO 9系列化合物。还报道了使用微波辐射的替代合成。La 2 W2 O 9有两种晶型，低温相（α）在1077°C转变为高温相（β）。用差热分析研究了W/Mo替代对该相变的影响。对于x≥0.7的成分，β结构可以在任何冷却速率下制备为单相。对于0.3≤x≤0.7的化合物，通过淬火可以得到单相的β相，而通过缓慢冷却可以得到α和β相的混合物。两种共存相中的W/Mo比不同，其中β相具有较高的Mo含量。x=0.1和0.2的化合物已被制备为相的混合物。用Rietveld方法分析了β-La_2W_（1.7）Mo_（0.3）O_9在P213空间群下的室温结构。最终的R-因子为RWP=9.0%和RF=5.6%，具有与β-La 2 Mo 2 O 9类似的结构。最后，这两种类型的结构的热膨胀已被确定从热衍射研究。α-La 2 W2 O 9和β-La2W1.2Mo0.8O9的热膨胀系数分别为2.9×10−6和9.7×10−6°C−1。
Abstract The La2W2−xMoxO9 series has been synthesized by the ceramic method. An alternative synthesis using microwave radiation is also reported. La2W2O9 has two polymorphs and the low-temperature phase (α) transforms to the high-temperature form (β) at 1077°C. The influence of the W/Mo substitution in this phase transition has been investigated by DTA. The β structure for x≥0.7 compositions can be prepared as single phase at any cooling rate. The β phase for 0.3≤x≤0.7 compounds can be prepared as single phase by quenching, whereas a mixture of α and β phases is obtained by slow cooling. The W/Mo ratio in both coexisting phases is different with the β-phase having a higher Mo content. The x=0.1 and 0.2 compounds have been prepared as mixtures of phases. The room temperature structure of β-La2W1.7Mo0.3O9 has been analyzed by the Rietveld method in P213 space group. The final R-factors were RWP=9.0% and RF=5.6% with a structure similar to that of β-La2Mo2O9. Finally, the thermal expansion of both types of structures has been determined from a thermodiffractometric study. The thermal expansion coefficients were 2.9×10−6 and 9.7×10−6°C−1 for α-La2W2O9 and β-La2W1.2Mo0.8O9, respectively.