Synthesis, Structures, and Thermal Expansion of the La 2W 2− xMo xO 9 Series

Abstract

The La 2W 2− x Mo x O 9 series has been synthesized by the ceramic method. An alternative synthesis using microwave radiation is also reported. La 2W 2O 9 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-La 2W 1.7Mo 0.3O 9 has been analyzed by the Rietveld method in P2 13 space group. The final R-factors were R WP=9.0% and R F=5.6% with a structure similar to that of β-La 2Mo 2O 9. 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-La 2W 2O 9 and β-La 2W 1.2Mo 0.8O 9, respectively.