Synthesis, structures, and thermal expansion of the La2W2-xMoxO9 series

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 (at) transforms to the high-temperature form (beta) at 1077degreesC. The influence of the W/Mo substitution in this phase transition has been investigated by DTA. The beta structure for x greater than or equal to 0.7 compositions can be prepared as single phase at any cooling rate. The beta phase for 0.3less than or equal toxless than or equal to0.7 compounds can be prepared as single phase by quenching, whereas a mixture of alpha and beta phases is obtained by slow cooling. The W/Mo ratio in both coexisting phases is different with the beta-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 beta-La2W1.7Mo0.3O9 has been analyzed by the Rietveld method in P2(1)3 space group. The final R-factors were R-WP = 9.0% and R-F = 5.6% with a structure similar to that of beta-La2Mo2O9. Finally, the thermal expansion of both types of structures has been determined from a thermodiffractometric study. The thermal expansion coefficients were 2.9 x 10(-6) and 9.7 x 10(-6)degreesC(-1) for alpha-La2W2O9 and beta-La2W1.2 Mo0.8O9, respectively. (C) 2002 Elsevier Science (USA).