Structural study of negative thermal expansion in yttrium and rare earth molybdates

Molybdates with formula A 2 (MoO 4 ) 3 show a very intense negative thermal expansion (NTE) above 450K, when A=Y, Ho-Lu with Sc 2 (WO 4 ) 3 -type structure (Pbcn) [1].There is not a detailed structural investigation at lower temperatures because they are hygroscopic [2].Three samples Y 2 (MoO 4 ) 3 , Er 2 (MoO 4 ) 3 and Lu 2 (MoO 4 ) 3 have been prepared by the conventional solid-state synthesis with preheated oxides.We have analysed the diffraction data of dehydrated compounds (A= Y, Er, Yb and Lu) from an ILL experiment (5-24-360) at the D2B powder diffractometer.We have found that all compounds show negative thermal expansion (NTE) behaviour in the temperature range of 150-400K.All the structures have been well refined by multipattern procedure [3], through X-ray powder diffraction data too, obtained on the laboratory equipment.Given an accurate and precise crystal structure we have measured the exact scope polyhedral distortion with the temperature and ionic radii and the role of the transverse vibration of binding A-O-Mo oxygen atoms in the structure, since several authors have related such vibrations with the NTE behavior [4].Also, we have studied the structural changes depending on the temperature and the ionic radii using the symmetry mode amplitude formalism [5], by comparing the distortions of lower temperatures respect to those at the highest temperature.