C. Guzmán-Afonso

Structural investigation of the negative thermal expansion in yttrium and rare earth molybdates

The Sc(2)(WO(4))(3)-type phase (Pbcn) of Y(2)(MoO(4))(3), Er(2)(MoO(4))(3) and Lu(2)(MoO(4))(3) has been prepared by the conventional solid-state synthesis with preheated oxides and the negative thermal expansion (NTE) has been investigated along with an exhaustive structural study, after water loss. Their crystal structures have been refined using the neutron and x-ray powder diffraction data of dehydrated samples from 150 to 400 K. The multi-pattern Rietveld method, using atomic displacements with respect to a known structure as parameters to refine, has been applied to facilitate the interpretation of the NTE behavior. Polyhedral distortions, transverse vibrations of A· · ·O-Mo (A = Y and rare earths) binding oxygen atoms, non-bonded distances A· · ·Mo and atomic displacements from the high temperature structure, have been evaluated as a function of the temperature and the ionic radii.

Experimental and theoretical study of α-Eu2(MoO4)3 under compression.

The compression process in the α-phase of europium trimolybdate was revised employing several experimental techniques. X-ray diffraction (using synchrotron and laboratory radiation sources), Raman scattering and photoluminescence experiments were performed up to a maximum pressure of 21 GPa. In addition, the crystal structure and Raman mode frequencies have been studied by means of first-principles density functional based methods. Results suggest that the compression process of α-Eu2(MoO4)3 can be described by three stages. Below 8 GPa, the α-phase suffers an isotropic contraction of the crystal structure. Between 8 and 12 GPa, the compound undergoes an anisotropic compression due to distortion and rotation of the MoO4 tetrahedra. At pressures above 12 GPa, the amorphization process starts without any previous occurrence of a crystalline-crystalline phase transition in the whole range of pressure. This behavior clearly differs from the process of compression and amorphization in trimolybdates with [Formula: see text]-phase and tritungstates with α-phase.

Polymorphism in Ho2(MoO4)3

1 Dpto. Fisica Fundamental II, Universidad de La Laguna, Avda. Astrofisico Fco. Sanchez, s/n, 38206 La Laguna, Tenerife, Spain 2 Dpto. Fisica Fundamental y Experimental, Electronica y Sistemas, Universidad de La Laguna, Avda. Astrofisico Fco. Sanchez, s/n, 38206 La Laguna, Tenerife, Spain 3 Dpto. Fisica Basica, Universidad de La Laguna, Avda. Astrofisico Fco. Sanchez, s/n, 38206 La Laguna, Tenerife, Spain 4 C.A.I. Difraccion de Rayos X, Universidad Complutense de Madrid, 28040 Madrid, Spain 5 Institut Laue-Langevin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble Cedex 9, France

Pressure evolution of two polymorphs of Tb 2(MoO 4)3

We have studied the high pressure behavior of the α and β′-phases of Tb 2(MoO 4)3 using a combination of powder X-ray diffraction and ab initio calculations. The α-Tb 2(MoO 4)3 phase did not undergo any structural phase transition in the pressure range from 0 up to the maximum experimental pressure of 21 GPa. We observed line broadening of the diffraction patterns at pressures above 7 GPa, which may be due to non-hydrostatic conditions. The complete amorphization of the sample was not reached in the pressure range studied, as expected from previous Raman studies. The behavior under pressure of the β′-Tb 2(MoO 4)3 phase is similar to that of other rare-earths trimolybdates with the same structure at room temperature. A phase transition was observed at 2 GPa. The new phase, which can be identified as the δ-phase, has never been completely characterized by diffraction studies. A tentative indexation has been performed and good refined cell parameters were obtained. We detect indications of amorphization of the δ-Tb 2(MoO 4)3 phase at 5 GPa.

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

25th European Crystallographic Meeting, ECM 25, İstanbul, 2009 Acta Cryst. (2009). A65, s 329 Page s 329 FA5-MS01-P30 Structural Study of Negative Thermal Expansion in Yttrium and Rare Earth Molybdates. Cristina González-Silgoa, Manuel E. Torresb, Javier GonzálezPlatasa,c, Javier Campod. V. Sánchez Fajardob, Diego Lozano-Gorrínc, Candelaria Guzmán-Afonsob, Nanci Sabalisckb. aDepartamento de Fisica Fundamental II. Universidad de La Laguna. España. bDepartamento de Física Básica. Universidad de La Laguna, España. cServicios Generales de Apoyo a la Investigación. Universidad de La Laguna. España. dInstituto de Materiales de Aragón. CSIC. España. E-mail: metorres@ull.es