Nuevas investigaciones sobre la inestabilidad estructural y mecanismos de transición en compuestos A2BX4 vía soluciones sólidas. Extensión al comportamiento de la sal de Rochelle

Estudio sobre compuestos de la familia bk2s04 con transición paraeléctrica- ferroeléctrica, soluciones sólidas de los mencionados compuestos y con la sal de Rochelle a fin de poder determinar las causas que generan la inestabilidad estructural y provocan los diferentes tipos de transiciones que presentan estos compuestos. El método de trabajo consiste en el análisis térmico diferencial, análisis por difracción de rayos X sobre muestras no nocristalinas, a varias temperaturas a fin de observar la evolución de la estructura con la temperatura. A partir del resultado de las estructuras cristalinas, se analiza el movimiento libracional y traslacional de las moléculas o grupos iónicos y se aplica el método de la valencia de enlace a fin de determinar la inestabilidad de los compuestos. Los resultados que obtenido han sido: a) determinarlas diferencias estructurales entre los compuestos de la misma familia, que definen un comportamiento diferente; b) determinar la causa de la inestabilidad estructural, la cual esta relacionada con las cavidades asimetricas y más voluminosas que ocupan los iones; c) formular un modelo termodinámico, basado en el tratamiento de las soluciones solidas por un modelo regular, con el cual se explican los estados intermedios que presentan estas transiciones; d) parametrizar las causas que producen la transición a fin de poder prever que compuestos pueden presentar la mencionada transición y cuales no

Poly[[μ4-tartrato-cadmium(II)] 0.167-hydrate]

The title compound, {[Cd(C4H4O6)]·0.167H2O}n, adopts a three-dimensional network structure in which each CdII ion is chelated by two pairs of carboxyl­ate and hydroxyl O atoms from two tartrate anions, and is additionally linked to two O atoms of two carboxyl­ate groups that are not involved in chelation. The asymmetric unit has four independent cadmium atoms, two of which lie on special positions of 2 site symmetry. The tartrate anions all lie on general positions. All hydroxyl groups are engaged in O—H⋯O hydrogen-bonds, one of which is also bifurcated. The non-coordinating water molecule is situated on a site with half-occupation

X-ray diffraction, thermal analysis, and Raman scattering study of K2BeF4 and comparation to other member of the (beta)-K2SO4 family with ferroelectric -paraelectric transition

Thermal analysis, powder diffraction, and Raman scattering as a function of the temperature were carried out on K2BeF4. Moreover, the crystal structure was determined at 293 K from powder diffraction. The compound shows a transition from Pna21 to Pnam space group at 921 K with a transition enthalpy of 5 kJ/mol. The transition is assumed to be first order because the compound shows metastability. Structurally and spectroscopically the transition is similar to those observed in (NH4)2SO4, which suggests that the low-temperature phase is ferroelectric. In order to confirm it, the spontaneous polarization has been computed using an ionic model

Solid-State Synthesis and Phase Transitions in the RE2(MoO4)3 Family Monitored by Thermodiffraction

Solid-state synthesis and phase transitions of RE2(MoO4)3 (RE ≡ Nd, Sm, Eu, and Gd) samples have been monitored by X-ray thermodiffraction with synchrotron radiation. The experiment was divided in two stages. In the first heating, different non-stoichiometric molybdates (Eu4Mo7O27, Eu2Mo4O15, and Pr2Mo4O15 structure types) emerged from the RE2O3 and MoO3 oxides before the expected phases (with α-Eu2(WO4)3 and La2(MoO4)3 structure types and the β-Gd2(MoO4)3 phase). The formation and coexistence of intermediate phases have been explained by common structural motifs with unit cell volumes per atom among those with the formula RE2(MoO4)3. Subsequent heating–cooling cycles showed the occurrence of the reversible and reconstructive α [La2(MoO4)3] ↔ β phase transition, including the less common transition β → α [La2(MoO4)3] obtained by heating the β′-Gd2(MoO4)3 phase from room temperature and clarifying much of the controversy in the literature. The transition mechanisms were studied by proposing a common supercell and comparing the RE and vacancy ordering within similar layers of MoO42– tetrahedra. The possible formation of stacking faults in Nd2(MoO4)3 was explained as a mixture of modulated scheelite phases. This research supports the importance of a directed and rational synthesis analyzing the intermediate products and their phase transitions for the enrichment of materials with new or improved properties.This work has been partially supported by the Agencia Canaria de Investigación, the Innovación y Sociedad de la Información of Gobierno Autónomo de Canarias (PROID2021010027, PROID202101010, and PROID2020010067), and by Cajacanarias Foundation (from Caixa Bank, Spain) with code 2021ECO5. We also wish to thank the European Synchrotron Radiation Facility (ESRF) organization, in particular, the BM25A beamline

Role of rare earth sites and vacancies in the anomalous compression of modulated scheelite tungstates

X-ray powder diffraction experiments at high pressures combining conventional sources and synchrotron radiation, together with theoretical simulations have allowed us to study the anomalous compression of the entire α-RE2(WO4)3 (RE = La-Ho) family with modulated scheelite structure (α phase). The investigated class of materials is of great interest due to their peculiar structural behavior with temperature and pressure, which is highly sought after for specialized high-tech applications. Experimental data were analyzed using full-profile refinements and were complemented with computational methods based on density functional theory (DFT) total energy calculations for a subset of the samples investigated. An unusual change in the compression curves of the lattice parameters a, c, and β was observed in both the experiments and theoretical simulations. In particular, in all the studied compounds the lattice parameter a decreased with pressure to a minimum value and then increased upon further compression. Pressure evolution of the experimental x-ray diffraction (XRD) patterns and cell parameters is correlated with the ionic radius of the rare earth element: (1) the lighter La-Nd tungstates underwent two phase transitions, and both transition pressures decreased as the rare earth's ionic radius increased. The XRD patterns of the first high pressure phase could be indexed with propagation vectors parallel to the a axis (tripling the unit cell). At higher pressures, the lattice parameters for the second phase (referred to as the preamorphous phase) showed little variation with pressure. (2) The heavier tungstates, from Sm to Dy, undergo a transition to the preamorphous phase without any intermediate phase. The reversibility of both phase transitions was investigated. DFT calculations support this unusual response of the crystal structures under pressure and shed light on the structural mechanism of negative linear compressibility (NLC) and the resulting softening. The pressure dependence of the structural modifications is related to tilting, along with small elongation and alignment, of the WO2−4 tetrahedrons. These changes correlate with those in the alternating RE…RE…RE chains and blocks of cationic vacancies arranged along the a axis. Possible stacking defects, which emerge between them, helped to explain this anomalous compression and the pressure induced amorphization. Such mechanisms were compared with other ferroelastic families of molybdates, niobates, vanadates, and other compounds with similar structural motifs classified as having “hinge frames.

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 β′-phase and tritungstates with α-phase.We thank Diamond Light Source for access to beamline I15 (EE1746) that contributed to the results presented here. Part of the diffraction measurements were performed at the 'Servicio Integrado de Difraccion de Rayos X (SIDIX)' of University of La Laguna. This work has been supported by Ministerio de Economia y Competitividad of Spain (MINECO) for the research projects through the National Program of Materials (MAT2010-21270-C04-01/02/03/04, MAT2013-46649-C41/2/3/4-P and MAT2013-43319-P), the Consolider-Ingenio 2010 MALTA (CSD2007-00045), the project of Generalitat Valenciana (GVA-ACOMP/2014/243) and by the European Union FEDER funds. C Guzman-Afonso wishes to thank ACIISI and FSE for a fellowship. J A Sans thanks the FPI and 'Juan de la Cierva' programs for fellowships.Guzmán-Afonso, C.; León-Luis, S.; Sans-Tresserras, JÁ.; González -Silgo, C.; Rodríguez-Hernández, P.; Radescu, S.;  muñoz, A.... (2015). Experimental and theoretical study of α–Eu2(MoO4)3 under compression. Journal of Physics: Condensed Matter. 27(46):465401-1-465401-11. https://doi.org/10.1088/0953-8984/27/46/465401S465401-1465401-11274

Effect of pressure on La-2(WO4)(3) with a modulated scheelite-type structure

We have studied the effect of pressure on the structural and vibrational properties of lanthanum tritungstate La2(WO4)3. This compound crystallizes under ambient conditions in the modulated scheelite-type structure known as the α phase. We have performed x-ray diffraction and Raman scattering measurements up to a pressure of 20 GPa, as well as ab initio calculations within the framework of the density functional theory. Up to 5 GPa, the three methods provide a similar picture of the evolution under pressure of α-La2(WO4)3. At 5 GPa, we begin to observe some structural changes, and above 6 GPa we find that the x-ray patterns cannot be indexed as a single phase. However, we find that a mixture of two phases with C2/c symmetry accounts for all diffraction peaks. Our ab initio study confirms the existence of several C2/c structures, which are very close in energy in this compression range. According to our measurements, a state with medium-range order appears at pressures above 9 and 11 GPa, from x-ray diffraction and Raman experiments, respectively. Based upon our theoretical calculations we propose several high-pressure candidates with high cationic coordinations at these pressures. The compound evolves into a partially amorphous phase at pressures above 20 GPa.We acknowledge the financial support of the Spanish Ministerio de Economia y Competitividad under Grants MAT2010-21270-C04-02/03/04, CTQ2009-14596-C02-01, CSD2007-00045 and the Comunidad de Madrid and European Social Fund S2009/PPQ-1551-4161893. Access to the MALTA Cluster Computer (Universidad de Oviedo), the Atlante Super-computer (Instituto Tecnologico de Canarias, Red Espanola de Supercomputacion), and the MALTA Xcalibur Diffractometer (Universidad Complutense de Madrid) is gratefully acknowledged. C. G. A. wishes to thank the Agencia Canaria de Investigacion, Innovacion y Sociedad de la Informacion, and the European Social Fund of the Gobierno de Canarias for a fellowship. J.A.S. acknowledges financial support through the Juan de la Cierva fellowship program.Sabalisck, N.; Lopez Solano, J.; Guzmán-Afonso, C.; Santamaría Pérez, D.; González-Silgo, C.; Mújica, A.; Muñoz, A.... (2014). Effect of pressure on La-2(WO4)(3) with a modulated scheelite-type structure. Physical Review B (Condensed Matter). 89:1741121-17411211. https://doi.org/10.1103/PhysRevB.89.174112S17411211741121189Maczka, M., Souza Filho, A. G., Paraguassu, W., Freire, P. T. C., Mendes Filho, J., & Hanuza, J. (2012). Pressure-induced structural phase transitions and amorphization in selected molybdates and tungstates. Progress in Materials Science, 57(7), 1335-1381. doi:10.1016/j.pmatsci.2012.01.001Boulahya, K., Parras, M., & González-Calbet, J. M. (2005). A Structural Study of the Solid Solution Eu2(Mo1-xWx)3O12. 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Journal of Physics: Condensed Matter, 23(32), 325402. doi:10.1088/0953-8984/23/32/325402Jayaraman, A., Sharma, S. K., Wang, Z., & Wang, S. Y. (1997). Pressure-induced amorphization in the α-phase of Nd2(MoO4)3 and Tb2(MoO4)3. Solid State Communications, 101(4), 237-241. doi:10.1016/s0038-1098(96)00587-xLucazeau, G., Le Bacq, O., Pasturel, A., Bouvier, P., & Pagnier, T. (2011). High-pressure polarized Raman spectra of Gd2(MoO4)3: phase transitions and amorphization. Journal of Raman Spectroscopy, 42(3), 452-460. doi:10.1002/jrs.2731Le Bacq, O., Machon, D., Testemale, D., & Pasturel, A. (2011). Pressure-induced amorphization mechanism in Eu2(MoO4)3. Physical Review B, 83(21). doi:10.1103/physrevb.83.214101Machon, D., Dmitriev, V. P., Sinitsyn, V. V., & Lucazeau, G. (2004). Eu2(MoO4)3single crystal at high pressure: Structural phase transitions and amorphization probed by fluorescence spectroscopy. 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Computer Physics Communications, 180(12), 2622-2633. doi:10.1016/j.cpc.2009.03.010Sabalisck, N., Mestres, L., Vendrell, X., Cerdeiras, E., Santamaría, D., Lavin, V., … Guzman-Afonso, M. C. (2011). Amorphization in rare earth tungstates with modulated scheelite-type structure under pressure. Acta Crystallographica Section A Foundations of Crystallography, 67(a1), C504-C505. doi:10.1107/s0108767311087228Logvinovich, D., Arakcheeva, A., Pattison, P., Eliseeva, S., Tomeš, P., Marozau, I., & Chapuis, G. (2010). Crystal Structure and Optical and Magnetic Properties of Pr2(MoO4)3. Inorganic Chemistry, 49(4), 1587-1594. doi:10.1021/ic9019876Garg, N., Murli, C., Tyagi, A. K., & Sharma, S. M. (2005). Phase transitions inSc2(WO4)3under high pressure. Physical Review B, 72(6). doi:10.1103/physrevb.72.064106Belsky, A., Hellenbrandt, M., Karen, V. L., & Luksch, P. (2002). New developments in the Inorganic Crystal Structure Database (ICSD): accessibility in support of materials research and design. 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X-ray diffraction, thermal analysis, and Raman scattering study of K2BeF4 and comparation to other member of the (beta)-K2SO4 family with ferroelectric -paraelectric transition

Thermal analysis, powder diffraction, and Raman scattering as a function of the temperature were carried out on K2BeF4. Moreover, the crystal structure was determined at 293 K from powder diffraction. The compound shows a transition from Pna21 to Pnam space group at 921 K with a transition enthalpy of 5 kJ/mol. The transition is assumed to be first order because the compound shows metastability. Structurally and spectroscopically the transition is similar to those observed in (NH4)2SO4, which suggests that the low-temperature phase is ferroelectric. In order to confirm it, the spontaneous polarization has been computed using an ionic model