Maria Luisa Martinez-Sarrion

Phase Diagram at Low Temperature of the System ZrO2/Nb2O5

Various compositions of the ZrO2/Nb2O5 system were synthesized and the experimental conditions for obtaining reproducible results were established. The gel was precipitated at constant pH = 10, aged at room temperature for 18 h and, after filtration, dried at 110 °C for 24 h. The phase diagram of the ZrO2/Nb2O5 system was established between 600 and 1300 °C. Phase transitions of ZrO2 and Nb2O5 were observed with XRD; and two ternary compounds, 6 ZrO2 · Nb2O5 and 12 Nb2O5 · ZrO2, were identified. The samples with a Nb2O5 content between approximately 10 and 40 mole% showed the greatest specific area and are thus best for use as catalyst supports. Phasendiagramm des Systems ZrO2/Nb2O5 bei niedriger Temperatur Das System ZrO2/Nb2O5 wurde in verschiedener Zusammensetzung dargestellt und die experimentellen Bedingungen fur reproduzierbare Ergebnisse bestimmt. Das Gel wurde bei konstantem pH = 10 gefallt, 18 Stunden bei 20 °C gealtert und nach Filtration bei 110 °C getrocknet. Das Phasendiagramm des ZrO2/Nb2O5-Systems wurde zwischen 600° und 1300 °C bestimmt. Phasenubergange von ZrO2 und Nb2O5 wurden rontgenographisch untersucht und zwei ternare Phasen, namlich 6 ZrO2 · Nb2O5 und 12 Nb2O5 · ZrO2, identifiziert. Die Proben mit einem Nb2O5-Gehalt zwischen 10 und 40 mol-% haben die groste spezifische Oberflache und sind somit am besten fur Katalysatoren geeignet

Analysis of the Eu3+ emission in a SrWO4 laser matrix under pressure

Optical properties of the SrWO4 matrix doped with different Eu3+ concentrations (10%, 13% and 20%) have been studied. At ambient pressure, the temporal evolution of the emissions from the 5D1 level has been analyzed in the different doped crystals in order to discuss the energy transfer processes between the Eu3+ ions. This study lets conclude that the Eu3+ ions are randomly distributed in the matrix. Moreover, the 5D0→7F1, 2 emissions have been measured increasing the pressure up to 15 GPa in order to analyze the changes in the Eu3+ local structure. The results confirm the existence of a structural phase transition around 8.5 GPa.

Study on K2SeO4 at different temperatures

The crystal structure determination of K2SeO4 and the X-ray powder diffraction analysis have been carried out at different temperatures. The X-ray powder diffraction corroborate the two transitions from paraelectric to incommensurate and from this to ferroelectric phase. This study shows, also, an inhomogeneous behavior of K2SeO4 when the temperature decreases. The crystal structure determination shows the oxygen atoms of the selenate ion do not show the same effective charge. The study at different temperature shown a rotation of SeO-24 ion around the c-axis and the lengthening of Se—O(2) bond when the temperature is lower. From the structural data, the librational study and the distortion theory we deduce that K(1) is the main ion that produces the different transitions. The incommensurate phase is essentially produced by the freedom of K(1) ion to have a displacement along the a-axis together with a libration of selenate ion about the b-axis. A rotation of the selenate ion around the b-axis together wi...

Study on the mixed crystals

A study on the mixed crystals of formula (1>x>0) has been carried out, using thermal analyses and x-ray diffraction on powder and single-crystal samples at different temperatures. It is shown that mixed crystals with have a transition from the paraelectric to the ferroelectric phase; decreases as the value of x increases. The orientation of the sulphate ion with respect to the crystallographic axes depends on the value of x. Mixed crystals with the paraelectric - ferroelectric transition turn around the b axis when the temperature decreases in the paraelectric phase, while in mixed crystals without a transition the orientation of the sulphate ion does not alter when the temperature varies. The transition is displacive, increasing the value of the parameter a without a soft mode. This is deduced from a study of the crystal structures, the translational and libration tensors and a density map of the sum of the bond valences.

Powder neutron diffraction of Tl2BeF4 at six temperatures from room temperature to 1.5 K.

The structure of thallium fluoroberyllate, Tl2BeF4, has been analysed by the Rietveld method on neutron diffraction patterns collected at 1.5, 50, 100, 150, 200 and 300 K, with the aim of detecting low-temperature instabilities. Atomic parameters based on the isomorphic beta-K2SO4 crystal in the paraelectric phase were used as the starting model at room temperature; no evidence for any phase transition has been detected at lower temperature. The structure was determined in the orthorhombic space group Pnma. All the atoms (except one F atom) occupy sites with m symmetry. We have compared the structure with those of other compounds of the beta-K2SO4 family, at room temperature, in order to gain insight into their observed instabilities. The irregular coordination of the cations may indicate stereochemical activity of the TlI lone pair but does not indicate a possible structural instability.

Effects on the dielectric and ferroelectric properties of an O2 atmosphere during sintering of Pb(Fe0.5Nb0.5)O3 ceramics

Multifunctional materials such as the single phase compound Pb(Fe0.5Nb0.5)O3 (PFN), where ferroelectric and magnetic order coexist, are very promising and have great interest from the academic and technological points of view. However, as was established in previous work, the occurrence of Fe2+ and oxygen vacancies originated during the sintering process increase electrical conductivity, dielectric loss, and space-charge accumulation at the grain boundary, all of them detrimental to optimal device performance.1-3 Here, a report on the effects of an O2 atmosphere during the calcination and sintering stages of the preparation process using the traditional ceramic method is presented. A comparative study of structural, morphological and small signal dielectric frequency-temperature response study, where appropriate microstructural and equivalent circuit models were established using the impedance spectroscopy technique is presented. Big differences in the hysteret!ic behavior of the samples prepared in the presence and without the O2 atmosphere were observed and are discussed. The O2 atmosphere improves the dielectric and ferroelectric properties largely, in comparison with previous reports on PFN ceramics where other methods such as sintering in air, Pb atmospheres or fabricated from sol-gel-derived powders were used. 1 O. Raymond et al, J. Appl. Phys. 97, 084107 (2005). 2 O. Raymond et al. J. Appl. Phys. 97, 084108 (2005). 3 O. Raymond et al. J. Appl. Phys. 99, 124101 (2006). This work was partially supported by DGAPA-UNAM Project IN109305-3 and CoNaCyT Projects No. 47714-F and 49986-F. The authors thank E. Aparicio, P. Casillas, I. Gradilla, and J. Peralta for their technical assistance.

X-Ray and Neutron Powder Diffraction Study of The Rb2-x(NH4)xSO4 System.

X-ray and neutron powder diffraction studies of Rb 2- x (NH 4 ) x SO 4 solid solutions throughout the concentration range 0≤ x ≤2 and in the temperature range from 10 up to 300 K have been carried out. From the concentration dependencies of the lattice parameters obtained for protonated and deuterated samples, the tricritical point have been determined near the concentration x =1.6. The β-γ phase transition is of the first order for compounds with x >1.6 and of the second order for x <1.6.