C. González-Silgo

Structural and dielectric characterization of cadmium tartrate

Polycrystalline samples of dimeric cadmium tartrate, [(Cd,C4H4O6)2H2O)]3H2O [labeled CdT(I)], were studied using impedance measurements and x-ray powder diffraction. The dependence of the real part of the dielectric constant on temperature showed a sharp peak at about 65 °C, revealing a structural phase transition, while the other broad peak in the temperature range (70<T<85 °C) was due to the loss of water molecules. The x-ray powder diffraction patterns at three temperatures (25, 60, and 70 °C) are consistent with three nonequivalent space groups. According to these results, it seems that this compound undergoes two successive phase transitions: P212121→P21→Pnmn, suggesting an intermediate ferroelectric behavior, labeled CdT(II) between a paraelectric CdT(I) and an anhydrous phase, labeled CdT(III).

Bond-valence parameters for ammonium-anion interactions.

Bond-valence parameters r0 and b, relating bond valence and bond length, are calculated for interactions between the ammonium ion and anions X = O, F, Cl. Searches in the Cambridge Structural Database (CSD) and in the Inorganic Crystal Structural Database (ICSD) were performed to obtain the lengths of NH4+...X contacts for ammonium ion environments in different structures. The procedure, which represents an extension of previous methods, allows certain environments to be rejected and enables the calculation of r0 and b from a reasonable range of interaction distances. Results are in very good agreement with the expected values on the basis of the assumed bond-valence model and their overall applicability to ammonium ion interactions is discussed.

VALMAP2.0: contour maps using the bond-valence-sum method

VALMAP2.0 is a Microsoft-Windows-based program designed to assist material scientists in accurate structural investigations. The aim of VALMAP is to calculate the sum of bond valences that a particular atom would have if it were placed at any arbitrary point in the crystal. By movement of this atom through all possible points, its valence-sum contour map can be displayed. Parameters of the bond-valence model are available and may be modified. The program was tested in a number of cases and two examples of applications are reported: (i) finding probable atom sites in crystal structures; (ii) displacive and order–disorder phase transition mechanisms, analysing steric effects.

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.

Polymeric (aqua-1κo)bis[(R,R)-tartrato-1κ2O1,O2 :2κ2O3,O4]dimanganese(II) trihydrate

The structure of the title compound, {[Mn 2 (C 4 H 2 O 6 ) 2 -(H 2 O)].3H 2 O} n , is comprised of corrugated polymeric sheets of dimeric [Mn 2 {(R,R)-C 4 H 2 O 6 } 2 (H 2 O)] units and water molecules (O2W, O3W and O4W). The manganese ions are coordinated by two (R,R)-tartrate ligands in a cis arrangement. The Mn1...Mn2 intradimeric distance is 5.527 (2) A.

Polymeric aqua-1κo-bis[μ-(R, R)-tartrato-1κ2o1, o2 :2κ2o3, o4]dicadmium(II) trihydrate

The structure of the title compound, {[Cd 2 (C 4 H 4 O 6 ) 2 -(H 2 O)].3H 2 O} n , consists of corrugated polymeric sheets of dimeric [Cd 2 (C 4 H 4 O 6 ) 2 (H 2 O)] units and three water molecules of crystallization. Both cadmium ions are coordinated by two (R,R)-tartrate ligands in a cis arrangement, and the octahedral geometry for each cation is completed by two carboxyl O atoms of different neighbouring dimers or by one carboxyl O atom and a water molecule. Additional water molecules are held in the crystal lattice, forming a hydrogen-bonding network to keep the dimers together in the non-planar sheets.

Barium L-tartrate

Barium L-tartrate, Ba 2+ .C 4 H 4 O 2- 6 , has been grown in silica-gel medium for the first time and its structure has been solved. A new coordination of the cation with the tartrate anion is observed. The cation exhibits ninefold coordination without the presence of water molecules and the tartrate groups are linked through Ba…O contacts to form a three-dimensional network.

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...

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.