Ana Šantić

Structure of sodium phosphate glasses containing Al2O3 and/or Fe2O3. Part I

Abstract The relationship between the composition and structure for three series of glasses: Na2O–Al2O3–P2O5 (NAP); Na2O–Al2O3–Fe2O3–P2O5 (NAFP), and Na2O–Fe2O3–P2O5 (NFP) has been studied. The structural changes in the NAP, NAFP and NFP glasses have been investigated by Raman and by Raman difference spectroscopy (RDS). The Raman spectra show that the addition of Al2O3 content to sodium phosphate glasses has a different effect on the phosphate network than the addition of Fe2O3. With increasing Fe2O3 content up to 20 mol% the structure changes from the chain-like metaphosphate to the pyrophosphate structure. The iron ions play an important role in forming P–O–Fe bonds that strengthen the cross-bonding of shorter pyrophosphate chains. On the other hand, with increasing Al2O3 content up to 20 mol%, the sodium metaphosphate is replaced by aluminium metaphosphate where Al(OP)6 units cross-link phosphate chains. The addition of Fe2O3 to Al2O3 in phosphate glasses containing both Al2O3 and Fe2O3 (NAFP series), enhances the formation of pyrophosphate units because iron ions have stronger effect on the depolymerization of metaphosphate chains if compared to the aluminium ions.

First and Second Universalities: Expeditions Towards and Beyond

Abstract Understanding the mechanisms of translational and localised ionic movements in disordered materials has seen intense activity spanning several decades. This article attempts to convey a concise overview of our contribution to this field over the period from 2005 to 2010 and to place it in its broad context.

Single-Step Preparation of the Mixed BaII−NbV Oxides from a Heteropolynuclear Oxalate Complex

A novel oxalate-based complex of the formula {Ba(2)(H(2)O)(5)[NbO(C(2)O(4))(3)]HC(2)O(4)}·H(2)O (1) was prepared from an aqueous solution containing the [NbO(C(2)O(4))(3)](3-) and Ba(2+) entities in the molar ratio 1:2, and characterized by X-ray single-crystal diffraction, IR spectroscopy, and thermal analysis. The crystal packing of 1 reveals a complex three-dimensional (3D) network: the Nb polyhedron is connected to eight neighboring Ba polyhedra through the oxalate ligands and the oxo-oxygen group, whereas the Ba polyhedra share edges and vertices. The ability of compound 1 to act as a single-source precursor for the formation of bimetallic oxides was investigated by the thermal analysis (TGA and DSC) and X-ray powder diffraction. Thermal processing of 1 resulted in the formation of mixed-metal oxide phases, Ba(4)Nb(2)O(9) and Ba(5)Nb(4)O(15). Three stable polymorphs of Ba(4)Nb(2)O(9) were isolated: the known, hexagonal α- and orthorhombic γ-Ba(4)Nb(2)O(9), and another one, not previously reported, hexagonal δ-Ba(4)Nb(2)O(9) polymorph. The new, δ-Ba(4)Nb(2)O(9) polymorph has the 6H-perovskite structure (space group P6(3)/m), in which the Nb(2)O(9)(8-) face-sharing octahedral dimers are interconnected via corners to the regular BaO(6)(10-) octahedra. Formation of the mixed-metal oxides takes place at different temperatures: the Ba(5)Nb(4)O(15) oxide occurred at ∼700 °C, as the major crystalline oxide phase; by heating the sample up to 1135 °C, the α-Ba(4)Nb(2)O(9) form was obtained, whereas the heating at 1175 °C caused the crystallization of two polymorphs, γ-Ba(4)Nb(2)O(9) and δ-Ba(4)Nb(2)O(9). Special focus was set on the electrical properties of the prepared mixed Ba(II)-Nb(V) oxides obtained by this molecular pathway in a single-step preparation.

Supramolecular Ionic-Liquid Gels with High Ionic Conductivity.

Supramolecular ionogels were prepared by the gelation of room-temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4 ]) with (S,S)-bis(leucinol)oxalamide. Remarkably, the ionic conductivity of solutions and ionogels with low gelator concentrations is higher than that of neat [BMIm][BF4 ]. On the basis of molecular dynamics simulations and quantum mechanical calculations, the origin of this phenomenon is attributed to the higher affinity of gelator molecules towards [BF4 ](-) ions, which reduces the electrostatic attraction between [BMIm](+) and [BF4 ](-) and thus increases their mobility. With increasing gelator concentration, the ionic conductivity decreases due to the formation of a denser gelator matrix, which hinders the pathways for ionic transport. However, even for very dense ionogels, this decrease is less than one order of magnitude relative to neat [BMIm][BF4 ], and thus they can be classified as highly conductive materials with strong potential for application as functional electrolytes.

Electrical properties of sodium phosphate glasses containing Al2O3 and/or Fe2O3. Part II☆

Abstract The electrical properties for three series of glasses: xNa2O–(40−x)Al2O3–60P2O5, (20⩽x⩽35), (NAP); 20Na2O–xAl2O3–(20−x)Fe2O3–60P2O5, (5⩽x⩽15), (NAFP), and xNa2O–(40−x)Fe2O3–60P2O5, (20⩽x⩽35), (NFP) glasses were measured by impedance spectroscopy in the frequency range from 1 Hz to 1 MHz and over the temperature range from 303 to 473 K. It was shown (in Part I) that the addition of Fe2O3 has significant effects on the structure of these glasses and Fe ions play a different structural role in phosphate network than that of Al ions. Such effects reflect changes in the origin of electrical conduction. With increasing Fe2O3 content in NAFP and NFP glasses the dc conductivity depends upon distance between iron ions and the activation energy of 53.1 kJ mol −1 indicates electronic conduction. On the other hand, the decrease in dc conductivity and activation energy for glasses in NAP series is attributed to the decrease in Na2O content from 35 to 20 mol%. The activation energy varies from 80.1 to 72.4 kJ mol −1 for NAP glasses suggesting ionic conduction. The impedance analysis for these glasses shows that the changes in the electrical conduction mechanisms coincide with the changes in the structure.

Ba4Ta2O9 Oxide Prepared from an Oxalate-Based Molecular Precursor—Characterization and Properties

A novel heterometallic oxalate-based compound, {Ba2(H2O)5[TaO(C2O4)3]HC2O4}·H2O (1), was obtained by using an (oxalato)tantalate(V) aqueous solution as a source of the complex anion and characterized by X-ray single-crystal diffraction, IR spectroscopy, and thermal analysis. Compound 1 is a three-dimensional (3D) coordination polymer with the Ta atom connected to eight neighboring Ba atoms through the oxalate ligands and the oxo oxygen group. Thermal treatment of 1 up to 1200 °C leads to molecular precursor-to-material conversion that yields the mixed-metal γ-Ba4Ta2O9 phase. This high-temperature γ-Ba4Ta2O9 polymorph has the 6H-perovskite structure (space group P6(3)/m), in which the Ta2O9 face-sharing octahedral dimers are interconnected via corners to the regular BaO6 octahedra. To date, γ-Ba4Ta2O9 has never been obtained at room temperature, because of the reduction of symmetry (P6(3)/m → P2(1)/c) that usually occurs during the cooling. Spectroscopic, optical, photocatalytic, and electrical properties of the obtained γ-Ba4Ta2O9 phase were investigated. In addition to the experimental data, an absorption spectrum and band structure of the γ-Ba4Ta2O9 polymorph were calculated using density functional theory.

Impedance changes during setting of amorphous calcium phosphate composites.

OBJECTIVES To investigate the electrical properties of experimental light-curable composite materials based on amorphous calcium phosphate (ACP) with the admixture of silanized barium glass and silica fillers. METHODS Short-term setting was investigated by impedance measurements at a frequency of 1kHz, while for the long-term setting the impedance spectra were measured consecutively over a frequency range of 0.05Hz to 1MHz for 24h. The analysis of electrical resistivity changes during curing allowed the extraction of relevant kinetic parameters. The impedance results were correlated to the degree of conversion assessed by Raman spectroscopy, water content determined by gravimetry, light transmittance measured by CCD spectrometer and microstructural features observed by scanning electron microscopy. RESULTS ACP-based composites have shown higher immediate degree of conversion and less post-cure polymerization than the control composites, but lower polymerization rate. The polymerization rate assessed by impedance measurements correlated well with the light transmittance. The differences in the electrical conductivity values observed among the materials were correlated to the amount of water introduced into composites by the ACP filler. High correlation was found between the degree of conversion and electrical resistivity. Equivalent circuit modeling revealed two electrical contributions for the ACP-based composites and a single contribution for the control composites. SIGNIFICANCE The impedance spectroscopy has proven a valuable method for gaining insight into various features of ACP-based composites. Better understanding of the properties of ACP-based composites should further the development of these promising bioactive materials.

A polar/π model of interactions explains face-to-face stacked quinoid rings: a case study of the crystal of potassium hydrogen chloranilate dihydrate

The nature of interactions between face-to-face staggered stacked quinoid rings with π-systems, observed with a short inter-ring centroid⋯centroid distance, is analyzed by experimental and theoretical methods. Charge density studies based on X-ray diffraction and DFT calculations, complemented by impedance spectroscopy, were employed to define the electronic and structural characteristics of the quinoid rings responsible for their interactions within the crystal packing. The crystal packing is mainly stabilized by strong electrostatic interactions between the K+ cation and the hydrogen chloranilate anion. The proximity and orientation of the stacked quinoid rings in parallel glide-plane arrangement are partly governed by the non-covalent interactions within the dimer. The estimated contribution of dispersion energy to the stacking of the rings about −10 kcal mol−1, as calculated by DFT methods, is comparable to medium–strong hydrogen bonding. The electronic structure of 3,6-dichloro-2,5-dihydroxyquinone monoanion exhibits alternating electron-rich and electron-poor regions. The calculated electrostatic energy shows variations with the relative orientation of rings within dimers reaching ca. 10–15 kcal mol−1. Thus, the nature of interactions between π-systems of quinoid rings can be described by a polar/π model where electrostatic complementarity plays a determinant role in π-stacking orientation. These interactions have great potential in crystal engineering and may be employed in the design of functional materials.

Giant persistent photoconductivity in BaTiO3/TiO2 heterostructures

The persistent photoconductivity (PPC) effect in nanotube arrays of barium titanate and TiO2 (BTO/TiO2NT) was studied at room temperature under daylight illumination. The BTO/TiO2NT heterostructures exhibited a giant PPC effect that was six orders of magnitude higher than the dark conductivity, followed by a slow relaxation for 3 h. The PPC in this material was explained by the existence of defects at the surfaces and the interfaces of the investigated heterostructures. The sample was prepared using a two-step synthesis: the anodization of a Ti-foil and a subsequent hydrothermal synthesis. The structural and electrical characteristics were studied by micro-Raman spectroscopy, field-emission-gun scanning electron microscopy, and impedance spectroscopy.

Probing semiconductivity in crystals of stable semiquinone radicals: organic salts of 5,6-dichloro-2,3-dicyanosemiquinone (DDQ) radical anions

The structural parameters and semiconductivity of crystals with stacked 5,6-dichloro-2,3-dicyanosemiquinone (DDQ) radicals were studied for a series of the first nine salts of DDQ with substituted N-ethyl- and N-methylpyridinium cations. Structures with stacks of equidistant radicals (interplanar separation 3.4 A between the dimers) are insulators. DFT calculations indicate the covalent character (pancake bonding) of the interactions between the DDQ radical anions. The inductive effects of electronegative substituents (halogeno and cyano) on the quinoid ring stabilise the radicals, but also increase the HOMO–LUMO gap, resulting in lower electrical conductivity.