M. Davidović

Thermally induced phase transformations of 12-tungstophosphoric acid 29-hydrate : synthesis and characterization of PW8O26-type bronzes

The phase transformations of 12-tungstophosphoric H3PW12O40-29H2O (29-WPA) acid in the temperature range from ambient temperature to 1150°C were investigated and characterized by differential thermal analysis (DTA), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), infrared (i.r.) and Raman spectroscopies. From room temperature to 550°C, 29-WPA passes through a dehydration process, which characterizes the formation of different crystallohydrates, in anhydrous form as well as “denuded” Keggins anions, the D-phase (PW12O38). During these processes, Keggins anions are not disturbed too much and they are preserved up to about 550°C. The “D” phase is transformed by solid-solid recrystallization at about 600°C in a new monophosphate bronze type compound PW8O26. Unit cell dimensions were calculated from XRPD data (ao=0.7515 nm). With the temperature increasing up to 1150°C, novel synthesized cubic bronze passed through three polymorphous phase transitions. According to a general formula for monophosphate tungsten bronzes (WO3)2m (PO4)4 all four polymorphs have m=16.

Equilibrium of the protonic species in hydrates of some heteropolyacids at elevated temperatures

Abstract Hydrates of three heteropolyacids: 12-molybdophosphoric H3PMo12O40·nH2O (MoPA·nH2O), 12-tungstophosphoric H3PW12O40·nH2O (WPA·nH2O) and 12-tungstosilisilic acid H4SiW12O40·nH2O (WSiA·nH2O) were examined by using thermal analysis, infrared spectroscopy and impedance measurements. These compounds are known as superionic proton conductors and their behaviour was examined as a function of temperature. At the TGA and DSC curves, in the 300–500 K temperature range, transformations were observed at several temperatures. These transformations are partially due to the dehydration process and partially to the change of equilibrium of protonic species (H3O+, H2O and OH- and their possible association: H5O+2, H7O+3, ...), as shown by thermal analysis and infrared spectroscopy.

Inelastic neutron scattering study of protonic species during the thermal dehydration of 12-tungstophosphoric hexahydrate

Abstract 12-Tungstophosphoric acid hexahydrate and its derivatives dehydrated at 473, 573 and 873 K have been studied by incoherent inelastic neutron scattering. Comparison is made with the sodium salts of this compound. Sharp spectra assigned to H 5 O + 2 species (hexahydrate at 25 K), H 3 O + (473 K dehydrated) and a lone proton (573 K) have been obtained. Protons fixed to the host lattice are possible minor species in the hexahydrate and in the material dehydrated at 573 K. The strong background above 300 cm −1 in the hexahydrate and above 50 cm −1 in the dehydrated materials may indicate the presence of mobile protons (proton gas).

Complex dielectric permittivity, bulk and surface conductivity of 12-tungstophosphoric acid hexahydrate and its dehydrated forms

Abstract Impedance measurements on pelletised 12-tungstophosphoric acid hydrates were performed in the frequency range 20 Hz-1 GHz, at temperatures from 20°C to 400°C, in the air of different humidities. Both the dielectric relaxations and conductivity of hexahydrate vary with the amount of water in surface layer. Bulk protonic conductivity of hexahydrate at room temperature was found to be about 3 × 10−11 S cm−1. Interesting behaviour of conductivity of anhydrous samples was observed.

Structural modifications of dodecatungstophosphoric acid hexahydrate induced by temperature in the 10–358 K range. In situ high-resolution neutron powder diffraction investigation

Abstract Dodecatungstophosphoric acid hexahydrate H 3 PW 12 O 40 ·6H 2 O crystal structure has been investigated by neutron powder diffraction (NPD) at different temperatures in the 10–358 K range. A nonconvergent reversible phase transition has been noticed at about 320 K. This transition is associated with a change in dynamic equilibrium of hydrate species and partial reduction/oxidation (redox) W 6+ ↔W 5+ . Expressive structure changes lie in the PO bonding inside Keggins anion and the H 5 O 2 + conformational angle.

Dielectric investigation of electrical conductivity in the copper salt of the 12-tungstophosphoric acid hydrate

Abstract The electrical and dielectric properties of the copper salt of 12-tungstophosphoric acid (WPA) hydrate have been investigated by broadband dielectric relaxation spectroscopy (DRS). This work is a continuation of our previous investigations on the influence of monovalent and bivalent cations on the dielectric properties of WPA hydrates salts. Measurements were made in the frequency range 10 mHz–1 MHz in order to explore the long-range dynamics, following recent microwave measurements aiming at the local dynamics properties. Experiments were done in the temperature interval 293–350 K. Data treatment included complex permittivity e *, complex electric modulus M * and complex impedance Z * formalism, Arrhenius diagram and Cole–Cole diagrams. The results are discussed in terms of conductivity relaxation and conductivity current relaxation. Rather long conductivity relaxation times measured seem to correspond to ion jumping processes. A phase transition was observed at 323 K.

Structure and proton conductivity in a magnesium salt of 12-tungstophosphoric acid

Abstract Some structural properties of a magnesium salt of 12-tungstophosphoric acid, MgHPW12O40·nH2O (MgHWPA-n), are reported. The influence of the Mg2+ ion on the structure, dynamics of the protonic species, and dielectric and conductive characteristics of the salt have been studied. The interaction of protonic species with Keggin anion was discussed. Preliminary results of the crystal structure indicate that the structure is composed of Keggin anions and Mg(OH2)6 aqua-complexes. Some comparisons are made with other alkaline-earth and alkaline salts of WPA. The results of thermal analysis (TGA and DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared spectroscopy (IR), conductivity and permittivity measurements of MgHWPA-n are reported and discussed.

Dielectric relaxation and conductivity in barium salt of 12-tungstophosphoric acid hydrate

Abstract Barium salt of 12-tungstophosphoric acid (WPA) hydrate BaHPW12O40·7H2O (BaHWPA·7H2O) has been investigated both in the microwave (X-band) region and in the lower frequency (5 Hz–500 kHz) region. This work is a continuation of our previous investigations on the influence of monovalent and bivalent cations on the dielectric properties of WPA hydrate salts. The real and imaginary parts of the permittivity (e′, e″) as a function of frequency and temperature, were determined in both frequency regions. The observed relaxations were attributed to specific relaxation processes. In the lower frequency range, the relaxation time τ=1.0×10−6 s was assigned to barium ion jumps. In the microwave region, the dielectric relaxation time τ=1.7×10−11 s was attributed to polyatomic ions (H3O+) and/or H2O molecules reorientations. A phase transition was observed at 330 K. The low value of the activation energy for reorientation in the microwave frequency region is discussed. The d.c. conductivity and its temperature dependence were determined. Measurements were made in the temperature interval from 286 to 353 K. In the upper part of this temperature interval a PTCR effect was observed.

Dielectric investigation of magnesium salt of 12-tungstophosphoric acid hydrate

Abstract Magnesium salt of 12-tungstophosphoric acid (WPA) hydrate MgHPW 12 O 40 –14H 2 O has been investigated in the microwave (X-band) and in the lower frequency (5 Hz–500 kHz) region. The specimens were obtained by pressing powder. Real and imaginary parts of permittivity were obtained for the specimens in both frequency regions. In the microwave region dipolar relaxation was observed with the relaxation time τ =1.6×10 −11 s. This relaxation is attributed to the fast reorientation of H 2 O molecules. In the lower frequency region ionic jump relaxation was observed with the relaxation time τ =3.9×10 −7 s. Results on ac and dc conductivity are discussed.

Synthesis, conductivity and structural characterization of phosphorous bronzes originating from heteropolyacids. Relation with similar proton containing phases

Abstract The phosphorus doped molybdenum oxide compound (P 0.18 Mo 4.00 O 12.66 ), isostructural with molybdite was synthesized in the process of thermally induced phase transformation of 12-molybdophosphoric acid. The synthesized phases (low and high temperature) and the precursor compound ( Q -phase) have been characterized by thermal (DTA and TGA), X-ray powder diffraction, electron scanning microscopy and spectroscopic (IR) methods. The temperature dependence of electric conductivity σ ( ω → 0) is discussed. It was concluded that the electronic conductivities of bronzes obtained from heteropolyacids are very similar to those of corresponding anhydrous polyacids.