Synthesis of metallic ReO3 nanowires

We present the synthesis of highly crystalline metallic rhenium trioxide (ReO₃) nanowires via a simple physical vapor transport at 300 °C for the first time. Based on HRTEM, the ReO₃ nanowires exhibit a core of perfect cubic perovskite‐type single crystal structure with a shell of thin amorphous and disordered structures of less than 2 nm in the near surface layers. Possibly this is due to proton intercalation induced by the surface reaction of single crystal ReO₃ with water.

     

Direct growth of single crystalline ReO3 nanorods on a tungsten (W) microwire for enhanced electron-transfer reaction kinetics

We report the direct growth of highly single crystalline rhenium trioxide (ReO₃) nanorods on a tungsten (W) microwire electrode via the physical vapor transport process without any catalyst. In our growth mode, ReO₃ nanocubes were initially formed on the surface of a tungsten (W) microwire electrode and further they were anisotropically grown along the [001] crystallographic direction. Furthermore, we performed the fundamental electrochemical experiments so that from cyclic voltammetric measurements, ReO₃ nanorods on a W microwire exhibit a good capacitance and Nernstian behavior for a Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox couple in 1 M KNO₃ aqueous solution, indicating a promising electrode material for electrochemical applications.     

Positron wave function in ReO3 by the APW method

The wave function of a positron in ReO₃ is calculated using the augmented-plane-wave method. Due to the loosely-packed structure of ReO₃, the ground-stateГ ₁ wave function exhibits a marked anisotropy particularly around the oxygen ions, and a large fraction of a positron is distributed in the interstitial region. Experimental results of the positron annihilation 2γ correlations and the positron annihilation rates in ReO₃ are discussed based on the positron wavefunction.     

Pressure study of monoclinic ReO2 up to 1.2 GPa using X‐ray absorption spectroscopy and X‐ray diffraction

The crystal and local atomic structure of monoclinic ReO₂ (α‐ReO₂) under hydrostatic pressure up to 1.2 GPa was investigated for the first time using both X‐ray absorption spectroscopy and high‐resolution synchrotron X‐ray powder diffraction and a home‐built B₄C anvil pressure cell developed for this purpose. Extended X‐ray absorption fine‐structure (EXAFS) data analysis at pressures from ambient up to 1.2 GPa indicates that there are two distinct Re—Re distances and a distorted ReO₆ octahedron in the α‐ReO₂ structure. X‐ray diffraction analysis at ambient pressure revealed an unambiguous solution for the crystal structure of the α‐phase, demonstrating a modulation of the Re—Re distances. The relatively small portion of the diffraction pattern accessed in the pressure‐dependent measurements does not allow for a detailed study of the crystal structure of α‐ReO₂ under pressure. Nonetheless, a shift and reduction in the (011) Bragg peak intensity between 0.4 and 1.2 GPa is observed, with correlation to a decrease in Re—Re distance modulation, as confirmed by EXAFS analysis in the same pressure range. This behavior reveals that α‐ReO₂ is a possible inner pressure gauge for future experiments up to 1.2 GPa.     

Heat capacity and magnetic susceptibility of ReO3

The specific heat and magnetic susceptibility of the transition metal oxide ReO₃ have been measured. The specific heat results give a Debye temperature Θ_D = 460 ± 10 K and an electronic specific heat coefficient γ = 6.45 ± 0.07 cal/mole K² which are in good agreement with similar measurements on the cubic sodium tungsten bronzes. The magnetic susceptibility and the electronic contribution to the specific heat are within a few percent of the corresponding parameters calculated from the free electron model with one electron per unit cell. Our results show that ReO₃ behaves much like a simple metal. No experimental evidence for narrow d-band effects was observed.     

Cationic Order in Double Perovskite Oxide, Sr2Fe1−x Sc x ReO6 (x = 0.05, 0.1)

We have synthesized by sol–gel method the following polycrystalline double perovskite samples: Sr₂Fe_1?x Sc _x ReO₆ (x = 0, 0.05, 0.1). The results of the Rietveld refinements presented single double perovskite phases with orthorhombic symmetry for the system Sr₂Fe_1?x Sc _x ReO₆, the differences in atomic radii between Fe³⁺ and Sc³⁺ cause a lowering in symmetry with respect to the parent Sr₂FeReO₆ tetragonal compound. The Curie temperatures are found at about 426 and 436 (±5) K for Sr₂Fe_0.9Sc_0.1ReO₆ and Sr₂Fe_0.9Sc_0.05ReO₆, respectively. The Mössbauer spectra measured at 77 K show complex hyperfine structures resulting from different magnetic contributions at Fe³⁺ sites; the average hyperfine field is estimated 50 T and the isomer shift at 0.5 mm/s. At room temperature an intermediate valence state for Fe is also observed.     

Generation of CO2 pulses by free induction decay in KCI:KReO4

Subnanosecond pulses have been produced by optical free induction decay in KCI:KReO₄. Here the ν₃ vibrational mode of the perrhenate ion (ReO^-₄) acts as a resonant absorber for the 10.6μm, P(26) CO₂ laser line. As the phase relaxation time T₂ of the substitutional molecule ion is very small, efficient picosecond pulses can be produced by this medium if faster shuttering can be devised.     

Unconventional field and angle dependences of the Shubnikov-de Haas oscillations spectra in the quasi two-dimensional organic superconductor (BEDO-TTF) 2ReO 4H 2O

We report on the inter-layer oscillatory conductance of the two-dimensional organic superconductor (BEDO-TTF)₂ReO₄H₂O measured in static and pulsed magnetic fields of up to 15 and 52 T, respectively. In agreement with previous in-plane studies, two Shubnikov-de Haas oscillation series linked to the two electron and the hole orbits are observed. The influence of the magnitude and orientation of the magnetic field with respect to the conducting plane is studied in the framework of the conventional two- and three-dimensional Lifshits-Kosevich (LK) model. Deviations of the data from this model are observed in low fields strongly tilted with respect to the normal to the conducting plane. In this latter case, the observed behaviour is consistent with an unexplained lowering of the cyclotron effective mass. At high magnetic field, the oscillatory data could have been compatible with the occurrence of a magnetic breakdown orbit built from the hole and electron orbits. However, the increase of the cyclotron effective mass, linked to the electron orbits, as the magnetic field increases above ∼12 T is consistent with a field-induced phase transition. In the lower field range, where the conventional LK model holds, the analysis of the angle dependence of the oscillations amplitude suggests significant renormalisation of the effective Landé factor. Received 22 August 2000 and Received in final form 20 December 2000     

A model for electrical conductivity in (TMTSF)2ClO4, (TMTSF)2ReO4 and related materials

To describe quantitatively the diverse variations of the temperature-dependent electrical conductivities of tetramethyltetraselenafulvenium (TMTSF) salts, a model based on small Fermi (E_f) and small gap (E_g − E_f) energies and T⁻ⁿ dependence of scattering is presented. The difference between the conductivities of quenched and slowly cooled (TMTSF)₂ClO₄ is described by different values of E_f and E_g − E_f in the two cases. The model illustrates, in terms of variables E_f and E_g − E_f, the effect of pressure on the metal-insulator transition in (TMTSF)₂ReO₄, for example, and indicates a relation between E_f and E_g − E_f at the onset of superconductivity.     

Infrared and Raman spectroscopic studies of the charge localization in one‐dimensional organic metals (DMtTTF)2X (X = ReO4, ClO4) with regular organic stacks

A novel selective synthesis of the unsymmetrically substituted tetrathiafulvalene dimethyltrimethylene‐tetrathiafulvalene (DMtTTF) is described together with its electrocrystallization to the known conducting mixed‐valence ClO₄^– and ReO₄^– salts. Infrared (IR) and Raman spectra of the two isostructural quasi‐one‐dimensional cation radical salts (DMtTTF)₂X (X = ReO₄^–, ClO₄^–) are investigated as a function of temperature (T = 5–300 K). At ambient temperature, these salts show metallic‐like properties and below T_ρ = 100–150 K, they undergo a smeared transition to semiconducting state. To study this charge localization, we measured temperature dependence of polarized IR reflectance spectra (700–16 000 cm^–1) and Raman spectra (150–3500 cm^–1, excitation λ = 632.8 nm) of single crystals. For both compounds, the Raman data and especially the bands related to the C=C stretching vibration of the DMtTTF molecule show that the charge distribution on molecules is uniform down to the lowest temperatures. Similarly, IR data confirm that down to the lowest temperatures, there is neither charge ordering nor important modification of the electronic structure. However, the temperature dependence of Raman spectra of both salts reveals a regime change at about 150 K. Additionally, using Density Functional Theory (DFT) methods, the normal vibrational modes of the neutral DMtTTF⁰ and cationic DMtTTF⁺ species and also their theoretical IR and Raman spectra were calculated. The theoretical data were compared with the experimental IR and Raman spectra of neutral DMtTTF⁰ molecule. Copyright © 2013 John Wiley & Sons, Ltd.     

Cyclotron resonance in the organic conductor (BEDO-TTF)2ReO4(H2O) in the millimeter wavelength band

Resonant microwave absorption in a (BEDO-TTF)₂ReO₄(H₂O) organic conductor single crystal at a temperature of 1.9 K, a magnetic field of up to 70 kOe, and in the frequency band between 30 and 120 GHz has been studied. A spectral component due to the cyclotron resonance (CR) of two-dimensional charge carriers has been identified for ν⩾80 GHz and H⩾10 kOe. The effective mass m(ω) increases with the frequency from m≈0.8m ₀ at ν=80 GHz to m≈0.95m ₀ at ν=120 GHz. Measurements of the CR line position and FWHM as functions of frequency yield an independently determined imaginary part of the memory function M(ω), which controls the dynamic magnetoconductivity, and the relaxation time τ(ν=100 GHz)≈2.9×10⁻¹¹ s, which is more than thirty times the value of this parameter in the low-frequency limit τ(ν→0). The anomalous behavior of the CR line position and FWHM is caused by the dispersion of both real and imaginary parts of M(ω), which are probably due to strong Fermi-liquid effects. Zh. éksp. Teor. Fiz. 111, 979–987 (March 1997)     

Magnetic breakdown above the “compressibility collapse” transition in ReO3

We report a new cross-sectional area of the ReO₃ Fermi surface above the compressibility collapse transition. Magnetic breakdown at low fields destroys the gaps at the new Brillouin zone faces of the high-pressure phase so that the observed Fermi surface is essentially identical in both phases at the fields where it can be observed.     

Radionuclide 188Re-Loaded Photothermal Hydrogel for Cancer Theranostics

Herein, an injectable photothermal hydrogel system containing a therapeutic radionuclide ¹⁸⁸Re is studied for combined radioisotope therapy and photothermal therapy (PTT) of cancer. A dopamine-conjugated poly(α,β-aspartic acid) copolymer (PDAEA) is used to trigger a sol–gel phase transition in mixture with Fe³⁺ ions, rapidly forming a gel by simply mixing PDAEA and FeCl₃ phosphate buffer saline solutions. The injectable hydrogel exhibits strong near-infrared light absorbance and can efficiently convert light into a heating effect for local PTT treatment. The obtained hydrogel possesses a porous 3D microstructure, and can be utilized for radionuclide loading. After the Na¹⁸⁸ReO₄ loading, the hydrogel is intratumorally injected into the tumor of mice bearing 4T1 murine breast cancer cells for studying the tumor retention and therapeutic efficiency. In vivo results show that Na¹⁸⁸ReO₄-loaded hydrogel exhibits significantly longer time in the tumor sites than that of free Na¹⁸⁸ReO₄. The tumor growth of mice treated with Na¹⁸⁸ReO₄-loaded hydrogel under near-infrared radiation is significantly inhibited compared with control groups. Therefore, the results show that the developed strategy using an injectable and biocompatible hydrogel may promote the applications of radioisotope therapy and photothermal therapy for cancer.     

A pressure induced anomaly in the Fermi surface of ReO3

De Haas-van Alphen frequencies of ReO₃ have been measured as a function of pressure up to 5 kbar with the magnetic field along [111] and [001] crystallographic directions. The variation of the frequencies with pressure is linear at low pressure and for the α frequency follows free electron scaling predictions. Above 3 kbar, all cross-sectional areas increase much faster with pressure. The area compressibilities are one order of magnitude higher than the low-pressure area compressibilities. This anomalous increase may be attributed to an increase in covalent bonding or a rotation of the oxygen atoms at high pressure.     

The band electronic structures of (BEDTTTF)2X,X− = BrO4− and ReO4−

The strictly isostructural organic metals, (BEDTTTF)₂ReO₄ and (BEDTTTF)₂BrO₄ consist of sheets of BEDT-TTF (abbreviated ET) donor molecules which form a novel two-dimensional network. Despite the similar structures these salts possess different physical properties. (ET)₂ReO₄ is a highly anisotropic metal down to 80 K but becomes insulating below this temperature. An applied pressure greater than 4.5 kbar reportedly suppresses the MI transition and gives superconductivity at 2 K. However, no superconductivity has yet been observed for (ET)₂BrO₄. Band electronic structure calculations derived from their ET molecule network geometries at 298 and 120 K, and intrastack and interstack energies between nearest neighbor ET molecules, show that (ET)₂ReO₄ and (ET)₂BrO₄ are highly anisotropic metals, and both possess very similar band structures.     

Determination of bond lengths in some compounds of rhenium from the x-ray LIII absorption discontinuity

The shapes and the fine structure of the x-ray L_III absorption discontinuity of rhenium have been studied in seven octahedral (K₂ReCl₆, K₂ReBr₆, (pyH)₂ReCl₆, (dipyH)₂ReCl₆, Re(dipy)Cl₄, K₃ReO₂(CN)₄ and ReO₃) and three tetrahedral (KReO₄, NH₄ReO₄ and NaReO₄) compounds. The bond lengths in these compounds have been determined by applying Levy’s theory to the data on the fine structure associated with the edge beyond the molecular orbital region.     

Scaling behavior of plasmon coupling in Au and ReO3 nanoparticles incorporated in polymer matrices

Polymer nanocomposites containing different concentrations of Au nanoparticles have been investigated by small angle X‐ray scattering and electronic absorption spectroscopy. The variation in the surface plasmon resonance (SPR) band of Au nanoparticles with concentration is described by a scaling law. The variation in the plasmon band of ReO₃ nanoparticles embedded in polymers also follows a similar scaling law.

     

Electronic structure of rhenium oxides

The photoelectron spectra of Re 4? electrons observed in the course of reduction of ReO₃ and oxidation of metallic rhenium are reported. On the basis of SCF-Xα calculations for double-octahedral models of different rhenium oxides, the formation of multiple metal-metal bonds in clusters of edge-sharing octahedra on shear planes and the presence of 2+ and 5+ apparent oxidation states in ReO₂ and ReO₃ respectively, are inferred.     

Characterisation of the Fermi surface and phase transitions of (BEDO-TTF)2 ReO4·(H2O) by physical property measurements and electronic band structure calculations

The electronic properties of the organic superconductor (BEDO-TTF)₂ ReO₄·(H₂O) were investigated by temperature dependent resistivity, ESR, Hall effect and magnetoresistance measurements. Shubnikov-de Haas (SdH) oscillations were observed in magnetic fields up to 24 T in the temperature range 0.5 K to 4.2 K. The electronic band structure of (BEDO-TTF)₂ ReO₄·(H₂O) was calculated by employing the extended Hückel tight binding method on the basis of its room temperature crystal structure. The two observed SdH frequencies of 75 T and 37 T correspond very well with two cross-sectional areas of the hole and electron Fermi surface pockets obtained from the tight binding calculation. From the temperature dependence of the SdH oscillation amplitudes, the cyclotron effective mass (m_c) belonging to the larger and smaller pockets were found to be 0.9 m₀ and m_c=1.15 m₀ respectively. Measurements of the angular dependence of the SdH frequencies show no deviation from that expected for a cylindrical Fermi surface. In terms of our tight binding calculations and experimental measurements, probable causes for the 213 K and 35 K phase transitions are discussed. The calculations show that (BEDO-TTF)₂ ReO₄·(H₂O) is a two dimensional semimetal but possesses a hidden nesting. The latter is likely to cause an SDW instability leading to the 35 K transition. The resistivity drop associated with the 213 K transition is likely to be induced by an abrupt increase in the relaxation time. The excellent agreement between the calculated and experimentally observed Fermi surface implies that, with decreasing temperature below 35 K, (BEDO-TTF)₂ ReO₄·(H₂O) gradually gets out of the SDW state and re-enters the original metallic state, in which it becomes superconducting below 2.4 K.Reported at the 13th Genral Conference of the Condensed Matter Division of the European Physical Society, Regensburg, March 1993