Structural evolution in iron tellurates

Three new members within the iron tellurate family have been synthesized and characterized by single-crystal diffraction. Fe₂Te₃O₉ is orthorhombic, , , , Z=4, space group Pnma, final agreement factors R₁=0.0261(wR₂=0.0688) for 1271 independent reflections. Fe₃Te₄O₁₂ is monoclinic, , , , β=107.950(10)^°, Z=4, space group P2₁/c, final agreement factors R₁=0.0380(wR₂=0.0281) for 3302 independent reflections. FeTe₆O₁₃ is trigonal, , , Z=6, space group , final agreement factors R₁=0.0309(wR₂=0.0641) for 1264 independent reflections. Together with the four already known members of the family, Fe₂TeO₅, Fe₂TeO₆, and Fe₂Te₃O₉ (a dimorphic variant of the afore-mentioned structure with the same chemical formula), and Fe₂Te₄O₁₁, the iron tellurates now span from relatively Fe-rich and Te-poor to relatively Fe-poor to Te-rich compounds. The structural diversity within Fe-Te-O system is discussed in terms of the lone-pair stereochemistry of the Te⁴⁺ anion and the cross-over from Fe³⁺ to mixed-valence Fe³⁺/Fe²⁺ and Fe²⁺ coordination polyhedra compounds.     

Synthesis and VUV-UV spectroscopic properties of rare earth borosilicate oxyapatite: RE5Si2BO13:Ln (RE=La, Gd, Y; Ln=Eu, Tb)

Three rare earth borosilicate oxyapatites, RE₅Si₂BO₁₃ (RE=La, Gd, Y), were synthesized via wet chemical method, of which RE₅Si₂BO₁₃ (RE=Gd, Y) were first reported in this work. In the three oxyapatites, [BO₄] and [SiO₄] share the [TO₄] tetrahedral oxyanion site, and RE³⁺ ions occupy all metal sites. The differential scanning calorimetry-thermo gravimetry measurements and high temperature powder X-ray diffraction pattern revealed a vitrification process within 300-1200 °C, which was due to the glass-forming nature of borosilicates. From the VUV excitation spectra of Eu³⁺ and Tb³⁺ in RE₅Si₂BO₁₃, the optical band gaps were found to be 6.31, 6.54 and 6.72 eV for RE₅Si₂BO₁₃ (RE=La, Gd, Y), respectively. The emission and excitation bands of Eu³⁺ and Tb³⁺ are discussed relating with their coordination environments. Among the three hosts, Y₅Si₂BO₁₃ would be the best for Eu³⁺ and Tb³⁺-doped phosphors.     

Synthesis and characterization of rare-earth doped SrBi2Nb2O9 phase in lithium borate based nanocrystallized glasses

Glass composites comprising of un-doped and samarium-doped SrBi₂Nb₂O₉ nanocrystallites are fabricated in the glass system 16.66SrO-16.66[(1−x)Bi₂O₃-xSm₂O₃]-16.66Nb₂O₅-50Li₂B₄O₇ (0?x?0.5, in mol%) via the melt quenching technique. The glassy nature of the as-quenched samples is established by differential thermal analyses. Transmission electron microscopic studies reveal the presence of about 15 nm sized spherical crystallites of the fluorite-like SrBi_1.9Sm_0.1Nb₂O₉ phase in the samples heat treated at 530 °C. The formation of layered perovskite-type un-doped and samarium-doped SrBi₂Nb₂O₉ nanocrystallites with an orthorhombic structure through the intermediate fluorite phase is confirmed by X-ray powder diffraction and micro-Raman spectroscopic studies. The influence of samarium doping on the lattice parameters, lattice distortions, and the Raman peak positions of SrBi₂Nb₂O₉ perovskite phase is clarified. The dielectric constants of the perovskite SrBi₂Nb₂O₉ and SrBi_1.9Sm_0.1Nb₂O₉ nanocrystals are relatively larger than those of the corresponding fluorite-like phase and the precursor glass.     

Ultrafast relaxation and exciton–exciton annihilation in PTCDA thin films at high excitation densities

Femtosecond pump–probe spectroscopy is applied to thin films of the quasi-one-dimensional organic semiconductor 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA). We present transient absorption spectra over a broad spectral range. Ultrafast intraband relaxation in the S₁ manifold towards the border of the Brillouin zone is shown to depend on temperature and excitation density. The intraband relaxation time is of the order of 100 fs. At high excitation densities (>10¹⁹ cm⁻³), the major de-excitation mechanism for the relaxed excitons is exciton–exciton annihilation. The experimental decay dynamics can be explained very well by two alternative annihilation models: one-dimensional diffusion limited bimolecular recombination or single-step long range Förster-type annihilation. In contrast, a three-dimensional diffusion limited annihilation model is significantly inferior. For all three models, we extract annihilation rates, diffusion constants, diffusion lengths, and Förster radii for room and liquid Helium temperature.     

Dynamics of superionic Rb3H(SeO4)2 single crystals studied by H and Rb spin-lattice relaxation

The ¹H and ⁸⁷Rb spin-lattice relaxation and spin-spin relaxation times in superionic Rb₃H(SeO₄)₂ single crystals grown by the slow evaporation method were measured over the temperature range 160-450 K. The temperature dependencies of the ¹H T₁, T_1ρ, and T₂ are measured. In the ferroelastic phase, T₁ differs from T_1ρ, which is in turn different from T₂, although these three relaxation times converge to similar values near 410 K. This transition seems to occur at temperature which is about 40 K lower than the superionic transition temperature. The observation of liquid-like values of the ¹H T₁, T_1ρ, and T₂ in the high temperature is compatible with the phase being superionic, indicating that the destruction and reconstruction of hydrogen bonds does indeed occur at high temperature. In addition, the ⁸⁷Rb T₁ and T₂ values at high temperature were similar (on the order of milliseconds), a trend that was also observed for ¹H T₁ and T₂. This behavior is expected for most hopping-type ionic conductors, and could be attributed to interactions between the mobile ions and the neighboring group ions within the crystal. The motion giving rise to this liquid-like behavior is related to the superionic motion.     

Synthesis of crystalline yttrium oxycarbonate in a single phase

Crystalline yttrium oxycarbonate, Y₂O₂CO₃, was synthesized by our original flux method using the 0.476Li₂CO₃-0.270Na₂CO₃-0.254K₂CO₃ eutectic mixture at 450 °C. The oxycarbonate was characterized by X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), and Fourier-transfer infrared spectroscopy (FT-IR). The Y₂O₂CO₃ prepared by the flux method was a crystalline single phase of hexagonal rare earth oxycarbonate (type-II). This is the first report on the single-phase synthesis of crystalline hexagonal type-II Y₂O₂CO₃.     

Preparation of the cubic-type La2O3 phase by thermal decomposition of LaI3

Cubic lanthanum oxide was prepared by the oxidation of lanthanum iodide at 700 °C in air atmosphere. The oxide was characterized by X-ray fluorescence analysis, X-ray diffraction, and Fourier-transformed infrared spectroscopy. The cubic La₂O₃ is most likely a single lanthanum oxide phase containing periodate hydrate and hydroxycarbonate species. The cubic lanthanum oxide is found to be chemically stable even if they are dispersed in water because of the presence of hydroxycarbonate and periodate hydrate species which inhibit the bulk hydroxylation.     

Growth and characterization of 2-amino-4-picolinium toluene sulfonate single crystal

2-Amino-4-picolinium toluene sulfonate (2A4PTS), a new organic material, was synthesized and grown as single crystals in room temperature by slow evaporation solution growth technique using water as solvent. The crystal structure of 2A4PTS has been determined using single crystal X-ray diffraction studies. 2A4PTS belongs to monoclinic crystal system. The molecular arrangements in the crystal were studied. The structural perfection of the grown crystals has been analysed by high-resolution X-ray diffraction (HRXRD) rocking curve measurements. Fourier transform infrared (FTIR) spectral studies have been performed to identify the functional groups. The optical transmittance window and the lower cutoff wavelength of the 2A4PTS have been identified by UV–Vis–NIR studies. The nonlinear optical properties have been investigated by Z-scan method. The nonlinear refractive index and linear absorption coefficient of the 2A4PTS are found to be in the order of 10⁻⁸ cm²/W and 10⁻⁴ cm/W, respectively. The laser induced surface damage threshold for the grown crystal was measured using Nd:YAG laser. Thermal analysis carried out on the compound reveals that 2A4PTS is stable up to 133 °C. The microhardness test was carried out and the load dependent hardness was measured.     

Lattice dynamics of TTF–CA across the neutral–ionic transition

We present the temperature evolution of polarised Raman and IR reflectivity spectra of Tetrathiafulvalene–Chloranil (TTF–CA) in the lattice phonon region (10–200 cm⁻¹). Quasi Harmonic Lattice Dynamics (QHLD) calculations prove to be essential in discussing the assignment of the phonons and their evolution across the temperature induced neutral–ionic phase transition.     

Atomistic simulation on the site preference and mechanical properties of Th3Co4+xAl12−x and U3Co4+xAl12−x

An atomistic study is presented on the phase stability, site preference and lattice constants of the actinide intermetallic compounds Th₃Co_4+xAl_12−x and U₃Co_4+xAl_12−x. Calculations are based on a series of interatomic pair potentials related to the actinides and transition metals, which are obtained by a strict lattice inversion method. The lattice constants of Th₃Co_4+xAl_12−x and U₃Co_4+xAl_12−x are calculated for different values of x. The site preference of Co atoms at Al sites is also evaluated and the order is given as 6h, 4f, 2b and 12k for Th₃Co_4+xAl_12−x, and 6h, 4f, 12k and 2b for U₃Co_4+xAl_12−x. In addition, some simple mechanical properties such as the elastic constants and bulk modulus are investigated for the actinide compounds with complex structures.     

First electrochemical growth of Tb16O30 single crystal

Single crystals of Tb₁₆O₃₀ (TbO_1.875) were successfully grown for the first time by DC electrolysis of the Tb³⁺ ion conducting Tb₂(MoO₄)₃ solid electrolyte at 11 V, 900 °C. The Tb₁₆O₃₀ phase is the intermediate phase of fluorite-related rare earth oxides and it is extremely difficult to grow in a single crystal form, because this intermediate phase is usually obtained as one of the mixture of the fluorite related TbO_x phases. Because there are many non-stoichiometric phases in the terbium oxide system, it is impossible to grow a specific intermediate phase in a single crystal form by the conventional methods via melt. Although single crystals of TbO_x have been recently obtained by anodic electrocrystallization from alkaline hydroxide melts containing TbCl₃, the composition has been confirmed to be TbO_x with 1.75<x<1.82. On the contrary, the presently developed DC electrolysis method can be simply applicable at moderate temperatures around 900 °C to artificially grow an intermediate phase of Tb₁₆O₃₀ (x=1.875) in a single crystal form, which was evidenced by the electron diffraction patterns for each particle.     

Thickness dependence of PL efficiency of organic thin films

Thickness dependence of photoluminescence (PL) efficiency and spectral shape of phosphorescent organic thin films is investigated and theoretically analyzed. The PL efficiency increases with increasing thickness to reach the maximum 92% at around 50 nm. It reduces to 77% at the thickness of 130–140 nm and oscillates between the values upon further increment of thickness. The quenching of excitons at the surface of organic layer significantly reduces the PL efficiency when the film is very thin. If the film is thicker than the critical thickness for the waveguiding of emitted light, the waveguided power is absorbed during the propagation through the organic layer so that apparent PL efficiency is reduced by the amount. Microcavity effect formed by quartz/organic layer/air also affects the PL efficiency. The appropriate thickness to obtain the PL efficiency close to the intrinsic value of a film is just the critical thickness for waveguiding through the film.