P. Prabhakar Rao

Electrical conductivity and impedance spectroscopy studies of cerium based aeschynite type semiconducting oxides: CeTiMO6 (M=Nb or Ta)

Complex ceramic oxides, CeTiMO6 (M=Nb or Ta) having aeschynite type mineral structure were prepared by the conventional ceramic route. Complex impedance analysis in the frequency range 10 Hz–1 MHz over a wide temperature range (30–600 °C) indicates the presence of grain boundary effect along with the bulk contribution and also confirms the presence of non-Debye type of multiple relaxations in the material. The frequency dependent conductivity plots exhibit double power law dependence suggesting three types of conduction mechanisms: low frequency (10 Hz–1 kHz) conductivity owing to long range translational motion of electrons (frequency independent), mid-frequency conductivity (1–10 kHz) due to short-range hopping, and high frequency (10 kHz–1 MHz) conduction due to localized orientation hopping mechanism. The hopping model can explain the nature of the conduction mechanism completely. The electrical conductivity measurements with temperature suggest the negative temperature coefficient of resistance behav...

Influence of cation substitution and activator site exchange on the photoluminescence properties of Eu3+-doped quaternary pyrochlore oxides.

Stannate-based pyrochlore-type red phosphors CaGd(1-x)SnNbO7:xEu(3+), Ca(1-y)Sr(y)Gd(1-x)SnNbO7:xEu(3+), and Ca(0.8-x)Sr0.2GdSnNbO(7+δ): xEu(3+) were prepared via conventional solid-state method. Influence of cation substitution and activator site control on the photoluminescence properties of these phosphors are elucidated using powder X-ray diffraction, Rietveld analysis, Raman spectrum analysis, and photoluminescence excitation and emission spectra. The Eu(3+) luminescence in quaternary pyrochlore lattice exemplifies as a very good structural probe for the detection of short-range disorder in the lattice, which otherwise is not detected by normal powder X-ray diffraction technique. The Eu(3+) emission due to magnetic dipole transition ((5)D0-(7)F1 MD) is modified with the increase in europium concentration in the quaternary pyrochlore red phosphors. (5)D0-(7)F1 MD transition splitting is not observable for low Eu(3+) doping because of the short-range disorder in the pyrochlore lattice. Appearance of narrow peaks in Raman spectra confirms that short-range disorder in the crystal lattice disappears with progressive europium doping. By using Sr as a network modifier ion in place of Ca we were able to increase the f-f transition intensities and europium quenching concentration. The influence of effective positive charge of the central Eu(3+) ions when it replaces a metal ion having lower oxidation state such as Ca(2+) was also investigated. The relative intensities of A1g (∼500 cm(-1)) and F2g (∼330 cm(-1)) Raman vibrational modes get inverted when Eu(3+) ions replaces Ca(2+) ions instead of Gd(3+) as trivalent europium ions can attract the electron cloud of oxygen ions strongly in comparison with divalent calcium ions. The influence of positive charge effect of Eu(3+) in Ca0.7Sr0.2GdSnNbO7+δ:0.1Eu(3+) phosphor is greatly strengthened the charge transfer band and (7)F0-(5)L6 transition intensities than that of the Ca0.8Sr0.2Gd0.9SnNbO7:0.1Eu(3+) phosphor. Our results suggest that the photoluminescence properties can be enhanced by simple compositional adjustments in the quaternary pyrochlore-type red phosphors.

Multiband Orange-Red-Emitting Phosphors SrY3SiP5O20 : Eu3 + under Near-UV Irradiation

The photoluminescence properties of silicon-based xenotime-type rare-earth phosphate as an orange-red-emitting phosphor material, Sry 3 Sip 5 O 20 :xEu 3+ (0.2, 0.6, 1, 2, 3, 5, 10, and 15 mol %), are reported. The photoluminescence spectra of the silicon phosphate indicated the simultaneous occurrence of six predominant orange-red band emissions due to doubly split magnetic-dipole ( 5 D 0 - 7 F 1 ), electric-dipole ( 5 D 0 - 7 F 2 ), and unusual ( 5 D 0 - 7 F 4 ) transitions under a near-UV wavelength excitation. The multiband emission of Eu 3+ at 5 D 0 - 7 F 1,2,4 is attributed to the odd-parity distortions of the Eu 3+ surrounding environment. With high Eu3+ concentration, the phosphor has a strong excitation due to f-f transitions appearing at around 396 nm, which correspond to the popular emission line from a near-UV light-emitting diode (LED) chip. Thus, the intense orange-red emission of the silicon phosphate phosphors under near-UV excitation suggests them to be a potential candidate for white light generation by using near-UV LEDs.

Role of bond strength on the lattice thermal expansion and oxide ion conductivity in quaternary pyrochlore solid solutions.

Quaternary pyrochlore-type solid solutions, CaGdZrNb(1-x)Ta(x)O(7) (x = 0, 0.2, 0.4, 0.6, 0.8, 1), were prepared by a high-temperature ceramic route. The pyrochlore phases of the compounds were confirmed by powder X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy. The crystallographic parameters of the pyrochlore compounds were accurately determined by Rietveld analysis of the powder XRD data. The isovalent substitution of Ta in place of Nb at the B site can reveal the effect of chemical bonding on lattice thermal expansion and oxide ion conductivity because both Nb and Ta have the same ionic radius (0.64 Å). Lattice thermal expansion coefficients of the samples were calculated from high-temperature XRD measurements, and it was found that the thermal expansion coefficient decreases with substitution of Ta. Oxide ion conductivity measured by a two-probe method also shows the same trend with substitution of Ta, and this can be attributed to the high bond strength of the Ta-O bond compared to that of the Nb-O bond. Microstructural characterization using scanning electron microscopy proves that the size of the grains has a small effect on the oxide ion conductivity. Our studies established the role of chemical bonding in deciding the conductivity of pyrochlore oxides and confirmed that the 48f-48f mechanism of oxide ion conduction is dominant in pyrochlore oxides.

Ba3Ce3Ti5Nb5O30—a novel ceramic oxide semiconductor

A novel ceramic oxide semiconductor Ba3Ce3Ti5Nb5O30 was prepared by solid state route. XRD shows that it has tetragonal tungsten bronze-type crystalline structure. The electrical conductivity at 30 °C is 3.49×10−7 ohm−1 cm−1 and at 600 °C is 1.2×10−2 ohm−1 cm−1. The activation energy of the carriers has been found to be 0.41 eV Ce+3 and is probably structurally stabilized in this compound.

Influence of Ce substitution on the order-to-disorder structural transition, thermal expansion and electrical properties in Sm2Zr2−xCexO7 system

Cerium substituted rare earth zirconates of the form Sm2Zr2−xCexO7 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) were synthesized via solid state reaction route. Structural, morphological and electrical characterizations were conducted using high temperature X-ray diffraction, FT Raman spectroscopy, thermo-mechanical analysis, scanning electron microscopy and impedance spectroscopy techniques. Addition of Ce ions into the crystal induced structural disorder to both cationic and anionic sublattices, gradually transforming the unit cell from an ordered pyrochlore to a defect fluorite structure. Substitution of bigger Ce ions to the Zr sites led to an increase in the lattice parameter and a decrease in thermal expansion coefficient of the material. The ionic disorder led to a decreased energy barrier for the thermally activated conduction process thereby increasing the overall conductivity of the materials. The maximum value of total conductivity observed in this study is 3.93 × 10−3 S cm−1 at 1023 K, which is higher than the recently reported values in a similar Sm2Zr2O7 system. Beyond x = 0.4, the ion–ion interaction in the disordered lattice began to dominate, leading to an increased activation energy and decreased total conductivity. The results demonstrate that the thermal expansion of these oxides is predominantly influenced by the B–O bond energy rather than the Madelung binding energy.

Monoclinic LaGa1−xMnxGe2O7: a new blue chromophore based on Mn3+ in the trigonal bipyramidal coordination with longer apical bond lengths

New blue inorganic oxide materials LaGa1−xMnxGe2O7 are developed by a solid state reaction method. Substitution of Mn3+ in LaGaGe2O7 changes the color from white (x = 0) to blue (x = 0.1–0.4). The blue color is due to the absorption in the energy region of 1.7–2.5 eV.

Preparation and characterisation of Ba3RE3Ti5Ta5O30 ceramics

Abstract Polycrystalline samples of Ba3RE3Ti5Ta5O30 (RE = Nd, Sm) were prepared by conventional solid state powder processing. The calcined compounds were pelletised and sintered at 1320, 1340, and 1360°C for 4 h. XRD structural and SEM microstructural characterisation showed the formation of single phase material with tetragonal crystal structure. Detailed studies of dielectric properties (ϵ, tanδ) as a function of frequency (100 Hz to 13 MHz) showed that the compounds had a dielectric constant around 120 and a loss factor of 6 × 10 - 3 at 1 MHz. At 1 MHz, the Nd and Sm compounds had dielectric constants of 123 and 117 respectively.

Pyrochlore-based oxides with small temperature coefficient of dielectric constant

Pyrochlore based oxides with small temperature coefficient of dielectric constant (TCK) in Ca–RE–Ti–M–O (RE=Sm or Dy and M=Nb or Ta) system are reported. The 1MHz dielectric constants of these oxides vary from 26 to 102. The TCK in the vicinity of room temperature (20°C) dramatically decreases, from more than −200 for CaRETiMO7 to less than −3ppm∕°C in the solid solutions of CaRETiNbO7–CaRETiTaO7 phases. Rietveld analysis of the x-ray diffraction data establishes a cubic pyrochlore-type phase in the space group Fd3m (No. 227).

Influence of phase transition from order to disorder and Philip's ionicity on the thermal expansion coefficient of pyrochlore type compositions with a multivalent environment

The present study involves a detailed investigation on interrelationships among Philips ionicity, thermal expansion coefficient and ionic conductivity in a new series of pyrochlore type compositions, RE2Y2/3Zr2/3Nb2/3O7 (RE = La, Nd, Sm, Gd, Dy, Y) using high temperature X-ray diffraction studies, a transmission electron microscope, a Raman spectroscopic technique, electrical conductivity measurements and crystal chemistry principles. The system undergoes structural transition from an ordered pyrochlore structure to a disordered defect fluorite structure in the series with a phase boundary upon Gd substitution. The ionicity difference of M–O bonds follows the thermal expansion trend in the fluorite phase but opposes it in the pyrochlore phase. The ionicity of the A–O bond contributes more to the thermal expansion coefficient in the pyrochlore system unlike in perovskites where it is due to the cumulative effect of the A–O and B–O bonds. On the contrary, the ionic conductivity increases with the decrease in the ionicity difference associated with the charge carrier concentrations. The best conductivity obtained in the series is 7.9356 × 10−6 S cm−1 for Y2(Y1/3Zr1/3Nb1/3)2O7 at 1023 K. The minimum thermal expansion coefficient is obtained at the phase boundary for Gd2Y2/3Zr2/3Nb2/3O7 and the value is 5.2 × 10−6 K−1. Furthermore, the anomalous behaviour of Sm and Nd compounds is explained based on the ionicity of M–O bonds. These results demonstrate that the ionicity difference plays a great role in determining the thermal expansion and ionic conductivity of pyrochlore based materials.