Sunil Kumar

Investigation of La and Al substitution on the spontaneous polarization and lattice dynamics of the Pb(1-x)LaxTi(1-x)AlxO3 ceramics

The phase purity and crystal structure of PLTA samples (synthesized via sol-gel process) were confirmed using synchrotron x-ray powder diffraction (wavelength, lmbda= 0.44573 A. Rietveld analyses of powder x-ray diffraction data confirmed the tetragonal structure for compositions with more than 0.18 and cubic structure for the sample with 0.25 composition. Temperature-dependent XRD was performed to investigate the structural change from tetragonal to cubic structure phase transition. Raman spectroscopy at room temperature also confirmed this phase transition with composition. Field emission scanning electron provided information about surface morphology while an energy dispersive x-ray spectrometer attached with FESEM confirmed the chemical compositions of samples. Temperature and frequency dependent dielectric studies showed that the tetragonal to cubic phase transition decreased from 680 K to 175 K with the increase in the x from 0.03 to 0.25, respectively. This is correlated with the structural studies. Electric field dependent spontaneous polarization showed proper ferroelectric loop for 0.06 to 0.18 belonging to a tetragonal phase while after 0.25 composition the spontaneous polarization vanishes.Bipolar strain versus electric field revealed a butterfly loop for 0.06 to 0.18 compositions. Energy storage efficiency initially increases nominally with substitution but beyond 0.18 composition enhances considerably.

Role of compensating Li/Fe incorporation in Cu0.945Fe0.055−xLixO: structural, vibrational and magnetic properties

Doped transition metal oxides, like CuO, are spintronic materials. An increase of magnetic moment has been reported in Fe-doped CuO. Additional secondary doping elements such as Li may further modify magnetism in transition metal oxides due to changes in valence state and size. Such complex doping may generate secondary impurity phases, restricting the success of such applications. Enhancement of magnetic properties strengthens applicability of the materials in devices. Single phase monoclinic Cu0.945Fe0.055−xLixO crystalline powders without any impurity are synthesized. Structural studies, electronic valence state, fine structure, and local geometry of constituent elements confirm the purity of the materials and provide clues to probable reasons leading to enhanced magnetism. Ferromagnetic coupling between neighboring spins is most likely dependent on the spin and separation between the spin species.

Sol–gel synthesis and characterization of a new four-layer K0.5Gd0.5Bi4Ti4O15 Aurivillius phase

Monophasic K0.5Gd0.5Bi4Ti4O15 powders were synthesized via a citrate-based sol–gel route. Rietveld analysis of powder X-ray diffraction data confirmed the composition to be a pure four-layer Aurivillius phase with an orthorhombic structure (space group A21am and unit cell parameters au2009=u20095.42798(52)xa0Å, bu2009=u20095.41821(41)xa0Å, and cu2009=u200941.2723(33)xa0Å) at room temperature. The chemical composition and the morphology of the sintered pellets were examined by field emission scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS). The dielectric properties were investigated as a function of temperature (27–600u2009°C) at various frequencies (10xa0Hz–1xa0MHz), and the phase transition was observed at 560u2009°C. Ferroelectric nature of K0.5Gd0.5Bi4Ti4O15 was demonstrated by the polarization–electric field (P–E) hysteresis loop. Equivalent circuit modeling of the complex impedance data was employed to determine the conduction behavior. The temperature dependence of dc conductivity was found to follow the Arrhenius law associated with the activation energy of 1.22u2009±u20090.02xa0eV and was attributed to the long-range movement of oxygen vacancies.

X-ray structural studies on solubility of Fe substituted CuO

CuO is a promising material for the spintronic industry for which lattice distortions/defects play an important role in determining its magnetic and various other physical properties. The ionic radii and charge of Cu2+[VI] (0.73 A) and Fe3+[VI] (0.64 A) are quite different. Hence high Fe substitution in CuO in place of Cu may generate strain/distortions. Fe substitution may enhance magnetic properties, even at room temperature, making such materials interesting for device applications. A detailed structural study on Fe incorporated CuO lattices to confirm phase purity, supported by evidence of the absence of a secondary phase is absolutely essential especially when considering a considerable substitution of up to ∼12.5%. The electronic valence state, fine structure and local neighborhood/geometry of constituent elements need to be investigated using synchrotron based X-ray absorption spectroscopy (XAS). We report, for the first time, such a detailed study on understanding this magnetically and electronically important material: Cu1−xFexO, 0 ≤ x ≤ 0.125.

Cu1–xFexO: hopping transport and ferromagnetism

Single phase, sol–gel prepared Cu1–xFexO (0u2009≤u2009xu2009≤u20090.125) powders are characterized in terms of structural, electronic and magnetic properties. Using dielectric and magnetic studies we investigate the coupling of electron and spin. The electrical conductivities and activation energies are studied with increasing Fe content. Modelling of experimental conductivity data emphasizes a single hopping mechanism for all samples except xu2009=u20090.125, which have two activation energies. Hole doping is confirmed by confirming a majority Fe3+ substitution of Cu2+ in CuO from X-ray photoelectron spectroscopy studies (XPS). Such a substitution results in stabilized ferromagnetism. Fe substitution introduces variation in coercivity as an intrinsic magnetic property in Fe-doped CuO, and not as a secondary impurity phase.

Structure, dielectric, and piezoelectric properties of K0.5Na0.5NbO3-based lead-free ceramics

Lead-free ceramics based on the (1 − x)K0.5Na0.5NbO3–xBi(Zn0.5Ti0.5)O3 (KNN–BZT) system obtained via the conventional solid-state processing technique were characterized for their crystal structure, microstructure, and electrical properties. Rietveld analysis of X-ray diffraction data confirmed the formation of a stable perovskite phase for Bi(Zn0.5Ti0.5)O3 substitutions up to 30 mol%. The crystal structure was found to transform from orthorhombic Amm2 to cubic Pmm through mixed rhombohedral and tetragonal phases with the increase in Bi(Zn0.5Ti0.5)O3 content. Temperature-dependent dielectric behavior indicated an increase in diffuseness of both orthorhombic to tetragonal and tetragonal to cubic phase transitions as well as a gradual shift towards room temperature. The sample with x ≈ 0.02 exhibited a mixed rhombohedral and orthorhombic phase at room temperature. A high-temperature X-ray diffraction study confirmed the strong temperature dependence of the phase coexistence. The sample with the composition 0.98(K0.5Na0.5NbO3)–0.02(BiZn0.5Ti0.5O3) showed an improved room temperature piezoelectric coefficient d33 = 109 pC/N and a high Curie temperature TC = 383 °C.

Increase in depolarization temperature and improvement in ferroelectric properties by V5+ doping in lead-free 0.94(Na0.50Bi0.50)TiO3-0.06BaTiO3 ceramics

A detailed study was carried out to investigate the effects of poling on structure, vibrational, dielectric, and ferroelectric properties of donor-doped (V5+ at Ti4+-site) lead-free Na0.47Bi0.47Ba0.06Ti(1-x)VxO3, (xu2009=u20090, 0.01, and 0.03) ceramics fabricated via a modified sol-gel method. Rietveld refinement of synchrotron radiation source powder x-ray diffraction data showed that unpoled samples are in rhombohedral R3c phase whereas poled samples showed a mix rhombohedral R3c and tetragonal P4mm phases at ambient temperature, due to a long-range order established in lattice system after poling. V+5 doping increases the rhombohedral distortion in unpoled and poled samples while it reduces the tetragonality in poled samples. Vibrational study revealed that unpoled samples have more lattice disorder compared to poled samples. X-ray absorption near edge spectroscopy measurement confirmed that Ti and V are in 4+ and 5+ oxidation states, respectively, for all poled and unpoled samples. The average grain size was found to decrease from 5.6u2009±u20090.5u2009μm for xu2009=u20090 to 1.0u2009±u20090.2u2009μm for xu2009=u20090.03. Depolarization temperature was found to increase significantly in poled samples from ∼104u2009°C for undoped sample to 150u2009°C for the sample with 1% vanadium substitution. Drastic improvements in ferroelectric and dielectric properties are explained in terms of structural changes. High remnant polarization Pru2009∼u200931.4 μC/cm2 and moderately low coercive field Ec ∼ 20u2009kV/cm have been observed at an applied electric field of ∼35u2009kV/cm for the sample with 1% vanadium substitution which makes it an attractive candidate for ferroelectric applications.A detailed study was carried out to investigate the effects of poling on structure, vibrational, dielectric, and ferroelectric properties of donor-doped (V5+ at Ti4+-site) lead-free Na0.47Bi0.47Ba0.06Ti(1-x)VxO3, (xu2009=u20090, 0.01, and 0.03) ceramics fabricated via a modified sol-gel method. Rietveld refinement of synchrotron radiation source powder x-ray diffraction data showed that unpoled samples are in rhombohedral R3c phase whereas poled samples showed a mix rhombohedral R3c and tetragonal P4mm phases at ambient temperature, due to a long-range order established in lattice system after poling. V+5 doping increases the rhombohedral distortion in unpoled and poled samples while it reduces the tetragonality in poled samples. Vibrational study revealed that unpoled samples have more lattice disorder compared to poled samples. X-ray absorption near edge spectroscopy measurement confirmed that Ti and V are in 4+ and 5+ oxidation states, respectively, for all poled and unpoled samples. The average grain size was...

Structure, electronic and photoluminescence study of Si doped ZnO nano-particles

A detailed structural and electronicstudy of sol-gel synthesized and 1100°C calcined Zn1-xSixO (0 ≤ x ≤ 0.06) nano-particles were carried out using XRD, FESEM, UV- vis/PL spectroscopy studies. XRD reveals wurzite structure at (0 ≤ x ≤ 0.03) after which some extra phase is seen which is also supported by FESEM images.UVs spectroscopy revealed that band gap increases and urbach energy decreases with substitution. Due to doping of Si4+, oxygen deficiency is decreased which improve structural properties in spite of introduction of strains due to lattice contraction.Photoluminescence studiesrevealed that the reduction in defects state in the sample with Si4+ substitution.