Kumar Brajesh

Orthorhombic-tetragonal phase coexistence and enhanced piezo-response at room temperature in Zr, Sn, and Hf modified BaTiO3

The effect of Zr, Hf, and Sn in BaTiO3 has been investigated at close composition intervals in the dilute concentration limit. Detailed structural analysis by x-ray and neutron powder diffraction revealed that merely 2mol. % of Zr, Sn, and Hf stabilizes a coexistence of orthorhombic (Amm2) and tetragonal (P4mm) phases at room temperature. As a consequence, all the three systems show substantial enhancement in the longitudinal piezoelectric coefficient (d(33)), with Sn modification exhibiting the highest value similar to 425 pC/N

Structural perspective on the anomalous weak-field piezoelectric response at the polymorphic phase boundaries of (Ba,Ca)(Ti,M)O3 lead-free piezoelectrics ( M=Zr , Sn, Hf)

Although, as part of a general phenomenon, the piezoelectric response of Ba(Ti-1 M-y(y))O-3 (M = Zr, Sn, Hf) increases in the vicinity of the orthorhombic (Amm2)-tetragonal(P4mm) and orthorhombic (Amm2)-rhombohedral (R3m) polymorphic phase boundaries, experiments in the last few years have shown that the same phase boundaries show significantly enhanced weak-field piezoproperties in the Ca-modified variants of these ferroelectric alloys, i.e., (Ba, Ca)(Ti, M)O-3. So far there is a lack of clarity with regard to the unique feature(s) which Ca modification brings about that enables this significant enhancement. Here, we examine this issue from a structural standpoint with M = Sn as a case study. We carried out a comprehensive comparative structural, ferroelectric, and piezoelectric analysis of the Amm2 phase in the immediate vicinity of the P4mm-Amm2 phase boundaries of (i) Ca-modified Ba(Ti, Sn)O-3, as per the nominal formula (1-x)BaTi0.88Sn0.12O3-(x)Ba0.7Ca0.3TiO3 and (ii) without Ca modification, i.e., Ba(Ti1-ySny) O3. We found that the spontaneous lattice strain of the Amm2 phase is noticeably smaller in the Ca-modified counterpart. Interestingly, this happens along with an improved spontaneous polarization by enhancing the covalent character of the Ti-O bond. Our study suggests that the unique role of Ca modification lies in its ability to induce these seemingly contrasting features (reduction in spontaneous lattice strain but increase in polarization).

Phase transitions in double perovskite Sr2ScSbO6: An Ab-initio study

First Principles study of the electronic properties of recently synthesized double perovskite Sr2ScSbO6 have been performed using density functional theory. With increasing temperature, the Sr compound undergoes three structural phase transitions at 400K, 550K and 650K approximately, leading to the following sequence of phases: P21/n → I2/m → I4/m → Fm-3m. Starting from the monoclinic phase P21/n at room temperature, resulting from the Sc/Sb ordering, the electronic structure for the tetragonal I4/m at 613K and cubic Fm-3m for T≥660K has been studied in terms of the density of states and band-structure. Presence of large band gap, both direct and indirect, has been reported and analyzed.

Electronic structure of ordered double perovskite Ba2CoWO6

Ba2CoWO6 (BCoW) has been synthesized in polycrystalline form by solid state reaction at 1200 °C. Structural characterization of the compound was done through X-ray diffraction (XRD) followed by Rietveld analysis of the XRD pattern. The crystal structure is cubic, space group Fm-3m (No. 225) with the lattice parameter, a = 8.210. Optical band-gap of the present system has been calculated using the UV-Vis Spectroscopy and Kubelka-Munk function, it’s value being 2.45 eV. A detailed study of the electronic properties has also been carried out using the density functional theory (DFT) techniques implemented on WIEN2k. Importance of electron-electron interaction between the Co ions leading to half-metallic behavior, crystal and exchange splitting together with the hybridization between O and Co, W has been investigated using the total and partial density of states.

Dielectric Studies of Ba Substitution in Lead Zinc Niobate-Lead Zirconium Titanate Ceramics by Precipitation Method

The polycrystalline samples of (Pb0.90Ba0.10) [(Zn1/3Nb2/3)0.20-(Zr0.51Ti0.49)0.80]O3 [PBZN-PZT] compositions ceramics have been synthesized by substitution of barium in PZN-PZT by precipitation method and finally by modified columbite method. X-ray diffraction (XRD) analysis of (Pb0.90Ba0.10)(Zn1/3Nb2/3)O3 (PBZN) and PBZN-PZT shows a cubic and mixed phase of tetragonal and rhombohedral pure perovskite structure at room temperature. The dielectric dispersion of the solid solutions of PBZN-PZT has been studied as a function of temperature for the frequency range from 100 Hz to 1 MHz. The temperature (T) variation of the real components (ϵ′) at different frequencies of the dielectric permittivity shows a broad maximum. The diffuse peaks in permittivity versus temperature confirm the relaxor behaviour of PBZN-PZT. The frequency dependence of maximum temperature (T m) in these compounds with barium substitution is modeled using Vogel-Fulcher relation.

Structural refinement and optical band gap studies of manganese-doped modified sodium potassium lithium niobate lead — piezoelectric ceramics

Li-doped NKLN ceramic (Na0.5 K0.5)0.935Li0.065NbO3 (NKLN935) in pure and MnO2 doped compositions have been revisited to carry out a detailed analysis of the structural and optical properties. Rietveld analysis of the X-ray diffraction (XRD) pattern reveals the system to be tetragonal (space group P4mm). UV–Visible (UV–Vis) spectroscopy and an equivalent Kubelka–Munk function is used to obtain the optical band gap values. It is reported that with increasing Mn doping, the band gap values decreases, which has been analyzed and understood in terms of the tetragonal structure and is found to be consistent with dielectric properties.

Dielectric Relaxation and Phase Transition in [Pb0.94Sr.06][(Mn1/3Sb2/3)0.05(Zr0.55Ti0.45)0.95]O3 Ceramics

A piezoelectric ceramic composition [Pb0.94Sr0.06][(Mn1/3Sb2/3)0.05 (Zr0.55Ti00.45)0.95]O3 which is 6% Sr2+ substituted PZT ceramics with lead manganese antimonite as an additional hardener dopant is synthesized in pure perovskite phase by ceramic route. The X-ray diffraction of the sample at the room temperature shows an orthorhombic phase. The precursor [Pb0.94Sr.06]CO3 is prepared by coprecipitation method to ensure homogeneous distribution of Sr2+ ions at A-site while the other columbite precursor MnSb2O6 is prepared by conventional solid state reaction method to prevent the formation of the unwanted pyrochlore phase. The field dependences of the dielectric response and conductivity are measured in a frequency range from 100 Hz to 1 MHz and in a temperature range from 300 k to about 775 k The dielectric measurements (real and imaginary parts) of this composition with temperature (300 k–775 k) at different frequencies (100 Hz–1 MHz) unambiguously point towards relaxor behaviour of the material. The real part of the dielectric constant is found to decrease with increasing frequency at different temperatures while the position of dielectric loss peak shifts to higher frequencies with increasing temperature indicating a strong dispersion beyond the transition temperature, a feature known for relaxational systems such as dipole glasses. The frequency dependence of the loss peak obeys an Arrhenius law with an activation energy 0.14 eV. The distribution of relaxation times is confirmed by Cole-Cole plots. The frequency-dependent electrical data are also analyzed in the framework of the conductivity and modulus formalisms. Both these formalisms yield qualitative similarities in the relaxation times. The SEM photographs of the sintered specimens present the homogenous structures and well-grown grains with a sharp grain boundary. The equality of the Curie and transition temperatures reveals that it undergoes second order phase transition