Poorva Sharma

Study of Elastic Properties and Their Pressure Dependence of Semi Magnetic Semiconductors

A theoretical study of the elastic behavior in diluted magnetic semiconductors Zn 1- x Mn x Se ( x =0.016, 0.026 and 0.053) using a three-body interaction (TBI) caused by the electron-shell deformation of the overlapping ions is carried out. The estimated values of phase transition pressure and the vast volume discontinuity in pressure–volume (PV) phase diagram indicate the structural phase transition from zinc blende ( B 3) to rock salt ( B 1). The variation of second-order elastic constants with pressure resembles that observed in other compounds of zinc blende structure family. However, the inconsistency in the value of pressure derivative of theoretical and the experimental value of bulk modulus is attributed to the fact that we have derived our expressions neglecting thermal effects and assuming the overlap repulsion significant only up to nearest neighbors. The present approach has also succeeded in predicting the Born and relative stability criteria. It is revealed that interionic potential approac...

Structural phase transition and elastic properties of ZnSe at high pressure

We have evolved an effective interionic interaction potential to investigate the pressure-induced phase transitions from zinc blende (B3) to rock salt (B1) structure in II-VI [ZnSe] semiconductors. The elastic constants, including the long-range Coulomb and van der Waals (vdW) interactions and the short-range repulsive interaction of up to second-neighbor ions within the Hafemeister and Flygare approach, are deduced. Keeping in mind that both of the ions are polarisable, we employed the Slater-Kirkwood variational method to estimate the vdW coefficients. The estimated value of the phase transition pressure (P t ) is higher than in the reported data, and the magnitude of the discontinuity in volume at the transition pressure is consistent with that data. The major volume discontinuity in the pressure-volume phase diagram identifies the structural phase transition from zinc blende to rock salt structure. The variation of second-order elastic constants with pressure resembles that observed in some binary semiconductors. It is inferred that the vdW interaction is effective in obtaining the thermodynamic parameters such as the Debye temperature, the Gruneisen parameter, the thermal expansion coefficient and the compressibility. However, the inconsistency between the thermodynamic parameters as obtained from present model calculations and their experimental values is attributed to the fact that we have derived our expressions by assuming the overlap repulsion to be significant only up to the nearest second-neighbor ions, as well as neglecting thermal effects. It is thus argued that full analysis of the many physical interactions that are essential to binary semiconductors will lead to a consistent explanation of the structural and elastic properties of II–VI semiconductors.

Pressure dependence of elastic properties of ZnX (X = Se,S and Te): Role of charge transfer

An effective interaction potential (EIOP) is developed to invoke the pressure induced phase transition from zinc blende (B3) to rocksalt (B1) structure and anharmonic properties in ZnX (X = Se, S, Te) semiconductors. The effective interaction potential incorporates the long range Coulomb interaction, van der Waals interaction and short-range repulsive interaction up to second neighbour ions within the Hafemeister and Flygare approach as well as the charge transfer effects caused by the electron-shell deformation of the overlapping ions. The van der Waals coefficients are computed by the Slater Kirkwood variation method as a first step. Later on, we evaluate volume collapse, second order and third order elastic constants with pressure pointing to the systematic trends in all compounds of zinc blende structure and their thermal properties such as force constant, Gruneisen parameter, compressibility, Debye temperature etc. The vast volume discontinuity in pressure-volume (PV) phase diagram identifies the structural phase transition from zinc blende (B3) to rock salt (B1) structure and is consistent with those revealed from earlier reports.

Room temperature structure vibrational and dielectric properties of Ho modified YMnO3

The structural, vibrational, and dielectric properties of bulk Ho-doped Y1−xHoxMnO3 (x = 0, 0.03, 0.05) solids prepared by standard solid-state reaction route were investigated. X-ray diffraction (XRD) patterns confirmed the hexagonal P63cm structure of Y1−xHoxMnO3 (x = 0.0, 0.03, 0.05) ceramics. Rietveld refinements of XRD data revealed that the doping ions led to unit cell contraction in three directions due to nearly equal ionic radii of Ho3+ ion (0.901 A) substituted at the Y-site ion. The grain size of Ho-doped solids varied from 5 to 10 μm. For pristine h-YMnO3, the experimentally observed Raman scattering lines at around 151, 305, 460, and 682 cm−1 are of A1 symmetry, those at 410 cm−1 are of E1 symmetry, and the lines at 139 and 219 cm−1 are of E2 symmetry. Another interesting observation is the existence of an A1 line at 682 cm−1 and an E1 line at about 410 cm−1 which are much stronger than the remaining lines of A1 and E1 symmetries, respectively. The high value of dielectric constant and dielectric loss tangent at low frequency is explained by space charge polarization and the saturation in the high-frequency region is due to the electric dipoles not being in pace with the frequency of the applied electric field.

Structural and Raman scattering study of Ni-doped CoFe2O4

Raman scattering measurements were made on polycrystalline CoFe2O4 and Co0.5Ni0.5Fe2O4 ferrites as prepared by solid-state reaction route. Rietveld refined X-ray diffraction pattern confirmed the formation of single-phase and both of the samples perfectly indexed in cubic spinel structure with Fd3m space group. Slight reduction in the lattice parameter of Co0.5Ni0.5Fe2O4 has been observed as compared to CoFe2O4. From Raman scattering spectra, a shoulder like feature has been observed in both of the compounds reveals that octahedral site is occupied by Co, Ni and Fe ions and tetrahedral site is occupied by only Fe ion.

Structural, dielectric and ferroelectric properties of La and Ni codoped BiFeO3

The codoped Bi1-xLaxFe1-yNiyO3 (x = 0.0, 0.1; y = 0.0, 0.05) samples are processed by solid-state reaction route in order to study structural, dielectric and ferroelectric properties. X-ray diffraction patterns of La/Ni doped samples have been performed using the rhombohedral (R3c) symmetry. Raman scattering measurement infers nine Raman active phonon modes. The substitution of Ni ion at Fe-site in BiFeO3 essentially changes the modes position i.e. all the modes are observed to shift at lower wave number. Frequency variation of the dielectric constant (e′) and dielectric loss (tanδ) has been investigated. It decreases with increasing frequency of the applied alternating field and become constant at high frequencies signifies substantial role of interfacial polarization. The remnant polarization and coercive field for BLFO, BFNO, BLFNO are 0.08, 0.03, 0.64 μC/cm2 and 0.24, 0.22, 0.25 kV/cm, respectively. La/Ni ion doping in BFO has successfully enhanced the polarization.

X-ray diffraction and Raman scattering study of Cr-doped ZnFe2O4 spinel ferrites

XRD and Raman scattering measurements were made on polycrystalline ZnFe2-xCrxO4 (x=0.0, 0.1, 0.2) spinel ferrites as prepared by solid-state reaction route. Rietveld refined X-ray diffraction pattern confirmed the formation of single-phase and are indexed in cubic structure with Fd3m space group. Slight reduction in the lattice parameter from 8.435 to 8.410 Ǻ, with Cr3+ ion substitution has been observed. From Raman scattering spectra, the modes of ZnFe1.9Cr0.1O4 and ZnFe1.8Cr0.2O4 are shifted towards the lower frequency side as compared to ZnFe2O4, this red shift is attributed to higher atomic mass of Zn (65.39 amu) as compared to Fe (55.84 amu).

Structural and Ferroic Properties of La, Nd, and Dy Doped BiFeO3 Ceramics

Polycrystalline samples of Bi0.8RE0.2FeO3 (RE = La, Nd, and Dy) have been synthesized by solid-state reaction route. X-ray diffraction (XRD) patterns of Bi0.8La0.2FeO3 and Bi0.8Nd0.2FeO3 were indexed in rhombohedral (R3c) and triclinic (P1) structure, respectively. Rietveld refined XRD pattern of Bi0.8Dy0.2FeO3 confirms the biphasic (Pnma

Rare earth (La) and metal ion (Pb) substitution induced structural and multiferroic properties of bismuth ferrite

In this study, bulk samples of multiferroic with compositional formula, BiFeO3 and Bi0.825A0.175FeO3 (A = La, Pb), were synthesized by solid state reaction route. X-ray diffraction (XRD) along with the Rietveld refinement revealed the distorted rhombohedral (R3c) structure for pristine BiFeO3 and Bi0.825La0.175FeO3 and tetragonal (P4/mmm) for Bi0.825Pb0.175FeO3 ceramic. To support the structural results, bond length between atoms for both of the compounds was calculated. A change in Raman mode position in BiFeO3 (BFO) has been observed with La and Pb substitution from Raman scattering measurements and also recommended a structural change with rare earth and metal ion substitution at Bi site. From the frequency dependent dielectric constant and dielectric loss plots, a decrease in dielectric values with increase in frequency was observed for both of the samples. For microelectronic devices, porous ceramics with lower value of dielectric constant are most useful. Thus, further studies are also needed to carefully tune the magnetoelectric properties and structural distortion after La/Pb substitution in BFO.

Structural study of Ti-doped CoFe2O4 mixed spinel ferrite

We present the results on atomic and lattice structure of the polycrystalline spinel ferrites system Co1-x TixFe2O4 (x = 0.0, 0.25, 0.50) synthesized by following the conventional solid-state reaction route. The observed X-ray diffraction (XRD) data confirms that all the prepared samples are indexed in cubic crystal structure (space group Fd3m). Diffraction pattern showed TiO2 phase due to presence of Ti+4 ions. Four Raman active phonon modes are observed for CoFe2O4 sample existing around 295, 462, 585, 689, cm−1 as Eg, T2g(2), T2g(3), and A1g, respectively. With 25 % Ti ion doping, the peak T2g(3) disappears, while to that T2g(1) emerges. This is an indication of presence of TiO2 phase in Co0.75Ti0.25Fe2O4 and Co0.5Ti0.5Fe2O4 ceramics.