Raman spectra of ZnO and Co substituted Zn 1−x Co x O ͑ZCO͒ were carried out using the Raman microprobe system with an Ar + ion laser source of 514.5 nm wavelength. The shift towards the lower frequency side of the nonpolar E 2 low mode and the broadening due to Co substitution in ZnO were analyzed using the phonon confinement model. The magnetic measurements showed ferromagnetic behavior with the maximum saturation magnetization ͑1.2 B /Co͒ for 10% Co substitution, which decreased with at further increase in Co concentrations. The intensities of E 1 ͑LO͒ at 584 cm −1 and multiphonon modes at 540 cm −1 were increased with an increase in Co substitution. The additional Raman modes in ceramic targets of ZCO spectra for higher concentration of Co substitution ͑x =15%-20%͒ were identified to be due to the spinel ZnCo 2 O 4 secondary phase.

The multi phonon Raman scattering in Mn doped (1%–10%) ZnO was observed at room temperature using 514.5nm Ar+ laser. The additional optical modes at 327, 332, 482, 532, and 680cm−1 in Zn1−xMnxO targets were identified as the second order Raman modes in the disordered lattice and the precipitation of the secondary phase ZnMn2O4. The crystalline grain sizes of 1%, 3%, 5%, and 10% Mn doped ZnO samples were calculated by phonon confinement model as 31.8, 18.3, 15.9, and 14.1nm, respectively. The optical band gap was found to be increased (3.27–3.41eV) due to the Mn doping.

Antimony doped p-type ZnO films were grown on Al2O3 (0001) substrate by pulsed laser deposition. The structural properties of Zn1−xSbxO (3% and 5%) thin films were investigated by Raman scattering studies. The softening of local lattice due to the formation of (SbZn−2VZn) acceptor complexes was detected as the shift in E2high mode toward lower frequency side in ZnSbO thin films. Additional optical modes observed at 277, 333, 483, and 534 cm−1 are due to the breaking of translational symmetry in w-ZnO by Sb doping. The Zn–Sb related local vibrational mode was detected around 237 cm−1 in 5% Sb doped ZnO thin film. Room temperature Hall measurements exhibited low resistivity of 0.017 Ω cm, high hole concentration of 6.25×1018 cm−3, and mobility of 57.44 cm2/V s in the 5% Sb-doped ZnO thin film.

Two intriguing unresolved issues of iridate physics are the avoided metallization under applied pressure of undoped Sr2IrO4 and related materials, and the apparent absence of superconductivity under electron doping despite the similarity of the fermiology of these materials with respect to cuprates. Here, we investigate the crystal structure and lattice vibrations of Sr2IrO4 by a combined phonon Raman scattering and x-ray powder diffraction experiment under pressures up to 66 GPa and room temperature. Density functional theory (DFT) and ab-initio lattice dynamics calculations were also carried out. A first-order structural phase transition associated with an 8 % collapse of the c-axis is observed at high pressures, with phase coexistence being observed between ∼ 40 and 55 GPa. At lower pressures and still within the high-symmetry tetragonal phase, a number of lattice and phonon anomalies were observed, reflecting crossovers between isostructural competing states. A critical pressure of P1 = 17 GPa is associated with the following anomalies: (i) a reduction of lattice volume compressibility and a change of behavior of the tetragonal c/a ratio take place above P1; (ii) a four-fold symmetry-breaking lattice strain associated with lattice disorder is observed above P1; (iii) two strong Raman active modes at ambient conditions (at ∼ 180 and ∼ 260 cm −1 ) are washed out at P1; and (iv) an asymmetric Fano lineshape is observed for the ∼ 390 cm −1 mode above P1, revealing a coupling of this phonon with electronic excitations. DFT indicates that the Ir 4+ in-plane canted magnetic moment is unstable against a volume compression, indicating that the phase above P1 is most likely non-magnetic. Exploring the similarities between iridate and cuprate physics, we argue that these observations are consistent with the emergence of a rotational symmetry-breaking electronic instability at P1, providing hints for the avoided metallization under pressure and supporting the hypothesis of possible competing orders that are detrimental to superconductivity in this family. Alternative scenarios for the transition at P1 are also suggested and critically discussed. Additional phonon and lattice anomalies in the tetragonal phase are observed at P2 = 30 and P3 = 40 GPa, indicating further competing phases that are stabilized at high pressures.

The investigation of structural phase transition and anharmonic behavior of Yb2O3 has been carried out by high-pressure and temperature dependent Raman scattering studies respectively. In situ Raman studies under high pressure were carried out in a diamond anvil cell at room temperature which indicate a structural transition from cubic to hexagonal phase at and above 20.6 GPa. In the decompression cycle, Yb2O3 retained its high pressure phase. We have observed a Stark line in the Raman spectra at 337.5 cm−1 which arises from the electronic transition between 2F5/2 and 2F7/2 multiplates of Yb3+ (4f13) levels. These were followed by temperature dependent Raman studies in the range of 80–440 K, which show an unusual mode hardening with increasing temperature. The hardening of the most dominant mode (Tg + Ag) was analyzed in light of the theory of anharmonic phonon-phonon interaction and thermal expansion of the lattice. Using the mode Grüneisen parameter obtained from high pressure Raman measurements; we have calculated total anharmonicity of the Tg + Ag mode from the temperature dependent Raman data.

Abstract:(In 1-x Fe x ) 2 O 3 polycrystalline samples with x = (0. 0, 0.05, 0.10, 0.15, 0.20 and 0.25) have been synthesized by a gel combustion method. Reitveld refinement analysis of X ray diffraction data indicated the formation of single phase cubic bixbyite structure without any parasitic phases. This observation is further confirmed by high resolution transmission electron microscopy (HRTEM) imaging, and indexing of the selected-area electron diffraction (SAED) patterns, X-ray Absorption Spectroscopy (XAS) and Raman Spectroscopy. DC Magnetization studies as a function of temperature and field indicate that they are ferromagnetic with Curie temperature (T C ) well above room temperature. a Corresponding author Email: ikgopal@barc.gov.in

Thin films of Zn1−xCoxO (x=1–15%) were grown on an Al2O3 (0001) substrate by pulsed laser deposition and characterized by investigating their photoluminescence (PL) and other optical properties. The films were highly (0001) oriented without any impurity phases as observed in x-ray diffraction. The optical transmittance spectra showed that the band edge of Zn1−xCoxO was decreased with increase of Co concentration along with a subbandgap absorption in the range of 1.8to2.2eV. The near band-edge PL spectrum of ZnO thin films at 77K showed a strong peak of donor bound exciton at 3.307eV along with the free excitons. However, the PL spectra for free excitons of the Zn1−xCoxO films at 77K did not show any significant redshift due to Co incorporation.

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