Structural, optical and electronic properties of Ni1-x Co x O in the complete composition range.

Crystallographic and electronic structures of phase pure ternary solid solutions of Ni1-x Co x O (x = 0 to 1) have been studied using XRD, EXAFS and XAS measurements. The lattice parameter of the cubic rock-salt (RS) Ni1-x Co x O solid solutions increases linearly with increasing Co content and follows Vegard's law, in the complete composition range. A linear increase in the bond lengths (Ni/Co-O, Ni-Ni and Ni-Co) with "x", closely following the bond lengths determined from virtual crystal approximation (VCA), is observed, which implies that there is only a minimal local distortion of the lattice in the mixed crystal. The optical gap of the ternary solid solution determined from diffuse reflectivity measurements shows neither a linear variation with Co composition nor bowing, as observed in many ternary semiconductors. This trend in the variation of optical gaps is explained by probing the conduction band using XAS at the O K-edge. We have observed that the variation in the onset energy of the conduction band edge with "x" is very similar to the variation in the optical gap with "x", thus clearly indicating the dominant role played by the conduction band position in determining the optical gap. The variation in the intensities of the pre-edge peak in the XANES spectra measured at Ni and Co K-edges, and the L1/2 peak in XAS spectra measured at Ni and Co L-edges, is found to depend on the unoccupied O 2p-metal-(Ni/Co) 3d hybridized states and the bond lengths.

Structural analysis of lead magnesium niobate using synchrotron powder X-ray diffraction and the Rietveld method.

The room-temperature synchrotron powder X-ray diffraction pattern of the single phase perovskite lead magnesium niobate (PMN) has shown significant broadening in the q range ∼ 5-7 Å(-1) compared with standard LaB6 synchrotron powder X-ray diffraction data, taken under similar conditions. This broadening/asymmetry lies mainly towards the lower 2θ side of the Bragg peaks. Attempts to fit this data with the paraelectric cubic phase (Pm\bar 3m) and the local rhombohedral phase (R3m) corresponding to polar nanoregions (PNRs) are made using the Rietveld method. Rietveld refinements show that neither cubic (Pm\bar 3m) nor rhombohedral (R3m) symmetry can fit this XRD pattern satisfactorily. The two-phase refinement fits the experimental data satisfactorily and suggests that the weight percentage of the PNRs is approximately 12-16% at room temperature. The unit-cell volume of these rhombohedral PNRs is approximately 0.15% larger than that of the unit cell volume of the paraelectric cubic phase.

Determination of the optical gap bowing parameter for ternary Ni1-xZnxO cubic rocksalt solid solutions.

The lattice parameter variation for phase pure cubic rocksalt (RS) Ni1-xZnxO ternary solid solutions is observed to be perfectly governed by Vegard's law. X-ray absorption near edge spectroscopy confirms the RS symmetry of Zn atoms in the cubic lattice of Ni1-xZnxO. The optical gap bowing parameter for RS Ni1-xZnxO ternary solid solutions is determined, using diffuse reflectivity, to be -0.93 ± 0.05 eV. The negative value of the bowing indicates a repulsive interaction between the ligand O-2p and the metal Ni-3d orbitals. The determined value of the bowing parameter can be useful in designing UV photodetectors based on Ni1-xZnxO solid solutions.