Abstract
Crystallographic and electronic structures of phase pure ternary solid solutions of Ni1−xCoxO (x = 0 to 1) have been studied using XRD, EXAFS and XAS measurements. The lattice parameter of the cubic rock-salt (RS) Ni1−xCoxO 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.
Highlights
Transition metal oxides (TMOs) exhibit a wide range of properties, most of which originate from the strong 3d electron– electron correlation in the transition metals
The normalized EXAFS signals for the wave vector (k) values ranging from 3.43 AÀ1 to 10.25 AÀ1 (3.41 AÀ1 to 10.22 AÀ1) obtained at the Ni (Co) K-edges, have been Fourier transformed to obtain the Fourier components in real space coordinate (R) using hanning window function
The variation of these bond lengths nearly follow the variation of bond lengths evaluated from virtual crystal approximation (VCA), which is unlike most conventional ternary semiconductors, where the distortion in the local structure around the constituent atom is observed
Summary
Transition metal oxides (TMOs) exhibit a wide range of properties, most of which originate from the strong 3d electron– electron correlation in the transition metals. These properties are interesting from a fundamental point of view, and have been behind several applications. A few of them include giant magneto resistance (GMR), high Tc superconductors, metal to insulator transition, p-type transparent semiconductors and systems for anti-ferromagnetic spintronics.[1,2] In the large class of TMOs, the bivalent oxides, NiO and CoO, and their ternaries, have applications in photodetection, resistive switching, energy storage in batteries, transparent electrodes, electrochromic smart windows, antiferromagnetic spintronics and solar cells.[2,3,4,5] Pure NiO and CoO belong to the group of type-II anti-ferromagnetic (AF2) materials with a magnetic moment of 1.9 mB on the Ni site and aSynchrotrons Utilisation Section, RRCAT, Indore 452013, India. E-mail: kiranb@ rrcat.gov.in bHigh Pressure and Synchrotron Radiation Physics Division, BARC, India cLaser and Functional Materials Division, RRCAT, Indore, 452013, India dLaser Technology Division, RRCAT, Indore 452013, India eUGC-DAE CSR, Indore 452001, India fAtomic and Molecular Physics Division, BARC, India gHomi Bhabha National Institute, Anushakti Nagar, Mumbai 400094, India
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