Local structures of polar wurtzites Zn1-xMgxO studied by Raman and Zn-67/Mg-25 NMR spectroscopies and by total neutron scattering

Article

关键词: POLARIZATION;    ZNO;    PARAMETERS;    OXIDE;   

DOI  :  10.1103/PhysRevB.78.195205

来源: SCIE

【 摘 要 】

Local compositions and structures of Zn1-xMgxO alloys have been investigated by Raman and solid-state Zn-67/Mg-25 nuclear-magnetic-resonance (NMR) spectroscopies and by neutron pair-distribution-function (PDF) analyses. The E-2(low) and E-2(high) Raman modes of Zn1-xMgxO display Gaussian- and Lorentzian-type profiles, respectively. At higher Mg substitutions, both modes become broader, while their peak positions shift in opposite directions. The evolution of Raman spectra from Zn1-xMgxO solid solutions is discussed in terms of lattice deformation associated with the distinct coordination preferences of Zn and Mg. Solid-state magic-angle-spinning (MAS) NMR studies suggest that the local electronic environments of Zn-67 in ZnO are only weakly modified by the 15% substitution of Mg for Zn. Mg-25 MAS spectra of Zn0.85Mg0.15O show an unusual upfield shift, demonstrating the prominent shielding ability of Zn in the nearby oxidic coordination sphere. Neutron PDF analyses of Zn0.875Mg0.125O using a 2x2x1 supercell corresponding to Zn7MgO8 suggest that the mean local geometry of MgO4 fragments concurs with previous density-functional-theory-based structural relaxations of hexagonal wurtzite MgO. MgO4 tetrahedra are markedly compressed along their c axes and are smaller in volume than ZnO4 units by approximate to 6%. Mg atoms in Zn1-xMgxO have a shorter bond to the c-axial oxygen atom than to the three lateral oxygen atoms, which is distinct from the coordination of Zn. The precise structure, both local and average, of Zn0.875Mg0.125O obtained from time-of-flight total neutron scattering supports the view that Mg substitution in ZnO results in increased total spontaneous polarization.

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