Local structures of polar wurtzites Zn_1-xMg_xO studied by Raman and ⁶⁷Zn/²⁵Mg NMR spectroscopies and by total neutron scattering

Local compositions and structures of Zn1-x Mgx O alloys have been investigated by Raman and solid-state Z 67 n/ M 25 g nuclear-magnetic-resonance (NMR) spectroscopies and by neutron pair-distribution-function (PDF) analyses. The E2 low and E2 high Raman modes of Zn1-x Mgx O 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-x Mgx O 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 Z 67 n in ZnO are only weakly modified by the 15% substitution of Mg for Zn. M 25 g MAS spectra of Zn0.85 Mg0.15 O show an unusual upfield shift, demonstrating the prominent shielding ability of Zn in the nearby oxidic coordination sphere. Neutron PDF analyses of Zn0.875 Mg0.125 O using a 2×2×1 supercell corresponding to Zn7 MgO8 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 ≈6%. Mg atoms in Zn1-x Mgx O 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 Zn0.875 Mg0.125 O obtained from time-of-flight total neutron scattering supports the view that Mg substitution in ZnO results in increased total spontaneous polarization.