Local structures of polar wurtzitesZn1−xMgxOstudied by Raman andZ67n/M25gNMR spectroscopies and by total neutron scattering

Abstract

Local compositions and structures of ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mg}}_{x}\text{O}$ alloys have been investigated by Raman and solid-state $^{67}\text{Z}\text{n}/^{25}\text{M}\text{g}$ nuclear-magnetic-resonance (NMR) spectroscopies and by neutron pair-distribution-function (PDF) analyses. The ${E}_{2}^{\text{low}}$ and ${E}_{2}^{\text{high}}$ Raman modes of ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mg}}_{x}\text{O}$ 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 ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mg}}_{x}\text{O}$ 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 $^{67}\text{Z}\text{n}$ in ZnO are only weakly modified by the 15% substitution of Mg for Zn. $^{25}\text{M}\text{g}$ MAS spectra of ${\text{Zn}}_{0.85}{\text{Mg}}_{0.15}\text{O}$ show an unusual upfield shift, demonstrating the prominent shielding ability of Zn in the nearby oxidic coordination sphere. Neutron PDF analyses of ${\text{Zn}}_{0.875}{\text{Mg}}_{0.125}\text{O}$ using a $2\ifmmode\times\else\texttimes\fi{}2\ifmmode\times\else\texttimes\fi{}1$ supercell corresponding to ${\text{Zn}}_{7}{\text{MgO}}_{8}$ suggest that the mean local geometry of ${\text{MgO}}_{4}$ fragments concurs with previous density-functional-theory-based structural relaxations of hexagonal wurtzite MgO. ${\text{MgO}}_{4}$ tetrahedra are markedly compressed along their $c$ axes and are smaller in volume than ${\text{ZnO}}_{4}$ units by $\ensuremath{\approx}6%$. Mg atoms in ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mg}}_{x}\text{O}$ 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 ${\text{Zn}}_{0.875}{\text{Mg}}_{0.125}\text{O}$ obtained from time-of-flight total neutron scattering supports the view that Mg substitution in ZnO results in increased total spontaneous polarization.