Investigation of Local Structure and Cation Ordering in Dielectric Oxide Microwave Ceramics with stoichiometry A(Li$_{\mathrm{x}}$(Nb,Ta)$_{\mathrm{y}})$O$_{\mathrm{3\thinspace }}$Using $^{\mathrm{7}}$Li and $^{\mathrm{93}}$Nb solid-state NMR spectroscopy.

The local structure and cation ordering in dielectric oxide microwave ceramics with stoichiometry A(Li$_{\mathrm{x}}$(Nb,Ta)$_{\mathrm{y}})$O$_{\mathrm{3\thinspace }}$are investigated using $^{\mathrm{7}}$Li and $^{\mathrm{93}}$Nb solid-state NMR spectroscopy. For all samples, $^{\mathrm{7}}$Li MAS NMR spectra show one strong and sharp resonance peak indicating one unique environment which corresponds to local lithium environment of nearest B-site neighbor (nBn) surrounded by 4 LiO$_{\mathrm{6\thinspace }}$octahedra and 2 NbO$_{\mathrm{6}}$ octahedra (TaO$_{\mathrm{6}}$ in some samples). In addition to this, $^{\mathrm{7}}$Li MAS NMR spectrum of (Ca$_{\mathrm{2/3}}$La$_{\mathrm{1/3}})$(Li$_{\mathrm{1/3}}$Nb$_{\mathrm{2/3}})$O$_{\mathrm{3}}$ show one additional weak and broad resonance peak which can be assigned to nBn of 3 LiO$_{\mathrm{6\thinspace }}$octahedra and 3 NbO$_{\mathrm{6}}$ octahedra. $^{\mathrm{93}}$Nb MAS NMR spectra of samples with niobium content, show a resonance peak with tail toward the low frequency limit, an evidence to the existence of chemical shifts and quadrupole couplings distributions. Both (Sr$_{\mathrm{2/3}}$La$_{\mathrm{1/3}})$(Li$_{\mathrm{1/3}}$Nb$_{\mathrm{2/3}})$O$_{\mathrm{3}}$ and Ca(Li$_{\mathrm{1/4}}$Nb$_{\mathrm{3/4}})$O$_{\mathrm{3}}$ spectra show one broad resonance peak, which can be interpreted as one NbO$_{\mathrm{6}}$ octahedron nBn with many slight variations through out the sample. While (Ca$_{\mathrm{2/3}}$La$_{\mathrm{1/3}})$(Li$_{\mathrm{1/3}}$Nb$_{\mathrm{2/3}})$O$_{\mathrm{3}}$ spectra show four peaks correspond to four distinct NbO$_{\mathrm{6}}$ octahedra local nBn environments with the nBn configuration as: (i) 3 LiO$_{\mathrm{6}}$ and 3 NbO$_{\mathrm{6}}$; (ii) 2 LiO$_{\mathrm{6}}$ and 4 NbO$_{\mathrm{6}}$; (iii) 1 LiO$_{\mathrm{6}}$ and 5 NbO$_{\mathrm{6}}$; (iv) all 6 NbO$_{\mathrm{6}}$