Solid-State 25Mg, 45Sc and 93Nb MAS NMR Studies: Local B-Site Chemical Environments and Ordering in PB(MG1/3NB2/3)O3 (PMN) and Related Relaxor Ferroelectrics

Solid-state 25Mg, 45Sc and 93Nb NMR spectroscopic studies of the local chemical environments and relative degree of B-site ordering in lead-based perovskite relaxor ferroelectrics are reported. 93Nb MAS NMR spectra of PMN and PZN provide conclusive evidence for significant B-site disorder, while 93Nb and 45Sc MAS NMR indicate B-site ordering in PSN. 93Nb MAS NMR spectra of PMN show resonances with different CQ values assigned to different types of Nb(V) B-sites of pseudo-cubic, axial and rhombic symmetry. For PSN, 93Nb and 45Sc MAS NMR spectra support the presence of predominantly a single Nb(V) and Sc(III) site with intermediate CQ values assigned to axially distorted B-sites. 25Mg MAS NMR of PMN also show evidence for at least two different Mg(II) sites of local cubic and axial symmetry due to partial B-site disorder in PMN. 25Mg and 93Nb NMR spectra of La3+-substituted PMN (at 25 mol%) show that A-site substitution by La(III) leads to increased B-site ordering in the Mg-rich and Nb-rich nanodomains of PMN. By contrast, the 25Mg and 93Nb NMR spectra of BMN, a Ba2+ A-site analog of PMN, demonstrate that BMN has a more highly ordered Mg/Nb B-site distribution than PMN. These multinuclear NMR approaches demonstrate the enormous potential of solid-state NMR spectroscopy to provide both qualitative and quantitative information about the chemical environments, cation ordering and motional behavior of the B-site cations in a diverse range of perovskite relaxor ferroelectrics.