Combined 135/137Ba Solid-State NMR at an Ultrahigh Magnetic Field and Computational Study of β-Barium Borate

Abstract

β-Barium Borate (β-BBO or β-BaB2O4) is an important nonlinear optical material with many practical applications. In this work, we have directly probed the local Ba environment in β-BBO by acquiring static 135Ba and 137Ba solid-state NMR spectra at an ultrahigh magnetic field of 21.1 T. The observed spectra are completely dominated by the second-order quadrupolar interaction. The measured quadrupolar coupling constants are 14.9 and 22.8 MHz for 135Ba and 137Ba, respectively. A computational study of the electric field gradient tensor at Ba2+ was also performed using both plane-wave pseudopotentials (CASTEP) and ab initio (Gaussian 03) methods. Combining the experimental and theoretical results allows one to shed some light on the controversy regarding the true space group of β-BBO. We conclude that the space group R3c represents the true structure of β-BBO rather than does the space group R3. This work demonstrates that a combination of 135/137Ba NMR and theoretical calculations is a useful tool for charac...