Intrinsic Proton NMR Studies of Mg(OH)$_2$ and Ca(OH)$_2$

TL;DR

This study used proton NMR to analyze Mg(OH)2 and Ca(OH)2 powders, revealing insights into their local proton dynamics and relaxation mechanisms across a range of temperatures.

Contribution

It provides more precise NMR spectral data and explores the underlying proton relaxation mechanisms in Mg(OH)2 and Ca(OH)2, highlighting differences in their local hopping and rattling behaviors.

Findings

Refined second moments of NMR spectra compared to previous data.

Large decrease in proton spin-lattice relaxation rate not explained by second moments.

Evidence of local hopping and anharmonic rattling mechanisms in proton relaxation.

Abstract

We studied the short proton free induction decay signals and the broad ^1H NMR spectra of Mg(OH)2 and Ca(OH)2 powders at 77-355 K and 42 MHz using pulsed NMR techniques. Using a Gaussian-type back extrapolation procedure for the obscured data of the proton free induction decay signals, we obtained more precise values of the second moments of the Fourier-transformed broad NMR spectra than those in a previous report [Y. Itoh and M. Isobe, J. Phys. Soc. Jpn. Vol. 84, 113601 (2015)] and compared with the theoretical second moments. The decrease in the second moment could not account for the large decrease in the magnitude of the intrinsic proton spin-lattice relaxation rate 1/T_1 from Mg(OH)2 to Ca(OH)2. The analysis of 1/T_1 $propto$ exp(-E_g/k_BT) with E_g~0.01 eV points to a local hopping mechanism, and that of 1/T_1 $propto$ T^n with n~0.5 points to an anharmonic rattling mechanism.