Asymptotic and intermediate long-time behavior of nuclear free induction decays in polycrystalline solids and powders

TL;DR

This paper investigates the long-time behavior of nuclear magnetic resonance free induction decays in polycrystalline solids, revealing that the asymptotic form is universal but only observable at very low signal levels, with intermediate behavior influenced by crystal structure.

Contribution

First principles calculations demonstrate the universal long-time exponential decay oscillations in polycrystalline FIDs, clarifying the influence of crystal structure on intermediate-time behaviors.

Findings

Asymptotic long-time FID behavior is universally exponential oscillations.

Intermediate FID behavior depends on distribution of decay constants and frequencies.

Solid xenon shows more clustered parameters, leading to clearer oscillations in experiments.

Abstract

Free induction decay (FID) measured by nuclear magnetic resonance (NMR) in a polycrystalline solid is the isotropic average of the FIDs for individual single crystallites. It has been recently proposed theoretically and verified experimentally that the long-time behavior of single-crystal FIDs has the universal form of exponentially decaying sinusoidal oscillations. Polycrystalline averaging complicates the situation theoretically, while the available experimental evidence is also ambiguous. Exponentially decaying sinusoidal oscillations have been observed for Xe-129 in polycrystalline solid xenon but not for F-19 in the powder of CaF2. In this paper, we present the first principles FID calculations for the powders of both CaF2 and solid xenon. In both cases, the asymptotic long-time behavior has the expected form of exponentially decaying sinusoidal oscillations, which is determined by the single crystallite FID with the slowest exponential decay. However, this behavior appears only at rather small values of the signal that have not yet been measured in experiments. At intermediate times accessible experimentally, a polycrystalline FID depends on the distribution of the exponential decay constants and oscillation frequencies for single crystallite FIDs. In CaF2, these parameters are relatively broadly distributed, and as a result, the sinusoidal long-time oscillations become somewhat washed out. In contrast, the single crystallite parameters are more clustered in solid xenon, and, as a result, the experimentally observable range is characterized by well-defined oscillation frequency and exponential decay constant even though both of these parameters do not represent the true long-time behavior. The above difference of the intermediate FID behavior originates from the difference of the crystal structures of solid xenon and CaF2.