Inelastic neutron scattering: A novel approach towards determination of equilibrium isotopic fractionation factors. Size effects on heat capacity and beta-factor of diamond

TL;DR

This paper introduces a new INS-based method to determine isotopic fractionation factors in solids, demonstrating size-dependent effects on heat capacity and beta-factors in nanodiamonds compared to bulk diamond.

Contribution

It presents a novel experimental approach using Inelastic Neutron Scattering to calculate isotopic properties from phonon density of states, including for nanomaterials.

Findings

Size-dependent increase in heat capacities of nanodiamonds.

Decrease in beta-factors for nanodiamonds relative to bulk diamond.

Evaluation of surface and phonon confinement effects on size-dependent properties.

Abstract

A new experimental method of determination of equilibrium isotopic properties of substances based on Inelastic Neutron Scattering (INS) is proposed. We present mathematical formalism allowing calculation of beta-factor of single-element solids based on INS-derived Phonon Density of States (PDOS). PDOS data for nanodiamonds of widely different sizes and of macroscopic diamond were determined from Inelastic Neutron Scattering experiment. This allowed determination of heat capacities and, for the first time, \b{eta}-factors for the diamond nanoparticles. We demonstrate considerable size-dependent increase of the heat capacities and decrease of the beta-factors for nanodiamonds relative to bulk diamond. Contributions of surface impurities/phases and phonon confinement to the size effects are evaluated. Applications to formation of diamond nanoparticles in nature are briefly discussed.