Formation of solid thorium monoxide at near-ambient conditions as observed by neutron reflectometry and interpreted by screened hybrid functional calculations

Abstract

Oxidation of a ~1000Å sputter-deposited thorium thin film at 150°C in 100ppm of flowing oxygen in argon produces the long-sought solid form of thorium monoxide. Changes in the scattering length density (SLD) distribution in the film over the 700-min experiment measured by in-situ, dynamic neutron reflectometry (NR) shows the densities, compositions and thickness of the various thorium oxides layers formed. Screened, hybrid density-functional theory calculations of potential thorium oxides aid interpretation, providing atomic-level picture and energetics for understanding oxygen migration. NR provided evidence of the formation of substoichiometric thorium oxide, ThO y (y<1) at the interface between the unreacted thorium metal and its dioxide overcoat which grows inward, consuming the thorium at a rate of 2.1 Å/min while y increases until reaching 1:1 oxygen-tothorium. Its presence indicates that kinetically-favored solid-phase ThO can be preferentially generated as a majority phase under the thermodynamically-favored ThO 2 top layer at conditions close to ambient.

Chemistry of Actinide Centers in Heterogeneous Catalytic Transformations of Small Molecules

“…Reactive sputter deposition of thorium metal in an Ar/O 2 atmosphere at 150 °C generated mixed ThO/ThO 2 films with a thickness of ∼100 nm. The presence of the metastable ThO phase was confirmed by time-resolved in situ dynamic neutron reflectometry coupled with hybrid functional density calculations . In contrast, sputtering at 100 °C led to the formation of ThO 2– x thin films with a thickness of ∼360 nm .…”

“…The presence of the metastable ThO phase was confirmed by time-resolved in situ dynamic neutron reflectometry coupled with hybrid functional density calculations. 79 In contrast, sputtering at 100 °C led to the formation of ThO 2−x thin films with a thickness of ∼360 nm. 80 Using Si (111) substrates, polycrystalline ThO 2 films with a preferential orientation along the (111) plane were generated.…”

Thorium and Rare Earth Monoxides and Related Phases

“…They did not cite, however, experimental work on ThO and UO published by Ackerman and Rauh [ 77 ] from the Argonne National Laboratory. New experimental evidence of ThO formation in thin films at near-ambient conditions has been recently published [ 28 ], and bulk thorium monoxide was predicted to be stable in the NaCl structure at high pressure ( Figure 1 C) [ 22 ].…”

“…Seventy years after Rundle’s [ 60 ] report, the growth of ThO thin films was reported by the Los Alamos Laboratory. He et al [ 28 ] performed experiments on the controlled oxidation of metallic thorium with in situ neutron reflectometry measurements emerging from the study of oxidation of metallic Th using neutron reflectometry. ThO was formed after one hour of exposure of metallic Th to a ≈100 ppm O 2 in an Ar mixture at 150 °C.…”

“…The thermodynamic properties of gaseous lanthanide and actinide monoxides were studied extensively and recently summarized by Konings et al [ 26 ]. The first report suggesting the synthesis of solid thorium monoxide dates to 1908 [ 27 ]; however, it is still considered a “long-sought” phase [ 28 ] with unknown properties. The properties of solid thorium monoxide are of interest not only as insight into bonding in actinides, but for the thermodynamic modeling of oxygen solubility in rocksalt-type ThC and ThN, studied as advanced nuclear fuels [ 29 , 30 , 31 , 32 ].…”