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# Integrated 1D XRD profiles of ThC powder at various pressures ranging from 1.2 GPa to 71.0 GPa. (Notes: blue diamond represents ThC-B1 phase, red triangle represents ThC-P4/nmm phase, solid black circle represents ThC 2-C2/c phase and hollow black circle represents ThC 2-C2/m phase).

Source publication

Thorium monocarbide (ThC) as a potential fuel for next generation nuclear reactor has been subjected to its structural stability investigation under high pressure, and so far no one reported the observation of structure phase transition induced by pressure. Here, utilizing the synchrotron X-ray diffraction technique, we for the first time, experime...

## Contexts in source publication

**Context 1**

... was gradually increased up to 71.0 GPa and then released. The diffraction profiles of ThC at pressures ranged from 1.2-71.0 GPa and back to ambient pressure are shown in Fig. 2 after the background was subtracted. The signals remains the same up to 53.2 GPa except for the major diffraction peaks becoming broader and shifting to larger angles systematically as pressure goes up, indicating the lattices are under distortion, partially resulting from the possible shear stresses in the sample due to the imperfect ...

**Context 2**

... that ThC has a phase transition at around 58 GPa from B1 to P4/nmm. This is the first time ever that the high pressure transition of ThC has been experimentally reported. Our results agree well with the prediction by Guo et al. 5 . After the pressure was released, the sample returned back to the ambient phase B1, as the top curve shows in Fig. 2. Compared to the 1.2 GPa B1 phase, its peaks shift to the smaller angles as d space slightly increases under ambient condition, and become much broader as a result of many crystalline defects produced during the high pressure ...

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## Citations

... In addition, the high-pressure phase transition of ThC has been studied experimentally and theoretically. Yu et al. [19] experimentally revealed the phase transition of ThC from B1 to P4/nmm at~58 GPa by synchronous X-ray diffraction. There is no phase transition in ThC under high pressures at 36 GPa [20] and 40-45 GPa [21], but a transitional P4/nmm phase is produced at 60-120 GPa [22], theoretically. ...

... This is consistent with most theoretical values (5.335-5.388 Å) [7,13,17,22,30,31] and is close to the experimental values of 5.344 Å [14] and 5.430 Å [19]. ThC1−x (x = 0.03125, 0.0625, 0.125, 0.1875, 0.25, or 0.3125) with specific vacancy concentrations was created by random substitution method obeying Lowenstein's rule [32], corresponding to the replacement of 1, 2, 4, 6, 8, or 10 carbon atoms with vacancies in a 64-atom supercell, respectively. ...

... [17] 0.29 f [7] 0.32 f [36] 0.215 Given in Refs. [11,14,19,41] are available experimental data. a for ThC 0.975 [41]. ...

Thorium monocarbide (ThC) is interesting as an alternative fertile material to be used in nuclear breeder systems and thorium molten salt reactors because of its high thermal conductivity, good irradiation performance, and wide homogeneous composition range. Here, the influence of carbon vacancy site and concentration on lattice distortions in non-stoichiometric ThC1−x (x = 0, 0.03125, 0.0625, 0.125, 0.1875, 0.25, or 0.3125) is systematically investigated using first-principle calculations by the projector augmented wave (PAW) method. The energy, mechanical parameters, and thermodynamic properties of the ThC1-x system are calculated. The results show that vacancy disordering has little influence on the total energy of the system at a constant carbon vacancy concentration using the random substitution method. As the concentration of carbon vacancies increases, significant lattice distortion occurs, leading to poor structural stability in ThC1−x systems. The changes in lattice constant and volume indicate that ThC0.75 and ThC0.96875 represent the boundaries between two-phase and single-phase regions, which is consistent with our experiments. Furthermore, the structural phase of ThC1−x (x = 0.25–0.3125) transforms from a cubic to a tetragonal structure due to its ‘over-deficient’ composition. In addition, the elastic moduli, Poisson’s ratio, Zener anisotropic factor, and Debye temperature of ThC1-x approximately exhibit a linear downward trend as x increases. The thermal expansion coefficient of ThC1−x (x = 0–0.3125) exhibits an obvious ‘size effect’ and follows the same trend at high temperatures, except for x = 0.03125. Heat capacity and Helmholtz free energy were also calculated using the Debye model; the results showed the C vacancy defect has the greatest influence on non-stoichiometric ThC1−x. Our results can serve as a theoretical basis for studying the radiation damage behavior of ThC and other thorium-based nuclear fuels in reactors.

... For the thorium monocarbide ThC, we theoretically identified a phase transition sequence at different pressures, and predicted a new phase P4/nmm at 60 GPa [22]. Afterward, this new P4/nmm was observed through synchrotron x-ray diffraction technique by our co-workers [23]. The elastic and thermodynamic properties of ThC 2 were studied at ambient pressure from the first-principles, and the calculated results show that the vibrational property of the C 2 dimer in ThC 2 is similar to that of a free standing C 2 dimer, indicating that the C 2 dimers are weakly bonded to Th atoms [24]. ...

... The corresponding mechanical criteria of the tetragonal structures are C 11 > 0, C 33 > 0, C 44 > 0, C 66 > 0, C 11 > C 12 , C 33 + C 11 > 2 C 13 and C 11 + C 12 + 1 2 C 33 + 2 C 13 > 0 [45]. The Cmcm, Cmce and Immm phases all belong to orthorhombic crystal with nine independent elastic constants, namely C 11 , C 22 , C 33 , C 44 , C 55 , C 66 , C 12 , C 13 and C 23 . The corresponding mechanical criteria of the orthogonal structures are C 11 > 0, C 33 > 0, C 44 > 0, C 66 > 0, C 22 > 0, C 55 > 0, 1 2 (C 11 + C 22 + C 33 ) + C 12 + C 13 + C 23 > 0, C 12 < 1 2 (C 11 + C 22 ), C 13 < 1 2 (C 11 + C 33 ) and C 23 < 1 2 (C 33 + C 22 ) [45]. ...

First-principles calculations and particle swarm optimization algorithm are combined to predict the crystal structures in the pressure range from 0 to 100 GPa. Four phases of ThBC are determined, including the P 4 1 22, Cmcm , Cmce and Immm phases, in which the Cmcm , Cmce and Immm phases are newly predicted structures. The mechanical, electronic and thermodynamic properties of the four phases are investigated. According to the enthalpy–pressure and volume–pressure curves, the phase transition pressure from the P 4 1 22 phase to the Cmcm phase is 15 GPa, the Cmcm to the Cmce is 36 GPa and the Cmce to the Immm is 69 GPa. All the transitions belong to the first-order phase transition. Based on the calculated elastic constants, the P 4 1 22, Cmcm , Cmce and Immm phases exhibit brittle nature. The Young’s moduli show that the P 4 1 22 phase has the largest degree of anisotropy, and the Immm phase has the smallest. The calculated density of states reveal that the P 4 1 22, Cmcm , Cmce and Immm phases are all metallic.

... For instance, binary carbides/nitrides/silicides of thorium and uranium have attracted much research attention [22,[31][32][33][34][35][36][37][38][39][40][41][42][43] both for fundamental and technological implications, such as power efficient and environment-friendly nuclear energy fuels for fourth generation nuclear power reactors, etc. This owes mainly to their high melting point and density, high thermal and electrical conductivity arising from their peculiar structural, mechanical, electronic and thermophysical properties. ...

Thorium compounds have attracted immense scientific and technological attention with regard to both fundamental and practical implications, owing to unique chemical and physical properties like high melting point, high density and thermal conductivity. Hereby, we investigate the mechanical and thermodynamic stability and report on the structural, electronic and magnetic properties of new silicon-doped cubic ternary thorium phosphides ThSixP1-x (x = 0, 0.25, 0.5, 0.75 and 1). The first-principles density functional theory procedure was adopted within full–potential linearized augmented plane wave (FP-LAPW) method. The exchange and correlation potential terms were treated within Generalized-Gradient-Approximation functional modified by Perdew-Burke-Ernzerrhof parameterizations. The proposed compounds showed mechanical and thermodynamic stable structure and hence can be synthesized experimentally. The calculated lattice parameters, bulk modulus, total energy, density of states, electronic band structure and spin magnetic moments of the compounds revealed considerable correlation to the Si substitution for P and the relative Si/P doping concentration. The electronic and magnetic properties of the doped compounds rendered them non-magnetic but metallic in nature. The main orbital contribution to the Fermi level arises from the hybridization of Th(6d+5f) and (Si+P)3p states. Reported results may have potential implications with regard to both fundamental point of view and technological prospects such as fuel materials for clean nuclear energy.

... In the search after better nuclear fuels for Generation-IV reactors [1] there has been a boost in the research work on thorium and its compounds (ThC and ThN) [2,3,4,5,6,7,8,9,10,11]. They have higher melting points, lower thermal expansion coefficients and larger thermal conductivities than their uranium counterparts [12]. ...

Protactinium contamination is a mayor issue in the thorium fuel cycle. We investigate, in this work, the consequences of Pa incorporation in vacancy defects and interstitials in Th, ThC and ThN. We calculate charge transfers and lattice distortions due to these incorporations as well as migration paths and energies involved in the diffusion of Pa.

... It is therefore crucial to determine the correct structures of materials. Previous research suggests various crystalline structures of ThC under high pressure 11,12,18 . For ThC 2 , although its structures have been obtained at ambient pressure and high temperature 10,[13][14][15] , the structural evolution under high pressure has not been fully understood until now. ...

Thorium-carbon systems have been thought as promising nuclear fuel for Generation IV reactors which require high-burnup and safe nuclear fuel. Existing knowledge on thorium carbides under extreme condition remains insufficient and some is controversial due to limited studies. Here we systematically predict all stable structures of thorium dicarbide (ThC2) under the pressure ranging from ambient to 300 GPa by merging ab initio total energy calculations and unbiased structure searching method, which are in sequence of C2/c, C2/m, Cmmm, Immm and P6/mmm phases. Among these phases, the C2/m is successfully observed for the first time via in situ synchrotron XRD measurements, which exhibits an excellent structural correspondence to our theoretical predictions. The transition sequence and the critical pressures are predicted. The calculated results also reveal the polymerization behaviors of the carbon atoms and the corresponding characteristic C-C bonding under various pressures. Our work provides key information on the fundamental material behavior and insights into the underlying mechanisms that lay the foundation for further exploration and application of ThC2.

Thorium was a part of energy infrastructure in the 19th century due to the refractory and electronic properties of its dioxide. It will be a part of future energy infrastructure as the most abundant energy reserve based on nuclear fission. This paper discusses the solid-state chemistry of the monoxides and related rocksalt phases of thorium and the rare earths, both at atmospheric and at high pressure. The existence of solid thorium monoxide was first suggested more than 100 years ago; however, it was never obtained in bulk and has been studied mostly theoretically. Monoxides of lanthanides from Eu to Ho are ferromagnetic semiconductors sought for spintronics and were studied in thin films. La to Sm metallic monoxides were synthesized in bulk at pressures below 5 GPa. Recently, ThO formation in thin films has been reported and the stability of bulk ThO at high pressure was theoretically predicted based on first principles computations at 0 K. New ab initio computations were performed accounting for temperature effects up to 1000 K using lattice dynamics in the quasi-harmonic approximation. New computational results confirm the stabilization of pure ThO above 30 GPa and suggest the possibility of high-pressure synthesis of (Th,Nd)O at 1000 K and 5 GPa.

The electronic, mechanical and thermodynamic properties of layered ThB 2 C are investigated using the first-principles calculations with generalized gradient and local density approximations. The equilibrium geometry and elastic stiffness constants of ThB 2 C are studied, and various elastic moduli, Poisson’s ratios and velocities are estimated from the elastic stiffness constants. ThB 2 C exhibits brittle characteristics. The phonon dispersion relationship verifies the thermodynamic stability of ThB 2 C. Considering the effect of phonon vibration on the thermodynamic properties of ThB 2 C, the variation of Gibbs free energy, bulk modulus and heat capacity at constant pressure with temperature are calculated using quasi-harmonic approximation in the temperature range of [Formula: see text][Formula: see text]K.

Actinide carbides, like ThC and UC, have been widely studied in recent years, on the contrary, protactinium carbide has merited less attention. One issue of PaC that has not been approached, neither experimentally nor theoretically is the formation of defects and the incorporation of impurities. Therefore, we will focus on these topics through a numerically study. To accomplish this, we calculate formation energies of vacancy, tetrahedral interstitial, Frenkel pair, Schottky and antisite defects. We find that C vacancies are the energetically preferred defect. We analyze density of states, charge densities and distortion of lattices with these kind of defects. We also study the effect of incorporation of oxygen and helium in vacancy and interstitial defects. In view of the lack of both theoretical and experimental results in PaC, we compared with UC and ThC. As a measure of the efficacy of the protactinium pseudopotential, which we generate, we calculate structural parameters and phonon spectrum.

The thorium compounds are prospective candidates for the next generation nuclear materials. Using the first-principles and particle swarm optimization methods, we have investigated structures and physical properties of four atomic ratios of thorium and silicon hybrids under various ranges from ambient pressure to 200 GPa. A series of new phases have been predicted and thermodynamics, mechanical stabilities, elastic properties, electronic properties and chemical bonds of all the stable structures have been studied in details. Furthermore, we utilize potential energy surface to investigate the vibration property of Si2 dimer in Th3Si2. The result indicates that the Si2 dimer in Th3Si2 compound have the similar vibration property as a free standing Si2. This also means the silicon atoms do not have a strong interaction with thorium atoms in thorium silicide.