The Structural Transitions And Stabilities Of Advanced Thorium-based Nuclear Materials Under High Pressures

Thorium and its compounds have received considerable attention in recent years due to the renewed interest in developing the thorium fuel cycle as an alternative nuclear energy technology.Currently,there is pressing need to explore the physical properties which are essential to the fundamental understanding and practical application of these materials.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.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.The phase transition of ThC at high pressure has been studied by means of density functional theory.Through structure search,a new phase with space group P4/nmm has been predicted.The calculated phonons demonstrate that this new phase and the previous B2 phase are dynamically stable as the external pressure is greater than 60 GPa and 120 GPa,respectively.The transformation from B1 to P4/nmm is predicted to be a first-order transition,while that from P4/nmm to B2 is found to be a second-order transition.Furthermore,we also revealed the phase transition of ThC from B1 to P4/nmm at pressure of?5 8 GPa at ambient temperature for the first time by using experimental method of the synchrotron X-ray diffraction technique.A volume collapse of 10.2%was estimated during the phase transition.For the thorium dicarbide(ThC2),we systematically predict all stable structures of 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.We also report on a computational study of thorium disulfide(ThS2),which plays an important role in the thorium fuel reprocessing cycle.We have employed the density functional theory and evolutionary structure search methods to determine the crystal structures,electronic band structures,phonon dispersions and density of states,and thermodynamic properties of ThS2 under various pressure and temperature conditions.Our calculations identify several crystalline phases of ThS2 and a series of structural phase transitions induced by pressure and temperature.The calculated results also reveal electronic phase transitions from the semiconducting state in the low-pressure phases of ThS2 in the Pnma and Fm3m symmetry to the metallic state in the high-pressure phases of ThS2 in the Pnma and I4/mmm symmetry.These results explain the experimental observation of the thermodynamic stability of the Pnma phase of ThS2 at the ambient conditions and a pressure-induced structural phase transition in ThS2 around 40 GPa.Moreover,the present study reveals considerable additional information on the structural and electronic properties of ThS2 in a wide range of pressure and temperature.Such information provides key insights into the fundamental material behavior and the underlying mechanisms that lay the foundation for further exploration and application of ThS2.