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Pressure-induced structural transformations and polymerization in ThC 2

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ABSTRACT

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.

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XRD patterns and volume versus pressure relation.Integrated X-ray diffraction patterns (black lines) obtained with subtraction of the background for (a) C2/c and (b) C2/m phases of ThC2 at selected pressure. Simulated powder diffraction patterns (red lines) using atomic positions derived from DFT-optimized structures are shown for comparison. Solid and hollow triangles indicate peaks that belong to the C2/c and the C2/m phases, respectively. (c) Experimental measured and DFT calculated volume versus pressure relation for the C2/c and C2/m phases. ∆V is the volume collapse at the transition pressure.
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f5: XRD patterns and volume versus pressure relation.Integrated X-ray diffraction patterns (black lines) obtained with subtraction of the background for (a) C2/c and (b) C2/m phases of ThC2 at selected pressure. Simulated powder diffraction patterns (red lines) using atomic positions derived from DFT-optimized structures are shown for comparison. Solid and hollow triangles indicate peaks that belong to the C2/c and the C2/m phases, respectively. (c) Experimental measured and DFT calculated volume versus pressure relation for the C2/c and C2/m phases. ∆V is the volume collapse at the transition pressure.

Mentions: We compare the diffraction pattern obtained from the experiment with the simulated one for the C2/m phase as well as the ground phase in Fig. 5(a) and (b). The parameters of the crystal structure are obtained through the Rietveld refinement and compared with calculations in Table 1 together with previous work13141521. The experimental results agree very well with our theoretical predications, with discrepancy within 0.3%. The volume per f.u. in two states are also calculated based on the experimental data and the theoretical simulation, as seen in Fig. 5(c). The abrupt drop of this value at transition point indicates the occurring of a first order transition. The volume collapse through the transition is estimated to be around 10%, more specifically 4.10 Å3 by experiment and 4.33 Å3 by simulation. More structural parameters of the high pressure phases of ThC2 at the corresponding pressures are listed in Table 1 and the atomic Wychoff positions of these phases are listed in Table 2.


Pressure-induced structural transformations and polymerization in ThC 2
XRD patterns and volume versus pressure relation.Integrated X-ray diffraction patterns (black lines) obtained with subtraction of the background for (a) C2/c and (b) C2/m phases of ThC2 at selected pressure. Simulated powder diffraction patterns (red lines) using atomic positions derived from DFT-optimized structures are shown for comparison. Solid and hollow triangles indicate peaks that belong to the C2/c and the C2/m phases, respectively. (c) Experimental measured and DFT calculated volume versus pressure relation for the C2/c and C2/m phases. ∆V is the volume collapse at the transition pressure.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5382917&req=5

f5: XRD patterns and volume versus pressure relation.Integrated X-ray diffraction patterns (black lines) obtained with subtraction of the background for (a) C2/c and (b) C2/m phases of ThC2 at selected pressure. Simulated powder diffraction patterns (red lines) using atomic positions derived from DFT-optimized structures are shown for comparison. Solid and hollow triangles indicate peaks that belong to the C2/c and the C2/m phases, respectively. (c) Experimental measured and DFT calculated volume versus pressure relation for the C2/c and C2/m phases. ∆V is the volume collapse at the transition pressure.
Mentions: We compare the diffraction pattern obtained from the experiment with the simulated one for the C2/m phase as well as the ground phase in Fig. 5(a) and (b). The parameters of the crystal structure are obtained through the Rietveld refinement and compared with calculations in Table 1 together with previous work13141521. The experimental results agree very well with our theoretical predications, with discrepancy within 0.3%. The volume per f.u. in two states are also calculated based on the experimental data and the theoretical simulation, as seen in Fig. 5(c). The abrupt drop of this value at transition point indicates the occurring of a first order transition. The volume collapse through the transition is estimated to be around 10%, more specifically 4.10 Å3 by experiment and 4.33 Å3 by simulation. More structural parameters of the high pressure phases of ThC2 at the corresponding pressures are listed in Table 1 and the atomic Wychoff positions of these phases are listed in Table 2.

View Article: PubMed Central - PubMed

ABSTRACT

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.

No MeSH data available.


Related in: MedlinePlus