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Stability of a new cubic monoxide of Thorium under pressure.

Sun W, Luo W, Ahuja R - Sci Rep (2015)

Bottom Line: It is found out that the pressure can stabilize the rocksalt phase of ThO, and the transition pressure is estimated between 14 and 22 GPa.The phonon dispersion curves of the rocksalt phase show the positive frequencies which indicates its dynamical stability.Our successful prediction of the stabilization of the metallic ThO has proposed a route to synthesize novel actinide monoxides.

View Article: PubMed Central - PubMed

Affiliation: Department of Material Science and Engineering, KTH-Royal Institute of Technology, Stockholm SE-10044, Sweden.

ABSTRACT
Density functional theory has been applied to elucidate the stability of thorium monoxide (ThO). It is found out that the pressure can stabilize the rocksalt phase of ThO, and the transition pressure is estimated between 14 and 22 GPa. The stability of ThO can be attributed due to the gradually filling 5f orbitals at the expense of 7s and 6d electrons in Th metal. For ThO, the pressure induces stronger Th-O bond reflected by the newly established 6d-2p hybridization which is the dominant cause of its stability. The phonon dispersion curves of the rocksalt phase show the positive frequencies which indicates its dynamical stability. Our successful prediction of the stabilization of the metallic ThO has proposed a route to synthesize novel actinide monoxides.

No MeSH data available.


The phonon dispersion curves of ThO at 20 GPa.
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f1: The phonon dispersion curves of ThO at 20 GPa.

Mentions: The reaction enthalpy defined as has been computed as a function of pressure up to 35 GPa. The reaction enthalpy is positive at ambient pressure which suggests that ThO is unstable. Now, when pressure increases and reaches around 14 GPa (LDA) and 22 GPa (PBE), the reaction enthalpy becomes negative which implies the stabilization of ThO. These two stabilization pressure with two different exchange correlation potentials can be used as lower and upper limits of stability range of ThO. Based on these results, we can confidently say that the transition pressure lies within this range. We can take an intermediate value of 20 GPa (LDA) as the prototype pressure to carry out the following analysis. To begin with, the dynamical stability of energetically stable ThO is tested in the full phonon calculations as described in Fig. 1. This figure shows that the phonon dispersion curves of ThO have no imaginary frequencies. In the phonon dispersion curve, the flatness along L-X-W also reminds that the chemical bonding in this path is rather isotropic (the XY plane). Another feature is the big frequency gaps between higher frequencies and acoustic modes that can be explained by the big mass ratio between Th and O. The longitudinal acoustic (LA) vibration at point L formed the frequency edge, the value of higher frequency is precisely double LA frequency. Therefore, ThO is stable in rocksalt structure.


Stability of a new cubic monoxide of Thorium under pressure.

Sun W, Luo W, Ahuja R - Sci Rep (2015)

The phonon dispersion curves of ThO at 20 GPa.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4559911&req=5

f1: The phonon dispersion curves of ThO at 20 GPa.
Mentions: The reaction enthalpy defined as has been computed as a function of pressure up to 35 GPa. The reaction enthalpy is positive at ambient pressure which suggests that ThO is unstable. Now, when pressure increases and reaches around 14 GPa (LDA) and 22 GPa (PBE), the reaction enthalpy becomes negative which implies the stabilization of ThO. These two stabilization pressure with two different exchange correlation potentials can be used as lower and upper limits of stability range of ThO. Based on these results, we can confidently say that the transition pressure lies within this range. We can take an intermediate value of 20 GPa (LDA) as the prototype pressure to carry out the following analysis. To begin with, the dynamical stability of energetically stable ThO is tested in the full phonon calculations as described in Fig. 1. This figure shows that the phonon dispersion curves of ThO have no imaginary frequencies. In the phonon dispersion curve, the flatness along L-X-W also reminds that the chemical bonding in this path is rather isotropic (the XY plane). Another feature is the big frequency gaps between higher frequencies and acoustic modes that can be explained by the big mass ratio between Th and O. The longitudinal acoustic (LA) vibration at point L formed the frequency edge, the value of higher frequency is precisely double LA frequency. Therefore, ThO is stable in rocksalt structure.

Bottom Line: It is found out that the pressure can stabilize the rocksalt phase of ThO, and the transition pressure is estimated between 14 and 22 GPa.The phonon dispersion curves of the rocksalt phase show the positive frequencies which indicates its dynamical stability.Our successful prediction of the stabilization of the metallic ThO has proposed a route to synthesize novel actinide monoxides.

View Article: PubMed Central - PubMed

Affiliation: Department of Material Science and Engineering, KTH-Royal Institute of Technology, Stockholm SE-10044, Sweden.

ABSTRACT
Density functional theory has been applied to elucidate the stability of thorium monoxide (ThO). It is found out that the pressure can stabilize the rocksalt phase of ThO, and the transition pressure is estimated between 14 and 22 GPa. The stability of ThO can be attributed due to the gradually filling 5f orbitals at the expense of 7s and 6d electrons in Th metal. For ThO, the pressure induces stronger Th-O bond reflected by the newly established 6d-2p hybridization which is the dominant cause of its stability. The phonon dispersion curves of the rocksalt phase show the positive frequencies which indicates its dynamical stability. Our successful prediction of the stabilization of the metallic ThO has proposed a route to synthesize novel actinide monoxides.

No MeSH data available.