First Principles Study of Lattice Thermal Conductivity and Sound Velocity Characteristics of FeO2 and FeO2He
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摘要: 高温高压实验研究发现了一种新型铁氧化物FeO2，其可以稳定地存在于地球内部。理论研究表明，FeO2可以与He在高温高压下发生反应，形成FeO2He，借此解释了地球内部He的赋存机制。采用第一性原理方法，对比研究了FeO2和FeO2He两种矿物在下地幔条件下的晶格热导率和声速特征。计算结果表明，FeO2He的晶格热导率比FeO2的晶格热导率高很多，并且其随压力的变化也比FeO2大。两种矿物的晶格热导率与温度的变化关系都接近T −1，与传统半导体的结论一致。分析表明，两种矿物的群速度差异较小，对晶格热导率的影响有限；两种矿物的非谐散射率有较大的差异，是导致两者晶格热导率差异的主要原因。FeO2He的剪切和压缩波波速都比FeO2大一些，但两者的波速都比下地幔主要矿物钙钛矿的波速小，符合D″层超低声速的特征。Abstract: Recent experimental studies at high temperature and high pressure reported a new iron oxide, FeO2, which is stable from 74 GPa to the core-mantle boundary (CMB) pressure. Theoretical investigation also indicated that FeO2 can react with He at high temperature and high pressure to form FeO2He, which can explain the enigmatic He reservoir in the Earth. In this work, the lattice thermal conductivities and wave velocities of two minerals have been studied using first principles combined with lattice dynamics method. The calculations show that the lattice thermal conductivity of FeO2He is larger than that of FeO2, meanwhile, and the pressure dependence of lattice thermal conductivity of FeO2He is stronger than that of FeO2. The temperature dependence of lattice thermal conductivity of both minerals is close to T −1 relation, which is similar with those of traditional semiconductor. The group velocities have limited effect on the difference in lattice thermal conductivity between two minerals, and which is determined by the giant discrepancy in the anharmonic scattering rates. The compressive velocity and shear velocity of FeO2He are larger than those of FeO2. Their velocities are smaller than the values of perovskite and post-perovskite at the same condition, which implies that FeO2 and FeO2He are of the character of ultra-low sound velocity in D" layer.
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Key words:
- FeO2 /
- FeO2He /
- lattice thermal conductivity /
- wave velocity /
- first principles
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图 1 (a) 300 K下FeO2和FeO2He的热导率（
$ \kappa$ ）随压力的变化关系；(b) FeO2和FeO2He的热导率随温度的变化关系Figure 1. (a) Pressure dependence of the thermal conductivity (
$\kappa$ ) of FeO2 and FeO2He at 300 K; (b) temperature dependence of the thermal conductivity of FeO2 and FeO2He
图 2 FeO2和FeO2He在300 K下的累计晶格热导率随频率的变化关系
Figure 2. Frequency dependence of cumulative thermal conductivity (
${\kappa}$ ) of FeO2 and FeO2He at 300 K
图 3 FeO2 (a)和FeO2He (b)在135 GPa下的声子谱
Figure 3. Phonon dispersions of FeO2 (a) and FeO2He (b) at 135 GPa
图 4 FeO2和FeO2He在135 GPa下的群速度(a)和散射率(b)随频率的变化关系
Figure 4. Frequency dependence of the group velocity (
${v_{{q}}}$ ) (a) and scattering rate (${\tau ^{ - 1}_{{q}}}$ ) (b) of FeO2 and FeO2He at 135 GPa
图 5 135 GPa下FeO2和FeO2He的
$W_q $ 随频率的变化关系Figure 5. Frequency dependence of
$W_q $ of FeO2 and FeO2He at 135 GPa -
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