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arXiv 1502.04667cond-mat.mtrl-scicond-mat.stat-mech
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金红石型TiO $_2$晶格膨胀过程中声子径迹杂化变化引起的声子四度性及其稳定化

Phonon quarticity induced by changes in phonon-tracked hybridization during lattice expansion and its stabilization of rutile TiO$_2$

Tian Lan, C. W. Li, O. Hellman, D. Kim, J. A. Muñoz, H. Smith, D. L. Abernathy, B. Fultz

摘要:

虽然TiO $_2$的金红石结构在高温下是稳定的,但是传统的准谐波近似预测随着热膨胀,几个声学声子不利地减少到零频率,错误地预测了在远低于1000 K的温度下的结构崩溃。用非弹性中子散射测量了金红石型TiO $_2$在300 ~ 1373 K范围内的声子态密度随温度的变化关系。令人惊讶的是,发现这些异常声学声子的频率随温度增加。第一性原理计算表明,随着晶格膨胀,反常声学声子的势从二次转变为四次,使金红石相在高温下稳定。在这些模式中,相邻的Ti和O原子的振动位移引起Ti的$3d$电子和O原子的$2p$电子的杂化变化。随着热膨胀,这种“声子跟踪杂化”中的能量变化使Ti和O原子之间的原子间势阱的底部变薄,并诱导声子势的四阶性。

英文摘要:

Although the rutile structure of TiO$_2$ is stable at high temperatures, the conventional quasiharmonic approximation predicts that several acoustic phonons decrease anomalously to zero frequency with thermal expansion, incorrectly predicting a structural collapse at temperatures well below 1000\,K. Inelastic neutron scattering was used to measure the temperature dependence of the phonon density of states (DOS) of rutile TiO$_2$ from 300 to 1373\,K. Surprisingly, these anomalous acoustic phonons were found to increase in frequency with temperature. First-principles calculations showed that with lattice expansion, the potentials for the anomalous acoustic phonons transform from quadratic to quartic, stabilizing the rutile phase at high temperatures. In these modes, the vibrational displacements of adjacent Ti and O atoms cause variations in hybridization of $3d$ electrons of Ti and $2p$ electrons of O atoms. With thermal expansion, the energy variation in this "phonon-tracked hybridization" flattens the bottom of the interatomic potential well between Ti and O atoms, and induces a quarticity in the phonon potential.

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