Theoretical study for anharmonicity in the phonon spectra for a distorted perovskite crystal structure in the photo exitted states

本项目利用基于第一性原理计算的声子色散关系，研究了畸变钙钛矿晶体结构（AgaO_3（A=Nd，La））在光激发态下光学光谱的温度依赖性。利用这些计算，我们还阐明了非谐效应对几种转动模（如平动模）线宽的影响。对于NdGaO_3的红外光谱，<2u>在174 cm ~（-1）处的B_i模<-1>是Nd原子和GaOG八面体的混合振动，而在300 cm ~（-1）以上的高频模不涉及Nd原子的运动<-1>。通过<2u><-1>与其它声子峰（356-cm ~（-1）和595-cm ~（-1））的比较，我们发现B_i模（174-cm ~（-1））的线宽随温度的升高而变化<-1><-1>。对于LaGaO_3，某些转动模[低温相（LT）的Ag（57-cm ~（-1）<-1>）和B_<2g>（118-cm ~（-1<-1>）），高温相（HT）的E_g（52-cm ~（-1<-1>））]的线宽与温度的关系主要是由热声子被观测声子散射所决定的，而弯曲模[HT的E_g（160-cm ~（-1<-1>））]的线宽与温度的关系主要是由观测声子散射和衰变成两个能量较低的声子所决定的。用与LaGaO_3类似的方法研究了LaAlO_3低温的拉曼谱，发现这是由于在DOS中存在较大的频率间隙（带隙非谐效应），从而导致LaGaO_3高温和LaAlO_3低温中各峰道上的非谐效应不同。为了用晶格动力学计算检验这种带隙非谐效应对其它晶体的有效性，我们研究了白云母型晶体。我们发现Ag模的非谐性是由CaWO_4中的大带隙非谐效应解释的。

In this project the observed temperature dependence of the optirahactive spectra for a distorted perovskite crystal structure (AGaO_3 (A=Nd, La)) in the photo-exited states is examined in terms of phonon-dispersion relations based on the first-principles calculations. We also clarify the origin of anharmonic effects on the linewidth of several rotational modes (e.g, librational mode) using these calculations. For IR spectra in NdGaO_3 the B_<2u>, mode at 174-cm-^<-1> involves mixed vibrations of the Nd atoms and the GaOG octahedra, and the higher frequency mode do not involve motion of Nd atoms above 300-cm-^<-1>. We found that the linewidth of the B_<2u>, mode (174-cm-^<-1>) varied with increasing temperature by comparison with other phonon peaks (356-cm-^<-1>and 595-cm-^<-1>). For the LaGaO_3 temperature dependence of the linewidth for some rotational modes [the Ag (57-cm-^<-1>) and the B_<2g> (118-cm-^<-1> for the low-temperature phase (LT), and the.E_g (52-cm-^<-1>) for the high-temperature phase (HT)] is dominated by the scattering of a thermal phonon by the observed phonon, while that for the bending mode [the E_g (160-cm-^<-1>) for the HT] is dominated by both the scattering and the decay of the observed phonons into two phonons having lower energies. We also have investigated Raman spectra in the LT of LaA1O_3.by similar approach as that in LaGaO_3 We found that this originates in the large frequency gap (band-gap-anharmonic effects) in the DOS that produces a difference in anharmonic effects on each peaks channels in the HT of LaGaO_3 and the LT of LaA1O_3, respectively. In order to examine validlY for this band-gap-ahnarmonic effects on other crystal with lattice dymamical calculation, we have investigated sheelite type crystals. We found that anharmonicity in the Agmodes was explained by a large band-gap-ahnarmonic effects in CaWO_4.