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

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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.