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Elastic and dynamical structural properties of La and Mn-doped SrTiO$$_{3}$$ studied by neutron scattering and their relation with thermal conductivities

LaおよびMnを置換したSrTiO$$_{3}$$の静的・動的構造およびそれらの熱伝導度との関係に関する中性子散乱による研究

梶本 亮一  ; 中村 充孝  ; 村井 直樹 ; 社本 真一  ; 本田 孝志*; 池田 一貴*; 大友 季哉*; 畑 博人*; 江藤 貴弘*; 野田 正亮*; 桑原 英樹*; 奥田 哲治*

Kajimoto, Ryoichi; Nakamura, Mitsutaka; Murai, Naoki; Shamoto, Shinichi; Honda, Takashi*; Ikeda, Kazutaka*; Otomo, Toshiya*; Hata, Hiroto*; Eto, Takahiro*; Noda, Masaaki*; Kuwahara, Hideki*; Okuda, Tetsuji*

The electron-doped SrTiO$$_{3}$$ exhibits good thermoelectric properties. Recent studies indicated that only a few percent co-doping of La and Mn in SrTiO$$_{3}$$ substantially reduces the thermal conductivity. Then, we investigated the static as well as dynamical structural properties of La and Mn-doped SrTiO$$_{3}$$ by time-of-flight elastic and inelastic neutron scattering. We found that the La-Mn co-doped samples show tetragonal structures similar to the low-temperature structure of SrTiO$$_{3}$$, while the La-doped and Mn-doped samples maintain cubic structures with the same symmetry as the high-temperature structure of SrTiO$$_{3}$$. However, it is unlikely that it is the origin of the low thermal conductivity, since the structural transition in SrTiO$$_{3}$$ does not significantly change the thermal conductivity. On the other hand, we found that the inelastic scattering spectrum shows a momentum-independent increase in the low-energy spectral weight by the co-doping. Interestingly, the increase in the low-energy spectral weight exhibits a clear correlation with thermal conductivity. The substitution of Sr$$^{2+}$$ ions by La$$^{3+}$$ ions results in the electron doping, and further doping of the Mn ions are expected to create Jahn-Teller active Mn$$^{3+}$$ ions. We consider that dynamical and local structural fluctuations caused by the Jahn-Teller instability in Mn$$^{3+}$$ ions coupled with the incipient ferroelectric nature of SrTiO$$_{3}$$ induce additional low-energy scatterings and correspond to the origin of low thermal conductivity in the La and Mn co-doped compounds.

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