Structural relations between two ground states of NaVO under high pressure; A Synchrotron X-ray diffraction study

NaVOにおける二つの基底状態の構造的関係; 放射光X線回折研究

大和田 謙二; 藤井 保彦; 村岡 次郎*; 中尾 裕則*; 村上 洋一; 野田 幸男*; 大隅 寛幸*; 池田 直*; 菖蒲 敬久  ; 礒部 正彦*; 上田 寛*

Owada, Kenji; Fujii, Yasuhiko; Muraoka, Jiro*; Nakao, Hironori*; Murakami, Yoichi; Noda, Yukio*; Osumi, Hiroyuki*; Ikeda, Naoshi*; Shobu, Takahisa; Isobe, Masahiko*; Ueda, Yutaka*

ANNNI(Axial Next Nearest Neighbor Ising)物質, NaVOの基底状態C, C相の構造的関係をX線回折で調べた。C相の構造はC相の4層(AAA'A')の一つ(AもしくはA')で説明できることがわかった。ただし、原子変位は27%にまで押さえられる。一方、電荷秩序は完全に起きている。このことは電荷格子結合定数が圧力によって変わっていることを示しており、悪魔の相図を生み出す競合する相互作用比の変化の起源を考えるうえで興味深い。

Structural relations between two ground states of the ANNNI (Axial Next Nearest Neighbor Ising) compound NaVO, C- and C-phases below and above the transition pressure = 1 GPa, were investigated by X-ray diffraction and scattering techniques. The structure of the C-phase is well explained by the () pattern which is one of four layers () of the C-phase, however, the amount of the atomic shifts under the conditions 1.6 GPa and 6 K is 27% that under ambient pressure. On the other hand, resonant X-ray scattering showed that the charges are disproportionated under high pressure. Based on these facts, it was concluded that charge disproportionation corresponds to the Ising variable in NaVO, where the atomic shifts are regarded as linearly coupled to the Ising spins. These results lead to the hypothesis that the competitive interactions between the Ising spins may result from the Ising spin-phonon coupling.