HoBaCo4O7中四面体刚体振动模的凝聚：355 K下相变的起源
Condensation of a tetrahedra rigid-body libration mode in HoBaCo4O7 : the origin of phase transition at 355 K
摘要：
通过HoBaCo 4 O 7在Ts = 355 K时的结构相变，分析了Rietveld剖面、Moessbauer谱和X射线吸收精细结构（XAFS）。通过在600 ℃的氩气流中退火样品，避免了氧含量超过O7。空间群（S. G.）Pbn21c和P63mc分别用于细化低温和高温相的结构参数。此外，Cmc 21对称性被认为是低温相结构的并发模型。在高温阶段，观察到大部分氧原子的热运动的严重各向异性。当样品冷却到Ts以下时，这种各向异性运动被淬灭。四极分裂在Ts附近的变化是不类似的阶梯状异常经常看到的电荷排序过渡。相反，我们观察到的平均四极分裂Ts附近的倾角异常。观察到不同钴位上电场梯度（EFG）的窄分布，并基于点电荷模型进行了解释。XAFS谱表明，YBaCo_4O_7（T > Ts）和HoBaCo_4O_7（T < Ts）之间没有明显的差别。在Ts处的转变的起源归因于与星形四面体单元的刚体旋转运动相关的天平动声子模的凝聚，星形四面体单元是kagome网络的构建块。结果表明，对于六角S. G.，平动模的凝聚与平移对称性不相容，但与正交S.G.相容正交晶格参数和EFG分量（Vxx，Vyy，Vzz）在接近Ts时随温度平滑变化，并且彼此紧密跟随。
英文摘要：
Rietveld profiles, Moessbauer spectra and x-ray absorption fine structure (XAFS) were analyzed through the structural phase transition at Ts = 355 K in HoBaCo4O7. Excess of the oxygen content over O7 was avoided via annealing the samples in argon flow at 600 degree C. Space groups (S.G.) Pbn21c and P63mc were used to refine the structure parameters in the low- and high-temperature phases, respectively. Additionally, the Cmc21 symmetry was considered as a concurrent model of structure of the low-temperature phase. In the high-temperature phase, severe anisotropy of thermal motion of the major part of the oxygen atoms was observed. This anisotropic motion turns to be quenched as the sample is cooled below Ts. The variation of quadrupole splitting near Ts is not similar to a steplike anomaly frequently seen at the charge-ordering transition. We observe instead a dip-like anomaly of the average quadrupole splitting near Ts. Narrow distribution of the electric field gradient (EFG) over different cobalt sites is observed and explained on the basis of point-charge model. XAFS spectra show no evidence of significant difference between YBaCo4O7 (T > Ts) and HoBaCo4O7 (T < Ts). The origin of the transition at Ts is ascribed to the condensation of the libration phonon mode associated with the rigid-body rotational movements of the starlike tetrahedral units, the building blocks of kagome network. It is shown that the condensation of the libration mode is not compatible with translation symmetry for the hexagonal S.G., but compatible for the orthorhombic S.G. The orthorhombic lattice parameters and EFG components (Vxx, Vyy, Vzz) vary smoothly with temperature at approaching Ts and closely follow each other.
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