Departamento de Química Inorgánica, Facultad de Químicas, Universidad Complutense Madrid, 28040 Spain.
Inorg Chem. 2011 Dec 5;50(23):12091-8. doi: 10.1021/ic2016098. Epub 2011 Oct 28.
An exhaustive temperature dependent structural and dielectric study of the tetragonal tungsten bronze-type Sr(2)NaNb(5)O(15) (SNN) compound has been performed in the 300-100 K temperature range, by combining X-ray, neutron diffraction, and transmission electron microscopy with dielectric measurements, in order to clarify the structural effects responsible for the observed low temperature dielectric properties. Interestingly, a relevant second anomaly in the dielectric constant, in addition to the ferroelectric (FE) to paraelectric (PE) transition at T(C) = 518 K is found at T ≈ 240 K, revealing a relaxor-like behavior of the material at low temperature. This phenomenon has been previously observed in FE perovskite-type phases and referred to as the re-entrant phenomenon. However, FE polarization tends to vanish below this low temperature dielectric anomaly and this fact is not expected for a classical relaxor-ferroelectric phase. Although there is no structural transition from RT to 100 K, there is a change in the elastic properties of the material in the considered temperature range and the intense anomaly at ~240 K could be associated to a smeared-out phase transition to a frustrated FE/ferroelastic (FEL) low temperature state in correlation with subtle structural effects.
我们对四方钨青铜型 Sr(2)NaNb(5)O(15)（SNN）化合物进行了详尽的温度相关结构和介电研究，在 300-100 K 的温度范围内，通过结合 X 射线、中子衍射和透射电子显微镜与介电测量，以阐明导致观察到的低温介电性质的结构效应。有趣的是，除了在 T(C) = 518 K 时发生铁电（FE）到顺电（PE）转变外，在 T ≈ 240 K 时还发现介电常数的一个相关的第二个异常，表明材料在低温下具有弛豫体行为。这种现象以前在 FE 钙钛矿型相中观察到，并称为再进入现象。然而，低于这个低温介电异常，FE 极化趋于消失，这对于经典弛豫体-铁电体相来说是不期望的。尽管从 RT 到 100 K 没有结构转变，但在考虑的温度范围内材料的弹性性质发生了变化，并且在~240 K 时的强烈异常可能与细微的结构效应相关联，与受挫的 FE/铁弹性（FEL）低温状态有关。
