Structural Effects Behind the Low Temperature Nonconventional Relaxor Behavior of the Sr₂NaNb₅O₁₅ Bronze

An exhaustive temperature dependent structural and dielectric study of the tetragonal tungsten bronze-type Sr2NaNb5O15 (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 TC = 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.本研究针对四方钨青铜型（tetragonal tungsten bronze-type）Sr₂NaNb₅O₁₅（SNN）化合物，在300 K至100 K的温度区间内开展了详尽的温度依赖结构与介电性能研究。研究结合X射线衍射（X-ray diffraction）、中子衍射（neutron diffraction）与透射电子显微镜（transmission electron microscopy）表征手段及介电性能测试，旨在阐明导致该材料低温介电特性的结构成因。值得注意的是，除了在T_C=518 K处发生的铁电（ferroelectric, FE）-顺电（paraelectric, PE）相变之外，研究还在约240 K处观测到介电常数的第二处显著异常峰，表明该材料在低温区间表现出类弛豫铁电行为。该现象此前已在钙钛矿型铁电相中被观测到，并被称为重入相变现象。然而，在该低温介电异常峰以下，铁电极化趋于消失，这一事实与经典弛豫铁电相的预期行为不符。尽管在室温（room temperature, RT）至100 K的区间内未观测到结构相变，但该材料在本研究涉及的温度范围内的弹性性能发生了变化；而约240 K处的强介电异常峰，可能与细微结构效应诱导的弥散相变至受挫铁电/铁弹性（ferroelastic, FEL）低温态的过程相关。