晶粒尺寸对PMN-35 % PT陶瓷中极性纳米团簇动力学的影响：宽带介电和红外光谱
Grain Size Influence on Dynamics of Polar Nanoclusters in PMN-35%PT Ceramics: Broadband Dielectric and Infrared Spectra
摘要：
研究了10 - 900 K温度范围内粗晶（晶粒尺寸~ 4 mkm）和细晶（晶粒尺寸~ 150 nm）PbMg_（1/3）Nb_（2/3）O_3 - 35%PbTiO_3陶瓷的介电响应e ~*（f，T）和极性声子谱。粗晶陶瓷中的e*（f，T）在高温下表现出弛豫行为，在铁电相变时表现出明显的反常。细晶陶瓷主要表现为弛豫铁电行为，介电常数较小。这种差异是由极性纳米团簇的不同弛豫动力学来解释的，这些纳米团簇在高温下通过在晶界处的钉扎而在细晶粒陶瓷中显得更加稳定。低于Tc，铁电畴的生长被抑制在细晶粒陶瓷支持也由二次谐波产生。另一方面，极性声子频率和它们的温度依赖性几乎是独立的晶粒尺寸，但立方对称性的选择规则不服从和所有的声子分裂，由于局部破缺的对称性由极性nanoregions和化学无序。在这两种陶瓷中，最低频率的极性声子在Tc = 440 K附近部分软化至~ 0.1 THz，但介电异常主要是由极性纳米团簇的翻转和呼吸引起的。由于软声子模和介电弛豫对介电常数的贡献，铁电相变（对应于极性纳米区域进入宏观畴的渗透阈值）可以被认为是位移型和有序-无序型之间交叉的特殊情况。
英文摘要：
Dielectric response e*(f,T) and polar phonon spectra of coarse grain (grain size ~ 4 mkm) and fine grain (grain size ~ 150 nm) ceramics of PbMg_(1/3)Nb_(2/3)O3-35%PbTiO3 were investigated at temperatures 10 - 900 K. e*(f,T) in coarse-grain ceramics exhibits relaxor behavior at high temperatures and a sharp anomaly at the ferroelectric phase transition. The fine-grain ceramics exhibit mainly relaxor ferroelectric behavior with a smaller dielectric constant. The difference is explained by different relaxational dynamics of polar nanoclusters, which appear to be more stabilized at high temperatures in the fine-grain ceramics by pinning at grain boundaries. Below Tc, the growth of ferroelectric domains is suppressed in fine-grain ceramics as supported also by a second harmonic generation. On the other hand, polar phonon frequencies and their temperature dependences are almost independent of the grain size, but the selection rules for the cubic symmetry are not obeyed and all phonons are split due to a locally broken symmetry by polar nanoregions and chemical disorder. The lowest-frequency polar phonon undergoes partial softening down to ~ 0.1 THz near Tc = 440 K in both ceramics, but the dielectric anomaly is caused predominantly by flipping and breathing of polar nanoclusters. Due to contribution of both the soft phonon mode and dielectric relaxations into the dielectric constant, the ferroelectric phase transition, which corresponds to the percolation threshold of the polar nanoregions into macroscopic domains, can be considered as a special case of crossover between the displacive and order-disorder type.
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