Ca_(0.28)Ba_(0.72)Nb_2O_6单晶中无公度调制结构、电畴结构与相变的TEM研究  

TEM study of incommensurate modulation structure,domain structure and phase transition in ferroelectric Ca_(0.28)Ba_(0.72)Nb_2O_6 single crystals

作　　者：祁亚军[1] 杨虎[1] 卢朝靖[1] 

机构地区：[1]湖北大学材料科学与工程学院,湖北武汉430062

出　　处：《电子显微学报》2007年第2期103-109,共7页Journal of Chinese Electron Microscopy Society

基　　金：教育部"新世纪优秀人才支持计划";国家自然科学基金资助项目和武汉市学科带头人计划

摘　　要：介电温谱测量结果表明,钨青铜型Ca0.28Ba0.72Nb2O6(CBN28)单晶在252℃附近经历了从正常铁电体到弛豫铁电体的相变。用透射电子显微镜研究了该单晶中的无公度调制结构、畴结构及其演变。由室温[110]带轴电子衍射花样确定无公度调制波矢q=(1/2)c*+(1+δ)(a*-b*)/4,其中无公度参数δ=0.09。高分辨像给出了无公度调制结构的直接证据。这种无公度超晶格结构是两种正交超晶胞结构均匀混合的结果。CBN28单晶中存在高密度沿c轴走向直径约50nm^500 nm的长钉状180°电畴。在电镜中原位升温至220℃左右,开始出现许多直径仅几纳米的细条状畴;温度继续上升时这种细条状畴渐增多。这种宏畴到微畴转变对应于在铁电相变点Tc附近的正常到弛豫铁电转变。刚好在Tc以下,还观察到经零场冷却的纳米针状畴。Dielectric measurement revealed that the CBN28 crystals experienced a normal-relaxor ferroelectric phase transition around 252℃ on heating. The modulation structures, domain structures and their evolution of CBN28 crystals were studied by transmission electron microscopy. A wave-modulation vector can be expressed as q = （1/2）c^＊ ＋ （1 ＋δ）（ a^＊ - b^＊ ）/4, where the incomensurability parameter δ was determined to be about 0.09 in the [ 110] diffraction patterns at room temperature. High-resolution TEM images along [ 110] zone provided direct evidence of the incommensurate superstructure. The incommensurate superlattice structure of CBN is caused by the uniform mixing of slabs of two orthorhombic cells （ao, bo, co ） and （a＇o, b＇o,c＇o ）. There exists a high density of 180° domain walls in the crystals. The domains appear predominantly spike-shaped along the c-axis and have a typical diameter of 50 nm - 500 nm. Macrodomains-to- microdomains transition, corresponding to the normal-to-relaxor ferroelectrics transition, has been observed near the ferroelectric phase transition temperature Tc , At temperature just below Tc, zero-field-cooled needle-like nanodomains were also observed.

关 键 词：Ca0.28Ba0.72Nb2O6 透射电镜 无公度调制结构 铁电畴 

分 类 号：O482[理学—固体物理]