Structural evolution of Na0.5K0.5NbO3 at high temperatures
Structural evolution of Na0.5K0.5NbO3 at high temperatures
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DOI：
10.1016/j.jssc.2010.09.018
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发表时间：
2010-11-01
影响因子：
3.3
通讯作者：
Kakimoto, Ken-ichi
中科院分区：
文献类型：
--
作者：
Ishizawa, Nobuo;Wang, Jun;Kakimoto, Ken-ichi
本文研究了助熔剂法生长的NbO3Na0.5K0.5NbO_3单晶在高温下结构和介电性能的变化。单晶X射线衍射研究表明，在465和671K的升温温度和446和666K的降温温度下，晶体分别发生了正交-四方和四方-立方相的相变。这两个转变都伴随着加热时坍塌和冷却时膨胀的体积不连续，表明这两个转变是一级的。随着温度的降低，Nb的配位数呈下降趋势，即立方配位数为6，四方配位数为5+1，正交配位数为4+2。在12次配位中，安娜原子与K原子的配位略有不同，立方和四方的配位数较少，立方和四方的配位数分别为8+4和7+5。由原子位置和形式电荷估算的自发极化(P-S)在正交晶系中约为0.29Cm(-2)，在四方晶系中约为0.18Cm(-2)。碱性氧化物组分对P-S的贡献估计在这两种铁电形式中约为15%。通过不同频率下的介电常数和介电损耗以及差示扫描量热分析，也证实了温度诱导相变的存在。(C)2010 Elsevier Inc.保留所有权利。
Changes in structure and dielectric properties at elevated temperatures have been investigated on single-crystals of sodium potassium niobate, Na0.5K0.5NbO3, grown by the flux method. Single-crystal X-ray diffraction studies revealed that the crystals underwent orthorhombic-tetragonal and tetragonal-cubic phase transitions at 465 and 671 K during heating and 446 and 666 K during cooling, respectively. Both transitions were accompanied by volumetric discontinuities of collapse upon heating and expansion upon cooling, suggesting that the transitions were of the first order. The coordination numbers of an Nb showed a decreasing tendency with decreasing temperature, i.e., 6 in cubic, 5+1 in tetragonal and 4+2 in orthorhombic. An Na atom occupied a slightly different position from the K atom in 12-fold coordination, resulting in fewer coordination numbers of 8+4 in cubic and tetragonal and 7+5 in orthorhombic. The spontaneous polarisation (P-s) estimated from the atom positions and formal charges were approximately 0.29 C m(-2) in orthorhombic and 0.18 C m(-2) in tetragonal. The contribution of the alkaline oxide components to P-s was estimated to be approximately 15% in both ferroelectric forms. The temperature-induced transitions were also confirmed through the dielectric constant and dielectric loss at various frequencies and the differential scanning calorimetry. (C) 2010 Elsevier Inc. All rights reserved.