Negative thermal expansion emerging upon structural phase transition in ZrV2O7 and HfV2O7
Negative thermal expansion emerging upon structural phase transition in ZrV2O7 and HfV2O7
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DOI：
10.1039/c0dt01087a
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发表时间：
2011-01-01
影响因子：
4
通讯作者：
Saito, Kazuya
中科院分区：
文献类型：
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作者：
Yamamura, Yasuhisa;Horikoshi, Aruto;Saito, Kazuya
利用X射线衍射和热容量热法研究了在高温阶段表现出负热膨胀（NTE）的ZrV2O7和HfV2O7。两个尖锐的异常，由于连续的相变，观察到在345.5 K和373.4 K的ZrV2O7和341.8 K和370.3 K的HfV2O7的热容的温度依赖性。它们的联合相变熵都很小，说明它们的相变是位移型的。通过对ZrV_2O_7和HfV_2O_7热容的谱分析，得到了用简单模型描述的有效声子态密度和低温相的模Gruneisen参数。它们的有效声子态密度显示了NTE化合物的三个共同特征：低能声子模、高能声子模和其间的宽声子能隙。组成多面体的平移和平动振动对应的低能模的模Gruneisen参数为负，但由于低温相V2O7基团的畸变，导致正的热膨胀，具有小的绝对值。结果表明，ZrV_2O_7和HfV_2O_7在高温相的NTE是由于在连续相变过程中结构畸变的释放而导致的。
ZrV2O7 and HfV2O7, which show negative thermal expansion (NTE) in the high-temperature phase, were investigated using X-ray diffraction and heat capacity calorimetry. Two sharp anomalies due to successive phase transitions were observed in the temperature dependence of heat capacity at 345.5 K and 373.4 K for ZrV2O7 and 341.8 K and 370.3 K for HfV2O7. The smallness of their combined entropies of transition suggested that the phase transitions are of displacive type. Effective phonon densities of states (DOS) described by a simple model, and mode-Gruneisen parameters of the low-temperature phase were obtained through the spectrum analyses of heat capacities of ZrV2O7 and HfV2O7. Their effective phonon DOS's show the three features common to NTE compounds: low-energy phonon mode, high-energy phonon mode, and a wide phonon gap in between. The mode-Gruneisen parameter of low-energy modes corresponding to translational and librational vibrations of the constituent polyhedra is negative but with a small absolute value due to the distortion of V2O7 group in the low-temperature phase, resulting in positive thermal expansion. It is revealed that the release of the structural distortion upon the successive phase transitions with large volume increase leads to the NTE of ZrV2O7 and HfV2O7 in the high-temperature phase.