Synthesis, Crystal Structure, and High-Temperature Phase Transition of the Novel Plumbide Na2MgPb
Synthesis, Crystal Structure, and High-Temperature Phase Transition of the Novel Plumbide Na2MgPb
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新型铅化物Na2MgPb的合成、晶体结构和高温相变
DOI：
10.1021/ic500466w
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发表时间：
2014
影响因子：
4.6
通讯作者：
H. Yamane
中科院分区：
文献类型：
--
作者：
T. Yamada;T. Ikeda;R. P. Stoffel;V. L. Deringer;R. Dronskowski;H. Yamane
通过加热组成元素，合成了一种迄今未知的钠镁铅，Na 2 MgPb。Na_2MgPb晶体为六方晶系，具有Li_2CuAs型结构（P_63/mmc，Z= 2，a= 5.110（2）Ω，c= 10.171（4）Ω，293 K）。该化合物还表现出多晶型：高温粉末XRD测量显示六方Na 2 MgPb（称为“α”相）在493-553 K转变为另一种六方相（β），β相在533-633 K进一步转变为立方结构（γ）。γ-Na 2 MgPb的摩尔体积分别比α相和β相的摩尔体积小约9%和13%（543 K）。Na 2 MgPb的电阻率在300 K时为0.39 mΩ，在300 ~ 491 K范围内随温度升高而升高，在491和523 K时下降。这些电阻率的突变可能分别归因于α → β和β → γ相变。为了进一步了解立方γ-Na 2 MgPb的结构，采用基于密度泛函理论（DFT）的第一性原理计算方法，对立方γ-Na 2 MgPb的正Heusler型（Cu 2 MnAl型）和反Heusler型（Li 2AgSb型）排列模型进行了研究.计算结果表明，对于立方γ相，逆Heusler型结构明显比规则Heusler型结构稳定（0 K）;在此基础上，利用从头算热化学方法成功地验证了稳定性有序性（α-Na_2MgPb在低温下是有利的，γ-Na_2MgPb在高温下是有利的），尽管理论上预测的转变温度为900 K，高于实验中观察到的转变温度。
A hitherto unknown sodium magnesium plumbide, Na2MgPb, was synthesized by heating the constituent elements. Na2MgPb crystallizes in a hexagonal unit cell with the Li2CuAs-type structure (P63/mmc,Z= 2,a= 5.110(2) Å,c= 10.171(4) Å at 293 K). The compound furthermore displays polymorphism: high-temperature powder XRD measurements revealed that hexagonal Na2MgPb (dubbed the “α” phase) transforms to another hexagonal phase (β) which is existent at 493–553 K, and the β phase changes to a cubic structure (γ) at 533–633 K further. The molar volume of γ-Na2MgPb is approximately 9% and 13% smaller than the molar volumes of the α phase and the β phase, respectively (at 543 K). The electrical resistivity of Na2MgPb is 0.39 mΩ at 300 K; it rises with increasing temperature from 300 to 491 K, and then drops at 491 and 523 K. These abrupt changes in resistivity may be attributed to the α → β and β → γ phase transitions, respectively. To gain further insight into the structure of cubic γ-Na2MgPb, putative models with regular Heusler-type (Cu2MnAl-type) and inverse Heusler-type (Li2AgSb-type) arrangements were probed using first-principles computations based on density functional theory (DFT). These computations indicate that, for the cubic γ phase, an inverse Heusler-type structure is distinctly more stable than the alternative regular Heusler type (at 0 K); beyond that,ab initiothermochemistry was successfully used to verify the stability ordering (α-Na2MgPb being favorable at low temperature, γ-Na2MgPb at high temperature), albeit the theoretically predicted transition temperature of 900 K which is higher than observed in experiment.