典型富氢化合物及其相关体系的高压行为研究
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摘要
氢是元素周期表中第一位的元素，在所有元素中具有最简单的原子结构。对于氢的相关研究不仅具有学术方面的意义，更有重要的应用价值。一方面，氢气作为一种可替代性的未来的清洁能源，用于汽车等的燃料，但是在氢能源推广的进程中，汽车装载能源的储氢问题是一大难题，因此对于含氢量较高的化合物的研究是热点问题。以往理论和实验研究发现，这类化合物的高压相可能具有更高的含氢量，从而具有更好的储氢性能，因此研究高压下的富氢化合物具有重大的意义。近年来，关于富氢化合物在高压下的相变次序、高压结构以及相变机制的研究，还有在金属化转变及提高超导温度等方面的探索均受到人们的极大重视。本论文中，我们选取了富氢化合物中的三元氢化物（碱金属氮氢化物）及二元氢化物（ZrH_2）作为主要研究对象。另一方面，氢被认为在高压下变为金属氢，金属氢是二十一世纪最为重要的十大物理问题之一，是潜在的室温超导体，但是一直没有在实验中获得，其中高压下氢分子的解离过程与机制是关键，因此我们选择相关的分子体系SnBr4，对其在高压下的行为进行研究，以便获得有助于氢高压研究的物理图像与规律。本文主要采用原位高压实验测量技术，辅助第一性原理计算的结果，对这些富氢化合物及分子体系进行了研究，获得了如下创新性成果：
（1）高压下碱金属氮氢化物的结构变化。在碱金属氮氢化物这个体系中，着重研究了LiNH_2和NaNH_2这两种储氢材料。为了使这类体系中的氢气更容易释放出来，目前主要是通过加入催化剂，或者球磨的方法。高压作为一种发现新型材料的有效方法，对碱金属氮氢化物进行高压下的研究以期获得具有更优异性能的储氢材料，早期的研究者对其进行了高压拉曼光谱的测量，由于Raman光谱不能完全确定物质的晶体结构，所以无法给出新相的结构。我们通过同步辐射XRD的实验测量及第一性原理计算研究了高压下LiNH_2的结构稳定性，最高压力为28.0GPa。发现其在压力为10.3–15.0GPa的范围内，发生了从四方相（空间群为I-4）到单斜相（空间群为P21）的结构转变。在这个相变过程中，伴随着大约11%的体积坍塌，而且这个体积压缩率比许多其他的复杂氢化物的要大一些。另外，通过对这两相态密度的计算，揭示了相变过程中伴随较大体积坍塌的原因。对于NaNH_2的实验研究表明常压相α-NaNH_2（正交晶系，空间群为Fddd）在1.12GPa转变为β-NaNH_2相，在1.93GPa又转变为γ-NaNH_2，从而解决了前人对于其相变顺序的争议，新相β-NaNH_2和γ-NaNH_2所属晶系分别为单斜晶系和正交晶系。
（2）不同静水压条件对ZrH_2高压行为的影响。为了极大的提高储氢材料的性能，对ZrH_2在高压下的行为进行研究也是必不可少的。另外，高压也是研究金属和氢之间相互作用的一种有效的手段。到目前为止，对于二元过渡金属氢化物的高压研究相对较少，大多都集中在对其常压下的结构及性质的研究。我们通过对ZrH_2在不同静水压条件下的角散同步辐射XRD的研究，发现其晶胞参数及其体积呈现不
II同的变化规律。我们主要研究了非静水压及静水压这两种情况，对于非静水压条件即样品腔中未加任何传压介质；静水压条件是用Ne作传压介质。对晶胞参数及其体积与压力关系的曲线进行拟合，发现非静水压条件下，样品的晶胞参数、键长随着压力是线性变化的，但是静水压条件下，晶胞参数及键长在压力大约为22.0GPa发生了明显的反常变化。并且在压力高于22.0GPa时，静水压条件下的晶胞体积大于非静水压条件下的晶胞体积，呈现反常变化，我们认为静水压条件下的样品发生了等结构相变。
（3）高压下分子晶体SnBr4的结构变化及电学性质的研究。作为重要的分子晶体，第四主族的卤化物在晶体结构变化及压致非晶化等方面已引起了极大的关注。尽管现有的理论及实验研究对于非晶化的机制提出了模型，但是仍存在很大争议。前人对于SnBr4的研究还仅仅局限在光谱测量及较低的压力范围内，因此，我们通过原位X射线光谱，拉曼光谱测量，光学吸收测量方法，以及理论计算的方法很全面地研究了SnBr4在高压下的行为。通过高压下XRD谱的测量发现在10.8GPa时，样品开始发生了晶体-晶体的结构转变，在21.5GPa时相变的完成。结合拉曼光谱的实验结果及理论计算的方法，我们认为这个结构变化是由SnBr4分子的二聚化引起的结构变化，理论计算的结果表明新相的空间群为P-1。在压力高于34.7GPa时，样品经历了向非晶态转变的过程，XRD谱的实验结果表明一直到43.8GPa样品还没有完全变成非晶相。实验和理论计算的结果都说明压致非晶化的过程是伴随分子解离的过程，即由分子相变为非分子相。通过高压下光学吸收光谱的测量发现，样品的带隙随着压力的增加在不断减小，当压力增加至21.0GPa时，带隙由常压下的3.4eV减小至1.5eV。更高压力下的透射光谱表明在大约40.3GPa时样品由绝缘体转变为金属。
Hydrogen is the first element in the periodic table of elements, whichalso has the simplest atomic structure in all elements. The research ofhydrogen not only has academic significance, but also is crucial inapplication. On one hand, the hydrogen gas is considered as one kind ofalternative clean energy in the future, especially used in automobile as fuel,but in the process of hydrogen energy promotion, the hydrogen storage inautomobile is still unsolved. So the hydrogen-rich compounds havebecome a hot issue. The previous theoretical and experimental studieshave found that the new high pressure phases with higher hydrogencontent probably exhibited better hydrogen storage properties, so theresearch on hydrogen-rich compounds under high pressure is of greatsignificance. In recent years, the research of hydrogen-rich compoundsunder high pressure have attracted much attention, including thetransformation sequence, high pressure structures and phase transitionmechanism, the insulator-metal transition and improving the superconductor temperature. We have selected alkli metal nitrogenhydride in the ternary hydrides and ZrH_2in the binary hydrides as themain objects of this study. On the other hand, hydrogen will become metalhydrogen under high pressure, and metal hydrogen is one of the mostimportant physical problems. As a potential room-temperaturesuperconductor, metal hydrogen has not been obtained in experiment.However, the dissociation process and mechanism of hydrogen under highpressure is critical, so we choose the related molecular system (SnBr4) tostudy its behavior under high pressure for obtaining the useful physicalpatterns and laws. In this article, we are going to study hydrogen-richcompounds and molecular system by in situ experimental measurementsand first principle calculations. And the obtained results are as follows:
(1) Study of alkali metal nitrogen hydride under high pressure. Inthe alkali metal nitrogen hydride system, we focus on LiNH_2and NaNH_2.In this system, some methods are being used in order to make it easier forthe hydrogen released, including adding catalyst or ball milling method.High pressure is one kind of effective method to find new materials, soalkali metal nitrogen hydrides have been investigated under high pressurefor obtaining more hydrogen storage materials with excellentperformances. The previous high pressure studies have explored thehydrides using the Raman spetra measurements. However, Raman spectracan not determine the crystal structure of the new phases. In this work, wehave studied the structural stability of LiNH_2under high pressure bymeans of synchrotron radiation XRD and first-principles calculation, andthe maximum pressure is28.0GPa. It is found that the sample changesfrom a tetragonal phase (space group I-4) to the monoclinic phase (spacegroup P21) in the range of10.3–15.0GPa. In this transformation process,the volume collapse is about11%, which is larger than many othercomplex hydride ones. In addition, through the calculation of the density of state of the two phases, we have revealed the reasons of the largevolume collapse. By in situ Raman spectrum measurement andsynchrotron radiation XRD study of NaNH_2, two structural changes werefound at about1.12GPa and1.93GPa, respectively, and it is proposedthat that the two crystal structures of the two new phases belong tomonoclinic and orthorhombic system, respectively.
(2) Effect of hydrostatic conditions on high-pressure behavior ofZrH_2. In order to improve the performance of hydrogen storage, a fullunderstanding of its behaviors under pressure is considered as essential.Moreover, the application of high pressure is an effective way to explorethe hydrogen-metal interactions which are of great relevance to specialproperties as well as hydrogen storage applications. As for the transitionmetal dihydrides, although many literatures have reported their groundstate properties and electronic properties by theoretical calculations,high-pressure experiments have not been intensively carried out. As one ofthe most important hydrogen storage material, the ZrH_2sample has beenmeasured by in situ synchrotron XRD under hydrostatic andnonhydrostatic compressions, respectively. By comparing the latticeconstants as a function of pressure under these two compressions, twodistinct compression regimes can be identified around22.0GPa in thehydrostatic compression while the lattice constants are linearly dependenton high pressure. So we have observed that ZrH_2has unusual compressionbehaviors under hydrostatic pressure, but the tetragonal structure of ZrH_2is stable up to43.8GPa under nonhydrostatic compression. Further, thevolume reduction under hydrostatic pressure is nearly equal with that ofnonhydrostatic compression below22.0GPa, above which the volumereduction starts to lower than that of nonhydrostatic pressure. Weexplained the remarkable phenomena that the sample underwent anisostructural phase transition under hydrostatic pressure.
(3) Structural and electronic changes of SnBr4under highpressure. As important molecular crystals, group IV halides have recentlybeen investigated for understanding structural changes and amorphousmechanism. Although several models have been proposed, the mechanismof pressure-induced amorphization observed in group IV halides is stillunder debate. As one of the group IV halides, previous high pressurestudies are only limited to low pressure and Spectral measurement. In thispaper, we have explored the pressure induced amorphization of SnBr4bysynchrotron X-ray diffraction, Raman spectra measurements, opticalabsorption/transmission measurements and ab initio calculations. Ourresult shows that there is a crystal-crystal transition induced by themolecular dimerization at about10.8GPa, which completes at about21.5GPa. Combining the Raman spectroscopy results with theoreticalcalculations, it is found that the crystal-crystal transition happens due tothe molecular dimerization. And the crystal structure of the dimeric phaseis obtained by the theoretical calculations with space group P-1. Thegradual amorphization is found to transform into non-molecularamorphous phase above34.7GPa, and the amorphization process does notcomplete until43.8GPa. The joint of experimental and theoretical resultsshows that the SnBr4molecular crystal becomes the non-molecular phaseupon compression. The band gap of the sample deceases with increasingpressure (from3.4eV at ambient pressure to1.5eV at21.0GPa). Ourexperimentally optical transmission spectra indicate a possibleinsulator-metal transition at about40.3GPa.
（1）高压下碱金属氮氢化物的结构变化。在碱金属氮氢化物这个体系中，着重研究了LiNH_2和NaNH_2这两种储氢材料。为了使这类体系中的氢气更容易释放出来，目前主要是通过加入催化剂，或者球磨的方法。高压作为一种发现新型材料的有效方法，对碱金属氮氢化物进行高压下的研究以期获得具有更优异性能的储氢材料，早期的研究者对其进行了高压拉曼光谱的测量，由于Raman光谱不能完全确定物质的晶体结构，所以无法给出新相的结构。我们通过同步辐射XRD的实验测量及第一性原理计算研究了高压下LiNH_2的结构稳定性，最高压力为28.0GPa。发现其在压力为10.3–15.0GPa的范围内，发生了从四方相（空间群为I-4）到单斜相（空间群为P21）的结构转变。在这个相变过程中，伴随着大约11%的体积坍塌，而且这个体积压缩率比许多其他的复杂氢化物的要大一些。另外，通过对这两相态密度的计算，揭示了相变过程中伴随较大体积坍塌的原因。对于NaNH_2的实验研究表明常压相α-NaNH_2（正交晶系，空间群为Fddd）在1.12GPa转变为β-NaNH_2相，在1.93GPa又转变为γ-NaNH_2，从而解决了前人对于其相变顺序的争议，新相β-NaNH_2和γ-NaNH_2所属晶系分别为单斜晶系和正交晶系。
（2）不同静水压条件对ZrH_2高压行为的影响。为了极大的提高储氢材料的性能，对ZrH_2在高压下的行为进行研究也是必不可少的。另外，高压也是研究金属和氢之间相互作用的一种有效的手段。到目前为止，对于二元过渡金属氢化物的高压研究相对较少，大多都集中在对其常压下的结构及性质的研究。我们通过对ZrH_2在不同静水压条件下的角散同步辐射XRD的研究，发现其晶胞参数及其体积呈现不
II同的变化规律。我们主要研究了非静水压及静水压这两种情况，对于非静水压条件即样品腔中未加任何传压介质；静水压条件是用Ne作传压介质。对晶胞参数及其体积与压力关系的曲线进行拟合，发现非静水压条件下，样品的晶胞参数、键长随着压力是线性变化的，但是静水压条件下，晶胞参数及键长在压力大约为22.0GPa发生了明显的反常变化。并且在压力高于22.0GPa时，静水压条件下的晶胞体积大于非静水压条件下的晶胞体积，呈现反常变化，我们认为静水压条件下的样品发生了等结构相变。
（3）高压下分子晶体SnBr4的结构变化及电学性质的研究。作为重要的分子晶体，第四主族的卤化物在晶体结构变化及压致非晶化等方面已引起了极大的关注。尽管现有的理论及实验研究对于非晶化的机制提出了模型，但是仍存在很大争议。前人对于SnBr4的研究还仅仅局限在光谱测量及较低的压力范围内，因此，我们通过原位X射线光谱，拉曼光谱测量，光学吸收测量方法，以及理论计算的方法很全面地研究了SnBr4在高压下的行为。通过高压下XRD谱的测量发现在10.8GPa时，样品开始发生了晶体-晶体的结构转变，在21.5GPa时相变的完成。结合拉曼光谱的实验结果及理论计算的方法，我们认为这个结构变化是由SnBr4分子的二聚化引起的结构变化，理论计算的结果表明新相的空间群为P-1。在压力高于34.7GPa时，样品经历了向非晶态转变的过程，XRD谱的实验结果表明一直到43.8GPa样品还没有完全变成非晶相。实验和理论计算的结果都说明压致非晶化的过程是伴随分子解离的过程，即由分子相变为非分子相。通过高压下光学吸收光谱的测量发现，样品的带隙随着压力的增加在不断减小，当压力增加至21.0GPa时，带隙由常压下的3.4eV减小至1.5eV。更高压力下的透射光谱表明在大约40.3GPa时样品由绝缘体转变为金属。
Hydrogen is the first element in the periodic table of elements, whichalso has the simplest atomic structure in all elements. The research ofhydrogen not only has academic significance, but also is crucial inapplication. On one hand, the hydrogen gas is considered as one kind ofalternative clean energy in the future, especially used in automobile as fuel,but in the process of hydrogen energy promotion, the hydrogen storage inautomobile is still unsolved. So the hydrogen-rich compounds havebecome a hot issue. The previous theoretical and experimental studieshave found that the new high pressure phases with higher hydrogencontent probably exhibited better hydrogen storage properties, so theresearch on hydrogen-rich compounds under high pressure is of greatsignificance. In recent years, the research of hydrogen-rich compoundsunder high pressure have attracted much attention, including thetransformation sequence, high pressure structures and phase transitionmechanism, the insulator-metal transition and improving the superconductor temperature. We have selected alkli metal nitrogenhydride in the ternary hydrides and ZrH_2in the binary hydrides as themain objects of this study. On the other hand, hydrogen will become metalhydrogen under high pressure, and metal hydrogen is one of the mostimportant physical problems. As a potential room-temperaturesuperconductor, metal hydrogen has not been obtained in experiment.However, the dissociation process and mechanism of hydrogen under highpressure is critical, so we choose the related molecular system (SnBr4) tostudy its behavior under high pressure for obtaining the useful physicalpatterns and laws. In this article, we are going to study hydrogen-richcompounds and molecular system by in situ experimental measurementsand first principle calculations. And the obtained results are as follows:
(1) Study of alkali metal nitrogen hydride under high pressure. Inthe alkali metal nitrogen hydride system, we focus on LiNH_2and NaNH_2.In this system, some methods are being used in order to make it easier forthe hydrogen released, including adding catalyst or ball milling method.High pressure is one kind of effective method to find new materials, soalkali metal nitrogen hydrides have been investigated under high pressurefor obtaining more hydrogen storage materials with excellentperformances. The previous high pressure studies have explored thehydrides using the Raman spetra measurements. However, Raman spectracan not determine the crystal structure of the new phases. In this work, wehave studied the structural stability of LiNH_2under high pressure bymeans of synchrotron radiation XRD and first-principles calculation, andthe maximum pressure is28.0GPa. It is found that the sample changesfrom a tetragonal phase (space group I-4) to the monoclinic phase (spacegroup P21) in the range of10.3–15.0GPa. In this transformation process,the volume collapse is about11%, which is larger than many othercomplex hydride ones. In addition, through the calculation of the density of state of the two phases, we have revealed the reasons of the largevolume collapse. By in situ Raman spectrum measurement andsynchrotron radiation XRD study of NaNH_2, two structural changes werefound at about1.12GPa and1.93GPa, respectively, and it is proposedthat that the two crystal structures of the two new phases belong tomonoclinic and orthorhombic system, respectively.
(2) Effect of hydrostatic conditions on high-pressure behavior ofZrH_2. In order to improve the performance of hydrogen storage, a fullunderstanding of its behaviors under pressure is considered as essential.Moreover, the application of high pressure is an effective way to explorethe hydrogen-metal interactions which are of great relevance to specialproperties as well as hydrogen storage applications. As for the transitionmetal dihydrides, although many literatures have reported their groundstate properties and electronic properties by theoretical calculations,high-pressure experiments have not been intensively carried out. As one ofthe most important hydrogen storage material, the ZrH_2sample has beenmeasured by in situ synchrotron XRD under hydrostatic andnonhydrostatic compressions, respectively. By comparing the latticeconstants as a function of pressure under these two compressions, twodistinct compression regimes can be identified around22.0GPa in thehydrostatic compression while the lattice constants are linearly dependenton high pressure. So we have observed that ZrH_2has unusual compressionbehaviors under hydrostatic pressure, but the tetragonal structure of ZrH_2is stable up to43.8GPa under nonhydrostatic compression. Further, thevolume reduction under hydrostatic pressure is nearly equal with that ofnonhydrostatic compression below22.0GPa, above which the volumereduction starts to lower than that of nonhydrostatic pressure. Weexplained the remarkable phenomena that the sample underwent anisostructural phase transition under hydrostatic pressure.
(3) Structural and electronic changes of SnBr4under highpressure. As important molecular crystals, group IV halides have recentlybeen investigated for understanding structural changes and amorphousmechanism. Although several models have been proposed, the mechanismof pressure-induced amorphization observed in group IV halides is stillunder debate. As one of the group IV halides, previous high pressurestudies are only limited to low pressure and Spectral measurement. In thispaper, we have explored the pressure induced amorphization of SnBr4bysynchrotron X-ray diffraction, Raman spectra measurements, opticalabsorption/transmission measurements and ab initio calculations. Ourresult shows that there is a crystal-crystal transition induced by themolecular dimerization at about10.8GPa, which completes at about21.5GPa. Combining the Raman spectroscopy results with theoreticalcalculations, it is found that the crystal-crystal transition happens due tothe molecular dimerization. And the crystal structure of the dimeric phaseis obtained by the theoretical calculations with space group P-1. Thegradual amorphization is found to transform into non-molecularamorphous phase above34.7GPa, and the amorphization process does notcomplete until43.8GPa. The joint of experimental and theoretical resultsshows that the SnBr4molecular crystal becomes the non-molecular phaseupon compression. The band gap of the sample deceases with increasingpressure (from3.4eV at ambient pressure to1.5eV at21.0GPa). Ourexperimentally optical transmission spectra indicate a possibleinsulator-metal transition at about40.3GPa.
引文
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