Phase Transformations in Rare-Earth Metals and Alloys Subjected to Ultra High Pressures and Synthetic Diamond Anvils

9704428 Vohra该研究调查了稀土金属及合金在兆巴压力下的相变，并为利用化学气相沉积技术(CVD)制造宝石级金刚石顶锤提供了必要的科学基础。实验研究是在阿拉巴马大学伯明翰分校的高压和CVD实验室以及位于布鲁克海文国家实验室的Chess、康奈尔大学和NSLS的国家高压光束线进行的。这项研究的重点是建立选定的原型稀土金属和合金在兆巴制度(100-400 Gpa)的相图。重点放在重稀土金属、钚和Yb上，这些金属的相变以及电子结构和价态在超高压下预计会发生戏剧性变化。压力也会导致f壳层的离域，从而导致“坍塌的低对称晶体结构”。利用高分辨率象板X射线衍射技术，结合同步辐射光源，研究了稀土金属中低对称性的晶体结构和最近发现的晶体颗粒长大现象。还研究了一种相关的过渡金属Y，它有望模拟稀土在高压下的行为，最近被预测在280 Gpa时将转变为体心立方结构。测量的相界和压力-体积(P-V)关系为各种压力下的f离域模型提供了严格的测试。研究的第二个方向是CVD钻石生长的基础研究，用于制造用于钻石顶座装置的低氮杂质合成钻石，此外，这笔资金支持的高压材料研究设施被用于夏季本科生(REU)项目和UAB阿拉巴马州矿产参与联盟(AMP)项目的研究体验。%这项研究有可能对理解稀土材料的结构和电子性质做出重大贡献。如果钻石研究成功，将具有广阔的技术应用前景。***

9704428 Vohra The research investigates phase transformations in rare- earth metals and alloys subjected to megabar pressures and develops the scientific base necessary to fabricate gem quality diamond anvils by chemical vapor deposition techniques (CVD). The experimental investigations are carried out at the High Pressure and CVD Laboratory at the University of Alabama at Birmingham (UAB) and the National High Pressure beamlines at CHESS, Cornell University and NSLS, at Brookhaven National Laboratory. The focus of this study is to establish phase diagrams for selected prototype rare-earth metals and alloys in the megabar regime (100-400 Gpa). The focus is on heavy rare-earth metals, lutetium and ytterbium, where phase transitions and dramatic changes in electronic structure and valence are expected at ultra high pressures. Pressure also causes delocalization of the f- shell giving rise to "collapsed - low symmetry crystal structures." A high resolution image plate x-ray diffraction technique is employed in conjunction with a synchrotron source to investigate low symmetry crystal structures as well as the phenomenon of crystal grain growth discovered recently in rare-earth metals. Also examined is a related transition metal, yttrium, which is expected to simulate rare-earth behavior at high pressures, and recently has been predicted to transform to the bcc structure at 280 Gpa. The measured phase boundaries and pressure-volume (P-V) relationships provide stringent tests for various models of f-delocalization under pressure. A second direction in the research is on fundamental studies on CVD diamond growth for fabrication of synthetic diamonds with low levels of nitrogen impurity for applications in diamond anvil cell devices, In addition, high pressure materials research facilities supported by this grant are employed in research experiences for undergraduates (REU) projects during the summer and for undergraduates in the Alabama Alliance for Minori ty Participation (AMP) Program at UAB. %%% This research has the potential for making significant contributions to the understanding of the structural and electronic properties of rare-earth materials. The research on diamond has broad technological applications if successful. ***