NSF-Germany Materials Collaboration: Defects and Orientation-Dependent Transport in Orthosilicates

具有Me = Co， Mn和Ni的Me2SiO4+d型正硅酸盐是以橄榄石结构结晶的固体。这种结构具有非常显著的晶格各向异性，这导致物质和电荷的输运具有很大的方向依赖性。到目前为止，关于这个问题的详细情况只知道硅酸铁的铁矾石。在非层状晶体结构中，观察到的取向依赖性是迄今为止最大的。一个完全开放的问题是，用Co， Mn和Ni取代Fe将如何影响物质和电荷输运的取向依赖。这个问题将被系统地研究在Me2SiO4+d型正硅酸盐中，Me = Co， Mn和Ni的物质和电荷的输运。这项研究将与德国布伦瑞克工业大学的Klaus-Dieter Becker小组进行国际合作。康奈尔大学的重点将是建立阳离子示踪剂扩散系数的晶体取向和氧活度依赖关系。电导率和氧活度跳跃后再平衡动力学的补充研究将在德国进行。此外，为了对观察到的氧活度依赖性进行建模，必要时将在康奈尔大学进行热重研究。为了实现所提出的取向依赖性的研究，单晶生长将在康奈尔大学通过使用浮动区方法进行。该项目将提供研究生和本科生水平的培训，并将提供国际合作的经验。这一基础研究的结果将极大地提高人们对一类重要的以橄榄石结构结晶的物质和电荷的输运的基本认识。例如，在应用地球科学中，橄榄石结构矿物是地球上地幔中常见的物质。对这类物质的进一步了解将有助于更好地了解地球的历史。另一方面，磷酸铁锂是一种具有非常高导电性的橄榄石结构材料，使其成为锂电池阴极的有希望的候选者。因此，固态电化学和其他领域也将受益于这项研究计划。这种广泛的相关性清楚地证明了公众对拟议研究的支持。研究将集中在具有橄榄石结构的正硅酸盐中物质和电荷的输运，并将包括单晶的生长和与阳离子和电荷输运有关的测量。由于其合作性质，该项目涉及不伦瑞克技术大学（德国）和康奈尔大学之间的研究生交流。此外，本科生将有重要的研究经验。将继续向当地学校和其他机构伸出援手。

Orthosilicates of the type Me2SiO4+d with Me = Co, Mn and Ni are solids which crystallize in the olivine structure. This structure has a very significant lattice anisotropy, which causes a large orientation-dependence of the transport of matter and charge. Details on this subject are known so far only for the iron silicate fayalite. The observed orientation dependences for fayalite are among the largest so far known for non-layered crystal structures. A completely open question is how the replacement of Fe by Co, Mn and Ni will influence the orientation dependence of the transport of matter and charge. This question will be systematically investigated for the transport of matter and charge in orthosilicates of the type Me2SiO4+d with Me = Co, Mn and Ni. The research will be performed in an international collaboration with the group of Klaus-Dieter Becker at the Technical University of Braunschweig (Germany). The emphasis at Cornell will be to establish the crystal orientation and oxygen activity dependence of tracer diffusion coefficients of the cations. Complementary studies of the electrical conductivity and on the kinetics of the reequilibration after oxygen activity jumps will be performed in Germany. In addition, to allow modeling of the observed oxygen activity dependences, thermogravimetric studies will be performed at Cornell University whenever necessary. To enable the proposed studies of the orientation dependence, single crystal growth will be performed at Cornell by using the floating zone method. The program will provide training at the graduate and undergraduate level and will also provide experiences in international collaboration.The results of this fundamental research will very significantly improve the basic understanding of transport of matter and charge in an important class of materials, which crystallize in the olivine structure. For example, in the applied geosciences, olivine structure minerals are common materials in the upper mantle of the Earth. An improved knowledge of this type of materials will aid to better understand the history of the Earth. On the other hand, lithium iron phosphate is an olivine structure material with a very high electrical conduction, making it a promising candidate for being used as the cathode in lithium batteries. Therefore, the field of solid-state electrochemistry and others will also benefit from this research program. This broad relevance clearly justifies public support of the proposed research. The research will focus on the transport of matter and charge in orthosilicates with the olivine structure and will include the growth of single crystals and measurements related to the transport of cations and of electrical charge. Due to its collaborative nature, the program involves an exchange of graduate students between the Technical University of Braunschweig (Germany) and Cornell University. In addition, there will be significant research experiences for undergraduates. Outreach to local schools and to other institutions will continue.