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

Me=Co，Mn，Ni的Me_2SiO_4+d类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与取向有很大的相关性。到目前为止，关于这个问题的细节只知道铁硅酸盐铁闪石。观察到的铁闪石的取向依赖关系是迄今为止已知的非层状晶体结构中最大的。一个完全悬而未决的问题是，Co、Mn和Ni取代Fe将如何影响物质和电荷输运的取向相关性。系统地研究了Me=Co，Mn，Ni的Me_2SiO_4+d正硅酸盐中的物质和电荷输运问题。这项研究将与德国布伦斯韦格工业大学的克劳斯-迪特尔·贝克尔小组进行国际合作。康奈尔大学的重点将是建立离子示踪剂扩散系数的晶体取向和氧活度依赖关系。德国将对电导率和氧活度跃升后再平衡的动力学进行补充研究。此外，为了能够对观察到的氧活度依赖关系进行建模，康奈尔大学将在必要时进行热重研究。为了使所提出的取向依赖关系的研究成为可能，将在康奈尔大学使用浮动区方法进行单晶生长。该计划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。这项基础研究的结果将极大地提高对一类重要材料的物质和电荷输运的基本理解，这些材料在橄榄石结构中结晶。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。对这类材料的了解的提高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很高导电性的橄榄石结构材料，有望用作锂电池的正极。因此，固态电化学和其他领域也将受益于这一研究计划。这种广泛的相关性显然证明了公众对拟议研究的支持是合理的。这项研究将集中在具有橄榄石结构的正硅酸盐中的物质和电荷的传输，并将包括单晶的生长以及与阳离子和电荷传输有关的测量。由于其合作性质，该项目涉及德国布伦斯韦格工业大学和康奈尔大学之间的研究生交换。此外，还将为本科生提供重要的研究经验。将继续与当地学校和其他机构进行接触。

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.