Phase formation and crystal-chemical characterization of low- and high-alumina mullite-type compounds

本项目旨在了解以硅线石（Al2O3）为端元的莫来石型化合物的铝硅酸盐体系中相的完整结构演变。组成系列Al4+2xSi2-2xO10-x中的SiO2 （x=0）和jota-氧化铝（Al2O3, x=1）。硅线石和合成莫来石相含量最高，为2:1-莫来石（2Al2O3）。SiO2, x=0.4)和3:2莫来石（3Al2O3）。在过去的几十年里，人们对2SiO2， x=0.25)进行了广泛的研究，但基本上没有关于这些化合物在该系列的低（x < 0.2）和高（x > 0.8）侧的晶体结构的信息。已知矿物（通常含有一些铁）存在于低氧化铝范围内，但没有晶体结构研究。约塔-氧化铝是否存在仍是一个有争议的问题。在由DFT计算得出的假设模型中，提出了一个高度无序的结构（Aryal等人，物理学家）。与AlO6和AlO5多面体形成由AlO4四面体的二、三、四团簇交联的多面体链。相反，我们认为它可能由AlO6八面体和AlO4形成的四面体四团簇组成。然而，实验证据仍然缺失。我们建议研究一系列已经可用的天然莫来石，并将在Eifel地区收集作为低氧化铝化合物的成员。由于我们最近已经证明（Fischer et al.，美国）。矿物。2015,in press)发现存在一种矿物，它具有硅线石（Si/Al有序）和莫来石（O空位）的结构特征，对这些Eifel矿物进行结构和微化学表征将特别有趣。还计划在活塞缸装置中通过高温高压退火硅线石来合成这些中间低铝相。包括jota-氧化铝在内的高铝化合物将按照已知的路线在微波系统中合成。这将是首次对这些化合物进行晶体结构研究。由于莫来石型材料是最重要的陶瓷成分，因此将我们对这些化合物的知识扩展到迄今为止未知的低（x<0.2）和高（x>0.8）氧化铝莫来石和莫来石型相的晶体结构将是非常重要的。

It is the aim of this project to understand the complete structural evolution of the phases in the alumino-silicate system of mullite-type compounds with the endmembers sillimanite (Al2O3 . SiO2, x=0) and jota-alumina (Al2O3, x=1) in the compositional series Al4+2xSi2-2xO10-x. While sillimanite and synthetic mullite with its most abundant phases 2:1-mullite (2Al2O3 . SiO2, x=0.4) and 3:2-mullite (3Al2O3 . 2SiO2, x=0.25) have been studied extensively within the last decades, there is essentially no information on the crystal structures of these compounds at the low (x < 0.2) and high-alumina (x > 0.8) side of the series. Minerals (usually containing some Fe) are known to exist in the low-alumina range, but there are no crystal-structure investigations available. The existence or nonexistence of jota-alumina is still a matter of debate. In hypothetical models derived from DFT calculations a highly disordered structure was proposed (Aryal et al., Phys. Rev. B84, 2011, 174123) with AlO6 and AlO5 polyhedra forming the polyhedral chain crosslinked by di-, tri-, and tetraclusters of AlO4 tetrahedra. Instead, we suggest that it could consist of AlO6 octahedra and of AlO4 forming tetrahedral tetraclusters only. However, experimental proof is still missing. We propose to study a series of natural mullites already available and to be collected in the Eifel area as members of the low-alumina compounds. Since we have shown just recently (Fischer et al., Amer. Mineral. 2015, in press) that a mineral exists which has structural features from both sillimanite (Si/Al ordering) and mullite (O vacancies) it will be especially interesting to characterize these Eifel minerals structurally and microchemically. It is also planned to synthesize these intermediate low-alumina phases by annealing sillimanite at high temperatures and pressures in a piston cylinder apparatus. High-alumina compounds including jota-alumina will be synthesized in a microwave system following known routes. It will be the first time that crystal-structure investigations are performed on these compounds. Since mullite-type materials are most important ceramic constituents it will be extremely important to extend our knowledge on these compounds towards the hitherto unknown crystal structures of the low- (x<0.2) and high- (x>0.8) alumina mullites and mullite-type phases.