Extended defects in natural and synthesized perovskite oxides: nanogeochemcial indicators and functional interfaces

Crystal growth in diverse geological environments is a complex process governed by a combination of physicochemical factors. Geochemical and geophysical changes during the growth are imprinted into crystal grains in the form of inclusions, crystallographic defects, intergrowths and other patterns. One of the most informative are growth-type extended defects, such as twin or antiphase boundaries. These 2D interfaces are characterized by locally different atomic structure and chemical composition. Atomic-scale analyses of such interfaces provide key information about mineral and rock crystallization history (nanogeochemcial indicators) and help mineralogists and petrologists to decipher the sequence of crystallization processes. Formation of growth-type defects occurs also during crystal growth in functional materials prepared under controlled synthesis conditions. Here, these complex interfaces exhibit a range of important effects, from influence on crystal/grain growth and overall microstructure development to quantum effects emerging from their local atomic structure. In this project we will focus our investigations on extended defects in perovskite oxides, which are one of the most intriguing groups of compounds in terms of structural and chemical diversity. They occur as natural minerals in a wide variety of parageneses and are being increasingly recognized as important geoenvironmental indicators. They are even more widespread as functional materials with a range of interesting properties. One of the main characteristics of perovskite oxides is their capacity to incorporate many elements in the form of solid solubility, point defects, extended defects, modulated phases and similar. For our studies we selected natural perovskite oxides with different geological background and chemical composition: perovskite (CaTiO3) crystals from Zlatoust, Ural; loparite ((Na,REE)Ti2O6) interpenetration twins from Mt. Khibiny, Kola peninsula (both Russia); and perovskite-magnetite intergrowths from alkaline basalts, East Eifel, Germany. All crystals are rich in different types of extended defects and contain many trace elements. The atomic structure of extended defects will be studied down to the atomic-scale using quantitative scanning transmission electron microscopy (STEM) in combination with spectroscopic techniques for determination of local chemical composition of the interfaces (EDS, EELS). Most challenging task will be detection of trace elements at different types of interfaces. Our primary objective will be related to the nanogeochemical aspect of extended defects in natural perovskites. Based on the nanoscale structure and chemical composition of different types of extended defects in natural samples, we will reconstruct their formation mechanism (via transformation or growth) and further, determine geochemical and geophysical conditions during crystallization. The studies on natural crystals will be complemented with synthesis of nature-mimicked compositions under controlled laboratory conditions. The influence of trace elements (detected in natural samples) on the formation of single perovskites and modulated phases from the perovskite-loparite-tausonite system and the formation mechanism of oxygen deficient brownmillerite phases in Fe-doped perovskite will be studied. Objectives related to the functionality of chemically induced interfaces will be to reveal the relationship between the local atomic structure of extended defects and modulated phases and functional properties of natural single crystals and polycrystalline samples. The local electrical properties of chemically-induced extended defects will be evaluated using in-situ measurements. The project team involves specialists from complementary research fields like mineralogy, petrology, chemistry and materials science and leading experts in advanced electron microscopy.