The Power of Order

Crystals are everywhere. Nearly our entire planet is composed of crystallised material. The crystallographers at the University of Vienna are delving deep into the nanoworld of crystal structure and exploring its transformability.

Feb. 1, 2016

What makes car paint glitter and paper appear extra white, and why do glass ceramic hobs not break at high temperatures? The answer is crystals, or more precisely, their regular crystal structure with its atomic configuration. “The crystalline state has a number of specific properties that are often very useful, making it increasingly relevant for humankind,” says Ronald Miletich, Head of the Department of Mineralogy and Crystallography. One of the key research areas of his Department comprise the analysis of the stability of crystals, their phase transitions and how the transition processes work.

We investigate structure property relationships in crystals, as we want to know: Can we predict when which of the crystalline structures react, in what form and in which direction they change?”

Ronald Miletich, Professor of Mineralogy and Crystallography

Extreme conditions can change the crystal structure, i.e. the arrangement of atoms in a crystal. Even under “normal” conditions, crystals can be unstable, at times even fragile: Sometimes, even minute changes in ambient temperature or pressure can cause a transition in the crystal structure. This “phase transition”, as researchers call it, changes the physical behaviour of the material.

In one project, the crystallographers at the University of Vienna are studying the behaviour of potentially toxic heavy metal hydrates, which are sometimes brought to the surface by pit water in mining areas. The results will allow researchers to better determine their potential danger for humans and the environment. Another group will study transition mechanisms of sulphate hydrates in exceptional conditions, like those on Mars, in a project granted by the Austrian Science Fund (FWF). The spectroscopic data of certain sulphate hydrates on our neighbouring planet still hold some mysteries for astrophysicists.

Crystalline paths

About a century ago, Max von Laue discovered the diffraction of X-rays by crystals. This discovery gained him the Nobel Prize in Physics and started the era of modern crystallography. Since his discovery, X-ray structure analysis has been used to infer the arrangement of atoms in crystals and changes to this atomic configuration.

“Today, we are interested in the structure property relationships in crystals. With our experimental tools and computer simulations, we are now able to determine when crystal structures change,” says Ronald Miletich, adding: “We understand the structure at the starting point and at the end, but we are not always clear on the details of the process between them.” Miletich’s Research Group Crystallography is increasingly focusing on the transition states of the crystal structure and their role in the phase transition. A focus has been placed on carbonates such as calcium carbonate, CaCO₃, the mineral that forms the Limestone Alps – the mountain ranges of the Alps that stretch across Austria.

One of the highlights for the crystallographers at the University of Vienna in 2015 was the discovery of the probable stability range of a new high-pressure form of CaCO₃ – an important intermediary step in CaCO₃ transition in temperatures below 40°C and at pressure of geological relevance. The researchers discovered the significance of this phase in their laboratory experiments, shedding new light on the transitions of the calcite. In higher temperature ranges, the phase does not play a role under hydrostatic pressure, so the structural changes of the calcite and their processes remain a mystery.

Radiation damage & inclusions

How does the irradiation of minerals influence their structural composition? Which mechanisms underlie the colour changes of diamonds exposed to radioactive radiation, an effect that has long been used by the gem industry?

These are questions that the Research Group Mineralogy, led by Lutz Nasdala, seeks to answer. To find answers to these questions, the researchers are subjecting natural minerals and their synthetic analogues to ion beams to intentionally cause structural alterations. Their main tool is a Focused Ion Beam, or FIB, available at the Department. It is also used to prepare samples: The ion beam with extremely high energy can be used like a scalpel to cut minerals without influencing the material too much. The resulting cross-sections (“foils”) are used for irradiation experiments. This research is funded by the FWF via a project and is carried out in close cooperation with the Institute of Ion Beam Physics and Materials Research at the German Helmholtz-Zentrum Dresden-Rossendorf.

We make use of spectroscopy to analyse minerals. With this method, we receive manifold information about minerals in the micrometer range.”

Lutz Nasdala, Professor of Mineralogy and Spectroscopy

The researchers are also investigating the relationship between light and matter. The spectroscopic analysis of a diamond formed 600 km under the Earth’s surface that was found in Brazil caused a scientific sensation in 2014: An international team of researchers, including Lutz Nasdala, was able to prove that there is water in the Earth’s interior, disproving an over thirty-year-old hypothesis. An inclusion of just 1/30 mm in the diamond contained the rare high-pressure mineral ringwoodite. The Department’s Raman spectrometer helped provide the first direct evidence of the terrestrial occurrence of this mineral phase. The water in the crystal lattice of the ringwoodite was found by colleagues from Bavaria. The results were published in the scientific journal Nature.

Gems can also be used in research: Zircons are used as reference material for geochronology, i.e. for determining the age of rocks. “Zircon is extremely resistant and, at the same time, it occurs in small quantities in nearly all rocks,” Lutz Nasdala explains. Due to its special properties, zircon can be used for the uranium-lead dating method, which analyses the isotope ratio of the two materials. Currently, the researchers are analysing two high-quality zircons in cooperation with globally leading laboratories with the goal of providing new reference material for dating.

Modern mineralogy

For their analyses, the mineralogists and crystallographers use state-of-the-art equipment: Ion beam probes, highly sensitive detectors and strong X-ray sources are just three examples. Mineralogy and crystallography not only make use of modern technology – they are themselves part of the technological progress.