There is an increased interest in probing the properties of condensed matter at extreme conditions of pressure and temperature. Nowadays, it is possible to perform experiments on materials subjected to an extended range of pressures, exceeding the conditions found at the center of Earth (363 GPa) and reaching the ones of other planets (around 1 TPa). High-pressure research has always benefited from the use of synchrotron light. The size of the samples used in diamond anvil cells coped well with the micro-focused high-intensity X-ray beams in order to obtain high-quality diffraction patterns, especially at pressure above 100 GPa.
The high-pressure behavior of III-A metals (e.g. Al, In, Ga, and Tl) has become a challenging problem because of unusual properties driven by compression is this simple metals. Thallium (Tl, Z=81) is also particularly interesting for technological applications since it forms part of products used in the electronics industry, glass lenses, semi-conductors, dyes and pigments. Due to its high toxicity and its tendency to oxidize and chemical reactivity, thallium has been hardly studied in the last decades.
Recently, a combined team used micro-focused X-ray diffraction at Materials Science and Powder Diffraction beamline (MSPD) of the ALBA Synchrotron, together with ab initio calculations, to probe the thallium behavior under high-pressure and high-temperature conditions. "Studying materials at extreme pressure and temperature give us insights into the deep interiors of large planets or possible new uses in advanced chemistry", says Catalin Popescu, one of the authors of the study and scientist at the MSPD beamline.
At the MSPD beamline, researchers conducted X-ray diffraction (XRD) experiments at different pressure and temperature conditions, ranging from nearly 0 GPa (ambient pressure) to 125 GPa (more than one million times the atmospheric pressure found on Earth's surface). These extreme pressure conditions, that nearly doubled that reached in previous experiments on thallium, were obtained using diamond-anvil cells (DAC), in which materials are squeezed between two diamond anvils, while X-rays are passing through them. "Identifying and understanding the behavior of materials under extreme conditions is key for developing materials with new or superior characteristics", says Catalin Popescu. It has to be stressed out that this is the first structural study by powder X-ray diffraction at high temperature at the MSPD beamline. The recent available device consists in a custom-designed vacuum vessel where the DACs were contained and heated with external resistance heaters. The temperature is measured by using a K-type thermocouple attached to one of the diamond, close to the gasket.
Researchers confirmed a first transition of thallium at 3.5 GPa and the structure stability of the metal at higher pressures up to 125 GPa. They also tested the existence of high-pressure and high-temperature phases and obtained accurate information on different physical properties of thallium. Theoretical calculations were consistent with the experimental results and predicted that novel physics should be discovered at Tera pascal (1000 GPa) pressures. With this experiment, new information on thallium has been published, unveiling new details of this material.
Figure: Selected X-ray diffraction (XRD) patterns of thallium at different pressures measured at room temperature (left), the XRD patterns measured at high pressure and high temperature corresponding to the three phases experimentally found (center top), phase diagram of thallium (top right), high temperature experimental device using a resistively-heated high-pressure membrane diamond anvil cell available at MSPD beamline (bottom right).
