high pressure barium structure

What is it about?

During the last decade, revolutionary developments in single crystal, high-pressure X-ray diffraction experiments are revealing completely new aspects in the phase diagrams of elements. These developments have shown that a surprising degree of complexity exists in the structures of some elements and in their physical behaviour. Two main areas of physics, chemistry and material sciences, are directly concerned: they provide a rich testing ground for fundamental physics and lead to the appearance of new structural types which exist at pressures around 100 GPa. Many recently reported crystal structures of metals at high pressures are not only very complex, but, in addition, they often loose their 3-dimensional periodicity. This means that any structural description using the classical approach can result in crude and inappropriate approximations. The real challenge is to recover the correct symmetrical properties hidden behind the complex appearances of the diffraction patterns, and to reveal their detailed and true atomic structures. A very instructive and clear example of the problem is barium, which is currently the focus of intense studies and debates. The 3D structure approximant of one of the high pressure modifications of Ba-IV (stable between 12 and 90 GPa) was recently published using a model which required a total of 99 independent Ba atoms. The proposed approximant was selected by the authors as the most probable among 64 possible models. Such surprising results lead (or mislead) us into a regime of strange and improbable physics. In principle, the independence of 99 Ba atoms implies the existence of an equal number of different electronic states, which simply looks unrealistic. Resolving the precise structure of this phase of barium is a therefore a key point in developing some new ideas in solid state physics. Based on a new series of high-resolution, synchrotron single crystal diffraction data collected at high pressure, we could unravel the complexity of one particular phase in barium in the pressure range between 16 and 20 GPa. For the first time, we could reveal the true symmetrical properties of the structure, apply the recent developments in higher dimensional space crystallography, and obtain a description of the structure compatible with the highest standards of precision. We demonstrate that the structure may be completely described using only 3 independent atoms (contrary to 99!). In addition, the high structural resolution reveals an unexpected phenomenon; namely the existence of incommensurate density waves of Ba atoms located in channels formed by a host barium matrix. These density waves are very sensitive to pressure changes, which in turn provide mechanism for understanding the evolution of the modulation. Our model has the property that it can be applied to predict the structure of other Ba phases stable at various pressures. Moreover, our methodological approach and detailed model will contribute to a better understanding of the physical mechanisms governing the phase transitions of many different elements under high pressure and provide a solid basis for further theoretical developments.

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Why is it important?

For the first time we present a very accurate structure model of the high pressure phase of the incommensurately modulated structure of barium IVb. Based on our results, it is thus possible to propose a theoretical model to understand the formation of atomic density waves not only in HP barium IV but also in many other elements under high pressure.

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