Tuning the phonon contribution to control thermal expansion

Thermal expansion is a ubiquitous phenomenon that represents an issue in engineering design contexts. Even if its control is of paramount importance and has been pursued for a long time, it is still an open problem with only particular solutions, mainly involving composite materials or components with a shape designed to counteract the thermal expansion. The existence of materials with anomalous thermal expansion properties has been acknowledged long ago; recently it was found that there are compounds with values of the coefficient of thermal expansion with absolute value at least large as those of common metals, but negative in sign. This sparked great scientific interest, leading to the discovery that the coefficient of thermal expansion can radically change when the atoms occupying specific crystal sites are substituted, even going from negative to positive. An outstanding application of this anomalous behavior could be that of tuning the thermal expansion. This would be achieved by preparing single phase materials with a coefficient of thermal expansion tailored to a particular engineering setting. However, it was also found that the intercalation of small chemical species into certain framework structured compounds turns their thermal expansion from negative to positive or almost zero. Further, in materials displaying order to disorder phase transitions, a close relationship is observed between the phase transition and anomalous thermal expansion. Other aspects of materials design (e.g. nanostructuration, valence state change) have been found to be relevant in this respect. It is expected that a variety of microscopic mechanisms are necessary to describe them. In this work we will focus on the vibrational contribution to thermal expansion: EXAFS spectroscopy offers extraordinary insight on the local dynamics of atomic pairs. This is due to the EXAFS sensitivity to the atomic species and to the correlation of the motions of atoms: the former allows to independently study the local neighborhoods of different atoms, while the latter, combined with diffraction measurements, can give information on the anisotropy of thermal vibrations. We have obtained information on the local dynamics of some analogues of the Prussian blue with brute formula MM’(CN)6. Intercalating these compounds with small chemical species can turn thermal expansion from negative to positive. EXAFS spectroscopy has been employed to ascertain a suppression of the transverse vibrations has been observed upon intercalation, with a predominant role of the vibrations of M-N atomic pairs. We have also studied a series of zirconium alloys of brute formula Zr2M (M=Fe, Co, Ni) with body tetragonal crystal structure. The relation between the thermal expansion coefficient along the c-axis, with values ranging from largely negative to positive and the anisotropy of thermal expansion of ZrM atomic pairs; a negative correlation between the anisotropy and both the average atomic volume and the c-axis lattice parameter is found. EXAFS studies of analogues of copper pyrophosphate Cu2P2O7, inexpensive and facile to synthesize, are a challenging subject. Substituting copper with zinc and/or phosphorus with vanadium has striking effects on the crystal structure, on the thermal expansion and on the local dynamics of atomic pairs. Our analysis corroborates the microscopic mechanism of hindering of vibrations transverse to bonds in correspondence to suppression of negative thermal expansion. Classical molecular dynamics simulations of spherical gold nanoparticles have been performed. Molecular dynamics can study the vibrational dynamics in solids and is thus complementary to EXAFS analysis. The spherical gold nanoparticles of different diameters that have been investigated are simple yet non-trivial systems, a case study of the effect of nanostructuration on the phonon contribution to thermal expansion.