Abstract :
[en] In this thesis, we study the Negative Thermal Expansion (NTE) in tellurium based liquid alloys (GeTe6, GeTe12 and As2Te3), through their structural and vibrational properties. These alloys exhibit a volume decrease of 4% in a 200 K range above the meltingtemperature (partially in the undercooled liquid for GeTe12). The density anomaly is accompagnied by other unusual behaviors, such a maximum in the specific heat, a minimumin the sound velocity and a steep decrease of the isothermal compressibility [1, 2].Moreover, in these covalent liquid alloys, the NTE often takes place together with a semiconductorto metal (SC-M) transition [3, 4]. Several experiments [5, 6, 7] or calculations[8, 9, 10] were performed to try to understand the structural changes giving rise to a NTEin some liquid chalcogenides. Despite these numerous studies, no driving mechanism couldbe clearly identified up to now to explain the observed trends.We evidence the structural evolution by measuring neutron diffraction spectra at severaltemperatures in the NTE range and perform First Principles Molecular Dynamics(FPMD) simulations at the same temperatures and densities to study the local order evolutionin the liquid. The obtained structures show an increase of the coordination numbersand a symmetrization of the first neighbors shell around atoms when the temperature rises.To confirm these results, we performed inelastic neutron scattering (INS) to obtain de vibrationaldensity of state (VDOS) along the NTE. We see a clear change of the VDOS,consisting in a red-shift of the highest frequencies with temperature. These experimentalresults, in addition to the FPMD simulations of the liquids, lead us to propose a model forthe density anomaly observed in some of these tellurium based systems : it correspondsto a structural change between a ‘low temperature’ liquid, characterized by a low densitystructure with an octahedral local order distorted locally by a Peierls-like mechanism, anda ‘high temperature’ liquid in which the vibrational entropy gain favors a more symmetric(less distorted), still octahedral, local order. Finally, electrical conductivity evolution isobtained from the As2Te3 simulated structures, to compare with the semi-conductor tometal transition measured in the experiments. We extended our study of the dynamicsof these Te-based compounds to the materials, called ‘phase-change’, Ge2Sb2Te5 andGe1Sb2Te4.The phase-change materials (PCMs), to the contrary of the compounds cited above,exhibit a normal volume expansion in the liquid phase. They possess however an unusualcombination of properties, which permits their use as memories in commercial applications,such as Blue-Ray disks. The PCMs show, besides the capacity to switch rapidly (in sometens of nanoseconds) from the crystal to the amorphous (and reverse), a large contrastbetween optical or electrical properties of both phases [11]. It is quite rare for a compoundto gather these physical properties, and most of the earliest researchs on PCMs were donewith the aim to find the most suitable material for applications. Nowadays, the alloysused in memories are generally based on Te, such as the pseudo-binary (GeTe)xSb2Te3 orthe Ag, In-doped Sb2Te [12, 13]. The development of PCMs for non volatile data storage(the future PC-RAM) needs a better theoretical understanding of their structure, stabilityand origin of the large property contrast between crystalline and amorphous phases.We present a theoretical study on the structural evolution with temperature of theSb2Te and Sb2Te3 binary alloys, from the liquid to the amorphous phase. These compoundsare considered as the basic components of most PCMs. We performed FPMDsimulations to obtain Sb2Te and Sb2Te3 structures above and below the melting temperature.We describe the local order around atoms in the liquid and amorphous structures,and compare it to the crystal. Our results could help to understand the ability for thosetwo coumpounds to change from the amorphous to the crystal so easily, because of thestrong local order observed in the quench. On the basis of FPMD simulations, carefullyassessed by comparison with experimental data, we will also study the atomic local orderof the Ge2Sb2Te5 and Ge1Sb2Te4 phase-change alloys, and analyze a model for their amorphousstructure. We show that the amorphous structure undergoes a Peierls-like distortionof the local atomic environment that results in a gap opening. Finally, by analyzing allour FPMD simulations, we develop a new method for enumerating mechanical constraintsin the amorphous phase and show that the phase diagram of the GeSbT e system can besplit into two compositional regions having a well-defined mechanical character: a Te-richflexible phase, and a stressed rigid phase that encompasses the known PCMs. This atomicscale insight should open new avenues for the understanding of PCMs and other complexamorphous materials from the viewpoint of rigidity.
