BICTEL/e - ULg
Serveur institutionnel des thèses de doctorat

• thermal evolution
• ab initio calculations
• Peierls
• liquids
• amorphous
• neutron scattering

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 melting

temperature (partially in the undercooled liquid for GeTe12). The density anomaly is accompagnied by other unusual behaviors, such a maximum in the specific heat, a minimum

in 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 semiconductor

to 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 NTE

in some liquid chalcogenides. Despite these numerous studies, no driving mechanism could

be clearly identified up to now to explain the observed trends.

We evidence the structural evolution by measuring neutron diffraction spectra at several

temperatures in the NTE range and perform First Principles Molecular Dynamics

(FPMD) simulations at the same temperatures and densities to study the local order evolution

in the liquid. The obtained structures show an increase of the coordination numbers

and 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 vibrational

density 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 experimental

results, in addition to the FPMD simulations of the liquids, lead us to propose a model for

the density anomaly observed in some of these tellurium based systems : it corresponds

to a structural change between a ‘low temperature’ liquid, characterized by a low density

structure with an octahedral local order distorted locally by a Peierls-like mechanism, and

a ‘high temperature’ liquid in which the vibrational entropy gain favors a more symmetric

(less distorted), still octahedral, local order. Finally, electrical conductivity evolution is

obtained from the As2Te3 simulated structures, to compare with the semi-conductor to

metal transition measured in the experiments. We extended our study of the dynamics

of these Te-based compounds to the materials, called ‘phase-change’, Ge2Sb2Te5 and

Ge1Sb2Te4.

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 unusual

combination 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 contrast

between optical or electrical properties of both phases [11]. It is quite rare for a compound

to gather these physical properties, and most of the earliest researchs on PCMs were done

with the aim to find the most suitable material for applications. Nowadays, the alloys

used in memories are generally based on Te, such as the pseudo-binary (GeTe)xSb2Te3 or

the 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, stability

and origin of the large property contrast between crystalline and amorphous phases.

We present a theoretical study on the structural evolution with temperature of the

Sb2Te and Sb2Te3 binary alloys, from the liquid to the amorphous phase. These compounds

are considered as the basic components of most PCMs. We performed FPMD

simulations 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 those

two coumpounds to change from the amorphous to the crystal so easily, because of the

strong local order observed in the quench. On the basis of FPMD simulations, carefully

assessed by comparison with experimental data, we will also study the atomic local order

of the Ge2Sb2Te5 and Ge1Sb2Te4 phase-change alloys, and analyze a model for their amorphous

structure. We show that the amorphous structure undergoes a Peierls-like distortion

of the local atomic environment that results in a gap opening. Finally, by analyzing all

our FPMD simulations, we develop a new method for enumerating mechanical constraints

in the amorphous phase and show that the phase diagram of the GeSbT e system can be

split into two compositional regions having a well-defined mechanical character: a Te-rich

flexible phase, and a stressed rigid phase that encompasses the known PCMs. This atomic

scale insight should open new avenues for the understanding of PCMs and other complex

amorphous materials from the viewpoint of rigidity.

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