Geoffroy DE LAITRE on the dynamical properties of aperiodic crystals

Jury

Alain Pautrat (CNRS Paris-Normandie), rapporteur
Philipe Rabiller (Université de Rennes), rapporteur
Pascale Launois (CNRS Paris Saclay), Examinatrice
Emilio Lorenzo (CNRS Alpes), Examinateur
 

Abstract

Aperiodic crystals are characterised by there long-range ordered that lack periodicity in at least one direction. Although their structure are in general well understood, especially with the superspace description, their dynamics are more debated. Indeed, the translational symmetry breaking occurring in these systems leads to the prediction of a new type of excitation called a phason mode and which propetrties and atomic realisations are still to be established in many cases. In particular, few experimental observation of phasons were obtained for incommensurably modulated crystals and incommensurate composites. The Rb2ZnCl4 phase displays a crystal structure consisting of an arrangement of Rb atoms and ZnCl4 tetrahedrons. The orientations of the later are incommensurately modulated between Ti=303.5 and TC=195K, breaking the periodicity of the mean lattice in the c direction. Its diffraction diagram is characterised by satellite reflections specific to the aperiodic order. We undertook to evidence the phason mode in this system through inelastic neutron scattering with the IN6-SHARP, IN5, IN12, IN22 and ThALES instruments of the ILL, and with the 1T spectrometer of the LLB. Working temperatures ranged from 350K to 140K, in order to reach the periodic “high symmetry” phase above  Ti, as well as the “lock-in” phase below TC where periodicity is restored. We show that the acoustic phonon modes measured near main Bragg reflections are standard, remaining insensitive to the phase transitions. In the incommensurate phase, we present the analysis of a quasi-elastic signal evidenced around satellite reflections. We show that this signal differs significantly from  phnon modes and that it disappears in the lock-in phase. Our observation leads us to identify it as a diffusive phason mode although its behaviour differs from predictions in the hydrodynamic limit. We propose an interpretation of the data that makes our observations compatible with theoretical models.