Study On The Anomalous Elastic Behaviour Of Typical Double Perovskites And Gete Associated With Their Phase Transitions

Resonant ultrasound spectroscopy（RUS） machine is a very powerful tool to study the elastic and anelastic properties of functional materials as a function of temperature across phase transitions. RUS technique can also investigate mechanisms of dissipation associated with any phase transitions（structural, magnetic, electric, dielectric etc.） of interest. In this thesis, two typical double perovskites and one alloy have been investigated using RUS and a full description of the method for carrying out RUS experiments as a function of temperature including sample preparation and data analysis, is also given.The ordered double perovskite Sr₂FeMoO₆（SFMO） possesses remarkable room-temperature low-field colossal magnetoresistivity and transport properties which are related, at least in part, to combine structural and magnetic instabilities that are responsible for a cubic – tetragonal phase transition near 420K. A formal strain analysis combined with new measurements of elastic properties from resonant ultrasound spectroscopy reveal a system with weak biquadratic coupling between two order parameters belonging to and of parent space group Fm3m. Observed softening of the shear modulus by⁵0 % is due to the classical effects of strain/order parameter coupling at an improper ferroelastic（F₄⁺） transition which is second order in character, while the ferromagnetic order parameter（mF₄⁺） couples only with volume strain. The influence of a third order parameter, for ordering of Fe and Mo on crystallographic B-sites, is to change the strength of coupling between the order parameter and the tetragonal shear strain due to the influence of changes in local strain heterogeneity at a unit cell scale. High anelastic loss below the transition point reveals the presence of mobile ferroelastic twin walls which become pinned by oxygen vacancies in a temperature interval near 340 K. The twin walls must be both ferroelastic and ferromagnetic, but due to the weak coupling between the magnetic and structural order parameters it should be possible to pull them apart with a weak magnetic field. These insights into the role of strain coupling and relaxational effects in a system with only weak coupling between three order parameters allow rationalization and prediction of how static and dynamic properties of the material might be tuned in thin film form by choice of strain contrast with a substrate.Ge Te is a material of intense topical interest due to its potential in the context of phase-change and nanowire memory devices, as a base for thermoelectric materials and as a ferroelectric. The combination of a soft optic mode and a Peierls distortion contributes large strains at the cubic – rhombohedral phase transition near 625 K and the role of these has been investigated through their influence on elastic and anelastic properties by resonant ultrasound spectroscopy. The underlying physics is revealed by softening of the elastic constants by ³0-45%, due to strong coupling of shear and volume strains with the driving order parameter and consistent with an improper ferroelastic transition which is weakly first order. The magnitude of the softening is permissive of the transition mechanism involving a significant order/disorder component. A Debye loss peak in the vicinity of 180 K is attributed to freezing of the motion of ferroelastic twin walls and the activation energy of ⁰.07 e V is attributed to control by switching of the configuration of long and short Ge-Te bonds in the first coordination sphere around Ge. Precursor softening as the transition is approached from above can be described with a Vogel-Fulcher expression with a similar activation energy, which is attributed to coupling of acoustic modes with an unseen central mode that arises from dynamical clusters with local ordering of the Peierls distortion. The strain relaxation and ferroelastic behaviour of Ge Te depend on both displacive and order/disorder effects but the dynamics of switching will be determined by changes in the configuration of distorted Ge Te6 octahedra, with a rather small activation energy barrier.Resonant Ultrasound Spectroscopy（RUS） has been used to characterize elastic and anelastic anomalies in a polycrystalline sample of multiferroic La2 Ni Mn O6（LNMO）. Based on previous investigations, the partially disordered ferromagnetic LNMO possesses both magnetocapacitance and magnetoresistance effects which are related, at least in part, to combine structural, spontaneous elastic, magnetic, electronic and dielectric anomalies that arise together near the room temperature. In this study, ferroelastic aspects of the coupling mechanisms, nature of two phase coexistence, local inhomogeneities, and first order transition P21/n- R ? have been characterized for the first time by RUS. This aspect study clearly signifies that overlapping strains from magnetic and dielectric order parameters could result in a strong magnetodielectric effect. In the stability field of the rhombohedral phase the crystals had nearly constant values of the shear modulus and high invariable values of tan δ, and the low acoustic loss can be observed in stability field of monoclinic phase after a freezing process. There is a broad typical Debye peak near the ferromagnetic transitions due to the freezing behaviour of ferroelastic twin walls induced by some magnetic domain defects which is related to the dielectric and elastic instabilities. The overlapping acoustic losses between ³00 and ⁶50 K confirm that two structure coexisting and the anti-site defects are presumably related to local inhomogeneities, not the intrinsic nature of a homogeneous system. The boundary movement between two different space groups induces extra friction, which also make a contribution to the acoustic loss.