Athermal Character of Structural Phase Transitions

Abstract

The significance of thermal fluctuations in nucleation in structural first-order phase transitions has been examined. The prototypical case of martensitic transitions has been experimentally investigated by means of acoustic emission techniques. We propose a model based on the mean first-passage time to account for the experimental observations. Our study provides a unified framework to establish the conditions for isothermal and athermal transitions to be observed. DOI: 10.1103/PhysRevLett.87.195701 PACS numbers: 64.60.My, 64.60.Qb, 81.30.Kf Fluctuations are considered as essential for a phase transition to take place. However, at moderate temperatures many solids undergo first-order phase transitions which are regarded as being athermal. The kinetics of these transitions is not dominated by thermal fluctuations and, consequently, they can take place only under the change of an external parameter (stress, magnetic field, temperature, etc.) which modifies the free energy difference between high and low symmetry phases. This behavior is in contrast to thermally activated transitions for which the relaxation from a metastable state can occur at constant external conditions due to thermal fluctuations. Examples of athermal transitions are found within the families of magnetic and structural phase transitions which involve long-range interactions (dipolar, elastic, etc.). The martensitic transition (MT) undergone by many solids is an interesting case that will be the prototype system experimentally investigated in the present Letter. It is a diffusionless first-order transition [1] mainly caused by a shear mechanism [2]. Often MT is preceded by premonitory effects such as softening of long and short wavelength acoustic vibrational modes. The anharmonic coupling of these modes is at the origin of the instability of the high temperature phase [3].In general, when a system is externally driven through a first-order phase transition, the response to the external control parameter is determined by the characteristics of the energy barriers separating the two phases. When thermal fluctuations are not relevant (athermal transition), the system remains in a given configuration as long as the state corresponds to a free energy local minimum. When driven, the system jumps towards a different configuration, once the local stability limit is reached. The path followed by the system is influenced by the existence of disorder (dislocations, grain boundaries, vacancies, local composition, atomic configuration, etc.), which controls the actual distribution of energy barriers. As the system evolves, it passes through a sequence of metastable states [4]. The kinetics is characterized by jumps (avalanches) from one metastable state to another which occur with a certain relaxation time. In a number of cases this time is power law distributed [5]. The jerky response for MT is related to sudden changes in the local strain field which are at the origin of the emission of acoustic waves in the range from kHz to MHz [6]. This effect is ...

Acoustic emission across the magnetostructural transition of the giant magnetocaloricGd5Si2Ge2

“…The effect of thermal fluctuations ͑athermal degree͒ and also the effect of driving rate on the power-law exponents has been explained in terms of the different time scales involved in driving the transition. 15,33 Comparison of the amplitude distributions obtained for Ṫ = 1 K / min and Ṫ = 6 K / min shows that the power-law exponent for cooling runs increases for increasing temperature rate. This is the behavior expected for those transitions for which the characteristic time of thermal fluctuations is not very different from the characteristic time associated with the change of the external field ͑temperature in the present case͒.…”

“…The transition was thermally induced using the experimental setup described in Ref. 15. The relative oscillations of the sample temperature were less than 0.01%.…”

“…12 Prototypical solidsolid phase transitions with associated AE are martensitic transitions, for which the analysis of the AE has provided valuable information on the mechanisms and kinetics of the transition. [12][13][14][15][16] The mechanisms of the crystallographic change in Gd 5 ͑Si x Ge 1−x ͒ 4 involve shearing of planes perpendicular to the long b axis. 6,17 Since this mechanism shares some similarities with martensitic transitions, it has been termed "martensiticlike."…”

Criticality of Tuning in Athermal Phase Transitions

“…In contrast, athermal first order phase transitions are not influenced by thermal fluctuations and proceed through a set of metastable states of free energy local minima, and hence as the external field (temperature, magnetic field, stress etc.) is varied, display bursty and discrete avalanches [2]. Theoretical models such as random field ising model and the renormalization group analysis [3,4] map these non-equilibrium first order phase transitions to equilibrium critical phenomenon, although the divergence of the correlation length at the critical field has never been clearly demonstrated.…”

Avalanche criticalities and elastic and calorimetric anomalies of the transition from cubic Cu-Al-Ni to a mixture of18Rand2Hstructures

“…The existence of this coexistence region is also favored by the fact that the stresses associated with the occurrence of orthorhombic variants at the onset of the temperature-driven transition may induce the transformation towards the monoclinic phase [25]. Thermally induced transitions equally occur so that the methodology of the investigation of ferroelastic phase transitions becomes applicable [26] while the athermal nature of the phases remains approximately preserved over limited temperature intervals [27].…”