Acoustic Detection of Phase Transitions at the Nanoscale

Abstract

Materials near structural phase transitions find applications in a wide range of devices. Typically, phase transitions are determined macroscopically through measurements of relevant order parameters and related property coefficients. Here, a method for understanding electric field induced phase transitions in ferroelectrically active materials at the nanometer scale via acoustic detection with band‐excitation piezoresponse force microscopy (BE‐PFM) is introduced. Specifically, the field‐induced rhombohedral (R) to tetragonal (T) phase transition in single crystal 0.72PbMg1/3Nb2/3O3‐0.28PbTiO3 (PMN‐PT) is mapped. It is shown that due to sample heterogeneity, some regions are more prone to the R–T transition, and display signatures in the acquired piezoresponse loops, as well as pronounced softening in the elastic modulus (monitored via the resonant frequency and calibrated with models of cantilever dynamics) that occurs just prior to phase switching. Landau–Devonshire thermodynamic theory confirms the stability of the tetragonal phase under applied fields in PMN‐PT, while phase‐field modeling suggests that the transition evolves smoothly in the probed volume of the tip, both in agreement with the BE‐PFM results. These results confirm the validity and utility of utilizing acoustic changes at phase transitions to detect their onset in nanoscale probed volumes, allowing spatial mapping of their onset with unprecedented resolution.

Acoustic Emission from Organic Martensites

“…Common features,s uch as rapid progression of the phase front (the habit plane), [1] pronouncedly anisotropic thermal expansion preceding the transition, [1,8] preservation of crystal symmetry, [1] and conformational and packing similarity between the two phases, [1,3,7,8] hint at an analogy between the transitions in these organic-containing molecular materials and the martensitic transitions-diffusionless,d isplacive first-order phase transformations observed with steel and some shape-memory alloys.H owever,b ecause of extremely fast rates and frequent disintegration, direct experimental evidence of this hypothesis has not been provided to date. Thea ssociated strong waves carry important information on the dynamics and mechanism of transformation; [43] not only do the results provide deeper understanding of the mechanistic and kinetic profile of the thermosalient phenomenon, but in addition to the previous conclusions,they confirm that the thermosalient materials are molecular analogues of the inorganic martensites and that the molecular solids can behave in am anner similar to metals and alloys. [38][39][40][41][42] However,unlike the continuous acoustic emission that occurs during plastic deformation with slow progression of dislocations,the response from the martensitic transitions is discontinuous.T he intensity of the acoustic bursts is several orders of magnitude higher than that of the continuous emission, and is akin to the acoustic emission generated by an avalanche or the seismic waves that precede an earthquake.H erein, we provide direct evidence that, in addition to only one brief previously reported case, [5] the phase transitions in thermosalient crystals are generally associated with outbursts of elastic energy that translate into acoustic waves.T he energy,i ntensity,a nd time scale of the thermoacoustic response is scrutinized with the mechanism of the thermosalient transition.…”

Ferroelectric or non-ferroelectric: Why so many materials exhibit “ferroelectricity” on the nanoscale

“…Clearly, the slopes of the response differ due to the different contact stiffness values of the cantilevers. After using a correction factor accounting for individual cantilever geometry (so-called shape factor λ), it becomes possible to compute the displacement directly, as in ( 358 Similar experiments on a BiFeO3 thin films are shown in (d). Here, the piezoresponse is plotted as a function of negative bias applied to the PFM tip, along with the resonant frequency.…”

“…Combined with relatively large (10-100 pm/V) piezoelectric constants, this made PFM the leading technique for almost two decades and enabled the rapid development of this field. An important component of this progress was the emergence of effective phase-field models that allowed simulation of ferroelectric domain dynamics as a function of global (temperature, pressure) [171][172][173]394 and local (tip bias, tip pressure) 164,172,358,359,395 parameters and hence provide insight into local switching mechanisms. 306,396,397 Recent emergence of phase field models for combined ferroelectric-semiconductor and ferroelectric-surface electrochemical behaviors offers numerous future opportunities.…”

“…Moreover, these effects are substantial in the case of field-induced phase transitions, when the elastic compliance diverges as the structural phase transition is approached, and have been observed experimentally. [357][358][359][360] For example, Li et al 359 showed that for strained BiFeO3 thin films, a transition from the rhombohedral to tetragonal phase could be induced at high fields, which could further be correlated to increased dissipation (≈1/Q), presumably from the motion of the interfaces in the probed volume of the tip. These results are outlined in Figure 16.…”

Machine learning–enabled identification of material phase transitions based on experimental data: Exploring collective dynamics in ferroelectric relaxors

“…We have previously argued for the possibility of the field-induced phase transition in these systems, which is supported by analytical theory, phase-field modeling, and acoustic measurements ( 34 ). Here, we investigate the relaxation response as a function of temperature and fully delineate the temperature-bias phase diagram.…”