Yuanchao Ji

Strain glass in ferroelastic systems: Premartensitic tweed versus strain glass

Cluster-spin glass and ferroelectric relaxors have been observed in defect-containing ferromagnetic systems and ferroelectric systems, respectively. However, it is unclear whether or not an analogous glass state exists in the physically parallel ferroelastic (or martensitic) systems. In the 1990s, theoretical studies suggested that premartensitic tweed could be viewed as a strain glass. However, there has been no experimental verification of this hypothesis. In this paper, we provide an experimental test of this hypothesis by measuring the possible glass signatures in two well-known premartensitic tweed systems prior to their martensitic transformation: one Ni63Al37 and the other Ti50Ni47Fe3 martensitic alloy. Our experiments show that no glass signatures exist for the premartensitic tweed in both systems. There is no mechanical susceptibility/modulus anomaly in the tweed temperature regime, suggesting no glass transition exists. The tweed remains ergodic, inconsistent with a frozen glass. These two critical experiments show that premartensitic tweed is not a frozen glass state. We demonstrate that strain glass exists in ferroelastic/martensitic systems but only in defect-containing ferroelastic/martensitic systems with defect concentration exceeding a critical value. This strain glass is a mechanical analogue of cluster-spin glass or ferroelectric relaxors, and possesses all the features of a glass. We further show that the tweed is equivalent to an ‘unfrozen state’ of a strain glass. Finally, we demonstrate that the microscopic origin of the strain glass can be easily understood in analogy with the behavior of a ‘defect-containing domino array’.

Evolution of the tetragonal to rhombohedral transition in (1 − x)(Bi1/2Na1/2)TiO3 − xBaTiO3 (x 7%)

Abstract (1 − x)(Bi1/2Na1/2)TiO3 − xBaTiO3 has been the most studied Pb-free piezoelectric material in the last decade; however, puzzles still remain about its phase transitions, especially around the important morphotropic phase boundary (MPB). By introducing the strain glass transition concept from the ferroelastic field, it was found that the phase transition from tetragonal (T, P4bm) to rhombohedral (R, R3c) was affected by a strain glass transition at higher temperature for x ≥ 4%. In these compositions, the T–R transition was delayed or even totally suppressed and displayed huge thermal hysteresis upon cooling and heating. Also, isothermal phase transitions were predicted and realized successfully in the crossover region, where the interaction between the T–R transition and the strain glass transition was strong. Our results revealed the strain glass nature in compositions around the MPB in this important material, and also provide new clues for understanding the transition complexity in other (Bi1/2Na1/2)TiO3-based Pb-free piezoelectric materials.

Time-dependent ferroelectric transition in Pb(1−x)(Zr0.4Ti0.6)(1−x/4)O3 − xLa system

Ferroelectric transition involves tiny shift of ions within unit cell, thus being intrinsically a very fast process without apparent time-dependence. Contrary to this conventional wisdom, here we report a time-dependent ferroelectric transition, which occurs in hours. The system studied was Pb(1−x)(Zr0.4Ti0.6)(1−x/4)O3 − xLa system with relaxor-forming dopant La3+. The time-dependent ferroelectric transition occurs at the ferroelectric/relaxor crossover composition range of 0.09 < x ≤ 0.16. In these compositions, in situ Raman spectroscopy and transmission electron microscopy reveal very slow growth of ferroelectric phase. Dielectric measurement shows isothermal kinetics of the transition. The slow ferroelectric transition can be understood as being caused by the slowing-down of the otherwise fast growth of polar nano-domains due to the random local field caused by La3+, so that long time is needed to achieve long-range order macroscopic ferroelectric phase.

Evolution from successive phase transitions to "morphotropic phase boundary" in BaTiO3-based ferroelectrics

Obtaining superior physical properties for ferroic materials by manipulating the phase transitions is a key concern in solid state physics. Here, we investigated the dielectric permittivity, piezoelectric coefficient d33, storage modulus, and crystal symmetry of (1-x)Ba(Ti0.8Zr0.2)O3-x(Ba1−yCay)TiO3 (BZT-xBCyT) systems to demonstrate the gradual evolution process from successive phase transitions in BaTiO3 to the morphotropic phase boundary (MPB) regime in BZT-xBC0.3T. Furthermore, we analysed with a Landau-type theoretical model to show that the high field-sensitive response (dielectric permittivity) originates from a small polarization anisotropy and low energy barrier at the quadruple point. Together, the intermediate orthorhombic phase regime and the tetragonal-orthorhombic and orthorhombic-rhombohedral phase boundaries constitute the MPB. Our work not only reconciles the arguments regarding whether the structural state around the MPB corresponds to a single-phase regime or a multiple-phase-coexistence regime but also suggests an effective method to design high-performance functional ferroic materials by tailoring the successive phase transitions.Obtaining superior physical properties for ferroic materials by manipulating the phase transitions is a key concern in solid state physics. Here, we investigated the dielectric permittivity, piezoelectric coefficient d33, storage modulus, and crystal symmetry of (1-x)Ba(Ti0.8Zr0.2)O3-x(Ba1−yCay)TiO3 (BZT-xBCyT) systems to demonstrate the gradual evolution process from successive phase transitions in BaTiO3 to the morphotropic phase boundary (MPB) regime in BZT-xBC0.3T. Furthermore, we analysed with a Landau-type theoretical model to show that the high field-sensitive response (dielectric permittivity) originates from a small polarization anisotropy and low energy barrier at the quadruple point. Together, the intermediate orthorhombic phase regime and the tetragonal-orthorhombic and orthorhombic-rhombohedral phase boundaries constitute the MPB. Our work not only reconciles the arguments regarding whether the structural state around the MPB corresponds to a single-phase regime or a multiple-phase-coexistenc...