PHYSICAL REVIEW B 93, 104103 (2016) Anomalous reduction in domain wall displacement at the morphotropic phase boundary of the piezoelectric alloy system PbTiO 3 -BiScO 3 Dipak Kumar Khatua, 1 Lalitha K. V., 1 Chris M. Fancher, 2 Jacob L. Jones, 2 and Rajeev Ranjan 1 , * 1 Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India 2 Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695, USA (Received 5 June 2015; revised manuscript received 4 February 2016; published 2 March 2016) A comparative study of field-induced domain switching and lattice strain was carried out by in situ electric-field-dependent high-energy synchrotron x-ray diffraction on a morphotropic phase boundary (MPB) and a near-MPB rhombohedral/pseudomonoclinic composition of a high-performance piezoelectric alloy (1 − x )PbTiO 3 -(x )BiScO 3 . It is demonstrated that the MPB composition showing large d 33 ∼ 425 pC/N exhibits significantly reduced propensity of field-induced domain switching as compared to the non-MPB rhombohedral composition (d 33 ∼ 260 pC/N ). These experimental observations contradict the basic premise of the martensitic-theory-based explanation which emphasizes on enhanced domain wall motion as the primary factor for the anomalous piezoelectric response in MPB piezoelectrics. Our results favor field-induced structural transformation to be the primary mechanism contributing to the large piezoresponse of the critical MPB composition of this system. DOI: 10.1103/PhysRevB.93.104103 I. INTRODUCTION Morphotropic phase boundary (MPB) ferroelectrics are widely used as actuators, sensors, and transducers by virtue of their exceptionally large piezoelectric response. A first intuitive explanation for the large electromechanical response was attributed to the availability of a large number of domain variants, which was supposed to enable efficient poling of the specimen [1]. This idea received theoretical support from a Devonshire-Ginzburg-Landau-based multiscale calculation which predicted enhanced domain switching in MPB systems [2]. The same scenario is considered to be applicable in single phase low-symmetry ferroelectrics [2]. In contrast, first-principles [3,4] and phenomenological free-energy cal- culations [5–7] have shown a correlation between anisotropic flattening of a free-energy profile and polarization rotation, as- sisted by low-symmetry phases as the fundamental mechanism for an enhanced piezoelectric response in ferroelectrics [3–10]. Martensitic-based theory on the other hand, attributes the large piezoelectric response of the MPB systems to enhanced density and mobility of domain walls [11–16]. In situ electric-field diffraction experiments enable direct estimation of domain switching and lattice strain in ferroelectrics [17–23]. Ghosh et al. have shown that the relatively enhanced dielectric and piezoelectric responses of polycrystalline BaTiO 3 in the grain size range of ∼1 to 2 μm is associated with an enhanced domain wall displacement [24]. The authors concluded this by comparing the domain switching fraction in BaTiO 3 ceramics of different grain sizes and found it to be largest in the size range of 1 to 2 μm. Apart from studying domain switching, an in situ field-dependent diffraction experiment can also ascertain the occurrence of field-induced interferroelectric transformation, if any. However, the combined effect of preferred orientation and severe overlapping of Bragg peaks corresponding to the different phases makes structural analysis very challenging. Using a special diffraction geometry to * rajeev@materials.iisc.ernet.in avoid a field-induced preferred orientation effect, Hinterstein et al. have shown evidence of field-induced tetragonal to rhombohedral/monoclinic transformation in a soft lead zir- conate titanate [Pb(Ti,Zr)O 3 (PZT)] [25]. A similar result was obtained by Kalyani et al. by an ex situ technique [26]. More recently, Hinterstein et al. have suggested that field-induced phase transformation is the dominant factor in determining the overall piezoelectric strain of a soft PZT [27]. Field- induced phase transformations have also been reported in other ferroelectric systems exhibiting a high piezoelectric response, such as BiScO 3 -PbTiO 3 [28], BaTiO 3 -based systems [29–31], Na 1/2 Bi 1/2 TiO 3 -based systems [32–34], and (K,Na)NbO 3 - based systems [35]. Because of the complications associated with overlapping of Bragg profiles, domain switching studies have mostly been reported for compositions away from the critical MPB, i.e., exhibiting single phase. So far it has not been established how the two phenomena (domain switching and phase transformation) influence each other in the core MPB compositions of piezoelectric alloys. The understanding of this issue is of great fundamental significance since it has direct bearing on our current understanding of the fundamental mechanisms at work in MPB compositions exhibiting a very high piezoelectric response. In the present paper, we have addressed this issue in detail by studying important com- positions spanning the BiScO 3 -PbTiO 3 morphotropic phase boundary. Similar to PZT, this alloy system is known for its high piezoelectric performance at the MPB [36]. Our results show that the propensity of domain switching is significantly reduced in the MPB composition as compared to the neighboring non-MPB composition. II. EXPERIMENT The details related to the specimen synthesis can be found in Ref. [28]. An in situ electric-field high-energy x-ray diffraction (XRD) experiment was carried out at the Advanced Photon Source at Argonne National Laboratory in transmission geometry (beamline 11-ID-C), which ensured 2469-9950/2016/93(10)/104103(6) 104103-1 ©2016 American Physical Society