A first-principles study reveals effects of cation ordered nanoregions in a lead-free relaxor ferroelectric, K_1/2Bi_1/2TiO₃

Lead-based oxides such as lead zirconate titanate (PZT) feature in a wide variety of technological applications, such as high-performance sensors, actuators and transducers. However, they also pose serious health and environmental risks due to the hazardous nature of lead.

Some oxides containing bismuth are emerging more recently as lead-free substitutes to lead-based oxides with comparable piezoelectric and dielectric properties, but with much less concern about their environmental impact. Results published in the Journal of Applied Physics present a first-principles study of cation ordering and its influence on Raman intensity and infrared reflectivity spectra for one of these promising piezoceramics, K_1/2Bi_1/2TiO₃ (KBT).

The octahedral A-site of KBT is shared by two cations, potassium and bismuth, while the B-site cation is always a titanium atom. The arrangement of the two A-site cations could be completely random, partially random or completely ordered. Since cation ordering can critically influence a material’s physical properties and behavior, the authors sought to answer this important question.

They used density functional theory to investigate the lattice dynamics, dielectric and ferroelectric properties, infrared reflectivity spectra, and Raman intensity spectra of KBT. The results revealed that some degree of A-site cation ordering in KBT does exist, and this ordering significantly influences its Raman spectrum. The comparison of theoretical and experimental Raman spectra suggests that nanoscale regions with different A-site orderings are formed in KBT samples. The overall arrangement of these ordered nanoregions may be fully or partially random.

The authors plan to perform experiments with neutron diffraction and high-resolution transmission electron microscopy to provide further complementary evidence of their findings. The hope is that understanding the fundamental mechanisms of KBT will open doors to tailoring its material properties for various technological applications.

Source: “Randomly arranged cation-ordered nanoregions in lead-free relaxor ferroelectric K_1/2Bi_1/2TiO₃: Prediction from a first-principles study,” by Manish K. Niranjan, P. Karuna Kumari, Krishnarjun Banerjee, and Saket Asthana, Journal of Applied Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5021410 .