Abstract
Defects, in particular vacancies, play a crucial role in substituted perovskite systems, influencing the structural features that underpin ferroelectricity. B-site vacancies introduce cation disorder in the perovskite lattice and are in fact one of the main driving forces for relaxor behaviour in barium titanate (BaTiO3, BT) based ferroelectrics. In this work, material systems are carefully selected to qualitatively study the change in B-site vacancy concentration for heterovalent substituted BT-based ferroelectric polycrystals. Raman spectroscopy was used to investigate those systems, and B-site vacancy specific Raman modes were identified unambiguously by comparison with charge-compensated BT, where B-site vacancies are absent. This study validates the hypothesis that vacancies induce Raman scattering because of symmetry breaking in the BT lattice, establishing this method as a vital tool to study substitutional defects in ceramic materials.
Highlights
Barium titanate (BaTiO3, both Fig. 1 (BT)), a prototypical room-temperature ferroelectric (FE) material with ABO3 perovskite form, undergoes a sequence of structural phase transitions before entering into the paraelectric phase above the Curie temperature (125 °C) [1]
In hard mode Raman spectroscopy, the phase transitions are usually studied by the change in vibrational energy of the crystal lattice that are evident in the observable phonons [18,24]
For tetragonal BT, Raman selection rules predict 13 first-order Raman modes from 100 cm−1 to 900 cm−1 [25]
Summary
Barium titanate (BaTiO3, BT), a prototypical room-temperature ferroelectric (FE) material with ABO3 perovskite form, undergoes a sequence of structural phase transitions before entering into the paraelectric phase above the Curie temperature (125 °C) [1]. Ferroelectricity in BT is due to the long-range correlation of Ti (B-site) cation displacements and can be modulated by chemical modification upon substitution of alternative chemical species in the equivalent perovskite lattice sites This has been a common scientific practice to explore the possibilities of finding better performing material systems [2,3,4,5,6], and is an opportunity to tune material properties such as the polarization response, loss mechanisms, electro-mechanical responses, Curie temperature, temperature stability of the permittivity, among others [7,8,9]. In heterovalent systems, vacancies (A-, B-site or oxygen) are expected to play a decisive role as charge compensating defects inducing disorder and modifying the functional properties [15]. We present here a way to qualitatively study the defects in perovskites, which constitutes a relevant input for the design of BT perovskite solid solutions with desired properties
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