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Photoferroelectric perovskites - an investigation of compositional and processing influence on microstructure and properties
Keywords: ferroelectric, perovskite, multi-functional
Ba(Ni0.5Nb0.5)O2.75 (BNNO) doped KNbO3 (KN) and (K0.5Na0.5)NbO3 (KNN) have been reported to exhibit narrow bandgaps and recognisable or giant piezoelectric and pyroelectric effects simultaneously within the same material (Grinberg et al., 2013; Bai et al., 2017a,b). It has been found that defect dipoles, Ni2+- oxygen vacancy combinations introduced with the dopant BNNO, act in the crucial role of easing the charge transfer from the oxygen 2p states at the maximum level of the valence band to the transition-metal (Nb) d states at the minimum level of the conduction band in the parental compositions KN or KNN (Grinberg et al., 2013; Bai et al., 2017a,b). This is expected to trigger the development of truly multi-source energy harvesters or multi-functional sensors where the key component would be designed utilizing only a single piece of material from which solar, kinetic and thermal energies could simultaneously be harvested/detected (Bai et al., 2017a,b). The working principles of such devices, whose functions are fully integrated within a material without degrading interactions, would be fundamentally different from those of previously reported hybrid devices which are simply physical combinations of individual components (Xu et al., 2009 & 2011; Yang et al., 2013; Lee et al., 2016). It is widely known that, due to the introduction of oxygen vacancies (referred to as hard doping or acceptor doping in piezoelectrics), the domain wall mobility is usually restricted. This is known as the domain wall pinning effect. Increased concentrations of oxygen vacancies will, in most cases, lead to even more restricted domain wall mobility and a suppressed piezoelectric and pyroelectric response. Fortunately, it has been found that for the BNNO doped KN (KBNNO), the change in the bandgap value is in a non-monotonic relationship with the BNNO fraction (Grinberg et al., 2013). That is to say, with 10-40 mol.% of dopant, the bandgaps of the KBNNO ceramics, ranging from 1.1 eV to 1.4 eV, are not increased or decreased with the amount of the dopant (i.e. the concentration of oxygen vacancies) (Grinberg et al., 2013). Meanwhile, all the above mentioned bandgaps are at the same level (smaller than 1.65 eV, the lowest photon energy of the visible light spectrum) which is preferred in the application of solar/visible light absorption and harvesting. A similar trend is also found in the BNNO doped KNN (KNBNNO) (Bai et al., 2017b)[3]. Such a phenomenon provides an opportunity to minimize the concentration of introduced oxygen vacancies, thus maximising the piezoelectric and pyroelectric properties while maintaining a narrow bandgap. It has been proved that KNN doped with 2 mol.% of BNNO can exhibit a narrow bandgap (1.6 eV) as well as large piezoelectric (100 pm/V) and pyroelectric (130 C/m2K) coefficients (Bai et al., 2017b). However, besides their advantages of simultaneously exhibiting good piezoelectric, pyroelectric and optical properties, detailed information e.g. the correlation of the compositions, processing conditions, microstructure and properties remains to be investigated. In this paper, such a detailed investigation is carried out. Compositions of 10 mol.% of BNNO doped KN (with and without potassium deficit) and 2-10 mol.% of BNNO doped KNN are studied. The inter-influence of different doping amounts of BNNO, calcination and sintering temperatures, phase structures and defects (potassium loss and oxygen vacancy) on the dielectric, ferroelectric and photovoltaic properties, are studied for the first time. Briefly, the results show that different calcination and sintering temperatures of the powders and ceramics have, by affecting the rate of potassium loss, resulted in different microstructures of the compositions. These different microstructures have determined the varying hygroscopicity of the ceramics as well as their different dielectric, ferroelectric and photovoltaic properties. In these ferroelectric-photovoltaic multi- functional perovskites, the remanent polarization at the operating temperature should be as large as possible. This can benefit not only the potentially strong piezoelectric and pyroelectric response but also boost the photovoltaic effect. A large polarization, and thus a large concentration of polarization-induced free charges, is preferable to a high concentration of defects or defect dipoles for a good feasibility of multi- functional applications. Together with references (Bai et al., 2017a,b), this paper serves as a comprehensive and detailed report of the KBNNO and KNBNNO multi-source energy harvesting/sensing materials, providing guidance for the working principles, fabrication, optimisation and characterisation of these materials.
Yang Bai,
University of Oulu, Finland
Oulu, Oulu

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