Electronic structure and transport properties of A-site co-substituted SrTiO3-δ ceramics with enhanced configuration entropy
KANAS N. 1, MADATHIL R. 2, SHARMA A. 3, SRDIC V. 4, MILJEVIC B. 4, RAKI? S. 5, BHATTACHARYA S. 3, ARMAKOVIC S. 5
1 Institute BioSense, Novi Sad, Serbia; 2 Department of Chemistry, University of Oslo, Oslo, Norway; 3 Nanofunctional Materials Technology Centre, Department of MME, IIT Madras, Madras, India; 4 Department of Materials Engineering, Faculty of Technology, University of Novi Sad, Novi Sad, Serbia; 5 Department of Physics, Faculty of Sciences, University of Novi Sad, Novi Sad, Serbia
Ceramics with an enhanced entropy became attractive due to their tunneable functional properties. Among them, SrTiO3 is interesting due to its versatile electronic properties ranging from dielectric to semiconducting behaviour. Here we analyze the effect of systematic co-substitution of various A-site cations on the electronic structure and transport properties of oxygen deficient SrTiO3-δ bulk ceramics.
The materials were synthesized by reverse co-precipitation method and densified by spark plasma sintering (SPS) in vacuum, followed by a post-thermal treatment in reducing atmosphere (5% H2 in Ar). An initial charge neutrality was maintained as in case of pristine SrTiO3 (no donor or acceptor doping) and n-type conductivity was obtained by introduction of oxygen vacancies during the SPS and post-thermal treatment. The obtained ceramics, Pb0.33Sr0.33Ca0.33TiO3-δ, Pb0.20Sr0.20Ca0.20Na0.20Nd0.20TiO3-δ, Sr0.25Ca0.25Na0.25Nd0.25TiO3-δ, Sr0.25Ca0.25Na0.25Pr0.25TiO3-δ and Sr0.25Ca0.25Na0.25La0.25TiO3-δ possess different microstructures that were revealed by Scanning Electron Microscopy (SEM). All of them, together with a reference sample Sr0.9La0.1TiO3-δ, were characterized in terms of electrical conductivity and Seebeck coefficient between 100 and 800 °C in 5% H2/Ar, while the ionic conductivity was monitored isothermally at 450 °C in ambient air at a frequency of 1 Hz. The highest Seebeck coefficient of about -440 µV·K-1 was obtained with Pb0.33Sr0.33Ca0.33TiO3-δ at 800 °C, while the highest electrical conductivity of about 1.9 S·cm-1 at 800 °C and ionic conductivity of about 5.2·10?4 S·cm-1 at 450 °C were obtained with Pb0.20Sr0.20Ca0.20Na0.20Nd0.20TiO3-δ. DFT calculations were applied to obtain information on band structures, the projected density of states (PDOS) and electron density differences (EDD). Band structures indicated that the oxygen deficiency essentially changed the conductive nature of the studied materials by shifting the Fermi level to the conductive zone, while PDOS plots helped determining the role of each element located at the A site. It appears that the electronic structure is affected to the greatest extent in the cases when Nd and Pr are present at the A site. EDD plots gave important information about the charge distribution near Ti atoms whose oxidation state was manipulated by oxygen deficiencies. When Nd was present at the A sites, the physical shift of other oxygen atoms is significantly higher, thus affecting the oxidation state of Ti.
This work reports on the relation between the degree of entropy at A-site followed by the influence of oxygen vacancies and the electronic transport properties in the selected perovskites.