Structural Changes from Conventional SrSnO3 to Ruddlesden-Popper Sr2SnO4 Perovskites and Its Implication on Photoluminescence and Optoelectronic Properties

Symmetry and structure studies of perovskites have attracted significant attention owing to their many scientific and technological applications in multiferroic and optoelectronics. This work highlights symmetry/structural changes in transformation of conventional ABO(3) (CP) to Ruddlesden-Popper perovskites A(2)BO(4) (RPP) and its effect on Eu3+ photoluminescence. Raman spectroscopy and Rietveld refined X-ray diffraction have suggested high symmetry of conventional SrSnO3: Eu3+ (CP-SrE) compared to the Ruddlesden-Popper Sr2SnO4:Eu3+ perovskite (RPP-Sr2E). Positron annihilation lifetime (PAL) has further suggested a high defect concentration in RPP-Sr2E compared to CP-SrE endowed by the extra SrO layer in the former, making it structurally more distorted. These changes are easily picked up in emission spectra through a very low asymmetry ratio (D-5(0) -> F-7(2)/D-5(0) -> F-7(1)) and higher excited lifetime in CP-SrE compared to RPP-Sr2E. DFT calculations suggest a very high negative formation energy for europium substitution at both Sr2+ and Sn4+ in CP-SrE and RPP-Sr2E, respectively, which supports different asymmetry ratios of europium emission under charge transfer and f-f excitation bands. Polarization versus electric field (hysteresis loop) measurements have suggested a typical behavior of the linear capacitor for both CP-SrE and RPP-Sr2E with comparable maximum polarization values. Cyclic voltammetry suggested higher concentrations of oxygen vacancies in RPP-Sr2E compared to CP-SrE. This coupled with high defect density as predicted by PAL leads to the enhanced current density, higher specific capacitance, and lower charge transfer resistance in RPP-Sr2E compared to CP-SrE, making the former a better optoelectronic candidate. This ability to directly monitor the local site symmetry within perovskites via photoluminescence spectroscopy can further be extended in the future for monitoring defect/temperature and pressure-induced phase transitions in perovskites.