Band Gap Engineering and Trap Depths of Intrinsic Point Defects in RAlO3 (R = Y, La, Gd, Yb, Lu) Perovskites

Yaroslav Zhydachevskyy,Yuriy Hizhnyi,Vasyl Stasiv,Aleksandr Lushchik,Andrzej Suchocki,Sergii G Nedilko,Leonid Vasylechko,Dmytro Sugak,Vladimir Pankratov,Nickolai Klyui,Irina Kudryavtseva,Marek Berkowski

doi:10.1021/acs.jpcc.1c06573

Abstract

The possibility of band gap engineering (BGE) in RAlO3 (R = Y, La, Gd, Yb, Lu) perovskites in the context of trap depths of intrinsic point defects was investigated comprehensively using experimental and theoretical approaches. The optical band gap of the materials, Eg, was determined via both the absorption measurements in the VUV spectral range and the spectra of recombination luminescence excitation by synchrotron radiation. The experimentally observed effect of Eg reduction from ∼8.5 to ∼5.5 eV in RAlO3 perovskites with increasing R3+ ionic radius was confirmed by the DFT electronic structure calculations performed for RMIIIO3 crystals (R = Lu, Y, La; MIII = Al, Ga, In). The possibility of BGE was also proved by the analysis of thermally stimulated luminescence (TSL) measured above room temperature for the far-red emitting (Y/Gd/La)AlO3:Mn4+ phosphors, which confirmed decreasing of the trap depths in the cation sequence Y → Gd → La. Calculations of the trap depths performed within the super cell approach for a number of intrinsic point defects and their complexes allowed recognizing specific trapping centers that can be responsible for the observed TSL. In particular, the electron traps of 1.33 and 1.43 eV (in YAlO3) were considered to be formed by the energy level of oxygen vacancy (VO) with different arrangement of neighboring YAl and VY, while shallower electron traps of 0.9–1.0 eV were related to the energy level of YAl antisite complexes with neighboring VO or (VO + VY). The effect of the lowering of electron trap depths in RAlO3 was demonstrated for the VO-related level of the (YAl + VO + VY) complex defect for the particular case of La substituting Y.

Highlights

Yttrium−aluminum perovskite (YAlO3 or YAP) is a well-known host material for solid-state lasers, scintillators, and various kinds of converting and storage phosphors
(2) The gradual decrease of band gap value (Eg) of RAlO3 perovskites from ∼8.5 to ∼5.5 eV with increase of cationic radius, i.e., in sequence of R cations Lu → Yb → Y → Gd → La, has been shown experimentally using both the optical absorption measurements in VUV spectral range and the spectra of luminescence excitation by synchrotron radiation. Such a wide (∼3 eV) variation of band gap values obviously provides a strong potential for the band gap engineering of RAlO3 perovskite compounds
(3) The Density functional theory (DFT) electronic structure calculations confirm perspectives of the Band gap engineering (BGE) approach in perovskites: the band gaps of RMIIIO3 crystals gradually decrease in the Lu−Y−La sequence of R cations and Al−Ga−In sequence of MIII cations

Summary

INTRODUCTION

Yttrium−aluminum perovskite (YAlO3 or YAP) is a well-known host material for solid-state lasers, scintillators, and various kinds of converting and storage phosphors (see, e.g., refs 1−6 and references therein). From the luminescence studies under synchrotron radiation excitation, it is known that LuAlO3 has a band gap width at least 0.6 eV larger than YAlO3,18,19 whereas GdAlO3 probably has a narrower band gap than YAlO3.20 The replacement of Gd by La gradually decreases the Eg of Gd1−xLaxAlO3.15 there is an obvious lack of a systematic study demonstrating the effect of various rare-earth (R) cations on the band gap width of RAlO3 perovskites This knowledge is very important for controllable tuning of the defect- or dopant-related trap depths and for improvement thereby of the performance of scintillator materials, storage, or persistent luminescence phosphors.

EXPERIMENTAL AND CALCULATION METHODS

RESULTS AND DISCUSSION

CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES