Understanding the short-range structure of an amorphous material is the first step in predicting its macroscopic properties. Amorphous strontium titanate (a-STO) presents a unique challenge due to contradictory experimental findings regarding the local oxygen environment of titanium, concluded to be either tetrahedral or octahedral. To elucidate the discrepancy, 72 models of a-STO with density ranging from the crystalline value 5.12 to 3.07g/cm3 were prepared using ab initio molecular dynamics liquid-quench simulations and characterized by extended x-ray absorption fine structure (EXAFS) for both Ti and Sr K edge. An excellent agreement between the calculated and two independent experimental EXAFS measurements demonstrates that the discrepancy in the Ti coordination stems from differences in the material's density. Next, density-dependent structural characteristics, including Ti-O and Sr-O coordination, distances, angles, polyhedral sharing, and vibrational density of states in a-STO are thoroughly analyzed and correlated with disorder-induced changes in the electronic properties that are calculated using a hybrid density functional. The obtained increase in the band gap and broadening of Ti-deg-orbital contributions in the conduction band are in excellent agreement with our x-ray absorption spectroscopy for Ti L-edge spectra and optical absorption measurements for crystalline and amorphous STO grown by pulsed laser deposition. The derived microscopic understanding of the structure-property relationship in amorphous "perovskite" serves as a foundation for further investigations of a-STO and related materials.
J. E. Medvedeva et al., "Structure and Electronic Properties of Amorphous Strontium Titanate," Physical Review Materials, vol. 6, no. 7, article no. 075605, American Physical Society, Jul 2022.
The definitive version is available at https://doi.org/10.1103/PhysRevMaterials.6.075605
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01 Jul 2022
National Science Foundation, Grant DMR-1729779