Structure and electronic properties of amorphous strontium titanate

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 3.07 g/cm(3) 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-d e(g)-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.

Automated summary

Understanding the short-range structure of amorphous materials is crucial for predicting their macroscopic properties. This study investigates the contradictory experimental findings regarding the oxygen environment of titanium in amorphous strontium titanate and reveals that the discrepancy is caused by differences in the material's density. The study also analyzes the density-dependent structural characteristics and electronic properties of amorphous strontium titanate, providing insights into the structure-property relationship.