Reversible control of the chromium valence in chemically reduced Cr-doped SrTiO sub(3) bulk powders

• Published in: Dalton transactions : an international journal of inorganic chemistry Vol. 45; no. 24; pp. 10034 - 10041
• Main Authors: Lehuta, Keith A, Kittilstved, Kevin R
• Format: Journal Article
• Language: English
• Published: 01.06.2016
• Subjects: Chromium; Dopants; Electronic structure; Lattice vacancies; Oxygen; Reduction; Reduction (chemical); Strontium titanates
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c6dt00706f

The effect of chemical reduction by NaBH sub(4) on the electronic structure of Cr-doped SrTiO sub(3-) delta bulk powders prepared by a solid-state reaction was systematically studied as a function of reduction temperature. Electron paramagnetic resonance (EPR) and diffuse reflectance spectroscopies (DRS) were utilized to monitor changes in the electronic structures of both intrinsic defects (oxygen vacancies and/or Ti super(3+)) and extrinsic dopants (Cr super(3+)) at different reduction temperatures. We identify the existence of two temperature regimes where changes occur within 30 min. The first temperature regime occurs between 300-375 degree C and results in (1) reduction of oxygen-related surface defects, and (2) an increase in the concentration of Cr super(3+) by over an order of magnitude, suggesting that EPR-silent Cr super(4+) or Cr super(6+) is being reduced to Cr super(3+) by NaBH sub(4). The second temperature regime occurs between 375-430 degree C where we observe clear evidence of Ti super(3+) formation by EPR spectroscopy that indicates chemical reduction of the SrTiO sub(3) lattice. In addition, the oxygen-related surface defects observed in regime 1 are not formed in regime 2, but instead lattice oxygen vacancies (V sub(O)) are observed by EPR. The changes to the Cr-doped SrTiO sub(3) electronic structure after chemical reduction in regime 1 are quantitatively reversible after aerobic annealing at 400 degree C for 30 min. The internal oxygen vacancies formed during the higher temperature reductions in regime 2 require increased temperatures of at least 600 degree C to be fully reoxidized in 30 min. The effect of these different oxygen-related defects on the EPR spectrum of substitutional Cr super(3+) dopants is discussed. These results allow us to independently tune the dopant and host electronic structures of a technologically-relevant multifunctional material by a simple ex situchemical perturbation.