1 California Institute of Technology, Pasadena, California 91125, United States
2 Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
Under reducing conditions, rutile TiO2
develops
O vacancies (VO) coupled to Ti3+
centers. It is favorable
for H atoms to enter this system, either forming OH groups or occupying
vacancy
sites (denoted HO) that bond to two Ti next to
the vacancy. OH
defects are well documented by the presence of infrared modes at ~3300
cm‑1,
while HO is relatively under investigated. We
report the energies,
geometries, and vibrational frequencies of hydrogen defects in rutile
predicted
from Quantum Mechanics calculations, focusing on the coexistence of OH
and HO.
We find that HO is more stable than OH by 1.42
eV, leading to an
infrared mode at ~1200 cm‑1. Introducing a
second H forms an OH bond
with an infrared mode at ~3300 cm-1. These
results suggest that
assessments of the hydrogen storage in mantle phases of rutile and
similar
minerals based on OH bands may significantly underestimate H
concentrations.