Infrared spectroscopic properties of goethite: anharmonic broadening, long-range electrostatic effects and Al substitution

TY - JOUR

T1 - Infrared spectroscopic properties of goethite

T2 - anharmonic broadening, long-range electrostatic effects and Al substitution

AU - Blanchard, Marc

AU - Balan, Etienne

AU - Giura, Paola

AU - Beneut, Keevin

AU - Yi, Haohao

AU - Morin, Guillaume

AU - Pinilla, Carlos

AU - Lazzeri, Michele

AU - Floris, Andrea

PY - 2014/4

Y1 - 2014/4

N2 - The infrared spectrum and its temperature dependence (20-320 K) were collected on a synthetic goethite sample (alpha-FeOOH). In addition, the infrared powder absorption spectrum of goethite and aluminum-substituted goethite was computed using ab initio quantum mechanical calculations based on density functional theory. This combined experimental and theoretical approach allows (1) the unequivocal assignment of absorption bands to the corresponding vibrational modes, (2) separate identification of the effects of the particle shape and of the aluminum substitution on the infrared spectrum, and (3) a discussion of the anharmonic properties and the origin of the line broadening in goethite. In particular, the two well-resolved OH bending absorption bands show different temperature evolution. Their detailed analysis suggests that the broadening of the band at similar to 800 cm(-1) cannot be described solely by a usual three-phonon process. The strong anharmonic behavior of this mode implies the addition of a four-phonon process, such as a pure dephasing process. In our calculations, the effect of the Hubbard U correction is also investigated and found to be most visible on the OH stretching and bending modes, in relation to the associated structural relaxation. The OH stretching frequencies decrease, leading to a better agreement with experimental frequencies, while the OH bending frequencies increase.

AB - The infrared spectrum and its temperature dependence (20-320 K) were collected on a synthetic goethite sample (alpha-FeOOH). In addition, the infrared powder absorption spectrum of goethite and aluminum-substituted goethite was computed using ab initio quantum mechanical calculations based on density functional theory. This combined experimental and theoretical approach allows (1) the unequivocal assignment of absorption bands to the corresponding vibrational modes, (2) separate identification of the effects of the particle shape and of the aluminum substitution on the infrared spectrum, and (3) a discussion of the anharmonic properties and the origin of the line broadening in goethite. In particular, the two well-resolved OH bending absorption bands show different temperature evolution. Their detailed analysis suggests that the broadening of the band at similar to 800 cm(-1) cannot be described solely by a usual three-phonon process. The strong anharmonic behavior of this mode implies the addition of a four-phonon process, such as a pure dephasing process. In our calculations, the effect of the Hubbard U correction is also investigated and found to be most visible on the OH stretching and bending modes, in relation to the associated structural relaxation. The OH stretching frequencies decrease, leading to a better agreement with experimental frequencies, while the OH bending frequencies increase.

KW - Goethite

KW - Aluminum substitution

KW - Density functional theory

KW - Infrared spectroscopy

KW - OH STRETCHING MODES

KW - ALPHA-FEOOH

KW - X-RAY

KW - IRON-OXIDES

KW - ALUMINUM SUBSTITUTION

KW - VARYING CRYSTALLINITY

KW - RAMAN-SPECTROSCOPY

KW - MOSSBAUER-SPECTRA

KW - AB-INITIO

KW - IN-SITU

U2 - 10.1007/s00269-013-0648-7

DO - 10.1007/s00269-013-0648-7

M3 - Article

SN - 0342-1791

VL - 41

SP - 289

EP - 302

JO - PHYSICS AND CHEMISTRY OF MINERALS

JF - PHYSICS AND CHEMISTRY OF MINERALS

IS - 4

ER -