Version Date: 7/14/97

Inelastic Neutron Scattering Study of the (O4H4) Substitution in Garnet.

G.A. Lager(l), J.C. Nipko(2) and C.-K. Loong(2), (1) Dept. of Geography and

Geosciences, University of Louisville, Louisville, KY 40292 USA, (2) IPNS Division,

Argonne National Laboratory, Argonne, IL 60439 USA.

Abstract I

Inelastic scattering data have been collected at incident neutron energies of 50, 150,

300 and 600 meV for hydrogarnet [Sr3Al2(04Hq)3], a model compound for silicate

hydrogarents found in the Earths crust and mantle. The vibrational spectrum is

characterized by a relatively sharp 0-H stretch at - 460 meV, which is consistent with the

weak O...H bond (- 2.65 A) and other complex features in the 20-100 meV region. The

mode assignments for isostructural grosslar and the infrared spectra for hydrogarnet were

used as a basis for the intreperation of many of the neutron spectral features.

Keywords: hydrogarnet, vibrational spectroscopy, hydrogen bond

Presenting Author: George A. Lager Session Preferred: E (Materials)

Address: Dept. Geography and Geosciences University of Louisville

City and Province/State: Louisville, KY

Country and Postal Code: USA 40292

Telephone: 502-852-6844 Fax: 502-852-4560

E-mail Address: galage0 1 @ homer.louisville.edu

The submitted manuscnpt has been created by the University of Chicago as Operator of Argonne Natlonal Laboratory ("Argonne') under Contract No. W-31-109-ENG-38 with the U S Department of Energy. The U.S. Government retalns for itself, and others act- ing on its behalf, a paid-up. nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, dis- tribute copies to the public, and perform pub- ltcly and display publicly, by or on behalf of the Government.

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DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or use- fulncss of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spc- cific commercial product, process, or service by trade name, trademark, manufac- turer, or otherwise does not necessarily constitute or imply its endorsement, mom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Introduction

One of the fundamental goals of geology is to predict from first principles

thermodynamic properties and mineral stability. Because the thermodynamic properties are

related to vibrational frequencies, numerous attempts have been made to calculate

thermodynamic properties from spectroscopic data. In mineralogy, this research has

primarily involved optical spectroscopies such as infrared and Rwnan. Neutron

spectroscopy has been used less frequently, in part because of the need for large sample

volumes. In this study, inelastic neutron scattering has been collected for synthetic

hydrogarnet (Sr3A12(04H4)J, a model compound for silicate hydrogarnets that occur in the

Earth's crust and mantle.

The general chemical formula for silicate hydrogarnet is rs1X'61Y ([41Si0,)3-,( 04HJx,

where the superscripts in brackets refer to the 0 coordination of the three different types of

cations in the structure. The structure is uniquely defined by the fractional coordinates (xyz)

of the 0 and H atoms and the unit-cell parameters (space group Ia-36). The cations occupy

special positions that are fixed by symmetry. The importance of this mineral is related to a

rather unusual chemical substitution involving Si"+ and H', which produces a defect garnet

structure consisting of (Siob" and (O,H,)" units. This substitution decreases the thermal

stability and bulk modulus and provides an effective mechanism for incorporating "water"

in the crystal structure. "Water" stored in garnets could be released in the Earth and act as a

catalyst for certain types of geochemical reactions. To determine if such a model is valid, it

is important to model the physical behavior of hydrogarnet at temperatures and pressures

similar to those found in the Earth. The ultimate goal of this project is to develop an

equation of state for hydrogarnet based on an analysis of the vibrational frequencies

obtained from inelastic neutron scattering measurements. Initial experiments have focused

on the vibrational spectrum of end member Sr3A12(04H4)3. This particular composition was

chosen because it is relatively easy to synthesize in large amounts and the spectrum is not

complicated by the effects of H bonding. A recent neutron powder diffraction study of the

2

deuteriated analog of this compound at ambient conditions has shown that there is only a

weak H-bond interaction (0.. .D = 2.65 A). [ 11

Experimental

The time-of-flight experiments were performed using the High-Resolution Medium-

Energy Chopper Spectrometer (HRMECS) at the Intense Pulsed Neutron Source (IPNS) of

Argonne National Laboratory. A polycrystalline sample of Sr,Al,(O,H,), (-20 g ) was

prepared by reacting high-purity N20, and SrCO, at 1573 K and adding H20 to the

resulting finely powdered strontium aluminate. Infrared spectra were obtained at ambient

conditions using the KBr-pellet method on a Nicolet 60SX FI'IR. [Z]

The HRMECS spectrometer is equipped with wide-angle multidetector banks

which, using an incident neutron energies ( E o ) of 50, 150, 300, and 600 meV, allows

measurements of inelastic scattering over a wide range of momentum and energy transfer.

The energy resolution AE in full-width-at-half-maximum of the HRMECS spectrometer

varies from approximately 4% of Eo in the elastic region to -2% near the end of the

neutron-energy-loss spectrum. The powder was contained in an duminum planar cell

mounted at a 45" angle to the incident neutron beam. Such a geometry decreases the

neutron traverse length in the sample to < 5 mm for all detector angles - thereby reducing

multiple-scattering effects. Multiple scattering was estimated to be less than 5% of the total

measured intensity and was corrected by removing a small, flat background from the data.

To reduce multiple-phonon excitations, the samples were cooled to 6 K for the

experiments. Normal background scattering was subtracted from the data by using empty-

container runs. Measurements of elastic incoherent scattering from a vanadium standard

provided detector calibration.

Results and Discussion

Fig. 2 displays the observed neutron scattering function and IR absorption

spectrum for hydrogarnet. The inset reproduces the low frequency portion of the

spectrum. Four incident energies were used to generate the spectrum from 0 to 500 meV

energy transfer. The observed frequency bands are labeled and are given in Table 1. Many

of the low frequency bands correspond to dispersive lattice modes and are not easily

assigned to a particular type of lattice motion; however, at about 60 meV we see an intense

feature in the infrared spectrum that is much weaker in the neutron spectrum. A similar

feature was observed in isostructural grossular (CqAl, (SiO,), ) 131 where it has been

attributed primarily to translational motion of the Al sublattice. This interpretation is

consistent with our neutron data since the neutron cross section for aluminum is much

smaller than that of hydrogen (which will dominate the neutron spectrum). Similarly we

observed a narrow band in the neutron spectrum at 76.7 meV (label K) that appears to be

absent in the IR spectrum. A mode of similar frequency is observed with Raman

spectroscopy in grossular, and one possible interpretation for this mode is a symmetric

bond bending mode involving the AI-0-H bond, [3]

The two modes labeled L and M appear in both the infrared and neutron spectrum.

Modes of comparable intensity and frequency were also observed in the infrared spectrum

of grossular. In grossular they have been assigned as internal v,-type modes of the

(SiO4X4 units. In hydrogarnet the Si& ion is absent; however, (04H4)" units have the same

symmetry as (Si04)4 units and occupy sites in the crystal lattice of hydrogarnet identical to

that of (SO,)' in grossular. Therefore, we believe there may be some similarity to the

internal modes of the (O,H,)', and (sio4)-4 and propose that the L and M modes may be

similar to the v,-type modes of (Si0,)-4.

4

- _

The mode labeled 0 corresponds to the 0-H stretch and is resolution limited in both

the IR and neutron spectrum. The 0-H bond length is 0.914 8, at ambient conditions, and

the next nearest neighbor oxygen distance is over 2.65 8,. Therefore, a very weak H-bond

exists in Sr hydrogranet, consistent with the narrow width of the 0-H stretch feature.

Acknowledgements

Work performed at Argonne National Laboratory is supported by US. DOE-BES

under contract No. W-3 1-109-ENG-38.

References

[ 11. B. C. Chakoumakos, G. A. Lager, and J. A. Fernandez-Baca, Acta Cryst. C48,

414-19 (1992).

[2]. L. Miura and G. R. Rossman, personal communication.

[3]. A. M. Hofmeister and A. Chopelas, Phys. Chem. Minerals 17,503-26 (1991).

Table I. Neutron and infrared frequencies of Hydrogamet.

Feature Label

A B C D E F G H I

J

K

L M N 0

Neutron Modes (mew 9.4 12.4 20.3 23.1 25.5 28.6 33.1 38.0 42.1

60 -

76.7

95.6 107 20 1 459

30

39.7

47.9 63 67

96 104 20 1 454

Assignment

Lattice Mode Lattice Mode Lattice Mode Lattice Mode Lattice Mode Lattice Mode Lattice Mode Lattice Mode Lattice Mode

A1 imiational

Symmetric AL - 0 - H Bond bending Mode

v3 - tYPe (O,H4) group v3 - type (O,H,) group

Lattice Mode 0-H Stretch

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Figure Captions:

Figure 1. The observed neutron (symbols) and infrared (line) spectra of hydrogarnet at

6 K. "he neutron data were composed of runs obtained with different incident energies as

labeled on top. Inset: an enlarged portion of the low energy spectrum.

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Infrared Intensity d cv 0 00 CB * cv 0

m m m . l- 0 0 0 0

0-