An in situ Raman study on katoite Ca3Al2(O4H4)3at high pressure

Masato KATO and Atsushi KYONO

Division of Earth Evolution Sciences, Graduate School of Life and EnvironmentalSciences, University of Tsukuba, Tsukuba 305–8572, Japan

We report on in situ high–pressure Raman spectroscopic study of hydrogrossular, katoite Ca3Al2(O4H4)3, in adiamond–anvil cell under hydrostatic conditions at room temperature. The variations of vibration modes wereanalyzed theoretically by factor group analysis. Three characteristic Raman peaks observed at 341, 541, and3649 cm−1 at 1.3 GPa were obtained continuously up to 8.3 GPa. The Raman peak at 3649 cm−1 was assignedto the O–H stretching vibration modes of A1g + Eg symmetry at 3634 cm−1 and F2g symmetry at 3656 cm−1. Thepressure dependence of the two O–H stretching vibration modes showed negative pressure shifts, indicating thathydrogen bond became shorter and stronger with compression. The most striking characteristic was that above5.1 GPa pressure derivative of the O–H stretching vibration mode of F2g symmetry started to decrease signifi-cantly. This change suggests that symmetry of the H4O4 tetrahedron changes at 5.1 GPa, implying the phasetransition of katoite to its high–pressure phase. Pressure derivatives of the two O–H stretching vibration modesof A1g + Eg and F2g symmetries are −7.2 and −1.1 cm−1/GPa, respectively, which yields negative Grüneisenparameters. In the frequency region of lattice mode, the lower frequency peak observed at 341 cm−1 wasassigned to librational R(O4H4) vibration mode of A1g symmetry, whose frequency increased continuouslyup to 5.1 GPa with pressure derivatives of 6.5 cm−1/GPa. Meanwhile, the higher frequency peak at 541cm−1 was assigned to T(OH) motions of A1g + Eg symmetry at 523 cm−1 and F2g symmetry at 545 cm−1, whosefrequencies increased with pressure derivatives of 4.4 and 4.9 cm−1/GPa, respectively. These pressure coeffi-cients in the lattice mode lead to the isothermal mode Grüneisen parameters varying from 0.49 to 1.11. Valuesof the full width at half maximum (FWHM) of all observed Raman bands were continuously increased up to 5.1GPa, but their increasing rates became higher above this pressure. The result is also indicative that katoitetransforms to the high–pressure phase above 5.1 GPa.

Keywords: Hydrogrossular, Raman spectroscopy, High pressure, Phase transition

INTRODUCTION

There is a great interest in understanding interactionsamong H2O, H+, and OH− in Earth’s materials. Severalearly works (Martin and Donnay, 1972; Wilkins and Sa-bine, 1973) have pointed out that a small amount of hy-drous components can be incorporated into some miner-als whose chemical formula would be nominally writtenwithout any hydrogen, namely the nominally anhydrousminerals (NAMs). Since then, a lot of attention has beendevoted to the understanding of the incorporation of hy-drogen into the NAMs, such as olivine, pyroxene, garnet,and their high–pressure polymorphs (Bell and Rossman,

1992; Ingrin and Skogby, 2000; Hirschmann et al., 2005;Li et al., 2008; Weis et al., 2017). From a geophysicalstandpoint, the NAMs are especially important becausethey may potentially introduce a large amount of water inthe Earth mantle thus significantly modifying its elasticproperties (Zhang et al., 2004; Zhang and Green, 2007;Karato, 2008; Demouchy and Bolfan–Casanova, 2016).

A silicate garnet with a formula A3B2(SiO4)3, whereA = Ca2+, Mg2+, Mn2+, Fe2+ and B = Cr3+, Fe3+, Al3+,crystallizes in the cubic system, space group Ia�3d. Thecalcium aluminum garnet, grossular Ca3Al2(SiO4)3, withthe largest divalent cation Ca2+ in A site and the smallesttrivalent cation Al3+ in B site exhibits a solid solutionwith the Si–free end member of hydrogrossular, katoiteCa3Al2(O4H4)3 (Flint et al., 1941; Cohen–Addad et al.,1964, 1967; Passaglia and Rinaldi, 1984; Sacerdoti and

doi:10.2465/jmps.180530A. Kyono, kyono@geol.tsukuba.ac.jp Corresponding author

Journal of Mineralogical and Petrological Sciences, Volume 114, page 18–25, 2019

Passaglia, 1985; Lager et al., 1989). Katoite is known asa typical model for the hydrogarnet substitution (Si4+ ↔4H+) in garnets and other silicates, and hence garnets canbe considered as one of the most important hydrogen res-ervoirs in the Earth’s mantle due to their abundance andstability. The replacement of Si4+ by 4H+ results in pro-found changes in the physical properties and thermody-namic stability of garnets.

Various experimental attempts have been so farmade to investigate the physical properties and stabilityof katoite at high–pressure condition (Olijnyk et al., 1991;Lager and VonDreele, 1996; Lager et al., 2002; Lager etal., 2005; Kyono et al., 2019). Lager et al. (2002) firstreported that katoite undergoes a phase transition fromspace group Ia�3d to its non–centrosymmetric subgroupI�43d at a pressure between 5.09 and 5.38 GPa. Takingthe results of neutron diffraction experiment (Lager andVonDreele, 1996) into consideration, Lager et al. (2002)proposed that H–H repulsion due to the compression ofthe inter–tetrahedral H–H distance might initiate the phasetransition from Ia�3d to I�43d. Lager et al. (2005) subse-quently reinvestigated the structural changes using thehigh–pressure neutron diffraction technique. Althoughthey observed two very weak reflections (730 and 530)violating the a–glide operation in space group Ia�3d above7.5 GPa, the reflections are too faint to reliably discrim-inate between katoite and its high–pressure phase. Conse-quently, the high–pressure neutron diffraction data on thepressure dependence of the D–D distances provided noexplanation as to the role of D–D interactions in the phasetransition (Lager et al. 2005). The recent ab–initio inves-tigation showed that the structures of katoite crystallizingin the space groups Ia�3d and I�43d are highly unstable atpressures between about 5 GPa and 15 GPa (Erba et al.,2015). The authors have suggested either an existence ofa third phase or a possible second order phase transition.Thus, there is still an argument about the high–pressurephase of katoite.

In the present study, in situ high–pressure Ramanspectroscopic study was performed to confirm the pres-sure–induced phase transition of katoite.

EXPERIMENTAL METHODS

Sample preparation

Commercially available Ca(OH)2 (FUJIFILM Wako PureChemicals Corp., purity ≥96.0%) and Al (Nirako Co.,Ltd., purity ≥99.8%) were used as starting materials. First,the Ca(OH)2 was dehydroxylated in an alumina cruciblein an electric furnace at 800 °C for 24 h. The CaO and Alpowder were then mixed in the molar proportion 3:2. Sec-

ond, the mixture was placed in a Teflon vessel and filledwith distilled water until 70% of a total volume of 28 ml.The vessel was subsequently placed in a stainless steelautoclave and subject to the hydrothermal treatment at250 °C for 5 days. After cooling down to room temper-ature, the products were finally recovered by filtration anddried at room temperature in a desiccator. Characteriza-tion of the products was performed using a high resolu-tion laboratory X–ray powder diffractometer (BrukerAXS Inc., D8 Advance diffractometer). Consequently,the product consisted of katoite up to roughly 10–20 µmin size, together with a small amount of calcite.

High–pressure Raman spectroscopy

The Raman spectroscopy measurements were performedon a micro Raman spectrometer (Photon design Corp.,Mars stabulite 2017). Raman spectra were collected inthe backscattering geometry using the 514.53 nm line ofan Ar+ laser as excitation source. The Raman frequencyshifts were accurately calibrated using the known threeRaman bands, 266.3, 520.3, and 1100.7 cm−1 from a sil-icon standard. High–pressure Raman measurements wereconducted at room temperature using a high–pressure di-amond–anvil cell (DAC) with 300 µm culet. The samplewas filled in a steel gasket hole of 100 µm in diameter,with a few ruby chips of approximately 2 µm in size. Hy-drostatic conditions in the sample chamber of the DACwere achieved by using a 16:3:1 methanol–ethanol–watermixture. Pressures were determined before and after eachmeasurement using the pressure–induced shift of the R1

ruby luminescence line (Mao et al., 1986). The laser beamwas focused on the sample to a 10 µm spot through amicroscope objective (20×). Raman spectra were acquiredwith an exposure time of 100 s. To improve signal/noiseratios, the spectra were averaged over 10 accumulations.The high–pressure Raman spectra were obtained on thecompression process. The least–squares Peak–fitting soft-ware Peak–Fit (AISN Software Inc., Chicago) was used toperform the analysis of the Raman spectra data. The base-lines fitting was conducted using a hyperbolic model.Band positions were determined by fitting the Ramanpeaks with a Lorentzian peak shape function.

RESULTS AND DISCUSSION

Pressure dependence of Raman spectra in katoite

The crystal structure of katoite consists of a three–dimen-sional network of alternating corner–sharing H4O4 tetra-hedra and Al(OH)6 octahedra, with larger Ca2+ cationsoccupying interstitial dodecahedral sites (Fig. 1). The Ra-

High–pressure Raman spectroscopy of katoite 19

man spectrum of katoite is composed of four modes: (1)Ca–motions, (2) external translation (T–) and librational(R–) modes of O4H4 clusters, (3) T– and R–modes of OHmotions, and (4) internal O–H stretching modes derivingfrom the O4H4 groups (Kolesov and Geiger, 2005). Inthe previous Raman spectroscopic study (Kolesov andGeiger, 2005), three characteristic Raman peaks wereobserved around 330, 535, and 3650 cm−1 under ambientconditions. In the region of lattice modes, the Ramanpeak at 330 cm−1 is assigned to the R(O4H4) mode ofA1g symmetry, whereas the peak at 535 cm−1 is due tothe T(OH) motions of both A1g + Eg and F2g symmetries.The Raman peak at 3650 cm−1 is assigned to the O–Hstretching modes of A1g + Eg and F2g symmetries.

Figure 2 presents the high–pressure Raman spectraof katoite and their peak fitting results in the frequencyregions of lattice mode and the OH stretching vibrationmode at 1.3 GPa. The three Raman peaks are clearly ob-served at 341, 541, and 3649 cm−1, which are approxi-mately consistent with those observed under ambient con-ditions (Kolesov and Geiger, 2005). In the frequencyregion of lattice mode, the lower frequency peak observedat 341 cm−1 is assigned to the librational R(O4H4) vibra-tion mode of A1g symmetry (Fig. 2a). Meanwhile, thehigher frequency peak at 541 cm−1 is assigned to theT(OH) motions of A1g + Eg symmetry at 523 cm−1 andF2g symmetry at 545 cm−1 (Fig. 2a). The Raman peak at3649 cm−1 are assigned to the O–H stretching vibrationmodes of A1g + Eg symmetry at 3634 cm−1 and F2g sym-

metry at 3656 cm−1 (Fig. 2b). In fact, a small Raman peak,which at least can’t be assigned to any Raman band ofcalcite, was also observed at 450 cm−1 in the present study(Fig. 2a).

Figures 3 and 4 present the in situ high–pressureRaman spectra between 1.3 and 8.3 GPa. The pressuredependence of Raman frequencies is shown in Figures 5and 6. The small Raman peak at 450 cm−1 becomes ob-scure and broader with increasing pressure, but becomesvisible and stronger above 6.3 GPa (Fig. 3). Although theRaman peak at 450 cm−1 could be assigned to the R–mode of OH motions, it showed very little pressure de-pendence compared with the other Raman bands. Theremay be the possibility of the orientational effect of sam-ple. Since the Raman peak at 450 cm−1 could not be con-tinuously obtained with pressure, it was therefore exclud-ed from our discussion.

The present study yielded several noteworthy find-ings. First, the three characteristic Raman peaks observedat 341, 541, and 3649 cm−1 at 1.3 GPa were obtained con-

Figure 1. The coordination polyhedra of katoite Ca3Al2(H4O4)3.Ca, Al, and H atoms are distributed at dodecahedra, octahedra,and tetrahedra, respectively.

Figure 2. Observed Raman spectra of katoite and peak fitting re-sults in frequency regions of (a) the lattice modes and (b) theOH stretching vibration modes at 1.3 GPa. Cross mark, dottedline, and solid lines represent the observed Raman spectra, theresolved Raman peaks, and the line profile fitted to the exper-imental data, respectively.

M. Kato and A. Kyono20

tinuously up to 8.3 GPa. The result implies that thecrystal structure of katoite described above remains essen-tially unchanged up to at least 8.3 GPa. Second, the pres-sure dependence of the two O–H stretching vibrationmodes shows negative pressure shifts. It has been alreadyknown that the synthetic hydrogrossular, hibschiteCa3Al2(SiO3)1.5(O4H4)1.5, also shows the negative pres-sure shifts of the O–H stretching vibration modes (Knittleet al., 1992). The negative pressure dependence indicatesthat hydrogen bond becomes shorter and stronger withcompression (Nakamoto et al., 1955). The present high–pressure Raman spectroscopic study therefore revealedthat in katoite the strength of hydrogen bonding increaseswith pressure, which is in good agreement with the resultsobtained by the previous high–pressure XRD and neutrondiffraction studies (Lager and VonDreele, 1996; Lager etal. 2002, 2005). The most striking characteristic is that thepressure derivative of the O–H stretching vibration mode

of F2g symmetry significantly decreases above 5.1 GPa(Fig. 6). The O–H stretching vibration is governed bythe symmetry (point group) of the H4O4 tetrahedron inkatoite. Furthermore, a constant contraction rate of thetetrahedral volume with pressure must result in a constantpressure dependence of the measured Raman frequenciesif the coordination geometry (symmetry) remains un-changed. On the contrary, change in the pressure deriva-tive suggests that symmetry of the H4O4 tetrahedral coor-dination is changed. In the present study, the discontinuityin pressure derivative of O–H stretching vibration modeobserved at 5.1 GPa implies that the katoite was trans-formed to its high–pressure phase.

Using the high–pressure single–crystal XRD tech-nique, Lager et al. (2002) showed that katoite undergoesa phase transition from space group Ia�3d to I�43d non–centrosymmetric subgroup. This pressure boundary waslocated between 5.09 and 5.38 GPa, which is strictly con-

Figure 3. Pressure dependence of Raman spectra in frequency re-gion of the lattice modes at pressure range from 1.3 to 8.3 GPa.

Figure 4. Pressure dependence of Raman spectra in frequency re-gion of the OH stretching vibration modes at pressure rangefrom 1.3 to 8.3 GPa.

High–pressure Raman spectroscopy of katoite 21

sistent with that determined by the present high–pressureRaman spectroscopic study. Lager et al. (2002) proposedthat compression of the H–H distance between O4H4

groups may destabilize the katoite structure and initiatethe phase transition to I�43d. The subsequent neutrondiffraction study (Lager et al. 2005), however, couldn’tprovide sufficient experimental results to explain thatthe shortened D–D distance triggers the phase transition.Compared to the previous neutron diffraction studies(Lager and VonDreele, 1996; Lager et al. 2005), the pres-ent high–pressure Raman spectroscopic study offers clearevidence that the O–H stretching vibration mode ischanged above 5.1 GPa, suggesting the symmetry changeof the O4H4 tetrahedral coordination. It is therefore cer-tain that compression of the O–H bond is associated withthe phase transition of katoite.

The observed full widths at half maximum (FWHM)are given in Figure 7. The FWHM values in the frequen-

cy regions of the lattice modes and OH stretching vibra-tion modes were increased with pressure. In particular,the latter increases significantly up to 8.3 GPa, but thetrends seem to change around 5 GPa. The similar situa-tion is also observable in the former. An increase ofFWHM values, that is broadening of Raman vibrationallines, is commonly due to three main reasons: (1) heter-ogeneous stress–strain distribution within the sample,(2) degradation of crystalline quality by an increase ofdefects, and (3) coupling and mixing between variousvibration modes. We considered the three possibilities.First, the heterogeneous stress–strain distribution causedby non–hydrostaticity in the pressure medium was sys-tematically and comparatively investigated by Klotz etal. (2009). The authors have reported that the 16:3:1methanol–ethanol–water mixture remained fully hydro-static up to 10.5 GPa. Thus, there must be no broadeningof Raman lines caused by heterogeneous stress–strain dis-tribution within the sample. Next, the previous X–ray andneutron diffraction studies have confirmed that little pres-sure effect severely degrades the crystallinity of katoite(Olijnyk et al., 1991; Lager and VonDreele, 1996; Lageret al. 2005). The broadenings of the O–H stretching vi-bration modes were nevertheless observed from 1.3 to 8.3GPa (Fig. 7b), which should be attributed to an increaseof the concentration of point defects at the H4O4 tetrahe-dra in the garnet structure. The point defect concentrationis related to the number of H atoms that are not locatedright at their crystallographic positions. Third, the previ-ous study on the assignment of the Raman modes (Kole-sov and Geiger, 2005) indicated that coupling and mixingbetween various modes occur in katoite. In the presentstudy, all Raman peaks observed at 1.3 GPa were fittedand separated by each Raman mode. Thereafter, we in-vestigated the pressure–induced Raman spectral changes.

Figure 7. The variations of full width at half maximum (FWHM)of (a) the lattice modes and (b) the OH stretching vibrationmodes as a function of pressure. Cross marks and filled dia-monds represent the translation T(OH) motion of A1g + Eg,and F2g symmetries, respectively. Open circles show the libra-tional R(O4H4) vibration mode of A1g symmetry. Open squaresand filled triangles correspond to the O–H stretching modes ofA1g + Eg and F2g symmetries, respectively.

Figure 6. Pressure dependence of peak positions of the O–Hstretching mode of (a) A1g + Eg and (b) F2g symmetries.

Figure 5. Pressure dependence of peak positions of the librationalR(O4H4) vibration mode of (a) A1g symmetry, the translationT(OH) motion of (b) A1g + Eg, and (c) F2g symmetries.

M. Kato and A. Kyono22

As it can be seen in Figures 3 and 4, the observed Ramanbands at 1.3 GPa are never intersected with each other asincreasing pressure. The change in the trends of pressuredependence of the FWHM values (Fig. 7) can thereforeoriginate in an increase of Raman active modes, whichis due to the symmetry reduction of katoite from cubicIa�3d to its subgroups.

Based on the assumption that the katoite transformsto its high–pressure phase, least square regression linesfitted to the data are divided below and above 5.1 GPa(Figs. 5–7). The pressure derivatives of peak positionsof the two O–H stretching vibration modes in katoiteare −7.2 and −1.1 cm−1/GPa, respectively, which yieldsnegative Grüneisen parameters (Table 1). In the frequen-cy region of lattice mode (Fig. 3), the lower frequencypeak observed at 341 cm−1 was assigned to the librationalR(O4H4) vibration mode of A1g symmetry, whose fre-quency increases continuously up to 5.1 GPa with pres-sure derivatives of 6.5 cm−1/GPa (Table 1). Meanwhile,the higher frequency peak at 541 cm−1 was assigned tothe T(OH) motions of A1g + Eg and F2g symmetries,whose frequencies increase with pressure derivatives of4.4 and 4.9 cm−1/GPa, respectively (Table 1). These pres-sure coefficients in the lattice mode region consequentlylead to the isothermal mode Grüneisen parameters vary-ing from 0.49 to 1.11 (Table 1).

Candidate structure of high–pressure phase of katoite

Katoite, Ca3Al2(H4O4)3, crystallizes in cubic space groupIa�3d at ambient conditions (Lager et al., 2002). Subgroupsof the cubic Ia�3d phase (point group Oh) include the fol-lowing five space groups: cubic I�43d (point group Td),cubic I4132 (point group O), cubic Ia�3 (point group Th),tetragonal I41/acd (point group D4h), and rhombohedralR�3c (point group D3d). The correlating relationships ofirreducible representations between cubic Ia�3d phase

and its subgroup are shown in Table 2. In the space groupIa�3d, the Ca and Al cations are located at the special crys-tallographic positions of the 24c and 16a Wyckoff posi-tions, respectively. The O and H atoms are on the generalpositions of 96hWyckoff positions. The theoretical factorgroup analysis for katoite in the cubic space group Ia�3dand the subgroups is summarized in Table 3. The factorgroup analysis theoretically indicates that 39 Raman–ac-tive vibrational modes (6A1g + 13Eg + 20F2g) are present-ed in katoite (Table 3). By the symmetry reductions fromcubic Ia�3d to cubic I�43d, cubic I4132, and cubic Ia�3, theRaman–active modes increase to 85 (14A1 + 28E + 43F2),84 (13A1 + 28E + 43F2), and 80 (13Ag + 26Eg + 41Fg),respectively. By the symmetry reduction to tetragonalI41/acd and rhombohedral R�3c, the Raman–active modesincrease to 100 (19A1g + 20B1g + 20B2g + 41Eg) and 80(26A1g + 54Eg), respectively. The results of factor groupanalysis imply that the number of Raman bands is remark-ably increased by the phase transition from the cubic Ia�3dto the subgroups. In the present study, a new Raman bandseems to emerge at 440 cm−1 at 6.3 GPa, and its intensitygradually increases with pressure (Fig. 3). The Ramanband is apparently different from that observed at 450cm−1 at 1.3 GPa. Lager et al. (2002) observed some weakreflections of the type hk0 with h, k ≠ 2n, indicating loss ofthe a–glide above 5.38 GPa. To our knowledge, their argu-ment that katoite transforms from Ia�3d to I�43d is onlybased on the observation of these reflections. The lossof the a–glide operation in the cubic Ia�3d leads to the threelower symmetry space groups, I�43d, I4132, and R�3c. Theyare distinguishable by the presence of additional reflectionconditions, but indistinguishable by Raman spectroscopicstudy. In the present study, these three, cubic I�43d (Td),cubic I4132 (O), and rhombohedral R�3c (D3d) can beconsequently considered as a candidate structure of thehigh–pressure phase of katoite.

ACKNOWLEDGMENTS

The authors must acknowledge and appreciate two anon-ymous reviewers for their constructive comments and in-

Table 1. Pressure dependence of Raman frequencies of katoite

These constants are fitted to the equation of the line νP = νi0 + βPand γiT = KT (∂lnνi/∂P)T. R2 is the correlation coefficient. Grünesienparameter γiTwas calculated with isothermal bulk modulus of KT =58(1) GPa (Lager et al., 2002).

Table 2. Correlation of irreducible representations betweenspace group Ia�3d and its subgroups

High–pressure Raman spectroscopy of katoite 23

Table 3. Wyckoff notations and irreducible representations for the atoms in katoite, space group Ia�3d, and possible candidates for the high–pressure phase

M. Kato and A. Kyono24

sightful suggestions to improve the manuscript. The au-thors also express thanks to T. Tsunogae and Y. Saito forkindly giving a use of the Raman spectrometer. The workwas partially supported by a Grant–in–Aid for ScientificResearch (C) from the Japan Society for the Promotion ofScience (project no. JP26400511).

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Manuscript received May 30, 2018Manuscript accepted December 16, 2018Published online February 14, 2019Manuscript handled by Keiji Shinoda

High–pressure Raman spectroscopy of katoite 25