A theoretical investigation on pressure(cid:1)induced changes in the vibrational spectrum of zeolite bikitaite

In this paper, the vibrational spectra of the natural zeolite bikitaite obtained from ab initio MD simulations are discussed. The calculated spectra, in line with experimental IR and Raman spectra of other zeolitic systems, predict that applied pressure significantly affects the O-H and T-O vibrational frequencies. The observed broadening of the OH stretching band is attributed to pressure induced changes in the host-guest and guest-guest hydrogen bond interactions.


INTRODUCTION
Con nement of materials in ordered matrices is currently of primary interest for applied research. It is known that a low-dimensional system shows chemical and electrooptical properties which are, in general, signi cantly di erent from the ones of the corresponding bulk material and can beof relevant technological interest for the tailoring of new kinds of materials 1]. Zeolitic frameworks hosting low-dimensional systems may be taken as useful models to study the factors governing at atomic level the stability and the properties of con ned materials 2].
One of these interesting model structures has been found in the natural zeolite bikitaite (Li 2 Al 2 Si 4 O 12 ] 2H 2 O) 3,4]. This is a rare lithium zeolite characterized by a high framework density a n d b y a monodimensional system of channels in which w ater molecules and Li cations are hosted. It is composed by sheets of 6-memberedTO 4 rings laying in the ab plane and connected to each other by pyroxene-chains of tetrahedra. Non-crossing channels, whose section in the ac plane is an eight membered ring of tetrahedra, run parallel to the b direction ( Figure 1a). Each extraframework Li + cation is tetrahedrally coordinated to three framework oxygens and one water oxygen. Experimental and theoretical investigations on bikitaite at ambient conditions proved that water molecules form one dimensional chains held together by hydrogen bonds(HB), whereas no water-framework HB exist 5,6]. Such a peculiar one-dimensional water chain, that runs parallel to the channel direction, has been found to date only in another high density lithium zeolite characterized by the same one-dimensional 8-ring channels, the synthetic Li- ABW 7,8]. Figure 1: Ball-and-stick representation of the bikitaite structure projected on the ac plane, at ambient pressure (a) and at 9.0 GPa (b). Black spheres represent H atoms, light g r e y spheres Si atoms, grey spheres O atoms and dark grey spheres Al and Li atoms.
Theoretical studies on bikitaite 6] allowed to explain the stability of the guest \ oating" water chain on the basis of long-range electrostatic host-guest interactions. At difference from bikitaite, water molecules in Li-ABW are hydrogen bonded to each other as well as to framework oxygens, thus indicating as additional stabilization factor the presence of short-range interactions 9]. Both experimental and simulated vibrational spectra account for the di erent dynamical properties of water in these two related zeolites and have allowed to assess the leading role of weak interactions (such as hydrogen bonds) in in uencing its vibrational behaviour 9,10].
In this respect, it would be of interest to study these water chains under di erent conditions with respect to the ambient ones. In particular, we are interested to investigate how the stretching and bending frequencies of water molecules in bikitaite are a ected by high pressure (HP).
Little is known about the behaviour of zeolites under hydrostatic pressure. This is partly due to the di culty in nding non-penetrating pressure transmitting media that behave hydrostatically over a wide range of pressure. Moreover, crystal structure re nements are often prevented by a numberof factors that cope to decrease resolution, thus allowing only the experimental determination of cell parameters. This problem is particularly severe for X-ray powder di raction experiments 11,12].
The usefulness of techniques that can integrate the experimental data with atomicscale information is therefore clearly evident. In this context, the use of ab-initio methods may lead to signi cant progress in this largely unexplored area, providing additional information not accessible through experiment. A c o m bined theoretical-experimental approach has already been successfully applied to the study of zeolites scolecite 11] and bikitaite 12] under HP. As far as bikitaite is concerned, both experiment and calculations have proved that this zeolite is remarkably stable under HP, as no amorphization or pressure induced phase transition has been observed up to 10 GPa 12].

DETAILS OF THE CALCULATIONS
Simulations on bikitaite have been performed using the Car Parrinello ab initio molecular dynamics method (CPMD) 13], which has proved to provide a satisfactory description of many condensed-phase systems, including zeolites. The method allows one to obtain reliable information at microscopic level on both static and dynamical properties. In particular, simulated IR spectra satisfactorily reproduce shifts in the O-H stretching and bending frequencies resulting from changes in the chemical environment 6]. Moreover, they allow to single out the contributions of distinct modes or groups of atoms to the total spectra, leading to an increased resolution and providing additional information not accessible through experiment.
We report here only the technical details adopted in the simulation runs. For a more detailed description of this methodology, t h e reader is referred to Ref. 14].
Two constant volume CPMD 15] simulations of bikitaite were performed using the experimentally determined cell parameters at the pressures of 5.7 GPa and 9.0 GPa, reported in Ref. 12]. A periodically repeated triclinic supercell containing two crystallographic unit cells along the b direction was adopted. We used the same plane-waves cuto , density functional approximations, pseudopotentials and MD simulation parameters (i.e. integration time step, ctitious mass) adopted in previous theoretical studies on bikitaite at ambient conditions ( 5,6]). After equilibration, the time evolution of the system was followed for 5.0 ps for both simulations.
Simulated vibrational spectra were calculated by F ourier transforming the velocity autocorrelation function obtained from the MD trajectory, and compared with the ambient pressure ones reported in Ref. 5].

RESULTS AND DISCUSSION
In zeolites, strong covalent T-O bonds with a partial ionic character are present, therefore long range electrostatic forces play a dominant role. On the other hand, the presence of short-range forces arising from HB can also beof signi cant relevance 16]. Hydrogen is incorporated in zeolites mainly in the form of guest water molecules, which are in most cases hydrogen bonded to framework oxygens and extra-framework cations. Information on the strength of these bondsare generally drawn from vibrational frequencies of O-H covalent bonds 17]. As far as HP conditions are concerned, studies on a series of minerals have evidenced a decrease in the OH stretching frequency, which is attributed to an increase of hydrogen bonding character 16,17]. In fact, the presence of an HB results in a broadening of the potential energy curve of the covalent OH bond and a decrease of spacing between vibrational levels. As an e ect, the stretching frequency decreases with the strengthening of the O...H interaction. However, this simpli ed picture is inadequate for three centered hydrogen bonds 16], which are often found in minerals at HP. V olume shrinking due to applied pressure can in fact bring two oxygens close enough to a guest water proton to allow formation of two w eak HB's. Presence of bifurcated hydrogen bonds may signi cantly a ect O-H stretching frequencies.
Before discussing our simulated bikitaite spectra, let us brie y summarize the results of IR and Raman studies on zeolites under HP. V elde and Besson 18] found that the OH stretching band in analcime was splitted in two components with increasing pressure and attributed such change to a pressure-induced deformation of the framework, resulting in two di erent HB lengths. Huang 19] reported a red shift of the three O-H stretching peaks in zeolite Y for pressures lower than 1.9 GPa, while for higher pressure the three bands collapsed to a broad, unresolved pro le. Also the water bending modes are in uenced by pressure, and such changes are attributed to the increased strength of hydrogen bonds. On the whole, these experimental data on vibrational behaviour of zeolites under HP [19][20][21][22] seem to support the idea that tetrahedral units are indeed not rigid. Rather, the T-O bonds slightly contract as a response to external pressure, thus resulting in a decrease of the tetrahedral volume. The results of our calculations on bikitaite 12] are in agreement with such description, predicting a negative change in the SiO 4 and AlO 4 volumes of the order of 2% and 3% respectively at a pressure of 9.0 GPa.
The simulated bikitaite spectra at 9.0 and 5.7 GPa are compared with the ambient pressure one in Figure 2. It is to point o u t that the simulated bikitaite spectra at 1 atm were found to bein good qualitative agreement with the micro-IR one reported in Ref. 5]. Underestimation of the absolute values of the stretching frequencies is a well-known artifact due to the use of ctitious masses in CP simulations, and prevents quantitative agreement with the experiment. On the other hand, relative frequencies calculated with respect to a reference state (for instance, an isolated water molecule in the gas phase) have proved to well compare with the corresponding experimental frequency shifts.
We rst examine the region typical of framework modes, i.e. between 200 and 1200 cm ;1 , focusing our attention on the three bands at highest frequency (i.e. 980, 880 and 660 cm ;1 at ambient pressure) commonly attributed to the T-O stretching modes 19,20]. Remarkably, they undergo a signi cant blue shift at 5.6 GPa, passing to 1000, 920 and 740 cm ;1 respectively, while the frequency shift from 5.7 to 9.0 GPa is lower (1020, 920 and 740 cm ;1 ). On the whole, these data indicate that pressure-induced strenghtening of the T-O bonds occurin bikitaite, in line with the ndings of recent HP IR and Raman studies on other zeolites 19,20] and with the calculated shortening of T-O distances at HP in bikitaite 12].
In general, the HP-induced volume contraction in silicates is attributed to three mechanisms. The rst one involves the rotation of rigid TO 4 , t h e second one the distortion of intra-tetrahedral O-T-O angles, while the third and less signi cant one, implies a decrease of the T-O bond lenght 2 3 ] . Since the less energetically costly mechanism is the rotation of rigid structural units 24], current models describing the HP behaviour of silicates normally treat the e ects of the third mechanism as negligible. Our calculations, together with the above quoted experimental data [19][20][21][22], indicate that shortening of TO bonds in zeolites under HP seems to be more e ective than what predicted by such models.
At a m bient pressure, the OH stretching band is broadened in the region betweeen 2900 and 3400 cm ;1 . The band pro le is structured, in agreement with the experimental one, owing to the presence of four crystallographically di erent protons experiencing di erent interactions with their environment. At HP, the band still shows distinct peaks but is signi cantly broader than at 1 atm. The full width at half maximum, 300 cm ;1 at 1 atm, increases to 410 cm ;1 at 5.7 GPa and to 560 cm ;1 at 9.0 GPa. Moreover, the band is broadened in the low frequency region, indicating that the compression leads to stronger perturbing e ects of HB to the H 2 O vibrational modes. This may be due to di erent  Figure 3: Contributions of the four crystallographically di erent OH bonds to the vibrational spectra at 1 atm, 5.7 GPa a n d 9.0 GPa.
mechanisms, as at HP both a strenghtening and an increase in the number of HB are observed.
In bikitaite, the a and c cell parameters decrease under compression and two framework oxygens come close enough to a water hydrogen to form bifurcated hydrogen bonds. In addition, the decrease of the b parameter is accompanied by a shortening of the O-O separation between water molecules and by a consequent strengthening of the inter-water hydrogen bonds already present at ambient pressure.
In order to have a better understanding of the role played by these mechanisms, we calculated the distinct contributions of the four hydrogens to the total spectra ( Figure   Stud 3). W1 and W2 represent the two water molecules in the unit cell. In each molecule, a proton (H a ) is hydrogen bonded to the adjacent molecule in the chain, while the other one (H b ) points towards framework oxygens and at ambient pressure is not involved in HB. At ambient pressure, the four stretching bands are centered at distinct frequencies.
OH bonds involving an hydrogen-bonded proton have lower stretching frequencies, in line with the previous discussion. The O-H a bands are also broader, as both O and H a are involved in strong HB with two adjacent water molecules. We also notice that the stretching frequencies of the two O-H bonds in the same molecule are rather close to each other in W1, and much more separated in W2. This strikingly di erent behaviour of the two water molecules could berationalized by considering the di erence between the two O-H bonddistances in each molecule, that in W2 is three times larger than in W1 (i.e. 0.018 A vs. 0.006 A) 12]. At 5.7 GPa, the H b atoms come at distances from O frame short enough to form hostguest HB. As an e ect, both the O-H b peaks are red-shifted. The frequency shift is slightly more pronounced for W1, which at 5.7 GPa f o r m s a s i g n i c a n tly stronger waterframework HB (the calculated H b ..O frame distances are 1.885 and 2.081 A for W1 and W2 respectively) 12]. Moreover, hydrogen bonds between water molecules, already present at 1 atm, are signi cantly shorter at 5.7 GPa (from 1.886 to 1.714 A) 12]. This leads to a red-shift of the O-H a stretching frequencies as well. The global result is a broadening of the O-H stretching band with respect to 1 atm ( Figure 2).
Remarkably, a further increase of the pressure, from 5.7 to 9.0 GPa, brings about a rather unexpected blue-shift of boththe O-H b stretching frequencies. We justify this result by considering that, even though at 9.0 GPa the H b atoms form a higher number of HB they are indeed weaker than those at 5.7 GPa. In fact, the MD simulation showed that at 9.0 GPa t wo and sometimes three framework oxygens are in competition to form HB's with H b . As a consequence, water-framework HB's involving di erent framework oxygens are continuously broken and formed, resulting on average, in a decrease of the hydrogen bonding strenght 12]. In particular, W1, that at 5.7 GPa was involved in a single and rather strong water-framework HB, at 9.0 GPa forms a three centered bond, characterized by signi cantly larger O frame -H b distances (2.163 and 2.079 A), therefore suggesting weaker HB interactions. The inter-water average HB distance is 1.719 A at 9.0 GPa, therefore the average H a -O stretching frequency undergoes no signi cant shift with respect to 5.7 GPa. On the whole, the above mentioned e ects result in a further broadening of the total stretching band at 9.0 GPa ( Figure 2).
Applied pressure also a ects OH bending frequencies. Figure 2 shows that at 5.7 GPa the bending band is shifted towards higher frequencies with respect to 1 atm, and it is also signi cantly broader. The opposite trend is observed in passing from 5.7 to 9.0 GPa: the bending peak is red-shifted and the band becomes narrower. Again, these e ects nd a microscopic explanation in the pressure-induced formation of new HB. As discussed above, the perturbing e ect of water-framework HB on the vibrational modes of water molecules is larger at 5.7 GPa, when a lower number of stronger hydrogen bonds are formed.
Work is in progress in order to perform experimental micro-IR spectra of bikitaite