In the last 15–20 years, studies on the behavior of both natural and synthetic microporous materials under high pressure (HP) have multiplied noticeably, providing not only important information on their elastic behavior and stability, but also opening new perspectives for technological applications. For instance, among the physical properties of microporous materials investigated under compression, worthy of mention are: the so called P-induced amorphization processes (PIA) (e.g., [1
]), the effect of pressure on the ionic conductivity (e.g., [7
]), the P-induced over-hydration (PIH) (e.g., [9
]) and the penetration of gas, like Ar, Xe, and CO2
]. High pressure experiments on porous materials have recently led to the synthesis of linear carbon based polymers in pure silica zeolites. Linear polymers like polyacetylene (PA), polyethylene (PE), and polycarbonyl (pCO) have been obtained from compression resulting in nanocomposite organic/inorganic materials, which are good candidates for developing highly directional semiconductors and high energy materials [18
The HP behavior of zeolites when compressed in non-penetrating fluids has recently been reviewed by Gatta and Lee (2014) [19
] and summarized in the following way: (i) microporosity does not necessarily imply high compressibility, in fact the range of compressibility is wide, with bulk modulus K0
ranging from ~15 to ~70 GPa; (ii) the flexibility observed in zeolites is based mainly on tetrahedra tilting; (iii) the deformation mechanisms are dictated by the framework topology; (v) the extraframework content (cations and water molecules) governs the compressibility level in isotypic structures.
Zeolites with GIS topology [20
] and GIS-like materials have been studied under both high temperature and high pressure, revealing widely variable degrees and mechanisms of deformation as a function of the non-ambient experimental conditions and the chemical composition of both the framework and extraframework. The study of gismondine dehydration [21
] showed that this framework is particularly flexible.
The HP behavior of a natural gismondine was studied using both “non-penetrating” (i.e., silicone oil, s.o.) [22
] and “penetrating” (methanol:ethanol:water = 16:3:1, m.e.w.) [11
] pressure-transmitting media (PTM). In the latter case, a PIH effect was observed at a very low P, inducing full occupation of originally partially occupied water sites. On the whole, both experiments revealed an unexpected low compressibility of gismondine, notwithstanding the high flexibility showed by this framework during dehydration and the similar framework deformation mechanisms [21
Lee et al. [23
] and Jang et al. [24
] studied the HP behavior of two synthetic phases with GIS topology, both compressed in penetrating media: a K-gallosilicate (K-GaSi-GIS) and a K-aluminogermanate (K-AlGe-GIS), respectively. These studies highlighted a very different response to hydrostatic pressure in materials sharing the same GIS topology, but with considerably different framework and extraframework compositions.
Two microporous mixed octahedral-pentahedral-tetrahedral (OPT; [25
]) framework silicates, structurally related to the GIS topology, were studied under HP [26
]: cavansite and pentagonite, the orthorhombic dimorphs of Ca(VO)(Si4
O. When compressed in m.e.w., these two phases exhibit rather different behaviors: pentagonite undergoes PIH, thanks to the crucial role of the seven-fold coordinated Ca, suitable for accepting an additional H2
O molecule. In contrast, in cavansite the eight-fold coordinated Ca cations do not allow further water penetration and thus PIH is not observed. The higher compressibility in s.o. of cavansite compared to gismondine is attributed to the presence of VO5
pyramids connecting the tetrahedral layers of the vanadosilicate.
This paper presents a study, performed by in-situ synchrotron X-ray Powder Diffraction (XRPD), of the HP stability and behavior of the natural zeolite amicite [K4Na4(Al8Si8O32)·10H2O], the GIS phase with ordered (Si, Al) and (Na, K) distribution. The investigation aimed in particular to understand: (i) the relationships between compressibility and framework/extraframework content; (ii) the influence of different PTM (penetrating 16:3.1 m.e.w. and non-penetrating s.o., respectively) on the compressibility and HP deformation mechanisms of this zeolite.
2. Amicite Structure
Amicite [ideal formula K4
O] is a rare natural zeolite, classified as the ordered K, Na member of the gismondine group [27
]. The sample used for this study is from the type locality (Höwenegg in Hegau, southern West Germany)—where amicite was discovered associated with merlinoite in a basaltic rock—and is the same studied by Alberti and Vezzalini [28
] (chemical formula: K3.75
O). Its GIS framework topology is shared by the other natural zeolites gismondine, garronite, gobbinsite, and by several other synthetic phases. Amicite structure [28
] was determined in the monoclinic I
2 s.g. The cell parameters are a
= 10.226(1), b
= 10.422(1), c
= 9.884(1) Å, β = 88° 19(1). The framework can be described as intersecting ribbons of 4-membered rings of tetrahedra (defined as double-crankshaft chains) running in the a
directions (Figure 1
and Figure 2
), laterally linked to form two sets of channels delimited by 8-membered rings running parallel to  and . The ordered distribution of Si and Al in the tetrahedra, and of Na and K in the channels, induces a lowering in symmetry from the topological I
space group to the real one I
2. Na and K are distributed in two different and fully occupied sites, with the water molecules in four sites, three of which are fully occupied (W1, W2, W3). Na is coordinated to three framework oxygen atoms and to all the water molecules, while K is coordinated to four framework oxygen atoms and the three fully occupied water sites.
3. Experimental Methods
In-situ HP XRPD experiments were performed at the SNBL1 (BM01a) beamline at ESRF, using an ETHZ modified Merril-Basset diamond anvil cell (DAC) [29
] with flat culets of 600 µm in diameter. Powders were loaded into a pre-indented gasket hole (i.e., a stainless steel foil of 60–80 µm thickness) with 250 µm diameter. The experiments were performed using two different PTM: m.e.w. as nominally penetrating, and s.o. as non-penetrating media, respectively. Pressure was measured before and after data collection at each pressure using the ruby fluorescence method [30
] on the non-linear hydrostatic pressure scale [31
]. The diffraction data were collected at a wavelength of 0.6825 Å in the Debye–Scherrer geometry on an area detector. One-dimensional diffraction patterns were obtained by integrating the two dimensional images with the program FIT2D [32
Amicite was compressed up to 8.13(5) GPa in m.e.w. and 8.68(5) GPa in s.o. In the latter case a partial loss of the hydrostatic conditions above 2.8 GPa was observed. In both experiments about 20 images were collected at increasing pressure values. Moreover, some patterns (labeled (rev) in Tables and Figures) were collected upon decompression down to ambient conditions. Figure 3
a,b reports selected integrated patterns obtained in m.e.w. and s.o., respectively.
The structural refinements of the data collected in m.e.w. converged successfully up to 4.71(5) GPa. At higher pressure (up to 6.9 GPa) the refinements were still possible, but some framework bond distances and angles produced unreliable values. As a consequence, above 4.71(5) GPa, only the unit-cell parameters were refined by the Rietveld method in the 2°–40° 2θ range.
For amicite in s.o., the low data quality did not allow complete structural refinements. The cell parameters were refined successfully up to 5.48(5) GPa, notwithstanding the previously cited hydrostaticity loss observed above 2.8 GPa.
Rietveld profile fitting was performed using the GSAS package [33
] with the EXPGUI [34
] interface. The initial structural model is as reported in [28
]. The background curve was fitted by a Chebyshev polynomial with 20 coefficients. The pseudo-Voigt profile function proposed by [35
] was applied, and the peak intensity cut-off was set to 0.1% of the peak maximum. Soft-restraints were applied to the T–O distances [Si–O = 1.58(2) − 1.62(2); Al–O = 1.72(2) − 1.74(2)] and their weights were gradually decreased after the initial stages of refinement (up to F = 1 in GSAS terminology). The isotropic displacement parameters were constrained in the following way: the same value for all the tetrahedral cations, a second value for all the framework oxygen atoms, a third value for the extraframework cations, and a fourth value for the water molecule oxygen atoms. The unit-cell parameters were allowed to vary in all the refinement cycles. Details of the structural refinements are reported in Table 1
5. Comparison between Amicite Compressibility in Aqueous Medium and Silicone Oil
The main difference between amicite HP behavior in m.e.w. and s.o. is the higher compressibility in the aqueous medium (see Figure 4
and Table 2
). This is clear by comparing the unit cell volume decrease in the two PTM at similar pressure values: 5.1% at 5.43 GPa in s.o., and 5.9% at 5.35 GPa in m.e.w. This effect is anomalous compared to what is generally observed for zeolites, when water penetration provides a support against the effects of pressure (e.g., see a review in Table 4
]. Although detailed structural data for the ramp in s.o. are lacking, this result can be ascribed to formation, during compression in m.e.w., of rather strong bonds between the additional water molecules and the framework oxygen atoms, which contribute to the shrinkage of the a
parameter. In particular, the distance O3–O6, which is parallel to the a
axis and corresponds to the shortest diameter of the 8-ring perpendicular to c
, undergoes a 10% reduction passing from 6.20 Å at Pamb
to 5.58 Å at 4.71 GPa (see Figure 1
6. Compressibility Behavior of Microporous Materials with GIS Topology
A number of microporous materials with GIS topology have been investigated under HP. Among the natural zeolites, amicite, the K-Na member of the GIS family, can be compared to gismondine [11
], the Ca member, producing the following observations:
Compression of gismondine in both m.e.w. and s.o. favors the tetragonalization of the unit cell; in amicite the a and c axes also tend to become more similar at HP, but the beta angle does not substantially change;
Gismondine compressed in m.e.w. undergoes a transition to a triclinic phase at about 3 GPa; the original symmetry of amicite, by contrast, is maintained in both the experiments;
The HP framework deformation mechanism is the same in the two zeolites, essentially being driven by the distortion of the “double crankshaft” chains and the consequent change in the 8-ring channel shape;
Amicite’s compressibility increases at HP both in m.e.w. and s.o.; by contrast, gismondine’s compressibility in s.o. slightly decreases while in m.e.w. it remains constant;
PIH occurs in both amicite and in gismondine compressed in m.e.w. However, it induces different reorganizations in the water molecule systems: in amicite there is both the filling of partially occupied sites and the appearance of two new water sites; in gismondine four partially occupied water sites reduce to only two fully occupied sites, giving rise to a more ordered water system;
In amicite 5.34 water molecules enter the zeolite porosities when compressed in m.e.w., while in gismondine only one additional molecule penetrates. This result can be explained by the higher channel stuffing of gismondine at Pamb compared to amicite;
Both amicite and gismondine are more compressible in m.e.w. than in s.o., but for different reasons. In gismondine this effect has been justified by the re-organization of the water molecule system, which leaves a larger free volume inside the pores compared to the phase compressed in s.o. In amicite the higher compressibility at HP results from the strong bonds between framework oxygen atoms and the new water molecules;
Overall, gismondine is more compressible than amicite, both in m.e.w. and in s.o. Comparing the unit cell volume decrease of the two phases at a similar pressure value—about 5.5 GPa—we find ΔV = −7.5% and −6.4% for gismondine in m.e.w. and s.o., respectively, while for amicite these values are −5.9% and −5.1%, respectively. The presence of the large potassium cations and the higher number of extraframework sites after PIH in amicite compared to gismondine probably contribute to better supporting the amicite structure.
Lee and co-workers [23
] studied the compressibility in m.e.w. of the potassium gallo silicate K-GaSi-GIS with GIS framework type (ideal formula K5.76
O, s.g. I
). When the results of this study are compared with those obtained from the natural phases, the following observations can be made:
The main feature of the P-induced evolution of cell parameters of K-GaSi-GIS is the noticeable squashing of the c axis, which is perpendicular to the dense plane and corresponds to the b axis of gismondine and amicite. This response to hydrostatic pressure corresponds to a gradual flattening of the double crankshaft chains and a reduction in the ellipticity of the 8-ring windows. The different behavior compared to amicite and gismondine, where the b axis slightly increases or remains almost unvaried, could be explained by the lower channel stuffing of the K-GaSi-GIS phase related to the high Si/Ga ratio;
In K-GaSi-GIS a PIH effect is again observed, with the penetration of about two water molecules at P < 1 GPa, but in this case the overhydration induces a disordering of the K-water system along the channels.
The potassium alumino germanate K-AlGe-GIS with GIS topology (ideal formula K8
O, s.g. I
) was studied under HP by Jang et al. [24
]. Its structure is similar to amicite for the ordered distribution of the tetrahedral cations and the same number of extraframework cations. However, there are eight instead of 10 water molecules in the synthetic phase. The variation in the unit cell parameters was determined in m.e.w. up to 3.22 GPa, but no structural refinements were reported, so no hypotheses were made concerning a possible PIH. The compressibility is anisotropic with a decrease in the a
axes, parallel to the channels, of 1.3% and 1.0%, respectively, while the b
parameter, perpendicular to the channels and the double crankshaft chains, decreased by 5.4% resulting in an almost linear volume contraction of 7.5%. The large b
variation is strictly related to a flattening of the double crankshaft chains under P, as already observed for K-GaSi-GIS. Again in this case, the different compressibility behavior compared to amicite and gismondine can be explained by the lower water content and consequent channel stuffing.
The high-pressure behavior of amicite, a GIS framework type zeolite, was investigated and the strong influence on compressibility of the chemical composition of both the framework and extraframework species was also confirmed for this variety. In particular, the study confirms that the compressibility of microporous materials is not simply related to their framework density and topology, but is also greatly affected by the type, amount, and location of the extra-framework species.
The HP framework deformation mechanism is the same in all the phases with GIS topology and is essentially driven by the distortion of the “double crankshaft” chains and the consequent shape change of the 8-ring channels. However, the degree of compressibility varies due to the different chemical compositions. In these zeolites the pressure-induced penetration of water molecules does not induce a unit cell volume expansion, and in the natural phases, when the structure after P release was determined, the overhydration effects are reversible following the return to ambient conditions.