Crystallization of LiAlSiO4 Glass in Hydrothermal Environments at

Aug 2, 2016 - At pressures of 0.25–2 GPa metastable zeolite Li-ABW and stable α-eucryptite are obtained at low and high temperatures, respectively,...
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Crystallization of LiAlSiO4 Glass in Hydrothermal Environments at Gigapascal Pressures−Dense Hydrous Aluminosilicates Kristina Spektor,†,§ Andreas Fischer,‡ and Ulrich Haü ssermann*,† †

Department of Materials and Environmental Chemistry, Stockholm University, SE-10691 Stockholm, Sweden Department of Physics, Augsburg University, D-86135 Augsburg, Germany

‡

S Supporting Information *

ABSTRACT: High-pressure hydrothermal environments can drastically reduce the kinetic constraints of phase transitions and afford high-pressure modifications of oxides at comparatively low temperatures. Under certain circumstances such environments allow access to kinetically favored phases, including hydrous ones with water incorporated as hydroxyl. We studied the crystallization of glass in the presence of a large excess of water in the pressure range of 0.25− 10 GPa and at temperatures from 200 to 600 °C. The p and T quenched samples were analyzed by powder X-ray diffraction, scanning electron microscopy, and IR spectroscopy. At pressures of 0.25−2 GPa metastable zeolite Li-ABW and stable α-eucryptite are obtained at low and high temperatures, respectively, with crystal structures based on tetrahedrally coordinated Al and Si atoms. At 5 GPa a new, hydrous phase of LiAlSiO4, LiAlSiO3(OH)2 = LiAlSiO4·H2O, is produced. Its crystal structure was characterized from single-crystal X-ray diffraction data (space group P21/c, a = 9.547(3) Å, b = 14.461(5) Å, c = 5.062(2) Å, β = 104.36(1)°). The monoclinic structure resembles that of α-spodumene (LiAlSi2O6) and constitutes alternating layers of chains of corner-condensed SiO4 tetrahedra and chains of edge-sharing AlO6 octahedra. OH groups are part of the octahedral Al coordination and extend into channels provided within the SiO4 tetrahedron chain layers. At 10 GPa another hydrous phase of LiAlSiO4 with presently unknown structure is produced. The formation of hydrous forms of LiAlSiO4 shows the potential of hydrothermal environments at gigapascal pressures for creating truly new materials. In this particular case it indicates the possibility of generally accessing pyroxene-type aluminosilicates with crystallographic amounts of hydroxyl incorporated. This could also have implications to geosciences by representing a mechanism of water storage and transport in the depths of the Earth.

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INTRODUCTION Hydrothermal conditions are widely applied in materials synthesis, where the hugely modified chemical reactivity of usually insoluble reagents is exploited.1 Reactants (typically metal salts administered in the form of colloids, gels, suspensions, or simply as bulk materials) go into solution as complexes, and products (typically oxides) are crystallized directly from aqueous solution under elevated pressure and temperature conditions. Hydrothermal conditions provide excellent possibilities for synthesizing advanced ceramic materials whether they are bulk single crystals, fine particles, or nanoparticles.2−4 Hydrothermal processing is normally performed at subcritical conditions (that is, at p and T conditions below the critical point of water (T = 374 °C and p = 22.1 MPa)). However, water can exist at very high pressures and temperatures, and it is interesting to imagine the extension of hydrothermal processing to extreme conditions, involving gigapascal pressures and temperatures up to 1000 °C. Some of water’s physicochemical properties will be drastically changed. The melting point increases from room temperature at 1 GPa to ∼75, ∼250, and ∼425 °C at 2, 5, and 10 GPa, respectively.5 The autoionization © 2016 American Chemical Society

constant, Kw, increases vastly with increasing p,T and attains values as high as 1 × 10−3 at 10 GPa and 730 °C (translating to a pH of 1.5).6 The dielectric constant ε0, which is drastically reduced at the critical point, changes comparatively little upon further varying p and T of the supercritical fluid. Recent molecular dynamics simulations predict a modest increase of ε0 from 10 to ∼30 when increasing pressure from 1 to 10 GPa at 730 °C.7 These values are similar to those of dipolar solvents, such as acetone. Also, the viscosity of extreme wateran important parameter for mass transportappears to remain comparatively low (below 3 mPa·s).8 Because of water’s changed properties one can expect large changes in thermodynamic relations that are key for hydrothermal processing (e.g., solubilities, mixing behaviors, surface-fluid interactions) as well as for rates of reactions and phase transitions (e.g., through modified (hydroxylated) surfaces). In this respect very little is known, and the potential of hydrothermal environments at pressures approaching 10 GPa Received: May 13, 2016 Published: August 2, 2016 8048

DOI: 10.1021/acs.inorgchem.6b01181 Inorg. Chem. 2016, 55, 8048−8058

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Inorganic Chemistry (i.e., conditions that would be also accessible in industrial settings) for creating new materials is virtually unexplored. There have been a few reports on the application of gigapascal pressures for hydrothermal crystal growth, notably by Lityagina, Bendelani, Dyuzheva et al., who investigated the growth of single crystals of the high-pressure oxide phases αPbO2, TiO2−II,9 ε-FeOOH,10 β-GaOOH,11 SiO2-coesite,12 and SiO2-stishovite13 by performing slow cooling experiments of water−oxide mixtures. Later Shatskiy et al. elaborated on the crystallization of stishovite from silica−water solutions at 11 GPa by using considerably higher temperatures than Lityagina et al. (up to 1600 °C), which afforded millimeter-sized single crystals.14 Rutile-structured stishovite is one of the hardest oxide materials and the prototype for phases with sixfold (octahedrally) coordinated Si. Interestingly, hydrothermal treatment of silica glass SiO2 near 10 GPa at low temperatures (350−550 °C) produces hydrous stishovite.15 In this form of stishovite up to 5% of Si are replaced by four protons, Si1−xH4xO2, which represents a major chemical functionalization of this hard material. The mechanism behind the formation of hydrous stishovite is not clear, but it appears that water is incorporated at the same time coesite with tetrahedrally coordinated Si transforms to denser stishovite (“transformation-reaction”). Thus, extreme water acts simultaneously as a catalyst (in the coesite−stishovite transition) and reactant (by being incorporated in stishovite). Remarkably, the formation of hydrous stishovite is not bound to a liquid water environment but also proceeds with appreciable rates in “hot” ice at temperatures as low as 350 °C at 10 GPa.15 The finding of hydrous stishovite raises expectations into the discovery of new forms of silicates and aluminosilicates by exploiting extreme water’s potential of catalyzing phase transitions (and thus allowing access to kinetically favored high-pressure phases) as well as acting as reactant in the formation of hydrous phases. Here we report on the hydrothermal treatment of LiAlSiO4 glass at pressures up to 10 GPa. Similar to SiO2, LiAlSiO4 represents a well-investigated system with respect to structures and phase relations at ambient and high pressures. Also, LiAlSiO4 phases have interesting materials properties, relating to high ion mobility and near zero coefficients of thermal expansion.16,17

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Figure 1. Hydrothermal treatment of LiAlSiO4 glass at gigapascal pressures. (a) Pieces of LiAlSiO4 glass starting material before grinding, (b) photograph of a Pd−Au sample capsule used in multi anvil experiments before (top) and after a run (bottom), (c) sketch of the employed 18/12 MA cell assembly (see ref 18 for details). (Figure 1b). Capsules were sealed using a LaserStar 1900 laser welding station. The quality of seals was tested by heating capsules in an oven at 100 °C for several hours and subsequent weighing. For the piston− cylinder experiments, the capsules and the lids were annealed in a furnace at 900 °C for 1 h prior to loading the sample. Experiments at 5 and 10 GPa were conducted in a 6−8 Kawai-type multi anvil device using the large volume 18/12 assembly developed by Stoyanov et al.18 A sketch of this assembly is shown in Figure 1c. Capsules with the outer sides of the lids filled with BN powder were inserted into a boron nitride sleeve and subsequently positioned in a graphite resistance furnace. The samples were pressurized to the target pressure at 400 psi/h and afterward heated to the desired temperature (200, 400, or 600 °C) at a rate of ∼20 °C/min followed by 8 h dwelling time. The temperature was measured close to the sample using a type C thermocouple (W5%Re−W26%Re wire) in an Al2O3 4bore sleeve. The temperature gradient was estimated to be ∼20 °C along and 10 °C across the sample.18 After the dwell time, samples were quenched by turning off the power to the furnace (∼50 °C/s), and the pressure was released at a rate of ∼0.5 GPa/h. A nonend loaded piston−cylinder apparatus (Depths of the Earth Inc, Cave Creek, AZ) was employed for experiments in the 0.25−2 GPa pressure range. Runs at 0.25 GPa utilized a pressure plate with a 25 mm (1 in.) bore diameter, whereas larger pressures were reached with a 13 mm (1/2 in.) diameter plate. For all piston−cylinder experiments the standard assemblies from Depths of the Earth Inc. were used. The pressure medium corresponded to unfired pyrophyllite, which was tightly packed around the sample capsules. Samples were heated with a graphite resistance furnace. The temperature was measured with a type C thermocouple. The samples were pressurized at a rate of ∼100 psi/min. When reaching ∼85% of the target pressure, heating was started, and the temperature continuously increased at a rate of 25 °C/min. At this point the compression speed was adjusted in a way that both targeted pressure and temperature would be reached at approximately the same time. At target p and T conditions, samples were dwelled for 8 h. After the dwell time the samples were isobarically quenched and subsequently depressurized.

EXPERIMENTAL METHODS

Synthesis. LiAlSiO4 glass was prepared from a mixture of Li2CO3 (ChemPur, 99.995%), Al 2 O 3 (ChemPur, 99.999%), and SiO 2 (Umicore, 99.99%). The dried powders were finely ground in an agate mortar and thoroughly mixed with a molar ratio of Li2CO3/ Al2O3/2SiO2. The mixture was transferred to a Pt crucible and melted in a furnace using the following heating protocol: from 300 to 700 °C at a rate of 1200 °C/h, from 700 to 1000 °C at 100 °C/h, and from 1000 to 1600 °C at 300 °C/h. Finally, the molten mixture was dwelled for 1 h at 1600 °C and quenched by placing the crucible onto a copper table. The resulting LiAlSiO4 glass came out as a colorless single piece without visible opaque regions. The glass piece was shattered (Figure 1a), and a part of it was ground using tungsten carbide cubes. The powder X-ray diffraction pattern constituted a broad halo at 2θ ≈ 23− 24° (Cu Kα1), thus confirming a completely amorphous nature. The Al/Si molar ratio, as quantified by energy-dispersive X-ray spectroscopy, was 1:1.075 ± 0.077. Additional analysis using inductively coupled plasma atomic emission spectroscopy with a Varian VistaMPX instrument yielded a Li/Al molar ratio of ∼1:0.99. Ground LiAlSiO4 glass (55−60 mg) and 20−35 mg of Milli-Q purity water (type I water, with a stated resistivity 18.2 MΩ·cm at 25 °C) were loaded into cylindrical Au/Pd (80:20) capsules having 6.2 mm length, 5 mm outer diameter, and 0.125 mm wall thickness 8049

DOI: 10.1021/acs.inorgchem.6b01181 Inorg. Chem. 2016, 55, 8048−8058

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Inorganic Chemistry Table 1. Experimental Conditions and Synthesis Products

a

sample/run IDa

pressure (GPa)

temperature (°C)

products (fractions are estimates from PXRD patterns and SEM observations)

0.25−200 (PC) 0.25−400 (PC) 1−200 (PC) 1−400 (PC) 1−600 (PC) 2−200 (PC) 2−400 (PC) 2−600 (PC) 5−200 (MA) 5−400 (MA) 5−600 (MA) 10−400 (MA) 10−600 (MA)

0.25 0.25 1 1 1 2 2 2 5 5 5 10 10

200 400 200 400 600 200 400 600 200 400 600 400 600

Li-ABW (∼100%) Li-ABW (major), β-eucryptite (30%), α-eucryptite (minor), unknown (minor) Li-ABW (∼100%) Li-ABW (66%), α-eucryptite (33%) α-eucryptite (100%) Li-ABW (100% crystalline phase) + untransformed glass α-eucryptite (>95%) + cookeite (byproduct) α-eucryptite (>95%) + cookeite (byproduct) amorphous (LiAlSiO4 glass with modified surface) LiAlSiO3(OH)2 (∼100%) LiAlSiO3(OH)2 (major), α-spodumene (10−15%), diaspore (50%)

PC = piston cylinder, MA = multi anvil. Scanning Electron Microscopy Investigations. Scanning electron microscopy (SEM) imaging and energy-dispersive X-ray spectroscopy (EDS) was performed using a JEOL JSM 7000F microscope equipped with a Schottky-type field emission gun. For imaging, powder samples were dispersed over a sticky carbon tape mounted on an aluminum stub and partially coated with 10−15 nm gold layer to decrease the charging. An accelerating voltage of 2−3 kV and a probe current of less than 10 pA were applied, which also allowed to examine the uncoated areas of the samples. Gold-coating did not lead to any noticeable surface alterations. All LiAlSiO4 samples, except the glass starting material and 5−600 and 10−600 products, were prepared for EDS analysis by ion beam cross section polishing using a JEOL SM-09010 cross section polisher equipped with a Penning-type argon ion gun. LiAlSiO4 glass was prepared by mechanically polishing several pieces (3−5 mm across) embedded in plastic mounts. The mounts were coated with ∼15 nm carbon layer prior to the measurements. To collect EDS data from the 5−600 and 10−600 samples, which represented fine powders, a small amount ( 2 were used. The volume fraction of the minor domain was determined to be 44.7(4)%.

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RESULTS AND DISCUSSION Overview of Phases and Phase Relations of LiAlSiO4. Compositionally homogeneous LixAlxSi1−xO2 glasses can be easily synthesized over a broad range of x.27 Here x = 0.5, LiAlSiO4, was chosen. Known crystalline phases with this composition include a zeolite, Li-ABW, as well as the “dense” aluminosilicates α- and β-eucryptite. Figure 2 provides an 8050

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environment. Establishing the α−β equilibrium phase boundary required hydrothermal conditions.39 Dry LiAlSiO4 has been the subject of several high-pressure studies. With pressure, β-eucryptite undergoes progressive amorphization at room temperature above 4.5 GPa, and becomes completely amorphous above 17 GPa.40 It has been argued that the pressure-induced amorphization of β-eucryptite is associated with a large activation energy barrier for coordination number changes toward a new high-pressure phase. When heated at pressures of ∼2 GPa β-eucryptite transforms into dense α-eucryptite,41 which in turn undergoes phase segregation into spodumene LiAlSi2O6 and LiAlO2 when increasing pressure beyond 3 GPa.40 Roy et al. and Norby et al. showed the significance of hydrothermal conditions for establishing the phase relations of LiAlSiO4 at ambient and nearly ambient pressure. One may expect a similar significance when extending investigations into the gigapascal pressure range. An obvious question is whether a hydrothermal environment prevents phase segregation at higher pressures and makes high-pressure forms of LiAlSiO4 accessible. Hydrothermal Crystallization of LiAlSiO4 Glass. Mixtures of LiAlSiO4 glass and water with molar ratios between 1:1 and 1:2 were compressed to five different pressures, 0.25, 1, 2, 5, and 10 GPa and heated to a temperature between 200 and 600 °C at which the system was dwelled for 8 h prior to p and T quenching (cf. Table 1). The experiments employed a rather large proportion of water to ensure a constant activity in possible high-p and T processes. Because the melting point of ice-VII at 5 and 10 GPa is near 250 °C (530 K) and 425 °C (700 K), respectively,5 the water environment was expected to be solid for the experiments 5−200 and 10−400. Figures 3 and 4 show the evolution of products with temperature at the various pressures as obtained from SEM (morphology and composition) and PXRD analysis, respectively (see also Table 1). Figure 3a depicts the glass starting material, constituting irregularly shaped pieces with sizes of 50−500 μm. At 0.25 GPa and 200 °C LiAlSiO4 glass converted quantitatively into zeolite Li-ABW, which occurred as elongated prismatic crystals, reaching 10−15 μm in length (Figure 3b). At 400 °C a mixture of Li-ABW and β-eucryptite, along with a small amount of α-eucryptite, was obtained (Figure 4a). LiABW was also produced at 1 GPa and 200 and 400 °C and 2 GPa and 200 °C. The PXRD pattern of sample 2−200 has an amorphous background (Figure 4c), which indicates a substantial amount of untransformed glass because of a slower kinetics of the glass−zeolite conversion. SEM analysis (as detailed in the Supporting Information, Figure S1) revealed that in samples 2−200 glass particles are essentially preserved, and only their surface transformed to Li-ABW. Samples 1−400 represented a mixture of α-eucryptite and Li-ABW, whereas 1− 600, 2−400, and 2−600 corresponded to roentgenographically pure samples of α-eucryptite (Figures 3c and 4b,c). As described by Norby et al., at nearly ambient pressure and in an aqueous environment Li-ABW transforms at 350 °C to αeucryptite, which in turn transforms between 800 and 900 °C to β-eucryptite.34,36,39 The comparatively low thermal stability of Li-ABW is confirmed by our experiments, which in addition showed that the instability is further enhanced with increasing pressure. Somewhat surprising is the presence of β-eucryptite as a substantial fraction of the 0.25−400 sample, which suggests a direct formation from Li-ABW. β-eucryptite should not be stable at 400 °C. However, Norby et al. observed that βeucryptite forms as intermediate during the hydrothermal

Figure 2. Crystal structures of the common LiAlSiO4 polymorphs. They are all built from corner-connected tetrahedral LiO4, AlO4, and SiO4 units. Li atoms and LiO4 tetrahedra are depicted with gray color; AlO4 and SiO4 units are depicted with blue and yellow colors, respectively. (a) Orthorhombic zeolite Li-ABW (LiAlSiO4·H2O): the framework contains channels that run along the c direction and host water molecules (red ●). Li atoms coordinate to three framework O atoms and the water molecule. (b) Rhombohedral α-eucryptite: tetrahedral units are arranged as tubes (indicated with red ○) that run along the c direction with centers at (0,0), (1/3,2/3), and (2/3,1/3) (left). A perpendicular view of a tube is shown in the right-hand figure. (c) Hexagonal β-eucryptite: The structure corresponds to a variant of β-quartz with stuffing Li atoms located in the channels running along the c direction.

overview of their structures. They are all built from cornersharing AlO4 and SiO4 tetrahedra, and they all observe strict Si−Al ordering. Also Li+ ions are exclusively in a tetrahedral coordination. Hexagonal β-eucryptite corresponds to stuffed βquartz in which the open channels are filled with Li+ ions.28 The β-eucryptite typically forms when LiAlSiO4 glass crystallizes in dry conditions at ambient pressure. The orthorhombic zeolite Li-ABW (LiAlSiO4·H2O) forms during hydrothermal treatment of glass or gels (made, e.g., of LiOH, Al(OH)3, and SiO2) with Li2O·Al2O3·2SiO2 composition at nearly ambient pressure conditions and temperatures below 350 °C.29−32 Higher temperatures afford rhombohedral αeucryptite.29,30,32,33 Li-ABW is a useful precursor for other (metastable) LiAlSiO4 modifications upon thermal conversion. Water loss leads first to anhydrous Li-ABW (at ∼300 °C) with narrowed, elongated, eight-ring channels in which Li+ is four-coordinated by framework O atoms, and then (at ∼600 °C) to monoclinic, cristobalite-like, γ-LiAlSiO4. Finally (at 900−1000 °C) βeucryptite is obtained. 34,35 This last transformation is accompanied by a considerable increase in the molar volume (from 81.7 to 88.8 Å3). In contrast, when heating Li-ABW in an aqueous environment at temperatures above 350 °C αeucryptite is obtained (hydrothermal conversion).34,36 The αeucryptite represents the densest LiAlSiO4 structure (at ambient conditions) with a molar volume ∼79 Å3.37 The thermodynamic relations between LiAlSiO4 phases were investigated by Roy et al. and Norby et al.34,36,38 Anhydrous Li-ABW and γ-LiAlSiO4 are metastable. At ambient pressure αeucryptite is considered the thermodynamic ground state of LiAlSiO4, and β-eucryptite is a high-temperature form, stable at temperatures above 850 °C.38,39 Clearly, the β-to-α conversion is kinetically hindered but catalyzed in a hydrothermal 8051

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its considerably lower molar volume (∼79 vs ∼88.5 Å3 at ambient pressure). Our experiments indicate an onset of compositional instability of α-eucryptite at 2 GPa. Although barely noticeable in PXRD (Figure 4c), SEM analysis uncovered the presence of another phase, especially at 400 °C, which subsequently was identified as cookeite by a TEM study (see Supporting Information, Figure S1d). Cookeite is a layer-structured aluminosilicate with flexible compositions and Al/Si ratios smaller than 1.42 When increasing pressure to 5 GPa α-eucryptite is not stable anymore. At 200 °C the sample remained amorphous (note that water was solid at these conditions). The diffraction pattern of 5−400 (Figure 4d) revealed a new phase, which was afforded as fine, aggregated, needles with lengths up to 15 μm (Figure 3e). Remarkably, the EDS-probed Al/Si ratio was close to 1:1, indicating that the original glass composition was retained (see Supporting Information, Figure S1e). Crystals of the new phase, which was subsequently characterized as a monoclinic oxyhydroxide LiAlSiO3(OH)2 (see below), appeared considerably thicker and with prismatic shape in the 5− 600 sample (Figure 3f). At 600 °C an onset of phase segregation was noticed by identifying the byproducts αspodumene (LiAlSi2O6) and diaspore (α-AlOOH) in the powder pattern of 5−600. At 10 GPa the situation is changed again. The PXRD pattern of 10−400 shows that a crystalline product was obtained even in a solid water environment (Figure 4e). The pattern resembles that of LiAlSiO3(OH)2. SEM analysis, however, disclosed that only parts of the surface of the original glass particles had transformed (see Supporting Information, Figure S 1f). At 600 °C a mixture of LiAlSiO3(OH)2 and yet another phase with Al/Si ratio of 1:1 was obtained. We summarize this section with the p and T product map shown in Figure 5. We emphasize that this map does not attempt to sketch a phase diagram, as we did not ascertain whether mixtures were equilibrated after 8 h of dwelling. Specific results from the analysis of PXRD patterns are presented in Table 2. A complete compilation for all runs is given as Supporting Information, Table S1. H2O Incorporation into LiAlSiO4. IR spectroscopy is a powerful method to probe water in oxides, in particular, minerals.43 It allows the detection of low amounts of water (100 ppm level), and to discriminate forms of hydrated components (e.g., OH− and H2O). Figure 6 shows the IR spectra of the various products obtained from the hydrothermally assisted crystallization of LiAlSiO4 glass at high pressures. The glass itself displays broad bands with absorption maxima at 936 and 695 cm−1. The structure of the glass corresponds to a network of cornercondensed tetrahedra, and the bands have been previously assigned to T(Si,Al)-O stretches and Si−O−Al intertetrahedral bending modes (i.e., bending of T-O-T angles).44 The broad bands of the glass represent an envelope to the spectra of LiABW and α-eucryptite (samples 0.25−200 and 1−600, respectively) whose structures are also built from cornercondensed AlO4 and SiO4 tetrahedra. In the crystalline materials various T-O stretching and T-O-T bending modes are resolved as narrow bands. It is then clear that the product of 5−600 (i.e., monoclinic LiAlSiO3(OH)2) represents a structurally different material. T-O-T bending bands at ∼700 cm−1 are very weak, which indicates that the proportion of cornercondensed TO4 tetrahedra is considerably reduced. It is also clear that the experiments at 10 GPa yielded yet another

Figure 3. SEM images of the starting material, ground glass LiAlSiO4 (a) and various products (b−j). (b) Li-ABW crystals obtained at 0.25 GPa/200 °C. (c) α-Eucryptite crystals obtained at 1 GPa/600 °C. (d) α-Eucryptite crystal covered with cookeite platelets obtained at 2 GPa/ 600 °C. (e) Needle-shaped crystals of LiAlSiO3(OH)2 obtained 5 GPa/400 °C. (f) Mixture of prismatic-shaped crystals of LiAlSiO3(OH)2 and more needle-shaped crystals of α-spodumene obtained 5 GPa/600 °C. (g) Close-up of twinned LiAlSiO3(OH)2 crystals obtained 5 GPa/600 °C. (h) Beam-damaged LiAlSiO3(OH)2 crystals after EDS analysis. (i) Glass particle with a partially crystalline surface obtained 10 GPa/400 °C. (j) Crystalline product corresponding to a mixture of LiAlSiO3(OH)2 and another, not characterized, hydrous LiAlSiO4 phase obtained 10 GPa/600 °C. For more details, see Supporting Information, Figure S1).

conversion of Li-ABW to α-eucryptite.36 Therefore, we assume that the 0.25−400 run was not equilibrated and that obtained β-eucryptite represents a transient phase. α-Eucryptite is clearly favored over β-eucryptite at high pressures, which follows from 8052

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Figure 4. PXRD patterns of products obtained from an 8 h hydrothermal treatment of LiAlSiO4 glass at 0.25 (a), 1 (b), 2 (c), 5 (d), and 10 GPa (e). The arrows in (c) mark weak reflections from cookeite.

product, which is characterized by intense bands at 857 and 878 cm−1. Figure 6b depicts the spectral region of 2900−3800 cm−1. LiABW has two broad bands in this region with maxima at 3600 and 3426 cm−1, which is in good agreement with earlier reports.29,45 Those bands stem from O−H vibrations of the water molecules in the channels, which form a peculiar onedimensional chain, parallel to the channel direction. The broadening has been attributed to strong hydrogen bonding between water molecules.46 Spectra of samples corresponding to α-eucryptite are featureless in the 2900−3800 cm −1

wavenumber range (not shown). In contrast, the spectra of the samples 5−400 and 5−600 reveal sharp and intense bands at 3372, 3558, and 3640 cm−1, which manifests a hydrous nature. Different to Li-ABW, these sharp bands suggest distinct O−H stretches from structural hydroxy groups. Monoclinic LiAlSiO3(OH)2 obtained at 5 GPa is also present in the 10− 600 sample. We assign the bands at 3373 and 3641 cm−1 in the spectrum of 10−600 to this phase. The main product in 10− 600, however, is a different hydrous phase with even sharper bands at 3484, 3508, 3547, and 3600 cm−1. 8053

DOI: 10.1021/acs.inorgchem.6b01181 Inorg. Chem. 2016, 55, 8048−8058

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crystals for single-crystal X-ray diffraction studies. Although obscured by twinning, the diffraction patterns could be indexed according to a monoclinic unit cell (a ≈ 9.57 Å, b ≈ 14.43 Å, c ≈ 5.06 Å, β ≈ 105°), and reflection conditions suggested the presence of a glide plane perpendicular to a 21 axis. Accordingly, the space group P21/c was assigned. The structure was solved by direct methods, and an ordered, twinned model was refined to an agreement 6.7% R1 (I > 2σ(I)). Table 3 lists a Table 3. Crystal Data and Structure Refinement for LiAlSiO3(OH)2 from Single-Crystal X-ray Diffraction empirical formula formula weight temperature, K crystal system space group a, Å b, Å c, Å ß, deg volume, Å3 Z ρcalc g/cm3 μ, mm−1 F(000) crystal size, mm3 radiation, Å 2θ range for data collection, deg index ranges reflections collected independent reflections data/restraints/parameters goodness-of-fit on F2 final R indexes [I ≥ 2σ(I)] final R indexes [all data] largest diff peak/hole/e Å−3

Figure 5. p and T map of products obtained from high-pressure hydrothermal treatment of LiAlSiO4 glass.

Table 2. Crystal Data and Refinement Results of PXRD Data product p (GPa)/ T (°C) space group a (Å) b (Å) c (Å) β (deg) V (Å3) Rp Rwp Rexp

zeolite LiABW LiAlSiO4·H2O

βeucryptite LiAlSiO4

α-eucryptite LiAlSiO4

hydrous LiAlSiO4 LiAlSiO3(OH)2

0.25/200

0.25/400

1.0/600

5.0/400

Pna21

P6222

R-3

P21/c

10.3102(1) 8.1991(1) 5.0027(1)

5.2475(5) 5.2475(5) 11.189(1)

13.470 27(5) 13.470 27(5) 8.999 43(4)

422.90(1) 12.4 11.7 6.35

266.82(6) 20.9 23.4 5.33

1414.160(10) 8.14 9.63 2.64

9.5535(5) 14.4501(7) 5.0446(4) 104.597(10) 673.91(7) 12.6 13.7 6.17

Hydrous LiAlSiO4: New Dense Hydroxy Aluminosilicate Phases. The PXRD patterns of products obtained at 5 and 10 GPa could not be matched to any of the known phases in the Li−Al−Si-O system. It was then possible to select

LiAlSiO3(OH)2 144.03 296.15 monoclinic P21/c 9.547(3) 14.461(5) 5.062(2) 104.36(1) 677.1(4) 8 2.826 0.433 576 0.06 × 0.01 × 0.01 Ag Kα (λ = 0.560 87) 3.476 to 42.624 −11 ≤ h ≤ 10, 0 ≤ k ≤ 18, 0 ≤ l ≤ 5 2522 1065 [Rint = 0.0520, Rsigma = 0.0611] 1065/0/66 1.019 R1 = 0.0666, wR2 = 0.1804 R1 = 0.0909, wR2 = 0.1951 1.85/−0.93

Figure 6. FTIR spectra showing the wavenumber range of M−O vibrations of the starting material and samples 0.25−200, 1−600, 5−600, and 10− 600 (a) and the spectral range of O−H vibrations for samples 0.25−200, 5−400, 5−600, and 10−600 (b). The complete range of each spectrum is normalized with respect to the strongest absorbing band (intensity 1). The broken horizontal lines indicate the presence of LiAlSiO3(OH)2 in the 10−600 sample. 8054

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the four O atoms that are not coordinated to Si atoms (sites O1−O4) as OH. As a result, the crystallographic formula unit is LiAlSiO3(OH)2, and there are eight units in the unit cell (Z = 8). The crystal structure of monoclinic LiAlSiO 3 (OH) 2 resembles that of pyroxene minerals.47 The two central building units of pyroxene structures are shown in Figure 7. They

summary of the refinement results. The structure consists of 16 atomic positions, all corresponding to general sites 4e. Ten sites are O atom positions. Of the remaining sites, two showed a tetrahedral coordination of O atoms and were assigned to Si atom positions. Al atom positions were assigned to the two sites associated with a similar electron density and octahedral coordination. The remaining two sites correspond to Li atom positons. Li1 attains a rather regular coordination environment by six O atoms with Li−O distances between 2.1 and 2.2 Å. For Li2 one Li−O distance is considerably elongated. Atomic positions and interatomic distances are compiled in Tables 4 Table 4. Fractional Atomic Coordinates and Isotropic Equivalent Displacement Parameters (103 Å2) for LiAlSiO3(OH)2 a atom

x

y

z

U(iso)

Si1 Si2 Al1 Al2 O(H)1 O(H)2 O(H)3 O(H)4 O5 O6 O7 O8 O9 O10 Li1 Li2

0.0473(2) 0.4566(3) 0.7504(3) 0.2516(3) 0.8538(6) 0.6456(6) 0.6492(6) 0.8549(6) 0.8735(6) 0.0970(6) 0.1281(6) 0.4046(6) 0.3781(6) 0.6315(6) 0.754(2) 0.245(2)

0.1936(2) 0.1937(2) 0.0968(2) 0.0130(2) −0.0030(4) −0.0006(4) 0.0881(4) 0.0858(4) 0.1852(3) 0.2795(4) 0.1006(4) 0.2767(4) 0.0970(3) 0.1862(4) 0.202(1) 0.111(1)

0.8223(5) 0.2562(5) 0.5291(5) 0.9691(5) 0.759(1) 0.302(1) 0.806(1) 0.260(1) 0.727(1) 0.648(1) 0.766(1) 0.034(1) 0.172(1) 0.333(1) 0.026(3) 0.0469(3)

1.6(6) 4.3(7) 2.7(6) 2.7(6) 4(1) 4(1) 4(1) 4(1) 2(1) 6(1) 7(2) 7(1) 4(1) 6(1) 17(4) 14(3)

Figure 7. Building units of pyroxenes: Chain of corner-condensed SiO4 tetrahedra (yellow, left) and chain of edge-condensed AlO6 octahedra (blue, right). In the octahedron chain O atoms are distinguished as peripheral “p” (not involved in edge-sharing) and central “c” (shared between two octahedra).

a P21/c, Z = 8, T = 295 K, estimated standard deviations in parentheses.

correspond to chains of corner-sharing SiO4 tetrahedra and edge-sharing AlO6 octahedra that run parallel to the c direction in the monoclinic unit cell. Tetrahedron chains bridge adjacent octahedron chains in three dimensions. In Figure 6 O atoms of the octahedron chain are divided into peripheral and central ones, the latter being shared between neighboring octahedra within the chain. The structure of LiAlSiO3(OH)2 is closely related to that of α-spodumene (LiAlSi2O6)in particular, when considering the high-pressure form of α-spodumene.48 Figure 8 compares both structures. Figure 8a,b shows how tetrahedron chains link octahedron ones. Note that tetrahedra within a chain assume the same orientation with one face (the “basal” face) parallel to the monoclinic bc plane and one atom (the “apex” O atom) pointing along the a* direction. In pyroxenes, basal O atoms of tetrahedron chains are condensed with peripheral O atoms of octahedron chains, and apex O atoms are condensed with central O atoms of octahedron chains. In contrast, all O atoms of tetrahedron chains in the LiAlSiO3(OH)2 structure are exclusively condensed with peripheral O atoms of octahedron chains. The linking of chains leads to layers along the monoclinic a* direction, consisting of alternating tetrahedra and octahedra (Figure 8c). The linking principle of the LiAlSiO3(OH)2 structure supports a framework with half the number of SiO4 chains compared to pyroxenes, that is, an Al/Si ratio of 1:1 (LiAlSiO3(OH)2 vs LiAlSi2O6). In this respect the structure may be considered as “diluted” α-spodumene. At the same time, tetrahedron layers now contain channels along the c

and 5, respectively. We emphasize that the assignment of atoms is without any ambiguity; the distribution of interatomic distances is typical for aluminosilicates containing octahedrally coordinated Al. The crystallographic composition conforms the EDS-determined Al/Si ratio near 1:1. As a last step we assigned Table 5. Interatomic Distancesa for LiAlSiO3(OH)2 atom

atom

distance, Å

atom

atom

distance, Å

Si1

O6 O6 O5 O7 O(H)1 O(H)2 O(H)3 O(H)4 O5 O10 O(H)4 O10 O(H)3 O5 O5 O10

1.660(6) 1.644(6) 1.614(6) 1.611(6) 1.959(6) 1.933(6) 1.892(7) 1.886(6) 1.853(6) 1.841(6) 2.14(2) 2.18(2) 2.10(2) 2.12(2) 2.11(2) 2.09(2)

Si2

O8 O8 O9 O10 O(H)1 O(H)2 O(H)3 O(H)4 O7 O9 O9 O(H)1 O6 O(H)2 O7 O8

1.657(6) 1.637(6) 1.594(5) 1.621(6) 1.902(6) 1.883(7) 1.950(6) 1.957(6) 1.857(6) 1.837(6) 2.20(2) 2.03(2) 2.46(2) 2.09(2) 2.09(2) 2.20(2)

Al1

Li1

a

Al2

Li2

Less than 3.0 Å, estimated standard deviations in parentheses. 8055

DOI: 10.1021/acs.inorgchem.6b01181 Inorg. Chem. 2016, 55, 8048−8058

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9 shows the TGA trace of sample 5−400, which corresponded to roentgenographically pure LiAlSiO3(OH)2. The weight loss

Figure 9. TGA traces for sample 5−400 (two runs). (insets) SEM images of LiAlSiO3(OH)2 crystals (lower left corner) and crystals of the product after dehydration (upper right corner).

Figure 8. Crystal structure of LiAlSiO3(OH)2 (left) compared to the crystal structure of the high-pressure form of α-spodumene (right). Both structures are monoclinic, space group P21/c. (a, b) Connectivity of the building units tetrahedron chain and octahedron chain (cf. Figure 7) along the a* and c directions, respectively. (c) Complete framework built from corner-connected building units viewed along the c directions. Oxygen atoms that are part of hydroxyl (i.e., “c”-type O atoms within octahedron chains) are presented as enlarged red circles with a green circle attached. (d) Same as (c) with Li atoms (octahedrally coordinated, gray) included.

below 300 °C (∼1%) is attributed to surface water, whereas dehydration takes place at 350−400 °C. The associated weight loss is in very good agreement with the crystallographic amount of water (12.5%). The anhydrous product retains the needlelike crystal shape of LiAlSiO3(OH)2 (inset in Figure 9), which is consistent with the observation of the SEM investigation. Preliminary EDS analysis of the dehydrated product shows an Al/Si ratio close to 1:1, and its PXRD pattern suggests indeed the formation of a new, anhydrous phase LiAlSiO4. The structure has yet to be resolved. To summarize, a hydrothermal environment prevents phase segregation or amorphization of LiAlSiO4 at pressures above 4 GPa. The compositional integrity of metal atoms can be maintained by the formation of dense hydrous phases LiAlSiO4· H2O that correspond to oxyhydroxides. Interestingly, it appears that these hydrous phases can be dehydrated without phase segregation, yielding new polymorphs of LiAlSiO4.

direction, because the central O atoms of octahedral chains are not involved in chain linking. Instead they will carry H. Compared to pyroxene-type linking, the periodicity along the b direction is longer, roughly doubling the b lattice parameter. The complete structures including also Li atoms are shown in Figure 8d. Li atoms in both structures are situated in the octahedron layers and attain an octahedral coordination by six O atoms. For LiAlSiO3(OH)2 two each correspond to OH (cf. Table 5). With respect to eucryptite LiAlSiO4, the oxyhydroxide LiAlSiO3(OH)2 represents a high-pressure form with two-thirds of the metal atoms attaining octahedral coordination. A corresponding structural high-pressure phase transition has not been observed when compressing dry LiAlSiO4.40 Rather amorphization (at low temperatures) or phase segregation (at temperatures above 600 °C) takes place. It is interesting to speculate whether one can access (new) anhydrous phases of LiAlSiO4 by thermal treatment of LiAlSiO3(OH)2. During EDS analysis crystals of LiAlSiO3(OH)2 regularly fractured, which points to the formation of an anhydrous phase in the electron beam (Figure 3h). Figure

CONCLUSIONS High-pressure hydrothermal environments have an unexplored potential for producing new aluminosilicate materials by catalyzing high-pressure phase transitions and/or the incorporation of water as hydroxyl in dense crystal structures. In this study, hydrothermal treatment of LiAlSiO4 glass at pressures between 5 and 10 GPa afforded new crystalline hydrous phases of LiAlSiO4. The crystal structure of one phase, monoclinic LiAlSiO3(OH)2, could be characterized. In the LiAlSiO3(OH)2 structure the accommodation of large concentrations of hydroxyl is accomplished by a peculiar linking principle of SiO4 tetrahedron and AlO6 octahedron chains. This linking principle, however, may be universal for creating hydrous pyroxene-type minerals and materials. Such dense oxyhydroxides based on Earth-abundant metals could constitute a new class of materials with interesting properties, for example, relating to proton/ion mobility. In addition, they represent precursors to new anhydrous aluminosilicate structures upon thermal conversion. An interesting conjecture is that hydrothermal environments in deep Earth afford hydrous pyroxenes rather generally, which would have large implications for H2O

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storage and transfer from subducted slabs to the upper mantle. An intriguing example is the recently discovered hydrous pyroxene Mg2Al(OH)2AlSiO6 (phase HAPY).49 Although compositionally rather different, in the HAPY structure, exactly the same linking principle of chains is realized as in LiAlSiO3(OH)2.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01181. Detailed compilation of scanning electron micrographs from the starting material and obtained products. Crystallographic data and refinement results of PXRD data for all samples. LeBail fits to the PXRD patterns of samples 5−400, 5−600, 10−400, and 10−600. (PDF) X-ray crystallographic information. (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

ESRF, The European Synchrotron Radiation Facility, 38000 Grenoble, France. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Swedish Research Council (VR) through Grant No. 2013-4690.

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