Anomalous Compressibility and Amorphization in AlPO4-17, the Oxide

Article pubs.acs.org/JPCC

Anomalous Compressibility and Amorphization in AlPO4‑17, the Oxide with the Highest Negative Thermal Expansion Frederico G. Alabarse,*,†,§,∇ Gilles Silly,† Jean-Blaise Brubach,§ Pascale Roy,§ Abel Haidoux,† Claire Levelut,∥ Jean-Louis Bantignies,∥ Shinji Kohara,○,⊥ Sylvie Le Floch,‡ Olivier Cambon,† and Julien Haines*,† †

ICGM, CNRS, ENSCM, Université de Montpellier, Montpellier, France Synchrotron Soleil, Saint Aubin-BP48, 91192 Gif sur Yvette, France ∇ Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Université Pierre et Marie Curie, Paris, France ∥ L2C, CNRS, Université de Montpellier, Montpellier, France ‡ Institut Lumière Matière, Université Claude Bernard Lyon 1, CNRS, F-69622, Villeurbanne Cedex, France ○ Research & Utilization Division, Japan Synchrotron Radiation Research Institute (JASRI, SPring-8), 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan ⊥ Synchrotron X-ray Group, Light/Quantum Beam Field Research Center for Advanced Measurement and Characterization, National Institute for Materials Science, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan §

ABSTRACT: AlPO4-17, known as the oxide with the highest negative thermal expansion (NTE), was studied under high pressure by angle-dispersive X-ray diffraction (XRD), mid- and far-infrared (IR) spectroscopy. Upon increasing pressure, the closure of the (P−O−Al) angle destabilizes the porous AlPO4-17 structure, which drives the amorphization process. On the basis of the decrease in intensity of the XRD lines and broadening of the IR modes, the material was found to begin to amorphize near 1 GPa. XRD, mid- and far-IR analysis evidenced pressure-induced framework softening and complete irreversible amorphization near 2.5 GPa corresponding to the collapse of the pores. The bulk modulus and its first pressure derivative (B0 = 31.2(5) GPa and B′0 = −10.1(3)) at ambient temperature were determined by fitting a third order Birch− Murnaghan equation of state (EOS) to the pressure−volume data. The material is extremely compressible and exhibits an elastic instability. Anomalous (negative) values of B′0 are very rare and have been observed previously for cyanides and metal−organic frameworks. Such an instability appears to be characteristic of materials, which exhibit strong NTE behavior and indicates a link between NTE and anomalous compressibility behavior. Mid-IR, far-IR, nuclear magnetic resonance, and pair distribution function analysis of the new amorphous form allow an amorphization mechanism to be proposed corresponding to a collapse of the structure around its pores retaining the columns built up of cancrinite cages and hexagonal prisms, based on alternating AlO4 and PO4 tetrahedra. An increase in coordination number of 10% of the Al atoms was observed. The pressure-induced amorphization in the strong NTE material AlPO4-17 opens the door to the development of new technological applications as crystal−amorphous nanocomposites with zero or specifically selected thermal expansion coefficients. been observed.8,10 In aluminosilicate and siliceous zeolites, the material may transform upon compression to two different amorphous states with the same chemical composition, but different densities and entropies: a low-pressure (∼2 GPa) lowdensity amorphous (LDA) phase, and a high-pressure (∼6 GPa) high-density amorphous (HDA) phase, which has the density of a densified aluminosilicate or silica glass.11−14 Flexibility is a typical response of open framework network solids due to structural distortions. In these open framework materials, which showed high NTE, unusual behavior upon

1. INTRODUCTION Pressure and temperature are basic thermodynamic variables, which transform matter from one state to another. Negative thermal expansion (NTE) is an unusual phenomenon that has been linked to thermally excited rigid unit vibrational modes and has been shown to occur in a different range of materials1 including open framework network solids like metal oxides such as ZrW2O8 (α = −27.3 × 10−6 K−1),2 metal cyanides such as zinc cyanide (Zn(CN)2), α = −50.7 × 10−6 K−1)3 and zeolites such as faujasite (α = −12.6 × 10−6 K−1).4 Pressureinduced amorphization (PIA), first reported by Brixner5 and Mishima et al.,6 has been largely studied and found to occurred in many materials and minerals.7−9 Recently, there has been special interest in PIA in zeolites, since polyamorphism has also © 2017 American Chemical Society

Received: January 31, 2017 Revised: March 8, 2017 Published: March 9, 2017 6852

DOI: 10.1021/acs.jpcc.7b00974 J. Phys. Chem. C 2017, 121, 6852−6863

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The Journal of Physical Chemistry C

PTFE-lined stainless steel autoclave and heated at 200 °C for 40 h. The crystallization process was stopped by quenching the autoclave in cold water. The solid recovered by filtration was washed five times with distilled water and air-dried at room temperature. Single crystals of AlPO4-17·xH2O, with maximum dimensions of 250 × 70 × 70 μm3, were synthesized. The synthesis products were first characterized using a polarizing microscope and scanning electron microscope (SEM, FEI Quanta 200 FEG), with accelerating voltage 200 V to 30 kV, probe current stage up to 2 μA (continuously adjustable), and magnification 20−106×. In order to eliminate the amine, the product was calcined in air at 500 °C for 24 h. The dehydrated phase was obtained after heating for 16 h at 150 °C under vacuum (about 10−2 mbar). The products were recovered at room temperature and transferred in a closed glass tube to a drybox (pH2O < 4 ppm), where the single crystals were gently ground to be loaded and sealed in a 0.3 mm diameter glass capillary for the X-ray powder diffraction analysis using a PANalytical X’Pert diffractometer equipped with an X’Celerator detector in transmission geometry using Cu Kα1 radiation. Angle dispersive X-ray diffraction under high pressure was performed in a diamond anvil cell (DAC). Silicone oil and quartz were used as pressure transmitting media (PTM) and calibrant, respectively. The single crystals were gently ground before being loaded in the DAC. The sample and PTM were placed in a tungsten gasket with a hole size of about 160 × 70 μm (diameter x thickness), loaded in the DAC inside a drybox (pH2O < 4 ppm). An X-ray capillary was used to provide a 100 μm diameter incident beam. X-ray diffraction patterns were obtained with exposure times in between 48 and 60 h on an image plate, placed at a distance of 143.61 mm from the sample using zirconium filtered molybdenum Kα radiation (λ = 0.71073 Å) from a microfocus tube (800 W). Lanthanum hexaboride (LaB6) was used to calibrate the XRD instrument. The intensities were integrated as a function of the diffraction angle 2θ to obtain a conventional one-dimensional diffraction pattern using the Fit2d software.21 The pressure was determined using the third order Birch−Murnaghan equation of state (EOS)22 of Quartz (B0 = 37.1 GPa, B′0 = 6.0).23 In order to normalize the intensity of the diffraction lines of the sample, the intensity of the 101 reflection of the pressure calibrant were used to correct for the reduction in sample thickness at high pressure. Mid- and far-Infrared (IR) spectra under high pressure were obtained using a DAC and the synchrotron light source at the Advanced Infrared Line Exploited for Spectroscopy beamline (AILES, Synchrotron SOLEIL), which is equipped with a Bruker instruments IFS 125 FT-IR spectrometer modified to operate with the synchrotron source.24 Because of its much higher brilliance in the far- and mid-infrared, the synchrotron light provides a better signal-to-noise ratio than usual laboratory source25 allowing measurements in the complete IR range on very small samples environments such as a DAC.14 Mid-IR spectra (4000−600 cm−1) were obtained using a Fourier transform infrared spectrometer (FT-IR) equipped with a MCT (5000−600 cm−1) detector and a KBr beamsplitter. The Far-IR domain was investigated using a 6 μm Mylar beamsplitter and a 4.2 K Si−bolometer detector (700−10 cm−1). Even though small features from the sample material may appear at the limit of the spectral range of the detectors, the higher level of noise in these regions prevents these data from being analyzed. Type IIa diamond were used in the DAC.

compression was observed due to the softening of a large number of rigid unit vibrational modes.15,16 Understand the flexibility effect under pressure, due to lower-energy vibrational modes, may clarify the contribution of lattice modes to NTE and provide a link between pressure and NTE behavior.15 This study investigates PIA in the porous aluminophosphate NTE material AlPO4-17. AlPO4-17, with an erionite structure (ERI), is built up from rings with 4- and 6 alternating AlO4 and PO4 tetrahedra that are linked to form sheets that contain rings of 6 AlO4 and PO4 tetrahedra, cancrinite cages and rings of 8 AlO4 and PO4 tetrahedra which forms the erionite cage (t-eri) empty pore structure,17,18 Figure 1. In the dehydrated state, all

Figure 1. 3D framework structure of AlPO4-17 showing the columns of alternating cancrinite cages connected along the c direction by hexagonal prisms forming the t-eri empty pore structural unit.

Al and P atoms are in 4-fold coordination. Following the NTE behavior predicted from computer simulations,19 anhydrous AlPO4-17 has been found to exhibit the strongest NTE effect in oxides with a thermal expansion coefficient of −35.1 × 10−6 K−1 over the temperature range 18−300 K.17 Because of the transverse thermal motion of oxygen in Al−O−P linkages,20 the NTE behavior in the three-dimensional porosity of AlPO4-17 results in a contraction along the a and b directions which is greater than that along the c direction.17 Both NTE and PIA confer unique physical properties to materials of this type. Understanding the impact of pressure on the structure of the exceptional NTE oxide AlPO4-17 may shed light not only on fundamental physics of NTE and PIA in this system, but also to the development of new technological materials.

2. EXPERIMENTAL SECTION The synthesis of AlPO4-17 was based on the work of Tuel et al.18 A single crystal of an aluminophosphate with the ERI framework topology, AlPO4-17, was synthesized in the presence of N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHDA) as the template. Aluminum triisopropoxide (Aldrich) and phosphoric acid (85 wt % Panreac) were used as aluminum and phosphorus sources, respectively. In order to prepare the initial gel, 136 g of aluminum isopropoxide were dispersed in 396 mL water and the suspension was stirred for 1 h. Then 45 mL of H3PO4 were added dropwise and the obtained gel was stirred for an additional hour. Finally, 71.26 mL of the template were slowly added and stirring was continued for an additional 1 h. To eliminate the isopropanol, the reaction mixture was boiled for about 20 min. The quantity of water lost during boiling was taken into account. The resulting gel, with the composition 1.0 Al2O3:1.0 P2O5:1.0 template:46.0 H2O was transferred into a 6853


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The Journal of Physical Chemistry C The gasket thickness was 50−60 μm with a hole diameter of 250 μm, which limited acquisition with a low noise level down to ∼50 cm−1 in the far-IR. The spectra were acquired with a resolution of 2 cm−1 with 400 and 800 scans in the far- and mid-IR, respectively. The sample and PTM were loaded in the DAC inside a glovebox (pH2O < 4 ppm). The PTM used was KBr for the mid-IR and polyethylene (PE) for the far-IR, which were dehydrated at 120 °C under 10−6 mbar and at ambient temperature under 10−6 mbar, respectively. Both were transferred in a closed glass tube to the glovebox after the dehydration process. As a pressure calibrant 5 μm ruby spheres were added to the sample and the pressure was determined based on the displacement of the R1 and R2 ruby fluorescence lines.26 The AILES beamline provides a setup, pumped down to a vacuum of ∼10−5 mbar, for measuring rapidly and reproducibly the mid- and far-IR spectra of samples under high pressure using DAC devices. Details can be found elsewhere.27,28 Using this set up, the synchrotron IR light is directed by gold coated mirrors through a double condenser system (Cassegrain objectives, N.A. 0.28) to the DAC in confocal configuration. The spectral profile of the framework lattice modes of AlPO4-17 can be very satisfactorily described by several overlapping Gaussian components after removal of a slowly rising linear background.14,28,29 This technique it was recent successful applied to AlPO4 molecular sieves to observe the correlation between structural and vibrational properties.28 During the fitting procedure, the background was evaluated by as a straight line between two regions where no modes are observed. This linear background determined at the lowest pressure was then subtracted prior to the Gaussian fit and this background function was fixed and used for spectra at all pressures. For clarity, we only present the framework band region prior to the background removal together with the fitted Gaussians and their assignment (see Figure 7). Such a fit provides a sound tool to quantitatively account for the changes of framework lattice modes, although the present Gaussian and background fit is not unique. High pressure large volume experiments were performed in a Belt type high-pressure chamber to prepare the amorphous form of microporous AlPO4-17 for the nuclear magnetic resonance and high energy X-ray total scattering analysis. A detailed description of this high-pressure apparatus is given elsewhere.30 The dehydrated powder was loaded in a 6 mm long, 4 mm internal diameter gold capsule inside a drybox (pH2O < 4 ppm) and placed in a pyrophyllite gasket. In this experiment, pressure was initially applied to the sample up to 5 GPa at room temperature. Finally, the pressure was decreased to ambient. The pressure in the sample chamber was calibrated using fixed pressure points based on the pressure induced phase transition of mercury, bismuth, and thallium30 and the pressure is considered to be accurate to ±0.3 GPa. The structure of AlPO4-17 and its amorphous form were investigated by magic angle spinning nuclear magnetic resonance (MAS NMR) on a VARIAN VNMRS 600 spectrometer equipped with a wide-bore magnet (B0 = 14 T) and a three channel ultrafast MAS probe (o.d. rotor 1.2 mm maximum spinning speed 60 kHz). The powder was placed in ZrO2 rotors and spun at 20 kHz. Spectra simulations were carried out with the PC DMfit program.31 Standard solutions of phosphoric acid (H3PO4), Al(NO3)3, and TMS (tetramethylsilane) were used as references for the 31P MAS, 27Al MAS, and 1 H MAS chemical shifts, respectively. The 31P and 27Al

isotropic chemical shifts (δ in ppm), 27Al quadrupole coupling constants (QCC in MHz) and asymmetry parameters (η) used for AlPO4-17 are given in Table 1 and are in agreement with Table 1. 31P and 27Al Isotropic Chemical Shifts (δ in ppm), 27 Al Quadrupole Coupling Constants (QCC in MHz), and Asymmetry Parameters (η) for AlPO4-17 P-1 P-2 Al-1 Al-2 a

δ iso-refa

δ iso-17

QCC

η

−29.0 −34.5 40.8 36.2

−29.3 −34.8 40.2 36.3

− − 4.4 2.2

− − 0.5 0.7

Zibrowius et al.32

the results of Zibrowius et al.32 Quantitative information on the local structure can be extracted from the REDOR (rotational echo double resonance) curves, produced by plotting the normalized difference (S0 − S)/S0 versus the echo delay,33−35 where the signal obtained after a rotor-synchronized spin−echo is termed S0 and the signal obtained after the same spin−echo, but modified by the reintroduction of the dipolar interaction with a second nucleus, is termed S. The REDOR sequence is sensitive to the number of neighbors and their distances to the observed nucleus. 27Al{1H}, 27Al{31P}, and 31P{1H} REDOR analyses were performed. The REDOR experiments 27Al{X} with X = 31P,1H were conducted at 14 T (ν0(27Al) = 156.33 MHz, ν0(31P) = 242.86 MHz and ν0(1H) = 599.92 MHz) with spinning frequencies of 20 kHz and radiofrequencies (rf) of 5.0, 83.3, and 62.5 kHz for 27Al, 31P, and 1H respectively. The 40 sets of S0 and S signals were recorded with 512 scans separated by a recycle delay (rd) of 1 s. High energy X-ray total scattering data from the recovered amorphous sample were obtained using the two axis horizontal diffractometer built for liquid and glass samples on the bending magnet beamline BL04B2 at the SPring-8 synchrotron. A bent Si (220) monochromator fixed at a Bragg angle of 3° in the horizontal plane was used to obtain 61.6 keV X-rays. The sample was placed in a 1 mm diameter glass capillary in the incident X-ray beam. The intensity of the scattered X-rays was measured using a Ge detector. The pair distribution function was obtained in the form of the total radial distribution function36 G(r) by direct Fourier transformation of the total scattering data S(Q) obtained up to a maximum Q of 20 Å−1. The details of experiment and standard data analysis are described elsewhere.37,38

3. RESULTS AND DISCUSSION 3.1. In Situ X-ray Powder Diffraction at High Pressure. The material obtained from the synthesis procedure used, AlPO4-17 single crystals in form of elongated hexagonal prisms (Figure 2), belongs to the ERI zeolite framework type. A Le Bail fit39 (Rp = 8.5% and Rwp = 11.6%) using the XRD data obtained at ambient pressure was performed with the software suite Fullprof.40 The results confirmed that the dehydrated starting material, space group P63/m, with cell parameters a and c of 13.1113(1) Å and 15.3600(1) Å, respectively at ambient pressure, was pure. Figure 3 shows a series of X-ray diffraction patterns of AlPO4-17 obtained in the diamond anvil cell between room pressure and 2.4 GPa. No pressure-induced phase transition was observed. The most intense reflection of AlPO4-17, the 100 located at small angles (at ∼3.5°, Figure 3) 6854


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reflection of AlPO4-17 decreases abruptly, to about 65 and 35%, at 1.3 and 2 GPa, respectively. Complete and irreversible amorphization is observed between 2 and 2.4 GPa. There is no evidence of the formation of stacking faults as the 101 reflection initially at 4.47° does not decrease in relative intensity and disappears at the same pressure as the other reflections upon amorphization. The material initially undergoes to isotropic compression (Figure 5). While the lattice parameter c decreases nearly Figure 2. Scanning electron microscope image of the as-synthesized AlPO4-17.

Figure 5. Lattice parameters as a function of pressure. Because of the strong increase in line width at 2 GPa, these data are not included in the fit.

linearly, the lattice parameter a decreases more rapidly with further increases with pressure until the complete amorphization, corresponding to the collapse of the pores. A reasonable hypothesis is that the columns of alternating cancrinite cages (tcan) and hexagonal prisms (t-hpr), formed by two rings of 6 AlO4 and PO4 tetrahedra, Figure 1, which are connected along the c direction, are responsible for lower compressibility along c. This is a good agreement with the NTE results17 indicating that the contraction along a is greater than along c. As flexibility is a property of the open framework structure in response of variation in pressure, the analogous mechanical response with temperature may open strategies for technological applications. Between room pressure and 0.4 GPa, a pressure routinely surpassed industrially, the hexagonal lattice parameters contracted by ∼0.5% at constant temperature (Figure 5) which corresponds to the same degree of relative thermal contraction in this direction over the temperature range from 18 to 300 K.17 Figure 6 shows the evolution of the relative volume (V/V0) as a function of pressure for AlPO4-17. The framework compresses rapidly by 4% at 0.9 GPa and 13% at 2 GPa, before complete amorphization. The bulk modulus and its first pressure derivative (B0 = 31.2(5) GPa and B′0 = −10.1(3)) at ambient temperature were determined by fitting a third order Birch−Murnaghan equation of state (EOS),22 to the pressure− volume data up to 1.3 GPa above which strong peak broadening occurs (Figure 3). The line width is essentially constant for the points up to and including the point at 1.3 GPa. Including the point at 2 GPa, at which pressure the reflections are very broad, results in a very poor fit. Similarly, an extremely poor fit with all of the data points lying far from the fitted curve was obtained with the second order equation of state corresponding to B′0 = 4. A good fit is again obtained

Figure 3. In situ powder X-ray diffraction patterns of AlPO4-17 in silicone oil at selected pressures: (∗) reflections corresponding to quartz pressure calibrant and (F) pressure release.

becomes progressively weaker starting at 1 GPa. Between 2 and 2.4 GPa, all the reflections clearly disappear. Figure 4 shows the evolution of the intensity of the 100 reflection as a function of pressure. On the basis of the rapid decrease in intensity of the diffraction lines and in the clear break observed in the evolution of the intensity of the 100 reflection as a function of pressure, the material was found to begin to amorphize near 1 GPa. The intensity of the 100

Figure 4. Normalized intensity of the 100 reflection of AlPO4-17 as a function of pressure. 6855


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observed B0 and B′0 were 6.5 GPa and −4.5, respectively. ZIF-8 is a factor of 5 more compressible than AlPO4-17. On the other hand, the bulk modulus of zinc cyanide, Zn(CN)2 and its pressure dependence are B0 = 34.2 GPa and B′0 = −6.015 and the compressibility is of the same order as AlPO4-17. The bulk modulus of AlPO4-17 is similar to that of berlinite (AlPO4, B0 = 36 GPa), an aluminophosphate isostructural with quartz,41 but with strong differences of B′0 values. Whereas the berlinite has a normal B′0 value, AlPO4-17 presents an elastic instability. 3.2. Anomalous Compressibility Behavior in AlPO417. In other zeolites studied and related materials, pressureinduced stiffening of the structure is the classical response, these materials become less compressible with positive values of B′0.12,13,22,23,28,41,42 In open framework zeolites that exhibit strong NTE, such as faujasite, anomalous compressibility behavior has not been reported. Under compression, NaX faujasite exhibits three pressure ranges with different compressibility values before becoming completely amorphous. The bulk modulus of hydrated, faujasite NaX is similar to that observed for berlinite, but with normal value of B′0 (B0 = 36 GPa and B′0 = 4)12 in contrast to AlPO4-17. However, molecular dynamics simulations on purely siliceous cubic zeolites exhibiting NTE suggest that the pressure-induced softening of these materials originates from the dependence of the frequencies of the NTE phonon modes on strain, in which the phonon modes will show Grüneisen parameters with negative first and second derivatives of the Grüneisen

Figure 6. Relative volume as a function of pressure.

using only the data below 1 GPa with similar values to those given above for the data up to 1.3 GPa. Upon comparison with other materials with negative B′0 values, AlPO4-17 has a high negative value of B′0. Negative B′0 values are linked to elastic softening giving rise to an elastic instability. The negative value of B′0 indicates that the framework structure of AlPO4-17 becomes more compressible as the pressure increases and exhibits an elastic instability (B0 = 31.2(5) GPa and B′0 = −10.1(3)). In ZIF-8,16 Zn(2methylimidazole)2, a metal−organic framework (MOF), the

Figure 7. a) Mid- (a) and far- (b) IR spectra in the region of the AlPO4-17 framework modes at selected pressures on compression and on decompression (∗). 6856


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Figure 8. Pressure dependence of the IR modes of the AlPO4-17 framework in the mid- (a) and far-IR (b). Open and solid symbols correspond to points obtained on compression and decompression, respectively.

direction.17 A similar structural response in this material was observed before the PIA. The lattice distortions induced by pressure in the AlPO4-17 structure may thus be related to lowenergy lattice vibrations52 and are responsible for the observed exotic behavior under pressure. As PIA in AlPO4-17 may involve disruption of long-range translational symmetry while retaining the local structure, framework connectivity and, possibly, porosity,53 analysis of the local structure in the amorphous form of AlPO4-17 in the following section can provide unique insights into the amorphization mechanism of this material. 3.3. In Situ Mid- and Far-Infrared Spectroscopy at High Pressure. AlPO4-17 was studied in situ under high pressure by mid- and far-infrared (IR) spectroscopy (Figures 7 and 8). AlPO4 molecular sieves display several bands in between 1400−100 cm−1 corresponding to framework lattice modes. Three specific regions are observed (1400−900, 900− 650, and 650−125 cm−1) characteristic of aluminophosphate molecular sieves.28,54−56 The bands in the 1400−900 cm−1 region correspond to the antisymmetric stretching vibrations (νa P−O−Al), Figure 7a (modes νa3, νa2, and νa1). The symmetric stretching mode (νs P−O−Al) is observed around 900−650 cm−1 (Figure 7a, modes νs2 and νs1). The 650−125 cm−1 region is more complex (Figure 7b, modes from δ12 to δ1). In dehydrated porous aluminophosphates, the higher spectral region of the far-IR (600−300 cm−1, modes from δ12 to δ5) correspond to characteristic bending of the rings (δ P−O−

parameter in their pressure dependence, which results in NTE and pressure-induced softening phenomena. 43 A close inspection of the published P−V data44 on empty pore siliceous faujasite are consistent with a negative value of B′0 of the order of −6.5 based on a fit of this data to a third-order Birch−Murnaghan equation of state as compared to the value of −2.6 obtained by molecular dynamics.43 As in zinc cyanide,15 the open framework structure of AlPO417 was found to be highly flexible under pressure. In Zn(CN)2, this was linked to the existence of low-energy lattice vibrations.45 In AlPO4-17, these vibrations involve the transverse displacement of the Al−O−P linkages, which are responsible for the high NTE behavior in this material.17 In ZrW2O8, which also shows strong isotropic NTE (−27.3 × 10−6 K−1),2 different mechanisms were proposed to explain their PIA, one, which also relates the PIA to the softening of low-energy modes, transforming the framework structure into a disordered state.46,47 The other mechanisms proposed correspond to a hindered decomposition of the parent structure into ZrO2 and WO3 and a kinetically hindered transition to a high-pressure, high-temperature crystalline phase.48,49 More recently, using reverse Monte Carlo modeling of neutron and X-ray total scattering data, the amorphization mechanism is linked to W−O bond formation in an ice-rules manner with an increase in the coordination number of tungsten.50,51 The NTE effect on the AlPO4-17 structure corresponds to a contraction along a, which is greater than along the c 6857


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The Journal of Physical Chemistry C Al) of four- and six-tetrahedra,28 while its lower frequency region (300−100 cm−1, modes from δ4 to δ1) have been assigned to external vibrations modes.54 Since there are no charge balancing cations present in the structure, these lower frequency modes therefore might be expected to principally involve the ring breathing of the four or six primary rings of T atoms, distortion modes and distinct modes arising from the larger ring of 8 AlO4 and PO4 tetrahedra forming the channels.28,54−56 In the far-IR, α-AlPO4 (berlinite) displays three bands at 112, 126, and 165 cm−1 (due to Γ3 modes, LOTO).57 Large ring (channel pore) mode vibrations in unidimensional AlPO4 molecular sieves are observed at 305− 260 cm−1,28,54,55 thus the strong candidates for an external modes associated with the large t-eri ring structure, which contains rings of 8 tetrahedra in AlPO4-17, are therefore the 285−175 cm−1 vibrational modes.54 Band assignments are given in Table 2.

pressures. In contrast, both lower frequency antisymmetric stretching modes (νa P−O−Al) and both symmetric stretching modes (νs P−O−Al) do not shift significantly until 0.9 GPa (Figure 8a, modes νa2, νa1, νs2, and νs1). Above this pressure, these modes shift more rapidly with further increases in pressure until complete amorphization at 2.3 GPa. Before amorphization, both (νs P−O−Al) lower wavenumber, νa1 and νa2 (1000 and 1100 cm−1, respectively), decrease in wavenumber at a rate of about ∼37 and ∼26 cm−1/GPa, respectively, while both (νs P−O−Al) modes, νs1 and νs2, shift by about ∼16 and ∼10 cm−1/GPa, respectively, Figure 8a. These frequency shifts are an indication of the collapse of the empty large pores and is linked to the pressure dependence of the a lattice parameter observed by XRD. Above this pressure, the pressure dependence of both modes exhibited slope changes. The νa P−O−Al and νs P−O−Al modes in the midIR correspond to bending and stretching involving bridging oxygens atoms. As the P−O−Al angle narrows, the bending character increases for νa P−O−Al mode causing a decrease in frequency, while similarly increasing the stretching character in the νs P−O−Al mode with a resulting increase in frequency. The shift of νas (P−O−Al) to lower frequencies and of the νs (P−O−Al) to higher frequencies is correlated to the closure of the (P−O−Al) angle [νas − νs = f(∠Al−O−P)].58 The closure of the (P−O−Al) angle was already observed during the PIA of others porous aluminophosphates, such as AlPO4-54·xH2O,55 confirmed by single-crystal synchrotron X-ray diffraction results,59 and its dehydrated form AlPO4-54.28 The far-IR modes of AlPO4-17 (Figure 7b) were also strongly affected during pressure-induced amorphization. Upon increasing pressure up to 0.9 GPa, these modes show several evolutions with δ2 and δ3 shifting to lower frequencies, δ5, δ6, δ7, δ9, and δ10, shifting to higher values, and δ1, δ4, and δ8, remaining constant. Above this pressure, stronger shifts are observed for all modes, in which a few modes become very weak and finally disappearing (δ2 to δ7). Again, while several far-IR modes shift to high frequencies, δ4 and δ3 shift in the opposite direction. At the amorphization pressure, 2.3 GPa, clear changes in slope are observed for the far-IR modes with δ5, δ6, δ8, and δ9 shifting to lower frequencies. In both mid- and far-IR, the weak dependence for most modes above the amorphization pressure is an indication of the lower compressibility of the amorphous form due to its higher density, as has been observed during the PIA of other AlPO4 molecular sieves.28 The δ4, δ3, and δ2 modes are linked to an external mode associated with the large t-eri ring structure and δ3 and δ2 might correspond to soft lattice modes.15 The latter shift to lower frequencies upon increasing pressure with the subsequent disappearance of the δ2 mode. The pressure dependence of these modes is representative of the pressureinduced framework softening with pore collapse as observed in the XRD results. 3.4. IR Spectra of the Amorphous AlPO4-17. Upon pressure release, both lower frequency νa P−O−Al modes are always found at lower values than the corresponding modes of the crystalline phase, showing that the amorphous form is retained during the pressure release. Additionally, both (νs P− O−Al) modes during the pressure release are found at higher frequencies than those of the crystalline phase. This smaller value of νa − νs indicates that the closure of the Al−O−P angle persists in the recovered amorphous form. In the far-IR region, upon pressure release, the high frequency modes return with higher values, while the low frequency modes return with lower

Table 2. Band Assignments for AlPO4-17 at Ambient Pressure and 300 K28,54−56 wavenumber (cm−1)

assignment

label

1221 1101 1004 766 696 636 561 517 485 453 422 367 314 266 219 198 154

antisymmetric stretching vibrations (νa P−O−Al) antisymmetric stretching vibrations (νa P−O−Al) antisymmetric stretching vibrations (νa P−O−Al) symmetric stretching mode (νs P−O−Al) symmetric stretching mode (νs P−O−Al) ring bending (δ P−O−Al) ring bending (δ P−O−Al) ring bending (δ P−O−Al) ring bending (δ P−O−Al) ring bending (δ P−O−Al) ring bending (δ P−O−Al) ring bending (δ P−O−Al) ring bending (δ P−O−Al) external mode external mode external mode external mode

νa3 νa2 νa1 νs2 νs1 δ12 δ11 δ10 δ9 δ8 δ7 δ6 δ5 δ4 δ3 δ2 δ1

The AlPO4-17 framework modes are well fitted by a sum of five Gaussians in the mid-IR (Figure 7a, modes νa3, νa2, νa1, νs2, and νs1), while 12 Gaussians are required in the far-IR (Figure 7b, modes from δ12 to δ1). The 700−600 cm−1 domain is the intersection of both mid- and far-IR regions, containing common modes, νs1 and δ12. The fit of both common features, νs1 and δ12, was performed for the spectra of both regions, but is presented separately in Figure 8. Upon compression at room temperature, the modes in the mid-IR region (Figure 7a) shift (Figure 8a) and a major change in amplitude is observed at the beginning of the amorphization at 0.9 GPa. The modes of the far-IR region (Figure 7b) were also strongly affected, decreasing in amplitude together with strong shifts (Figure 8b) and broadening around 1 GPa due to amorphization. At higher pressures, the characteristic modes of crystalline AlPO4-17 can no longer be distinguished. Figure 8 shows the pressure-induced frequency shift of select modes of AlPO4-17. In the antisymmetric stretching (νa P−O− Al) region, the feature above 1200 cm−1 is common in molecular sieves and the frequency depends on the pore size of the material.28,54 Up to 0.9 GPa, this mode shifts smoothly to lower frequencies and then remains constant at higher 6858


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Figure 9. 27Al{1H} (a), 31P {1H} (b), and 27Al{31P} (c) REDOR for 4-fold Al, P, and Al, respectively, for AlPO4-17 compared to AlPO4-54·xH2O.55

Figure 10. 27Al MAS (a) and 31P NMR (b) spectrum of AlPO4-17 at ambient pressure and after recovery from 5.5 GPa. 27Al MAS and 31P NMR AlPO4-17 amorphous spectrum has been multiplied by 10 to facilitate the comparison. (∗) VAl from impurity phases.

return to the initial crystalline structure with open pores. This irreversible pore collapse is linked with the irreversibility of the closure of the Al−O−P angle. 3.5. Study of the Recovered Amorphous AlPO4-17 by Nuclear Magnetic Resonance and X-ray Total Scattering. NMR REDOR investigations of crystalline aluminophosphates may give further information to understand the PIA mechanism.55 27Al{1H}, 31P{1H}, and 27Al{31P} REDOR spectra of AlPO4-17 were obtained on the dehydrated sample in order to determine if any H atoms are bonded to oxygen in AlO4 and PO4 tetrahedra and the number of P atoms linked to AlO4 units. For example, in the 27Al{1H} REDOR sequence, the 27Al signal obtained after a rotor-synchronized spin−echo (labeled S0) is compared to a signal obtained after the same

values compared to their initial wavenumber values. This is again a clear evidence that the new amorphous form is retained during the pressure release. The δ6 and δ5 mode are associated with four- and six-membered rings, these modes return to values similar to those at ambient pressure on pressure release together with an increase in amplitude observed in the 650− 275 cm−1 region. Bands in this region are also associated with rings of four and six tetrahedra, which is an indication that during PIA, the degree of crystallinity of AlPO4-17 decreases, but the local structure is preserved as has also been observed in the case of PIA in dehydrated porous aluminophosphates28 and other zeolites.13,14 In the latter materials, amorphization is also irreversible due to pore collapse, and in the absence of a structure directing agent, there is no mechanism available to 6859


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The Journal of Physical Chemistry C spin−echo (labeled S) modified by the reintroduction of the 27 Al /1H dipolar interaction. This reintroduction is achieved by the application of rotor-synchronized π-pulses on the 1H channel, leading to a decrease of the 27Al signal experiencing the reintroduction. 27 Al{1H} and 31P{1H} REDOR (Figure 9, parts a and b) indicate the absence of H atoms in the both Al and P tetrahedral environments, when compared to hydrated aluminophosphates,55 showing that the sample was fully dehydrated. The 27Al{31P} REDOR performed in AlPO4-17 (Figure 9c) indicated that the distance between both AlO4 and PO4 tetrahedral environments were the same as other aluminophosphates,55 and thus confirms that the structure is built of alternating AlO4 and PO4 tetrahedra. In AlPO4-17, the 27Al MAS NMR spectrum, Figure 10a, contains an asymmetric line at ca. 35.5 ppm can be assigned to tetrahedrally coordinated framework Al, corresponding to two overlapping sites with a 4-fold 27Al environment. The 27Al MAS NMR spectra of the amorphous sample (recovered from 5.5 GPa), Figure 10a, contains broader characteristic peaks of AlO4 tetrahedra due to the disorder in the system, when compared to the starting material. The 27Al MAS NMR spectrum of the initial sample (Figure 10a) also has a peak around 14 ppm, which corresponds to 5-fold coordinated Al due to a small amount of impurities in the starting material, but the 27Al {31P} REDOR NMR analyses showed is not linked to the AlPO4-17 structure (Figure 9c). The relative area of this peak indicates an impurity level of less than 4%. No crystalline secondary phase was detected by X-ray diffraction. The extremely weak broad feature on the background at 25° in 2θ would be consistent with this small amount of impurities being amorphous. A fit to the spectrum of the amorphous phase is consistent with 87% IVAl, 10% VAl, and 3% VIAl. The 3% could correspond to an increase in coordination number from 5 to 6 in the impurity. If this is the case, 10% of the Al atoms in AlPO4−17 would increase from 4-fold to 5-fold coordination in the amorphous form after compression to 5.5 GPa. The 31P NMR spectrum of AlPO4-17 (Figure 10b) contains two lines with approximate chemical shifts of −29 and −35 ppm with an intensity ratio of about 3:1. The existence of two signals in the 31P NMR spectrum also confirms the presence of two crystallographically distinct P sites in the framework. The 31 P NMR spectrum of AlPO4-17 (Figure 10) showed that the dehydrated sample is highly crystalline with no amorphous material as compared to the spectrum of amorphous AlPO4-17, which shows that the network of 31P was strongly affected by the high pressure treatment. As expected, the spectra of amorphous forms is much broader compared to the starting material, due to the broad distributions of chemical shifts. The total correlation function data obtained from X-ray total scattering from the pressure-amorphized AlPO4-17 form, Figure 11, provide information on the structure of the new amorphous form. The average intratetrahedral distances, 1.74 Å (P−O, Al−O) and 2.48 Å (O−O), are shifted with respect to those of the crystalline phase, 1.58 and 2.60 Å, respectively, which is a sign that the material was strongly modified. Note that it was not possible to resolve the P−O and Al−O peaks. The increase in the coordination number for a portion of the Al atoms would be consistent with an increased average Al−O and decreased average O−O distance. The intertetrahedral Al−P distance across the bridging angle in the crystalline phase at 3.08 Å corresponds to the shoulder 3.01 Å in the amorphous form. This lower value is consistent

Figure 11. Pair distribution functions G(r) of pressure-amorphized AlPO4-17 and that calculated based on the crystal structure of AlPO417 at ambient pressure17. (∗) intertetrahedral P−O, Al−O, and O−O distances.

with the decrease in the Al−O−P angle. The feature at around 3.31 Å (marked with an ∗ in Figure 11) cannot arise from the above intertetrahedral Al−P distances as it would correspond to a nonphysical Al−O−P angle of the order of 180°. This peak instead should have contributions from the shortening of intertetrahedral P−O, Al−O, and O−O distances corresponding to the strong distortion of the various rings in the structure. The small feature at 3.72 Å is much closer to that of 3.83 in the crystalline form indicating a proportion of rings that are only slightly distorted. The shoulder located at around 4.37 Å, common in both crystalline and amorphous forms, is linked to intracancrinite (t-can) and intra-t-hexagonal prism (t-hpr) Al− Al or P−P distances and its maxima, the feature at 4.6 Å, shifted to higher values, is linked to distortions at the t-can P−P and Al−Al distances. The distances above 5 Å are somewhat more difficult to assign. In the crystalline phase, the three major peaks at 5.46, 6.37, and 7.44 Å correspond principally to intercage P−P and Al−Al, inter-t-can Al−P, and further Al−P intercage distances, respectively. In the G(r) of the amorphous form, a shorter and a longer distance is found in the same range of the initial distance. These shorter and longer distances are an indication of significant distortion of the ring structures around the pores that link the columns of alternate cancrinite cages and hexagonal prisms. In contrast, the weak peaks of the crystalline phase at 5.16 and 6.98 Å corresponding respectively to Al−O distances in the t-can units and other t-can and t-hpr distances, respectively, remain present in the amorphous form. This could be evidence that distorted columns of alternate cages of cancrinites and hexagonal prims are present in the amorphous structure, although definite conclusions cannot be made. Pair distribution function (PDF) analysis of the new amorphous form is consistent with a model for the amorphization that could correspond to a collapse of the structure around its pores retaining the columns build up by alternate cancrinite cages and hexagonal prisms (see Figure 1). The total X-ray scattering results allow us to identify the 6860



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structural changes that occur upon amorphization, which give rise to the smaller Al−O−P angle probed in situ by infrared spectroscopy. As observed in the IR results, the closure of the Al−O−P angle persists in the recovered amorphous form and based on the PDF results, the closure of the Al−O−P angle occurs around the empty t-eri 8-membered ring pore structure. Amorphization corresponding to the collapse of the structure around the pores was already evidenced in the zeolites silicalite1 by PDF analysis13 and in faujasite by combined infrared and PDF analysis.14

Article

AUTHOR INFORMATION

Corresponding Authors

*(F.G.A.) E-mail: [email protected]. Telephone: 33(0)1 4427 3783. Fax: 33(0)1 4427 3785. *(J.H.) E-mail: [email protected]. Telephone: 33(0)4 6714 9349. Fax: 33(0)4 6714 4290. ORCID

Julien Haines: 0000-0002-7030-3213 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Agence Nationale de la Recherche (Project ANR09-BLAN-0018-01) for financing this study, supported by the French state funds managed by ANR within the Blanc International programme PACS (Reference ANR-13-IS040006-01). The synchrotron x-ray scattering experiments were performed at the BL04B2 in the SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Number 2013A1084). The large volume high pressure experiments were performed at the ILM-Tech platform in Lyon.

4. CONCLUSION XRD shows that microporous AlPO 4 -17 is extremely compressible. Unusual compression behavior was observed with a pressure-induced softening effect. Anomalous (negative) values of B′0 are very rare and have been observed previously for cyanides and metal-oganic frameworks. Such an instability appears to be characteristic of materials, which exhibit strong NTE behavior, indicating that NTE and anomalous compressibility behavior may have a common origin in the relative flexibility of such open-frameworks structures. The amorphization of AlPO4-17 was observed in XRD by the disappearance of the X-ray diffraction lines. In the IR, strong shifts in the mid-IR modes, changes in the amplitude and strong broadening of the modes was observed providing evidence for amorphization. In the FIR, the broadening of the characteristic peaks is observed at the amorphization pressure. The frequency shift of νas (P−O−Al) to lower frequencies and of the νs (P−O−Al) to higher frequencies is related to the closure of the (P−O−Al) angle. These frequency shifts are correlated to that of the pressure dependence of the a lattice parameter observed by XRD. In the FIR, low frequency modes associated with the t-eri structure, linked to low-energy lattice vibrations, shift in agreement with the pressure-induced framework softening observed by XRD. On pressure release, the MIR data indicate that closure of the Al−O−P angle persists in the recovered amorphous form, while FIR indicate that the local structure of four- and six-tetrahedra is preserved. NMR analysis provides evidence of complete disorder in the new amorphous AlPO4-17, with an increase in the coordination number of 10% of the Al atoms. PDF analysis support the XRD, mid- and far-IR results, which provide evidence of pressure-induced framework softening around the t-eri structural units due to closure of the (P−O−Al) angles in the 8-membered rings and indicate that the collapse of the pores with the preservation of distorted columns build up by alternate cancrinite cages and t-hpr’s is a plausible mechanism for amorphization process. Since PIA in AlPO4-17 was observed in a range of pressure routinely attained in industry, the present observation of PIA in the strong NTE material AlPO4-17 opens the door to the development of new technological applications as crystal− amorphous nanocomposites analogous to vitroceramics with zero or specifically targeted thermal expansion coefficients. As the dense amorphous material recovered from AlPO4-17 can be expected to have positive thermal expansion, a composite containing both forms could have a tunable thermal expansion coefficient as a function of the ratio of the crystalline and amorphous forms obtained at a given processing pressure.

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Anomalous Compressibility and Amorphization in AlPO4-17, the Oxide

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