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Pressure-Induced Structural Transitions in MOO3 · xH2O (x ) 1/2, 2) Molybdenum Trioxide Hydrates: A Raman Study Dan Liu, Weiwei Lei, Xiaohui Chen, Jian Hao, Yunxia Jin, Qiliang Cui,* and Guangtian Zou State Key Laboratory of Superhard Materials, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed: July 7, 2009; ReVised Manuscript ReceiVed: NoVember 5, 2009
The high-pressure behaviors of MOO3 · 1/2H2O and MOO3 · 2H2O have been investigated by Raman spectroscopy in a diamond anvil cell up to 31.3 and 30.3 GPa, respectively. In the pressure range up to around 30 GPa, both MOO3 · 1/2H2O and MOO3 · 2H2O undergo two reversible structural phase transitions. We observed a subtle structural transition due to O-H · · · O hydrogen bond in MOO3 · 1/2H2O at 3.3 GPa. We found a soft mode phase transition in MOO3 · 2H2O at 6.6 GPa. At higher pressures, a frequency discontinuity shift and appearance of new peaks occurred in both MOO3 · 1/2H2O and MOO3 · 2H2O, indicating that the second phase transition is a first-order transition. The frequency redshift of the O-H stretching bands of MOO3 · 1/2H2O and MOO3 · 2H2O are believed to be related to the enhancement of the O-H · · · O weak hydrogen bonds under high pressures. 1. Introduction In recent years, significant interest has been focused on molybdenum oxide (MOO3) and the related hydrates (MOO3 · xH2O) for specific applications, such as battery electrodes, largearea display devices, gas sensors, etc.1-9 Molybdenum trioxide hydrate has been regarded as an interesting precursor because its dehydration results in the formation of orthorhombic MOO3, which provides a convenient intermediate stage for the synthesis of the a new high-pressure phase of molybdenum trioxide.10,11 MOO3 with different crystalline modifications exhibits a ReO3-type structure, which consists of a three-dimensional array of corner-sharing distorted MoO6 octahedrons.12-17 MOO3 · xH2O also has arrangements of distorted MoO6 octahedrons, and the hydration of MoO3 is accomplished either by substitution of one oxygen of the octahedral by H2O or by intercalation of H2O between sheets of layered structure. This leads to a rich family of crystalline hydrates, including dihydrate MOO3 · 2H2O, monohydrate MOO3 · H2O, hemihydrates MOO3 · 1/2H2O, and MOO3 · 1/3H2O.18-24 In MOO3 · xH2O, the cohesion between layers is maintained by O-H · · · O hydrogen bond interactions. Several studies have shown that pressure has a significant effect on enhancing the bonding of molecular aggregates containing charges and controlling the strength of O-H · · · O hydrogen bond interactions.25 In a recent paper from this laboratory, two structure phase transitions of MOO3through studies of in situ high-pressure Raman spectra and X-ray diffraction were reported.26 Raman spectroscopy is one of the most convenient and powerful tools for studying hydrogen bonds and investigating conformations and dynamics of molecules in the condensed phase. To establish stable or metastable phase relation between different crystalline modifications and to obtain information on weak O-H · · · O interactions in molybdenum trioxide hydrates, we have carried out studies on Raman spectra of MOO3 · xH2O (x ) 1/2, 2) at various pressures. In this paper, we present an investigation on pressure-induced structural phase transitions of MOO3 · 1/2H2O and MOO3 · 2H2O at room temperature using in situ Raman spectra measurements * Corresponding author. E-mail:
[email protected].
in a diamond anvil cell up to pressures of 31.3 and 30.3 GPa, respectively. Our results show that both MOO3 · 1/2H2O and MOO3 · 2H2O undergo two structural phase transitions under high pressures. We also present and discuss the behavior of the O-H · · · O hydrogen bond of MOO3 · 1/2H2O and MOO3 · 2H2O under high pressures. 2. Experimental Section White powder of MOO3 · 1/2H2O was prepared with molybdenum powder and nitric acid following the methods by Benard.27 The yellow powder of MOO3 · 2H2O was synthesized with Na2MoO4 · 2H2O and nitric acid following Freedman et al.28 The structures of the synthetic materials were refined and easily indexed to monoclinic structure, corresponding to P21/m (phase Ih) for hemihydrate MOO3 · 1/2H2O and P21/n (phase Id) for dihydrate MOO3 · 2H2O. Molybdenum trioxide hydrate powder was loaded into a gasketed high pressure Diamond Anvil Cell (DAC) with a culet face of 500 µm in diameter using a mixture of 4:1 (methanol/ ethanol) as the pressure-transmitting medium. High-pressure Raman experiments of MOO3 · 1/2H2O and MOO3 · 2H2O were carried out at room temperature up to pressures of 31.3 and 30.3 GPa, respectively. The T301 stainless steel gasket was preindented by diamond anvils to an initial thickness of about 60 µm, and then a center hole of 120 µm diameter was drilled as the sample chamber. Pressure was determined from the frequency shift of the ruby R1 fluorescence line.29 By monitoring the separation and widths of both R1 and R2 lines, we confirmed that hydrostatic conditions were maintained throughout the experiments. The precision of our pressure measurements was estimated to be around 0.05 GPa. High-pressure Raman spectra were recorded on a Renishaw inVia Raman Microscope in the backscattering geometry using the 514.5 nm line of an argon ion laser with an incident power of 15 mW and a CCD detector system. The spectral resolution was about 1 cm-1. The average time used in the acquisition of a single spectrum was about 30 s. Pressure-induced shifts of overlapping Raman bands were analyzed by fitting the spectra to Lorentzian functions to determine the line shape parameters. Experimental details
10.1021/jp906423w 2009 American Chemical Society Published on Web 11/25/2009
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Figure 1. (a) A perspective view of the ab-plane shows the 3D structure of MOO3 · 1/2H2O with the intermolecular hydrogen bonding interactions. An alternation of linear double rows running along the b-axis direction and linked together through corner-sharing MoO6 and MoO5(OH2) octahedra. (b) Monoclinic MOO3 · 2H2O structure with layers of corner-sharing MoO5(OH2) distorted octahedra exchanging four oxygens in the equatorial plane and stacking along the b-axis direction. The second type of water molecule assumes the interlayer position, and the layers are maintained by the O-H · · · O hydrogen bond interactions. The yellow dashed lines represent O-H · · · O hydrogen bonds.
regarding the Raman and ruby fluorescence systems are presented below. 3. Results and Discussion A. Pressure Dependence of the Vibrational Modes in MOO3 · 1/2H2O. Raman spectroscopy provides valuable information on local and cooperative changes during pressureinduced transformations between phases. The structure projection parallel to the c-axis direction of MOO3 · 1/2H2O is shown in Figure 1a.23 We can see that the structure consists of two kinds of distorted MoO6 and MoO5(OH2) octahedra. Within a double chain, each MoO6 octahedron has two edges in common with the neighboring octahedral, and the successive octahedral double chains are linked together by sharing corners to form a layer. The site symmetry of the two water molecules in the unit cell is Cs, and this yields two vibrations in the C2h factor group for each internal mode. One of these vibrations is Raman-active, and the other, IR-active so that no splitting due to intermolecular coupling is expected in the Raman (or IR) spectra.30 In our experiments, all Raman-active phonon modes are discernible at ambient conditions, and all the observed modes correspond well to the band positions reported in the literature for MOO3 · 1/2H2O.30 Figure 2a shows representative Raman bands of MOO3 · 1/2H2O in the frequency range of 100-1200 cm-1 at different pressures under hydrostatic compression. Two bands are observed in the range 1500-3600 cm-1: the one at 1620 cm-1 is attributed to the δO-H bending vibration mode, and the other near 3461 cm-1 is assigned to νO-H stretching of H2O, as shown in Figure 2b. All Raman peak assignments are summarized in Table 1. In Figure 2b, we notice that the δO-H bending vibration mode of H2O displays continuous shifts to higher wavenumbers, and the intensity diminishes gradually and becomes indiscernible when the pressure is above 3.3 GPa. However, the νO-H stretching mode of H2O shifts to a lower frequency with increasing pressures up to 13.7 GPa. To obtain the pressure coefficient of mode frequencies in each phase, linear fits to data presented in Figure 3 were performed. We can see that the O-H stretching band exhibits anomalous pressure-induced frequency redshift as the pressure increases. In Figure 3, we obtain a negative slope (dν/dP ) -7.66 cm-1/GPa) of νO-H stretching
Liu et al.
Figure 2. Raman spectra of MOO3 · 1/2H2O at different pressures. For clarity, the spectra have been divided into two parts: (a) 100-1200 cm-1 and (b) 1500-3700 cm-1. Symbols 1 and * show the appearance of new Raman peaks corresponding to IIh and IIIh phases, respectively.
TABLE 1: Raman Frequencies (cm-1) of MOO3 · 2H2O and MOO3 · 1/2H2O at Ambient Conditions MOO3 · 2H2O
MOO3 · 1/2H2O
Assignent
3461 1620 984 979 923 906 816 ∼700-400
∼3500-3000
νO-H stretching δO-H bending νOdModO stretching νModO stretching νOdModO stretching Mo-O bridging νOMo2 stretching νOMo3 stretching Deformation modes νMo-OH2 stretching Deformation and lattice modes
∼400-300
