Molecular Dynamics in Supercooled P−Se Liquids near the Glass

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Molecular Dynamics in Supercooled P-Se Liquids near the Glass Transition: Results from 31P NMR Spectroscopy E. L. Gjersing and S. Sen* Department of Chemical Engineering and Materials Science, University of California Davis, Davis, California 95616, United States

B. G. Aitken Glass Research Division, Corning Incorporated, Corning, New York 14831, United States ABSTRACT: The structure of phosphorus selenide glasses with compositions close to the P4Se3 stoichiometry with and without doping with a few atom % Ge has been investigated with Raman and 31P NMR spectroscopic techniques. The results indicate that the structure of these glasses consists predominantly of P4Se3 cage molecules. However, in spite of this structural similarity, doping with Ge results in a remarkably large increase in Tg. The dynamical behavior of the constituent P4Se3 molecules in the Ge-free composition is investigated with a 31P NMR hole-burning technique in the supercooled liquid state. These molecules perform large angle rotational reorientations near and above the glass transition with time scales similar to those expected for shear relaxation. Such coupling between molecular rotation and shear relaxation processes near Tg is reminiscent of the dynamical behavior of organic molecular glass-forming liquids. However, this behavior is in stark contrast with the large temporal decoupling between molecular rotation and shear relaxation previously reported for a Ge-doped arsenic sulfide liquid that contained similarly structured As4S3 cage molecules.

’ INTRODUCTION The binary phosphorus selenides (P-Se) are some of the most widely studied chalcogenide glasses due to their large glass-forming region and the resulting rich variety of structural and topological motifs.1,2 The atomic structure of these glasses has been studied in detail using neutron diffraction and Raman, nuclear magnetic resonance (NMR), and extended X-ray absorption fine structure (EXAFS) spectroscopic techniques, and an overall consistent picture of the compositional dependence of structure has emerged.1-6 The selenium-rich glasses in this system in the compositional region between 0 and 40 atom % P form network structures typical of the covalently bonded chalcogenides with 4and 3-coordinated PSe4 tetrahedra and PSe3 pyramidal units linked together by selenium chains. On the other hand, glasses with 40-54 atom % P and with 64-84 atom % P have been shown to contain a significant fraction of molecular P4Se3 units that, in P-rich glasses (g67 atom % P), are believed to be embedded in a matrix of amorphous red phosphorus consisting of corner-shared P atoms. The concentration of the P4Se3 molecules is of course expected to be the highest in glasses with similar stoichiometry, i.e., with ∼57 atom % P. However, glass formation has proved to be difficult in the compositional range between 54 and 64 atom % P due to increasing tendency of crystallization. Although molecular glasses and liquids are quite common in organic systems (e.g., ortho-terphenyl, glycerol), no such glasses were known in the inorganic world until Verrall and Elliot reported the glass of composition P2Se (67 atom % P) to be r 2011 American Chemical Society

predominantly molecular with a structure composed largely of P4Se3 molecules.7,8 Subsequently and more recently we have shown the existence of molecular pseudobinary As-S glasses doped with Ge in the Ge-As-S system with compositions near Ge3As52S45 that are composed predominantly of As4S3 molecules, held together by van der Waals forces.9 The As4S3 molecules are topologically similar to P4Se3 molecules with a cagelike structure and a high degree of sphericity. These molecules consist of a three-membered As(P)3 ring surmounted by an As(P)S(Se)3 pyramid with each S(Se) atom being bonded to an As(P) atom in the As(P)3 ring that is directly below it. Besides their molecular structure, these Ge-doped As-sulfide glasses are also characterized by intriguing dynamical properties. Gjersing et al. had demonstrated that the rotational time scale of the As4S3 units in these molecular liquids was decoupled by several orders of magnitude from the time scale of shear relaxation near the glass transition.9 The frequency of rotational reorientation of these molecules was found to remain surprisingly fast (∼2 kHz) even at ∼70 K below Tg. Hence, dynamically this glass behaves like a system where the molecules reorient relatively freely over a time scale during which their average positions remain fixed in space. It is interesting to note that such dynamics are somewhat similar to the molecular dynamics encountered in plastic crystals Received: December 7, 2010 Revised: February 14, 2011 Published: March 09, 2011 2857

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The Journal of Physical Chemistry B where the molecules reorient, while their positions remain fixed at the lattice sites of the structure. In fact, high-temperature polymorphs of As4S3, P4S3, and P4Se3 are examples of such plastic crystalline phases.10-12 This observation is in contrast with the cases of organic molecular glass-forming liquids where the time scale of rotational diffusion of the constituent molecules remains closely coupled with the shear relaxation and freezes at experimental time scales below Tg.13,14 One potential reason behind this difference in the dynamical behavior between the As4S3 and the organic molecules may be that the former is a highly spherical molecule that may have a much lower activation energy barrier for rotational motion compared to that for nonspherical organic molecules. In this regard the dynamics of P4Se3 molecules can provide important additional information since both the sphericity and the moment of inertia of these molecules are expected to be different from those of the As4S3 molecules (vide infra). Detailed investigation of the high-temperature speciation equilibria and species exchange dynamics in P-Se liquids containing 5-48 atom % P was carried out by Maxwell and Eckert using both 31P and 77Se nuclear magnetic resonance (NMR) spectroscopy.15-17 However, little is known regarding dynamical processes in P-rich glass-forming liquids with large concentrations of P4Se3 molecules. Presented here are the results of structural and dynamical investigations on two novel phosphorus selenide glasses with P:Se ratios corresponding closely to P4Se3 stoichiometry. These glasses with ∼58 and 62 atom % P lie in the previously unexplored composition region demarcated by 54 and 64 atom % P where the binary P-Se liquids are found to readily devitrify upon cooling. The glass with ∼62% P was obtained via fast quenching, while the glass containing 57% P was stabilized via addition of 2.8 atom % Ge. These two glasses will be referred to as PSe and GPSe, respectively, in the subsequent discussion for the sake of brevity. The structures of these two glasses have been studied using Raman and 31P NMR spectroscopy. In addition, variable-temperature 31P single-pulse and holeburning NMR spectroscopy have been carried out to study the dynamic behavior of the P4Se3 molecules in these glasses and corresponding supercooled liquids.

’ EXPERIMENTAL SECTION Sample Synthesis and Characterization of Physical Properties. The PSe and GPSe glasses were synthesized by melting

mixtures of g99.995% purity (metals basis) elements in evacuated (10-6 Torr) and flame-sealed fused silica ampules at a temperature of 873 K for at least 13 h in a rocking furnace. The ampules were rapidly quenched in water to make transparent orange-red glass. Both glass samples were confirmed to be amorphous using powder X-ray diffraction. Chemical compositions of the samples were determined using electron microprobe analysis to be P62.3Se37.6 (PSe) and P57.7Se39.5Ge2.8 (GPSe). Differential scanning calorimetry with a heating rate of 10 K/min was employed to determine Tg values of 468 and 286 K for the GPSe and PSe glasses, respectively. Density measurements were performed using the Archimedes method; the density of the GPSe and PSe glasses at ambient temperature was found to be 3.09 and 2.93 g cm-3, respectively. The PSe glass sample, due to its low Tg, was kept in an airtight container and stored in a refrigerator at ∼283 K. Raman Spectroscopy. The unpolarized Raman spectra of the GPSe glass and the PSe supercooled liquid were collected at ambient temperature in a backscattering geometry using a Bruker

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Figure 1. Unpolarized Raman spectra of (a) GPSe glass and (b) PSe supercooled liquid at ambient temperature.

RFS 100/S Fourier-transform (FT) Raman spectrometer equipped with a frequency-doubled Nd:YAG laser operating at a wavelength of 1064 nm. The resolution of the solid-state liquid nitrogen cooled Ge detector was set to 2 cm-1, and laser power levels were varied between 20 and 50 mW for data collection. About 64-128 scans were averaged to obtain each spectrum. 31 P NMR Spectroscopy. All 31P NMR spectra were acquired using a Bruker Avance 500 spectrometer operating at a magnetic field of 11.7 T with a 31P resonance frequency of 202.5 MHz. Spectra were acquired under static conditions and under magicangle-spinning (MAS) with spinning rates of up to 15 kHz using a Bruker 4 mm triple-resonance MAS probe. Crushed samples were taken in ZrO2 rotors with KelF caps. These static and MAS one-pulse spectra were collected with a 90° pulse (2.0 μs) recycle delay of 5 and 64 scans. For collection of 31P MAS NMR spectra at sample spinning rates of 20 and 30 kHz, a Bruker 2.5 mm triple-resonance probe was used with a 90° pulse length of 1.2 μs. All variable-temperature 31P NMR measurements were carried out using the Bruker 4 mm triple-resonance MAS probe. Sample temperature was controlled using hot or cold N2 gas, and probe thermocouple temperature was calibrated externally using the well-known temperature dependence of the 207Pb chemical shift of Pb(NO3)2.18 The 31P static hole burning NMR experiments were performed using the 4 mm probe and the pulse sequence discussed in ref 19 with a burn pulse of 5 ms at a power level of 0.029 W, a π/4 acquisition pulse of 1 μs at a power level of 291.4 W, and a recycle delay of 5 s. Sixteen scans were averaged to obtain each spectrum. The 31P NMR spectral line shapes collected during heating and subsequent cooling were found to be practically identical, implying reversibility and absence of any crystallization over the duration of the NMR measurements.

’ RESULTS AND DISCUSSION The Raman spectra for the GPSe glass and the PSe supercooled liquid are shown in Figure 1. These two spectra are nearly identical and are in excellent agreement with those reported in previous studies of P4Se3 crystal20 and P2Se glass.21 The main features corresponding to the nine Raman-active intramolecular vibrational modes of the P4Se3 cage molecule are all present in these two spectra, including five E modes at ∼133, 174, 315, 353, and 408 cm-1 and four A1 modes at ∼214, 324, 365, and 487 cm-1.21 The lack of any other vibrational band with any significant intensity in these spectra implies that the structure of these two materials is predominantly molecular in nature, consisting of P4Se3 molecules. The main 2858

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Figure 3. 31P static and MAS NMR spectra of PSe supercooled liquid acquired at room temperature. Spinning rates for the MAS experiments are indicated alongside each spectrum. Spinning sidebands indicated by asterisks in the MAS spectra. Figure 2. 31P static and MAS NMR spectra of GPSe glass acquired at room temperature. Spinning rates for the MAS experiments are indicated alongside each spectrum. Spinning sidebands indicated by asterisks in the MAS spectra.

observable difference between the GPSe glass and the PSe supercooled liquid in their Raman spectra is a shoulder on the low frequency side of the 215 cm-1 peak in the spectrum of the GPSe glass, located near ∼205 cm-1. This shoulder near 205 cm-1 can be assigned, on the basis of previous Raman studies on GexSe100-x glasses, to the stretching mode of GeSe4/2 tetrahedra sharing Se atoms.22,23 Therefore, it may represent Ge-Se-Ge linkages in (GeP3Se3)2 dimers where the apical P atom in the P4Se3 molecule is replaced by a Ge, and to fulfill the four bonds needed for the Ge atom, two of these Ge-containing GeP3Se3 units link together via a shared Se atom resulting in Ge2P6Se7 units. The arsenic analogs of these units have previously been conjectured to be present in Gedoped molecular arsenic sulfide glasses containing As4S3 molecules.24 On the other hand, the Raman spectrum of the GPSe glass is also consistent with the presence of tetrahedrally coordinated Ge atoms locally forming regions of corner and edge-shared GeSe4 tetrahedra. Figure 2 displays the room-temperature 31P static and MAS NMR spectra for the GPSe glass. The latter were collected at different spinning speeds to isolate the spinning sidebands from isotropic peaks. The static spectrum displays a broad signal between -150 and 300 ppm that appears to consist of at least three overlapping resonances, and MAS was employed at speeds of 10, 15, 20, and 30 kHz to obtain higher resolution of these resonances. Relatively narrow isotropic peaks are clearly visible in these MAS spectra at 145, 60, and -78 ppm along with a broad resonance spanning between 50 and 150 ppm that is not narrowed significantly by MAS. On the basis of previous 31P NMR studies of P-Se glasses and P4Se3 crystal, the peaks at 60 and -78 ppm can be readily assigned to the apical and basal P atoms, respectively, in P4Se3 molecules.1,2,6,15-17 Line shape simulations of these 31P MAS NMR spectra in Figure 2 indicate that the relative fraction of P atoms in these apical and basal sites is 18 and 53%, respectively, thereby being consistent with the expected 1:3 ratio for these sites in P4Se3 molecules. These simulations also indicate that the basal P sites in these molecules are characterized by a rather large chemical shift anisotropy (CSA) of

about -170 ppm. The sharp peak at 145 ppm represents a small fraction of the P atoms (∼3%) that exist in highly symmetric sites with negligible CSA. The structural assignment of this peak is not clear to date, but it must represent a small concentration of P atoms in a very ordered, possibly molecular, environment in the glass structure. The broad peak between 50 and 150 ppm represents the remaining 26% of the P atoms in the sample. In a recent study Bytchkov et al.2 have assigned this broad resonance to threecoordinated P atoms in an amorphous red phosphorus like environment. It may be noted here that, unlike the P-Se glasses with g67 atom % P as reported in a previous study by Bytchkov et al., the 31P MAS spectra of the GPSe glass do not display significant peaks at 130, 30, and -125 ppm corresponding to molecular P4Se2 units. Therefore, the concentration of such molecules in the GPSe glass with ∼57.7 atom % P must be negligible. If all Ge atoms are involved in the formation of Ge2P6Se7 units and the rest of the Se atoms are tied up in P4Se3 molecular units, then the composition of the GPSe glass implies the presence of about 16-17% of P atoms in an amorphous red phosphorus like environment. This estimate is significantly lower than the value of ∼26% obtained from simulation of the 31P MAS NMR spectra, as mentioned above. On the other hand, calculations on the basis of a structural model where Ge atoms are present as corner- and edge-sharing GeSe4 tetrahedra forming locally GeSe2-like regions suggest that ∼22-23% P atoms can be present in amorphous red phosphorus like environments. This estimate is quite close to the experimentally observed value of 26% and suggests that the structure of the GPSe glass is composed predominantly of P4Se3 molecules coexisting with a small volume fraction of GeSe2 and red-phosphorus like structural regions. The room-temperature 31P static and MAS NMR spectra of the PSe supercooled liquid are shown in Figure 3. The static spectrum displays three peaks that are approximately in the same position as those in the static 31P NMR spectrum of the GPSe glass (Figure 2). However, these peaks are significantly narrower and better resolved in the case of the PSe supercooled liquid compared to those in the GPSe glass. The 31P MAS NMR spectra of the PSe supercooled liquid reveal three isotropic peaks at -77, 62, and 145 ppm that were also observed in the GPSe glass at approximately similar positions. The relative fractions of P atoms 2859

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Figure 4. 31P (a) static and (b) 10 kHz MAS NMR spectra of PSe glass and supercooled liquid acquired at the temperatures indicated alongside each spectrum. Spinning sidebands indicated by asterisks in the MAS spectra.

with chemical shifts at -77 and 62 ppm are ∼68 and 23%, respectively, indicating that nearly 91% of the P atoms in the PSe glass belong to P4Se3 molecules. This fraction is significantly higher than that (71%) observed in the GPSe glass. It is also noteworthy that the CSA of the basal P sites in the PSe glass is approximately -100 ppm, significantly lower than that (-170 ppm) characteristic of the GPSe glass as mentioned above. The peak at 145 ppm corresponds to nearly 9% of the P atoms, higher than the value of 3% for the GPSe glass. Notably absent from the room-temperature spectra of this PSe glass is the large, broad peak between 50 and 150 ppm, characteristic of the amorphous red-phosphorus like structural regions. In relation to the significant differences observed between the room-temperature 31P NMR spectra of the GPSe and PSe glasses, we would like to point out that the Tg values for the GPSe and PSe glasses are rather different (468 and 286 K, respectively) with the Tg of the PSe glass being 12 K lower than room temperature. Therefore, 31 P NMR spectra of PSe in Figure 3 were acquired above Tg in the supercooled liquid state, and these spectra may be affected by dynamical effects related to rotational motion of the P4Se3 molecular units in the liquid.7,9 To explore this possibility, variable-temperature 31 P static and MAS NMR experiments were conducted at temperatures above and below Tg for the PSe material, and the results are shown in Figure 4. The static 31P NMR spectra of PSe glass and supercooled liquid, collected over a temperature range of 223-323 K (Figure 4a), demonstrate that the room-temperature 31P NMR spectrum is indeed motionally narrowed and the static 31P NMR line shape of PSe glass at 223 K looks nearly identical to that of the roomtemperature spectrum of the GPSe glass. As the temperature is raised to 298 K, the three peaks become better resolved as the CSAs of the P4Se3 molecular peaks begin to average (vide infra) due to rotational motion of these molecules in the PSe supercooled liquid (Figure 4). It is to be noted here that although the orientational averaging of the CSAs can only be obtained via rotational reorientation of molecules such observation does not necessarily preclude the presence of any translational motion of the P4Se3 molecules. Random translational

motion can lead to line narrowing; however, NMR CSAs cannot get averaged without a time-dependent change in the orientation of the molecules with respect to the applied magnetic field. Upon further increase in temperature to 323 K, the two peaks corresponding to the apical and basal P sites in the P4Se3 molecules appear almost completely averaged, with positions of 62 and -80 ppm only a few parts per million away from their isotropic values of 61 and -77 ppm (Figure 4). The third broad peak in these spectra, spanning the range between 50 and 150 ppm corresponding to the amorphous red phosphorus type environments, shows very little narrowing with increasing temperature. The intensity of this peak therefore appears weaker at higher temperatures as the areas of the molecular peaks begin to collapse under narrower lines and producing higher intensities. For example, the widths of the molecular peaks at -77 and 61 ppm are reduced in half from 127 to 63 ppm and from 55 to 21 ppm, respectively, as the temperature increases from 298 to 323 K. The width of the broad peak corresponding to the amorphous red phosphorus type environments, on the other hand, decreases by only 10% in this temperature range, dropping from 99 ppm at 298 K to 90 ppm at 323 K. This relative lack of peak narrowing for the red phosphorus matrix indicates that these atoms remain effectively fixed in their positions; i.e., they do not participate in significant motion nor are they involved in chemical exchange with the molecular P4Se3 units. This broad peak is a large contributor to both the 298 and 323 K 31P NMR static spectra for the PSe supercooled liquid, and a question arises about the absence of this peak in the room-temperature 31P MAS spectra shown in Figure 3. To address this issue, 31P MAS NMR spectra were acquired for the PSe glass and supercooled liquid over the temperature range of 248-295 K (Figure 4b). For the two spectra that were collected below Tg at 248 and 273 K, the broad peak between 50 and 150 ppm is readily apparent and comparable to that in the room-temperature 31P MAS NMR spectrum of the GPSe glass sample. However, at only 2 K above Tg (288 K), the motional narrowing of the P4Se3 molecular peaks becomes 2860

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Figure 5. Normalized 31P NMR spectra of the PSe supercooled liquid at 295 K with hole burned at the basal P site (-77 ppm) of the P4Se3 molecule as a function of increasing delay time following the burning of the hole. From the lowest to the highest intensity of the peak at -77 ppm, the τ values are: 0.001, 0.003, 0.025, 0.05, 0.08, 0.09, 0.2, and 0.5 s.

significant enough that, combined with the splitting of the peaks under MAS that further concentrates the signal, the broad peak is no longer resolved (Figure 4b). The change in the CSA of the -77 ppm peak as the P4Se3 molecules experience rotational motion in the supercooled liquid state is also apparent from these variable-temperature 31P MAS NMR spectra in Figure 4b. For example, the spinning sidebands for this peak extend out to -300 ppm in the sub-Tg spectra, and this limit decreases to -277 ppm in the 288 K spectrum and finally to -250 ppm in the 295 K spectrum (Figure 4b). The sub-Tg 31P MAS NMR spectrum of the PSe glass collected at 248 K is remarkably similar to the room-temperature 31 P MAS NMR spectrum of the GPSe glass. Simulation of the 248 K spectrum of the PSe glass in Figure 4b yields nearly identical speciation to that of the GPSe glass with the relative fractions of 56, 19, and 4% for the peaks at -77, 60, and 145 ppm, respectively, and the remaining 21% under the broad peak ranging between 50 and 150 ppm. This result therefore confirms the findings of the Raman spectroscopy results which show nearly identical vibrational spectra for the two compositions. 31 P static NMR hole-burning experiments have been carried out to investigate the time scale of the rotational dynamics of P4Se3 molecules in the supercooled PSe liquid near Tg. As shown in Figure 3, the 31P static line shape of these materials is primarily inhomogeneously broadened due to the chemical shift anisotropy, and therefore rf pulses can be used to selectively saturate or invert spins that correspond to specific orientations of the P4Se3 molecules. This “hole-burning” in the NMR line shape allows tagging of these molecules, and subsequent rotational reorientation of the constituent molecules (both tagged and untagged) will result in spectral diffusion and recovery of the hole.19 The nature and time scale of recovery of the burnt hole therefore provides important information regarding the dynamics of the constituent molecules. The 31P static NMR spectra of the supercooled PSe liquid at 298 K with a hole burnt into the basal P site at -77 ppm and acquired with τ, the delay between burn pulse and acquisition pulse, ranging from 0.001 and 0.5 s, are shown in Figure 5. After proper normalization of these spectra to a constant integrated intensity to cancel out the effect of spin-lattice relxation, the remaining fraction of the burnt hole versus τ provides direct information on the rotational time scale of the P4Se3 molecules

Figure 6. Fraction of recovered hole (at -77 ppm) versus τ for the PSe supercooled liquid at 298 K (a) and at 305 K (b). Note the difference in scale for the x-axes in (a) and in (b).

and is shown in Figure 6a for the data presented in Figure 5. This experiment was also performed with the same hole width and position at 305 K, and the corresponding hole recovery data are presented in Figure 6b. These holes display nonexponential recovery behavior that can be simulated with a stretched exponential function f = exp[-(τ/τR)β] where f is the fraction of the burnt hole, τR the hole recovery time, and β the stretching or Kohlrausch-Williams-Watts (KWW) exponent that varies between 0 and 1 (Figure 6). The recovery times τR thus obtained are 0.12 s at 298 K and 0.02 s at 305 K with β = 0.73 and 0.99, respectively. Similar τR and β are obtained irrespective of the width of the burnt hole, consistent with the observations made previously in 13C NMR hole-burning experiments for isotropic rotational reorientation of glycerol molecules.19 These results therefore imply rotation of the P4Se3 molecules via large angular jumps in the supercooled PSe liquid near the glass transition, and the relaxation becomes increasingly nonexponential with decreasing temperature as Tg is approached from above. Holes were also burned at the apical P site at 60 ppm, and a relaxation time of 0.15 s, similar to that (0.12 s) for the basal P sites, is obtained at 298 K. This similarity between the two time scales confirms that the hole-burning experiment is measuring the reorientation of the entire P4Se3 molecule rather than localized motions of the individual P atoms. The time scales for rotational motion of P4Se3 molecules in the PSe supercooled liquid as determined from the hole-burning NMR are on the order of 10-1 and 10-2 s for 1000/T values of 3.35 and 3.28 K-1, respectively. Assumption of a linear dependence between log τR and 1000/T (i.e., a single activation energy) predicts a τR of 2861

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Figure 7. Comparison of the geometry of P4Se3 (left) and As4S3 (right) molecular units. One of the four P or As atoms is located at the apex, and the other three are located at the base of the molecule. Higher sphericity of the As4S3 molecule is apparent from higher coverage of the area of the circumscribing circle compared to that for the P4Se3 molecule.

∼101.1 s at 1000/Tg = 3.5 K-1 for this composition. This magnitude of τR is consistent with the fact that the shear relaxation time scale at Tg for a wide range of glass-forming liquids is expected to be on the order of 101-102 s. Although no viscosity data are available for the PSe liquid, the magnitude of the rotational time scales τR of the constituent molecules just above Tg indicates that such motion is most likely coupled to and controls the shear relaxation processes. Such coupling between the time scales of molecular rotation and shear relaxation is similar to the behavior displayed by organic molecular glass-forming liquids such as glycerol and o-terphenyl13,25 and is in stark contrast with the behavior of more structurally and chemically similar inorganic As4S3-based molecular liquids.9 As mentioned earlier, in the As4S3 glass-forming liquid the time scale of rotation of the constituent molecules is strongly decoupled from that of shear relaxation by several orders of magnitude, and the molecules are able to rotate freely even at temperatures significantly below Tg. The P4Se3 and As4S3 cage molecules appear structurally identical at first glance, but an investigation of the intramolecular bond angles and lengths reveals some important differences with shorter P-P bond lengths for the basal P atoms of 2.2 Å compared to the As-As bond length of 2.45 Å for basal As atoms and a smaller P-Se-P bond angle of 100° compared to 105° for the As-S-As angle.20,26 Moreover, from considering the fact that the atomic radii of the P atoms are smaller than those of the Se atoms, a bulge would be created around the equator of the P4Se3 molecule, whereas As atoms, being larger than the S atoms, would allow for a more spherical molecule as depicted in Figure 7. Sphericity (Ψ), or the ratio of the surface area of a sphere with the same volume as the molecule to the actual surface area of the molecule, is given by Ψ ¼ π1=3 ð6V Þ2=3 =A where V is the volume; A is the surface area; and Ψ = 1 indicates a perfect sphere. Ψ for the two molecules is calculated from the bond length and angle information provided in the literature.20,26,27 Such calculations confirm that the P4Se3 molecule is indeed less spherical, with Ψ = 0.74, than the As4S3 molecule which has Ψ = 0.82. Besides, as mentioned earlier, the moment of inertia of the P4Se3 molecule will be larger by 25% due to its larger mass (10%) and radius (5%) compared to that of the As4S3 molecule. It is unclear at this stage how a reduction in the sphericity and increase in the moment of inertia of this magnitude would affect the activation energy for rotational dynamics. However, such differences must play an important role in

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understanding the large change in the dynamic behavior between the two systems. An additional factor that may be responsible for the differences in molecular dynamics is that the previously studied As4S3 system was doped with Ge. This hypothesis corroborates the observation made in this study that, in spite of the similarity in the phosphorus speciation and molecular concentration, the Tg of the GPSe glass is significantly higher (468 K) than that of the PSe glass (286 K). It is likely that the presence of Ge results in partial polymerization of the molecular structure and/or formation of fragments of a corner-shared Ge(S/Se)4 tetrahedral network that exerts a strong control on the glass transition process of the liquid.24 This scenario is consistent with the consequent decoupling of the rotational dynamics of the isolated cage molecules from the glass transition process in Ge-doped molecular liquids. Future dynamical studies on the GPSe glass may shed more light on the role of the Ge atoms in the rotational and shear relaxation of P4Se3-based molecular glasses.

’ SUMMARY The structure and dynamical behavior of two phosphorus selenide glasses with compositions P62.26Se37.6 and P57.7Se39.5Ge2.8 are investigated. These two glasses are predominantly molecular, consisting of P4Se3 cage molecules. In addition, ∼25% of the phosphorus atoms in both glasses form amorphous red phosphorus like structural regions. The Ge atoms in the Ge-doped glass possibly form small domains of a three-dimensional network of cornershared GeSe4 tetrahedra that presumably is responsible for radically altering the Tg from 286 K for the Ge-free glass to 468 K for the Gedoped glass. The dynamical behavior of the P4Se3 cage molecules in the Ge-free composition is investigated in the supercooled liquid state with a 31P hole-burning NMR technique. The results indicate isotropic rotational reorientation of the P4Se3 molecules in the supercooled liquid with a time scale that is similar to that of shear relaxation. This dynamical behavior is in sharp contrast with the results of a previous study on a Ge-doped molecular arsenic sulfide liquid where the time scale of rotation of the constituent isomeric As4S3 cage molecules showed large decoupling from that of shear relaxation by several orders of magnitude. Such dynamical differences can be related to the differences in sphericity and moment of inertia between P4Se3 and As4S3 molecules and to the presence of Ge dopant in the arsenic sulfide liquid. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by NSF Grant DMR-0906070 to S.S. ’ REFERENCES (1) Georgiev, D. G.; Mitkova, M.; Boolchand, P.; Brunklaus, G.; Eckert, H.; Micolaut, M. Phys. Rev. 2001, B64, 134204. (2) Bytchkov, F.; Fayon, D.; Massiot, L.; Hennet; Price, D. L. Phys. Chem. Chem. Phys. 2010, 12, 1535. (3) Arai, M.; Johnson, R. W.; Price, D. L.; Susman, S.; Gay, M.; Enderby, J. E. J. Non-Cryst. Solids 1986, 83, 80. (4) Price, D. L.; Misawa, M.; Susman, S.; Morrison, T. I.; Shenoy, G. K.; Grimsditch, M. J. Non-Cryst. Solids 1984, 66, 443. (5) Lathrop, D.; Eckert, H. J. Non-Cryst. Solids 1988, 106, 417. 2862

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dx.doi.org/10.1021/jp111641f |J. Phys. Chem. B 2011, 115, 2857–2863