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Structural Evolution of Solid Phenanthrene at High Pressures Francesco Capitani, Marc Höppner, Lorenzo Malavasi, Carlo Marini, Gianluca A. Artioli, Michael Hanfland, Paolo Dore, Lilia Boeri, and Paolo Postorino J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04326 • Publication Date (Web): 15 Jun 2016 Downloaded from http://pubs.acs.org on June 20, 2016

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The Journal of Physical Chemistry

Structural Evolution of Solid Phenanthrene at High Pressures F. Capitani, ∗,†,4 M. Höppner, ‡ L. Malavasi, ¶ C. Marini, § G.A. Artioli, ¶ M. Hanand, k P. Dore, ⊥ L. Boeri, #,‡ and P. Postorino @ †Dipartimento di Fisica, Università di Roma Sapienza, P.le Aldo Moro 2, 00185 Roma, Italy ‡Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569, Stuttgart, Germany ¶Dipartimento di Chimica, Università di Pavia, Via Taramelli 16, 27100 Pavia, Italy §CELLS-ALBA, Carretera B.P. 1413, Cerdanyola del Vallès, 08290 Barcelona, Spain. kEuropean Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France ⊥CNR-SPIN and Dipartimento di Fisica, Università di Roma Sapienza, P.le Aldo Moro 2, 00185 Roma, Italy #Institute of Theoretical and Computational Physics, TU Graz, Petersgasse 16, 8010, Graz, Austria @CNR-IOM and Dipartimento di Fisica, Università di Roma Sapienza, P.le Aldo Moro 2, 00185 Roma, Italy 4Present Address: Synchrotron SOLEIL, L’Orme des Merisiers, F-91192 Gif-sur-Yvette, France E-mail: [email protected]

Phone: +330169359462

Abstract

1. Introduction

The effect of hydrostatic pressure on the structure of solid phenanthrene (C14 H10 ) was investigated up to 25.7 GPa through synchrotron X-ray powder diffraction and an evolutionary algorithm for crystal structure prediction based on van der Waals Density Functional calculations. We observed the onset of a phase transition around 8 GPa from the monoclinic P21 low pressure phase, with two molecules per unit cell arranged in a herringbone fashion, to a new high pressure phase. The best candidate structure for this phase exhibits three molecules in a P1 triclinic unit cell in a parallel arrangement, stabilized by dominant π − π intermolecular interactions. The P21 and P1 phases coexist in the pressure range from 8 to 13 GPa, whereas above 13 GPa only the P1 high pressure phase is present. At higher pressures (P > 20 GPa), experiments and first-principles calculations suggest a tendency towards amorphization.

Polycyclic Aromatic Hydrocarbons (PAHs) are receiving a special attention in several fields, since their intrinsic electronic mobilities and largely tunable semiconducting properties 1 anticipate a wide range of technological applications. Given the considerable interest, many chemical protocols have been set to optimize their chemical synthesis. 2,3 Moreover, superconductivity with Tc ’s up to 33 K has been reported in several phenacenes doped with alkaline and rare-earths; 4–10 several aspects of this discovery are currently debated. 11–15 One of the most attractive properties of these materials is their extreme tunability: because of the small energy scale involved in different inter- and intra-molecular interactions, small external perturbations can induce large changes in physical properties, accompanied by equally large modifications of the crystal structure. 16–19 Due to their strong interrelation, understanding the struc-

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tural properties of these solids is a crucial step in controlling their functionality. The crystal structures of most PAHs at ambient conditions have been fully characterized, whereas the study of their evolution at high pressure is still incomplete and the essential underlying mechanisms are not entirely understood. Structural studies of organic solids at high pressures are indeed still a challenging task, due to the practical limitations of X-Ray diffraction techniques (small cross sections of carbon and hydrogen atoms, reduced sample dimensions, pressure gradients,etc), the deficiency of standard Density Functional Theory (DFT) methods in describing intra- and intermolecular interactions on an equal footing, and the intrinsic complexity of the energy landscapes of molecular solids. 20–23 Most studies in literature employ a combination of empirical structural models, 24 XRD and ab-initio calculations, 25 which have limited predictive power. Vibrational spectroscopy, with or without the support of XRD, have been also exploited to study both homocyclic 16,18,26–29 and heterocyclic 30–33 aromatic systems under pressure. Although each system presents a slightly different behavior under pressure, a general trend can be identified. At low pressure molecules change their spatial configuration in order to achieve a closer packing that is often translated in a structural phase transition. 25,31,34 At higher pressures intermolecular distances are strongly reduced and, beyond a certain threshold, molcecular bonds break and irreversibles chemical reactions, such as polymerization 28,30,35 or amorphization, 16,26,27,31 takes place. Nevertheless, in many cases, the structural scenario behind these pressure-induced transformations has not been completely unveiled owing to the aforementioned limitations of both XRD and theoretical models. An exception is represented by benzene, the model aromatic system. In fact, it was demonstrated that benzene starts to react giving rise to hydrogenated amorphous carbon when the distance between the nearest neighbor molecules is about 2.6 Å. This distance is obtained at 41 GPa in annealed and at 23 GPa in nonannealed samples. 26 The same critical distance has been also observed for triazine but at lower pressure, i.e. 8 GPa, owing to the lower stability of heterocyclic rings 32

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In this work, we try to make a step forward in the study aromatic systems under pressure. We perform synchrotron XRD measurements as a function of pressure of the crystal structure of a PAH, phenanthrene (C14 H10 ), under very good hydrostatic conditions. The structural models used to fit the experimental spectra are generated by an Evolutionary Algorithm (EA) for crystal structure search based on van der Waals (vdW) DFT calculations. 36 This method, once applied to the ambient pressure solid, provides a structural model in agreement with that reported in the literature and with our present diffraction data. This made us confident about the predictive power of our EA search and allows us to have an unbiased description and a microscopic insight of the phase diagram of solid phenanthrene up to a pressure of 25 GPa. Phenanthrene is a phenacene, i.e. a PAH in which N benzene rings are juxtaposed in a zigzag fashion. At ambient pressure, it crystallizes in a P21 herringbone structure with 2 formula units, and it is a wide-band semiconductor. 37,38 Wang et al. 5 reported a superconducting transition with a Tc of 5 K upon potassium intercalation. A recent study 29 claimed a series of structural transitions between 0 and 30 GPa. We suspect that many of these transitions might be a spurious effect of non-hydrostatic pressure. In fact, our XRD data unambiguously point to a single structural transition at 8 GPa, to a phase with a partial molecular reorientation. Experimental and theoretical hints of the occurrence of a partial amorphization for P > 20 GPa have also been found for pressures above 20 GPa. This paper is organized as follows. In Section 2, we briefly describe our experimental and computational methods. In Section 3, we present a summary of the most important results. The following Section 4 contains a detailed discussion of the abinitio crystal structure search and of the general physical principles about the observed structural trend based on the results of our crystal structure search. In the end, we draw some general conclusions and provide a general perspective on the future studies of PAHs under pressure.


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2. Methods

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Experimental methods Polycrystalline phenanthrene powder was purchased from Sigma Aldrich and was further purified by sublimation. High pressure XRD measurements were performed at the ID-09 beamline of the ESRF synchrotron (Grenoble, France) with a fixed wavelength of 0.4127 Å. The sampleto-detector distance and the image plate orientation angles were calibrated using the CeO2 standard. The two-dimensional diffraction images, acquired from a MAR445 detector, were converted to one dimensional 2θ diffraction patterns using the FIT2D software. 39 Pressure on the sample was generated by a membrane diamond anvil cell equipped with a stainless steel gasket. The sample was loaded in the gasket with Helium, thus ensuring the best hydrostatic conditions, and a small ruby sphere was used for in situ pressure measurement. 40

25.7 GPa 15.5 GPa 13.5 GPa 8.7 GPa 12.0 GPa 10.3 GPa 8.7 GPa 7.8 GPa 7.8 GPa

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Figure 1: (a) Diffraction patterns of phenanthrene powder at selected pressures. (b) Detail of the 2θ region over the 7-10 GPa pressure range where the appearance of a new reflection due to the new high pressure phase is evident. (c) Le Bail profile fitting of the diffraction pattern at 15.5 GPa with the SHP identified by the EA search.

Computational methods To explore the complex state space of solid phenanthrene, we employed an evolutionary algorithm as implemented in the code XtalOpt 41–43 optimized for molecular crystals (E. Zurek, private communications) to search the structure with the lowest enthalpy for a given pressure. The enthalpy calculations and variable-cell structural relaxations of the crystal structures were performed with the DFT code QUANTUM ESPRESSO, which is based on plane waves and pseudopotentials. 44,45 To account for the weak intra-molecular vdW interactions, corrections 46–48 to standard DFT in the parametrization of Lee et al. 49 were included. More details and a reasoning for the choice of the vdW-DFT functional are provided in the Supporting Information. The structural models thus obtained were then used as a basis to fit the diffraction patterns with the method of Le Bail 50 thereby identifying the occurrent structure.

At ambient conditions, diffraction data are reproduced using the monoclinic P21 unit cell in agreement with Trotter et al. 51 The continuous shift of the Bragg peaks towards higher angles all over the pressure range shows the progressive volume reduction, whereas the invariance of the peak pattern indicates the persistence of the low pressure phase up to 8 GPa. Beyond 8 GPa, subtle changes appear, notably a new low-angle reflection around 2θ = 2.8◦ – see Fig. 1(b). These modifications evolve continuously up to ∼13 GPa, and above this pressure the shape and features of the diffraction patterns stabilize. These findings suggest that between 8 GPa and 13 GPa the system enters a region of phase coexistence between the low pressure monoclinic P21 structure (SLP ) and a new competing high pressure structure (SHP ) that gradually replaces the low pressure one. Above 13 GPa, the system stabilizes in the new SHP structure. The presence of two close reflections at low angle above 8 GPa – see Fig. 1(b) – suggests that SHP is characterized by a unit cell with two lattice parameters having close values. However, the limited number of reflections did not allow us to un-

3. Results XRD patterns of phenanthrene collected over the 0-25.7 GPa pressure range are shown in Fig. 1(a).


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The Journal of Physical Chemistry SLP

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ambiguously determine the crystal structure from experimental data only. In order to identify a reliable model for the high pressure phase, we performed a structure search based on an evolutionary algorithm (EA) 41–43 optimized for molecular crystals (E. Zurek, private communications). At a given pressure, the structure search generates a set of candidate structures with a fixed number of formula units (Z). At P = 0 GPa, we obtained, as expected, the (SLP ) structure given by the P21 unit cell with Z = 2 represented in Fig. 2 (a)-(b). At 16 GPa, no candidate structure with Z = 2 was able to reproduce the peak at 2.8◦ in the XRD data. On the other hand, several candidate structures predicted by the 16 GPa search with Z = 3 turned out to be compatible with the experimental data. These structures, which lie in a small enthalpy interval of ∼ 50 meV per formula unit around the minimum, differ from each other for minor structural distortions and are all characterized by an almost parallel arrangement of the molecules. Among them, we selected as structural model for all our XRD fits the lowest-enthalpy SHP structure shown in Fig. 2 (c)-(d). Fig. 1 (c) shows how the Le Bail fit based on this SHP structure provides an excellent description of the 15.5 GPa XRD data. This is always the case for data above 13 GPa, while in the range between 8 and 13 GPa we had to include both SLP and SHP in the fitting procedure, thus pointing out the occurrence of a phase coexistence. Based on our combined experimental and theoretical analysis we can thus derive the following sequence of transitions: (i) 0-8 GPa: phenanthrene remains in the SLP phase shown in Fig. 2 (a)-(b), with space group P21 and a herringbone arrangement of the molecules. 51 (ii) 8-13 GPa: the SLP coexists with a new SHP phase. (iii) 13-20 GPa: the SHP is stabilized. This structure is characterized by a unit cell with Z = 3 formula units, in which the molecules are arranged parallel to each other. Finally, for pressure above 20 GPa, XRD spectra and first-principles calculations indicate a strong tendency towards the formation of amorphous structures with a low compressibility. 26,52,53

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c a

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Figure 2: Unit cell of the low and high pressure phase of phenanthrene. In panel (a)/(b) the structure of the low pressure phase (SLP ) is shown with a view along the c / b axis of the unit cell. In panel (c)/(d) the structure of the high pressure phase (SHP ) is shown with a view along the c / a axis. The interplanar distances d010 and d001 are projections of the b / c axis onto c × a / a × b, respectively. The angle between the normal vectors of two neighboring molecules is labeled as ϑ .

4. Discussion XRD analysis The XRD data in Fig. 1 clearly show a structural transition for pressures higher than 8 GPa; however, the limited number of reflections in our XRD data does not allow an unambiguous determination of the new structure. To fit the experimental spectra we employed the lowest-enthalpy structure (SHP ) found by our EA search at P=16 GPa; the unit cell contains three molecules (Z = 3), which are almost parallel and stacked in a AAA fashion. Other structures with Z = 3 predicted by our EA search are also compatible with the experimental data; they differ from SHP for minor structural distortions, and are almost degenerate with it in enthalpy. This means that, although we cannot unambigously identify the high-pressure structure, we can confidently conclude that phenanthrene undergoes a motif transition from herringbone to face-to-face arrangement of the molecules as a result of volume compression. This is a common characteristic of all low-enthalpy structures, and signals a prominent role of π − π intermolecular interactions, consistently with the behavior of


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other PAHs at high pressures. 25,34 A more detailed analysis of the XRD spectra confirms that SHP is indeed a very good structural model for the high-pressure phase. Fig. 3 shows a comparison of experimental and theoretical results for the equation of state (EOS) and the pressuredependence of interplanar distances and angles. In the three panels, experimental data are shown as circles and vdW-DFT calculations as triangles; empty and full symbols indicate that the data refer to the SLP and SHP phases of Fig. 2. Additional details are contained in the Supporting Information. Panel (a) of Fig. 3 shows a plot of the volume vs pressure for the two phases. Note that the quantity plotted in the figure is the volume per molecule (V /Z). Its smooth behavior across the transition confirms the assumption that the unit cell of the SLP and SHP contain a different number of molecules (Z = 2 and Z = 3, respectively). The experimental low pressure data are very accurately reproduced by the vdW-DFT calculations. The agreement is rather good also for the highpressure phase, up to ∼ 20 GPa, thus showing that the chosen SHP is well representative of the actual high pressure structure. In the coexistence region, the volumes V/Z of the SLP and SHP phases do not significantly differ (less than 1 percent), suggesting that the transition is a weakly first-order one. The experimental and calculated data have been both fitted with a Vinet EOS 54 that reproduces well the data up to 20 GPa. The best fit curves are shown as solid lines in Fig. 3; the corresponding parameters for the experimental (theoretical) bulk modulus and its pressure derivative are B = 5.7(6) GPa (B = 11.2(3) GPa) and B0 = 9.7(6) (B0 = 6.8(1)) respectively. We notice that such a discrepancy between theory and experiment is not uncommon in soft materials like PAHs. 55 Above 20 GPa, significant deviations between the fit curves and the experimental data are observed (Fig. 3(a)) thus suggesting that the onset of a third, apparently less compressible, phase. Panels (b) and (c) of Fig. 3 show the pressure deV pendence of the interplanar distances d010 = ac sin β

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Figure 3: Pressure dependence of unit cell volume (a) and interplanar distances d001 (b) and d010 (c) (see text). Experimental values (circles) are compared to DFT calculation (triangles). Open symbols refer to SLP , filled symbols to SHP . Data shown in (a) and (c) are normalized to Z, the number of molecules per unit cell. Solid lines are the best fit curves from a Vinet equation of state (see text). Pressure dependence of the γ angle (see Fig. 2) is shown in the inset of (a). Vertical dashed lines at 8 GPa and 13 GPa mark the region of phase coexistence. and d010 is almost perpendicular to the molecular plane. Indeed, the pressure dependence of d001 shows a continuous crossover from SLP to SHP , i.e. from Z = 2 to Z = 3, whereas a continuous trend across the phase transition for d010 is only obtained dividing it by the number of molecules Z (see panels (b) and (c)). This behaviour thus suggests that the additional molecule in the SHP does not lie in the 001 direction but rather along the 010, consistently with what is shown in Fig.3(c). Finally, the pressure dependence of the angle γ in the SHP phase is plotted in the inset of panel (a) in Fig.3. The data show that the angle changes almost linearly up to 15 GPa while it keeps nearly constant on further increasing the pressure. Since γ is directly related to the orientation between

and d001 = ab Vsin γ . These two quantities allow for an independent test of the validity of the SHP model for the high pressure phase, considering that d001 is nearly parallel to the long molecular axis


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molecules in the ab plane (see Fig.2), this suggests that, once molecules have attained an almost parallel arrangement, pressure causes simply a volume compression, without a further reorientation of the molecules. For pressures exceeding 20 GPa, when the system is entering the third, less compressible, phase, we observe that the experimental d010 becomes nearly constant around a value of about 2.6 Å (see Fig.3(c)). In the SHP , a contraction of d010 can be remarkably limited by the strong repulsive interaction occurring between the π electrons of nearly parallel molecules. At high pressures, these interactions may become so strong to destabilize the molecular structure itself and lead the system towards an amorphous phase made up of disordered clusters of hydrogenated carbons. This reaction is further supported by the value d010 ∼ 2.6 Å which is about the same value already found for the ring opening of benzene 26 and triazine 32 at high pressure. Further indications about the nature of this third phase, characterized by a partial molecular collapse, can be found in the diffraction pattern collected at 0.3 GPa after pressure release shown in Fig. 1. This measurement is apparently less defined and a sensible peak broadening occurs. However, a comparison with the pattern from the pristine sample at P = 0 GPa shows that most of the structural transformations occurred during the pressure cycle are reversible. The overall shape modification of the diffraction pattern can be seen as a signature of the occurrence of irreversible structural processes. This irreversible behavior would be consistent with the presence of residual stress and lattice distortion induced by “locally amorphous” structures that are produced in the very high pressure regime and persist in the recovered sample.

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pressure behavior of phenanthrene. The three panels of Fig. 4 show the state space plots for P = 0 and P = 16 GPa, where each point represents a candidate structure. The color scale refers to the intermolecular angle ϑ defined in Fig. 1: red (yellow) points correspond to structures in which the relative molecular arrangement is parallel (perpendicular). The molecular structures which are collapsed or strongly bent, i.e. which have “locally amorphous” character, are represented by green dots. The arrows indicate the location of the SLP and SHP structures; S1 is the structure with Z = 2 which has the lowest enthalpy at P = 16 GPa. Further details on the definition of intermolecular angles and locally amorphous structures are contained in the Supporting Information. The main driving principle behind the pressure evolution of the state space is the minimization of the enthalpy through the decrease of the volume for increasing pressures. At ambient conditions (Fig. 4(a)), structures with a large angle ϑ between the molecules – as in the herringbone arrangement – lie lower in enthalpy than the structures with a parallel alignment. At higher pressures, structures with molecules in a parallel arrangement become favored by enthalpy over herringbone ones, due to the smaller unit cell volume. Indeed, with increasing pressure, SLP moves from the low to the intermediate enthalpy region of the state space, while several ϑ ≈ 0◦ structures with low volume occupy the low-enthalpy region. The effect is equally strong for Z = 2 (Fig. 4(b)) and Z = 3 (Fig. 4(c)). The XRD spectra contain indications of a irreversible transition to a locally amorphous structure at high pressures. The state space plots in Fig. 4 show that at 16 GPa the number of green dots with low enthalpies is much higher than at ambient pressure. It is foreseable that with increasing pressure some of these collapsed molecular structures will be stabilized in enthalpy, in agreement with the experimental trends. We tested this hypothesis performing a further state space calculation. Our goal in this case is not to identify a candidate structure for the highpressure phase, but to follow the evolution of representative structures from the ambient to the highpressure regime. For this reason, we restrict our search to Z = 2 and, instead of performing a sepa-

State space analysis: structural trends and molecular collapse under pressure The structure search generates a set of candidate structures which, at each pressure, can be used to construct a state space, representing the resulting structures in an enthalpy H vs. volume V diagram. The analysis of this state spaces offers interesting insights on the mechanism governing the high


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Figure 4: State space of phenanthrene generated by an evolutionary algorithm search (a) at ambient conditions for Z = 2, (b) at 16 GPa for Z = 2 and (c) at 16 GPa for Z = 3. The angle ϑ between adjacent molecules (see Fig. 2 for definition) is encoded by color – a parallel alignment is reflected by red and a perpendicular alignment by yellow. Green points refer to largely-bent / “locally amorphous” configurations. rate EA search for each pressure, we recalculate the location in the state space of all the structures generated by a single structural search at P = 16 GPa at ambient pressure (P = 0) and at a pressure higher than the amorphization pressure seen in experiment (P = 25 GPa). The resulting state space plots are shown in Fig. 5. At ambient conditions, most of the molecular collapsed structures (green dots) are well above the enthalpy of the two representative candidates SLP and S1 for the low and high-pressure phases, i.e. outside the displayed region. However, with pressure their relative enthalpy is reduced because of the smaller volume, and they become the ground state at pressures above 20 GPa. In particular, at P = 25 GPa, we find that the lowestenthalpy structure (Scollapsed in the plot) bears little resemblance to SLP , S1 and other molecular structures at lower pressures: the C-C and C-H bonds are so strongly bent that the local enviroment resembles amorphous carbon. More details can be found in the Supporting Information. Note that, since we restricted our EA search to Z = 2 and molecular crystal structures we discarded a priori from our search many collapsed phases, which might have a lower enthalpy than Scollapsed at P = 25 GPa. Therefore, although it is very unlikely that Scollapsed is the actual leading structure in the experiment, its occurrence signifies a strong tendency of the system to the formation of a collapsed

structure, in agreement with the signatures of irreversible phase transitions seen in the experiment. A similar behavior was recently observed for solvated fullerenes 52 and for benzene 35,53 at high pressure, indicating a general tendency towards a molecular collapse of hydrocarbons at extreme conditions.

5. Conclusions In this work we reported a joint experimental and theoretical study of solid phenanthrene under pressures up to 25.7 GPa. XRD spectra from synchrothron radiation were collected in a DAC under the optimal hydrostatic conditions ensured by the use of helium as pressurizing medium. Measured diffraction patterns were fitted using structural models generated by an evolutionary algorithm for crystal structure search based on vdWDFT calculations. We found that the P21 phase previously identified at ambient conditions is stable up to ∼8 GPa. In this SLP structure, the H-π intermolecular interaction is dominating, thus favoring a herringbone arrangement of the molecules. On increasing pressure, a volume reduction is first obtained by a reorientation of the molecules and by a shortening of the intermolecular distances. Then, π − π intermolecular interactions become dominant and lead to a parallel arrangement of the


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Figure 5: State space plots for P = [0, 16, 25] GPa of an EA run with Z = 2 (see text for details); arrows point to the locations of SLP , S1 , and Scollapsed . The thick grey arrow indicates the relative shift of Scollapsed with pressure in the state space. phenanthrene molecules. Above 13 GPa, a new high pressure triclinic P1 structure is identified by means of a detailed data analysis and a structural search based on an evolutionary algorithm. This SHP structure, which coexists with the SLP in the intermediate pressure range 8-13 GPa, has three molecules per unit cell disposed in a parallel arrangement with AAA stacking. In this phase, a further volume reduction can be attained only by a reduction of the intermolecular distances. At pressures higher than 20 GPa the system becomes much harder: applied pressure barely induces a volume compression since a further contraction is strongly limited by the repulsive interactions between the π electrons of nearly parallel molecules. This destabilizes the molecular structure itself and, on the basis of the results of the EA search, we suggest that very high pressures can lead the system towards “locally amorphous” structures, i.e. disorderd clusters of hydrogenated carbons. We found that this reaction occurs when an intermolecular distance of ∼ 2.6 Å is reached, i.e. the same critical distance reported for the formation of an amorphous state in benzene, 26 thus suggesting a similar mechanism for phenanthrene. Further experimental investigations aimed to achieve a full structural determination, and an extended EA search including structures with Z > 3, are needed for a full Rietveld refinement of the high-pressure phase. Studies on larger phenacenes and other hydrocarbons under pressure can be very interesting to verify the existence of the “locally

amorphous” state we suggested for phenanthrene and to investigate the relevance of the molecular dimension in this phenomenon. Acknowledgement We acknowledge ESRF for provision of beamtime. This work is supported by Fondazione Cariplo (grant no. 2013-0632). Supporting Information Available: The Supporting Information contains additional details on ab-initio vdW-DFT calculations, the construction of the state space, pictures and lattice parameters of all discussed structure. The cif files of all structures are provided as separate files. This material is available free of charge via the Internet at http://pubs.acs.org/ .

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Graphical TOC Entry π-π < π-H

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Structural Evolution of Solid Phenanthrene at High ... - ACS Publications

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