Structural and Electronic Competing Mechanisms in the Formation of

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Structural and Electronic Competing Mechanisms in the Formation of Amorphous Carbon Nitride by Compressing s-triazine Margherita Citroni, Samuele Fanetti, Carla Bazzicalupi, Kamil Dziubek, Marco Pagliai, Marcelo Medre Nobrega, Mohamed Mezouar, and Roberto Bini J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09538 • Publication Date (Web): 02 Dec 2015 Downloaded from http://pubs.acs.org on December 11, 2015

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The Journal of Physical Chemistry

Structural and Electronic Competing Mechanisms in the Formation of Amorphous Carbon Nitride by Compressing s-Triazine Margherita Citronia,b , Samuele Fanettib , Carla Bazzicalupia , Kamil Dziubekb,c , Marco Pagliaia,d , Marcelo Medre Nobregae , Mohamed Mezouarf , Roberto Binia,b,∗

a

Dipartimento di Chimica ”Ugo Schiff” dell’Università degli Studi di Firenze, Via della Lastruccia 3, I-50019 Sesto Fiorentino, Firenze, Italy b LENS, European Laboratory for Non-linear Spectroscopy, Via N. Carrara 1, I-50019 Sesto Fiorentino, Firenze, Italy c

Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Pozna´ n, P oland d present address: Scuola Normale Superiore, Piazza dei Cavalieri 7,

I-56126 Pisa, Italy e Departamento de Química Fundamental, Instituto de Química da Universidade de S¨ ao Paulo (USP), C.P. 26077- CEP 05513-970 - S¨ ao Paulo, SP, Brazil f

ESRF, 6 Rue Jules Horowitz BP220, F-38043 Grenoble Cedex, France

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Abstract The pressure induced transformation of molecular crystals can give rise to new materials characterized by intriguing hardness or energetic properties. Mechanisms regulating these reactions at molecular level result from a complex interplay between crystal structure and dynamics, and electronic properties. Here, we show that the formation of a three-dimensional amorphous carbon nitride by compressing phase II s-triazine is controlled by the competition between two different mechanisms, one entirely structural the other electronic, representing the first example where such occurrence is demonstrated. Temperature drives the reactivity below 8 GPa by ruling the lattice dynamics, whereas above 8 GPa the electronic modifications, uniquely governed by pressure, trigger the chemical transformation. The amorphous material synthesized has a bonding structure characterized by a bulk typical of a strongly conjugated three-dimensional carbon nitride with hydrogen atoms migrated to saturate C and N terminations.

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1

Introduction

The reactivity of simple molecules in the crystal phase became a very attractive topic only with the widespread diffusion of the diamond anvil cell (DAC) which made the GPa range easily accessible. Pressure is indeed much more powerful than temperature in modifying the intermolecular distances to the extent of driving the chemical instability. In addition, the possibility of controlling and independently finely tuning temperature and pressure allowed to gain insight about the activation and propagation mechanisms of the reactivity in many different molecular systems 1 . A complex interplay between structure, which rules intermolecular distances and orientations, lattice dynamics, governed by collective motions, and electronic configuration must be taken into account to explain the crystal reactivity. Therefore, understanding of the reaction mechanisms can be gained only through a precise characterization of all these aspects. This is not a trivial task because all the aforementioned contributions are affected by pressure, by temperature and, as the electronic properties are concerned, also by optical excitation. For these reasons, careful studies where the effects of pressure, temperature and electronic excitation are controlled and disentangled are mandatory to shed light on the reaction mechanisms at molecular level. In this respect, the characterization of the benzene reactivity at high pressure is a paradigmatic case study. In fact, in benzene it was demonstrated the primary role of the lattice phonons in triggering and propagating the reaction through the crystal 2 , and how reaction initiators can be created with and without the involvement of electronic excited states 3,4 . The importance of understanding the mechanisms regulating the reactiv3



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ity exceeds the pure fundamental aspect, representing also the preliminary step for the exploitation of high pressure methods for synthetic purposes. In this framework, the recent report about the purely pressure induced reactivity and photoinduced high pressure reactivity of crystalline s-triazine (1,3,5-triazine) is of particular interest 5 . An extended, likely amorphous, conjugated material incorporating a much larger amount of nitrogen with respect to that characterizing a-C:H:N obtained by different plasma deposition techniques 6 , was obtained by compressing s-triazine above 8 GPa or by increasing temperature or photoirradiating the sample at pressures as low as 4 GPa. This is a relevant result in the search and application of new high energy density materials (HEDMs) 7,8 and in the synthesis of layered graphitic (C,N) materials 9 . Also the production of novel carbon nitrides, excellent superhard materials with high thermal and chemical stability 10,11 , can benefit of these new synthetic routes. Melamine and cyanuric chloride, two s-triazine derivatives, have been employed to build extended condensed rings planar structures at high temperature and ambient pressure 12–15 , and in high-pressure syntheses of planar carbon nitrides 16–18 . Recently, further compression of one of these products, C6 N9 H3 ·HCl, brought to the formation around 20 GPa of interlayer bonds which give rise to a pillared-layer structure 19 . An overview of the high-pressure experiments performed so far is reported in ref. 20 . The high-pressure reaction in s-triazine, under different pressure, temperature and irradiation conditions, thus represents an interesting opportunity to approach the synthesis of three-dimensional denser carbon nitrides which have been predicted by ab initio calculations to have higher energy with re-

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spect to graphitic-like planar structures 9,21 . The differences in the mechanical properties reported for the materials recovered from low and high pressure reactions 5 , and the dependence on temperature of the reaction threshold pressure, suggestive of a reaction triggered by lattice phonons, hint at the fundamental role of the intermolecular interaction geometry and how this is modified by pressure. Therefore, in order to unveil the mechanism ruling the reactivity, a mandatory step to drive the reactivity towards a targeted material, we have performed: (i) a structural characterization of the triazine phase II as a function of pressure; (ii) a systematic study of the reaction at different P-T conditions also through kinetic studies; (iii) a characterization of the amorphous materials recovered in the different reaction conditions. The results indicate the formation of an amorphous carbon nitride and outline a reaction mechanism resulting from a competition, never reported so far, between structural and electronic factors.

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Methods

At ambient temperature triazine is a highly deliquescent solid which in presence of moisture becomes chemically unstable. Freshly sublimated large transparent crystallites obtained from s-triazine powder purchased from SigmaAldrich (97%) were loaded in a membrane diamond anvil cell (MDAC), equipped with IIa type diamonds and rhenium gaskets to contain the sample, together with a small ruby chip for pressure calibration by the ruby fluorescence method 22 . High-temperature experiments were performed by using resistively heated cells. The temperature was measured with an ac-

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curacy of ±0.1 K by a K-type thermocouple placed close to the diamonds. Heating rates close to the reaction onset are described in the following. Infrared absorption measurements were performed with a Bruker-IFS 120 HR spectrometer suitably modified for high pressure experiments 23,24 with an instrumental resolution better than 1 cm−1 . Ambient pressure low temperature angle dispersive X-ray diffraction (ADXRD) experiments were performed on colourless crystals grown by under vacuum sublimation and then mounted on a SCD Oxford Diffraction XcaliburPX diffractometer equipped with a copper anode (λ=1.5418 ˚ A) and then slowly cooled down to 120 K. High pressure ADXRD experiments were performed at the ESRF high pressure beam line ID27, using a monochromatic wavelength with λ=0.3738 ˚ A and a PerkinElmer flat panel detector. The focal spot diameter of the beam was smaller than 5 µm. The 2D diffraction patterns were analysed and integrated using the FIT2D software 25 . Structural optimizations of phase II triazine crystal, with molecules arranged in setting proposed in ref. 26 , have been performed for a series of pressure values in the range between 1.2 and 7.0 GPa with the CPMD package 27 , annealing both the ion and electron velocities with a 0.95 factor and an initial temperature of 1 K 28 . The simulation box sides have been obtained by doubling the experimental cell parameters along each direction and have been kept constant during the calculations. The BLYP exchange and correlation functional 29,30 has been employed, describing the core region of each atomic species with Martins-Troullier pseudopotentials 31 in conjunction with the Kleinman-Bylander 32 decomposition and expanding the plane waves until 80 Ry. The equations of motion have been integrated with a time step of 5 au (∼ 0.12 fs) with a fictitious mass of 700 au for

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electrons. Van der Waals interactions have been taken into account with the schema proposed by Grimme 33 .

3

Results

3.1

X-ray diffraction

S-triazine presents two different crystal structures. Phase I, trigonal R¯3c (D63d ; Z=2), exists at ambient pressure down to 198 K where it transforms to phaseII, monoclinic C2/c (C62h ; Z=2) 26,34,35 . The I-II phase boundary has been partly characterized with increasing pressure up to ambient temperature by different techniques 5,36–39 . Single crystal low temperature (120 K) ambient pressure XRD experiments performed in this work revealed a heterogeneous composition in which only 16% of the sample was converted to phase-II. Coordinates were analyzed and the packing of the minor component was recognized to be in agreement with the structure adopted for the low temperature phase-II of s-triazine 26,34,35 . The powder diffraction pattern, calculated from the coordinates properly converted in Smith setting 26 , was compared with the experimental diffraction pattern and then refined by Rietveld technique using TOPAS 4.2 software 40 . ADXRD patterns were collected at ambient temperature as a function of pressure using synchrotron light. S-triazine was pressurized in a MDAC without any compression medium and annealed at 400 K after crossing the III phase transition. In Figure 1 the two-dimensional x-ray diffraction pattern, measured at 1.2 GPa, is reported as a function of the 2θ scattering angle 7



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with the assignment and calculated peak positions according to the C2/c monoclinic structure. A Rietveld refinement was performed on this pattern using TOPAS 4.2 software 40 . The low temperature coordinates in Smith setting 26 were used as initial atomic positions. All the parameters relative to data collection, crystallographic data, refined atomic positions, bond lengths and molecular angles are reported in Tables 1 and 2. The diffraction patterns measured at higher pressures were reproduced extracting the cell parameters from the 2θ positions of the observed peaks (not less than seven) for each pattern recorded. At each pressure point, the relative hkl indexes were taken from those obtained in the previous XRD pattern. The results were further refined by using an iterative approach based on the following steps. First, the atomic positions were optimized by means of a CPMD simulation (see Methods section), maintaining fixed the cell parameters to the experimental values. Then, the optimized atomic coordinates were employed as input to perform a Rietveld refinement of the experimental XRPD data, improving the cell parameters. During this step, the atomic positions remain fixed. This iterative procedure was repeated twice. In Figure 2 the lattice parameters obtained from this procedure are reported as a function of pressure. Lattice parameters were employed to determine the volume data which have been fitted by a 3rd order Vinet equation of state 41 (see Figure 2): [

]

3 (1 − fν ) exp (C0 − 1)(1 − fν ) P = 3B0 2 fν 2

(1)

where fν = (V /V0 )1/3 , being V0 the cell volume at ambient pressure, B0 and C0 are the isothermal bulk modulus and its derivative against pressure at P=0, respectively. The value of V0 obtained by the fit, 384.5 ˚ A3 , nicely agrees 8


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with the volume data determined at ambient pressure and low temperature: ˚3 at 5 K 35 , 369.96 A ˚3 at 120 K (this work) and 375.058 A ˚3 at 150 367.394 A K 26 . Also the value of B0 , 6.3 GPa, well compares with that of the monoclinic P 21 /c phase II of benzene for which a value of 5.5 GPa is reported 42 . The fit of the volume data is good up to 5 GPa whereas above this pressure the data are relevantly scattered reflecting the behaviour of the b parameter and, to a lesser extent, of the monoclinic angle (see Figure 2). This occurrence does not depend on a overall deterioration of the diffraction data but it is ascribable to specific reflections. In Figure 3 we report the pressure evolution of the diffraction pattern in the low 2θ region. Here, a remarkable broadening and the appearance of a shoulder on the low scattering angle side is observed, for most peaks above 4 GPa. The only exception is the 20-2 reflection which does not show any appreciable pressure effect on the peak shape.

3.2

Reaction thresholds and kinetics

The chemical instability of s-triazine under pressure has already been reported at ambient temperature (above 8 GPa) and at 530 K and 4 GPa evidencing a clear temperature dependence of the pressure threshold for the reaction 5 . In this work we have determined the stability boundary of phase II performing three different isobaric heating studies (3.1, 4.6 and 6.0 GPa) and two isothermal compressions at 298 and 400 K. In the isobaric studies the temperature was increased approaching the supposed reaction onset in steps of 1-5 K waiting half hour at each step. The threshold temperature was determined by analysing the difference between absorption spectra measured 9



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in successive temperature steps. In Figure 4 the spectra collected during the isobaric heating performed at 3.1 GPa are reported together with the corresponding difference spectra. As reported in ref. 5 the onset of the reaction is observable through the appearance of a broad absorption between 900 and 1800 cm−1 and two sharper features at 811 and 992 cm−1 marked by asterisks in the difference spectra reported in Figure 4. Once the reaction onset was identified we also measured the corresponding kinetics at slightly higher temperature conditions because the reaction threshold could be identified only a posteriori. In Figure 5 three kinetics measured in phase II at different P-T conditions are reported. The amount of reacted triazine was determined by the absorbance decrease of the bands at 4000 and 4750 cm−1 , relative to combination modes involving the ν1 and ν6 C-H stretching modes 43 , measurable during all the reaction evolution and not overlapping to product bands. The absorption pattern was fitted using a Voigt profile for each band and the total absorbance, computed as the sum of the individual components, was taken as a measurement of the amount of residual triazine. Since triazine is entirely consumed at the end of the reaction, we reported the time evolution of the percentage of triazine reacted so that the three kinetics can be compared irrespective of the different sample thickness. The three set of data were analyzed with the Avrami model 44–46 , developed for the crystal growth from a liquid phase and later extended also to the interpretation of solid-state reactions 47 . The fitting equation, derived from ref. 48 , can be written as: (

A0 − At A0

)

[

= 100 1 − e[k(t−t0 )]

n

]

(2)

where A0 and At are the integrated absorption of the triazine band at the 10


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beginning of the reaction (t0 ) and after a time t, respectively, whereas n is a parameter which accounts for the growth dimensionality once set the nucleation law 47 . The kinetic curves built in this way could be nicely reproduced using the rate constants and n parameters reported in Table 3. The comparison of the rate constants determined in the three experiments emphasizes the temperature role. In spite of the remarkable different pressures (3.1 and 6.0 GPa), the two kinetics performed at similar temperatures (∼550 K) present almost the same rate constant 5 times larger than that obtained at intermediate pressure but lower temperature. The other remarkable result is that the n parameter ranges in the three kinetics from 2.40 to 2.99 which, in diffusion-controlled reactions, supports a nucleation-free 3D growth of the extended product 1 . The stability of triazine has also been characterized in phase I by isobarically heating the sample at 1 GPa. The reaction threshold has been identified at 445 K with a very slow kinetics, in fact after 160 h only 22% of the phase I triazine crystal was transformed in an extended dark product. Another sample was therefore brought, at almost the same pressure, to 465 K but in this case the characterization of the reaction kinetics was impossible for the sudden transformation of the sample into a black material. In spite of the only partial characterization of the phase I stability because of the limited accessible P-T range, the much lower threshold temperature for the reaction, with respect to that expected by extrapolating at this pressure the phase II onsets (see Figure 6), indicates the fundamental role of the structural arrangement in driving the chemical reactivity.

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3.3

Reaction dynamics

To rationalize the P-T stability boundary of s-triazine we may consider the treatment adopted for explaining the reactivity in crystalline benzene 2 where the structural arrangement, tuned by pressure, and the thermal displacement due to the lattice phonons combine to bring about a critical distance necessary for reaction to occur. Neglecting the molecular compressibility and the temperature effect on the EOS, using the one determined at 298 K, we calculate the intermolecular C(N)· · ·N(C) contacts by the atomic coordinates obtained from the refinement procedure. The shortest contacts are found to correspond to C· · ·N intermolecular contacts between molecules lying on adjacent planes parallel to the (20-2) family. The C· · ·N distances among molecules lying in the (20-2) planes are slightly longer (about 0.05 ˚ A) thus falling just in between the former contacts and those developing parallel to the c axis. All the other contacts are not discussed because longer than these in all the pressure range examined. For the sake of clarity, in Figure 7 we report the pressure evolution of the intermolecular C· · ·N distances among atoms lying on adjacent planes belonging to the (20-2) family, the shortest ones, and those developing parallel to the c axis. These distances, that will be hereafter referred to as structural distances, are between molecules fixed in their equilibrium positions. However, the atomic positions are continuously modified by translational and librational lattice modes whose amplitudes are temperature dependent. Translational modes are expected to be the most effective in reducing the intermolecular distances as indeed was demonstrated in the benzene case 2 . Therefore, in order to extract a minimum instantaneous distance we corrected the nearest 12


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neighbour structural distances by a thermal translational contribution neglecting, in a first approximation, the librational contribution. As reported in ref. 2 , the maximum instantaneous linear displacement from the equilibrium position (am ) can be estimated as 3σ, to account for more than 99% of the displacement amplitudes, where σ 2 is classically given by: σ 2 = ⟨x2 ⟩ =

kT m(2πcν)2

(3)

k is the Boltzmann constant, T the temperature, m the molecule mass and ν the frequency (cm−1 ) of the phonon mode. Since a phonon density of states (DOS) is not available for phase II triazine, we measured the room temperature Raman spectrum of the lattice modes as a function of pressure. The measured spectrum and the evolution with pressure of the frequencies of four lattice modes, out of six expected from group theory considerations, are in perfect agreement with those recently reported 49 . Since both translational (3) and librational (3) modes are expected to be Raman active, we determined, at all the pressures where the reaction was studied, the center frequency of this distribution that was employed in eq.3 for the calculation of the displacement amplitudes. In Table 1 we report the structural distances and the corresponding translational thermal correction 2am which should be subtracted to obtain the instantaneous shortest distances. Considering only the data relative to the experiments where the threshold was identified at temperatures ≥500 K (3.1, 4.6 and 6.0 GPa), the instantaneous distances are very similar in spite of the remarkable different pressure and temperature conditions where the reaction takes place. The dispersion of the thermally corrected da and db distances are in fact 0.014 (0.5%) and 0.026 (1.0%) ˚ A, respectively. These values must be 13



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compared to the dispersion of the structural values that is four to seven times larger being 0.103 (3.3%) and 0.115 (3.6%) ˚ A, respectively. The dispersion of the thermally corrected da and db increases to 0.050 and 0.036 ˚ A if also the point at 8.3 GPa and 400 K is considered, but these values still remain much smaller than the dispersion of the structural distances which are 0.144 and 0.170 ˚ A, respectively. On the contrary, the thermally corrected ambient temperature threshold distances (8.2 GPa) are definitely much larger (7-8 times) than the average distance obtained from the three experiments performed above 500 K. This discussion is better understood by looking at the stability diagram reported in Figure 6. The threshold found at 8.2 GPa and 400 K is characterized by a slightly lower temperature than expected by a linear extrapolation of the instability boundary which appears to hold in the 2-7 GPa pressure range. This discrepancy is extremely evident when also the ambient temperature point is considered, suggesting that other mechanisms beside the pure phonon dynamics must be taken into account to explain the reactivity above 8 GPa. As the thermal correction is concerned, it ranges between 13% at 298 and 20% at 549 K being therefore remarkably larger than the temperature variation of the cell parameters which is less than 2% for molecular systems in analogous pressure range 50 . The employment of the EOS measured at 298 K is therefore justified also at the highest temperatures considered. The reactive distance found for s-triazine by this approach is in excellent agreement with the one found for benzene, which also ranged between 2.5 and 2.6 ˚ A 2 . These values are considerably smaller than the van der Waals separation to which generally the molecular instability is associated 51 .

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3.4

Reaction products

The dark red materials synthesized at pressures lower than 8 GPa and high temperature appear homogeneous and mechanically resistant to scratching. The IR spectra of these products are all very similar and closely recall those of nitrogen-doped amorphous hydrogenated carbon (a-C:N:H) 5 . The main information gained by IR spectra is the strong intensification of the 1000-1700 cm−1 frequency range with respect to amorphous hydrogenated carbons not containing nitrogen atoms. This is a very characteristic behaviour ascribed to the symmetry breakage of the sp2 carbon domains and to the increase of the dynamic effective charge 6,52 . Minor differences with respect to the spectra of a-C:H:N 6 consist mainly of recognizable sharper features characterizing the s-triazine reaction product likely related to the presence of nitrogen bearing functional groups present as terminations 5 . However, important spectral differences with respect to the IR absorption spectra of a-C:H:N and a-C:N are observed in other spectral regions. The first regards the feature lying between 2000 and 2200 cm−1 in a-C:H:N and a-C:N consisting of two bands due to the stretching of nitrile groups (lower frequency) and cumulated double bonds (higher frequency) 53 . Although present in the IR absorption spectra measured on our recovered materials outside of the DAC reported in Figure 8 (the allowed two-phonon transition of diamond prevents the access to this region), this feature is barely visible resulting much weaker than in refs. 6,53 . The second difference regards the CH stretching modes. Only absorption bands below 3000 cm−1 are observed indicating that most of H atoms are bonded to saturated carbons that, as will be shown later, are localized as terminating groups. 15



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Because of the amorphous nature of the sample the analysis of the structure function, and of the corresponding radial distribution function g(r), can be extremely informative about the average sample arrangement. X-ray diffraction measurements of the recovered products exhibit a broad feature superimposed on the diffuse scattering from air which was subtracted by measuring carefully the empty gasket background intensity with the same instrumental settings and exposure time. The S(Q) was computed independently, with excellent agreement, by using the procedure described in ref. 55 and the program PDFgetX3 56 which was also employed to compute the g(r). The g(r) presents three broad peaks with maxima at 1.325, 2.305 and 3.637 ˚ A (see Figure 9). The first peak corresponds to the C-N bond length and it is slightly shorter than in s-triazine (1.34 ˚ A at 1.2 GPa). The second nearest neighbour distance, related to N(C)· · ·N(C) contacts, implies a dC) angle of 120.87◦ thus indicating a relaxation of the sp2 angle Nd CN (CN

distortion characteristic of s-triazine. Both data are suggestive of a smooth reduction of the delocalization characterizing the pristine six membered ring but isolated C=N bond distances are definitely shorter (1.29-1.30 ˚ A). The g(r) of our recovered product is characterized by much broader features with respect to those calculated using PLATON program 57 (RDFwidthPar=5.0) for the crystalline materials reported in Figure 9. Despite this difference, we can recognize a reasonable overlap, for the features below 3 ˚ A, both with phase II s-triazine and even better with the Cc-C3 N4 structure, which was recently proposed by systematic evolutionary structure search as one of the most stable phases at ambient pressure 11 . In s-triazine these distances are relative to nearest neighbour distances within the same molecule. The better

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agreement with the crystalline carbon nitride also extends at longer distances where, for example, the strong maximum of s-triazine extending between 3.2 and 3.6 ˚ A, related to all the contacts described in Figure 7, and that centred at 3.909 ˚ A are badly recognized in the g(r) of the recovered product. The latter presents a broad peak centred at 3.637 ˚ A suggestive of a distribution of C· · ·N contacts between C and N atoms belonging to adjacent sp2 hybridized C units (CNCN), found in the ordered Cc-C3 N4 at 3.52 ˚ A. At longer distances the g(r) of our product flattens contrary to those characterizing crystalline C2/c s-triazine and Cc carbon nitride. Due to the similarities between the IR absorption spectra of the recovered materials and those of nitrogen-doped amorphous hydrogenated carbon, we also considered the g(r) of a-C:N:H synthesized from ammonia 58 . For this material the first two peaks are shifted at longer distances, 1.43 and 2.44 ˚ A respectively. These distances are even longer in a-C:H where the first peak is observed around 1.54 ˚ A (C-C) with a shoulder at 1.42 ˚ A due to graphitic clusters 59,60 . Although the broadness of the peaks does not permit sharp conclusions, this comparison is suggestive of a significant reduction of the number of single bonds in the material synthesized in this work with respect to a-C:N:H.

4

Discussion

The information gained by the structural characterization of phase II triazine is essential for the identification of the reaction mechanism at the molecular level. The nearest neighbour contacts can be calculated and the most probable reaction paths identified on a pure structural basis and, eventually,

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further selected according to the reaction kinetics. As reported in the benzene case 2 , the crystal dynamics reduces these distances through the collective motions. In particular out-of-phase translational motions were found the most efficient in reducing the intermolecular contacts achieving instantaneous distances well below the van der Waals separation. Following this approach we have found that, in spite of the different P-T conditions, the reaction thresholds determined for T≥500 K present the same reactive distance, obtained by subtracting to the structural distance the thermal contribution, whose average values are 2.505±0.007 ˚ A for the shortest C· · ·N contacts relative to molecules lying on adjacent planes of the (20-2) family and 2.629 ±0.014 ˚ A for those arranged along the c axis, being the contacts within the (20-2) plane just in between. These findings sharply indicate a reaction activated by the lattice phonons which are responsible of the transport of reactants to the reactive sites. The shortest contacts are always relative to molecules lying on adjacent planes oriented as the (20-2) ones which are therefore those preferentially involved in the reaction. This interaction could be responsible of the stabilization of this crystal plane as indicated by the pressure evolution of the diffraction pattern (Figure 3) with all the peaks broadening and becoming asymmetric with the only exception of the one related to the (20-2) reflection. However, the small difference with the shortest contacts within the (20-2) planes (∼0.05 ˚ A) well agrees with the kinetic studies that indicate a three dimensional growth, excluding any oligomerization process selectively involving molecules lying on the same plane or along specific directions. Contrary to the benzene case, where the thermal correction is able to explain the instability boundary in a broader P-T range, 10-40 GPa and

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300-600 K, in phase II triazine we found that the agreement with this simple model decreases when we consider also the threshold at 8.3 GPa and 400 K, and macroscopically fails at ambient temperature where the threshold pressure is much lower than expected by the thermally corrected structural data. In a recent study of s-triazine, regarding the effect of pressure on the lowest electronic transitions, it was found that the energy separation between the ground and the lowest nπ ∗ state is not affected by the compression, whereas the ππ ∗ state remarkably red shifts with pressure bringing the two excited states closer and closer implying a destabilization of the π bonding molecular orbitals 61 . The red shift of the ππ ∗ state indeed completes at 8 GPa with the overlapping of the transitions to the nπ ∗ and ππ ∗ states, which marks the complete destabilization of the aromatic rings. At the same pressure we have observed the onset of the chemical reaction both at 298 and 400 K Interestingly, an opposite behaviour has been observed in pyridine, another N bearing aromatic molecule, where a stabilization with pressure of the nonbonding orbitals through the enforcement of N· · ·H hydrogen interaction, was reported 62 . These observations nicely account for the very different chemical stability of the two systems under compression, being much lower in s-triazine. An interpretation of the reactivity in the entire P-T range investigated can thus be provided only taking into account the competition between two different mechanisms. The first mechanism, related to the crystal structure and dynamics, well accounts for the reactivity for pressures lower than 8 GPa. Temperature appears here as the key parameter, ruling the oscillation amplitudes which determine the instantaneous shortest contacts and, consequently, the growth geometry of the extended carbon nitride. Above 8 GPa,

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the effects of temperature are overcome by those of pressure that modifies the electronic density driving the instability of the π bonding orbitals and finally causing the ring destabilization. The importance of the structural arrangement on the reaction evolution is readily grasped by the comparison of the instability threshold evolution with pressure in phase II and of the analogous P-T data determined in phase I. In fact, despite the reduced amount of data collected in phase I, because of the minor accessible P-T range, macroscopic differences mark the reactivity in phase I. In spite of the lower pressure, the instability boundary at 1 GPa is about 150 K lower than expected by a simple linear extrapolation of the instability boundary of phase II. Besides a deep insight in the reaction mechanism we also succeeded in a characterization of the reaction product. IR absorption spectra provide only qualitative information indicating a product very similar to an amorphous hydrogenated carbon incorporating nitrogen atoms in the bulk. However, minor, but important, spectral differences with this latter class of materials exist and are confirmed by the structure function, and the corresponding pair radial distribution function, obtained by synchrotron X-ray diffraction data. Strong similarities with amorphous carbon nitrides and a pronounced conjugation, therefore average C-N bond distances shorter than in a-C:N:H, are indeed evidenced. The first, second and third nearest neighbour distances are suggestive of a ring aperture and, also supported by the kinetic data, of a 3D net-like structure formation with most of the C and N atoms maintaining a sp2 hybridization. Interestingly, we have no indication of a sensible decomposition of the product, therefore a 1:1:1 amount of C, N and H could be

20


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reasonably assumed. By the comparison of the results obtained from the pair radial distribution function and the IR absorption data we deduce a strong conjugation of the bulk material with preponderant sp2 hybridization and a localization of the H atoms as -CHx and -NHx terminating groups.

5

Conclusions

The combined application of synchrotron X-ray diffraction and optical spectroscopic techniques allowed the characterization of the stability diagram of s-triazine and of the microscopic mechanism regulating the high-pressure transformation into an amorphous carbon nitride. Two competing mechanisms have been identified by combining the outcome of a careful identification of the reaction threshold temperatures at different pressures with a X-ray diffraction characterization of the reacting crystal phase (II), and the results of a two-photon study as a function of pressure of the lowest electronic excited states. Up to about 8 GPa the reactivity is entirely driven by the crystal structure and its dynamics: lattice phonons realize instantaneous configurations triggering the chemical reaction. The critical distance has been determined to be slightly larger than 2.5 ˚ A in close agreement with the benzene findings. The reaction is expected to develop involving molecules sitting on (20-2) crystal planes giving rise to a 3D extended carbon nitride. For pressures larger than 8 GPa the reaction occurs irrespective of temperature, which is the variable regulating the crystal dynamics, being likely related to the closure, driven by pressure, of the energy gap between ππ ∗ and nπ ∗ states which, on turn, determines the ring destabilization. This is to our

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

knowledge the first case where such competition is demonstrated and can represent a text book example in solid state reactivity. Besides the reaction mechanism the other valuable contribution of this study consists in the material synthesized by s-triazine. The stoichiometry of the starting material is maintained in the product and the chemical bonding resulting from the pair radial distribution function is quite different from that of a-C:N:H for the lack of dominant signatures from single bonds. The large N amount, the bonding structure, with C and N atoms mainly sp2 hybridized, and the compatible microscopic structure with recently computed crystalline structures points to the attainment of an amorphous extended carbon nitride. Once again this study remarks the capability of high-pressure studies of providing profound insight in the mechanism regulating the chemistry of simple molecules through experiments directed to disentangle the specific effect of all the variables affecting the reaction dynamics. This information is mandatory to figure out a possible practical exploitation of high pressure in the synthesis of novel materials. Acknowledgements Supported by Deep Carbon Observatory initiative (Physics and Chemistry of Carbon at Extreme Conditions from the Alfred P. Sloan Foundation) and by the Italian Ministero dell’Istruzione, dell’Università e della Ricerca MIUR (grants FIRB - Futuro in Ricerca 2010 RBFR109ZHQ). The authors acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities at ID27 beamline. M.M.N. thanks the Brazilian agencies CNPq and FAPESP (Grant Nos. 2010/18107-8 and 2013/05983-2) for fellowships and financial support, K.D. gratefully acknowl-

22


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edges the Polish Ministry of Science and Higher Education for financial support through the ”Mobilno´ sc´ Plus” program. Additional Informations Accession codes: The X-ray crystallographic coordinates for structures reported in this Article have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition number CCDC 1411901. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/datar equest/cif. Author Informations Corresponding Author: Roberto Bini, tel. +390554572489; E-mail: roberto.bini@unifi.it.

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References [1] Bini, R.; Schettino, V. Materials Under Extreme Conditions - Molecular Crystals at High Pressure; Imperial College Press: London, U. K., 2014. [2] Ciabini, L.; Santoro, M.; Gorelli, F. A.; Bini, R.; Schettino, V.; Raugei S. Triggering Dynamics of the High-Pressure Benzene Amorphization. Nat. Mater. 2007, 6, 39-43. [3] Ciabini, L.; Santoro, M.; Bini, R.; Schettino, V. High Pressure Photoinduced Ring Opening of Benzene. Phys. Rev. Lett. 2002, 88, 085505. [4] Citroni, M.; Bini, R.; Foggi, P.; Schettino, V. Role of Excited Electronic States in the High-Pressure Amorphization of Benzene. Proc. Natl. Acad. Sci. USA 2008, 105, 7658-7663. [5] Citroni, M.; Fanetti, S.; Bini, R. Pressure and Laser-Induced Reactivity in Crystalline s-Triazine. J. Phys. Chem. C 2014, 118, 10284-10290. [6] Ferrari, A. C.; Rodil, S.E.; Robertson, J. Interpretation of Infrared and Raman Spectra of Amorphous Carbon Nitrides. Phys. Rev. B 2003, 67, 155306. [7] Agrawal, J. P. High Energy Materials: Propellants, Explosives and Pyrotechnics; WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2010. [8] Gao, H.; Shreeve, J. M. Azole-Based Energetic Salts. Chem. Rev. 2011, 111, 7377-7436. [9] Teter, D. M.; Hemley, R. J. Low-Compressibility Carbon Nitrides. Science 1996, 271, 53-55. 24


Page 24 of 45

Page 25 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[10] Liu, A. Y.; Cohen, M. I. Prediction of New Low Compressibility Solids. Science 1989, 245, 841-842. [11] Dong, H.; Oganov, A. R.; Zhu, Q.; Qian, G.-R. The Phase Diagram and Hardness of Carbon Nitrides. Sci. Rep. 2015, 5, 9870. [12] Kroll, P.; Hoffmann, R. Theoretical Tracing of a Novel Route from Molecular Precursors through Polymers to Dense, Hard C3 N4 Solids. J. Am. Chem. Soc 1999, 121, 4696-4703. [13] Jurgens, B.; Irran, E.; Senker, J.; Kroll, P.; Muller, H.; Schnick, W. Melem (2,5,8-Triamino-tri-s-triazine), an Important Intermediate during Condensation of Melamine Rings to Graphitic Carbon Nitride: Synthesis, Structure Determination by X-ray Powder Diffractometry, SolidState NMR, and Theoretical Studies. J. Am. Chem. Soc. 2003, 125, 10288-10300. [14] Li, X.; Zhang, J.; Shen, L.; Ma, Y.; Lei, W.; Cui, Q.; Zou, G. Preparation and Characterization of Graphitic Carbon Nitride through Pyrolysis of Melamine. Appl. Phys. A 2009, 94, 387-392. [15] Lotsch, B. V.; D¨ oblinger, M.; Sehnert, J.; Seyfarth, L.; Senker, J.; Oeckler, O.; Schnick, W. Unmasking Melon by a Complementary Approach Employing Electron Diffraction, Solid-State NMR Spectroscopy, and Theoretical Calculations Structural Characterization of a Carbon Nitride Polymer. Chem. Eur. J. 2007, 13, 4969-4980. [16] Alves, I. ; Demazeau, G. ; Tanguy, B. ; Weill, F. On a New Model of the Graphitic Form of C3 N4 . Solid State Commun. 1999, 109, 697-701. 25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[17] Zhang, Z. ; Leinenweber, K. ; Bauer, M. ; Garvie, L. A. J. ; McMillan, P. F. ; Wolf, G. H. High-Pressure Bulk Synthesis of Crystalline C6 N9 H3 HCl: A Novel C3 N4 Graphitic Derivative. J. Am. Chem. Soc. 2001, 123, 7788-7796. [18] McMillan, P. F.; Lees, V.; Quirico, E.; Montagnac, G.; Sella, A.; Reynard, B.; Simon, P.; Bailey, E.; Deifallah, M.; Cor` a, F. Graphitic Carbon Nitride C6 N9 H3 ·HCl: Characterization by UV and Near-IR FT Raman Spectroscopy. J. Solid State Chem. 2009, 182, 2670-2677. [19] Horvat-Bordon, E.; Riedel, R.; Zerr, A.; McMillan, P.; Auffermann, G.; Prots, Y.; Bronger, W.; Kniep, R.; Kroll, P. High-Pressure Chemistry of Nitride-Based Materials. Chem. Soc. Rev. 2013, 3, 2122. [20] Salamat, A.; Deifallah, M.; Quesada Cabrera, R.; Cor` a, F.; McMillan, P. Identification of New Pillared-Layered Carbon Nitride Materials at High Pressure. Sci. Rep. 2006, 35, 987-1014. [21] Lowther, J. E. Relative Stability of Some Possible Phases of Graphitic Carbon Nitride. Phys. Rev. B 1999, 59, 11683-11686. [22] Mao, H. K.; Bell, P. M.; Shaner, J. V.; Steinberg, D. J. Specific Volume Measurements of Cu, Mo, Pd, and Ag and Calibration of the Ruby R1 Fluorescence Pressure Gauge from 0.06 to 1 Mbar. J. Appl. Phys. 1978, 49, 3276-3283. [23] Bini, R.; Ballerini, R.; Pratesi, G.; Jodl, H. J. Experimental Setup for Fourier Transform Infrared Spectroscopy Studies in Condensed Matter

26


Page 26 of 45

Page 27 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


at High Pressure and Low Temperatures. Rev. Sci. Instrum. 1997, 68, 3154-3160. [24] Gorelli, F. A.; Ulivi, L.; Santoro, M.; Bini, R. The ϵ Phase of Solid Oxygen: Evidence of an O4 Molecule Lattice. Phys. Rev. Lett. 1999, 83, 4093-4096. [25] Hammersley, A. P.; Svensson, S. O.; Hanfland, M.; Fitch, A. N.; H¨ ausermann, D. Two-Dimensional Detector Software: From Real Detector to Idealised Image or Two-Theta Scan. High Press. Res. 1996, 14, 235-248. [26] Smith, J. H.; Rae, A. I. M. The Structural Phase Change in s-Triazine. I. The Crystal Structure of the Low-Temperature Phase. J. Phys.C: Solid State Phys. 1978, 11, 1761-1770. [27] CPMD, Copyright IBM Corp 1990-2008,

Copyright MPI f¨ ur

Festkörperforschung Stuttgart 1997-2001, http://www.cpmd.org. [28] Car, R.; Parrinello, M. Unified Approach for Molecular Dynamics and Density-Functional Theory. Phys. Rev. Lett. 1985, 55, 2471-2474. [29] Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behaviour. Phys. Rev. A 1988, 38, 3098-3100. [30] Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785-789.

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[31] Troullier, N.; Martins, J. L. Efficient Pseudopotential for Plane-Wave Calculations. Phys. Rev. B 1991, 43, 1993-2006. [32] Kleinman, L.; Bylander, D. M. Efficacious Form for Model Pseudopotentials. Phys. Rev. Lett. 1982, 48, 1425-1428. [33] Grimme, S. Semiempirical GGA-Type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787-1799. [34] Rae, A. I. M. The Structural Phase Change in s-Triazine: Reconciliation of X-ray Diffraction and NQR Measurements. J. Chem. Phys. 1979, 70, 639-642. [35] Prasad, S. M.; Rae, A. I. M.; Hewatt, A. W.; Pawley, G. S. The Crystal Structure of s-Triazine at 5 K. J. Phys.C: Solid State Phys. 1981, 14, L929-L931. [36] Oron, M.; Zussman, A.; Rapoport, E.

14

N PNQR Study of the Effect

of Pressure on the Phase Transition and Molecular Reorientation in sTriazine. J. Chem. Phys. 1978, 68, 794-798. [37] Eckert, J.; Fincher Jr., C. R.; Heilmann, I. U. Neutron Scattering Study of the Phase Transition in s-Triazine at High Pressure . Solid State Commun. 1982, 41, 839-842. [38] Dove, M. T.; Ewen, P. J. S. A Raman Scattering Study of the Pressure Induced Phase Transition in s-Triazine. J. Chem. Phys. 1985, 82, 20262032. 28


Page 28 of 45

Page 29 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[39] Elliott, G. R.; Iqbal, Z. The Raman Spectrum and Phase Transition in Sym-Triazine. J. Chem. Phys. 1975, 63, 1914-1918. [40] Bruker AXS TOPAS version 4.2, Copyright (c), 1999, 2009 Bruker AXS. [41] Vinet, P.; Ferrante, J.; Smith, J. R.; Ross, J. H. A Universal Equation of State for Solids. J. Phys. C 1986, 19, L467-L473. [42] Ciabini, L.; Gorelli, F. A.; Santoro, M.; Bini, R.; Schettino, V.; Mezouar, M. High-Pressure and High-Temperature Equation of State and Phase Diagram of Solid Benzene. Phys. Rev. B 2005, 72, 094108. [43] Larkin, P. J.; Makowski, M. P.; Colthup, N. B. The Form of the Normal Modes of s-Triazine: Infrared and Raman Spectral Analysis and Ab Initio Force Field Calculations. Spectr. Acta A 1999, 55, 1011-1020. [44] Avrami, M. Kinetics of Phase Change. I General Theory. J. Chem. Phys. 1939, 7, 1103-1112. [45] Avrami, M. Kinetics of Phase Change. II Transformation-Time Relations for Random Distribution of Nuclei. J. Chem. Phys. 1940, 8, 212224. [46] Avrami, M. Granulation, Phase Change, and Microstructure Kinetics of Phase Change. III J. Chem. Phys. 1940, 9, 177-184. [47] Hulbert, F. Models for Solid-state Reactions in Powdered Compacts: A Review. J. Br. Ceram. Soc. 1969, 6, 11-20. [48] Citroni, M.; Ceppatelli, M.; Bini, R.; Schettino, V. High-Pressure Reactivity of Propene. J. Chem. Phys. 2005, 123, 194510. 29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[49] Li, S.; Li, Q.; Xiong, L.; Li, X.; Li, W.; Cui, W.; Liu, R.; Liu, J.; Yang, K.; Liu, B.; Zou, B. Effect of Pressure on Heterocyclic Compounds: Pyrimidine and s-Triazine. J. Chem. Phys. 2014, 141, 114902. [50] Mills, R. L.; Olinger, B.; Cromer, D. T. Structures and Phase Diagrams of N2 and CO to 13 GPa by X-ray Diffraction. J. Chem. Phys. 1986, 84, 2837-2845. [51] Scheffer, J. R. Crystal Lattice Control of Unimolecular Photorearrangements. Acc. Chem. Res. 1980, 13, 283-290. [52] Kaufman, J. H.; Metin, S.; Saperstein, D. D. Symmetry Breaking in Nitrogen-Doped Amorphous Carbon: Infrared Observation of the Raman-Active G and D Bands. Phys. Rev. B 1989, 39, 13053-13060. [53] Rodil, S. E.; Muhl, S. Bonding in Amorphous Carbon Nitride. Diam. Relat. Mater. 2004, 13, 1521-1531. [54] Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules. Academic Press, Inc. 1991 San Diego. [55] Santoro, M.; Gorelli, F. A. Structural Changes in Supercritical Fluids at High Pressures. Phys. Rev. B 2008, 77, 212103. [56] Juh´ as, P.; Davis, T.; Farrow, C. L.; Billinge, S. J. L. PDFgetX3: a Rapid and Highly Automatable Program for Processing Powder Diffraction Data into Total Scattering Pair Distribution Function. J. Appl. Cryst. 2013, 46, 560-566. 30


Page 30 of 45

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[57] Spek, A. L. Structure Validation in Chemical Crystallography. Acta Cryst. D 65, 148-155 (2009). [58] Walters, J. K.; Pickup, D. M.; Newport, R. J. Structure of a-C:N:H Prepared from Ammonia. J. Mater. Res. 2005, 20, 3338-3345. [59] Fitzgibbons, T. C.; Guthrie, M.; Xu, E.; Crespi, V. H.; Davidowski, S. K.; Cody, G. D.; Alem, N.; Badding, J. V. Benzene-Derived Carbon Nanothreads. Nat Mater. 2015, 14, 43-47. [60] Walters, J. K.; Honeybone, P. J. R.; Huxley, D. W.; Newport, R. J.; Howells, W. S. Structural Properties of Amorphous Hydrogenated Carbon. I A High-Resolution Neutron-Diffraction Study. Phys. Rev. B 1994, 50, 831-838. [61] Fanetti, S.; Citroni, M.; Bini, R. Tuning the Aromaticity of s-Triazine in the Crystal Phase by Pressure. J. Phys. Chem. C 2014, 118, 1376413768. [62] Fanetti, S.; Citroni, M.; Bini, R. Structure and Reactivity of Pyridine Crystal Under Pressure. J. Chem. Phys. 2011, 134, 204504.

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Space Group

C2/c

a (˚ A)

6.653(2)

b (˚ A)

9.364(7)

c (˚ A)

6.824(8)

β (◦ )

125.55(3)

Unit cell volume (˚ A3 )

345.9(3)

Z

4

wavelength (˚ A)

0.3738

2θ range (◦ )

3.5-15.0

Number of observations

672

Number of parameters

22

R(I)

0.007

Rwp

0.022

Rp

0.021

Rexp

0.011

Table 1: Crystallographic data, details of data collection and agreement indices for the final least-square cycles of the Rietveld refinement of s-triazine phase II at 1.2 GPa.

32


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


atom

x/a

y/b

z/c

Biso (˚ A2 )

N1

0

-0.1405(15)

0.25

3

C1

0

0.14180(48)

0.25

3

H1

0

0.2589(35)

0.25

3.6

N2

0.2106(12)

0.07105(53)

0.4052(15)

3

C2

0.21193(89)

-0.07193(52)

0.4062(14)

3

H2

0.3898(23)

-0.1215(59)

0.5366(21)

3.6

bond type

length (˚ A)

N1-C2

1.34(1)

C1-N2

1.341(9)

N2-C2

1.339(7)

C1-H1

1.10(4)

C2-H2

1.09(3)

bond angles

degrees

d1C2 C2N

123

d 2 N 1C2N

118

d 2 N 2C1N

121

d2C2 C1N

120

Table 2: Refined atomic positions, bond lenghts and selected bond angles of s-triazine phase II at 1.2 GPa.

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

P(GPa), T(K)

k(s−1 )

n

3.1 GPa, 551 K

0.229

2.86

4.6 GPa, 527 K

0.054

2.99

6.0 GPa, 549 K

0.333

2.40

Table 3: Rate constants k and n parameter obtained by the fit with equation 1 of the reaction kinetic data of phase II s-triazine reported in Figure 5.

34


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P(GPa), T(K)

8.2 GPa, 298 K

8.3 GPa, 400 K

6.0 GPa, 500 K

4.6 GPa, 520 K

3.1 GPa, 549 K

contact structural d (˚ A) thermal 2am (˚ A)

instantaneous d (˚ A)

da

3.000

0.389

2.611

db

3.104

0.389

2.715

da

2.999

0.451

2.548

db

3.102

0.451

2.651

da

3.040

0.542

2.498

db

3.157

0.542

2.615

da

3.080

0.574

2.506

db

3.205

0.574

2.631

da

3.143

0.631

2.512

db

3.272

0.631

2.641

Table 4: Nearest-neighbour CN distances determined at the different PT reaction thresholds. da and db represent the shortest contacts between molecules lying in adjacent (20-2) crystal planes and along the c axis, respectively, as indicated in Figure 7. In the column labelled as structural d, the nn distances determined by the structural parameters (see text for the description of the procedure adopted) are reported. The thermal contribution accounts for the total nn distance reduction due to translational modes and it is twice the single molecule displacement calculated by equation 2. Its value is subtracted from the structural distance to obtain the minimum instantaneous distance.

35



110 020 1-1-2

1.2 GPa; 298 K

20-2

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 45

220 040 022

3-1-4

2-2-2 130

4

6

112

8

10

132

12

4-2-2

14

2 (°)

Figure 1: X-ray diffraction pattern, measured with λ=0.3738 ˚ A, as a function of the 2θ scattering angle of s-triazine in phase II at 1.2 GPa and 298 K. Vertical bars report the calculated peak positions of the allowed hkl reflection, according to the C2/c monoclinic structure, with a square of the structure factor larger than 0.05. The hkl assignment of all the stronger observed diffraction peaks are also reported.

36


124

120 9.3 b

9.0

c

Å

6.9 6.6

a

6.3 6.0

350 340

3

3

V

= 384.5 Å

B

= 6.3 GPa

0

0

330

V (Å )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Degrees

Page 37 of 45

C

0

= 10.8

320 310 298 K

300 290 0

1

2

3

4

5

6

7

8

Pressure (GPa)

Figure 2: Upper panel: ambient temperature pressure evolution of the a, b, c lattice parameters and of the monoclinic angle β for a C2/c monoclinic unit cell. Lower panel: EOS of phase II s-triazine at 298 K. The data have been collected only in compression because of the chemical modifications 37 occurring at high pressure. The evolution with pressure of the volume has been fitted by a Vinet equation (full line), the values of V0 , B0 and C0 (see eq.1) employed in the fit are also reported.



20-2 110

1-1-2 220 040

020

1.7 GPa 2.1

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 45

2.8 3.6 4.4 4.8 5.3 5.7 6.3 6.6

4

5

6

7

8 2

9

10

11

(°)

Figure 3: Ambient temperature pressure evolution of the diffraction pattern of s-triazine phase II.

38


Page 39 of 45

0.4 543.2 K - 538.8 K

3.1 GPa

549.2 K - 543.2 K 550.9 K - 549.2 K

2 0.3

550.9 549.2

Abs

543.2 K

0.2 550.9 K

1

*

0.1

*

497.5 K

0.0

0 750

1000

1250

1500

1750

1000

1250

1500

1750

-1

Frequency (cm )

Figure 4: Left panel: infrared absorption spectra of s-triazine in phase II recorded at 3.1 GPa as a function of temperature. At each temperature the sample was thermalized at least for half hour. Right panel: difference spectra obtained by subtracting at each spectrum the one collected at the previous temperature.

39


Abs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



100

reacted triazine (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 45

75

50

3.1 GPa; 551.0 K 4.5 GPa; 526.8 K

25

6.0 GPa; 549.0 K

0

0

5

10

15

20

25

30

35

time (h)

Figure 5: Kinetic curves describing the time evolution of triazine consumption in phase II at three different P-T conditions. The full lines are the fit performed by using eq.2.

40


Page 41 of 45

600

reaction reaction 500

T (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


I 400

II

300

0

2

4

6

8

10

P (GPa)

Figure 6: Stability boundaries of phases I and II of s-triazine. Blue empty triangles are P-T conditions where the reaction was photoinduced 5 , empty red dots indicate the reaction thresholds in phase II in isothermal compression studies, full black dots indicate the reaction thresholds in phase II in isobaric heating studies, the associated bars indicate the temperature uncertainty in identifying the threshold, empty and full green squares indicate the reaction thresholds in phase I in isobaric heating studies corresponding, respectively, to slow and fast kinetics (see text). Empty red diamonds indicate the experimental data where the I-II phase transition was identified for T≤300 K by FTIR spectroscopy, and the red line the corresponding linear fit 5 . Dashed 41 lines are eye-guides describing the instability boundary of phase II.



3.6

(20-2)

3.5

nn distance (Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 45

3.4

d

3.3

c axis

b

3.2

d

3.1

a

3.0 1

2

3

4

5

6

7

8

9

Pressure (GPa)

Figure 7: Pressure evolution of representative shortest C· · ·N nearest neighbour distances. The contacts indicated as da (empty red dots) are relative to molecules lying on adjacent planes belonging to the (20-2) family whereas those labelled as db (full green dots) are relative to molecules arranged along the c axis. The two contacts are reported in the inset as dashed lines of the same color of the symbols (da red, db green). C· · ·N distances among molecules lying in the (20-2) planes fall just in between the da and db contacts whereas all the other contacts are larger than db .

42


Page 43 of 45

a

CH

b

0.04

2

0.4 CH

2

0.02

0.2

0.00

0.0

CH

CH

3

CH

3

2000

2100

2200

2300

2800

2900

3000

3

2

Abs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

a

0 b

1000

1500

2000

2500

3000

3500

4000

-1

Frequency (cm )

Figure 8: IR absorption spectra measured at ambient conditions on recovered samples removed from the DAC. Blue trace: reaction photoinduced (457 nm) at 4 GPa and 400 K (phase II), red trace: reaction at 1.1 GPa and 450 K (phase I), black trace: reaction performed at 4.5 GPa and 526.8 K. The relevant absorption differences among the spectra are due to the differences in the amount of material effectively recovered after its removal from the DAC. The small absorption around 2150 cm−1 (enlarged view a) is due to nitrile groups and cumulated double bonds 53 . In the enlarged view b the absorptions due to the CH stretching43modes and the relative assignments 54 are reported.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

recovered

C2/c s-triazine

(b)

Cc

-C N

(c)

(d)

0

Page 44 of 45

3

4

a-C:N:H

1

2

3

4

5

6

r(Å)

Figure 9: Comparison of the pair distribution functions: (a) extracted from the X-ray diffraction data relative to the product recovered from the reaction performed at 3.1 GPa and 549 K, blue and red traces are relative to two different background treatment in the X-ray data, (b) computed for the s-triazine crystal with the data deriving from the refinement at 1.2 GPa, (c) computed for the Cc-C3 N4 structure reported in ref. 11 as one of the most sta44 ble carbon nitrides structure at ambient pressure, (d) obtained from neutron diffraction experiments on nitrogen-doped amorphous hydrogenated a-C:N:H (7% of N) from ref. 58 . Dotted lines indicate the three maxima of the recovered products and are reported to make easier the comparison of the peak maxima.


Page 45 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10: TOC2

45


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