High-PressureâHigh-Temperature Study of Benzene: Refined Crystal

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High-pressure high-temperature study of benzene: refined crystal structure and new phase diagram up to 8 GPa and 923 K Artem D. Chanyshev, Konstantin D. Litasov, Sergey V. Rashchenko, Asami Sano-Furukawa, Hiroyuki Kagi, Takanori Hattori, Anton F. Shatskiy, Anna M. Dymshits, Igor S. Sharygin, and Yuji Higo Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00125 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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Crystal Growth & Design

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Title: High-pressure high-temperature study of benzene: refined crystal structure and new

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phase diagram up to 8 GPa and 923 K

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Authors: Artem D. Chanyshev,ab,* Konstantin D. Litasov,ab Sergey V. Rashchenko,ab Asami

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Sano-Furukawa,c Hiroyuki Kagi,d Takanori Hattori,c Anton F. Shatskiy,ab Anna M. Dymshits,a

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Igor S. Sharygin,a and Yuji Higoe

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Affiliations:

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a

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V.S. Sobolev Institute of Geology and Mineralogy SB RAS, 3 Ac. Koptyuga ave.,

Novosibirsk 630090, Russia

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b

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c

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Japan

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Novosibirsk State University, 2 Pirogova st., Novosibirsk, 630090, Russia

J-PARC Center, Japan Atomic Energy Agency, 2-4 Shirakata, Tokai-mura, Ibaraki 319-1195,

d

Geochemical Research Center, Graduate School of Science, The University of Tokyo, Hongo

7-3-1, Bunkyo 113-0033, Tokyo, Japan

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e

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*

Spring-8, Japan Synchrotron Radiation Research Institute, Kouto, Hyogo 678-5198, Japan Corresponding author: [email protected] (A.D. Chanyshev)

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Abstract

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The high-temperature structural properties of solid benzene were studied at 1.5–8.2 GPa up to

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melting or decomposition using multianvil apparatus and in situ neutron and X-ray diffraction.

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The crystal structure of deuterated benzene phase II (P21/c unit cell) was refined at 3.6–8.2 GPa

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and 473–873 K. Our data show a minor temperature effect on the change in the unit cell parameters

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of deuterated benzene at 7.8–8.2 GPa. At 3.6–4.0 GPa we observed the deviation of deuterium

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atoms from the benzene ring plane and minor «zigzag» deformation of the benzene ring, enhancing

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with the temperature increase caused by the displacement of benzene molecules and decrease of

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van der Waals bond length between the π-conjuncted carbon skeleton and the deuterium atom of

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adjacent molecule. Deformation of benzene molecule at 723–773 K and 3.9–4.0 GPa could be

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related to the benzene oligomerization at the same conditions. In the pressure range of 1.5–8.2 GPa

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benzene decomposition was defined between 773–923 K. Melting was identified at 2.2 GPa and

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573 K. Quenched products analyzed by Raman spectroscopy consist of carbonaceous material.

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The defined benzene phase diagram appears to be consistent with those of naphthalene, pyrene and

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coronene at 1.5–8 GPa.

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Introduction

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Benzene C6H6 is one of the fundamental compounds in organic chemistry, forming the building

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unit of aromatic compounds, that are believed to be the most abundant organic molecules in the

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Universe1-4 possibly due to electron delocalization over their carbon skeleton, which makes them

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remarkably stable.2 The formation of aromatic hydrocarbons in space is usually associated with

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irradiation-driven polymerization of smaller hydrocarbons.5 Large aromatic hydrocarbons have a

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huge influence on the heating of cosmic gas and the degree of interstellar space ionization. 4

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Moreover, aromatic hydrocarbons as well as amines and amino acids were found in meteorites.6-8

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Carbonaceous chondrites containing aromatic hydrocarbons in their matrix could serve as a source

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of extraterrestrial organic prebiotic material during the period of heavy bombardment of the inner

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Solar System 3.8–4.5 billion years ago.9-11

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Many investigations have been devoted to the study of benzene at ambient pressure and high

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temperature. The lowest temperature for conversion of benzene to biphenyl in a glass ampoule is

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573 K. At 700 K and higher (but undefined) pressure, a 75% conversion to biphenyl occurs within

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48 hours.12 Extensive pyrolysis of benzene at ambient pressure was defined at 1073–1273 K.13, 14

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Solid benzene has been investigated for the last 100 years from the pioneering research by P.

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Bridgman15 to newest studies.16-18 Several versions of benzene phase diagram were proposed at

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high pressures and temperatures19, 20 (Fig. S1). Under normal conditions, benzene is a volatile

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liquid. At ambient pressure, it crystallizes at 278.7 K (280.0 K for deuterated benzene) into the

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orthorhombic phase I (Pbca).21 It should be noted that this phase is stable down to the lowest

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temperature studied of 4 K.22 At room temperature liquid benzene crystallizes into phase I at 0.07

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GPa.23, 24 Phase I undergoes a phase transitions at 1.4 GPa and 298 K; however, this transition

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proceeds in a sluggish way at room temperature.25 The structure of high-pressure phase (II),

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indicated to be stable up to 20 GPa at 298 K,20 was determined as monoclinic P21/c with two

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molecules per unit cell.26 Detailed crystallographic studies of benzene phases I and II were

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performed at 0.15–1.1 GPa and 295–296 K.27, 28 The benzene molecules (both phase I and II) are

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assumed to have always a planar hexagonal structure. At pressure > 20 GPa crystalline benzene

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transforms to polymers or carbon-bearing nanomaterial (carbon nanothreads).16-18 A melting curve

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and boundary between phase I and phase II were defined at 0–4 GPa and 298–800 K using

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differential thermal analysis.29 High-pressure high-temperature polymerization of benzene was

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found at 15–23 GPa and 540–650 K.20, 30 Recent high-pressure study of benzene (both C6H6 and

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C6D6) has revealed polymerization and decomposition boundaries at 6–9 GPa and 900–1000 K.31

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However, in the pressure range 2–6 GPa the phase diagram is simply not constrained at high

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temperatures; also the existence of phase IV, determined in Cansell et al.19 at 2–7 GPa and 600–

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900 K remains unclear. ACS Paragon Plus Environment

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Low-temperature study of benzene (both C6H6 and C6D6) at ambient pressure displayed

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identical variations of the lattice and atomic position parameters of hydrogenous and deuterated

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phase I with decreasing temperature down to 4 K.22,

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investigations of solid hydrogenous and deuterated benzene at 293 K did not reveal any differences

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between compressibility of C-H and C-D bonds as well as entire molecules up to 20 GPa.25

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High-pressure computational

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An important and poorly studied feature of benzene is oligomerization at high pressures and

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temperatures. Pressure-induced oligomerization of benzene at room temperature was previously

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defined at 13 GPa.34 Benzene oligomerization was explained by the overlapping of π bonds and

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decrease of the intermolecular distances.34,

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temperature was also found for higher aromatic compounds: naphthalene,36, 37 anthracene37 and

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coronene37 at 15–16 GPa. In addition, significant oligomerization of aromatic compounds

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(naphthalene, anthracene, pyrene, coronene) was observed at 3.5–7.0 GPa and 773–873 K.37, 38

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One can suggest that benzene oligomerization occurs at similar conditions.

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Pressure-induced oligomerization at room

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Here we aimed to determine melting and decomposition parameters of benzene (both C6H6 and

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C6D6) and refine structure of stable phases of deuterated benzene at 2–8 GPa and high temperature

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up to 923 K using multianvil apparatus including in situ neutron and X-ray diffraction. Combined

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with previous data at 0–4 GPa these data provide further constraints on the benzene phase diagrams

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and systematics of aromatic compounds at high pressures.

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Experimental

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Laboratory experiment

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To verify the reliability of the polytetrafluoroethylene (PTFE) capsule as a container for

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benzene, an experiment was performed at 3.5 GPa and 773 K. Previously the suitability of PTFE

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as a container for volatile liquids was confirmed at 2–8 GPa and T < 873 K by Likhacheva et al.39

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We used 6-8 type 1500-tons multianvil apparatus “Discoverer” installed at IGM SB RAS

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(Novosibirsk, Russia). Cubic Fujilloy N05 WC anvils with a truncated edge length (TEL) of 12.0

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mm were used to compress the high-pressure cell. Pyrophyllite gaskets sealed the compressed

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volume and improve the stress distribution inside the cell. ZrO2 semi-sintered ceramics (OZ-8C,

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MinoYogyo Co., Ltd) were used as a pressure medium, and a cylindrical graphite heater as the

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heating element (Fig S2). Double PTFE capsule was used as a sample container. High-purity (99.8

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%, Sigma-Aldrich) hydrogenous benzene C6H6 was loaded at room temperature into PTFE

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capsule. Temperature was monitored with a W97Re3-W75Re25 thermocouple, inserted through the

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heater and electrically isolated by Al2O3 tubes. The lateral temperature variations across the charge

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did not exceed 10 K, whereas the vertical temperature gradient was negligible. The detailed

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temperature measurement procedure is described in Litasov and Ohtani.40 ACS Paragon Plus Environment


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The room-temperature pressure calibration was performed by monitoring the resistance

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changes in Bi at 2.5 and 7.7 GPa.41 The pressure calibration at high temperature was performed

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using known phase transitions in SiO2 (quartz-coesite)42 and CaGeO343 at 1100oC. The detailed

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pressure calibration procedures are described in Shatskiy et al.44

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The cell assembly was initially compressed to desired press load and then heated to the target

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temperature during 10–20 minutes and exposed for 6 hours. The decompression duration was 6

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hours.

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X-ray diffraction

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Powder X-ray diffraction measurements were conducted using a Kawai-type 1500-tons

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multianvil apparatus installed at a bending magnet beamline BL04B1 at the SPring-8 synchrotron

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radiation facility (Hyogo prefecture, Japan). This apparatus is equipped by an energy dispersive

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X-ray diffraction system with a Ge solid state detector (SSD) with a 4096-channel analyzer and a

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CCD camera for radiographic imaging of the sample. The incident X-ray collimated to dimensions

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of 50 µm horizontally and 200 µm vertically was directed at the sample through the gaps between

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the second stage anvils. The SSD analyzer was calibrated using the X-ray fluorescence lines of

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different metals before the measurements. The diffraction angle (2θ) was calibrated before each

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experiment with precision of 0.0001o, using MgO as the standard.

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We used the same type high-pressure cell described in «Laboratory experiment» section (Fig

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S2). The main difference was the need to use a material with a high X-ray transparency as windows

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in experimental cell and pyrophyllite gaskets. In the experiments we used insets made of MgO in

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the ZrO2 pressure medium and boron epoxy in the gasket. High-purity (99.8 %, Sigma-Aldrich)

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hydrogenous benzene C6H6 was used as a starting material. Sample capsule was PTFE. Pressure

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was estimated from the unit cell volume of Cu using equations of state from Wang et al.45

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The sample was compressed to desired pressure and then heated up to 1073 K. Diffraction

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patterns were collected every 100 K starting from 298 K. Exposure durations for collecting

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diffraction data were 200 sec. for pressure marker and 600 sec. for benzene. The temperature was

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hold within 0.5 K of the desired value using the temperature controlling program.46

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After decompression all samples were analyzed using STOE IPDS-2T X-ray diffractometer,

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equipped with a Mo source (graphite monochromator) and IP detector. Measurements were

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performed in Gandolfi mode.

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Neutron diffraction

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The experiments were performed using a six-axis multi-anvil press ATSUHIME installed in the

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high-pressure neutron diffractometer (PLANET) at BL11 of the spallation neutron source of the ACS Paragon Plus Environment

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Materials and Life Science Experimental Facility (MLF) at the J-PARC, Ibaraki, Japan.47, 48 The

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diffraction patterns were measured by the time-of-flight (TOF) method. The neutron beam was

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generated using a Hg target with pulse repetitions every 40 ms (25 Hz). The diffracted beam

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travelled through a 3 mm gauge radial collimator and was detected using 3He gas position-sensitive

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detectors. The detector coverage of each bank was 90±11.3o against the incident beam in the

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horizontal direction and 0±34.6o in the vertical direction. The power of accelerator was around 300

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kW. The sample position was aligned using an optical camera or more precisely by scanning to

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maximize the sample scattering intensity. Experimental details are given in Table S4.

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Six WC anvils with TEL of 7.0 and 10 mm were used for the experiments. The high-pressure

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cell contained Cr2O3-MgO pressure medium, ZrO2 end caps, cylindrical graphite heater, sample

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capsule, pressure marker and Au electrodes (Fig. S3). High-purity (99.8 %, Wako Co. Ltd)

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deuterated benzene (C6D6) was loaded at room temperature into PTFE capsules or sealed Pt

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capsules. To prevent evaporation the PTFE capsules were tightly closed by PTFE caps. Pressure

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was estimated from the NaCl unit cell volumes using equations of state from Sokolova et al.49

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Temperature calibrations were conducted in separate experimental runs before the measurement.

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The samples were compressed to selected values and then heated up to 873–1073 K. Diffraction

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patterns were collected every hundred degree in the temperature range 298–673 K and every fifty

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degree starting from 673 K. Exposure durations for collecting diffraction data were 5 h to 24 h at

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each step. The diffractometry data were collected in the interval 0.1–4.1 Å. The incident beam was

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truncated by a slit to be 2 mm wide and 4 mm high before its introduction to the sample. Separate

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data sets were collected for an empty cell and one containing a vanadium pellet at ambient

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conditions by placing them in the high-pressure cell. Sample data were normalized with the

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vanadium data to correct the energy profile of the incident neutron beam, detector sensitivity, and

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cell attenuation. The empty-cell data were subtracted from the individual data.

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Structure refinement

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The neutron powder diffraction patterns after necessary corrections were analyzed by the

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Rietveld method using GSAS-II software.50 The starting models of deuterated benzene phases I

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and II structures were constructed using atomic coordinates defined at 0.15–0.97 GPa and 295

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K.27, 28 Patterns collected at lower temperatures (up to 473 K) contained a metastable mixture of

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phase I and phase II; with temperature increase the transformation phase I → phase II was

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completed. For run 566 (1.5–2.2 GPa) loading in metallic ampoule caused an appearance of

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diffraction from capsule materials (MgO, Pt, Au) on the patterns.

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In the first series of Rietveld refinement, we fixed the geometry of C6D6 molecule by restraining

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all angles in the molecule to be 120±1°. The latter allowed us successfully refine the structures of ACS Paragon Plus Environment


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phases I and II even for problematic patterns (i.e. those contaminated by capsule materials,

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containing both benzene phases, or with poor quality due to short measurement time). We used

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these data of Rietveld refinement to determine the lattice parameters of benzene at 1.8–8.2 GPa

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and 298–923 K. In the second series of Rietveld refinement, we used high-quality patterns where

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only benzene-II is present in order to refine atomic positions without any restraints and observe

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the deviation of C6D6 molecule geometry from idealized one.

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Raman spectroscopic measurements

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The recovered samples from neutron diffraction experiments were analyzed using a Raman

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micro-spectrometer on the 514.5-nm line of an Ar-ion laser and an Olympus BX51 confocal

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microscope at the University of Tokyo (Japan). An Olympus MPlan-FL N 50x/NA=0.80 objective

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was used. Ar-ion laser (Ion Laser Technology), a single polycromator (500is Imaging

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Spectrograph, Brucker Optics) equipped with a 1200 grooves/mm grating, and a CCD detector

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(DU401A-BR-DD, Andor Technology). The spectra were collected at room temperature with

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resolution of approximately 2 cm-1. The power of laser beam was approximately 1.0–2.5 mW at

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the sample surface.

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The recovered samples from laboratory and SPring-8 experiments were analyzed using a Horiba

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Jobin LabRAM HR800 Raman micro-spectrometer on the 532-nm line of a Nd:YAG laser and an

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Olympus BX41 microscope at the IGM SB RAS (Novosibirsk, Russia). An Olympus MPlan

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100x/0.90 ∞/0/FN22 objective was used. The spectra were collected at room temperature with

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resolution of approximately 2 cm-1. The power of laser beam was approximately 40 mW.

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Acquisition time was 10–30 s depending on the signal intensity. Spectra calibration was carried

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out using the 520.6 cm-1 line of a silicon wafer. Raman spectra was decomposed into Lorentzian

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functions, after subtracting background as a linear baseline.

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Results and discussion

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Stability of benzene at high pressures and high temperatures

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In the laboratory experiment hydrogenous benzene was retained at 3.5 GPa and 773 K. After

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the decompression it was found that benzene was decomposed to black anthracite-like material

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(Fig. S4a left). The color of the inner PTFE capsule changed from white to blue, which indicates

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an increased PTFE transparency. Indeed, in a thin section both capsules are the same colorless

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(Fig. S4a right). Both capsules were unchanged and retained the plasticity. Raman spectroscopy

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studies of PTFE capsules after experiments have revealed slight deterioration of the spectrum with

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respect to the PTFE initial sample, however all PTFE modes were preserved (Fig S4b).


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The X-ray diffraction measurements were performed at 7–8 GPa and 298–1073 K using

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synchrotron radiation. Hydrogenous benzene was compressed to 7.0 GPa at room temperature and

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then heated to 1073 K in Run S3033 (Fig. S5). The insufficient quality of the diffraction profile

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and contamination from PTFE and MgO diffraction lines did not allow refining the benzene

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structure at high pressures and temperatures. However, several diffraction peaks were clearly

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correlated with the space group P21/c. Benzene decomposition was defined at 8.0 GPa and 873 K

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by the disappearance of major diffraction peaks (Fig S5). The PTFE decomposition was also

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observed at the same conditions.

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Three experiments were performed in the pressure range of 1.5–8.2 GPa using in situ neutron

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diffraction. In Run 566 deuterated benzene was loaded into sealed Pt capsule and compressed to

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1.9 GPa at 298 K and then heated to 1073 K (Fig 1a). Despite efforts to minimize contamination,

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considerable diffraction peaks derived from the pressure marker (Au), Pt capsule and MgO were

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observed. Mixtures of phases I and II were found at 298 and 473 K (I:II = 2:1 and 1:4, respectively).

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The unit cell volumes of phase I were changed from 418.76(20) to 414.52(99) Å3 with increasing

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temperature from 298 to 473 K at 1.8–1.9 GPa. The unit cell volumes of phase II increased from

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207.58(22) to 210.17(16) Å3 at the same conditions. Further heating of benzene to 573 K led to

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disappearance of the major diffraction peaks caused by the benzene melting. At 1.7 GPa and 823

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K a new graphite peak (002) was observed (Fig 1a).

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In Run 562 deuterated benzene was compressed to 3.5 GPa at 298 K and then heated to 873 K

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(Fig. 1b). Sample container was PTFE. Structural analysis of diffraction profiles revealed mixtures

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of phases I and II at 298 and 373 K (I:II = 2:1 and 1:1, respectively). The structures of phases I

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and II were successfully refined by multi-phase analysis. At 3.5 GPa the unit cell volumes of phase

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I were defined as 384.46(23) Å3 at 298 K and 386.05(17) Å3 at 373 K. Above 373 K the most

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intense peaks derived from phase I had disappeared, suggesting that the phase I had transformed

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into a monoclinic phase II. Structure refinement of phase II was performed at 300–823 K. The unit

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cell volume of phase II was changed from 189.84(15) to 195.68(16) Å3 with increasing

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temperature from 298 to 823 K at 3.5–4.0 GPa. Decomposition was defined between 823 and 873

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K by the disappearance of the major diffraction peaks (Fig. 1b). The disappearance of benzene

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diffraction peaks was accompanied by the appearance of a new graphite peak (002). After

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decompression we observed the graphitization of PTFE.

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In Run 565 deuterated benzene was compressed to 7.8 GPa at room temperature and then heated

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to 923 K (Fig. 1b). Sample container was PTFE. Structural analysis of diffraction profiles revealed

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mixture of phases I and II at 298, 373 and 473 K (I:II = 2:1, 2:1 and 1:1, respectively). The

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structures of phases I and II were successfully refined by multi-phase analysis. The unit cell

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volume of phase I was slightly increased from 345.24(9) to 346.15(18) Å3 with temperature ACS Paragon Plus Environment


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increase from 298 to 473 K at 7.8–8.2 GPa. At 573–923 K diffraction peaks assignable exclusively

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to the monoclinic phase II were identified. We successfully refined structure of phase II at 298–

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923 K. The unit cell volumes of phase II were 170.10(11) Å3 at 298 K and 7.8 GPa and 174.46(21)

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Å3 at 923 K and 7.8 GPa. During heating, the pressure was slightly changed within 7.8–8.2 GPa

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(Fig 1b). Decomposition was determined at 7.8 GPa and 923 K by the disappearance of major

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diffraction peaks (Fig 1b). After decompression the PTFE capsule was remained intact.

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Thus, we determined that PTFE was not decomposed at 7.8 GPa and 923 K, although previously the decomposition of PTFE was found at 7.7 GPa and 873 K.39

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Fig. 1 (color online). Representative neutron diffraction patterns of deuterated benzene (C6D6)

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depending on temperature at (a) 1.7–2.2 GPa, Run 566 (b) 3.5–4.0 GPa, Run 562 and (c) 7.8–8.2

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GPa, Run 565. (a) Melting was observed at 2.2 GPa and 573 K. Pt Miller indexes are black normal,

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MgO Miller indexes are blue underlined, Au Miller index is red italic. Decomposition was defined

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at (a) 1.6 GPa and 823 K, (b) ~4.0 GPa and 873 K and (c) 7.8 GPa and 923 K by the appearance

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of a new graphite peak (002) marked as 002 G.



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Structural evolution of deuterated benzene phase II (P21/c unit cell) at high pressures and high temperatures

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We carried out two series of crystal structure refinement of deuterated benzene phase II: with

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fixed geometry of C6D6 molecule (R1) and without any restraints (R2). First series of structure

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refinement (R1) was carried out to obtain lattice parameters of both phases even for problematic

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patterns. In the second series of Rietveld refinement, we used high-quality patterns of phase II,

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collected at 473–823 K in Run 562 and at 573–873 K in Run 565. The results of Rietveld

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refinement are summarized in Tables S1 (R1 refinement) and S2 (R2 refinement).

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Rietveld fitting results and the refined crystal structure (R2 refinement) of deuterated benzene at 673 K and 3.8 / ~8.1 GPa are shown in Figures 2 and 3, respectively.

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271 272 273

Fig. 2 (color online). Rietveld refinement of neutron diffraction patterns acquired at (a) 3.8 GPa and 673 K and (b) ~8.1 GPa and 673 K.

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Figures 3–5 were drawn using the Mercury CSD 2.0 program.51 The structure of deuterated

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benzene comprises hexagonal C6D6 rings, which lie in mutually intersecting planes (Fig. 3). It can

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be seen that two neighboring molecules within different layers interact with each other through ACS Paragon Plus Environment

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D4•••π intermolecular bond (Fig. 3). The reduction of the H2•••π (similar to D4•••π)

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intermolecular distance in benzene (C6H6) phase II at 295 K with increasing pressure from 0.9 to

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2.5 GPa was defined in Katrusiak et al.28 At different pressures and temperatures one can observe

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variations of interplanar angle and D4•••π bond distance (Fig. 3). The important structural features

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of benzene at high pressures are the deviation of deuterium atoms from the benzene ring plane and

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minor «zigzag» deformation of the benzene ring, enhancing with the temperature increase (Fig.

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S6).

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Figure 4 shows the evolution of interplanar angles, the shift of deuterium atoms relative to the

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benzene ring plane, D4•••π intermolecular distances and «zigzag» deformation of the benzene ring

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with increasing temperature at 3.6–8.2 GPa. Data obtained at 1.8–1.9 GPa and 298–473 K (Run

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566) were not shown due to insufficient quality of the diffraction spectra.

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Successive systematic changes of the crystallographic parameters at 3.6–4.0 GPa with

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increasing temperature to 723–773 K (Fig. 4, 5) are undoubtedly linked to each other. As

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temperature rises, the benzene rings gradually displace into the parallel planes (Fig. 5b) that

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coincides with the deviation of deuterium atoms from the ring plane (Fig. 5c) and the deformation

293

of carbon skeletons. The deformation of benzene ring including the deviation of D4 from the ring

294

plane with rising temperature is caused by the displacement of benzene molecules and decrease of

295

van der Waals bond length between the π-conjuncted carbon skeleton and the deuterium atom of

296

adjacent molecule (Fig. 5a). Previously high-pressure calculations revealed the crumple of

297

benzene ring in dimer molecules.52,

298

deformation at 723–773 K is related to the partial benzene oligomerization that should occurs at

299

the same conditions (similar to other aromatic hydrocarbons37, 38).

53

Therefore, one can suppose that observed skeleton

300

However, based on the available diffraction data, one cannot unequivocally define that the

301

positions of the carbon atoms at high pressures and temperatures are displaced with a «zigzag»

302

deformation of the benzene ring. Additional computational studies at high pressures and

303

temperatures are required to uniquely determine the position of the carbon atoms in the ring.

304



Page 12 of 35

305 306 307

Fig. 3 (color online). Crystal structures of deuterated benzene phase II (P21/c unit cell) at (a) 3.8 GPa and 673 K and (b) ~8.1 GPa and 673 K from two different viewpoints.

308

309 310

Fig. 4 (Color online). Temperature evolution at 3.5–8.2 GPa for a P21/c unit cell of the

311

interplanar angles, the shift of deuterium atoms relative to the benzene ring plane, the D4•••π

312

intermolecular distances and the «zigzag» deformation of the benzene ring observed in R2

313

refinement series. «Zigzag» deformation of the benzene ring was measured as the distance between

314

C2 atom and plane of C1-C3 atoms. Arrows indicate the temperature increase trends for Run 562

315

at 3.6–3.9 GPa. ACS Paragon Plus Environment

Page 13 of 35 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


316 317

Fig. 5 (Color online). Temperature evolution of (a) the D4•••π intermolecular distances, (b) the

318

interplanar angles and (c) the shift of deuterium atoms relative to the benzene ring plane for a P21/c

319

unit cell at 3.6–3.9 GPa.

320 321

To illustrate the distribution of close contact interactions in benzene molecules with rising

322

temperature at 3.6–4.0 GPa, Hirshfeld surface analysis54 was employed (Fig. 6). The Hirshfeld

323

surface is a tool to compare the averaged electron density of a molecule to that of the entire

324

crystal.55, 56 White color represent molecular contacts at the van der Waals distance, while red and

325

blue regions show lengths shorter and longer than the van der Waals distance, respectively (Fig.

326

6). The bar graph on the left side of Figure 6 shows that part of D•••D interactions are predominant

327

at 3.6–3.9 GPa and does not change with increasing temperature from 473 to 723 K. Part of C•••D

328

interactions slightly decreases from 42 % at 473 K to 41 % at 723 K, whereas part of C•••C

329

interactions increases from 0 % at 473 K to ~1 % at 723 K (Fig. 6). Figure 6 clearly shows the

330

weakening of D4•••C2 and D4•••C3 intermolecular interactions with increasing temperature from

331

473 K to 723 K at 3.6–3.9 GPa. This weakening is caused by the displacement of D4 atom to the

332

center of the adjacent benzene ring with increasing temperature (Fig. 6). At higher temperature

333

(773–823 K) the formation of oligomers at the grain boundaries is assumed, which can lead to the

334

distortion of crystallographic parameters at these conditions.

335

At 8.1–8.2 GPa the obtained data on the unit cell parameters do not correlate well with

336

temperature and have a smaller interval of variations in comparison with the data at 3.6–4.0 GPa

337

(Fig. 4). A slight change in the crystallographic parameters at 8.1–8.2 GPa is due to a specific

338

feature of molecular crystals like aromatic compounds: low energy of intermolecular interactions

339

and corresponding lattice phonons at pressure > 4 GPa.57, 58

340



Page 14 of 35

341 342

Fig. 6 (Color online). The Hirshfeld surface analysis of benzene molecules (P21/c unit cell) at

343

3.6–3.9 GPa and 473–723 K. The color scale on the Hirshfeld surface represents the close

344

intermolecular distances (red for the shortest, blue for the longest). The percentage contributions

345

of atomic contacts to the Hirshfeld surface are shown in the bar graphs. Part of C•••C interactions

346

is 0 % at 473 K and ~1 % at 723 K (top green part).

347


Page 15 of 35 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


348

Thermal expansion of deuterated benzene phase II (P21/c unit cell)

349

The unit cell volumes and lattice parameters of deuterated benzene at high pressures and

350

temperatures are summarized in Table S1. The evolution with pressure and temperature of the cell

351

parameters a, b, c, the monoclinic angle β and unit cell volume of C6D6 and C6H620 phase II (P21/c

352

unit cell) is presented in Figure 7. In the pressure range of 1.8–4.0 GPa the values of lattice

353

parameters a, b, c and the monoclinic angle β steadily increase with rising temperature up to 823

354

K (Fig. 7). At higher pressure (7.8–8.2 GPa) we observed very weak temperature effect on the

355

change of lattice parameters (Fig. 7). The anisotropy of thermal expansion of lattice parameters at

356

1.8–8.2 GPa and 298–923 K was not observed (Fig. 7). Obtained data at 473–573 K are consistent

357

with previous data at 540 K.20

358

359 360

Fig. 7. Temperature evolution of the lattice parameters for a P21/c unit cell at 1.8–8.2 GPa. Full

361

lines are the compressibility curves of lattice parameters defined by Ciabini et al.20 at 540 K.

362

Arrows indicate the temperature increase trends for Run 566 at 1.8–1.9 GPa and Run 562 at 3.5–

363

4.0 GPa. ACS Paragon Plus Environment


Page 16 of 35

364

It should also be noted that benzene monoclinic phase does not possess the compression

365

anisotropy at 540 K up to 27 GPa20 in contrast to polyacenes: the a-axis of naphthalene57 and

366

anthracene59 monoclinic unit cells is almost twice more compressible than the b-axis at 298 K up

367

to 13.0 and 20.7 GPa, respectively. The anisotropy of the polyacenes compression is related to the

368

linear arrangement of fused benzene rings in the structure.

369

Our data show the minor temperature effect onto the compression behavior of deuterated

370

benzene at 7.8–8.2 GPa. Previous high-pressure studies of aromatic compounds also revealed very

371

weak temperature effect on the unit cell volumes of naphthalene and coronene at P ≥ 3 GPa for the

372

temperature range of 298–873 K.57, 60 Such a diminishing of thermal effects with the pressure

373

increase is apparently a specific feature of molecular crystals like aromatic compounds.57, 61-63 This

374

may be related to low energy of intermolecular interactions and corresponding lattice phonons,

375

which can be easily “suppressed” by the applied pressure.57, 58

376 377

Raman spectra and X-ray diffraction patterns of decomposition products

378

The decomposition products in Runs 562, 565, 566 quenched from 973–1073 K were presented

379

by the solid carbonaceous soot-like materials that were analyzed by Raman spectroscopy. In the

380

range 0–2000 cm-1 we observed two broad peaks that decomposed into three modes at 1334–1347,

381

1524–1550 and 1581–1586 cm-1 (Fig. 8a). These modes correspond to the D1, D3 and G bands of

382

imperfect graphite, respectively.64-67

383

Soot-like material found after recovering in Run S3033 was also analyzed by Raman

384

spectroscopy. We observed five modes at 1229, 1340, 1510, 1576 and 1606 cm-1(Fig. S7), that

385

correspond to the D4, D1, D3, G and D2 peaks of carbonaceous material, respectively.64-67.

386

The decomposition product of benzene in laboratory experiment presented by the anthracite-

387

like material was also studied by Raman spectroscopy. Two peaks were observed that decomposed

388

into four modes at 1187, 1340, 1525 and 1590 cm-1 (Fig. S7). These modes correspond to the D4,

389

D1, D3 and G bands of carbonaceous material, respectively.64-67

390

D bands indicate the carbon amorphization; D3 and D4 modes were previously observed only

391

for soot 67. The I(D1)/I(G) = 1.15–1.52 for C6D6 decomposition products (Runs 562, 565, 566),

392

1.76 for Run S3033 decomposition product and 1.14 for decomposition product in laboratory

393

experiment. According to the «carbon amorphization trajectory»64, carbonaceous materials contain

394

up to 10 % of sp3 carbon. The other decomposition product should be deuterium or hydrogen,

395

which could escape through the capsules during the decomposition of benzene. Light

396

hydrocarbons (methane or deuterated methane) or other compounds could also be the

397

decomposition products. However, previously high-pressure study of aromatic hydrocarbons did

398

not reveal any sufficient amount of methane or higher hydrocarbons after decomposition of ACS Paragon Plus Environment

Page 17 of 35 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


399

anthracene at 5.7 GPa.68 Therefore, one can suppose that benzene decompose into carbonaceous

400

material and predominantly hydrogen (deuterium).

401

In addition, the quenched decomposition products in Runs 562 and 565 were analyzed by X-

402

ray powder diffractometry. We observed two clear peaks at d = 3.71 and 4.13 Å on both spectra

403

(Fig. 8b) and one more implicit peak at d = 4.79 Å on the bottom spectrum (Run 562). Observed

404

peaks do not correlate with 002 peak of crystalline graphite (Fig. 8b). Implicit peak at d = 4.79 Å

405

is according to 002 peak of deuterated graphite.31,

406

material can hardly be characterized as deuterated graphite, since the 002 peak at d = 4.79 Å should

407

be the most intense, which we do not observe. The decomposition products of deuterated benzene

408

are apparently consist of partially amorphized carbonaceous material, possibly with some amount

409

of deuterated graphite.

69

Nevertheless, the formed carbonaceous

410

411 412

Fig. 8 (Color online). (a) Raman spectra of recovered deuterated benzene decomposition

413

products at ~1.5 GPa, 1073 K (Run 566), ~4.0 GPa, 873 K (Run 562) and 7.8 GPa, 923 K (Run

414

565). Three modes of carbonaceous material were defined as D1 (1334–1347 cm-1), D3 (1524–

415

1550 cm-1) and G (1581–1586 cm-1). (b) X-ray diffraction patterns of recovered deuterated

416

benzene decomposition products at ~4.0 GPa, 873 K (Run 562) and 7.8 GPa, 923 K (Run 565).

417

Vertical solid lines indicate the 002 diffraction peaks of crystalline graphite and deuterated

418

graphite CD1.06.31, 69

419 420

Benzene phase diagram

421

We observed melting of deuterated benzene (C6D6) between 1.9 and 2.2 GPa at 475–573 K

422

(Fig. 1a), which is consistent with previous data on hydrogenous benzene (C6H6).29 Decomposition

423

of deuterated benzene was defined at 1.6 GPa and 823 K, ~4.0 GPa and 873 K, 7.8 GPa and 923 ACS Paragon Plus Environment


Page 18 of 35

424

K (Fig. 1). Decompostion of hydrogenous benzene (C6H6) was found at 3.5 GPa and 773 K and at

425

8.0 GPa and 873 K (Fig. S5). Taking into account the data on benzene melting we determined

426

decomposition curves of benzene (both C6H6 and C6D6) up to 8 GPa (Fig. 9a). Triple point (fluid,

427

solid and decomposed material) was defined around 3.5 GPa and 773 K (Fig. 9a). At low pressures

428

(< 3.5 GPa) decompostion line of fluid benzene has as a negative slope (dT/dP < 0). In the pressure

429

range of 3.5–6 GPa decompostion line of solid benzene (phase II) may follow to an extension of

430

the melting curve; at higher pressure (> 6 GPa) decomposition line is nearly isothermal (Fig. 9a).

431

The decrease in the carbonization temperature of benzene with increasing pressure to 3.5 GPa (Fig.

432

9a) could be explained by the pressure effect on the rate and parameters of decomposition. It was

433

clearly shown that pressure increase the decomposition rate of aromatic compounds and decrease

434

the carbonization temperature in the range of 0–2 GPa.70-74 Thus the negative slope of

435

carbonization line of fluid benzene indicates a difference in the volumes of fluid phase and forming

436

carbon solid phase (Vfluid benzene > Vcarb. material). The slope of reaction line for carbonization of solid

437

benzene is gradually changed from positive (dT/dP > 0) to neutral with increasing pressure (Fig.

438

9a). Neutral slope of carbonization line (dT/dP = 0) could be expained by the the equality of

439

volumes of reagents (benzene and oligomers) and products (carbonaceus material) at these

440

conditions. Deuterium (as well as hydrogen) is assumed to be escaped from the system.

441

It is important to note an existence of the liquid-vapor curve and triple point (liquid-vapor-

442

supercritical fluid) at low pressures near 4.9 MPa and 562 K;75 however, we do not display this

443

triple point due to scale mismatch (Fig. 9a).

444

A comparison of the obtained data with the previous results on melting and decomposition of

445

aromatic compounds is shown in Figure 9b. The benzene polymerization and decomposition

446

curves proposed by Ciabini et al.20 and Kondrin et al.31 are barely consistent with our data (Fig.

447

9b). The minor differences can be associated with methods of the temperature measurements in

448

our study and in Kondrin et al.31 or by catalytic effect of capsule materials (Cu and Pt in Ref 31).

449

However, we did not find significant differences in the decomposition parameters of polycyclic

450

aromatic hydrocarbons at 1.5–8.0 GPa using different types of capsules, including Pt,37 BN60, 70 or

451

annealed talc.38 The decomposition curve of benzene20 at 0–12 GPa is an extrapolation line of the

452

data obtained at pressures > 20 GPa. Phase IV19 was also not found in present study.

453

Using Raman spectroscopy, we found out that the benzene decomposition products consist of

454

carbonaceous materials. Previously, formation of polymer with the composition close to CH

455

(hydrographite69) from benzene was shown at the same PT-conditions: 6–9 GPa and 900–975 K.31

456

We suppose that the formation of benzene polymers at high pressures and temperatures close to

457

the decomposition parameters of aromatic compounds70 is possible at relatively short exposure


Page 19 of 35 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


458

durations. The durations of our experiments significantly exceeded 1 hour (the duration of the

459

previous experiments31).

460

461 462

Fig. 9 (Color online). (a) The phase diagram of benzene constrained from the present results.

463

The melting line and the I-II phase boundary are from Akella and Kennedy.29 CM - carbonaceous

464

material observed after sample recovering. (b) Comparison of defined P-T diagrams of benzene

465

transformations with those proposed by Ciabini et al.20 and Kondrin et al.31 as well as with P -T

466

diagrams of naphthalene, pyrene and coronene.70 The melting line and the I-II phase boundary are

467

from Akella and Kennedy.29 The hypothesized phase IV boundaries are from Cansell et al.19

468 469

Decomposition and melting curves of benzene (our study), naphthalene, pyrene and coronene70

470

are consistent with each other (Fig. 9b). With an increase the number of benzene rings from one

471

(benzene) to six (coronene), the positions of the triple point between fluid, solid and decomposed

472

material are shifted to lower pressures and higher temperatures reducing PT-field of liquid phase

473

(Fig. 9b). Carbonization lines of all fluid phases have a negative slope (dT/dP < 0) which indicates

474

a difference in the volume of the fluid phases and the forming carbon solid phases (Vfluid > Vcarb.

475

material).

476

to neutral (dT/dP = 0) with increasing pressure (Fig. 9b). It can be clearly seen that the pressure

477

interval for positive slope of carbonization curve decreases with an increase in the number of

478

benzene rings (Fig. 9b). Neutral slope of carbonization lines (dT/dP = 0) indicates the equality of

479

reagent (aromatic compounds and their oligomers) and product (carbonaceus material) volumes.

Slopes of solid phases carbonization lines are gradually changed from positive (dT/dP > 0)



Page 20 of 35

480

It is important to note that at pressures > 20 GPa benzene polymerization occurs both at room17

481

and high20, 30 temperatures up to 540 K. Nevertheless additional studies are required to determine

482

the position and slope of the polymerization curve at 15-25 GPa up to carbonization temperatures.

483

Comparison of aromatic compounds densities at standart conditions and high pressures (3.7–

484

4.0, 7.2–8.1 GPa) at 773–873 K (nearest to carbonization temperatures) are shown in Table S3.

485

The difference in the values of densities between benzene and coronene at standard conditions

486

reaches 35%, whereas at high pressures and 773–873 K (close to carbonization parameters) does

487

not exceed 5% (Table S3). Thus at high pressures and temperatures (close to carbonization

488

parameters) the densities of aromatic compounds are almost identical. We suppose that the

489

equalities of carbonization parameters for aromatic compounds at pressures > 4 GPa (Fig. 9b) are

490

related to the identity of molecular packing at these conditions.

491 492

Conclusions

493

We have refined crystal structure and defined melting and decomposition parameters of

494

benzene at 1.5–8.2 GPa and 298–923 K. At 1.5–4.0 GPa the expansion of unit cell parameters and

495

the minor reduction of the parts of C•••D interactions between the molecules with increasing

496

temperature up to 723 K were defined. We also observed the deviation of deuterium atoms from

497

the benzene ring plane and minor «zigzag» deformation of the benzene ring, enhancing with the

498

temperature increase at 3.6–4.0 GPa caused by the displacement of benzene molecules and

499

decrease of van der Waals bond length between the π-conjuncted carbon skeleton and the

500

deuterium atom of adjacent molecule. Defined carbon skeleton deformation at 723–773 K should

501

be related to the partial benzene oligomerization. At 7.8–8.2 GPa minor temperature effect onto

502

the compression behavior and intermolecular interactions of benzene monoclinic unit cell was

503

found, which can be attracted to the low energy of lattice phonons at P > 4 GPa. Benzene

504

decomposition was defined at 1.5–8.2 GPa between 773–923 K. The benzene decomposition

505

products consist of carbonaceous material containing up to 10 % of sp3 carbon. Triple point

506

between solid, fluid and decomposed state of benzene was defined at ~3.5 GPa and ~773 K.

507

Carbonization curve of fluid benzene has a negative slope (dT/dP < 0), whereas slope of

508

carbonization line of solid benzene (phase II) is gradually changed from positive (dT/dP > 0) to

509

neutral (dT/dP = 0) with increasing pressure. Phase diagrams of benzene, naphthalene, pyrene and

510

coronene show consistent shift of triple point between fluid, solid and decomposed state to lower

511

pressures. The equalities of carbonization parameters for aromatic compounds at pressures > 4

512

GPa are related to the identity of molecular packing at these conditions.

513


Page 21 of 35 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

514


Acknowledgements

515

This work was supported by the RF state assignment project No 0330-2016-0006. Experiments

516

were conducted under J-PARC general research proposal No 2016A0194 and SPring-8 general

517

research proposal No 2015A1496.

518 519

Supporting Information

520

Comparison of P-T diagrams of benzene transformations (Fig. S1); Cell assembly for Kawai-

521

type multianvil apparatus for laboratory and SPring-8 experiments (Fig. S2); Cell assembly for 6-

522

axis multianvil apparatus for J-PARC experiments (Fig. S3); Image of the quenched sample after

523

recovery from laboratory experiment and Raman spectra of PTFE capsules after experiments (Fig.

524

S4); Benzene (C6H6) X-ray diffraction patterns changes with temperature at 6.7–8.0 GPa (Fig. S5);

525

Different views of benzene molecule at ~3.9 GPa and 723 K (Fig. S6); Raman spectra of benzene

526

decomposition products (Fig. S7); The unit cell parameters of deuterated benzene phases I and II

527

at high pressures and high temperatures (Table S1); The unit cell and crystallographic parameters

528

of deuterated benzene phase II at high pressures and high temperatures (Table S2); Comparison of

529

unit cell volumes and density of benzene, naphthalene and coronene (Table S3); Experimental

530

details of structure refinement for C6D6 phase II (Table S4).



531

Page 22 of 35

Notes and references

532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582

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For Table of Contents Use Only Title: High-pressure high-temperature study of benzene: refined crystal structure and new

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phase diagram up to 8 GPa and 923 K

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Authors: Artem D. Chanyshev, Konstantin D. Litasov, Sergey V. Rashchenko, Asami Sano-

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Furukawa, Hiroyuki Kagi, Takanori Hattori, Anton F. Shatskiy, Anna M. Dymshits, Igor S.

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Sharygin, and Yuji Higo

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Synopsis: High-pressure study of benzene showed the deformation of benzene ring with

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increasing temperature at 3.6–3.9 GPa. Benzene decomposition into amorphous carbon and

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hydrogen was defined between 773–923 K and 1.5–8.2 GPa. A new phase diagram of benzene

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was proposed.

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Table of Contents Graphic:

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Fig. 1 (color online). Representative neutron diffraction patterns of deuterated benzene (C6D6) depending on temperature at (a) 1.7–2.2 GPa, Run 566 (b) 3.5–4.0 GPa, Run 562 and (c) 7.8–8.2 GPa, Run 565. (a) Melting was observed at 2.2 GPa and 573 K. Pt Miller indexes are black normal, MgO Miller indexes are blue underlined, Au Miller index is red italic. Decomposition was defined at (a) 1.6 GPa and 823 K, (b) ~4.0 GPa and 873 K and (c) 7.8 GPa and 923 K by the appearance of a new graphite peak (002) marked as 002 G. 84x208mm (300 x 300 DPI)



Fig. 2 (color online). Rietveld refinement of neutron diffraction patterns acquired at (a) 3.8 GPa and 673 K and (b) ~8.1 GPa and 673 K. 84x99mm (300 x 300 DPI)


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Fig. 3 (color online). Crystal structures of deuterated benzene phase II (P21/c unit cell) at (a) 3.8 GPa and 673 K and (b) ~8.1 GPa and 673 K from two different viewpoints. 177x58mm (300 x 300 DPI)



Fig. 4 (Color online). Temperature evolution at 3.5–8.2 GPa for a P21/c unit cell of the interplanar angles, the shift of deuterium atoms relative to the benzene ring plane, the D4•••π intermolecular distances and the «zigzag» deformation of the benzene ring observed in R2 refinement series. «Zigzag» deformation of the benzene ring was measured as the distance between C2 atom and plane of C1-C3 atoms. Arrows indicate the temperature increase trends for Run 562 at 3.6–3.9 GPa. 84x107mm (300 x 300 DPI)


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Fig. 5 (Color online). Temperature evolution of (a) the D4•••π intermolecular distances, (b) the interplanar angles and (c) the shift of deuterium atoms relative to the benzene ring plane for a P21/c unit cell at 3.6– 3.9 GPa. 158x73mm (300 x 300 DPI)



Fig. 6 (Color online). The Hirshfeld surface analysis of benzene molecules (P21/c unit cell) at 3.6–3.9 GPa and 473–723 K. The color scale on the Hirshfeld surface represents the close intermolecular distances (red for the shortest, blue for the longest). The percentage contributions of atomic contacts to the Hirshfeld surface are shown in the bar graphs. Part of C•••C interactions is 0 % at 473 K and ~1 % at 723 K (top green part). 84x97mm (300 x 300 DPI)


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Fig. 7. Temperature evolution of the lattice parameters for a P21/c unit cell at 1.8–8.2 GPa. Full lines are the compressibility curves of lattice parameters defined by Ciabini et al.20 at 540 K. Arrows indicate the temperature increase trends for Run 566 at 1.8–1.9 GPa and Run 562 at 3.5–4.0 GPa. 84x124mm (300 x 300 DPI)



(a) Raman spectra of recovered deuterated benzene decomposition products at ~1.5 GPa, 1073 K (Run 566), ~4.0 GPa, 873 K (Run 562) and 7.8 GPa, 923 K (Run 565). Three modes of carbonaceous material were defined as D1 (1334–1347 cm-1), D3 (1524–1550 cm-1) and G (1581–1586 cm-1). (b) X-ray diffraction patterns of recovered deuterated benzene decomposition products at ~4.0 GPa, 873 K (Run 562) and 7.8 GPa, 923 K (Run 565). Vertical solid lines indicate the 002 diffraction peaks of crystalline graphite and deuterated graphite CD1.06.31, 69 178x84mm (300 x 300 DPI)


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(a) The phase diagram of benzene constrained from the present results. The melting line and the I-II phase boundary are from Akella and Kennedy.29 CM - carbonaceous material observed after sample recovering. (b) Comparison of defined P-T diagrams of benzene transformations with those proposed by Ciabini et al.20 and Kondrin et al.31 as well as with P -T diagrams of naphthalene, pyrene and coronene.70 The melting line and the I-II phase boundary are from Akella and Kennedy.29 The hypothesized phase IV boundaries are from Cansell et al.19 190x95mm (300 x 300 DPI)


High-PressureâHigh-Temperature Study of Benzene: Refined Crystal

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