Francesco Capitani

Supervisor: Prof. P. Postorino

PhD in Physics

(Cycle XXVIII)

18 Febbraio 2013

Effect of high pressure on the structural and vibrational properties of

organic molecular crystals

Physics Dept.

High Pressure Spectroscopy Research Group

Interest in aromatic molecular crystals Discovery of superconductivity upon doping R. Mitsuhashi et al., Nature 464, 76 (2010)

Spectroscopic/Theoretical study of vibrational spectrum of undoped Picene under pressure

Picene: C22H14

Structure highly compressible, pressure modify structure

but rather rigid molecule and finely tune intermolecular interaction

Common feature in organic molecular systems!

Structure/intermolecular interactions strongly affect physical/chemical properties

But still not systematically investigated and understood

Premise Unusual birth of a project: from particular to general

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Organic Molecular Crystals

Crystals made up of organic molecules: 90% of the molecular crystals!

Characterized by weak intermolecular interactions

About 300,000 organic structures present in CSD (42.6%) Cambridge Structural Database Entries: Summary Statistics, 6 January 2014

Wide choice for the building blocks crystal with the desired properties

Van der Waals ~𝑟−6

Dipole-dipole ~𝑟−3

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Given the molecule, which will be its crystal structure? (Crystal Engineering) G.R. Desiraju, J. Am. Chem. Soc. 135, 9952 (2013)

(also considered that organic crystals can have hundreds of atoms in the unit cell)

Close Packing vs. Intermolecular Interactions

Crystallization driven by:

minimize voids between molecules geometrical model by Kitaigorodsky ('70s)

Intermolecular interactions (weak but fundamental)

Close packing

Complicated energy landscape

Metastable phases and polymorphism

Kinetics

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Project: Probing intermolecular interactions

Motivation:

We need a better understanding of intermolecular interactions

Vantages of HP:

Probing interactions /close packing interplay changing intermolecular distances

P-dependence of the studied physical properties: severe test to theoretical models

Study focused on a class of organic molecular crystals

Vibrational spectrum Structure

Spectroscopy (Raman and IR) X-Ray diffraction (XRD)

High Pressure (HP)

How?

Exp. Techniques:

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Aromatic Molecular Crystals (AMCs)

Molecules with (at least) a benzene ring

Polycyclic aromatic hydrocarbons (PAHs): Only fused benzene rings in a (quasi) planar structure

Crystallize in 4 Crystal Packings

Applications in organic electronics

JACS 130, 10470 (2008), PRL 108, 226401 (2012)

SUPERCONDUCTORS upon doping!!

Two main intermolecular interactions

π-π π-π

H-π

Delocalized π orbitals

G.R. Desiraju et al., Acta Crystallogr. B 85, 473 (1989) 6

PAHs doped with alkali metals (A), Alkali Earths (AE) or Rare Earths (RE):

New class of superconductors

A3/AE1.5/RE-Phenanthrene C14H10: Tc = 5 K

X. F. Wang et al., Nat. Commun. 2, 507 (2011)

X. F. Wang et al., Phys. Rev. B 84, 214523 (2011)

A3/AE1.5-Picene C22H14: Tc = 7 or 18K R. Mitsuhashi et al., Nature 464, 76 (2010) H. Mitamura et al., Phys. Chem. Chem. Phys. 13, 16476 (2011)

A-Dibenzopentacene C30H18: Tc = 33 K M. Xue et al., Sci. Rep. 2, 389 (2012)

A-Coronene: Tc = 7 K H. Mitamura et al., Phys. Chem. Chem. Phys. 13, 16476 (2011)

Linear isomer of these compounds are not superconducting

Molecular structure is important!

Superconductivity in PAHs

picene

phenanthrene

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Aromatic Superconductors: Open Issues

Theoretical Experimental

Superconducting mechanism debated:

Electron-phonon coupling?

T. Kato et al., Phys. Rev. Lett. 107, 077001 (2011)

A. Subedi and L. Boeri, Phys. Rev. B 84, 020508(2011)

Are intermolecular, intramolecular or intercalant phonons the most relevant? M. Casula et al., Phys. Rev. Lett. 107, 137006 (2011)

Electronic correlations?

G.Giovannetti and M.Capone, Phys. Rev. B 83, 134508(2011)

A.Ruff et al., Phys. Rev. Lett 110, 216403 (2013).

Lack of comparison with experimental data

Synthesis of superconducting doped samples not an easy task

Different synthesis methods lead to different phases in K-doped Picene T. Kambe et al., Phys. Rev. B 86, 214507 (2012)

Exact structure still not clear: where dopant atoms go?

There are not systematic studies

on doped samples , but even on pure systems!!

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Why study pure picene under pressure?

Spectroscopy + High Pressure: only effects of lattice compression

Two main effects of K-doping in picene:

Charge doping

Lattice expansion T. Kambe et al., Phys. Rev. B 86, 214507 (2012)

Diamond Anvil Cell

Raman spectrometer @ HPS Lab, Phys. Dept

9 IR @ SISSI - ELETTRA

Picene (C22H14) at ambient conditions

Experimental/Theoretical

characterization of the

vibrational spectrum at

ambient pressure needed

molecule

crystallization herringbone structure

2 molecules per unit cell

216 Raman/IR active modes

B. Joseph, L. Boeri, L. Malavasi, F. Capitani et al., J.Phys.: Condens. Matter 24, 252203 (2012)

Calculations by M. Höppner and L. Boeri

IR @ SISSI - ELETTRA

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Vibrational spectra under pressure

High quality Raman/IR experimental spectra

Theoretical spectra based on DFPT-LDA (by M. Höppner and L. Boeri)

Excellent theor/exp agreement

Phonons display frequency hardening and no anomalies (phase transitions)

Confident about calculations, deeper analysis is possibile

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Pressure vs. Doping: the Raman peak at 1380 cm-1

Spectral shape dominated by the a1 mode

Frequency hardening

Kambe et al. Data: K-doped picene Our Data: Pressure

Spectroscopic marker of K content

Frequency softening

Grüneisen parameters

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Pressure vs. Doping: the Raman peak at 1380 cm-1

* * T. Kambe et al., Phys. Rev. B 86, 214507 (2012)

Softening in K-doped picene not only related to volume expansion

(electronic states are involved)

Kambe et al. Data: K-doped picene Our Data: Pressure

Different Grüneisen parameters

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Phonons and structure under pressure

AΠ: average of the largest projections on molecular eigenstates

Picene at 8 GPa still a molecular crystal (𝐴𝜋 =0.6), but loses molecular character with P

P vs V relation experimentally not available

Theoretical Equation of State

V0= 613 Å3, B0= 18.5 GPa, B'0= 6.8

Structure

Crystalline Molecular

Molecular: 𝐴𝜋 = 1

Crystalline: 𝐴𝜋~ 0

Intermolecular interactions

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Picene: Main Outcomes

Pure Picene

Detailed understanding of the vibrational spectrum

Spectral evolution and Structural stability under pressure up to 8 Gpa

Intermolecular interactions: picene loses molecular character under pressure

Comparison with literature results on K-doped samples

Softening of the a1 Raman peak in K-doped samples is not only due to lattice expansion

F. Capitani, M. Hӧppner, B. Joseph, L. Malavasi, G. A. Artioli, L. Baldassarre, A. Perucchi, M. Piccinini, S. Lupi, P. Dore, L. Boeri and P. Postorino

"A combined experimental and computational study of the pressure dependence of the vibrational spectrum of solid picene C22H14"

Phys. Rev. B 88, 144303 (2013). 15

Present and future work

Moving the target:

From superconductivity to fundamental interactions in Aromatic Molecular Crystals under HP

X-Ray Diffraction main tool to probe the structure

HP XRD: Need of intense and collimated radiation

Use of Synchrotron radiation

Powder Diffraction

Bragg's Law

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HP XRD analysis

0 5 10 15 20

Inte

nsity (

arb

. u

nits)

2

Silicon Powder

Data collected @ESRF ID-09

Phenanthrene Powder

Diffraction signal from hydrocarbons not so good

Analysis not an easy task

Le Bail method: possible

Rietveld method: maybe possible using costraints

Low data-parameters ratio

Unit cell

Molecule as a rigid body

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First HP XRD measurements: Phenanthrene

Data collected @ESRF ID-09

Pressure range: 0-25 GPa

λ = 0.41 Å

Gas loading: Helium

600 800 1000 1200 1400 1600

8.0 GPa

6.0 GPa

4.1 GPa

2.0 GPa

Wavenumber (cm-1)

curve translate:

0.0 GPa

MIR experimental

We have also spectroscopic measurements

No hydrostatic medium

Complex spectra

(IR collected @ SISSI – ELETTRA)

Possible phase transitions (dependence from hydrostaticity?!)

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Summary and Perspectives

Phenantrene:

HP XRD data analysis and calculations in progress

Raman/IR pressure range to be extended

Chrysene

All to be performed

Picene

Spectroscopy and theory completed,

HP XRD to be performed

*Density Functional Perturbation Theory

Raman IR XRD HP Theory*

Phenanthrene ~ ~ ~ ~ ~

Chrysene r r r r r

Picene a a r a a

Aim Gain insight on

intermolecular interactions interplay (and new structural phases)

on PAHs by exploiting HP techniques 19

Side work: interactions of Butil-ammonium nitrate in water solutions

Ionic Liquid: salt with large size ions

Molecular dynamics (by E. Bodo) Raman

Water cannot destroy the ionic couple

Anion-Water interaction is stronger than cation-water one

Reduction of Coulombian interaction

Melting point at ambient temperature

Looks like a different situation from what we have talked about so far but...

(submitted to J.Chem. Phys.)

Organic Cation NH3 + C4H9

Anion NO3

Intermolecular interactions (especially H-bond) are important!!

+ -

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Thank you for the attention!

… and thanks to:

Stuttgard (Germany)

L.Boeri & M.Hӧppner

Physics Dept., Rome (Italy)

L. Baldassarre P. Dore

B. Joseph S. Mangialardo

P. Postorino

Dept. of Physical Chemistry Pavia (Italy)

G.A. Artioli L. Malavasi

SISSI beamline Trieste (Italy)

S. Lupi A. Perucchi

Alghero (Italy)

M. Piccinini

This research is supported by

Chemistry Dept., Rome (Italy)

E. Bodo

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Structure

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Intermolecular interactions

Single molecule eigenvalues and eigenvectors

Crystalline eigenvalues and eigenvectors

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Phenanthrene Refinement

P= 2.1 GPa P= 11.4 GPa

P= 15.5 GPa

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BAN

Radial distribution functions of the anion-cation center of mass. See Figure 1 for the definition of each atom label.

Number of H-bonds among the ions in BAN (black), among the water molecules (green), among water molecules and anions (red) and among water molecules and cations (blue) as a function of concentration. Inset: radial distribution functions on growing the water content for the distance of water molecules.