Carlo MassobrioInstitut de Physique et de Chimie des Matériaux de Strasbourg (France)

Bonding and structure in chalcogenides via first-principles atomic scale modelling

Purpose : Correlation atomic structure/diffusion/electronic properties

Indo-french workshop on glasses and glass ceramics, University of Lille, France, June 6-8, 2012

Systèmes “nano” (1)

Glasses and liquids Hybrid organic-inorganiclamellar solidsNano systems

Team « Atomic scale simulations via First-Principles MD»Carlo Massobrio et Mauro Boero at IPCMS

Clusters and moleculeson surfaces

Car-Parrinello Molecular Dynamics• We can generalize the classical MD lagrangean by adding the

electronic degrees of freedom ψi• a constraint for keeping the orthogonality of the wavefunctions• and any external additional variables αq (e.g. thermostats etc)

Atomic scale studies on a computerby using FPMD

HOW MUCH DOES IT COST..???HOW LONG DOES IT TAKE …???

?

Atomic-scale studies on a computerby using FPMD

100000 euros ~ 400 computational coresHOW LONG DOES IT TAKE …???

?

120 atoms, FPMD (CPMD code)IBM SP6 (IDRIS, Orsay)24 pc, 0.7 s/step1 ps of dynamics ~ 19 h/mpc (Δt=10 a.u.)

480 atoms, FPMD (CPMD code)IBM SP6 (IDRIS, Orsay)64 pc, 28 s/step1 ps of dynamics ~ 2100 h/mpc (Δt=10 a.u.)

HOW LONG DOES IT TAKE …???

Working on liquids and glasses: essential featuresFirst principles molecular dynamics (Car-Parrinello scheme)Density functional theory combined with GGA (PW, BLYP)Periodic cell, Plane waves basis setsNorm conserving pseudopotentials

kmin < 0.4 Å-1 kFSDP ≈ 1 Å-1

(intermediate range order scales)

Length of equilibrium trajectories : Liquids : up to 1000 ps (significant sampling ensured by diffusion)

Glasses : Quench of MANY uncorrelated liquid configurations (LC) followed by structural relaxation. This gives access toerror bars…(never do simulation on glasses without errorbars !!!!)

Sizes of the periodic boxes: L=15-25 Å (N=120-480)

How do we deal with space and time limitations..?

GGA Perdew-Wang (J. P. Perdew, Y. Wang PRB 45, 13244 (1992))

Bonding vs GGA choice…

Based on the εc( ρ) of the uniform electron gasbetter than LDA but still tendency to delocalize ..!!

GGA Becke-Lee-Yang-Parr(C. Lee, W, Yang, R. G. Parr, PRB 37, 785 (1988))

Built on « exact » calculation of the correlation energy forisolated systems…no reference made to the electron gas.

Is BLYP better suited for cases of close electronegativities..??(GeSe2 Δel =0.54, SiO2 Δel=1.54)

Circles: exp. By P. S. Salmon

dGeGe (exp) = 2.33 ±0.03 Å

dGeGe (BLYP) = 2.45 ±0.1 Å

PW 0.04 3.76 1.88 0.37BLYP 0.22 3.55 1.77 0.33Exp 0.25 3.5 1.75 0.23CON 0 4 2 0 RCN 2 2 1 1

Ngege Ngese Nsege Nsese

Liquid GeSe2 BLYP et PW

…localization leads to a betteraccount of Ge-Ge short range correlations

dGeGe (PW) = 2.7 ±0.1 ÅHigher chemical order amountsto lower mobility

D (exp, T=1050 K) < 10-6cm2/sD (PW, T=1050 K) ~10-5cm2/sD (BLYP, T=1050 K) ~10-6cm2/s

Neutron total structure factor Ge2Se3

Exp: P. S. Salmon, J. Non-Cryst. Sol., 353, 2959-2974 (2007)

[4] Van Roon F. and al, J. Chem. Phys. 113, 5425 (2000)

[3] Micoulault M. and al,Phys. Rev. B, 79:214205 (2009).

[3][4]

• Ge an Se atoms are more mobile in l-Ge2Se3 than in l-GeSe2• Ge-Ge homopolar bonds destabilize the tetrahedral network• Ge an Se atoms are more mobile in l-Ge2Se3 than in l-GeSe2• Ge-Ge homopolar bonds destabilize the tetrahedral network

Correlation Wannier Crs/ Ge sites

Correlation Wannier Crs/ Se sites

Atomic structure of two intermediatephase glasses: GeSe4 and SiSe4

CM, M. Celino, P.S. Salmon, R.A. Martin, M. Micoulaut, A. Pasquarello, PRB 79, 174201 (2009)

From k space tpr space by usingthe same kmax(16Å-1)

FPMD results

Higher kmax (40Å-1)

Exp vs FPMD:

Se-Se-Se (BB) 26 % 33%Se-Se-Ge (AB) 47% 21%Ge-Se-Ge (AA) 23 % 46%

FPMD FPMD-NMR

Structural composition of first-neighbor shells in GeSe2 and GeSe4 glasses from a first-principles analysis of NMR chemical shifts

M. Kibalchenko, J. R. Yates, CM and Alfredo Pasquarello, J. Chem.Phys C 2011, 115, 7755-7759

Electronic structure and localisation properties of C60Tan (n=1,3) clusters: A first-principles study,

M. Matsubara, C. Massobrio, L. M. Ramaniah,E. Ruiz and M. Boero, Phys. Rev. B 81, 195433 (2010).

My contact in India:Lavanya Ramaniah (BARC)

Active in the area of materials modelling:

Advanced plans to submit a CEFIPRA project on disordered materials

(one idea: under pressure)

Common work so far: