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Accurate bonding description in disordered chalcogenides: challenges and open issues

Accurate bonding description in disordered chalcogenides: challenges and open issues. Carlo Massobrio Institut de Physique et Chimie des Matériaux Strasbourg (France) (CNRS-Univ. L. Pasteur ). CECAM, Lyon, October 18, 2005. AX 2 disordered networks: (A=Ge, Si; X=Se,S,O).

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Accurate bonding description in disordered chalcogenides: challenges and open issues

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  1. Accurate bonding description in disordered chalcogenides: challenges and open issues Carlo Massobrio Institut de Physique et Chimie des Matériaux Strasbourg (France) (CNRS-Univ. L. Pasteur) CECAM, Lyon, October 18, 2005

  2. AX2 disordered networks: (A=Ge, Si; X=Se,S,O) How to build up a classical potential..?? Strong ionic character: coulomb interaction What else: two-body or three-body parts..?? Three-body: favoring edge-sharing Drawback of available potential models: Different network-forming disordered share similar structures: No homopolar bonds No miscoordinations same intermediate range order behavior: lack of concentration fluctuations (No FSDP in the Scc structure factor)

  3. GeSe2 SiSe2 Prototypical AX2 liquids and glasses featuring AX4 units and intermediate range order What is their atomic structure..?? Tool: first-principles molecular dynamics Disordered SiSe2, GeSe2 , GeSe4 : structure factors, building blocks and defects Intermediate range order (IRO): concentration fluctuations vs charge fluctuations chemical disorder

  4. FSDP FSDP What is so interesting about l- and g-GeSe2 ?? I.T. Penfold and P.S. Salmon, PRL 67, 97 (1991) FSDP in both the total... (first sharp diffraction peak) FSDP …and in the concentration- concentration structure factor Scc/cge cse = 1+cgecse(Sgege-Sgese) +cgecse(Ssese-Sgese) For point charges (classical MD): no FSDP in SCC charge-charge structure factor Scc/cgecse= SZZ

  5. SNN = cgecgeSgege +csecseSsese +2cgecsecgeSgese SCC/cge cse = 1+cgecse(Sgege-Sgese) +cgecse(Ssese-Sgese) First-principle molecular dynamics results  total neutron structure factor Theory: FSDP absent Note (dashed lines) LDA is inaccurate..!!!

  6. ngege ngese nsege nsese n Theory 0.04 3.76 1.88 0.37 2.77 0.02 Exp 0.25 3.5 1.75 0.23 2.57 0.2 CON 0 4 2 0 2.67 RCN 2 2 1 1 2.67 CON chemically ordered network RCN random covalent network Liquid GeSe2 Which kind of network..?? GeSe4 tetrahedra and miscoordinations n = cge(ngege + ngese) + cse (nsese+nsege)

  7. Facts about g-SiSe2 In the crystal ES (edge-sharing) tetrahedra only In the glass, both ES and corner-sharing (CS) present L.F. Gladden and S.R. Elliott, PRL 59, 908 (1987) M.Tenhover et al Solid State Comm. 65, 1517 (1988) D. Selvanathan et al PRB 61, 15061 (2000) Open issues: which is the predominant structural sequence of tetrahedra..?? How many Si(0), Si(1), Si(2)...?? Si(1) Si(0) Si(2) 0,1,2: number of fourfold rings for a given Si By classical MD, see G.A. Antonio et al PRB 45, 7455 (1992)

  8. Exp. Classical MD Our work Si(0) 26% 48% 30% Si(1) 52% 46% 61% Si(2) 22% 6 % 9% nsisi nsise nsesi nsese n Theory 0.06 3.89 1.94 0.10 2.68 CON 0 4 2 0 2.67 RCN 2 2 1 1 2.67 Amorphous SiSe2 M. Celino PhD thesis Enhanced chemical order

  9. Liquid GeSe4 M.J.Haye, CM et al PRB 58, 14661 (1998) Network chemically ordered GexSe(1-x) liquids IRO Se Ge 0 0.5 1 Total S(k) FSDP

  10. Understanding Scc and Szz structure factors Scc= cacx[1+cacx((SAA-SAX)+(SXX-SAX))] ziionic charges (Ge= 4, Se =6) POINT-LIKE CHARGE (PLC) MODEL zvipoint-like charges Ge= 4, Se =-2 for GeSe2, Ge= 4, Se=-1 for GeSe4 How close are Scc and Szz..??

  11. Scc l-GeSe4 FSDP Szz FPMD calculations : no FSDP in Szz General postulate: charge-charge correlations absent on IRO scales Charge neutrality on IRO scales is a primary constraint How to rationalize the appearance of FSDP in Scc..?? Scc l-GeSe2 Which atomic structure when FSDP is absent (or present) in Scc..?? Szz

  12. Correlation between FSDP in Scc and chemical disorder (I) Constraint: charge neutrality on IRO scales Case I: perfect chemical order: l-SiO2 (theory) Absence of FSDP in Scc No fluctuations of concentration on IRO scales in the Absence of deviations from tetrahedral order

  13. Correlation between FSDP in Scc and chemical disorder (II) Constraint: charge neutrality on IRO scales Case II: small deviations from chemical order: g-SiSe2 (theory) Distinct FSDP in Scc The same situation is encountered in liquid and amorphous GeSe4 (theory) and in the neutron scattering results for liquid GeSe2 (P.S. Salmon)

  14. Correlation between FSDP in Scc and chemical disorder (III) Constraint: charge neutrality on IRO scales Case III: large deviations from chemical order: l-GeSe2 (theory) case (II) (l-GeSe2, exp) compared to case (III) (l-GeSe2, theory) Vanishing FSDP in Scc

  15. Disordered networks: 3 classes of systems identified Perfect network: charge neutrality does not require any local variation of the concentration: no FSDP in Scc Ex: disordered SiO2 (case I) Moderate chemical disorder : Occurrence of different valence states Variations of concentration induce FSDP in Scc Ex: l-GeSe4, g-SiSe2, l-GeSe2 (exp) (case II) High chemical disorder: FSDP in the total S(k) but no FSDP in Scc Ex: l-GeSe2 (theory) (case III) Intensity of FSDP in Scc Case II Case III Case I Chemical disorder

  16. To summarize: Disordered network-forming materials: l-GeSe2 l-GeSe4 g-SiSe2 strong interplay bonding-structure Predominant structural motifs revealed IRO can coexist with structural defects: Correlation between chemical disorder and FSDP in the structure factor Scc(k): three classes of systems identified Beyond classical MD and cluster calculations: Ab-initio MD is crucial to quantify the kind of bond connectivity and the departures from chemical order

  17. Simulation of disordered structures: two additional points of methodology System size and periodicity: are they compatible with IRO..? Validity of phenomenological models: analysis via first-principles calculations

  18. R c sinkr = [ ] ò - 2 1 (r) 1 + r g 4 π dr S r αβ αβ kr O 1 - ( ) = a å ik R R i j β e S ij αβ N N α β Two ways are available to calculate the structure factor Real space Reciprocal space Total S(k) FSDP appears in the Ge-Ge S(K) when correlations beyond 6Å are accounted for

  19. Elliott theory remarkable but... ...A residual peak still noticeable in the S (voids-cations) at the FSDP location…! CC No evidence for plane formation

  20. Our theoretical model First principles molecular dynamics Density functional theory: GGA (PW91) Periodic cell, Plane waves basis set Ec = 20 Ry Norm conserving pseudopotentials Case of GeSe2 N=120 L= 15.7 Å liquid, T = 1040 K L= 15.1Å glass, T= 300 K Case of SiSe2 N= 120, 144 (48 Si, 96 Se) L= 15.6 Å (glass density) , N=120 (initially random) L= 16.6 Å (glass density) , N=144 (initially crystalline) kmin < 0.4 Å-1 kFSDP 1 Å-1 Equilibrium trajectories : 50-100 ps, liquids 3-5 ps, glasses (after quench) Computer code: norm-conserving version of ultra-soft FPMD written by A. Pasquarello (Lausanne) (PRL 69, 1982 (1992), PRB 47, 10142 (1993))

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