Spectroscopie vibrationnelle appliquée à la détermination de la structure locale des verres silicatés

Transcription

1 Spectroscopie vibrationnelle appliquée à la détermination de la structure locale des verres silicatés B. Hehlen 1,2 1 Laboratoire Charles Coulomb (L2C), UniversityMontpellier II, France. 2 CNRS, UMR5221, Montpellier, France.

2 Outline I-Vibrationalspectroscopies - Infrared, Raman and hyper-raman scattering -Nature of the vibrations in the glass formerssio 2 and B 2 O 3 Atomic displacements, polar or not polar, Quantitative description of the structural modifications in binary and ternary glasses II- Hyper-Raman scattering: Coherent vs incoherent excitations III- Si-O-Si angle distribution in silicates and borosilicates IV- Signature of network modifier cations in the Raman spectra of aluminosilicate glasses

3 Scattered intensity: Vibrational spectroscopies { P( r, t) ( 0,0 ) } I( ω, q) TF P Hyper-Raman Polarization: Dipole moment Induced polarization Polarizability Hyper-Polarizability ω i ω i ω d P T (r,t) = µ(r,t) + α(r,t) E i Infrared Raman (IR) (RS) + β(r,t) E i E i +... Hyper-Raman (HRS) ω i ω v Different selection rules in IR, RS, and HRS Only polar modes in IR Polar and non-polar excitations in RS and HRS Their exist excitations active in HRS not active in RS, and vice versa HRS complements IR and Raman techniques

4 Le spectromètre Hyper-Raman I I Y A G HRS RS Laser pulsé ns 10 6 Nécessité d un spectromètre très lumineux!! Diffractomètre CCD Diffractomètre Diffractomètre : -Haute résolution ( 2cm -1 ) - Haute luminosité CCD : -Très Sensible + faible bruit Polariseur -Spectres VV et VH λ=1064 nm Atténuateur CCD λ 532 nm Polariseur Doubleur de fréquence Microscope confocal - Résolution spatiale qqes µm Hyper-Raman µscope échantillon Doubleur de fréquence Spectromètre Raman très lumineux

5 v-sio 2 : Vibrational spectroscopy Only Polar modes Polar modes + BP Polar modes (but not TO4!) + BP Selection rules partly apply!!

6 Raman [Galeener et al. 70 s-80 s] [Pasquarello et al. PRL2003] The vibrations of v-sio 2 R-band n Ring modes D 1 Stretching of SiO 4 tetrahedra I RS (ω) 500 D 2 F2s F2b F1 n [Taraskin et al. PRB 1997] F2s F2b F1 Hyper-Raman Libration of rigid SiO 4 tetrahedra Frequency (cm -1 ) Rocking Si-O-Si [Kirk JPC1988] [Hehlen et al. PRL 2000] Motions of rigid SiO 4 tetrahedra Deformation of Si-O-Si units Deformation of SiO 4 tetrahedra Weak bonds Hard bonds

7 II Hyper-Raman spectroscopyin v-sio 2 : Localized vs delocalized excitations [B.Hehlen and G. Simon, JRS2012]

8 Hyper-Raman scattering 2 d order polarization fluctuation: In glasses β= β Av + β Loc Fluctuations from the average media [Denisov et al. Phys. Rep. 1987] Local fluctuations β Loc in liquidsand gases T d - Depends on the symmetry(point group) of the molecular units - Isotropic averaging over all orientation Incoherent D 3h Scatteringisindependenton the wavevectorq (intensityand depolarizationratio ρ=i VH /I VV )

9 HRS selection rules for β β Av : Isotropic average media ( m) symmetry group No LOs Only LOs The scatteringdependson q, intensities and depolarization ratios!! -LOs owing to their coupling with the long range electric field - Strongly delocalized vibrations βin glasses: A complicatedmixture of β Av and β Loc

10 q-dependence of the HRS spectra [B.Hehlen and G. Simon, JRS2012] Depolarization ratio ρ = I I VH VV TO4-LO4 : k S q k i ρand I HRS depend on q k i q k S HRS efficienciescontroledby β Av? HRS Boson peak: k i k S ρ BP = 0.63 ± 0.1 q whatever the scattering geometry!!! HRS efficiencycontroledby β Loc

11 HRS efficiencies of the (TO-LO) 4 doublet LO4: fullfilthe ( m) average media selection rules Si O Si Collective motions du to the coupling with the macroscopic E-field TO4 : intermediate between( m) and local selection rules Expe. ( m) I VV I VH I VV I VH I VV I VH β= β Loc + β Av Delocalizedexcitation!! [M. Wilson et al.prl 1996] Densityfluctuations in v-sio 2 ρ/ρ 1.2% in a volume of (2 nm) SiO 2 units Up to 100 Si-O-Si unitscouldbeinvolved!! [A.M. Levelutand A. Guinier, Bull. Soc. Fr. Miné. Crist. 1967]

12 The Boson Peak The HRSBoson Peak Scatteringindependenton q Constant depolarizationratio ρ= I VH /I VV = 0.63 Local or quasi-local excitations Librations of rigidsio 4 tetrahedra [B. Hehlen etal., PRL 2000] Importance of librations at low-frequency in v-sio 2 -Soft mode of the α-βtransition of α-quartz [Y. Tezukaet al., PRL 1991] -Its frequency extrapolate to that of the glass at Tg [B. Hehlenet al., JNCS 2002] - Supported by numerical simulations [B. Guillot et al., PRL1997] - They participate to the total excess of low-ω vibrations [U. Buchenau et al., PRL 1984] - Compatible with the Rigid Units Model (RUM) [K. Trachenko et al., PRL 1998] Boson Peak (excess of Cp/T 3 at low-t) : Rigid librations + Translations

13 III -O- bond angle of silicates extracted from their Raman spectra [B.Hehlen, JPCM2010] 1500 I RS (ω) R Bending Si-O-Si D 1 D Frequency (cm -1 )

14 Raman is highly sensitivity to local structural modifications and very simple to operate but, it hardly provides quantitative estimates!! One example : Bending modes R, D1, D2 What has to be known : Si-O-Si angle θ θ Coupling to light coefficient C(ω) Coherent or incoherent scattering Relation between the frequency or/and intensity and the structural property Effect of the surrounding on points 1-3 C(ω) ω 2 No cos(θ/2) = ω No -After normalization by C(ω), the frequency of the R, D1 and D2 bands relates to the Si-O-Si angle through 3 -The transformation is however an approximation du to the unknowns 2 and 4

15 ))(( Permanent densification Raman Scattering in permanently densifiedsilicas, d-sio 2 -Reduction of the Si-O-Si angle θin the network -SiO 4 tetrahedraremain unchanged [Y.Inamura et al. JNCS 2001] Si θ O Si Puckering of the ring network + bond redistribution Densityof states of bendingmodes O Si Si g B θ Raman Intensity ω 1 ω I ω )3 ρω ω C ( ω ) [n( ω ) + 1] (RS i s B Glass density Coupling function Boson peak g B For those C B (ω) ω 2 [B.Hehlen, JPCM2010] ω 1 I ω )3 ρω ω ω [n( ω ) + 1] (RS i s

16 Raman is highly sensitivity to local structural modifications and very simple to operate, but it hardly provides quantitative estimates!! One example : Bending modes R, D1, D2 What has to be known : Si-O-Si angle θ θ 1 2 Coupling to light coefficient C(ω) Coherent or incoherent scattering VDOS g(ω) C(ω) ω 2 No 3 4 Relation between the frequency or/and intensity and the structural property Effect of the surrounding on points 1-3 cos(θ/2) = ω No -After normalization by C(ω), the frequency of the R, D1 and D2 bands relates to the Si-O-Si angle through 3 -The transformation is however an approximation du to the unknowns 2 and 4

17 Si-O-Si angle θin d-sio 2 [B. Hehlen, J.Phys.: Cond Matter 2010] R-band Small rings : θ θ n= 3 n= 4 Network angle : θ n n 6 Max. of the distribution n> 6 Average angle

18 Si-O-Si angle θin d-sio 2 [B. Hehlen, J.Phys.: Cond Matter 2010] RMN (Devine et al. 1987) R-band Small rings : θ θ n= 3 n= 4 Network angle : θ n n 6 n> 6 Max. of the distribution Average angle

19 Si-O-Si angle θin d-sio 2 [B. Hehlen, J.Phys.: Cond Matter 2010] RMN (Devine et al. 1987) R-band Small rings : θ θ n= 3 n= 4 Network angle : θ n n 6 n> 6 Max. of the distribution Average angle Simulations [Rahmani et al. PRB,2003] [Matsubara, Ispas, Kob, 2009]

20 Si-O-Si angle in sodo-silicates Density of states of bending modes 0 x SiO 2 40%Na 2 O NS1.5 33%Na 2 O NS2 B(ω) g B Boson peak x 25%Na 2 O NS3 20%Na 2 O NS4 Bimodal angular population in sodo-silicates : -A narrow and peaked one at high frequency -A broad one at lower frequency

21 Si-O-Si angle in sodo-silicates Most probable angle (max. of the distribution) Angle ( ) SiO 2 NS4 NS3 experimental simulation NS2 NS1.5 [Truflandier, Ispas,Charpentier] [Ispaset al PRB 2001] Same trend!! % mol. Na 2 O Depolymerization reduces the Si-O-Si angle

22 bond angle in boro-silicate glasses Raman density of states after subtraction of the BP signal : BSN, BSNC 15% Na 2 O % Na 2 O % Na 2 O 15% CaO

23 Integrated intensity : Strong correlation with the concentration of SiO 2 in the glass SiO 2 densifiedsio 2 R-band NS BSN 15% Na 2 O BSN 30% Na 2 O BSNC 15% Na 2 O -15%CaO -The nature of the modes underlying the R-band (-O-bending) does not change with glass composition -B-O-B bending give a very weak Raman signal (not shown here) R-Band in borosilicates: -O-bending, mostly with adjacent Si atoms.

24 -O-bond angle vssio 2 mol% SiO 2 -O- bon nd angle θ 15% Iso-cation curves % alkali 30% NS BSN 15% Na 2 O BSN 30% Na 2 O BSNC 15% Na 2 O -15%CaO

25 -O-bond angle vsb 2 O 3 mol% θ -O-bond angle θ θ -O- bond angle variation θ

26 Distribution of Si-O-Si angles In sodo silicates - θ? - Na + Si θ O Si Close to cations NS2 NS4 Connected network From Raman scattering SiO 2 Inten P(θ Si-O-Si ) From Computer simulations (Truflandier, Ispas,Charpentier) (Ispas et al PRB 2001) Angle ( )

27 In boro-silicates Distribution of O bond angles 30% Na 2 O Depolymerized network 15% Na 2 O 15% CaO % cations Inhomogeneous angle distribution 15% Na 2 O Polymerized network 15% cations Homogeneous angle distribution Signature of an Inhomogeneous distribution of the cationsin the glass? [Greaves] [Mayer et al.prb 2001]

28 IV Signature of network modifier cations in the Raman spectra [B. Hehlen, D. Neuville] Alumino-silicate glasses Depolarized Raman spectra, VH : Bending modes are inactives, allowing a clear observation of the cationband near 350 cm -1 - AlO 4 - Na + or SiO 4??? Na + -

29 360 cm -1 Modifier/compensator state of cations (Raman VH) SiO 2 CAS xsio 2 : (1-y)CaO: yal 2 O 3 x=0.5 CaO Al 2 O cm -1 NAS xsio 2 : (1-y)Na 2 O: yal 2 O 3 x=0.67 SiO 2 Charge compensator region Na 2 O Al 2 O 3 NS xsio 2 : (1-x)Na 2 O SiO 2 Na : Network modifier Na 2 O Al 2 O 3

30 Raman intensity of the cation-band Assuming al Aluminum atoms in AlO4 - tetrahedra : xsio 2 :yna 2 O:zAl 2 O 3 1 Na + Compensates 1 AlO4 - Na N = 2y-2z Modif xsio 2 :ycao:zal 2 O 3 1 Ca 2+ Compensates 2 AlO4 - Ca N = y -z Modif I RS / 6.1 Intensity dependences collapse in one master curve Raman signal goes to 0 when all cations are charge compensators N Modif Raman-cationband near 340 cm -1 relates to modifiercations

31 Conclusions Hyper-Raman : Importance of librations of rigid SiO 4 tetrahedrain the boson peak of silica Spatial coherence of the vibrations TO4 : chain mode involving many rocking Si-O-Si units HRS BP: localized or quasi-localized modes Raman : Si-O-Si bond angle value and distribution in silica and sodo silicates -O- bond angle in borosilicates Role of cations(modifier/compensator) in aluminosilicateglasses

32 Thanks to : G. Simon (Ph.D L2C, present address : LADIR-Paris) O. Noguera(Post-Doc L2C, present address : SPCTS -Limoges) D. Neuville(IPGP) P. Richet (IPGP) E. Lecomte, O. Dargaud(Saint Gobain- Aubervillers) And Samples S. Clément, C. Dupas, and G. Prévot (L2C) Technical support Work supported by :