ROMA, September 2015 Giovanna Ruello Glass Systems Nanovoids Relaxations &

22 Void-Based Model: First Sharp Diffraction Peak in Alkaline Borate Glasses INTRODUCTION OUTLINES Voids & Densified Glasses OUTCOMES Glass Systems Nanovoids Relaxations & Voids & acoustic relaxations

GLASS  G <  C 3 Materials which solidify without crystallizing What are GLASSES? CRYSTAL  =  C Melting + fast cooling  Short Range Order  0 - 5Å;  Intermedium Range Order  5-20 Å  Long range order  >20 Å. Introduction Glass Systems Structural Voids: FSDP in Alkaline Borate Glasses Voids in densified glasses Voids and acoustic relaxations Outcomes

Borate matrix consists of BO 3 triangles Presence of larger complex boroxol rings B2O3B2O3 (M 2 O) x (B 2 O 3 ) 1-x Network modifiers M + are located inside structural voids BO 4 TETRAHEDRA  Same structural features in the short range order  Great differences in the medium range order I: The network composition is kept constant while the size of the modifier is changed Alkaline Borate Glasses: structure Two peaks, PK1 and PK2, in the low Q range of the S(Q) Introduction Glass Systems Structural Voids: FSDP in Alkaline Borate Glasses Voids in densified glasses Voids and acoustic relaxations Outcomes

II. The network composition is varied while the size of the modifier is kept constant Dependence of FSDP on alkaline concentration N PK1 increases as 2x Two larger cages per each molecule of alkaline oxide Compositional variation of the FSDP intensity is due to changes in the DISTRIBUTION OF CAGE SIZES within the 3D network. (Cs 2 O) x (B 2 O 3 ) 1-x Introduction Glass Systems Structural Voids: FSDP in Alkaline Borate Glasses Voids in densified glasses Voids and acoustic relaxations Outcomes

Void-based Model Diameter of voids Q PK1 of the pre-peak Q PK2 of the FSDP diameter of unfilled voids diameter of voids housing the alkali cations Introduction Glass Systems Structural Voids: FSDP in Alkaline Borate Glasses Voids in densified glasses Voids and acoustic relaxations Outcomes

Think to voids as spheres whose planar sections are circle Circumradius Empty symbols Full symbols Regular polygons having 2n vertices (n=coordination number of M) and side length s defined by the B-O distance. Regular polygons having 2n vertices (n=coordination number of M) and side length s defined by the B-O distance. Void-based Model s Introduction Glass Systems Structural Voids: FSDP in Alkaline Borate Glasses Voids in densified glasses Voids and acoustic relaxations Outcomes

Voids in densified tetrahedral glasses (I) Under compression The density ρ increases MOLAR VOLUME REDUCTION According to our model, in the normal glass where Number of tetrahedral structural units (of side l) in a mole of glass Tetrahedra volume Void radius in densified system Introduction Glass Systems Structural Voids: FSDP in Alkaline Borate Glasses Voids in densified glasses Voids and acoustic relaxations Outcomes

Voids in densified tetrahedral glasses (II) The densification causes the shrinkage of larger voids SiO 2 GeO 2 Lithium Silicates GeSe 2 LDA HDA EXPERIMENTAL vs THEORETICAL void radius ExperimentalTheoretical Increasing Pressure Introduction Glass Systems Structural Voids: FSDP in Alkaline Borate Glasses Voids in densified glasses Voids and acoustic relaxations Outcomes

Voids & structural relaxations (I) ACOUSTIC RELAXATION Bent B–O–B angles corrispond to two or more equivalent positions around the B–O–B straight line and small energy barriers separating the equivalent states. ACOUSTIC RELAXATION Bent B–O–B angles corrispond to two or more equivalent positions around the B–O–B straight line and small energy barriers separating the equivalent states. Ultrasound attenuation peak found at low temperature usually ascribed to the thermal activated relaxation of unknown locally mobile structural defects Anderson and Bommel, J. Am. Cerarn. Soc. 38,125 (1955) “Structural relaxation region" as a region delimited by the atoms around a void In alkaline borate glasses Number of the excess relaxing units Number of B-O-B bridges in the voids created by alkali ions Alkaline coordination number Pre-peak Area G. D’Angelo, to be published Introduction Glass Systems Structural Voids: FSDP in Alkaline Borate Glasses Voids in densified glasses Voids and acoustic relaxations Outcomes Since the short range structure is not affected by the nature of the metallic oxide, we associate the differences in the internal friction to changes in the medium range structure.

G. D’Angelo, to be published In densified B 2 O 3 Q FSDP increases Reduction of void diameter Reduction of void diameter In densified B 2 O 3 Q -1 decreases Reduction of relaxing defects Reduction of relaxing defects Voids & structural relaxations (II) Densified glasses Introduction Glass Systems Structural Voids: FSDP in Alkaline Borate Glasses Voids in densified glasses Voids and acoustic relaxations Outcomes

12 Outcomes Q PK2 of the FSDP gives information about the diameter of unfilled voids. Q PK1 of the pre-peak gives information about the diameter of voids housing the alkali cations NEW VOID-BASED MODEL: FSDP position gives information about the diameter of structural voids; FSDP intensity about the number of these voids. This model explain the increased Q FSDP and decreased I FSDP in densified tetrahedral glasses. PUCKERED RINGS are the SOURCE FOR RELAXING DEFECTS and thus for acoustic attenuation in glasses. Introduction Glass Systems Structural Voids: FSDP in Alkaline Borate Glasses Voids in densified glasses Voids and acoustic relaxations Outcomes

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Notes on the FSDP The FSDP IS NOT the expression of correlations between cation centered cluster and voids in the network structure ….But is due to the B-B, O-O and B-O partial static structural factors In Borate glasses D’Angelo et al., J. Non-Cryst. Solids 354, (2008)

B2O3B2O3 SiO 2 GeO 2 Broadened envelopes of the corresponding peaks of crystalline phase Notes on the FSDP The FSDP width IS NOT a correlation length Kohara &Suzuya, J. Phys.: Condens. Matter 17 (2005) S77-S86 Whereas the crystal exhibits voids or cavities of a specific size, causing the appearance of distinct peaks, the glass is characterized by a distribution of void sizes. D’Angelo et al., J. Phys. Chem. B 2010, 114, 12565–12571 ….but is due to a ring size distribution

FSDP In covalent glasses the FSDP implies the presence of intermediate range order due to the cages formed by the topological connection of tructural units in the network. [S. R. Elliott PRL 1991; J. H. Lee and S. R. Elliott, PRB 1994] The most reliable interpretation is the Elliott’s void-based model A pre-peak in the concentration-concentration structure factor due to the chemical short range ordering of interstitial voids around cation- centered clusters in the structure Borjesson

Rings in Glycerol Q=1.4 Å -1 D.C. Champeney et al., Mol. Phys. 58 (1986) 337. Glycerol Trimer Uchino et al., Science 273, (1996) 6-membered ring l= 2,77 Å D th =4.78 Å Q=1,31 Å -1 EXAGON …NOT ONLY GLASSES BUT ALSO… D Liu et al., Journal of Physics and Chemistry of Solids 66, (2005) Q=1,87 Å -1 D exp =3,36 Å Voids in DNA Separation between phosphate groups along the periphery of the double helix

Size of voids and network expansion