RICERCA ITALIANA     

Research program

Vibrational dynamics and relaxation in densified glasses and confined disordered systems

University Co-ordinator

Università degli Studi di MESSINA - FISICA - ()

Research Unit Leader

Giuseppe Carini

Description

The aim of the present research program is to obtain an insight on the microscopic origin of the anomalies in the acoustic, thermal and vibrational properties of amorphous solids, related to the local molecular mobility. This will be performed by an experimental study having the objective to analyse the influence of increasing permanent densification of a glass on these dynamical behaviours. The following archetypes of oxide glasses will be used as model systems: (i) GeO2 (network forming ion (NFI)=Ge, having coordination number equal to 4), whose network consists of tetrahedral units GeO4, all the oxygen bridging between two germanium atoms and (ii) B2O3 (NFI=B, having coordination number equal to 3), whose network consists mainly of boroxol rings, a 6 membered planar group B3O3(O-)3 where each oxygen bridges between two boron atoms. Anhydrous powders of (GeO2) and (B2O3) having a laboratory reagent 99.99 % purity grades will be used in order to prepare glasses with a negligible OH content (few ppm). This expedient is necessary because both germanium and boron oxides are quite hygroscopic and the presence of OH groups in the network of these glasses causes significant variations of their physical properties. After casting, each glass will be annealed and stabilized at about 20-30 K above its calorimetric Tg in a high purity nitrogen atmosphere in order to avoid undesired effects arising from different thermal histories and then cooled and stored at room temperature. The gradual increase of density (densification) will be obtained by loading the melt-quenched glasses in a multi-anvil apparatus, at room temperature or at temperatures lower than Tg, for the synthesis at pressures ranging between 1 GPa and 10 GPa.
Recent studies on the relaxation dynamics of alkali and silver borate glasses, where controlled structural variations have been obtained by changing the composition [2,3], revealed that the spectral density of tunneling or two level systems (TLS) is largely independent of the morphology of an amorphous solid, confirming its universal nature as intrinsic to the glassy state. Quite differently, thermally activated relaxing centres depend markedly on the connectivity (defined as the number of bridging oxygens (BO) per NFI; a BO is an oxygen bridging between two NFIs) and the bond strength characterizing the glassy network. The totality of observations in borate glasses lead to associate the defect modes with some kind of local motion of BØ3 groups within the borate network, also revealing that only a small fraction of relaxing particles are involved in tunnelling local motions below 10 K.
Differently from borate glasses, permanently densified glasses permit to increase the atomic packing and the connectivity of the network without changing the stoichiometry. Increasing density of glassy GeO2 leads to a growing packing of GeO4 tetrahedra and of oxygen ions building up the cages of the network, that is the system acquires a structure having a more efficient packing to fill in the lesser volume available to it [16]. In glassy B2O3, increasing densification obtained by loading the system with increasing pressures between 1 and 6 GPa results in the transformation of a fraction of triangular units BO3 in BO4 tetrahedra, whose concentration increases with increasing density [17]. It is believed that these structural changes should cause a significant reduction of the local atomic mobility and, consequently, of the anomalies characterizing the vibrational and relaxation dynamics. This should open new ways about the investigation of the microscopic origin of the thermally activated relaxation processes (T>10K) and of tunnelling effects (T<10K), also disclosing the possibility to answer to two fundamental open questions: (i) the nature of the mobile “particle” and (ii) if the same "particle" can be considered as being responsible for the two different kinds of local motion.
Detailed structural information concerning the pressure induced variations of the local and medium range order of such systems will be obtained by neutron and x-ray diffraction measurements and by NMR-MAS (magic angle spinning) spectroscopy of 11B element. The study of the vibrational and relaxation dynamics will be performed by neutron scattering (at ILL of Grenoble), Raman spectroscopy, mechanical and ultrasonic spectroscopy, dilatometry, DSC and low temperature calorimetry.

Improvements and expected results of the research program.

(i) Preparation of permanently densified B2O3 and GeO2 glasses (first year of the project).
The two glasses investigated in this research work, amorphous B2O3 and GeO2, are both highly hygroscopic. The samples will be prepared by procedures useful to remove or to minimize water content: they will be melted at about 1200 °C and bubbled with dry nitrogen. The high temperature is expected to favor bubbling and to affect only the water content. Moreover each cast specimen will be carefully annealed in order to have well stabilized glasses; this will ensure that (i) all the thermal history of the cast glass will be erased and (ii) all the residual stress in the cast glass will be removed. The annealed glasses will be densified by loading in a multianvil apparatus for synthesis at pressures ranging between 1 GPa and 10 GPa. Since it is expected that the pressure response of cold-pressed glasses is slow, an equilibration time of several hours under pressure will be necessary.

(ii) Improvement of the Low Temperature Physics Laboratory (first year of the project).
The dilution refrigerator purchased by funds of PRIN 2007 will be used to extend down to about 8 mK the temperature interval (0.3-300 K), at present explored by calorimetry, thermal conductivity and ultrasonic techniques. This extension will permit a more accurate investigation of the anomalies in the physical properties due to TLSs.

(iii) Papers on referred international Journals and oral presentations at International Conferences and Workshops (first and second year of the project).