Spectral editing based on scalar spin-spin interactions: new results on the structure of metathiophosphate glasses

The local structure of glassy 'NA'P'S IND.3' and 'AG'P'S IND.3' was analyzed based on quantitative 'ANTPOT.31 P' MAS-NMR spectroscopy. The glasses contain some oxide impurities, which could be quantified from the NMR spectral analysis. Four discrete resonances are observed in both glasses, which were assigned to four distinct types of phosphate groups 'P POT.(n)', where n is the number of P-S-P bridges(i.e., 'P POT.(0)', 'P POT.(1)', 'P POT.(2)', and 'P POT.(3)' units, respectively) with the help of 2D homonuclear J-resolved and INADEQUATE methods. Based on the results obtained, the interpretations of previous spectra obtained at low spinning speeds on lithium and silver thiophosphate glasses (Chem. Mater. 2 (1990), 273, and J. Am. Chem. Soc. 114 (1992), 5775) need to be revised. Contrary to the situation in alkali phosphate glasses, the corresponding sulfide analogs are characterized by a wide 'P POT.(n)' species distribution close to that predicted by a statistical charge distribution. INADEQUATE experiments fail to detect 'P POT.(n)' 'P - POT.(n-1)' connectivities, suggesting that the structure of these glasses is rather inhomogeneous, possibly featuring the different 'P POT.(n)' species in segregated domains

NMR and conductivity studies of the mixed glass former effect in lithium borophosphate glasses

Alkali ion charge transport has been studied in a series of mixed glass former lithium borophosphate glasses of composition 0.33Li2O + 0.67[xB2O3 + (1 – x)P2O5]. The entire concentration range, 0.0 ≤ x ≤ 1.0, from pure glassy Li2P4O11 to pure glassy Li2B4O7 has been examined while keeping the molar fraction of Li2O constant. Electrical conductivity measurements and nuclear magnetic resonance techniques such as spin relaxometry, line shape analysis, and stimulated-echo spectroscopy were used to examine the temperature and frequency dependence of the Li+ ion motion over wide ranges of time scale and temperature. By accurately determining motional time scales and activation energies over the entire composition range the ion dynamics and the charge transport are found to be fastest if the borate and the phosphate fractions are similar. The nonlinear variation of the charge conduction, the most notable feature of the mixed glass former effect, is discussed in terms of the composition dependence of network former units which determine the local glass structure

Mixed network former effect in ion-conducting Alkali borophosphate glasses: structure/property correlations in the system '['M IND. 2'O] IND. 1/3''['('B IND. 2''O IND. 3') IND. x''('P IND. 2''O IND. 5') IND. 1-x'] IND. 2/3' (M = 'LI', K, 'CS')

Glasses in the system '['M IND. 2'O] IND. 1/3''['('B IND. 2''O IND. 3') IND. x''('P IND. 2''O IND. 5') IND. 1-x'] IND. 2/3' (M = 'LI', K, 'CS') (0.0 'MENOR OU IGUAL' x 'MENOR OU IGUAL' 1.0) were prepared by standard melt-quenching procedures, and their physical properties were characterized by thermal analysis, density measurements, and impedance spectroscopy. Their atomic level structures were comprehensively characterized by Raman spectroscopy, by X-ray photoelectron spectroscopy (XPS), and by ' ANTPOT. 11 B', 'ANTPOT. 31 P', and 'ANTPOT. 7 LI' as well as 'ANTPOT. 133 CS' high resolution solid state magic-angle-spinning (MAS) NMR techniques. 'ANTPOT. 31 P' MAS NMR peak assignments were aided by the presence or absence of homonuclear indirect 'ANTPOT. 31 P'-'ANTPOT. 31 P' spin–spin interactions, "J-coupling", as detected via refocused INADEQUATE techniques. Consistent speciations of the phosphate and borate network former components in terms of the various 'P POT. n IND. mB' and 'B POT.n IND. mP' units, where n is the number of bridging oxygens (BOs) and m is the number of B or P units bonded to P or B, respectively, present in these glasses were derived from 'ANTPOT. 11 B' MAS NMR, combined with both 'ANTPOT. 31 P' MAS NMR and XPS line shape analyses, constrained by charge and mass balance considerations. The speciation of the BO species in the glassy network was quantified both by O 1s XPS and 'ANTPOT. 11 B'{'ANTPOT. 31 P'} rotational echo double resonance spectroscopy. Both experiments indicate a strong preference of heteroatomic B-O-P over homoatomic P-O-P and B-O-B linkages to the extent that close to the maximum number of possible 'B POT. 4'-O-P linkages is formed. Further, the structural speciations of the borate and phosphate species, together with bond valence (BV) analyses of the charge redistribution on the various structural units, indicate that the alkali network modifier oxide is not proportionally shared between the two network former components B and P in these systems. Rather, the amounts and types of the various borate and phosphate species are found to be consistent with the negative charge brought in by the alkali modifier 'M IND. 2'O being distributed more toward the phosphate structural units which are suggested to attract a larger concentration of network modifier species than predicted by the bulk composition. The experimental results obtained from these studies help in understanding the strongly nonlinear compositional dependence of the glass transition temperature and the ionic conductivity in terms of detailed atomic-level structural information. The emerging structural principles appear to be general to all of the alkali borophosphate glass systems, with the type of alkali ion network modifier producing only minor variations.Deutsche Forschungsgemeinschaft (SFB 458; Ionic Motion in Disordered Materials)National Science Foundation (DMR, Materials World Network NSFDMR 0701564

RR Lyrae mode switching in globular cluster M 68 (NGC 4590)

D.M.B. acknowledges NPRP grant # X-019-1-006 from the Qatar National Research Fund (a member of Qatar Foundation). A.A.F. acknowledges the support of DGAPA-UNAM through project IN106615-17.We build on our detailed analysis of time-series observations of the globular cluster M 68 to investigate the irregular pulsational behaviour of four of the RR Lyrae stars in this cluster. M 68 is one of only two globular clusters in which mode switching of RR Lyrae stars has previously been reported. We discuss one additional case, as well as a case of irregular behaviour, and we briefly revisit the two previously reported cases with a homogeneous analysis. We find that in 2013, V45 was pulsating in the first-overtone mode alone, despite being previously reported as a double-mode (fundamental and first overtone) pulsator in 1994, and that the amplitude of the fundamental mode in V7 is increasing with time. We also suggest that V21 might not have switched pulsation modes as previously reported, although the first overtone seems to be becoming less dominant. Finally, our analysis of available archival data confirms that V33 lost a pulsation mode between 1950 and 1986.Publisher PDFPeer reviewe

Long-term photometric monitoring of RR Lyr stars in M3

The period-change behaviour of 134 RR Lyrae stars in the globular cluster Messier 3 (M3) is investigated on the ~120-year time base of the photometric observations. The mean period-change rates (\beta \approx 0.01 d Myr^-1) of the subsamples of variables exhibiting the most regular behaviour are in good agreement with theoretical expectations based on Horizontal-Branch stellar evolution models. However, a large fraction of variables show period changes that contradict the evolutionary expectations. Among the 134 stars studied, the period-change behaviour of only 54 variables is regular (constant or linearly changing), slight irregularities are superimposed on the regular variations in 23 cases and the remaining 57 stars display irregular period variations. The light curve of ~50 per cent of the RRab stars is not stable, i.e., these variables exhibit Blazhko modulation. The large fraction of variables with peculiar behaviour (showing light-curve modulation and/or irregular O-C variation) indicate that, probably, variables with regular period changes incompatible with their evolutionary stages also could display some kind of instability of the pulsation light curve and/or period, but the available observations have not disclosed it yet. The temporal appearence of the Blazhko effect in some stars, and the 70-90 years long regular changes preceded or followed by irregular, rapid changes of the pulsation period in some cases support this hypothesis. [...] Abstract truncated due to the limitations of astroph. See full abstract in the paper.Comment: 22 pages, 14 figures, accepted for publication in MNRA

Effect of dissolution rate and subsequent ion release on cytocompatibility properties of borophosphate glasses

Present work explores the relationship between the composition, dissolution rate, ion release and cytocompatibility of a series of borophosphate glasses. While, the base glass was selected to be 40mol%P2O5-16mol%CaO-24mol%MgO-20mol%Na2O, three B2O3 modified glass compositions were formulated by replacing Na2O with 1, 5 and 10 mol% B2O3. Ion release study was conducted using inductively coupled plasma atomic emission spectroscopy (ICP-AES). The thermal scans of the glasses as determined by differential scanning calorimetry (DSC) revealed an increment in the thermal properties with increasing B2O3 content in the glasses. On the other hand, the dissolution rate of the glasses decreased with increasing B2O3 content. To identify the effect of boron ion release on the cytocompatibility properties of the glasses, MG63 cells were cultured on the surface of the glass discs. The in vitro cell culture study suggested that glasses with 5 mol% B2O3 (P40B5) showed better cell proliferation and metabolic activity as compares to the glasses with 10 mol% (P40B10) or with no B2O3 (P40B0). The confocal laser scanning microscopy (CLSM) images of live/dead stained MG63 cells attached to the surface of the glasses also revealed that the number of dead cells attached to P40B5 glasses were significantly lower than both P40B0 and P40B10 glasses

Structural, thermal, in vitro degradation and cytocompatibility properties of P2O5-B2O3-CaO-MgO-Na2O-Fe2O3 glasses

Borophosphate glasses with compositions of (48 − x)P2O5-(12 + x)B2O3-14CaO-20MgO-1Na2O-5Fe2O3 (where x = 0, 3, 8 mol%) were prepared via a melt-quenching process. The effects of replacing P2O5 with B2O3 on the structural, thermal, degradation properties and cytocompatibility were investigated. Fourier transform infrared (FTIR) spectroscopy analysis confirmed the existence of BO3 triangular units and BO4 tetrahedral units within all the glasses with an increase of B/P ratio from 0.25 to 0.5. The BO4 units within the glass structure were observed to cause an increase in density (ρ) as well as glass transition (Tg) temperature and to decrease the crystallisation temperature (Tc). A decrease in thermal stability which indicated by process window was also observed in the case of substitution of P2O5 with B2O3. Degradation analysis of the glasses indicated that the dissolution rate increased with the addition of B2O3. The decrease in the thermal stability and chemical durability were attributed to the increase of BO3 units, which could increase crystallisation tendency and be easily hydrolysed by solution. The effect of boron addition on the cytocompatibility of the glasses was analysed using Alamar Blue and alkaline phosphatase (ALP) assays and DNA quantification. MG63 osteosarcoma cells cultured in direct contact with the glass samples surface for 14 days showed better cytocompatibility, compared to the tissue culture plastic (TCP) control group. In summary, the glass formulation with 12 mol% B2O3 presented the best cytocompatibility and thermal stability, thus could be considered for continuous fibre fabrication in future research and downstream activities

Structure and Ionic Conductivity in the Mixed-Network Former Chalcogenide Glass System [Na2S]2/3[(B2S3)x(P2S5)1–x]1/3

Glasses in the system [Na2S]2/3[(B2S3)x(P2S5)1–x]1/3 (0.0 ≤ x ≤ 1.0) were prepared by the melt quenching technique, and their properties were characterized by thermal analysis and impedance spectroscopy. Their atomic-level structures were comprehensively characterized by Raman spectroscopy and 11B, 31P, and 23Na high resolution solid state magic-angle spinning (MAS) NMR techniques. 31P MAS NMR peak assignments were made by the presence or absence of homonuclear indirect 31P–31P spin–spin interactions as detected using homonuclear J-resolved and refocused INADEQUATE techniques. The extent of B–S–P connectivity in the glassy network was quantified by 31P{11B} and 11B{31P} rotational echo double resonance spectroscopy. The results clearly illustrate that the network modifier alkali sulfide, Na2S, is not proportionally shared between the two network former components, B and P. Rather, the thiophosphate (P) component tends to attract a larger concentration of network modifier species than predicted by the bulk composition, and this results in the conversion of P2S74–, pyrothiophosphate, Na/P = 2:1, units into PS43–, orthothiophosphate, Na/P = 3:1, groups. Charge balance is maintained by increasing the net degree of polymerization of the thioborate (B) units through the formation of covalent bridging sulfur (BS) units, B–S–B. Detailed inspection of the 11B MAS NMR spectra reveals that multiple thioborate units are formed, ranging from neutral BS3/2 groups all the way to the fully depolymerized orthothioborate (BS33–) species. On the basis of these results, a comprehensive and quantitative structural model is developed for these glasses, on the basis of which the compositional trends in the glass transition temperatures (Tg) and ionic conductivities can be rationalized. Up to x = 0.4, the dominant process can be described in a simplified way by the net reaction equation P1 + B1 P0 + B4, where the superscripts denote the number of BS atoms for the respective network former species. Above x = 0.4, all of the thiophosphate units are of the P0 type and both pyro- (B1) and orthothioborate (B0) species make increasing contributions to the network structure with increasing x. In sharp contrast to the situation in sodium borophosphate glasses, four-coordinated thioborate species are generally less abundant and heteroatomic B–S–P linkages appear to not exist. On the basis of this structural information, compositional trends in the ionic conductivities are discussed in relation to the nature of the charge-compensating anionic species and the spatial distribution of the charge carriers