Dörte Stachel

Structural specifics of phosphate glasses probed by diffraction methods: a review

The present work illustrates the different contributions to the structure of phosphate glasses which are made by diffraction studies. The resolution in real space of the neutron diffraction experiments resolves two P–O distances. The lengths of the P–O bonds to the terminal and to the bridging oxygen atoms change as a function of the P2O5 content and of the species of the modifier cation. The model about the role of the properties of the modifier atoms, Me, in the structure of phosphate glasses predicts three different types of structural incorporation of these atoms. The experimental findings of a network change at ≈20 mol% modifier oxide content in range I are explained by a change of the interaction between the Me sites and the twofold-linked PO4 groups. For the intermediate cations a change of the Me–O coordination number, NMeO, is obtained which indicates a stabilization of Me–O–P bridges in range II. With increasing modifier content a situation commonly described as a modified random network ensues (range III) where clusters of MeOn polyhedra are formed. The corresponding consequences for the Me–Me distances from the relation between NMeO and the available number of terminal oxygen atoms per modifier cation are simulated by the reverse Monte Carlo method. This approach which makes use of the scattering information about the medium-range order is applied to the structures of binary metaphosphate glasses with Me=Zn, Ca, Sr, Ba, Na and K.

31P NMR investigations of binary alkaline earth phosphate glasses of ultra phosphate composition

31P NMR CW and FT MAS measurements have been used to investigate the structural composition of x(MeO)·(1−x)(P2O5) glasses (Me = Ca, Sr, Ba, x = 0.25−0.50). The influence of the structural composition on components of the chemical shift tensors is determined and discussed by means of a correlation between isotropic chemical shifts in crystalline phosphates and molecular structure published recently by Sternberg et al. [Z. Phys. Chem. Neue Folge 168 (1990) 115].

The dependence of structural peculiarities in binary phosphate glasses on their network modifier content

X-ray diffraction measurements were performed on four (MeO)x(P2O5)(1 - x) glass series, with Me = Zn, Mg, Ca, Ba, and x ranging from about 0.05 to 0.6. The parameters for the MeO coordination sphere are determined. Abrupt changes in the ionic volume fraction are found for the glasses, with different Me ions, at certain compositions which correlate with the behaviour of the MeO coordination sphere. These phenomena can be explained in terms of a model based on the chemical characteristics of P2O5 and on the interplay between the number of non-bridging oxygen atoms per Me cation and changes in the type of the MeOm polyhedron.

Intermediate range order in MeOP2O5 glasses

Binary phosphate glasses with concentrations ranging from 2.5 to 60.2 mol% MeO, MeZn, Mg, Ca, Ba, including vitreous P2O5 have been studied by X-ray diffraction to extract information on their intermediate range structure. The P2O5-like network structure is maintained in the low concentration range up to ∼ 20 mol% MeO. For these compositions, the characteristic length of 2π/Smax = 0.47 nm identified with the interlayer-distance in the P2O5 network is slightly shortened with MeO additions. The parameter 2π/Smax does not significantly change for concentrations exceeding ∼ 20 mol% MeO when the network structure is dominated by isolated MeOn polyhedra. The formation of more compact structures by MeOn polyhedra sharing corners, edges, and possibly faces leads to a shortening of the characteristic length. The latter process of structural formation represents a preference for producing an overstructure in the intermediate range which manifests itself as a prepeak in the total structure factors of the calcium and barium phosphate glasses. The Fourier transform analysis of the wide angle X-ray scattering of the BaOP2O5 glasses allows correlations to be investigated beyond the nearest-neighbor coordination shell. The main peak in the structure factors of the barium phosphate glasses appears dominantly to originate from the BaBa first-neighbor distance correlations.

The P-O bond lengths in vitreous probed by neutron diffraction with high real-space resolution

Neutron diffraction has been used to determine the two P-O bond lengths which occur in the threefold-linked tetrahedral structural units of vitreous . The large maximum momentum transfer which is available on the LAD instrument at the ISIS spallation neutron source gives high real-space resolution which enables a clear separation to be made between P-O bond lengths involving terminal oxygens and those involving bridging oxygens . The two P-O distances in vitreous were found to have narrower distributions than have previously been observed, using the same experimental conditions, for any other modified phosphate glass.

Infrared spectra and structure of various crystalline ultraphosphates and their glasses

The crystalline compounds o′-P2O5, MgP4O11, ZnP4O11, CaP4O11, Ca2P6O17 and β-Ca(PO3)2 were prepared and characterized by x-ray diffraction of single crystals. On the basis of the determined crystallographic data an interpretation of the infrared spectra of the crystalline ultraphosphates in the region of the stretching vibrations was given considering the short-range and intermediate-range order of their polymeric structures. Comparing the infrared spectra of the crystalline and the glassy compounds a structural similarity of the glass and the crystal in the intermediate-range order but at most up to the dimension of a unit cell is evident.

Effects of atmospheric humidity on the infrared reflectivity of vitreous P2O5 and ultraphosphate glasses

Nearly anhydrous vitreous P2O5 and ultraphosphate glasses with up to 15 mol% MeO (Me = Ca, Zn) were prepared. All glasses were melted in evacuated and sealed SiO2 glass tubes at 1050°C for 2 h and were cooled by switching off the oven. The infrared spectroscopic properties of the nearly anhydrous glasses are given and the decomposition of the glasses in atmospheric humidity was followed by time-resolved measurements of the infrared reflectance spectra. The resistance of the ultraphosphate glasses to humidity is correlated with the type of interaction between the cation and the ultraphosphate network.

A neutron and X-ray diffraction study of the structure of the LaP3O9 glass

Abstract A study of La metaphosphate glass using a combination of neutron and X-ray diffraction has yielded reliable information on the La–O coordination and the spatial distribution of the La 3+ cations. Due to the real-space resolution of the neutron diffraction experiment, it was possible to observe two different P–O bond lengths in the PO 4 tetrahedron. The La–O coordination number of about 7 exceeds the number, 6, of terminal oxygen atoms available for the coordination of each of the La 3+ cations. This leads to a tendency for clustering of the LaO n polyhedra, which is also observed in complementary reverse Monte Carlo results. The first feature visible in the X-ray structure factor at a scattering vector magnitude, Q , of 12 nm −1 , is related to a La–La peak in real space at about 640 pm. This corresponds to La–La neighbours which are separated by a PO 4 unit.

Combined neutron and X-ray scattering study of phosphate glasses

Abstract The advantage of high resolving power and of different contrast of total X-ray and neutron diffraction (XRD and ND) experiments is demonstrated in studies of the short- and medium-range order of some phosphate glasses. Details of two different P–O bonds are detected using Q-ranges up to 500 nm −1 . A different contrast is used to resolve Me–O distances in the range of the O–O peaks at 0.25 nm where Me is the modifier atom. In many cases the Me–O peaks are asymmetric. Reverse Monte Carlo simulation is applied to combine X-ray and neutron scattering data for extracting features of the medium-range order. Increasing Me–O distances changes the mutual order of the Me atoms in metaphosphate glasses with the tendency of a clustering of Me sites due to MeOn polyhedra sharing oxygen atoms. But if compared with the alternating order of Me and P sites the clustering of the Me sites is a minor effect. Me–Me first-neighbor distances are little affected if the fraction of modifier oxide is varied while the number of Me–Me neighbors is clearly changed.

Oriented fluororichterite/diopsid glass-ceramics prepared by electrochemically induced nucleation

Abstract Glasses in the system Na 2 O–MgO–CaO–SiO 2 –F − were crystallized using electrochemically induced nucleation. For that purpose, a platinum crucible containing the melt was used as anode while a platinum wire inserted into the melt acted as cathode. A dc-potential of 1 V supplied between the electrodes led to a current flow of 10 mA. This resulted in nucleation at the cathode and subsequently to radial growth of fluororichterite and diopsid crystals. Both crystalline phases were oriented with their crystallographic c -axis radially, i.e., perpendicular to the electrode surface. The structures formed are described and the mechanism leading to nucleation and subsequent crystal growth is discussed.