R. Kranold

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.

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 change of the Ge–O coordination number in potassium germanate glasses probed by neutron diffraction with high real-space resolution

Abstract Neutron diffraction experiments of high real-space resolution are used for studying the Ge–O coordination numbers and the Ge–O bond lengths in two K 2 O–GeO 2 glasses (11 and 20 mol% K 2 O). On the long-distance side of the first-neighbour Ge-O peak lying at 175 pm, which is likewise found for vitreous GeO 2 , appears a clear shoulder. This feature indicates the existence of GeO n units larger than GeO 4 tetrahedra. The shoulder is not resolved as a separate peak which could be attributed to Ge–O bonds of GeO 6 octahedra unambiguously. Hence, it cannot be differentiated between GeO 5 or GeO 6 polyhedra being formed in addition to the GeO 4 tetrahedra. The increase of the Ge–O coordination number complies with the concept of a lack of non-bridging oxygen atoms in alkali germanate glasses up to concentrations of about 20 mol% K 2 O where the additional oxygen atoms are incorporated in Ge–O–Ge bridges.

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.

The structure of potassium germanate glasses – a combined X-ray and neutron scattering study

Abstract X-ray scattering experiments on potassium germanate glasses of 11 and 20 mol% K 2 O confirm the change of the Ge–O coordination state which was obtained by earlier neutron scattering studies. The O–O and Ge–Ge coordination numbers are extracted by help of the different contrast of the X-ray and neutron data. Their behavior agrees with the model assumption of the increase of N GeO and the absence of non-bridging oxygen atoms up to about 20 mol% K 2 O concentration. Reverse Monte Carlo (RMC) simulations are used to analyze the changes in the medium-range order (MRO). The modified glasses are assumed to be quasi-binary systems formed of K atoms and GeO n units where the latter are represented by their central Ge positions. According to the Bhatia/Thornton formalism the first diffraction peak is identified as a chemical order prepeak which indicates the preference of K–Ge neighbors in the packing of K and Ge sites. The Ge–Ge pair distributions show additional features which are explained by the needs of the network linkages and by steric effects of the K + cations in the environments of the GeO 4 tetrahedra.

Investigation of the primary crystallization of Ni-17 at. % P alloy by ASAXS

Primary crystallization of Ni(P) particles in hypoeutectic Ni-P amorphous alloy obtained by electroless deposition has been investigated with ASAXS. The particle size distribution, the size dependence of the particle composition and the amorphous matrix composition were found simultaneously. The size distribution consists of a peak at particle radius of ∼1 nm and a tail spanning from ∼2 to 15 nm. The composition of the particles of the peak changes from ∼14 to ∼2 at.% P as their radius grows from 0.7 to about 3 nm. The particles in the tail of the size distribution (2-15 nm) have nearly constant P content in the range of 0-2 at.%. The matrix composition tends to the eutectic one at the end of the primary crystallization process.

The non-magnetic surface of magnetic particles in nanostructured glass ceramics studied by SANS

Abstract Nanosized crystallites of ferrimagnetic magnetite, Fe 3 O 4 , were prepared by heat treatment of a rapidly quenched silicate glass melt of composition 27.4CaO–42.8SiO 2 –6.1B 2 O 3 –23.7Fe 2 O 3 (mol%). SANS experiments of samples annealed at different heat treatment conditions were performed to determine the evolution and nature of the non-magnetic surface layer of the nanocrystals. It is suggested that this layer is formed by the distorted crystalline structure and thermally induced effects at the surface region and by the formation of a segregation shell enriched with Fe 2 O 3 and subsequent surface oxidation of the Fe 3 O 4 nanocrystals as well.

Variation in P-O Bonding in Phosphate Glasses -A Neutron Diffraction Study

Two different lengths of P-O bonds in the PO4 units of phosphate glasses are found by neutron diffraction experiments of high resolution in real space. The two lengths are related to bonds of the phosphorus atom with the terminal and the bridging oxygen atoms. The mean lengths and widths of both P-0 distance peaks change as a function of the glass composition. In a large range, starting from vitreous P2O5 s up to the pyrophosphate composition, the behavior of the bond lengths is compared with that in the related crystals and with that resulting from ab initio calculations. The bond lengths depend not only on the species of the participating oxygen atoms and on the number of links of the concerning PO4 unit but also on the number of links of the neighboring PO4 unit and on the species of the modifier cation