Alkali Metaphosphate Glasses Research Articles

Structural features of (Li,Na,K)PO3 mixed alkali metaphosphate glass for significant anisotropy

AbstractThe mixed alkali metaphosphate glass of 16.7Li2O‐16.7Na2O‐16.7K2O‐50P2O5 (in mol%, (Li,Na,K)PO3) showed large birefringence (significant anisotropy) due to highly oriented ‐P‐O‐P‐ chains when the glass was deformed under uniaxial tensile stress above glass transition temperature (Tg). To investigate structural features causing the significant anisotropy, 31P magic‐angle spinning nuclear magnetic resonance (MAS NMR), Raman, and Fourier‐transform infrared spectra of alkali metaphosphate glasses were analyzed. In the (Li,Na,K)PO3 and the 50Na2O‐50P2O5 (NaPO3) glasses, which have comparable force constant between alkali cation and oxygen (FM‐O), the number of Q2 units in ‐P‐O‐P‐ chains was equivalent. On the other hand, various types of alkali cations in the (Li,Na,K)PO3 glass result in larger distribution of FM‐O compared to the NaPO3 glass. Therefore, formation of the significant anisotropy for the (Li,Na,K)PO3 glass results from orientation of local structures with small FM‐O during suppression of shear flow between chains by local structures with large FM‐O.

The glass-forming ability explained from local structural differences by NMR between glasses and crystals in alkali metaphosphates

The glass forming ability of inorganic substances has traditionally been evaluated in kinetic terms related to the rate of spontaneous crystallization of their melts, which is related to viscosity from the macroscopic point of view. However, differences or similarities between the local structures of glasses and crystals of the same composition could have an additional influence on the kinetic ability to form a stable glass or a crystal from a melt. In this work, the local structure of alkali metaphosphate glasses and crystals has been analyzed by means of 31P MAS-NMR to demonstrate that the ability to form a glass is directly related to the differences between the atomic organization in the glassy states and the structure of the stable crystals at lower temperatures. This implies that chemical compositions having a similar atomic order in their glass and crystal structures will not easily develop the disordered long-range structure of glass.

Ionic conductivity and mixed-ion effect in mixed alkali metaphosphate glasses.

In this work, mixed alkali metaphosphate glasses based on K-Na, Rb-Na, Rb-Li, Cs-Na and Cs-Li combinations were studied by differential scanning calorimetry (DSC), complex impedance spectroscopy, and Raman spectroscopy. DSC analyses show that both the glass transition (Tg) and melting temperatures (Tm) exhibit a clear mixed-ion effect. The ionic conductivity shows a strong mixed-ion effect and decreases by more than six orders of magnitude at room temperature for Rb-Na or Cs-Li alkali pairs. This study confirms that the mixed-ion effect may be explained as a natural consequence of random ion mixing because ion transport is favoured between well-matched energy sites and is impeded due to the structural mismatch between neighbouring sites for dissimilar ions.

Modifier Interaction and Mixed-Alkali Effect in Bond Constraint Theory Applied to Ternary Alkali Metaphosphate Glasses

Introducing an interaction parameter γ, we implement modifier interaction and the mixed-alkali effect into bond constraint theory, and apply this extension for simplistic property prediction on ternary phosphate glasses. The severity of the mixed alkali effect results from the interplay of two simultaneous contributions: Bond constraints on the modifier species soften or stiffen with decreasing or increasing γ, respectively. When the modifier size is not too dissimilar the decrease in γ reflects that the alkali ions can easily migrate between different sites, forcing the network to continuously re-accommodate for any subsequent distortions. With increasing size difference, migration becomes increasingly difficult without considerable network deformation. This holds even for smaller ions, where the sluggish dynamics of the larger constituent result in blocking of the fast ion movement, leading to the subsequent increase in γ. Beyond a certain size difference in the modifier pair, a value of γ exceeding unity may indicate the presence of steric hindrance due to the large surrounding modifiers impeding the phosphate network to re-accommodate deformation.

Dynamic light scattering in mixed alkali metaphosphate glass forming liquids

We report the first ever photon correlation spectroscopy performed on single alkali and mixed alkali metaphosphate glasses at refractory temperatures above the glass transition. We find not only a significant decrease in the glass transition temperature but also a decrease in fragility for the mixed alkali composition as compared with the single akali glasses. We argue that structural relaxation in these polymeric oxide glasses is largely controlled by the cross linking cations and that the changes in fragility that we observed are a reflection of changes in the cooperativity of structural relaxation wrought by the substantial decrease in the ion mobility that accompanies the mixing of alkali ions.

Analysis of dispersed frequency response for ionic glasses: influence of electrode and nearlyconstant loss effects

Analysis by D L Sidebottom of the dispersive frequency response of the real-part of the conductivity,σ′(ω), for many alkali phosphate and metaphosphate glasses, using a fitting modelinvolving a ‘universal dynamic response’ power law with an exponentn and a constant-lossterm, led to anomalous n behaviour that he explained as arising from variable constriction of the local cation conductionspace. In order to obtain adequate fits, he eliminated from the data all low-frequency decreases ofσ′(ω) below the dc plateau, ones actually associated with electrode effects. Such a cut-off doesnot, however, eliminate electrode effects possibly present in the high-frequency part of thedata range. The results of the present detailed analysis and fitting of both synthetic dataand several of his experimental data sets show unequivocally that his anomalousn behaviour arose from neglecting electrode effects. Their inclusion, with or without datacut-off in the fitting model, leads to the expected high-frequency slope value ofn = 2/3 associated with bulk conduction, as required by recently published topologicaleffective-dimension considerations for dielectric relaxation in conductive systems.Further, the effects of the inclusion in a full fitting model of series and possiblyparallel complex constant-phase-element contributions, representing electrode andnearly constant loss effects, respectively, have been investigated in detail. Suchcomposite models usually lead to best fitting of either the full or cut-off complex datawhen they include the semi-universal, topologically based K1 bulk model, oneindirectly derived from the assumption of stretched-exponential temporal behaviour.

Constriction effect in the nearly constant loss of alkali metaphosphate glasses

We report measurements of the dielectric loss in a series of monovalent metaphosphate glasses at cryogenic temperatures. An analysis of the scaling properties of the dielectric response reveals a nonionic contribution to the loss present only in glasses for which the conduction cation is constricted by the oxide network. As a result, our effort to examine recent conflicting reports for the cation mass dependence of the nearly constant loss in ion-conducting, disordered materials has been obfuscated. Instead, it is shown how this nonionic contribution may explain additional conflicting observations for the nearly constant loss reported over the last 10 years.

Medium-range order in alkali metaphosphate glasses and melts investigated by reverse Monte Carlo simulations and diffraction analysis

Reverse Monte Carlo simulations have been performed on the alkali metaphosphate glasses Na0.5Li0.5PO3 and LiPO3 concerning structural experimental data obtained by neutron and x-ray diffraction at 300 K for both systems and versus temperature up to the melting point for the mixed composition. It appears that the contrast effect due to the negative scattering length of Li is not the only reason for the difference in the intensity of the prepeak observed in both systems. The main structural difference lies in the intermediate-range order, while the short-range order is quite similar in both systems. Moreover, it is shown that the intensity increase of the prepeak in the Na0.5Li0.5PO3 structure factor is due to the partial structure factors of the PO4 tetrahedron, sustaining the hypothesis of an ordering between several PO4 tetrahedra and voids with temperature.

Nitridation of alkali metaphosphate glasses: a comparative structural analysis of the Na–P–O–N and Li–Na–P–O–N systems

Phosphates, as aluminosilicates, are a major family of glasses characterized by XO 4 tetrahedral units, in which nitrogen can substitute for oxygen, thus forming oxynitride glasses. As shown by X-ray photoelectron spectroscopy (XPS), nitrogen N 3− may exist in P(O,N) 4 tetrahedra as doubly coordinated (N) and triply coordinated (N<) atoms, respectively, bonded to two and three phosphorus atoms. They replace non-bridging (OP) as well as bridging (P–O–P) oxygen atoms, thus inducing a more covalent and cross-linked structure, and a correlation is observed between the O 1s and N 1s spectra. 31P magic angle spinning nuclear magnetic resonance (MAS NMR) results show that three kinds of tetrahedra, PO 4, PO 3N and PO 2N 2, coexist within the glass network. The thermal nitridation in flowing ammonia of alkali metaphosphate glasses is particularly studied here and a comparative XPS and NMR structural analysis of `NaPON' and `LiNaPON' phosphorus oxynitride glass series of increasing nitrogen content is presented.

First diffraction peak in alkali metaphosphate glasses

The structure factor of the alkali metaphosphate glasses LiPO 3 and Na 0.5Li 0.5PO 3 has been measured at 300 K for both systems and up to 800 K for the latter. The first diffraction peak of alkali metaphosphate glasses merges at lower Q (around 1.1 Å −1) than in other covalent glasses, with the highest intensity for Li-containing glasses. These points are discussed following the hypothesis of voids ordering in the structure and contrasts effects due to the negative neutron Li scattering length. The intensity of the prepeak is significantly growing with temperature, in case of Na 0.5Li 0.5PO 3, in contrast to the other peaks of the structure factor. This point is discussed in relation with literature data from flow birefringence indicating structural changes in these glassy systems.

Dynamics of mixed alkali metaphosphate glasses and liquids

Dynamic mechanical, stress relaxation, and oscillatory shear experiments were performed to examine relaxation processes in a series of mixed alkali metaphosphate glasses and liquids. At temperatures less than the glass transition temperature, T g, dynamic processes are dominated by hopping of alkali ions and local network relaxations that accommodate the motion of the cations. The average relaxation times associated with these dynamic processes were sensitive to the molar ratio of alkali ions in the glass. For T>T g , viscous relaxations were controlled primarily by the translational dynamics of P–O–P chains. These longer time viscous relaxation processes also appear to be affected by the dynamics of the alkali ions. The high frequency elastic modulus, G 0, the fragility of the liquid, the heat capacity changes at T g, Δ c p( T g), and the breadth of the distribution of relaxation times, W, were all dependent on the molar ratio of the alkali oxides. Correlations between the fragility, W and Δ c p( T g), which were found to describe the effects of single alkali oxide modified glasses, failed to describe related properties in these mixed alkali metaphosphates.

Brillouin scattering in alkali metaphosphate glasses and melts

We report the results of Brillouin scattering measurements performed on a series of alkali metaphosphate glasses and melts including the mixed alkali composition. Using independent viscosity data, we have applied a time–temperature superposition approach to obtain from the relevant features of the Brillouin spectra the relaxation characteristics of the complex longitudinal elastic modulus. A mixed alkali effect is evidenced in the Brillouin scattering which occurs in the gigahertz range. We find a relaxation process that can be described by a Kohlrausch function with β≈0.30 for single alkali compositions, but is narrower ( β≈0.40) at the mixed alkali composition.

Specific heat and transport “anomalies” in mixed alkali glasses

We show that changes in the relative mole fractions of Li2O and Na2O in alkali metaphosphate glasses lead to “anomalies” in the specific heat and structural relaxations. The heat capacity change between the liquid and glassy states, Δcp(Tg), at the calorimetric glass transition temperature, Tg, exhibits a minimum when the mole fractions of Li2O and Na2O are comparable. Moreover, systematic changes in the temperature dependence of the viscosity, η, i.e., changes in the “fragility” of the system, accompany these changes in mole fraction. This observed dependence of the “fragility” on the mixed alkali ion composition occurs in the absence of apparent changes in the covalent network connectivity which normally accounts for this behavior in glasses.

Structure of vitreous P 2O 5 and alkali phosphate glasses

The structure of vitreous P 2O 5 (v-P 2O 5), sodium ultraphosphate glasses, (Na 2O) x (P 2O 5) 100− x ( x=10, 20), and alkali metaphosphate glasses, MePO 3 (Me=Li, Na, K, Rb and Cs), have been studied by neutron diffraction. Structural features in the neutron structure factors, S( Q), characteristic of intermediate-range ( Q ≲ 3 Å −1) order were identified. The feature of intermediate-range order in v-P 2O 5 is accounted for by the P 4O 10 molecule packing model. The addition of the alkali metal modifier, Na, has an effect on the intermediate-range structure due to destruction of the PO 4 network structure. Around x ∼ 50 a new peak appears at lower Q than the intermediate-range order peak, which is found in the S( Q)'s of all alkali metaphosphate glasses, which may be associated with extended-range order. The length scale of the extended-range order increases with Me + size. These phenomena can be explained by the affects of oxygen atoms, i.e. PO 4 chain-like units, ordering around the Me +.

Mechanical relaxation anomalies in mixed alkali oxides

Mechanical relaxation (MR) processes were investigated in single and mixed alkali (MA) metaphosphate glasses using a dynamic mechanical analyzer (DMA) over a range of frequencies, 0.1–50 Hz, and temperatures, 24–250°C. The mechanical loss modulus, M″, of each mixed, sodium and lithium, alkali glass exhibited two characteristic maxima, a large maximum just below T g, and a well developed, yet considerably diminished in amplitude, maximum at a much lower temperature. The single alkali analogs, on the other hand, exhibited only a single maximum and this maximum appeared in the same location as the lower temperature peak observed in the MA glasses. The location of these maxima are identified with dynamic processes within the glass which occur with average frequencies, ν μ(Na, Li), for the high T maximum in the mixed glasses, and ν μ (Li) and ν μ (Na) for the lithium and sodium glasses, respectively. These frequencies ν μ(Na, Li), ν μ (Li) and ν μ (Na), varied exponentially with 1/ T; ν μ (Na, Li) had the largest activation energy. In addition ν μ(Na, Li) ≪ ν μ(Na) < ν μ(Li) for T < T g. Two other important observations were made, the high temperature maximum in M″ reached its largest amplitude when the mole fractions of Na 2O and Li 2O were comparable and ν μ (Na, Li) exhibited a maximum in the same composition range. Our observations are discussed in light of a local site-memory relaxation model based on the notion that below T g, cation hopping dynamics are intimately coupled with local glass network relaxations.

Ab initio molecular orbital calculations on the electronic structure of phosphate glasses. Binary alkali metaphosphate glasses

Ab initio molecular orbital calculations have been made for binary alkali metaphosphate glasses using H 2P 2O 7R 2 (R = Li, Na, K, Rb) as molecular models. The calculations have demonstrated that lithium and sodium ions interact equally with two terminal oxygens in each PO 2O − 2/2 tetrahedron, while potassium and rubidium ions interact favorably with one of the two terminal oxygens. The result indicates that the local environments of Na and Li are substantially different from those of K and Rb. The spectroscopic data observed for binary alkali metaphosphate glasses are also interpreted in terms of the molecular orbital concept.

Two contributions to the ac conductivity of alkali oxide glasses.

Although the frequency dependent conductivity of ion-containing glasses often displays scale invariant power law dispersion at high temperatures, the exponent increases to unity at lower temperatures. We report measurements of the conductivity of a series of alkali metaphosphate glasses including a mixed alkali composition and demonstrate that this temperature dependence results from the superposition of two power law dispersions originating from {ital separate} {ital mechanism}, and does not indicate any intrinsic change in scaling of the process which dominates at high temperatures.

Oxynitride surface layers on sodium and lithium metaphosphate glasses

Oxynitride layers can be created on the surface of NaPO 3 and LiPO 3 glasses by heat treating these glasses in dry ammonia near the glass transformation temperature. The chemical durability of these glasses increased several times with the formation of oxynitride surface layers, but the improvement was less than that for bulk nitrided glasses because of the small thickness of the oxynitride surface layer and the presence of the non-cross-linking NH and NH 2 species. When a glass is heated in flowing ammonia, an oxynitride surface layer forms by the dissolution and diffusion of NH 3. The maximum nitrogen content depends on the solubility of ammonia in the glass and the nitriding temperature. The bridging oxygen/non-bridging oxygen ratio in the oxynitride surface layer, as determined from the deconvoluted O1s and N1s spectra, is consistent with the mechanism proposed for the nitridation of bulk alkali metaphosphate glasses.

Oxygen bonding in nitrided sodium- and lithium-metaphosphate glasses

Oxygen bonding in nitrided alkali metaphosphate glasses was characterized by X-ray photoelectron spectroscopy. Bridging-to-nonbridging oxygen ratios (BO/NBO) were determined from deconvoluted O1s spectra. As the nitrogen contents of the metaphosphate glasses increased, the BO/NBO ratios decreased. A mechanism based on the equal reactivity of bridging and nonbridging oxygens during the nitridation process is used to explain the observed trends in the oxygen bonding.

Bond Dissociation Energies in the Alkali Disilicate, Digermanate and Metaphosphate Glasses

Bond Dissociation Energies in the Alkali Disilicate, Digermanate and Metaphosphate Glasses