Structural properties of iron-phosphate glasses: spectroscopic studies and ab initio simulations

Electrospun Polyphosphate Coacervate Glass Fibers in the System P2O5-CaO-MgO-Na2O-Fe2O3 for Wound Healing

This study investigates a series of phosphate-glass fibers (PGFs) in the system P2O5-CaO-MgO-Na2O-Fe2O3 with various Fe contents (0, 0.1, 0.5, 1, and 2 wt %) prepared via electrospinning of polyphosphate coacervate gels. This method is preferable over the traditional high-temperature melt-spinning technique used for PGF production as it represents a more cost-effective and sustainable route. Structural analysis performed via Fourier transform Infrared spectroscopy shows that PGFs are mainly formed by polyphosphate chains containing Q1 and Q2 units. Thermal analysis demonstrates that the amorphous nature of the PGFs can be preserved up to calcination temperatures in the range 450-520 °C, with crystallization temperatures increasing with the iron content. Dissolution studies were performed by immersing the PGFs in deionized water and analyzing the species released (P, Ca, Mg, Fe, and Na) via microwave plasma atomic emission spectroscopy at regular intervals up to 72 hours (h). Results show that both iron and phosphate anion release increases with iron loading, suggesting that the phosphate network is weakened by an increasing amount of iron. Given that PGFs are particularly advantageous in wound healing due to their fibrous morphology, their cytocompatibility was assessed by seeding human keratinocytes (HaCaTs) in contact with the dissolution products of PGFs after 24 h of immersion at three different ratios of dissolution products to cell medium (1:100, 3:100, and 5:100). No cytotoxicity was observed for any of the ratios studied. Moreover, the dissolution products of some PGFs resulted in an enhanced growth of HaCaTs, with the best result being observed when using dissolution products from PGFs containing 0.1 wt % of Fe and a dissolution product-cell medium ratio of 5:100. Dissolution products from PGFs with an Fe content up to 0.5 wt % have also demonstrated antibacterial activity against the bacterium Escherichia coli (E. coli). A preliminary test on the efficacy of PGFs in wound healing via ex vivo studies on human skin has demonstrated that the PGFs in direct contact with the wound promote 84% wound closure.

Medium-Range Order in Iron Phosphate Glass Models Obtained Using Various Randomization Techniques: A Molecular Dynamics Study

Glasses are known to have medium-range order (MRO), but their link to any experimentally measurable quantity is still ambiguous. The first sharp diffraction peak (FSDP) in structure factor S(q) obtained from diffraction experiments on glasses has been associated with this MRO (∼7-15 Å), but understanding the fundamental origin of this universal peak is still an open problem. We have addressed this issue for a complex glass, i.e., iron phosphate glass (IPG), through atomistic models generated from a hybrid approach (our in-house-developed MC code + molecular dynamics simulation). IPG is a technologically important glass with applications in waste vitrification, bioactive glass, laser glass material, anode material for batteries, etc., and is seen as a strengthened substitute for borosilicate glasses. We performed a comparative study by generating glass models from different initial configurations and randomization techniques. The developed IPG models were first validated with existing data on short-range order (SRO) and MRO through the study of pair correlation functions, bond angle distributions, and coordination number for SRO and rings distribution, FSDP in structure factor, and void size distribution for MRO. The study of coordination environment of oxygen is specifically shown to aid in understanding glass formation through topological constraint theory. Thereafter, to understand the fundamental origin of FSDP in S(q), structure factors were calculated corresponding to the individual ring sizes present in the model. The relative contribution of these individual S(q)'s in the total experimental S(q) is estimated using an inverse fitting approach. The contributions thus obtained directly correlated with ring size percentages in the models for the considered q-range. In particular, the melt-quenched model obtained from the MC model as an initial structure is found to reproduce most experimental features seen in IPG. Through this exercise, we can connect the rings distribution of an atomistic glass model with an experimentally measurable quantity like FSDP in S(q) for a complex glass-like IPG. This gives physical meaning to the rings distribution while also proving that this structural descriptor is a useful tool for validation of MRO in simulation-produced models of glass.

Neutron, X-ray diffraction, DSC, Raman, Mössbauer and leaching studies of iron phosphate glasses and crystalline phases

xFe2O3-(100 - x)P2O5 glasses were synthesized by melt quenching and structure-property correlation studies were carried. Glasses containing 25 to 40 mol% Fe2O3 were prepared while the sample with 50 mol% Fe2O3 formed a crystalline sample containing Fe3 2+Fe4 3+[PO4]6 3- and Fe2 2+[P2O7]4- phases on melt-quenching. Glass density increases from 2.98 to 3.20 g cm-3, ionic packing fraction is in the range of 0.63-0.65 and the glass transition temperature decreases from 500 °C to 493 °C on increasing Fe2O3 concentration from 25 to 40 mol%. Pair distribution function analysis and Reverse Monte Carlo simulations of neutron diffraction datasets were used to calculate the atomic pair distributions, interatomic distances and co-ordination environments. The P-O co-ordination is essentially tetrahedral and is in the range: 3.9-3.7 (±0.1), the Fe-O co-ordination number decreases steadily from 4.8 to 4.2 (±0.1) with an increase in Fe2O3 concentration in the phosphate network, while O-O co-ordination is in the range: 6.6-6.3(±0.1), the decrease in these co-ordination numbers are due to an increase in oxygen deficiency in the glass network with an increase in Fe2O3 mol%. Fe-O and P-O pair distributions are asymmetrical indicating short-range disorder due to the existence of a wide range of bond-lengths with maxima at 1.79 Å and in the range: 1.45-1.51 Å respectively. Mössbauer studies carried out at room temperature and 80 K found that Fe exists in 2+ and 3+ valence states, and the glass and crystalline samples contained Fe2+ at least at three different sites. Raman studies found that the meta and pyrophosphate structural units are dominant species up to 35 mol% Fe2O3 concentration, while the orthophosphate units are in majority at 40 mol% of Fe2O3. The crystalline sample is a two phase material and contained both orthophosphate and pyrophosphate units with the former being the dominant species. Leaching studies on two iron phosphate glasses carried out in purified water at 90 °C found that dissolution of glasses decreases and the chemical durability increases drastically with an increase in Fe2O3 mol%.

Structural study of strontium-containing iron-phosphate glasses for radioactive waste vitrification

Iron-phosphate glasses are a wide group of materials with a wide range of applications. Among others, they are promising materials in toxic waste vitrification because of their high chemical durability and relatively low processing temperature and time. They are a novel group of glasses that are considered in the vitrification of radioactive waste, especially those that cannot be treated using conventional borosilicate ones. Since strontium isotopes are one of the main fission products present in the waste, the influence of Sr on the structural properties of the glasses is an important factor. Strontium-containing iron-phosphate glasses were subjected to structural studies using FT-IR and Raman spectroscopies. The obtained spectra were described, and appropriate band assignments were done. Based on the research conducted, the structural features of the phosphate network and their changes were determined. The results obtained showed that strontium in relatively low content up to 20 mol% acts as the glass network charge compensator and can stabilize the network. Above this threshold, SrO can be treated as a pure modifier, leading to gradual depolymerization. Thus, this point may be treated as the maximum waste loading for effective strontium immobilization.

Toward Atomistic Understanding of Iron Phosphate Glass: A First-Principles-Based Density Functional Theory Modeling and Study of Its Physical Properties

Iron phosphate glasses (IPGs) have been proposed as futuristic materials for nuclear waste immobilization and anode materials for lithium batteries. Recently, many attempts have been made to propose atomistic models of IPGs to explain their properties from an atomistic viewpoint. In this paper, we seek to produce small scale models of IPG that can be handled within the scheme of density functional theory (DFT) to study its electronic structure. The starting models, generated using the Monte Carlo (MC) method, were subsequently annealed using ab initio molecular dynamics (AIMD) to minimize the coordination defects. The geometry of the equilibrated structure was then optimized at 0 K. This hybrid approach (MC + AIMD + DFT optimization) produced good atomistic models of IPG, which can reproduce the experimentally observed band gap, vibrational density of states, magnetic moment of Fe, and mechanical and optical properties. Computationally expensive melt-quench simulation can be avoided using the present approach, allowing the use of DFT for accurate calculations of properties.

Synthesis of Hyperbranched Flame Retardants with Varied Branched Chains' Rigidity and Performance of Modified Epoxy Resins

To overcome the high flammability and brittleness of epoxy resins without sacrificing their glass transition temperature (Tg) and mechanical properties, three epoxy-terminated hyperbranched flame retardants (EHBFRs) with a rigid central core and different branches, named EHBFR-HB, EHBFR-HCM, and EHBFR-HBM, were synthesized. After chemical structure characterization, the synthesized EHBFRs were introduced into the diglycidyl ether of bisphenol A (DGEBA) and cured with 4, 4-diaminodiphenylmethane (DDM). The compatibility, thermal stability, mechanical properties, and flame retardancy of the resultant resins were evaluated. Results showed that all three EHBFRs could significantly improve the fire safety of cured resins, and 30 wt. % of EHBFRs (less than 1.0 wt. % phosphorus content) endowed cured DGEBA with a UL-94 V-0 rating. In addition, the increased rigidity of branches in EHBFRs could increase the flexural strength and modulus of cured resins, and the branches with appropriate rigidity were also beneficial for improving their room temperature impact strength and Tg.

Vibrational characteristics of aluminum-phosphate compounds by an experimental and theoretical approach

Aluminum phosphates are materials with relatively wide potential applications in many industries. The vibrational features of selected compounds were established on Raman and infrared spectroscopy. The experimentally determined spectra are compared to those calculated by ab initio methods. This gives a unique possibility of a proper assignment of the experimental spectral features to specific modes of vibration. In the results, it was evidenced that the spectra are characterized by two specific intense bands in the mid- and high-frequency range due to the P–O–P and P–O bonds in [PO₄] tetrahedron vibrations. The position of the high-frequency band is related to the number of bridging oxygen atoms connecting [PO₄] tetrahedrons in the unit cell. Additionally, the differences in the spectra were evidenced as a result of different polymorphic forms of the selected compounds. Therefore, the results may be useful in determining the phase composition of polyphase materials or structural features of aluminum–phosphate glasses and glass–ceramic materials.

A new iron-phosphate compound (Fe7P11O38) obtained by pyrophosphate stoichiometric glass devitrification

Iron phosphates are a wide group of compounds that possess versatile applications. Their properties are strongly dependent on the role and position of iron in their structure. Iron, because of its chemical character, is able to easily change its redox state and accommodate different chemical surroundings. Thus, iron-phosphate crystallography is relatively complex. In addition, the compounds possess intriguing magnetic and electric properties. In this paper, we present crystal structure properties of a newly developed iron-phosphate compound that was obtained by devitrification from iron-phosphate glass of pyrophosphate stoichiometry. Based on X-ray diffraction (XRD) studies, the new compound (Fe₇P₁₁O₃₈) was shown to adopt the hexagonal space group P6₃ (No. 173) in which iron is present as Fe³⁺ in two inequivalent octahedral and one tetrahedral positions. The results were confirmed by Raman and Mössbauer spectroscopies, and appropriate band positions, as well as hyperfine interaction parameters, are assigned and discussed. The magnetic and electric properties of the compound were predicted by ab initio simulations. It was observed that iron magnetic moments are coupled antiferromagnetically and that the total magnetic moment of the unit cell has an integer value of 2 µ_B. Electronic band structure calculations showed that the material has half-metallic properties.

Iron in Hydroxyapatite: Interstitial or Substitution Sites?

Iron-doped hydroxyapatite (Fe-HAp) is regarded as a promising magnetic material with innate biocompatibility. Despite the many studies reported in the literature, a detailed theoretical description of Fe inclusions is still missing. There is even no consensual view on what kind of Fe defects take place in Fe-HAp-iron interstitial or calcium substitutions? In order to address these questions, we employ modern first-principles methodologies, including hybrid density functional theory, to find the geometry, electronic, magnetic and thermodynamic properties of iron impurities in Fe-HAp. We consider a total of 26 defect configurations, including substitutional (phosphorus and calcium sites) and interstitial defects. Formation energies are estimated considering the boundaries of chemical potentials in stable hydroxyapatite. We show that the most probable defect configurations are: Fe3+ and Fe2+ substitutions of Ca(I) and Ca(II) sites under Ca-poor conditions. Conversely, Fe interstitials near the edge of the hydroxyl channel are favored in Ca-rich material. Substitutional Fe on the P site is also a probable defect, and unlike the other forms of Fe, it adopts a low-spin state. The analysis of Fe K-XANES spectra available in the literature shows that Fe-HAp usually contains iron in different configurations.

Development of a New Sr-O Parameterization to Describe the Influence of SrO on Iron-Phosphate Glass Structural Properties Using Molecular Dynamics Simulations

Iron-phosphate glasses, due to their properties, have many potential applications. One of the most promising seems to be nuclear waste immobilization. Radioactive ⁹⁰Sr isotope is the main short-lived product of fission and, due to its high solubility, it can enter groundwater and pose a threat to the environment. On the other hand, Sr is an important element in hard tissue metabolic processes, and phosphate glasses containing Sr are considered bioactive. This study investigated the effect of SrO addition on a glass structure of nominal 30Fe₂O₃-70P₂O₅ chemical composition using classical molecular dynamics simulations. To describe the interaction between Sr-O ion pairs, new interatomic potential parameters of the Buckingham-type were developed and tested for crystalline compounds. The short-range structure of the simulated glasses is presented and is in agreement with previous experimental and theoretical studies. The simulations showed that an increase in SrO content in the glass led to phosphate network depolymerization. Analysis demonstrated that the non-network oxygen did not take part in the phosphate network depolymerization. Furthermore, strontium aggregation in the glass structure was observed to lead to the non-homogeneity of the glass network. It was demonstrated that Sr ions prefer to locate near to Fe(II), which may induce crystallization of strontium phosphates with divalent iron.

Innovative Formulations of Phosphate Glasses as Controlled-Release Fertilizers to Improve Tomato Crop Growth, Yield and Fruit Quality

Three phosphate glass compositions, VF1, VF2, and VF3, containing macro and micronutrients with different [K₂O/(CaO+MgO)] ratio, were formulated to be used as controlled release fertilizers for tomato crop, depending on their chemical durability in water and their propriety with respect to the standards of controlled-release fertilizers. This study investigated the influence of [K₂O/(CaO+MgO)] ratio variation on glass properties. For this, the elaborated glasses have undergone a chemical characterization using inductively coupled plasma atomic emission spectroscopy, a thermal characterization using differential thermal analysis, a physicochemical characterization based on density and molar volume measurements, and a structural characterization using Raman spectroscopy, Fourier-transform infrared spectroscopy, and X-ray diffraction. In addition, the chemical durability was determined by measuring the percentage of weight loss and the pH. Results revealed that the glass structure and composition have the mean role in controlling the release of nutrients in water. By increasing [K₂O/(CaO+MgO)] ratio, the dissolution rates of the glasses increased due to the shrinking in the rate of crosslinking between phosphate chains, accompanied with a diminution in transition and crystallization temperatures, and an increase in the molar volume. An agronomic valorization of VF1 and VF2 glass fertilizers, which showed dissolution profiles adequate to the criteria of controlled-release fertilizers, was carried out to evaluate their efficiency on tomato crops. These glass fertilizers improved soil mineral content and tomato performances in comparison to the control and NPK treatments with the distinction of VF2. The results highlight the effectiveness of these smart fertilizers toward their potential large-scale application to improve crop production and quality for high nutritional value foods.

Multifunctional Ca10(PO4)6F2 as a host for radioactive waste immobilization: Am3+/Eu3+ ions distribution, phosphor characteristics and radiation induced changes

Na⁺₂Eu³⁺₂:Ca₆(PO₄)₆F₂ is explored as a potential host for radioactive waste immobilization. Since Eu³⁺ ion is a surrogate of highly radioactive Am³⁺ ion, the photoluminescence (PL) characteristics of Eu³⁺ ion helped to investigate the possible distribution of hazardous and radioactive Am³⁺ ion among the two lattice sites in the matrix. It was observed that Am³⁺ will prefer to occupy the Ca2-site lattice which has a direct linkage to F atom. From DFT calculation we have found that both Eu³⁺ and Am³⁺ ions are following similar trend of distribution into the Ca2-site compared to Ca1-site which has no F atom linkage. The radiation stability of the compound was also investigated by PL study after irradiating it with a ⁶⁰Co gamma source with different doses starting from 2 kGy to as high as 1000 kGy. It was observed that radiation induced changes were more surrounding the Ca1-site than in Ca2-site.Considering all the experimental and theoretical observations it is concluded that from radioactive waste immobilization point of view it is more preferable to dope the Am³⁺ ion into the Ca2 site. The Eu³⁺ doped compound was also found to be red color emitting phosphor materials with color purity of 95.24%.

Depolymerization of sodium polyphosphates on an iron oxide surface at high temperature

Density functional theory (DFT) and first principles molecular dynamics (FPMD) studies of pyrophosphate cluster Na₄P₂O₇ and triphosphate cluster Na₅P₃O₁₀ absorbed and decomposed on an Fe₂O₃(0001) surface have been conducted. Comparative analyses of the structure properties and adsorption processes during the simulation at elevated temperature have been carried out. The results depict the key interactions including the covalent P-O bonds, pure ionic Na-O or Fe-O interactions. The iron oxide surface plays an important role in the bridging bond decomposition scheme which can both promote and suppress phosphate depolymerization. It is found that the chain length of polyphosphates does not have considerable effects on the decomposition of phosphate clusters. This study provides detailed insights into the interaction of a phosphate cluster on an iron oxide surface at high temperature, and in particular the depolymerization/polymerization of an inorganic phosphate glass lubricant, which has an important behavior under hot metal forming conditions.

The Relevance of Phosphorus and Iron Chemistry to the Recovery of Phosphorus from Wastewater: A Review

The addition of iron is a convenient way for removing phosphorus from wastewater, but this is often considered to limit phosphorus recovery. Struvite precipitation is currently used to recover phosphorus, and this approach has attracted much interest. However, it requires the use of enhanced biological phosphorus removal (EBPR). EBPR is not yet widely applied and the recovery potential is low. Other phosphorus recovery methods, including sludge application to agricultural land or recovering phosphorus from sludge ash, also have limitations. Energy-producing wastewater treatment plants increasingly rely on phosphorus removal using iron, but the problem (as in current processes) is the subsequent recovery of phosphorus from the iron. In contrast, phosphorus is efficiently mobilized from iron by natural processes in sediments and soils. Iron-phosphorus chemistry is diverse, and many parameters influence the binding and release of phosphorus, including redox conditions, pH, presence of organic substances, and particle morphology. We suggest that the current poor understanding of iron and phosphorus chemistry in wastewater systems is preventing processes being developed to recover phosphorus from iron-phosphorus rich wastes like municipal wastewater sludge. Parameters that affect phosphorus recovery are reviewed here, and methods are suggested for manipulating iron-phosphorus chemistry in wastewater treatment processes to allow phosphorus to be recovered.