Metaphosphate Composition Research Articles

Insight into the structure of magnesium and sodium mixed phosphate glasses: A molecular dynamics study

Three phosphate glasses from the series (50-x/2)Na2O–xMgO–(50-x/2)P2O5 (x = 20, 30 and 40) were studied using classical molecular dynamics simulation. In addition, the metaphosphate compositions of 50MgO-50P2O5 and 50Na2O-50P2O5 were also modelled. The model results showed good agreement with the experimental results considering reported experimental uncertainties. Both Na and Mg act as network modifiers. The magnesium metaphosphate glass model showed that the local environments of the magnesium consist of distorted structures with mean coordination number of 4.4, while the Na-O mean coordination number is found to be 5 in sodium metaphosphate composition. For sodium magnesium phosphate glasses, the models show that Mg-O coordination number has no change over the range of the target compositions with an average of NMgO =4.4 and the mean distance at 1.99 Å. There is also no noticeable change in Na-O coordination number ∼5 and the Na-O distance is found at 2.41 Å in sodium magnesium phosphate glasses. The depolymerization of the phosphate glass network is observed upon the addition of MgO. The relative amounts of bridging Ob and non-bridging Onb oxygen atoms change toward increasing Onb with addition of MgO content. The connectivity in the models has been studied and it is characterised by Q2 sites at metaphosphate composition, decreasing to Q1 sites upon the addition of MgO. The nearest neighbor distances of Mg-Mg and Na-Na in which modifier cations are bonded to non-bridging oxygens, Onb, are measured and show Mg-Mg correlations to be unfavorable in comparison to Na-Na correlations. In addition, Mg(Onb)N polyhedra are predominantly connected to each other by corner sharing.

Structures and Dissolution Behaviors of Quaternary CaO-SrO-P2O5-TiO2 Glasses.

Calcium phosphate glasses have a high potential for use as biomaterials because their composition is similar to that of the mineral phase of bone. Phosphate glasses can dissolve completely in aqueous solution and can contain various elements owing to their acidity. Thus, the glass can be a candidate for therapeutic ion carriers. Recently, we focused on the effect of strontium ions for bone formation, which exhibited dual effects of stimulating bone formation and inhibiting bone resorption. However, large amounts of strontium ions may induce a cytotoxic effect, and there is a need to control their releasing amount. This work reports fundamental data for designing quaternary CaO-SrO-P2O5-TiO2 glasses with pyro- and meta-phosphate compositions to control strontium ion-releasing behavior. The glasses were prepared by substituting CaO by SrO using the melt-quenching method. The SrO/CaO mixed composition exhibited a mixed cation effect on the glassification degree and ion-releasing behavior, which showed non-linear properties with mixed cation compositions of the glasses. Sr2+ ions have smaller field strength than Ca2+ ions, and the glass network structure may be weakened by the substitution of CaO by SrO. However, glassification degree and chemical durability of pyro- and meta-phosphate glasses increased with substituted all CaO by SrO. This is because titanium groups in the glasses are closely related to their glass network structure by SrO substitution. The P-O-Ti bonds in pyrophosphate glass series and TiO4 tetrahedra in metaphosphate glass series increased with substitution by SrO. The titanium groups in the glasses were crosslink and/or coordinate phosphate groups to improve glassification degree and chemical durability. Sr2+ ion releasing amount of pyrophosphate glasses with >83% SrO substitution was larger than 0.1 mM at day seven, an amount that reported enhanced bone formation by stimulation of osteogenic markers.

Structure of tin phosphate glasses by neutron and X-ray diffraction

Abstract Neutron and X-ray diffraction experiments with large ranges of scattering vector, up to Qmax of 400 nm−1 and 250 nm−1, respectively, were performed on (SnO)x(P2O5)1-x glasses with 0.5 ≤ x ≤ 0.7, and the structural parameters of the P O and Sn O polyhedra were determined. The fraction of P–NBO (non-bridging oxygen) bonds increases systematically at the expense of the P–BO (bridging oxygen) bonds with SnO additions. The Sn O coordination number decreases in a predictable way from four at the metaphosphate composition (x = 0.5) to about three at the diphosphate composition (x = 0.67). The structural units are assumed to be SnO4 trigonal bipyramids and SnO3 trigonal pyramids with the lone-pair of Sn(II) electrons occupying one of the corners, similar to those in related crystal structures. For all glasses up to x = 0.67, the SnOz pyramids and PO4 tetrahedra form a continuous network with P O P and P O Sn bonds and the Sn O polyhedra do not share oxygen neighbors. From comparisons of the compositional dependences of the Sn O and Zn O coordination numbers and the respective number densities of atoms in the binary tin and zinc phosphate glasses, it is presumed that the Sn O coordination increases further with NSnO > 4 in the ultraphosphate range (x

A highly efficient method of dehydroxylation and fining of Nd phosphate laser glasses

AbstractWe describe herein a method that shows to be highly efficient in the dehydroxylation and fining process of phosphate glasses and that has been tested on the Nd‐doped 15Na2O‐15K2O‐20BaO‐50P2O5 metaphosphate composition. The method consists of a two‐step procedure: first, the batch is melted in a ceramic crucible; then, the obtained glass is re‐melted under a constant N2 flow, while being placed inside a graphite crucible with a much bigger surface area as compared to its depth. This allows the melt to be spread over the crucible area resulting in a relatively thin layer with a high area in contact with the atmosphere at the temperature of remelting. It has been demonstrated that the water content decreases with both temperature and time length and that the glass samples obtained are completely free of bubbles and stria showing very high chemical and optical homogeneity. This method employed, has been proven to be very effective, as evidenced by the great increase in the lifetime of Nd3+ luminescence––reaching a maximum of 380 μs‐which is much higher than the one measured in the untreated glass.

Electrical conductivity and viscosity of phosphate glasses and melts

Phosphate glasses are of great interest for basic research and for special applications in various fields as technical and optical glasses. Metaphosphate compositions with PO4-chain structure are mostly used. The cations have a significant effect on the properties of the glasses and melts. This was studied in more details. High purity metaphosphate glasses, Mn(PO3)n with M = Na+, K+, Zn2+, Mg2+, Ca2+, Sr2+, Ba2+, Al3+ and Fe3+, were prepared and the optical and thermal properties were measured with different methods. The electrical conductivity of glasses and melts is the property of great practical importance. It was determined by impedance spectroscopy in a wide temperature range from 300 to 1250 °C in frequency range of 0.1 Hz to 6 MHz. The electrical conductivity of alkaline earth phosphate glasses and melts increases with increasing ion radius of the cation. NaPO3 and KPO3 glasses and melts have much higher conductivity. Viscosity measurements were carried out from transformation temperature, Tg, to the melt. The measured data were fitted and the activation energies of the conductivity and viscosity were calculated. Simple exponential behavior was found at temperatures below Tg, but above Tg only in very narrow temperature range. The relation between conductivity and viscosity was considered.

Structure properties relationship in calcium sodium metaphosphate and polyphosphate glasses

Two series of glasses, the first with the formula (50-x/2)Na2O-xCaO-(50-x/2)P2O5 and an oxygen over phosphorus O/P ratio between 3 and 3.33 and the second with the composition (50-x)Na2O-xCaO-50P2O5 with an O/P = 3, were obtained by melt quenching and studied in order to understand the effects of the CaO addition on the structure, physical properties and thermochemistry of polyphosphate and metaphosphate glasses. The spectroscopic investigations by Fourier Transform Infrared spectroscopy and 31P Magic Angle Spinning Nuclear Magnetic Resonance indicate in the first series, the shortening of the polyphosphate chains as CaO content increases. The spectra of the second series indicate the conservation of the metaphosphate chains. The density and thermal analysis measurement show the shrinking and the reticulation of the phosphate network as CaO is added in both series and the chemical durability of the glasses is enhanced by the substitution of the PONa and POP bonds by the POCa– bonds. The thermochemical study shows the decrease of the dissolution heat of the glasses when the average chain length decreases and when the chemical durability of the glass increases. The molar enthalpy of mixing indicates the presence of two ranges of interaction in the metaphosphate compositions around 0–20% and 25–50% of CaO.

A model for the construction of the network in phosphate glasses through viscosity-structure relationships

We have established a relationship between the viscosity activation energy of metaphosphate melts and the NMR chemical shift anisotropy of phosphorus that is measured in their corresponding glasses, and proposed a model by which the fragility of the glasses can be explained as dependent on the particular modified random network that is originated upon cooling. The increase of the modifier field strength gives rise to higher viscosity activation energy during the glass formation upon cooling, with a maximum at the calcium metaphosphate composition. The lower rates of increase of viscosity would produce network structures characterized by higher anisotropicity and be the responsible for the higher fragility of the glasses, which can be defined as a measure of the degree of difference between the structures of the glassy state and the melt from which this is obtained.

Structural Studies on Ag<sub>2</sub>O-P<sub>2</sub>O<sub>5</sub> Glasses

Silver phosphate glasses of general formula xAg2O·(100 - x)P2O5 have been investigated over compositional range from x = 40 to 62.5 mol%. The local structure around phosphorus atom has been studied via 31P nuclear magnetic resonance. The distribution of [PO4]Qn species as a function of composition has been shown to slightly deviate from the simple binary alkali phosphate model. An anomalous behavior has been recorded and interpreted in terms of mixed ring-chain effect in metaphosphate composition. The splitting of NMR spectra into sub resonances is assigned to different binding sites characterizing Q1 ring and Q1 chain structure. Higher Ag2O concentration (≥50 mol%) leads to formation of phosphate groups with specific resonance peaks which are mainly related to pyro and orthophosphate species. The rate of change of the chemical shift of the 31P NMR depends on the bond type, which in turn reflects the extent of double bonding between phosphorus and oxygen atoms. Increasing concentration of Q0 with increasing Ag2O content leads to decreasing quantities of bridging and double bonds. As a consequence, specific symmetric resonance peak of higher intensity and chemical shift (Q0) is a feature of silver rich glasses (orthophosphate). The latter species is therefore proposed to compose of separated membered rings, which cause deshielding of phosphate units. XRD and EDP studies have shown that, amorphous phosphate network is the dominant structure of glasses containing ≤ 55 mol% Ag2O. Some ordered and well crystallized phases are formed at higher Ag2O concentration. Increasing non-bridging oxygen atoms is shown to have the main effect on crystallization behavior. Orthophosphate composition is the most crystalline one among the other compositions (ultra, meta- and pyrophosphate). Presence of orthophosphate species which typically contains highest concentration from isolated Q0 units is the main reason for building up crystalline Ag3PO4phosphate phase.

Structural characterization and calorimetric dissolution behavior of Na2O[sbnd]CuO[sbnd]P2O5 glasses

Two series of glasses belonging to the ternary system Na2O - CuO - P2O5 where copper is introduced differently were synthesized by the conventional melt-quenching technique. Both series start from meta-phosphate composition NaPO3 (50Na2O-50P2O5). In the first series, the Na2O/P2O5 ratio is constant (Na2O/P2O5=1) and both Na2O and P2O5 oxides were replaced by copper oxide CuO: (50-x/2) Na2O - xCuO - (50-x/2) P2O5, 0≤x≤40. In the second series, the molar percentage of P2O5 is kept constant at 50mol% and Na2O is replaced by CuO: (50-x)Na2O – xCuO – 50P2O5, 0≤x≤50. The glasses of these two series belong respectively to NaPO3-CuO and NaPO3-Cu(PO3)2 lines of the ternary system. These materials were characterized by density measurements, differential scanning calorimetry (DSC), X-ray diffraction (XRD), Raman, diffuse reflectance and dissolution heat measurements carried out with micro-calorimetry technique. Introduction of copper in meta-phosphate glass induces a blue/green color, and strengthens the vitreous network, as shown by the decrease of the molar volume (VM) and the increase of the glass transition temperature (Tg) when the content of CuO rises. The crystallization of the glasses, followed by XRD, leads to the formation of NaPO3, Na2CuP2O7 or Na2Cu(PO3)4 depending on the composition. Raman spectroscopy shows the conservation of the metaphosphate chains for NaPO3-Cu(PO3)2 glasses characterized by a constant O/P ratio (O/P=3) and the depolymerization of these chains for NaPO3-CuO glasses where O/P ratio varies from 0 to 3.67. UV - visible study showed that the optical energy gap decreases by increasing the CuO concentration. Calorimetric study of the dissolution of glasses in acid solution shows a decrease of the dissolution enthalpy, for both series, when the CuO content increases. The dissolution phenomenon is endothermic for glasses with low content in CuO (x<30) and exothermic for glasses rich in CuO.

Production and Properties of Glassy Metaphosphate Composition for Protecting the Elements of Water-Heating Systems from Corrosion

The production from simple and easily accessible materials and use of an environmentally safe, glassy, metaphosphate composition (GMC) (the formula is patented) for comprehensive protection of the elements of water-heating systems from corrosion and salt deposition (scale) are examined. The results of experiments investigating the solubility and anticorrosion and antiscaling properties of the GMC are presented.

Structures and dissolution behaviors of MgO–CaO–P2O5–Nb2O5 glasses

MgO–CaO–P2O5–Nb2O5 glasses with the substitution of MgO for CaO consisting predominantly of meta-, pyro-, and orthophosphate groups were prepared using a melt-quenching method, and their structures and dissolution behaviors were investigated. Magnesium in the metaphosphate compositions was shown to act as a modifier oxide and coordinate phosphate groups. Magnesium in the pyro- and/or orthophosphate compositions was shown to act as a network former and to form P–O–Mg bonds, which crosslink short phosphate groups. The chemical durability of the metaphosphate glasses increased with increasing the substitution of MgO for CaO because magnesium exhibits a larger field strength than calcium. However, the durability of the pyro- and/or orthophosphate glasses decreased with increasing the MgO content: P–O–Mg bonds weakened the glass network structure and induced hydrolysis.

Investigation of CuI-Containing Molybdophosphate Glasses by Infrared Reflectance Spectroscopy

Superionic glasses xCuI–(l – x)[Cu2MoO4–CuPO3] were prepared and studied by infrared reflectance spectroscopy to investigate the structure of the oxyanion matrix and the type of sites occupied by copper ions. The study was complemented by the consideration of glasses 0.67CuI–0.33[Cu2MoO4–Cu3PO4] and xCuI–(l – x)[Cu2O-nP2O5] with n = 0.33, 0.50, 1.0. The oxyanion structure of glasses xCuI–(l – x)[Cu2MoO4–CuPO3] was found to involve discrete PO43-, P2O74- and MoO42- units, where P and Mo are 4-fold coordinated to oxygen, and molybdate octahedral species which have the MoO3 stoichiometry and are linked by Mo–O–Mo bridging bonds. Despite the nominal metaphosphate composition (CuPO3) of these glasses the spectra gave no signature for metaphosphate structures based on the PO3– unit. Also, increasing amounts of CuI were found to favor the creation of PO43- and MoO3 species at the expense of P2O74- and MoO42-. These findings were explained by the acidity order P2O5 > MoO3 and the need to accommodate the bulky CuI in the glassy matrix, a process facilitated by condensed MoO3 octahedral species. Cu ions were found to be present as monovalent cations and to occupy oxide and iodide sites. The latter sites organize into CuI-like pseudophases at high CuI contents, in agreement with the conduction pathway model for superionic glasses.

Chain-length-identification strategy in zinc polyphosphate glasses by means of XPS and ToF-SIMS

The surface chemistry of amorphous zinc polyphosphates of different compositions (ranging from zinc metaphosphate to zinc orthophosphate) has been investigated by means of X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary-ion mass spectroscopy (ToF-SIMS). The identification of the chain length of zinc polyphosphates by XPS was on the basis of the integrated intensity ratio of the bridging (P-O-P) and nonbridging (P = O and P-O-M) oxygen peaks used for fitting the oxygen 1s signal, the shift of the P 2p(3/2) signal towards lower binding energies and the modified Auger parameter towards higher values as the zinc content increases. The discrimination of the polyphosphate chain lengths was also achieved by ToF-SIMS, by comparing the intensities of selected characteristic phosphate fragments. Both techniques appear to be suitable for the investigation of polyphosphate glasses in applications such as tribology, where there is a need to identify the chain length present in the outermost monolayer of the film. Fourier-transform infrared (FT-IR) spectroscopy was used to characterize the bulk compounds. The FT-IR studies showed that long-chain structures linked through P-O-P bonds predominate in the metaphosphate composition, while when the zinc content is increased, the chains become shorter, ultimately being replaced by PO(4) monomers in the orthophosphate composition.

Structural investigation on silver phosphate glasses embedded with nanoparticles

Structural investigations of CdI2 doped silver phosphate glass-nanocomposites in the system xCdI2–(1−x)(yAg2O–(1−y)P2O5) are presented in this paper. Although, X-ray diffraction patterns show amorphous nature of the compositions, electron microscopic studies show several nanocrystalline phases such as CdI2, β-AgI, AgPO3 and Ag4P2O7 dispersed within the glass matrix. The presence of CdI2 nanoparticles in cluster form is dominant for the compositions in the limit of the glass forming domain. The particle size in polyphosphate compositions is higher than that of metaphosphate or ultraphosphate compositions. FTIR spectra of these compositions have been investigated. It has been observed that the presence of different nanocrystalline phases does not affect the basic glass network. Differential scanning calorimetric results show that the incorporation of CdI2 within the glassy matrix increases the glass transition temperature. It is observed that the structural characteristics of the CdI2 doped silver phosphate glasses are quite different from that of other divalent metal iodide doped glasses.

Local Structure of Cerium in Aluminophosphate and Silicophosphate Glasses

The local structure of cerium in two systematic compositional series of glasses, nominally CeP{sub 3}O{sub 9}-AlP{sub 3}O{sub 9} and CeP{sub 3}O{sub 9}-SiP{sub 2}O{sub 7}, was interrogated using X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) spectroscopy. XPS revealed that, for glasses melted in air, {>=}95% of cerium ions are Ce{sup 3+}. This was independently confirmed using X-ray absorption near edge spectroscopy (XANES). Ce K-edge extended X-ray absorption fine structure (EXAFS) has been used to determine the local structure of Ce{sup 3+}. Near the metaphosphate composition, cerium was found to have an average cerium coordination number of {approx}7.0 and an average cerium-oxygen bond length of 2.41 {angstrom}. The average cerium coordination number and average cerium-oxygen bond distance were found to increase with decreasing cerium concentration in both compositional series. Rare-earth clustering is suggested based on numerical calculations for glasses containing {>=}14 and {>=}15 mol% Ce{sub 2}O{sub 3} for the aluminophosphate and silicophosphate series, respectively.

Structural Characteristics of Antibacterial Bioresorbable Phosphate Glass

AbstractNeutron diffraction with isotopic substitution has been used to probe the local environment of silver in a melt‐quench derived bioresorbable phosphate glass. Samples enriched with 107Ag and 109Ag were prepared and neutron diffraction data collected. A first‐order difference was taken between the data sets to give detailed information about the silver environment in the glass matrix. The measured Ag‐O correlation has three components in the first coordination shell at 2.28 Å, 2.51 Å, and 2.58 Å with coordination numbers of 2.1, 2.7, and 1.1, respectively, consistent with silver occupying a distorted octahedral environment. These results have been correlated with those from Ag K‐edge X‐ray absorption near‐edge spectroscopy measurements. The results suggest that the addition of silver to the (CaO)0.3(Na2O)0.2(P2O5)0.5 system has a significant effect on the host phosphate network, with shorter and more branched chains replacing the long chains and rings normally associated with the metaphosphate composition.

Lead and Aluminum Bonding in Pb−Al Metaphosphate Glasses

The bonding properties of cations in phosphate glasses determine many short- and medium-range structural features in the glass network, hence influencing bulk properties. In this work, Pb-Al-metaphosphate glasses (1 - x)Pb(PO(3))(2).xAI(PO(3))(3) with 0 < or = x < or = 1 were analyzed to determine the effect of the substitution of Pb by AI on the glass structure in the metaphosphate composition. The glass transition temperature and density were measured as a function of the Al concentration. The vibrational and structural properties were probed by Raman spectroscopy and nuclear magnetic resonance of (31)P, (27)AI, and (207)Pb. Aluminum incorporates homogeneously in the glass creating a stiffer and less packed network. The average coordination number for AI decreases from 5.9 to 5.0 as x increases from 0.1 to 1, indicating more covalent AI-O bonds. The coordination number of Pb in these glasses is greater than 8, showing an increasing ionic behavior for compositions richer in AI. A quantitative analysis of the phosphate speciation shows definite trends in the bonding of AIO(n) groups and phosphate tetrahedra. In glasses with x < 0.48, phosphate groups share preferentially only one nonbridging O corner with an AIO(n) coordination polyhedron. For x > 0.48 more than one nonbridging O can be linked to AIO(n) polyhedra. There is no corner sharing of O between AIO(n) and PbO(n) polyhedra nor between AIO(n) themselves throughout the compositional range. The PbO(n) coordination polyhedra show considerable nonbridging O sharing, with each O participating in the coordination sphere of at least two Pb. The bonding preferences determined for Al are consistent with the behavior observed in Na-AI and Ca-AI metaphosphates, indicating this may be a general behavior for ternary phosphate glasses.

Direct observation of the R ⋯ R separation in rare-earth phosphate glasses

Rare-earth phosphate glasses exhibit exotic optical and magnetic properties, which are dictated by their structure, principally the nature of the closest R ⋯ R separation. Conventional structural characterization tools have enabled the comprehensive determination of the radial distribution from the rare-earth ion out to ∼4 Å. At or beyond this distance, however, lies the crucially important minimum R ⋯ R separation. This study illustrates three types of non-conventional diffraction experiments that overcome the radial limitations of regular structural probes. The results from such investigations are compiled with all other known estimates of R ⋯ R separations in rare-earth phosphate glasses, (R 2O 3) x (P 2O 5) 1− x , in the range 0.167 < x < 0.25, both from experimental and computational means. Coupled with tabulations of all known minimum R ⋯ R separations in analogous meta- and ultra-phosphate crystal structures, a collective comparison of all of these R ⋯ R separations permits a hypothesis by which all results are consistent. That hypothesis indicates that where rare-earth phosphate glasses are near meta-phosphate, their minimum R ⋯ R distance lies at ∼4.0 Å whilst a second, much broader, nearest-neighbour R ⋯ R correlations is centered between 6.0–6.4 Å. However, glasses that contain rare-earth ions at least as heavy as holmium are likely to exhibit a very different structure, with a minimum R ⋯ R separation of 5.4–5.6 Å. Structures that lie intermediate between the ultra-phosphate and meta-phosphate composition may display mixed characteristics of these two extremes. If present in high levels, aluminium contamination arising from the synthetic process may also affect the R ⋯ R distribution significantly. Overall, there is excellent agreement between experiment and theory.

Thermoluminescence of aluminophosphate glasses in the metaphosphate composition

Potassium aluminum phosphate (KAP) glasses in the system xKPO 3–(100 − x)Al(PO 3) 3 with x = 10, 30 and 50 mol% were prepared in the metaphosphate composition. The glasses were doped with MnO 2 and their thermoluminescent (TL) response was investigated. Raman spectra showed that these glasses did not undergo structural changes with the substitution of manganese ions. The glass composition x = 50 mol% doped with 1.0 mol% of MnO 2 presented the best TL response. The material displayed good sensitivity for γ-rays, X-rays and UV light. The emission curves exhibited two TL peaks, one at a low temperature (∼150 °C) and the other at a high temperature (∼365 °C), whose positions were dependent on the type of exciting radiation applied. The results of the present study indicated that the high temperature peak is a good candidate for TL dosimetric investigations.

Structure of rare-earth phosphate glasses by X-ray and neutron diffraction

Previous diffraction studies of the structures of rare-earth phosphate glasses (R2O3)x(P2O5)1−x are extended to glasses with smaller R3+ ions with R=Sm, Gd, Dy, Er, Yb, Y for x=∼0.25 and with R=Nd, Sm, Gd for x=∼0.15. Parameters for the P–O, R–O and O–O first-neighbor peaks were obtained by Gaussian fitting. P–P and R–P distances were estimated from the positions of peak maxima. Effects of residual silica or alumina contents present as a result of glass processing were taken into account for selected samples. The P–O coordination number, NPO, and the P–O, O–O, P–P distances are consistent with the presence of phosphate tetrahedra and are insensitive to the R species and the R2O3 content. Rare-earth coordination numbers, NRO, decrease from ∼8 to ∼6.5 when x is increased from ∼0.15 to ∼0.25. NOO and NPP decrease with increasing R2O3 content indicating the network disintegration. The numbers NRO of the metaphosphate glasses (x=∼0.25) decreases from ∼7 to ∼6 when R is changed from La to Yb. This change is also indicated by the behavior of the R–O distances and by constant number densities of atoms. The decrease in NRO with increasing R2O3 content is due to the reduction in the number of terminal O (OT) available for coordination of the R3+ ions (six at metaphosphate composition). Especially for smaller R3+ ions sharing OT between two R sites is not favored. The decrease by ∼0.04nm of the prominent R–R first-neighbor distance with a change of R from La to Yb at the metaphosphate composition is indicated by a shift to higher magnitude of scattering vector of the shoulder occurring in front of the first main diffraction peak.