Influence of B2O3 on viscosity and structure of the CaO–Al2O3-waste electrolyte (WE) based melt

The fluorides contained waste electrolyte (WE) from the electrolytic aluminum industry can be used as a substitution of fluorite (CaF₂) in the newly designed mold flux. In this study, the influence of B₂O₃ on viscosity and structure of CaO–Al₂O₃-WE based melt was investigated. Results show that the viscosity of mold flux melt decreases with both increasing temperature and B₂O₃ content. The apparent activation energy (E_a) also reduces from 78.96 ± 1.75 to 55.26 ± 2.79 kJ/mol with the addition of B₂O₃ from 0 to 7 wt%. The analyses of fourier transform infrared (FTIR) and Raman spectroscopies suggest that the lower symmetry of the original aluminate and silicate structure due to the insertion of [BO₄]-tetrahedral and [BO₃]-triangular, and the formation of more non-bridging oxygen (O⁻) and 2D structural units in the network with the addition of B₂O₃, deceases the viscosity and E_a of the CaO–Al₂O₃-WE Based Melt.     

Investigation of viscosity and structure of CaO-SiO2-MgO-Al2O3-BaO-B2O3 slag melt

The role of B₂O₃ in the viscosity and structural variations of CaO-SiO₂-MgO-Al₂O₃-BaO-B₂O₃ slag melt was examined using the rotating cylinder method, Raman spectroscopy, and ²⁷Al and ¹¹B magic angle spinning nuclear magnetic resonance (MAS-NMR) spectra. The results showed that the viscosity of the slag decreased constantly with an increment in the B₂O₃ content although the polymerisation degree of slag enhanced. The addition of B₂O₃ induced the transformation from Q⁰ and Q¹ units to Q² and Q³ units in the Si-related structure. The concentration of [AlO₄] structural units increased while that of [AlO₅] and [AlO₆] units decreased. ¹¹B MAS-NMR spectra revealed that [BO₃]-trihedral units with a two-dimensional (2D) structure dominated the B-related structural units. The structural analysis confirmed the decrease in strength and stability of the entire network structure had a more significant effect on the viscosity of the slag due to the addition of B₂O₃.     

Effect of Na2O on the High‐Temperature Thermal Conductivity and Structure of Na2O–B2O3 Melts

The effect of Na₂O and temperature on the thermal conductivity of the Na₂O–B₂O₃ binary system has been measured using the hot‐wire method to examine the relationship between the thermal conductivity and structure in high‐temperature melts. The thermal conductivity of the binary melt is measured from 1173 to 1473 K in the fully liquid state. The thermal conductivity slightly increases with Na₂O content up to 20 wt%. Above 20 wt% Na₂O, the thermal conductivity decreases with increasing Na₂O. The network structure of molten glass was analyzed using Fourier transform infrared (FTIR), Raman spectroscopy, and XPS. The FTIR analysis shows that 3‐D complex borate structures, such as tri‐, tetra‐, and pentaborate are made by [BO₄] tetrahedral units interconnected with 2‐D structure boroxol rings in the low Na₂O region. Above 20 wt% Na₂O content, nonbridged oxygen in [BO₂O^?] units and diborate groups increase with increase in Na₂O. The same tendency is shown by the Raman spectroscopy and XPS analyses. The Raman analysis shows that boroxol rings disappeared with large [BO₄] groups, such as tri‐, tetra‐, and pentaborate structures, which increase at low Na₂O content. Isolated diborate groups and nonbridged oxygen in [BO₂O^?] units increase at high Na₂O content. It can be inferred that single structure units, such as isolated diborate groups, interfere with conduction. The XPS analysis results show that free oxygen produced by the interconnection of Na₂O in the borate structure does not cause significant changes to O^2? in the low Na₂O region, but increases the O^o and decreases the O^?. Above 20 wt% Na₂O, O^? slightly increases and O^o shows a decreasing trend.     

Influence of MeF2 (Me = Mg, Ca, Sr, and Ba) on the electrical properties of glasses in the MeF2-Na2B4O7 system

Glasses in the MeF₂-Na₂B₄O₇ (Me = Mg, Ca, Sr, and Ba) system have been synthesized. It is shown that the glass formation is observed at a MeF₂ content of up to 40 mol %. The influence of the MeF₂ content on the electrical conductivity and the fluorine concentration in the glass bulk is examined. From the analysis of the concentration dependence of the electrical conductivity with due regard for the fluorine content, it is concluded that the glass structure is predominantly built up of the polar groupings Na⁺[BO_4/2]^-, Na⁺[F-BO_3/2], Me _1/2 ²⁺ [BO_4/2]⁻, Me _1/2 ²⁺ [F⁻BO_3/2], [MeF_4/2], and [MeF_6/3] and the BO_3/2 nonpolar structural-chemical units. The electricity transport is governed by the migration of sodium ions formed upon dissociation of the Na+[BO_4/2]^-and Na+[F^-BO_3/2] groupings. An increase in the MeF₂ content leads to a decrease in the total concentration of sodium ions, a decrease in the Na⁺[BO_4/2]^- concentration, and an increase in the Na⁺[F^-BO_3/2] concentration. Upon introduction of MeF₂ up to ∼20 mol %, the fluorine losses during the synthesis are caused by the dehydration of glass melt. An addition of 20–25 mol % MeF₂ brings about the saturation of the glass by the [F^-BO_3/2]-type structural units, so that the fluorine concentration reaches a saturation in the structures of calcium-, strontium-, and barium-containing glasses and increases in magnesium-containing glasses, owing to the formation of the [MgF+_6/3] groupings.     

Influence of MeF2 (Me = Mg, Ca, Sr, and Ba) on the electrical properties of glasses in the MeF2-Na2B4O7 system

Glasses in the MeF₂-Na₂B₄O₇ (Me = Mg, Ca, Sr, and Ba) system have been synthesized. It is shown that the glass formation is observed at a MeF₂ content of up to 40 mol %. The influence of the MeF₂ content on the electrical conductivity and the fluorine concentration in the glass bulk is examined. From the analysis of the concentration dependence of the electrical conductivity with due regard for the fluorine content, it is concluded that the glass structure is predominantly built up of the polar groupings Na⁺[BO_4/2]^-, Na⁺[F-BO_3/2], Me _1/2 ²⁺ [BO_4/2]⁻, Me _1/2 ²⁺ [F⁻BO_3/2], [MeF_4/2], and [MeF_6/3] and the BO_3/2 nonpolar structural-chemical units. The electricity transport is governed by the migration of sodium ions formed upon dissociation of the Na+[BO_4/2]^-and Na+[F^-BO_3/2] groupings. An increase in the MeF₂ content leads to a decrease in the total concentration of sodium ions, a decrease in the Na⁺[BO_4/2]^- concentration, and an increase in the Na⁺[F^-BO_3/2] concentration. Upon introduction of MeF₂ up to ∼20 mol %, the fluorine losses during the synthesis are caused by the dehydration of glass melt. An addition of 20–25 mol % MeF₂ brings about the saturation of the glass by the [F^-BO_3/2]-type structural units, so that the fluorine concentration reaches a saturation in the structures of calcium-, strontium-, and barium-containing glasses and increases in magnesium-containing glasses, owing to the formation of the [MgF+_6/3] groupings.     

Characterization of immiscibility in calcium borosilicates used for the immobilization of Mo6+ under Au-irradiation

The aim of this paper was to assess factors affecting primary and secondary phase separation in simplified calcium borosilicate glasses studied for nuclear waste applications. Several glasses with varying [MoO₃] and [B₂O₃] were synthesized and exposed to Au-irradiation to examine compositional effects on glass structure and domain size of separated phases induced by accumulated radiation damage resulting from α-decay over a ~1000 year timeframe. The produced glasses fell within the immiscibility dome of CaO−SiO₂−B₂O₃ and showed a unique microstructure of embedded immiscibility with three identifiable amorphous phases according to electron microscopy, Raman spectroscopy, and diffraction. These glasses were then bombarded with 7 MeV Au³⁺ ions to a dose of 3 × 10¹⁴ ions/cm² creating an estimated ~1 dpa of damage. Several changes to the morphology, spatial distribution, and size of secondary phases were observed, indicative of significant structural reorganization and changes to the chemical composition of each phase. A general mechanism of coalescence to form larger particles was observed for [MoO₃] < 2.5 mol%, whereas segregation to form smaller more evenly distributed particles was seen for [B₂O₃] ≤ 15 mol% and [MoO₃] ≥ 2.5 mol%. These microscopic changes were concurrent to surface-bulk diffusion of Ca and/or Mo ions, where the direction of diffusion was dependent on [B₂O₃] with a barrier identified at ~20 mol%, as well as cross-phase diffusion of said ions. These modifications occurred in part through the formation of distorted ring structures within the borosilicate network, which enabled the increased dissolution of isolated (MoO₄)²⁻ units. Au-irradiation was therefore able to increase the solubility of molybdenum and alter the structure and composition of secondary phases with the extent of modification varying with [MoO₃] and [B₂O₃]/[SiO₂], though glasses notably remained heterogeneous. The collective results suggest that radiation and composition can both be used as design tools to modulate the domain size and distribution of separated phases in heterogeneous glasses.     

Effect of topological structure on photoluminescence of PbSe quantum dot‐doped borosilicate glasses

Borosilicate glasses doped with PbSe quantum dots (QDs) were prepared by a conventional melt‐quenching process followed by heat treatment, which exhibit good thermal, chemical, and mechanical stabilities, and are amenable to fiber‐drawing. A broad near infrared (NIR) photoluminescence (PL) emission (1070‐1330 nm) band with large full‐width at half‐maximum (FWHM) values (189‐266 nm) and notable Stokes shift (100‐210 nm) was observed, which depended on the B₂O₃ concentration. The PL lifetime was about 1.42‐2.44 μs, and it showed a clear decrease with increasing the QDs size. The planar [BO₃] triangle units forming the two‐dimensional (2D) glass network structure clearly increased with increasing B₂O₃ concentration, which could accelerate the movement of Pb²⁺ and Se^2? ions and facilitate the growth of PbSe QDs. The tunable broadband NIR PL emission of the PbSe QD‐doped borosilicate glass may find potential application in ultra‐wideband fiber amplifiers.     

Effect of fluorine and CaO/Al2O3 mass ratio on the viscosity and structure of CaO–Al2O3-based mold fluxes

The effect of CaO/Al₂O₃ mass ratio (C/A) and fluorine content on the viscosity and structure of CaO–Al₂O₃-based mold fluxes has been researched in this paper. The viscosity results indicated that increasing fluorine only slightly decreases the viscosity of the slag melt, and higher C/A is also observed to decrease the viscosity of molten slag when the C/A changes from 1.3 to 1.7. Structural analysis of the as-quenched fluxes using the Raman spectroscopy showed that the amounts of Al–O⁰ and Si–O–Al structural units all decrease with higher fluorine content and C/A, indicating that a depolymerization of the molten structure is occurring. The results of ²⁷Al and ¹⁹F magic angle spinning nuclear magnetic resonance showed that fluorine tends to participate in the network structure and coordinate with Al³⁺ ions to form complex ionic clusters. The results suggested that the role of fluorine in the CaO–Al₂O₃-based slag system is different from the traditional slag system in which fluorine only acts as a diluent, thus reducing the effect of fluorine on lowering the viscosity. In addition, the coordination environment of Al³⁺ ions can be simplified by higher C/A through promoting the generation of [AlO₄]⁻ tetrahedral structures. Besides, the free O²⁻ ions provided by excess CaO would break the Al–O⁰ bonds and further depolymerize the network structure, thereby decrease the viscosity.     

Assessment of interatomic parameters for the reproduction of borosilicate glass structures via DFT-GIPAW calculations

Borates and borosilicates are potential candidates for the design and development of glass formulations with important industrial and technological applications. A major challenge that retards the pace of development of borate/borosilicate based glasses using predictive modeling is the lack of reliable computational models to predict the structure-property relationships in these glasses over a wide compositional space. A major hindrance in this pursuit has been the complexity of boron-oxygen bonding due to which it has been difficult to develop adequate B–O interatomic potentials. In this article, we have evaluated the performance of three B–O interatomic potential models recently developed by Bauchy et al [J. Non-Cryst. Solids, 2018, 498, 294–304], Du et al [J. Am. Ceram. Soc. https://doi.org/10.1111/jace.16082 ] and Edèn et al [Phys. Chem. Chem. Phys., 2018, 20, 8192–8209] aiming to reproduce the short-to-medium range structures of sodium borosilicate glasses in the system 25 Na₂O x B₂O₃ (75 − x) SiO₂ (x = 0-75 mol%). To evaluate the different force fields, we have computed at the density functional theory level the NMR parameters of ¹¹B, ²³Na, and ²⁹Si of the models generated with the three potentials and the simulated MAS NMR spectra compared with the experimental counterparts. It was observed that the rigid ionic models proposed by Bauchy and Du can both reliably reproduce the partitioning between BO₃ and BO₄ species of the investigated glasses, along with the local environment around sodium in the glass structure. However, they do not accurately reproduce the second coordination sphere of silicon ions and the Si–O–T (T = Si, B) and B-O-T distribution angles in the investigated compositional space which strongly affect the NMR parameters and final spectral shape. On the other hand, the core-shell parameterization model proposed by Edén underestimates the fraction of BO₄ species of the glass with composition 25Na₂O 18.4B₂O₃ 56.6SiO₂ but can accurately reproduce the shape of the ¹¹B and ²⁹Si MAS-NMR spectra of the glasses investigations due to the narrower B–O–T and Si-O-T bond angle distributions. Finally, the effect of the number of boron atoms (also distinguishing the BO₃ and BO₄ units) in the second coordination sphere of the network former cations on the NMR parameters have been evaluated.     

BaO−B2O3 system and its mysterious member Ba3B2O6

The first systematic study of the BaO–B₂O₃ system and barium orthoborate Ba₃B₂O₆ (3BaO·B₂O₃) was reported in 1949. Thereafter, the system was repeatedly refined but the structure of Ba₃B₂O₆ compound has not been adequately studied yet. In our study we have, for the first time, obtained the crystalline samples of Ba₃B₂O₆. The solved structure (Pbam, a = 13.5923(4) Å, b = 13.6702(4) Å, c = 14.8894(3) Å) belongs to the class of ‘anti‐zeolite’ borates with a pseudotetragonal [Ba₁₂(BO₃)₆]⁶⁺ cation pattern which contains channels along the c axis filled with anionic clusters. The Ba₃B₂O₆ compound may be regarded as a fluorine‐free end‐member of the Ba₃(BO₃)_2–xF_3x solid solution. The BaO–B₂O₃ phase diagram presented in our study is based on our research and literature data.     

Structure and crystallization behavior of complex mold flux glasses in the system CaO-Na2O-Li2O-CaF2-B2O3-SiO2: A multi-nuclear NMR spectroscopic study

The structure of mold flux glasses in the system CaO-(Na,Li)₂O-SiO₂-CaF₂ with unusually high modifier contents, stabilized by the addition of ∼4 mol% B₂O₃, is studied using ⁷Li, ²³Na, ¹⁹F, ¹¹B, and ²⁹Si magic-angle-spinning (MAS), and ⁷Li{¹⁹F} and ²³Na{¹⁹F} rotational echo double-resonance (REDOR) nuclear magnetic resonance (NMR) spectroscopy. When taken together, the spectroscopic results indicate that the structure of these glasses consists primarily of dimeric [Si₂O₇]⁻⁶ units that are linked to the (Ca,Na,Li)-O coordination polyhedra, and are interspersed with chains of corner-shared BO₃ units. The F atoms in the structure are exclusively bonded to Ca atoms, forming Ca(O,F)_n coordination polyhedra. This structural scenario is shown to be consistent with the crystallization of cuspidine (3CaO·2SiO₂·CaF₂) from the parent melts on slow supercooling. The progressive addition of Li to a Na-containing base composition results in a corresponding increase in the undercooling required for the nucleation of cuspidine in the melt, which is attributed to the frustrated local structure caused by the mixing of alkali ions.     

Incorporation limit of MoO3 in sodium borosilicate glasses

Optimizing the concentration of molybdenum incorporated in a borosilicate glass matrix is essential in the vitrification of high-level radioactive waste. However, the incorporation limit of MoO₃ in fundamental borosilicate systems has been rarely correlated with the local structure of the molybdenum cations. This study investigates the variations in the incorporation limit of MoO₃ in ternary sodium borosilicate glass upon varying the B₂O₃/(SiO₂ + B₂O₃) ratio (i.e., B)_. The incorporation limit of MoO₃ was less than 3 mol% in the low-B region (B < 0.7), where molybdenum cations mainly existed as [MoO₄]²⁻. However, when B was higher than 0.85, the incorporation limit was higher than 6 mol%, and the Raman spectra indicated the presence of octahedrally coordinated molybdenum cations, essential to stabilize the Mo–O–Mo linkage. The variation in the local structure of molybdenum cations can be explained by the available amount of non-framework cations compensating for the negative charge near [MoO₄]²⁻. These results allow the development of glass compositions with a high incorporation limit of MoO₃ simply by controlling the local structure near the molybdenum cations.     

A new polyborate anion, [B7O9(OH)6]3−: Self assembly,XRD and thermal properties of s-fac-[Co(dien)2][B7O9(OH)6]·9H2O

The title compound, s-fac-[Co(dien)₂][B₇O₉(OH)₆]·9H₂O (1) (dien = HN(CH₂CH₂NH₂)₂), has been prepared as a crystalline solid in moderate yield (35%) from the reaction of B(OH)₃ with [Co(dien)₂][OH]₃ in aqueous solution (10:1 ratio). The structure contains a novel polyborate anion [B₇O₉(OH)₆]^3 − which is structurally based on the known ‘ribbon’ isomer of [B₇O₉(OH)₅]^2 −, with an additional [OH]⁻ group coordinated to a B atom in one of the outer boroxole rings. Compound 1 is formed by a self-assembly process in which the cation and anion mutually template themselves from equilibrium mixtures under reaction conditions. The [B₇O₉(OH)₆]^3 − anions are H-bonded to each other in layers with ‘cavities’ suitable for the [Co(dien]₂]^3 + complex. Three [B₇O₉(OH)₆]^3 − anions are in the secondary coordination sphere (via H-bonds) of each cation, with each anion H-bonded to three cations.     

Local structure of network modifier to network former ions in soda‐lime alumino‐borosilicate glasses

The structure of soda‐lime alumino‐borosilicate glass was studied using molecular dynamics simulations of samples of varying compositions containing ~20 000 atoms each. Pair distribution functions (PDFs) of cations to oxygen were used for comparison to available experimental data to evaluate consistency between simulations and experiment. Additional PDFs and coordination of the network forming cations (Al/B/Si) to network modifiers (Ca/Na) were examined, which is difficult to measure experimentally. The results are consistent with available experimental data regarding cation‐oxygen bond lengths and network former to oxygen coordination numbers. Si and Al are predominantly 4‐coordinated, with a small concentration of overcoordinated species similar to experimental data. B varied as 3‐coordinated, BO₃, and 4‐coordinated, BO₄, as a function of the amount of Ca²⁺ and Na⁺ present, the ratio of Al₂O₃ to B₂O₃, and the fictive temperature of the sample, similar to experimental data. The simulations provide new information regarding the locations on the network modifiers to the +3 cations, Al and B. For instance, one Al ion can have multiple Na within 4 Å, but also the Na can be within 4 Å of several +3 cations. Such results would indicate a greater complexity of local structure that goes beyond the stoichiometric one +1 modifier ion near one +3 network former or one +2 modifier near two +3 formers in tetrahedral sites.     

Formation of Mixed Bond‐Angle Linkages in Zinc Boromolybdate Glasses

The short and medium range structure of glassy MoO₃–ZnO–B₂O₃ has been studied by neutron diffraction and reverse Monte Carlo simulation. The partial atomic pair correlation functions and coordination numbers are presented that are not yet reported for this system. We have established that the first neighbor distances do not depend on concentration within limit of error, the actual values are r_B‐O = 1.38 Å, r_Mo‐O = 1.72 Å, and r_Zn‐O = 1.97 Å. It is found that ZnO takes part in the glassy structure as network former, as ZnO₄ tetrahedral are linked both to MoO₄ and to BO₃ and BO₄ groups. It is revealed that BO₄/BO₃ increases with increasing B₂O₃ content. We have found that only small amount of boroxol ring is present, BO₃ and BO₄ groups are organized into superstructure units, and a small part is in isolated BO₃ triangles. The BO₃ and BO₄ units are linked to MoO₄ or ZnO₄ forming mixed ^[4]Mo‐O‐^[3]B, ^[4]Mo‐O‐^[4]B, ^[4]Mo‐O‐^[4]Zn, ^[3]B‐O‐^[4]Zn, ^[4]B‐O‐^[4]Zn bond linkages.     

Rheological behavior of the CaO-Al2O3-based mold fluxes with different Na2O contents

The CaO-Al₂O₃-based mold flux is expected during the casting of aluminum containing advanced high strength steels. In this study, the rheological behavior of the CaO-Al₂O₃-based mold fluxes with different Na₂O contents was investigated. The results show that the viscosity in the range of 1423–1573 K reduced sharply when the Na₂O content increased from 0 wt % to 10 wt %, but the tendency slowed down with further increasing Na₂O from 10 wt % to 15 wt %. The activation energy for viscous flow also decreased from 237.76 ± 4.88 kJ/mol to 180.37 ± 7.10 kJ/mol with increasing Na₂O. The structure analyses show that the melt networks were mainly constructed by [SiO₄]^4--tetrahedral, [AlO₄]⁵-tetrahedral and Si-O-Al structural units. These networks were depolymerized with the addition of Na₂O since the charge compensation effect from Na⁺ was relativity weaker comparing with the network breaking effect from O²⁻. In addition, the break temperature of the mold fluxes also decreased from 1406 K to 1198 K due to the more precipitation of low melting point Ca₂Al₂SiO₇, rather than MgAl₂O₄ in the mold flux, during the cooling cycle.     

Performance and transition mechanism from acidity to basicity of amphoteric oxides (Al2O3 and B2O3) in SiO2–CaO–Al2O3–B2O3 system: A molecular dynamics study

Amphoteric oxides (Al₂O₃ and B₂O₃) represent opposite effects on the structure and properties of silicate melts in different conditions, while the understanding about the transition from acidity to basicity is far from complete. Molecular dynamics simulation was adopted in the present study to investigate the performance and acidity-basicity transformation of Al₂O₃ and B₂O₃ in the SiO₂–CaO–Al₂O₃–B₂O₃ system. The results showed that, different from Ca²⁺ ions, excessive Al³⁺ or B³⁺ ions tend to destroy the bridge oxygen structures, showing the function of basic oxide. This is similar to the behavior of Ca²⁺ ions and other basicity ions. It was found that, on the one hand, B³⁺ ions tend to form [BO₃]^3- planar triangular structures with the increase of B³⁺ ions contents, on the other hand, B³⁺ ions could reduce the stability of Si–O bonds. Therefore, B³⁺ ions could make the system structure less stable, which is the reason why the B₂O₃ is a kind of active agent. In addition, because of the significant differences in lattice energy and atomic structure between Al₂O₃ and B₂O₃, the effects of Al₂O₃ and B₂O₃ on the thermodynamic properties of silicate melts are quite different.     

Electrical Properties and the Structure of Glasses in the xNa2O–(1 – x)2PbO · B2O3 System

The temperature–concentration dependences of the electrical conductivity and the activation energy for electrical conduction of glasses in the Na₂O–B₂O₃ and Na₂O–2PbO · B₂O₃ systems are studied. The investigation into the nature of the electrical conduction in these glasses reveals that the contribution from the electronic component (10^–3%) of the conductivity is within the sensitivity of the Liang–Wagner technique. A considerable alkali conductivity is observed upon introduction of more than 12 mol % Na₂O. The true transport number of sodium _Na is as large as unity at [Na₂O] 15 mol %. It is shown that the observed temperature–concentration dependences of the electrical and transport properties are governed by the ratio between the concentrations of polar and nonpolar structural–chemical units of the Na⁺[BO_4/2]^–, Na⁺[O^–BO_2/2] Na⁺[O^–BO_2/2], Pb²⁺ _1/2[BO_4/2]^–, Pb²⁺ _1/2[O^–BO_2/2], and [BO_3/2] types.     

Structure and crystallization of ZnO-B<Subscript>2</Subscript>O<Subscript>3</Subscript>-P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

Structure and crystalline behavior of the ternary system ZnO-B₂O₃-P₂O₅ glasses were investigated by means of X-ray diffraction (XRD) and infrared Raman spectra. The research showed that number of the planar [BO₃] units increases with the increase of B₂O₃ content. When the B₂O₃ content is above ≥10 mol %, the relative content of planar [BO₃] units increases rapidly and causes weakening of the glass structure and decrease in the chemical stability. In the crystallized glasses the predominant crystal phase Zn₂P₂O₇ decreases with the increase of B₂O₃ content, while the crystal phase BPO₄ increases with it, which cause the declining of chemical stability and the decrease of thermal coefficients of expansion.     

Optical properties of Eu3+/Pr3+ doped Zn4O(BO2)6 synthesized by solid-state reaction

Zn₄O(BO₂)₆ based on the [B₂₄O₄₈] sodalite-cage structure fixed by the inside [Zn₄O₁₃] clusters is expected to be a new class of solid-state lighting material with perfect thermal and mechanical stability. Herein, in the current work, we have respectively introduced non-equivalent rare-earth cations Eu³⁺ and Pr³⁺ into Zn₄O(BO₂)₆ host to design white and green emission materials by a novel solid-phase sintering method at lower temperatures. Zn₂B₆O₁₁ replacing B₂O₃ or H₃BO₃ as raw materials can effectively avoid the impure products caused by the uncontrollable volatilization of B₂O₃ or H₃BO_3. The newly designed light-emitting materials of Zn_4(1-x)O(BO₂)₆: xRe³⁺ (ReEu or Pr), including Zn₄O(BO₂)₆ host, have good absorption capacity in the ultraviolet region. Under ultraviolet irradiation, Zn₄O(BO₂)₆, Zn_4(1-x)O(BO₂)₆: xEu³⁺and Zn_4(1-x)O(BO₂)₆: xPr³⁺ emit the blue, white and green lights, respectively. In addition, all these materials can effectively degrade methylene blue, in which Zn_4(1-x)O(BO₂)₆: xPr³⁺ has the highest efficiency. The luminescence and degradation mechanisms of Zn₄O(BO₂)₆, Zn_4(1-x)O(BO₂)₆: xEu³⁺and Zn_4(1-x)O(BO₂)₆: xPr³⁺ have been adequately explained by their electronic structures based on the first principle calculations. The current study confirms that the doping of Eu³⁺/Pr³⁺ in Zn₄O(BO₂)₆ can broaden its applications as photoluminescent and photocatalytic materials.