Alkali Metal Borate Research Articles

Rb3MgB5O10 and LiBaAl(BO3)2: covalent tetrahedra MO4-containing borates with deep-ultraviolet cutoff edges.

Borates are favored by materials scientists and chemists because of the significant electronegativity difference between B and O atoms and their flexible assembly modes resulting in abundant structures and excellent properties. For the design of deep-ultraviolet (DUV) optical crystals with excellent macroscopic performance, it is crucial to choose appropriate cations and anionic groups and microscopically reasonable assembly patterns. Herein, by introducing covalent tetrahedra ([MO4], M = Mg, Al), two new mixed alkali metal and alkaline earth metal borates, Rb3MgB5O10 and LiBaAl(BO3)2, were synthesized using the melt method and high-temperature solution method. They contain M-B-O two-dimensional (2D) layers (2∞[MgB5O10] and 2∞[Al(BO3)2], respectively) composed of isolated B-O groups ([B5O10]5- and [BO3]3-, respectively) and metal-centered tetrahedral connectors ([MgO4]6- and [AlO4]5-, respectively). Combining experiments and theoretical calculations shows that the two compounds have short cutoff edges (<200 nm) and moderate birefringences. Further analysis manifests that the isolated [MO4] covalent tetrahedra can optimize the arrangement of anion groups, guarantee the balanced optical properties of materials, and point out the direction for further exploration of novel borate structures.

Evaluation of the Standard Entropy of Crystalline Alkali Metal Borates

A composition–property relationship has been developed that makes it possible to select the optimal values of the standard entropy of alkali metal borates, for which available experimental and reference data vary in wide ranges. This relationship allows one to estimate the standard entropy of unstudied alkali metal borates with sufficient validity. To ensure the reliability of the relationship, a critical analysis of the initial data borrowed from reference books and original experimental works has been carried out. Experimental measurements of low-temperature heat capacity were processed to verify the reliability of the standard entropy values of alkali metal borates presented in the literature.

Speciation and Phase Equilibria of Aqueous Boric Acid and Alkali Metal Borates from Ambient to Hydrothermal Conditions: A Comprehensive Thermodynamic Model

Speciation and Phase Equilibria of Aqueous Boric Acid and Alkali Metal Borates from Ambient to Hydrothermal Conditions: A Comprehensive Thermodynamic Model

Capture and Electrochemical Conversion of CO2 into Carbon Nanotubes Using Molten Alkali Metal Borates

Molten alkali metal borates have recently shown promise as high-temperature sorbents for capture of CO2 and acid gases. These molten salt sorbents enable realization of thermodynamic enhancements offered by conventional solid high temperature sorbents while resolving practical challenges such as morphological degradation. Prior studies have focused on regeneration of alkali borates through steam sweeping and thermal cycling. In this work, we demonstrate that mixed sodium-lithium borate salts as CO2 sorbents can also be regenerated electrochemically, producing valuable multiwalled carbon nanotubes (MWCNT) via electroreduction of captured CO2. Effects of cathode materials and operating conditions in CO2 electroreduction in molten sodium-lithium borate are quantified. By varying relative starting compositions of alkali borates and alkali carbonates, an optimal composition of borates and carbonates is determined, achieving high coulombic efficiencies and significantly higher CO2 uptake capacities than traditionally employed carbonate salts used for conversion of CO2 into CNTs in the desirable 550-650°C range.

Mixed Alkali Metal and Alkaline Earth Metal Scandium Borate Birefringence Material with Layered Structure and Short Ultraviolet Cutoff Edge.

Birefringent crystals are essential in the domains of linear and nonlinear optics that need light wave polarization control. Rare earth borate has become a popular study material for ultraviolet (UV) birefringence crystals due to its short cutoff edge in the UV area. RbBaScB6O12, a two-dimensional layered structure compound with the B3O6 group, was effectively synthesized through spontaneous crystallization. The UV cutoff edge of RbBaScB6O12 is shorter than 200 nm, and the experimental birefringence is 0.139 @ 550 nm. Theoretical research indicates that the large birefringence originates from the synergistic impact of the B3O6 group and the ScO6 octahedron. RbBaScB6O12 is an outstanding candidate material for birefringence crystals in the UV and even deep UV regions due to its short UV cutoff edge and significant birefringence.

Estimation of the Standard Heat Capacity of Crystalline Alkali Metal Borates

A structure–property correlation was developed that makes it possible to choose the optimal values of the standard heat capacity of alkali metal borates, for which, according to the data of various experimental works and reference publications, wide variations are observed. This correlation enables one to estimate the standard heat capacity of unstudied alkali metal borates with sufficient validity. To ensure the reliability of the correlation, a critical analysis was made of the initial data taken from reference publications and original experimental works. Our own processing of experimental measurements of the heat capacity and the enthalpy increment was performed to verify the reliability of the literature values of the standard heat capacity of alkali metal borates.

Estimation of the Standard Enthalpy of Formation of Crystalline Alkali Metal Borates

A structure–property correlation has been developed, which makes it possible to choose the optimal values of the enthalpy of formation of alkali metal borates, for which, according to the data of various experimental works and reference publications, wide variations are observed. Using this correlation, the enthalpy of formation of unstudied alkali metal borates can be reasonably estimated. It has been found that the contribution of B2O3 to the enthalpy of formation is the same not only for alkali metal borates but also for Ba, Ca, and Pb borates, for whose enthalpy of formation a structure–property correlation has also been established. This suggests the suitability of the obtained correlations for estimating the enthalpy of formation in the borate series, where the value is known only for one member of the series, as well as the possibility of estimating the enthalpy of formation of mixed borates of different metals.

Li3B8O13X (X = Cl and Br): Two New Noncentrosymmetric Crystals with Large Birefringence Induced by BO3 Units

Enthusiasm for the exploration of nonlinear alkali metal borates remains high. Focusing on the Li-B-O-X (X = Cl and Br) system, two examples of noncentrosymmetric borates, Li3B8O13Cl and Li3B8O13Br, were obtained using a high-temperature solution method under vacuum conditions. Structurally, the Li3B8O13X crystals exhibit two independent alternately arranged three-dimensional B-O network structures formed by the basic building block unit B8O16. The performance measurements demonstrate their short ultraviolet cutoff edges. The theoretical calculation indicates that the BO3 units dominate the contribution to their large optical anisotropy with the birefringence, 0.094 and 0.088@1064 nm for Li3B8O13Cl and Li3B8O13Br, respectively.

Two non-centrosymmetric mixed alkali metal and alkaline earth metal scandium borate nonlinear optical materials with short ultraviolet cutoff edges.

Rare earth borates, a subset of the essential nonlinear optical (NLO) materials, have sparked a significant amount of attention in recent years. In self-fluxing systems, two non-centrosymmetric scandium borates with classical B5O10 groups, namely Rb7SrSc2B15O30 (I) and Rb7CaSc2B15O30 (II), were successfully discovered. Both I and II exhibit a short ultraviolet (UV) cutoff edge (<200 nm) and appropriate second-harmonic generation efficiency (∼0.76 × KH2PO4, ∼0.88 × KH2PO4 at 1064 nm, respectively). According to theoretical calculations, it is speculated that the band gap and NLO characteristics of these two compounds are mostly derived from the B5O10 group and the ScO6 octahedron. Due to the short cutoff edges of I and II, they may be considered as potential NLO materials in the UV and even deep UV spectral ranges. Furthermore, the advent of I and II adds to the diversity of rare earth borates.

Two Mixed Alkali-Metal Borates Templated from Cations to Clusters

Two mixed alkali-metal borates, K0.5Li[B6O10]·0.5H3O (1) and [(μ5-OH)@(Na4Li)]0.5[B6O10]·0.5B(OH)3 (2), have been prepared under solvothermal conditions. Both of them are obtained in the same synthetic system and contain a B6O138- cluster as the structural-building unit (SBU) but exhibit quite different structural features. 1 is a centric three-dimensional (3D) oxoboron (B-O) framework, where templated mixed K+ and Li+ cations occupied the cavities of the structure. 2 crystallizes in an acentric space group under the templating effect of a unique acentric alkali-metal cluster [(μ5-OH)@(Na4Li)]4+. The SBU of 2 is also the B6O138- cluster, which acts as six-connected nodes linked together to form a 3D B-O framework, showing different characters from 1 because of two types of templates; the acentric [(μ5-OH)@(Na4Li)]4+ clusters and the electroneutral B (OH)3 groups fill in two different cages in the framework and further connect each other via Na-O-B bonds to build a novel two-dimensional (2D) wavy bricklike network, resulting in a 3D B-O framework interpenetrated by a 2D [(μ5-OH)@(Na4Li)]-B(OH)3 network. As a crystal material with an acentric space group, 2 shows a good second harmonic generation response of about 2.8 times that of KDP (KH2PO4) and has a cutoff edge below 190 nm, which suggests that 2 is a potential deep-UV NLO material.

Comments on the paper “Insight on the growth and property studies of inorganic hydrated borate (Na6[B4O5(OH)4]3·8H2O) single crystal ̶ An effective third order nonlinear optical (NLO) material for optical limiting application”

The authors of the title paper (Mater. Chem. Phy. 205 (2018) 138–146) claimed to have grown for the first time a new alkali metal borate with formula Na6[B4O5(OH)4]3·8H2O (I) by slow evaporation of an aqueous solution containing NaOH, H3BO3 and C3H4O4 in 3:1:1 mol ratio. In this letter to the Editor, many points of criticism concerning the crystal growth, structure characterization and interpretation of the spectral and thermal data of I are described to prove that it is improperly characterized.

Synthesis of Hexagonal Boron Nitride Coatings Using Concentrated Solutions of Alkali Metal Borates

Continuous nanocrystalline coatings of hexagonal boron nitride (h-BN) with a thickness of ~1000‒350 nm are prepared from highly concentrated aqueous solutions of lithium and sodium borates, which display good film-forming properties. The borate solutions were applied on silicon substrates by spin coating followed by drying in air and treatment in ammonia at a temperature of 1300°C. This method is deemed promising for creating protective coatings (e.g., for protecting carbon products from oxidation at elevated temperatures).

Understanding Material Compatibility in CO2 Capture Systems Using Molten Alkali Metal Borates.

Molten alkali metal borates have been proposed as energy-efficient sorbents for the low-cost capture of CO2 at high temperatures. The molten sorbents could help to mitigate global warming by capturing CO2 from industrial sources and preventing the release of CO2 into the atmosphere. However, these novel materials operate under harsh conditions, introducing challenges of which material compatibility is one of the most important. Other than platinum, where a less than 0.1% change in performance was observed over 1000 h of continuous use, few materials were found to be compatible with the molten salts. Common ceramics, steels, and superalloys were eliminated from consideration due to corrosive oxidation of the substrate and contamination of the melt resulting in chemical degradation and reduction in the sorbent's working capacity. A high-purity nickel alloy, Nickel 200/201, with a protective oxide layer was found to perform optimally with regards to both corrosive degradation and chemical degradation. Modest corrosion rates on the order of 0.3-0.5 mm/year were estimated, and the sorbent capacity was found to drop by between a manageable 0.5 and 20% over 100 h. Various protective measures are proposed, and future work suggested, to ensure that material compatibility does not limit the potential of molten alkali metal borates to reduce CO2 emissions and contribute to a clean energy future.

Structural response of alkali metal borates at Fe2O3 sliding interface: The effect of alkali cations

Structural transformation and chemical reaction induced by sliding motion is an interesting topic in material science and tribochemistry. In this paper, we used density functional theory (DFT) to study the adaptation of molecular and electronic structures at the iron oxide–alkali borate interface. Sodium borates with different sodium concentrations and two other common alkali metal borates of lithium and potassium have been confined between two Fe2O3 surfaces and subjected to the sliding at 1 and 38GPa. The results suggest a significant impact of alkali elements on the energy shape and bonding behavior in the systems. The low energy variation was obtained in the systems with sodium borate at 33% and 50% concentration of Na2O. Meanwhile, high variation energy was encountered with the sodium borate with low Na2O concentration (25% Na2O), lithium borate and potassium borates at 3 GPa. The formation of an “easy-shear” pattern was found in the lowest energy variation system with 50% Na2O at 1 GPa. The high variation energy was closely related to the dissociation of iron atoms from the oxide surface and the subsequent rupture of Fe−O bonds of the surface. Based on the current simulations, the possible interfacial layering has been proposed to rationalize experimental findings on the friction and wear of borate lubricated iron oxide surfaces.

Li4Ca2B8O16: A Borate with a Unique Fundamental Building Block and a Short Cutoff Edge.

A complex alkali and alkaline-earth metal borate Li4Ca2B8O16 has been synthesized successfully via the high-temperature solution method. Li4Ca2B8O16 crystallizes into the space group P1̅ (No. 2) of the triclinic crystal system. The crystal structure exhibits a three-dimensional framework consisting of the [B8O16]∞ chains connected by the [LiOn] (n = 3, 4, 6) and [CaO7] polyhedra. The fundamental building block [B8O18] differs from those of other anhydrous octa-borates, which can be identified as a unique one. Furthermore, Li4Ca2B8O16 shows three different Li-O units, which is unique among all non-disordered anhydrous borates. It shows a short UV cutoff edge less than 190 nm. To better study the relationships between the crystal structure and properties, the DFT calculations were used for the evaluations of the optical band gap and birefringence.

Bench-Scale Demonstration of Molten Alkali Metal Borates for High-Temperature CO2 Capture

The molten alkali metal borates represent an opportunity for carbon dioxide capture and the on-going challenge of adapting to a carbon-constrained world with the immediacy of climate change. We demonstrate the performance of this new class of materials at the bench scale, highlighting the ability to operate isothermally at high temperatures through the use of steam as a sweep gas. Breakthrough curves for a wide set of conditions are presented and linked to the sorbents’ underlying thermodynamic and kinetic properties. We demonstrate the ability to capture over 99.9% of incoming CO2 even at low concentrations or to capture ∼90% CO2 under high flow rates and utilize a greater portion of the sorbent capacity. The advantages of the molten alkali metal borates scale well, and their unique position as high-temperature liquid phase sorbents may enable new process designs for efficient low-cost CO2 capture facilities.

Acid Gas Capture at High Temperatures Using Molten Alkali Metal Borates.

Materials designed for CO2 capture provide both an opportunity and a challenge in that industrial emissions typically contain an assortment of acid gasses, which may include SOx and NOx alongside CO2. Growing pressure to reduce emissions of all acid gasses, CO2 included, presents an opportunity for simultaneous capture and a challenge in handling the resultant products. Molten alkali metal borates embody a new class of high-temperature liquid-phase materials for carbon dioxide capture and we propose here that they can also be used to address the more general challenge of acid gas capture. We examine the melt capture performance at industrially relevant concentrations and mixtures, identifying the various reaction mechanisms and products, and propose designs for separating these products efficiently at high temperatures, so that they outperform the state-of-the-art CO2 capture technologies in handling this opportunity challenge. We also discuss the conditions to avoid and the challenges that lie ahead for these materials in the context of emission reduction and environmental protection.

Neutron scattering studies of static and dynamic correlation lengths in alkali metal borate glasses

This paper reports the variation in the first sharp diffraction peak (FSDP) and the boson peak (BP) of lithium and cesium borate glasses xM2O-(1-x)B2O3 (M = Li and Cs, x = 0.08. 0.14, and 0,22) investigated by elastic and inelastic neutron scattering. The different content dependences between lithium and cesium borate glasses are discussed on the basis of the different modifications of the boron-oxide network structure by the small Li and large Cs ions. The static characteristic correlation lengths Rfsdp were determined from the width of the FSDP of the static structure factor S(Q). The dynamic characteristic correlation lengths Rbp were determined from the BP energy of the dynamic structure factor S(E). They are close in absolute value (about 2 nm). With the increase in the alkali metal contents, Rfsdp increases, while Rbp gradually decreases with the different rate in the lithium and cesium glasses. However, their ratio, Rfsdp/Rbp increases with the content of the metal in both the lithium and cesium glasses at the same rate. The decrease in the amplitude of elastic fluctuations with the increasing x due to the increase in fragility with the increasing x might restore the correlation between Rfsdp and Rbp in the lithium borate glasses, and this tendency may be the same for the cesium borate glasses.

Four new deep ultraviolet borates with isolated B12O24 groups: Synthesis, structure, and optical properties

Four new deep ultraviolet (DUV) borates Li7Na2KRb2B12O24, Li7.35Na2.36K1.50Cs0.78B12O24, Li6.97Na2.63K1.24Cs1.15B12O24, and Li7.27Na2.67Rb2.06B12O24 have been produced by high temperature solution method in open air. The structural analysis suggests that they all crystallize in the same trigonal space group R3¯ (No. 148), and possess the identical isolated B12O24 groups. Structural comparison among alkali-metal borates with BnO2n in their chemical formulas have been discussed. In addition, infrared spectra were used to verify their structural validity. Furthermore, the UV–Vis–NIR diffuse reflectance spectra reveal that they exhibit short cutoff edges below 180 nm (6.88 eV). The first-principles theoretical studies carried out to increase the understanding of electronic structure and linear optical property.

Molten ionic oxides for CO2 capture at medium to high temperatures

Molten ionic oxides based on sodium borate and mixed alkali-metal borates show remarkably fast sorption kinetics and intrinsic regenerability as liquid absorbents for CO2 capture at medium to high temperatures