(K,Na)2[AsB6O12]2[B3O3(OH)3], a New Microporous Material, and Its Comparison to Teruggite

Yulia A Pankova,Sergey V Krivovichev

doi:10.3390/min9120781

Abstract

Single crystals of the novel boroarsenate (K,Na)2[As2B12O24][B3O3(OH)3] (I) were obtained using the borax flux method. The crystal structure of I was found to be triclinic, P-1, a = 8.414(5), b = 10.173(6), c = 15.90(1) Å, α = 79.56(1), β = 78.68(1), γ = 70.91(1), V = 1251(1) Å3, Z = 2. The crystal structure of I is based upon the novel [AsB6O12]− microporous boroarsenate framework formed by B and As coordination polyhedra. This framework can be subdivided into borate units that are interlinked by AsO4 tetrahedra. In the case of I, the borate substructure is a chain consisting of triborate rings, ☐2Δ, formed by two (BO3) triangles and one (BO4) tetrahedron connected through shared common oxygen atoms. The chains are extended along [0 1 ¯ 1] and are interlinked by (AsO4) tetrahedra in the [011] direction. As a result, the framework has large channels parallel to [100], having an effective diameter of 4.2 × 5.6 Å2. The channels contain occluded electroneutral ring triborate complexes, [B3O3(OH)3]0, formed by three (BO2(OH−))− triangles sharing common O atoms, as well as K+ and Na+ cations. The triborate [B3O3(OH)3]0 units correspond to similar clusters found in the crystal structure of the α-form of metaboric acid, HBO2. According to information-based complexity calculations, the crystal structure of I should be described as complex, with IG = 5.781 bits/atom and IG,total = 625.950 bits/cell. Teruggite, Ca4Mg[B6As(OH)6O11]2(H2O)14, the only known boroarsenate of natural origin, has almost twice as much information per unit cell, with IG,total = 1201.992 bits/cell. The observed difference in structural complexity between I and teruggite is the consequence of their chemistry (hydration state) and different formation conditions.

Highlights

Borophosphates and metalloborophosphates constitute an important, chemically and structurally rich family of boron compounds, with more than 220 different structural architectures having been reported to date
The aim of this study is to report on the synthesis and crystal structure of (K,Na)2 [AsB6 O12 ]2 [B3 O3 (OH)3 ] (I), a new microporous compound, which is the first boroarsenate containing occluded electroneutral [B3 O3 (OH)3 ]0 moieties within its framework channels
The crystal structure of I was found to be based upon the novel [AsB6 O12 ]− microporous boroarsenate framework (Figure 1) formed by B and As coordination polyhedra

Summary

Introduction

Borophosphates and metalloborophosphates constitute an important, chemically and structurally rich family of boron compounds, with more than 220 different structural architectures having been reported to date. As the number of compounds steadily grew, the systematics was refined further, taking into account the type of boron polyhedra ((BO4 ) tetrahedra and/or (BO3 ) triangles), the degree of protonation (ratio O/OH), and the method of synthesis [2,3]. Minerals 2019, 9, 781 to that of borophosphates is undoubtedly due to the toxicity of arsenic, which drastically restricts the possible technological applications of boroarsenates. It is, interesting that no borophosphates sensu stricto (i.e., compounds containing polyions of linked B and P polyhedra) have been reported as minerals, whereas there is one natural boroarsenate—teruggite, Ca4 Mg[B6 As(OH) O11 ]2 (H2 O)14 —that was first found in the Loma Blanca borate deposit in the province of Jujuy, Argentina [12].

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