Hidden polymorphs drive vitrification in B2O3

Abstract

Understanding the conditions which favor crystallisation or vitrification of liquids has been a long-standing scientific problem [1][2][3]. Another connected, and not yet well understood question is the relationship between the glassy and the various possible crystalline forms a system may adopt [4,5]. In this context, B2O3 is a puzzling case of study since i) it is one of the best glass-forming systems despite an apparent lack of lowpressure polymorphism ii) it vitrifies in a glassy form abnormally different from the only known crystalline phase at ambient pressure [6] iii) it never crystallises from the melt unless pressure is applied, an intriguing behaviour known as the crystallisation anomaly [7][8][9]. Here, by means of ab-initio calculations, we discover the existence of novel B2O3 crystalline polymorphs with structural properties similar to the glass and formation energies comparable to the known ambient crystal. The resulting configurational degeneracy drives the system vitrification at ambient pressure. The degeneracy is lifted under pressure, unveiling the origin of the crystallisation anomaly. This work reconciles the behaviour of B2O3 with that from other glassy systems and reaffirms the role played by polymorphism in a system's ability to vitrify [10,11]. Some of the predicted crystals are cage-like materials entirely made of three-fold rings, opening new perspectives for the synthesis of boron-based nanoporous materials. PACS numbers:Polymorphism, the possibility for a substance to form several distinct crystalline phases of identical composition, is observed for a wide range of materials. This phenomenon has tremendous importance not only per se for understanding the crystallisation process but also because of practical implications such as the design and control of new materials with specific properties [12], a major issue for the pharmaceutical industry [13]. Indeed, the ability of molecular units to pack in various ways generates crystal phases which generally differ in their physical properties and cohesive energies. Another important implication of polymorphism is related to the glassy state: as pointed out earlier [10,11], glass formation is often prevalent for those materials which are found in a variety of crystalline forms. An obvious example is silica (SiO 2 ), the archetypal glass-former, which at low pressure is found as quartz, cristobalite, keatite, tridymite, coesite and moganite [14]. The existence of these many polytypes illustrate that the structural units, here the SiO 4 tetrahedra, can occur in several conformations with little difference in strain energy, allowing for the possibility of metastable states [14]. The ease of glass formation is usually understood as the result of the system frustration associated to the presence of many minima of comparable energy in the crystal energy landscape (CEL). In the context of organic chemistry, it has also been observed that systems with many almost equi-energetic structures containing a common interchangeable motif, correlate with a t...

Titania–silica mixed oxides investigated with density functional theory and molecular dynamics simulations

“…The presence of so many different structures might explain the fact that TiSiO 4 has often been produced in amorphous state [11]. A similar correlation between the tendency to form amorphous phases and the existence of many low-energy polymorphs was observed for B 2 O 3 as well [42]. The metastable nature of these phases forces us to conclude that, unless techniques carefully designed to reach a mixed state are employed, the two oxides will not mix.…”

m‐Metallaphenol: Synthesis and Reactivity Studies

“…The latter were found as the only motif in a trigonal crystalline form of B 2 O 3 , where they form a 3D net 13. Very recently, Ferlat et al14 proposed two planar models of B 2 O 3 , which were derived from defect‐ordered variants of a graphite‐like layer. One model is based on a net of corner‐sharing BO 3 units, the other on boroxol‐rings (Figure 9).…”

“… B 2 O 3 layer proposed by Ferlat et al;14 top: layer T0 with BO 3 , bottom: layer T0‐b with B 3 O 6 . …”

Thermodynamic stability of borophene, B2O3 and other B1−xOx sheets

“…For bulk phases the boron-oxygen system is well studied and several works on the phase diagram exist [13][14][15]. Thermodynamically stable are the pure 'icosahedral' boron phases in their various forms [16], boron suboxide B 6 O [17] and boron trioxide B O 2 3 that is a vitreous phase under ambient conditions [12] but can be crystalline when synthesized under pressure [15,18,19]. Sometimes boron monoxide B O 2 [20,21] is also considered but its stability and existence is strongly debated [13,14,22,23].…”

“…In the literature a variety of 2D boron-oxygen structures were studied. Two-dimensional variants of B O 2 3 were proposed by Ferlat et al [12,24], with building blocks formed by planar BO 3 units (see [20] and multiple hypothetical structural models for monolayers were previously considerd [26,27]. One possible B O 2 monolayer model is shown in figure 1(c).…”