Electron anions and the glass transition temperature

Abstract

Properties of glasses are typically controlled by judicious selection of the glass-forming and glass-modifying constituents. Through an experimental and computational study of the crystalline, molten, and amorphous [Ca 12 Al 14 O 32 ] 2+ · (e -) 2 , we demonstrate that electron anions in this system behave as glass modifiers that strongly affect solidification dynamics, the glass transition temperature, and spectroscopic properties of the resultant amorphous material. The concentration of such electron anions is a consequential control parameter: It invokes materials evolution pathways and properties not available in conventional glasses, which opens a unique avenue in rational materials design.amorphous materials | glass transition | electrides | molecular dynamics | density functional theory O ne of the key challenges in materials design is identification of degrees of freedom that ensure robust control of the properties of interest. Small changes in composition of crystalline materials can lead to large changes in their electronic properties, such as optical absorption and electrical conductivity (1). Similarly, a small concentration of (co)dopants in amorphous materials can be used to control their optical properties (2, 3). Glass properties are typically controlled by varying composition or processing conditions (4). However, delicate control of viscosity and glass transition temperature in multicomponent materials (T g ) has remained elusive due to the collective origin of these properties. Substitution of atomic anions with electron anions in materials to form electrides (5, 6) introduces an additional degree of freedom. Here we demonstrate that electron anions dramatically change dynamics of atoms in an inorganic amorphous material, which strongly affects its T g . Extending the concept of electron anions to other amorphous materials would provide a powerful instrument for the design of glasses with finely tuned characteristics.Calcium aluminates (CA), composed of CaO and Al 2 O 3 , represent a common family of oxide glasses. Whereas pure Al 2 O 3 is a poor glass former, blending it with CaO leads to the formation of stable glasses composed of AlO 4 tetrahedra with strong and directional covalent bonds instead of nondirectional ionic bonds in bulk Al 2 O 3 (7). In contrast, Ca-O bonds retain their ionic character; they are weaker than Al-O bonds and lack a preferred orientation, which enables motion of the AlO 4 tetrahedra relative to each other. Hence, the T g of stoichiometric CA systems decreases by ∼50 K as the CaO content increases from 57 mol% to 70 mol% (Fig. 1). This relatively narrow interval of T g variation with composition is typical of multicomponent glasses (8, 9). The T g for stoichiometric [Ca 12 (5,14,15). In crystalline C12A7:e -, the electrons occupy the cage conduction band (CCB) states associated with the lattice cages (16, 17), leading to polaron-type conduction at x < ∼0.5 and metallic conduction at x > 0.5 (13, 16). The exceptionally low work function of crystalline C12A7:e -...

Transparent amorphous oxide semiconductors for organic electronics: Application to inverted OLEDs

“…Although the work function of a-C12A7:e is larger than that of c-C12A7:e (2.4 eV), this value is still exceptionally low and close to that of metal lithium (2.9 eV). We attribute this low value to the presence of anionic electrons existing at interstitial positions without belonging to specific orbitals of the structure's ions (27). The work function of a-ZSO (3.5 eV) is significantly lower than that of typical transparent oxide semiconductors (20,28) including ZnO (4.3eV), as is summarized in Fig.…”

Electrides: a review

“…48,49 As such the material itself can exhibit a large range of tunable physical properties depending on the concentration of anionic electrons, which has already shown to be a consequential control parameter for macroscopic attributes, such as the glass transition temperature. 50 The mayenite framework has also proven robust with respect to presence of alternate ions in the pore space. Once the O 2À ions leave the structure they can be replaced with alternate counterions, such as OH, 51 F, Cl, 52 H, 53 Au, 54 Pt, 55 and even the rare-earth elements Gd, Er, Ce, Tb, Eu, Eu.…”

The Effect of Calcination Temperature on the Structure and Performance of Nanocrystalline Mayenite Powders

“…The calcium aluminates, CaO-Al 2 O 3 (C-A) system is a promising group of materials due to superior refractory properties. Especially, mayenite (12CaO·7Al 2 O 3 labelled as C12A7) is an auspicious functional material for usage in various engineering applications, such as catalysis [1,2,3,4], in particular, used in the synthesis of ammonia [5,6], batteries [7], white light-emitting diodes (W-LEDs) [8], electronic [9] and optoelectronic devices [10], which results from the discovery of oxygen mobility [11] and ionic conductivity [12,13,14]. The unique properties of C12A7 come from its special crystal structure.…”