Synthesis and Identification of Metastable Compounds: Black Arsenic—Science or Fiction?

Abstract

Dedicated to Professor Rudolf Hoppe on the occasion of his 90th birthdayThe prediction and identification of stable and particularly of metastable compounds is important in achieving innovative materials. [1] With this objective in mind, approaches containing both theoretical examinations of phase stabilities and concepts of a rational synthesis [2] are gaining increasing importance. Thus, an investigation of energy landscapes [3] provides important information about local and global minima to predict the existence of new compounds and structures.Herein, we introduce a combination of quantum-chemical calculations and thermodynamic considerations to realize target-oriented planning and optimization of chemical synthesis. The analysis of phase formation is acquired with an in situ method for monitoring gas-phase reactions. Using the system P-As, we investigated a textbook example of monotropic phase transitions, which features a variety of known and postulated allotropes. [4] To estimate the relative stabilities of the compounds under discussion, structure allotropes of N, P, and As were modeled by means of DFT calculations. The calculated electronic energies (normalized to one atom Pn) for molecular Pn 2 and Pn 4 , the black/orthorhombic (o-Pn), gray/ trigonal (t-Pn), and the simple-cubic (c-Pn) allotrope as well as the tubular polymeric forms of Hittorf (H-Pn), [5] Ruck (R-Pn), [6] and Pfitzner (P-Pn), [7] which are known for phosphorus, are shown in Figure 1. The high stability of N 2 can be observed as well as the preference for solid-state structures for P and As. Figure 1 b emphasizes the results for the t and o forms of P and As. The known stability of o-P compared to the high-pressure modifications t-P and c-P is correctly predicted as well as the stability of gray t-As compared to the known high-pressure modification c-As, hypothetical tubular As allotropes, and predicted o-As. Does this mean that o-As can be synthesized as a metastable compound? [8a] The calculated values of the total electronic energies correctly express the higher stability of o-P compared with the high-pressure phase t-P and also t-As compared to o-As. From the computed values DE el = E el (o)ÀE el (t) of À4 kJ mol À1 for P and + 2.5 kJ mol À1 for As, a transition from the o-to the tstructure in the ideal solution As x P 1Àx can be estimated to be about x = 0.6 ( Figure 2). Taking thermodynamic energy terms into consideration (for details, see the Supporting Information), a distinctive stabilization of the o phase is calculated and the o-t transition shifts to a higher As content (x = 0.9). According to the calculated values, very low energy differences decide whether or not o-As can actually be synthesized. [8b]

Van der Waals interactions in selected allotropes of phosphorus

“…To point out the changing stability order of the Pn = P, As crystal structures, calculations were perfomed on orthorhombic and trigonal layered structures of P and As [3,4]. Similar to our earlier work on TN 2 (T = C, Si, Ge) compounds [65], calculations with the LDA functional underestimate lattice parameters, which are mostly overestimated by the GGA.…”

“…One part of the project dealt with the relative stabilities of polymorphs of dipnictides NiPn 2 (Pn = N, P, As, Sb, Bi) [1] and related ternary ordering variants MPnS (M = Fe, Co, Ni; Pn = As, Sb) [2]. The structures of the pnictogen elements themselves were studied in another part of the project with a focus on the solid solution As x P 1-x [3] and the various allotropes of phosphorus [4]. Subsequently, some of the well-known and new phosphorus allotropes have been thoroughly investigated concerning the role of van der Waals interactions for their solid structures and relative stabilities.…”

“…As a first step, we studied the stability order of the boundary phases orthorhombic black phosphorus and trigonal grey arsenic in the solid solution P 1-x As x in a combined experimental and DFT investigation [3]. The respective stability of orthorhombic vs. high-pressure trigonal black P and vice versa of trigonal gray vs. orthorhombic black As was correctly described by GGA calculations.…”

Structural Transition in Layered As1–xPx Compounds: A Computational Study

“…Most stable and thus more abundant are the layered allotropes such as black phosphorus [12], with the designation A17 or α-P, and grey arsenic [4], with the designation A7 or β-As. [13] Since a conversion of the α to the β phase is possible [14,15], combining both elements in the same layer and changing the composition is bound to cause a structural transition [16,17]. Since both structures are almost equally stable, we may expect phase coexistence that should bring an unexpected richness in both structural and electronic properties [5,16,18,19].…”

Phase stabilities at a glance: Stability diagrams of nickel dipnictides

“…6 The concept was applied by our group to calculate the relative stability of black and red phosphorus and arsenic. 7 A recent contribution by Zunger et al dealing with stabilities of hypothetical Half-Heusler-type compounds emphasizes the role of competing phases with different atomic compositions in addition to thermodynamic stabilities. 8 Keeping in mind the works cited above, we want to present a graphical representation using the example of an expedient and comprehensible case.…”