Experimental Substantiation of the “Energy Landscape Concept” for Solids: Synthesis of a New Modification of LiBr

Abstract

Our approach towards synthesis planning in solid-state chemistry is based on the so-called energy landscape concept.[1] Here, each chemical compound capable of existing for a given period of time in a given (equilibrium) geometry is associated with a locally ergodic minimum region on the energy landscape over the configuration space.[2] As a special feature of this approach, full reference is given to metastable configurations. Thus, for the alkali-metal halides, which at ambient conditions preferably adopt the rock salt or CsCl structure type, many further polymorphs exhibiting various different structures have been predicted to be kinetically stable. One of the most conspicuous structures among those predicted is the 5-5 structure type, in which cations and anions coordinate each other trigonal-bipyramidally, forming commutative partial structures. [2][3][4] In particular for the lithium compounds of the heavy halogens bromine and iodine, the wurtzite or sphalerite modifications are predicted to be rather low in energy (Figure 1). [4] It was shown previously that LiI can exist in a hexagonal modification, in addition to the conventional rock salt structure. However, no further details about the crystal structure were reported.[5] More recently Wassermann et al. [6] found that both hexagonal and cubic LiI structures were contained in films at room temperature that had been deposited from the gas phase. Conclusive evidence for LiI to also exist in the wurtzite type structure has been provided by the "low-temperature-deposition" technique [7] combined with the Rietveld analysis of the X-ray diffraction patterns as taken from thick-layer samples.[8] During that investigation it was shown that solid solutions LiBr 1Àx I x (0.25 x 0.8) can also be obtained as hexagonal phases.[8] However, attempts to generate the respective modification for pure LiBr failed at that time.Here, we report on a systematic study of LiBr, exploring the full parameter space of our deposition technique, which has revealed formation of the wurtzite polymorph under appropriate conditions.In contrast to conventional molecular beam epitaxy (MBE) [9] and layer-by-layer deposition techniques, [10] where the substrates are heated to higher temperatures in order to allow the deposited matter to arrive at a structurally ordered state, [11] during the low-temperature-deposition approach the elemental starting materials are deposited atom by atom onto a cooled substrate. In this way amorphous starting mixtures are formed preferentially and the respective elements are homogeneously distributed and dispersed on the atomic level. This technique has been chosen to reduce as far as possible the diffusion paths during the solid-state reaction and in the crystallization of the desired compound, which would allow for running solid-state syntheses at extremely low thermal activation. This is a crucial prerequisite if one wants to realize the metastable solids predicted computationally. It has been demonstrated that solid-state reactions yielding crystalline pro...

A Universal Representation of the States of Chemical Matter Including Metastable Configurations in Phase Diagrams

“…The in-principle feasibility has been shown at the example of the quasi-binary system LiBr-NaBr. All computational results agree with the-admittedly scarce-experimental knowledge available: LiBr has been shown to exist as a metastable Wurtzite-type polymorph, [24] and LiBr-LiI form a metastable Wurtzite-type solid solution with a large miscibility gap. [25,26] Interestingly, just including the dominant contribution to the entropy of formation, that is, the ideal entropy of mixing, has yielded quite pleasing results.…”

In‐situ‐Verfolgung der Bildung kristalliner Feststoffe

“…Beispiele für solche Experimente sind die Untersuchung der Kristallisation von SAPO-34, [58] von Zeolith A, [59] von ALPO-4, [60] von ULM und MIL [61] und von borhaltigem ZSM-5. [62] [66] In den letzten Jahren sind weitere theoretische Arbeiten zur Syntheseplanung erschienen, in denen z. B. Hochdruckphasen durch eine systematische Erkundung der Energielandschaft mittels Ab-initio-Verfahren identifiziert wurden.…”

“…B. Lit. [19a, 64] [66] In den letzten Jahren sind weitere theoretische Arbeiten zur Syntheseplanung erschienen, in denen z. B. Hochdruckphasen durch eine systematische Erkundung der Energielandschaft mittels Ab-initio-Verfahren identifiziert wurden.…”

Nucleation and growth kinetics of co-deposited copper and selenium precursors to form metastable copper selenides

“…Providing energy into the system to overcome the smaller activation energy, E a,1 , for the metastable phase but not sufficient to overcome the larger activation energy, E a,2 , required to form the thermodynamic product traps the metastable product. In the 70s this concept was capitalized on by metallurgists seeking novel metallic glasses [2][3][4] and more recently metastable films precursors have been used to systematically prepare new compounds, understand nucleation barriers to compound formation and to better understand the energy landscapes made accessible by deposited films [5][6][7][8][9][10][11][12][13][14][15][16][17].…”