Stability of Alkali Metal Halide Polymorphs as a Function of Pressure

Čančarević, Ž.; Schön, J. Christian; Jansen, Martin

doi:10.1002/asia.200700323

“…The system considered (lithium fluoride) was chosen for several reasons: the small number of electrons leads to fast calculations, and the ionicity of the system makes convergence easy. Moreover, the system had earlier been studied with model potentials [17,18], and it turned out that the relevant minima were the same when full ab initio structure prediction was performed [16] (for a brief summary see also [19]). …”

Section: Introductionmentioning

confidence: 99%

Structure prediction based onab initiosimulated annealing for boron nitride

Doll

¹

,

Schön

²

,

Jansen

³

2008

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Possible crystalline modifications of chemical compounds at low temperatures correspond to local minima of the energy landscape. Determining these minima via simulated annealing is one method for the prediction of crystal structures, where the number of atoms per unit cell is the only information used. It is demonstrated that this method can be applied to covalent systems, at the example of boron nitride, using ab initio energies in all stages of the optimization, i.e. both during the global search and the subsequent local optimization. Ten low lying structure candidates are presented, including both layered structures and 3d-network structures such as the wurtzite and zincblende types, as well as a structure corresponding to the β-BeO type.

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“…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.…”

mentioning

confidence: 96%

“…[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.…”

mentioning

confidence: 96%

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

Liebold‐Ribeiro

¹

,

Fischer

²

,

Jansen

³

2008

Self Cite

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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...

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“…[4] The excessively high calculated values arise from the known systematic errors of such physical quantities, depending on the ab initio method employed.…”

mentioning

confidence: 99%

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

Liebold‐Ribeiro

¹

,

Fischer

²

,

Jansen

³

2008

Self Cite

View full text Add to dashboard Cite

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...

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Stability of Alkali Metal Halide Polymorphs as a Function of Pressure

Cited by 66 publications

References 92 publications

Structure prediction based onab initiosimulated annealing for boron nitride

Structure prediction based onab initiosimulated annealing for boron nitride

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

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

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