Auxetics are materials showing a negative Poisson's ratio. This characteristic leads to unusual mechanical properties that make this an interesting class of materials. So far no systematic approach for generating auxetic cellular materials has been reported. In this contribution, we present a systematic approach to identifying auxetic cellular materials based on eigenmode analysis. The fundamental mechanism generating auxetic behavior is identified as rotation. With this knowledge, a variety of complex two-dimensional (2D) and three-dimensional (3D) auxetic structures based on simple unit cells can be identified.

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...

The dynamic mechanical properties of finite two-dimensional periodic cellular materials are investigated by finite element eigenmode analysis for different architectures of the unit cell. Frequency band gaps are examined in quadratic and hexagonal lattice topologies with regular, inverted, and chiral architecture. Pronounced band gaps develop for chiral lattices. The formation of band gaps can be traced back to the resonance behavior of the elementary building blocks of the cellular structure for different boundary conditions (mode transition). Based on the findings of this work periodic lattice materials with specific band gaps can be designed. Fig. 11. Eigenmode analysis of a single curved strut (L ¼ 5.0 mm, t ¼ 0.2 mm) performed using hinged-hinged and free-free boundary conditions. The plot (left) shows the frequency as a function of the strut amplitude A, whereas the shapes of the different vibration modes of bands I-IV are depicted on the right.

The interaction of chloroform (CHCl(3)) with single-wall carbon nanotubes (SWCNT) is investigated using both first principles calculations based on Density Functional Theory and vibrational spectroscopy experiments. CHCl(3) adsorption on pristine, defective, and carboxylated SWCNTs is simulated, thereby gaining a good understanding of the adsorption process of this molecule on SWCNT surfaces. The results predict a physisorption regime in all cases. These calculations point out that SWCNTs are promising materials for extracting trihalomethanes from the environment. Theoretical predictions on the stability of the systems SWCNT-CCl(2) and SWCNT-COCCl(3) are confirmed by experimental TGA data and Fourier Transform Infrared Spectroscopy (FT-IR) experiments. Results from resonance Raman scattering experiments indicate that electrons are transferred from the SWCNTs to the attached groups and these results are in agreement with the predictions made by ab initio calculations.

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...