ABSTR ACTBrachiopods are a phylum of shell-forming sessile marine invertebrates which have existed since the early Cambrian. Two very different biomaterial design strategies for their shells evolved early in their history. Both strategies use hybrid fibre composites, however, one is based on mineral fibres embedded in~2 wt.% of organic biopolymer sheaths and the inorganic fibres are calcite single crystals. In the second strategy the fibres are biopolymers and are reinforced with Ca-phosphate nanoparticles to form a fibrous nanocomposite. Here the organic component (chitin) dominates. The Ca-phosphate nanoparticle-reinforcement strategy is not unlike that in vertebrate bone, however, the microscale structure is laminated with alternating laminae which have a different degree of mineralization.The calcitic shells feature an outer compact layer of calcite micro-and nanoparticles protecting the inner fibrous layer from the outside. Transmission electron microscopy of the fibrous layer reveals intercrystalline and intracrystalline biopolymers. The calcitic shell material is stiff with nanoindentation E-moduli of 63Ô8 GPa and relatively hard (Vickers microhardness up to 400 HV 0.0005/10 and nanohardness 4Ô0.5 GPa). Compared to inorganic calcite the microhardness is doubled and the nanohardness increases by 60%. We attribute this increased hardness to intracrystalline biopolymers. The nano-indentation E-moduli of the chitinophosphatic shells range from 3 to 55 GPa as a result of the varying degree of mineralization between their laminae, and similarly their nanohardness varies between 0.1 and 3 GPa. For brachiopods burrowing inside the sediment, the alternation of nonmineralized laminae with thin, more strongly mineralized laminae provides abrasion-resistance, hardness and longitudinal stiffness while it preserves the flexibility provided by the organic component for bending movements.

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[1] We investigated micromechanical properties and ultrastructure of the shells of the modern brachiopod species Lingula anatina, Discinisca laevis, and Discradisca stella with scanning electron microscopy (SEM, EDX), transmission electron microscopy (TEM) and Vickers microhardness indentation analyses. The shells are composed of two distinct layers, an outer primary layer and an inner secondary layer. Except for the primary layer in Lingula anatina, which is composed entirely of organic matter, all other shell layers are laminated organic/inorganic composites. The organic matter is built of chitin fibers, which provide the matrix for the incorporation of calcium phosphate. Amorphous calcium phosphate in the outer, primary layer and crystalline apatite is deposited into the inner, secondary layer of the shell. Apatite crystallite sizes in the umbonal region of the shell are about 50 Â 50 nm, while within the valves crystallite sizes are significantly smaller, averanging 10 Â 25 nm. There is great variation in hardness values between shell layers and between the investigated brachiopod species. The microhardness of the investigated shells is significantly lower than that of inorganic hydroxyapatite. This is caused by the predominantly organic material component that in these shells is either developed as purely organic layers or as an organic fibrous matrix reinforced by crystallites. Our results show that this particular fiber composite material is very efficient for the protection and the support of the soft animal tissue. It lowers the probability of crack formation and effectively impedes crack propagation perpendicular to the shell by crack-deviation mechanisms. The high degree of mechanical stability and toughness is achieved by two design features. First, there is the fiber composite material which overcomes some detrimental and enhances some advantageous properties of the single constituents, that is the softness and flexibility of chitin and the hardness and brittleness of apatite. Second, there is a hierarchical structuring from the nanometer to a micrometer level. We could identify at least seven levels of hierarchy within the shells.

The effects of silicic acids on calcite growth are a model for the effects of partially polymerized additives on crystal growth. At alkaline pH, silicic acid polymers coexist with small mono-and oligomers. Atomic force microscopy (AFM) showed that large polymers promote two-dimensional nucleation, while mono-and oligomers have a bimodal promoter/inhibitor effect on step propagation. This bimodality can be interpreted as the result of attachment of mono-and oligomers along the steps along with a modification of the kinetics of kink generation and/or propagation. The bimodal step kinematic effect is accompanied by a single morphologic effect: growth islands transform from a rhombus into an ellipse. This effect has been reported for other additives, indicating that many additives generate few morphologies. Such a convergence limits the versatility of chemical control on biomorphogenesis. Contrarily, the strong kinetic effect of silicic acids may make them very efficient controllers of biomorphogenesis, if coupled with a physical shape control, for example, by templates. Thus, silicic acids show a unique bimodality as a controller of calcite biomineralisation and as an abundant biomineral.

Abstract. Biologic structural materials for skeletons or teeth show a hierarchical architecture, where organic macromolecules and mineral substance form a hybrid composite material with its components inter-weaved on many length scales. On the nanostructure level brachiopods form hybrid composite mesocrystals of calcite with occluded organic molecules. On the microstructure level several types of materials are produced, on which the electron back-scatter diffraction (EBSD) technique gives insight in texture and architecture. We describe the calcite single-crystal fiber composite architecture of the secondary layer involving organic matrix membranes, the competitive-growth texture of the columnar layer and the nanostructuring of the primary layer. In the overall skeleton the organic biopolymers provide flexibility and tensile strength while the mineral provides a high elastic modulus, compressive strength, hardness and resistance to abrasion. The hierarchical composite architecture, from the nanostructure to the macroscopic level provides fracture toughness. The morphogenesis of the biomaterial as a whole and of the mineral crystals is guided by the organic matrix and most probably involves amorphous calcium carbonate (ACC) precursors. In this paper we review the hierarchical architecture of rhynchonelliform brachiopod shells, which is very distinct from mollusk nacre.

In the work structure mechanisms of Mn segregation from solid solutions with perovskite-like and spinel structure were investigated. The complex Mn-content oxides are often used as catalysts. Spinels (Mn 3-x Al x O 4) are known as catalysts of deep oxidation. The La 1-x Ca x MnO 3 solid solutions with the perovskite-like structure are catalysts for the partial oxidation of methane. In both systems an active component of catalysts is manganese oxide which is formed under catalytic reaction conditions. Spinel catalysts are activated by heating under oxidizing atmosphere. Perovskite catalysts are used under reducing atmosphere. To simulate real conditions of catalytic processes we applied high-temperature XRD in the air (oxidizing atmosphere) and in the vacuum (reducing atmosphere). Mn-content spinel (Mn 1.5 Al 1.5 O 4) is stable under heating in the vacuum: spinel keeps structure and composition. On the contrary decomposition is observed in the air during heating in the air. Nanocrysralline phases of β-Mn 3 O 4 and spinel Mn 2.4 Al 0.4 O 4 are the products of decomposition. The lattice parameter of cubic phase decreases from 8.285 Å to 8.043Å when sample was heated from ambient temperature to 700 o C. This fact shows change of spinel composition from Mn 1.5 A 1.5 O 4 to Mn 0.4 Al 2.4 O 4 , i.e. manganese ions leave the structure. TEM data indicates that small particles of β-Mn 3 O 4 are appeared on the surface of the well crystallized cubic spinel when sample was calcined at the temperature of 300-400 o C. After further increase calcination temperature β-Mn 3 O 4 particles grow. As a result we can conclude that Mn cations segregation from initial spinel occurs with formation of new phase β-Mn 3 O 4. At the same time spinel Mn 1.5 A 1.5 O 4 transforms to aluminum-enriched spinel Mn 0.4 Al 2.4 O 4 with statistical distribution of cation vacancies. Temperature stability of the series of samples La 1-x Ca x MnO 3 (0≤x≤1) was investigated using in situ XRD (vacuum and air atmosphere). According to TEM and XRD analysis the microstructure and phase structure of samples keeps during heating in the air. To model a process of catalysis reduction in the reactor we heated samples in the vacuum. According to TEM data multiple planar defects in perovskite structure were formed starting from x=0.3, the amount of the defects grows with the increase of x value. In the sample with the x=0.9 areas of the planar defects segregation and superstructure formation is observed. According to EDX analysis in these areas the deficit of Mn cations (~20%) is detected. The superstructure is characterized by quadruple period of the crystal lattice in [001] direction. Thus Mn cations segregation was observed in the both systems. In the spinel oxide segregation is accompanied by partial oxidation of Mn 2+ to Mn 3+ due to oxygen embedding. In the perovskite oxides part of Mn cations transform to Mn 2+ due to oxygen loss in the vacuum conditions. Microstructure of systems changes by different ways during high-temperature treatment. Planar defects are...

It is convenient to consider a change in the structure of crystalline material as a reason for remarkable modifications in its physical properties: a structural phase transition often results in the anomalous behaviour of macroscopic parameters in condensed matter. However, is it always true that a detected change in properties, discontinuous or smooth, manifests itself as a change in the crystal structure? Isostructural transformations exemplify a violation of such a rule. They connect phases with identical structures but with different properties; a volume collapse, giant magneto-elastic coupling, or negative thermal expansion may serve as examples of corresponding macroscopic responses. Isostructural phase transformations have two possible regimes: the transition can be discontinuous (first-order) or transformation can be smooth (crossover regime). One finds a limited number of examples of discontinuous isostructural phase transitions. At the same time, many examples of anomalous but continuous variation of material parameters within the stability range of the same phase have been reported. Normally structural information collected for nonambient conditions are limited and therefore an isostructural transformation occurring in the crossover regime may be easily mixed with a second order phase transition. In our talk, we show typical anomaly types for isostructural phase transformations. Generic model-free phase diagrams, with their specific features as critical lines and points, will be presented and analyzed. Phase transformations observed in rare-earth metals and their compounds provide an experimental illustration of different transformation regimes. Pseudoplastic or pseudoelastic shape memory is based on recoverable ferroelastic twin domain accomodation in stress fields. The twins are associated to a martensitic phase transition. Pseudoelasticity (superelasticity) occurs when the high-symmetry phase (paraelastic "austenite") is thermodynamically stable in the stress-free state, and the phase transition to the distorted phase (ferroelastic "martensite") is induced by the application of stress. Those ferroelastic twin variants which are suitably oriented in the stress field become stable and the spontaneous strain of the martensitic transition leads to a large shape change during the process. When stress is released, the system falls back to the (stable) austenite state and the original shape is "pseudoelastically" recovered. KeywordsPseudoplastic shape memory occurs when ferroelastic "martensite" is thermodynamically stable, and the effect is due to ferroelastic domain switching in the stress field. In this case a release of stress does not recover the original domain configuration. The original shape (and domain configuration) is then restored via a thermal transition to the paraelastic austenite and stress-free cooling to martensite. Microdiffraction experiments using 100keV x-rays at beamline BW5@HASYLAB reveal the formation of stress-induced martensite in the stress field concentrated at the tips of cr...