title: Ab initio study of thermodynamic, structural, and elastic properties of Mg-substituted crystalline calcite.
abstract: Arthropoda, which represent nearly 80% of all known animal species, are protected by an exoskeleton formed by their cuticle. The cuticle represents a hierarchically structured multifunctional biocomposite based on chitin and proteins. Some groups, such as Crustacea, reinforce the load-bearing parts of their cuticle with calcite. As the calcite sometimes contains Mg it was speculated that Mg may have a stiffening impact on the mechanical properties of the cuticle (Becker et al., Dalton Trans. (2005) 1814). Motivated by these facts, we present a theoretical parameter-free quantum-mechanical study of the phase stability and structural and elastic properties of Mg-substituted calcite crystals. The Mg-substitutions were chosen as examples of states that occur in complex chemical environments typical for biological systems in which calcite crystals contain impurities, the role of which is still the topic of debate. Density functional theory calculations of bulk (Ca,Mg)CO₃ were performed employing 30-atom supercells within the generalized gradient approximation as implemented in the Vienna Ab-initio Simulation Package. Based on the calculated thermodynamic results, low concentrations of Mg atoms are predicted to be stable in calcite crystals in agreement with experimental findings. Examining the structural characteristics, Mg additions nearly linearly reduce the volume of substituted crystals. The predicted elastic bulk modulus results reveal that the Mg substitution nearly linearly stiffens the calcite crystals. Due to the quite large size-mismatch of Mg and Ca atoms, Mg substitution results in local distortions such as off-planar tilting of the CO₃²⁻ group.
authors.affiliation: Department of Computational Materials Design, Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf 40237, Germany.
authors.forename: Pavlína
authors.initials: P
authors.lastname: Elstnerová
authors.forename: Martin
authors.initials: M
authors.lastname: Friák
authors.forename: Helge Otto
authors.initials: HO
authors.lastname: Fabritius
authors.forename: Liverios
authors.initials: L
authors.lastname: Lymperakis
authors.forename: Tilmann
authors.initials: T
authors.lastname: Hickel
authors.forename: Michal
authors.initials: M
authors.lastname: Petrov
authors.forename: Svetoslav
authors.initials: S
authors.lastname: Nikolov
authors.forename: Dierk
authors.initials: D
authors.lastname: Raabe
authors.forename: Andreas
authors.initials: A
authors.lastname: Ziegler
authors.forename: Sabine
authors.initials: S
authors.lastname: Hild
authors.forename: Jörg
authors.initials: J
authors.lastname: Neugebauer
mesh_terms: D000818:Animals; D003386:Brachyura / Q000737:chemistry / Q000648:ultrastructure; D002119:Calcium Carbonate / Q000737:chemistry*; D002412:Cations; D003460:Crystallization; D055119:Elastic Modulus; D004548:Elasticity; D004602:Elements; D034582:Integumentary System; D008274:Magnesium / Q000737:chemistry*; D008956:Models, Chemical; D013816:Thermodynamics
chemical_list: D002412:Cations; D004602:Elements; C005479:magnesium carbonate; D002119:Calcium Carbonate; D008274:Magnesium