Probing carbonate in bone forming minerals on the nanometre scale.

To devise new strategies to treat bone disease in an ageing society, a more detailed characterisation of the process by which bone mineralises is needed. In vitro studies have suggested that carbonated mineral might be a precursor for deposition of bone apatite. Increased carbonate content in bone may also have significant implications in altering the mechanical properties, for example in diseased bone. However, information about the chemistry and coordination environment of bone mineral, and their spatial distribution within healthy and diseased tissues, is lacking. Spatially resolved analytical transmission electron microscopy is the only method available to probe this information at the length scale of the collagen fibrils in bone. In this study, scanning transmission electron microscopy combined with electron energy-loss spectroscopy (STEM-EELS) was used to differentiate between calcium-containing biominerals (hydroxyapatite, carbonated hydroxyapatite, beta-tricalcium phosphate and calcite). A carbon K-edge peak at 290 eV is a direct marker of the presence of carbonate. We found that the oxygen K-edge structure changed most significantly between minerals allowing discrimination between calcium phosphates and calcium carbonates. The presence of carbonate in carbonated HA (CHA) was confirmed by the formation of peak at 533 eV in the oxygen K-edge. These observations were confirmed by simulations using density functional theory. Finally, we show that this method can be utilised to map carbonate from the crystallites in bone. We propose that our calibration library of EELS spectra could be extended to provide spatially resolved information about the coordination environment within bioceramic implants to stimulate the development of structural biomaterials.

Phonon-induced diamagnetic force and its effect on the lattice thermal conductivity.

Phonons are displacements of atoms around their rest positions in a crystalline solid. They carry sound and heat, but are not classically associated with magnetism. Here, we show that phonons are, in fact, sensitive to magnetic fields, even in diamagnetic materials. We do so by demonstrating experimentally that acoustic phonons in a diamagnetic semiconductor (InSb) scatter more strongly from one another when a magnetic field is applied. We attribute this observation to the magnetic-field sensitivity of the anharmonicity of the interatomic bonds that govern the probability of phonon-phonon interactions. The displacements of atoms locally affect the orbital motion of valence band electrons, which, in the presence of an external magnetic field, spatially modulates the orbital diamagnetism around the displaced atoms. The spatial gradient in magnetic moment results in an anharmonic magnetic force exerted on the displaced atom. The process is modelled by ab initio calculations that, without the use of a single adjustable parameter, reproduce the observed 12% decrease in the lattice thermal conductivity under a 7 T magnetic field at a temperature of 5.2 K.