The biomineral hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2 is the main mineral constituent of mammal bone. Hydroxyapatite crystallizes in the hexagonal and monoclinic phases, the main difference between them being the orientation of the hydroxyl groups. Using density functional theory we study the energetics of the hexagonal and monoclinic phases along with the several hypothetical crystal structures of hydroxyapatite. The monoclinic phase has the lowest energy, with the hexagonal phase being only 22meV/cell higher in energy. We identify a structural transition path from the hexagonal to monoclinic phase with the activation energy of 0.66 eV per hexagonal cell. At room temperature the transition occurs on a millisecond time scale. The electronic structures of the monoclinic and hexagonal phases are compared. For the hexagonal phase we calculate the phonon frequencies at the Γ-point and elastic constants. Both are in good agreement with available experiment.

The biomineral hydroxyapatite (HA) [Ca10(PO4)6(OH)2] is the main mineral constituent of mammal bone. We report a theoretical investigation of the HA surface. We identify the low energy surface orientations and stoichiometry under a variety of chemical environments. The surface most stable in the physiologically relevant OH-rich environment is the OH-terminated (1000) surface. We calculate the work function of HA and relate it to the surface composition. For the lowest energy OH-terminated surface we find the work function of 5.1 eV, in close agreement with the experimentally reported range of 4.7 eV-5.1 eV [V. S. Bystrov, E. Paramonova, Y. Dekhtyar, A. Katashev, A. Karlov, N. Polyaka, A. V. Bystrova, A. Patmalnieks, and A. L. Kholkin, J. Phys.: Condens. Matter 23, 065302 (2011)].

Articles you may be interested inMagnetic and structural properties of BiFeO3 thin films grown epitaxially on SrTiO3/Si substrates J. Appl. Phys. 113, 17D919 (2013) The (110) plane of Co 3 O 4 spinel exhibits significantly higher rates of carbon monoxide conversion due to the presence of active Co 3þ species at the surface. However, experimental studies of Co 3 O 4 (110) surfaces and interfaces have been limited by the difficulties in growing high-quality films. We report thin (10-250 Å ) Co 3 O 4 films grown by molecular beam epitaxy in the polar (110) direction on MgAl 2 O 4 substrates. Reflection high-energy electron diffraction, atomic force microscopy, x-ray diffraction, and transmission electron microscopy measurements attest to the high quality of the as-grown films. Furthermore, we investigate the electronic structure of this material by core level and valence band x-ray photoelectron spectroscopy, and first-principles density functional theory calculations. Ellipsometry reveals a direct band gap of 0.75 eV and other interband transitions at higher energies. A valence band offset of 3.2 eV is measured for the Co 3 O 4 /MgAl 2 O 4 heterostructure. Magnetic measurements show the signature of antiferromagnetic ordering at 49 K. FTIR ellipsometry finds three infrared-active phonons between 300 and 700 cm À1

A relatively low conductivity of PtSi is one of the impediments to its application as a contact material in semiconductor technology. In this paper we discuss a possible strategy to control the conductivity of PtSi by manipulating the density of states at the Fermi level through alloying. Using density functional theory, we demonstrate theoretically that alloying PtSi with Ti substantially increases the number of conducting electrons, and suggest possible ways to increase the Ti solubility limit. We identify a tertiary compound with the conducting electron concentration almost three times larger than that of bulk PtSi. We analyze the effect of Ti alloying on the work function of PtSi, and its Schottky barrier height to Si and we examine the effect of alloy scattering on PtSi conductivity.

We present a study of the electronic properties of chlorine-doped NiSi and ClNiSi/Si contacts by means of density functional theory (DFT). Using DFT, we consider the theoretical effect of Cl doping in orthorhombic Pbnm NiSi and its impact on the work function, Schottky barrier with Si, and on the change in conductivity caused by impurity scattering. Our calculations suggest that Cl substitution on the Si site is energetically preferable. The thermodynamic analysis shows that chlorine at the surface lowers the surface energy. We find that Cl can reduce the work function of NiSi by as much as ∼100 meV, and that the Schottky barrier with Si strongly depends on the position of the substitutional chlorine. Electric conductivity of NiSi is found to be significantly reduced even for the lowest calculated Cl concentration. Both findings are validated experimentally.