We use first principles calculations to study structural, vibrational and superconducting properties of H2S at pressures P ≥ 200 GPa. The inclusion of zero point energy leads to two different possible dissociations of H2S, namely 3H2S → 2H3S + S and 5H2S → 3H3S + HS2, where both H3S and HS2 are metallic. For H3S, we perform non-perturbative calculations of anharmonic effects within the self-consistent harmonic approximation and show that the harmonic approximation strongly overestimates the electron-phonon interaction (λ ≈ 2.64 at 200 GPa) and Tc. Anharmonicity hardens H-S bond-stretching modes and softens H-S bond-bending modes. As a result, the electronphonon coupling is suppressed by 30% (λ ≈ 1.84 at 200 GPa). Moreover, while at the harmonic level Tc decreases with increasing pressure, the inclusion of anharmonicity leads to a Tc that is almost independent of pressure. High pressure hydrogen sulfide is a strongly anharmonic superconductor.Cuprates [1] have for many years held the world record for the highest superconducting critical temperature (T c = 133 K) [2]. However, despite almost 30 years of intensive research, the physical mechanism responsible for such a high T c is still elusive, although the general consensus is that it is highly non-conventional. The discovery by Drozdov et al.[3] of T c = 190 K in a diamond anvil cell loaded with hydrogen sulfide (H 2 S) and compressed to about 200 GPa breaks the cuprates record and overturns the conventional wisdom that such a high T c cannot be obtained via phonon-mediated pairing.The claim that hydrogen at high pressure could be superconducting is not new [4] and it was recently supported by first principles calculations based on the harmonic approximation applied to dense hydrogen [5][6][7][8] and several hydrides [9][10][11][12][13][14][15]. More recently, two theoretical papers predicted the occurrence of high T c superconductivity in high-pressure sulfur-hydrides [16,17]. However, as shown in Refs. [18,19], anharmonicity can be crucial in these systems. For example, in PdH, the electron-phonon coupling λ parameter is found to be 1.55 at the harmonic level, while a proper inclusion of anharmonic effects leads to λ = 0.40 [18], in better agreement with experiments. Thus, in hydrogen-based compounds, the phonon spectra are strongly affected by anharmonic effects.Several first principles calculations [16,17,20,26] suggested that decomposition of the H 2 S sample occurs within the diamond-anvil cell at high pressures. The high-T c superconducting material is therefore very unlikely to be H 2 S, while H 3 S is the obvious candidate for the H-rich decomposition product.Here we study the structural, vibrational and superconducting properties of H 2 S above 200 GPa, where the highest T c occurs. We show that the inclusion of zero point motion in the convex hull at 200 and 250 GPa stabilizes two metallic structures, H 3 S and HS 2 . Finally, we show that, contrary to suggestions in previous work [16,20], the harmonic approximation does not explain the measured T c ...

We provide evidence that citrate anions bridge between mineral platelets in bone and hypothesize that their presence acts to maintain separate platelets with disordered regions between them rather than gradual transformations into larger, more ordered blocks of mineral. To assess this hypothesis, we take as a model for a citrate bridging between layers of calcium phosphate mineral a double salt octacalcium phosphate citrate (OCPcitrate). We use a combination of multinuclear solid-state NMR spectroscopy, powder X-ray diffraction, and first principles electronic structure calculations to propose a quantitative structure for this material, in which citrate anions reside in a hydrated layer, bridging between apatitic layers. To assess the relevance of such a structure in native bone mineral, we present for the first time, to our knowledge, 17 O NMR data on bone and compare them with 17 O NMR data for OCP-citrate and other calcium phosphate minerals relevant to bone. The proposed structural model that we deduce from this work for bone mineral is a layered structure with thin apatitic platelets sandwiched between OCP-citrate-like hydrated layers. Such a structure can explain a number of known structural features of bone mineral: the thin, plate-like morphology of mature bone mineral crystals, the presence of significant quantities of strongly bound water molecules, and the relatively high concentration of hydrogen phosphate as well as the maintenance of a disordered region between mineral platelets.NMR crystallography | biomineralization B one is a complex organic-inorganic composite material (1), in which calcium phosphate nanoparticles are held within a primarily collagen protein matrix. The mineral component is a poorly crystalline phase, closely related to hydroxyapatite. The currently accepted model of bone mineral is ∼50-to 150-nmthick stacks of very closely packed apatitic platelets, each of order 2.5-4 nm in thickness (1-4), arranged so that their large (100) faces are parallel to each other and their c axes are strongly ordered (parallel to collagen fibrils) (5). NMR studies show that, in addition to the largely ordered but nonstoichiometric apatitic phase, there is a substantial, highly hydrated, disordered phase containing up to 55% of the bone mineral phosphatic ions (6, 7) but in the form of hydrogen phosphate or phosphate strongly hydrogen-bonded to water rather than apatitic orthophosphate (8). This phase has been assigned as a surface phase, but whether the surface in question is that of individual mineral platelets or the surface of the overall structure formed by a stack of such platelets is not yet clear. There is, however, significant experimental evidence that is not explained by this model as it stands. First, there has never been any observation of an isolated mineral platelet in mature bone, even in preparations in which there have been attempts to disperse the mineral structures (9). This feature suggests that the mineral platelets are not independent structures-indeed, their ordered aggregati...

We propose here a two-dimensional material based on a single layer of violet or Hittorf's phosphorus. Using first-principles density functional theory, we find it to be energetically very stable, comparable to other previously proposed single-layered phosphorus structures. It requires only a small energetic cost of approximately 0.04 eV/atom to be created from its bulk structure, Hittorf's phosphorus, or a binding energy of 0.3-0.4 J/m(2) per layer, suggesting the possibility of exfoliation in experiments. We find single-layered Hittorf's phosphorus to be a wide band gap semiconductor with a direct band gap of approximately 2.5 eV, and our calculations show it is expected to have a high and highly anisotropic hole mobility with an upper bound lying between 3000-7000 cm(2) V(-1) s(-1). These combined properties make single-layered Hittorf's phosphorus a very good candidate for future applications in a wide variety of technologies, in particular for high frequency electronics, and optoelectronic devices operating in the low wavelength blue color range.