Structures and stability of calcium and magnesium carbonates at mantle pressures

Ab initio random structure searching (AIRSS) and density functional theory methods are used to predict structures of calcium and magnesium carbonate (CaCO$_3$ and MgCO$_3$) at high pressures. We find a previously unknown CaCO$_3$ structure which is more stable than the aragonite and "post aragonite" phases in the range 32--48 GPa. At pressures from 67 GPa to well over 100 GPa the most stable phase is a previously unknown CaCO$_3$ structure of the pyroxene type with fourfold coordinated carbon atoms. We also predict a stable structure of MgCO$_3$ in the range 85--101 GPa. Our results lead to a revision of the phase diagram of CaCO$_3$ over more than half the pressure range encountered within the Earth's mantle, and smaller changes to the phase diagram of MgCO$_3$. We predict CaCO$_3$ to be more stable than MgCO$_3$ in the Earth's mantle above 100 GPa, and that CO$_2$ is not a thermodynamically stable compound under deep mantle conditions. Our results have significant implications for understanding the Earth's deep carbon cycle.We acknowledge financial support from the Engineering and Physical Sciences Research Council United Kingdom (EPSRC) of the United Kingdom.This is the author accepted manuscript. The final version is available from APS via http://dx.doi.org/10.1103/PhysRevB.91.10410

Quasi-molecular and atomic phases of dense solid hydrogen

The high-pressure phases of solid hydrogen are of fundamental interest and relevant to the interior of giant planets; however, knowledge of these phases is far from complete. Particle swarm optimization (PSO) techniques were applied to a structural search, yielding hitherto unexpected high-pressure phases of solid hydrogen at pressures up to 5 TPa. An exotic quasi-molecular mC24 structure (space group C2/c, stable at 0.47-0.59 TPa) with two types of intramolecular bonds was predicted, providing a deeper understanding of molecular dissociation in solid hydrogen, which has been a mystery for decades. We further predicted the existence of two atomic phases: (i) the oC12 structure (space group Cmcm, stable at > 2.1 TPa), consisting of planar H3 clusters, and (ii) the cI16 structure, previously observed in lithium and sodium, stable above 3.5 TPa upon consideration of the zero-point energy. This work clearly revised the known zero-temperature and high-pressure (>0.47 TPa) phase diagram for solid hydrogen and has implications for the constituent structures of giant planets.Comment: accepted in The Journal of Physical Chemistr

What comes first? The dynamics of cerebral oxygenation and blood flow in response to changes in arterial pressure and intracranial pressure after head injury

Background Brain tissue partial oxygen pressure (PbtO2) and near-infrared spectroscopy (NIRS) are novel methods to evaluate cerebral oxygenation. We studied the response patterns of PbtO2, NIRS, and cerebral blood flow velocity (CBFV) to changes in arterial pressure (AP) and intracranial pressure (ICP). Methods Digital recordings of multimodal brain monitoring from 42 head-injured patients were retrospectively analysed. Response latencies and patterns of PbtO2, NIRS-derived parameters [tissue oxygenation index (TOI) and total haemoglobin index (THI)], and CBFV reactions to fluctuations of AP and ICP were studied. Results One hundred and twenty-one events were identified. In reaction to alterations of AP, ICP reacted first [4.3 s; inter-quartile range (IQR) −4.9 to 22.0 s, followed by NIRS-derived parameters and CBFV (10.9 s; IQR: −5.9 to 39.6 s, 12.1 s; IQR: −3.0 to 49.1 s, 14.7 s; IQR: −8.8 to 52.3 s for THI, CBFV, and TOI, respectively), with PbtO2 reacting last (39.6 s; IQR: 16.4 to 66.0 s). The differences in reaction time between NIRS parameters and PbtO2 were significant (P<0.001). Similarly when reactions to ICP changes were analysed, NIRS parameters preceded PbtO2 (7.1 s; IQR: −8.8 to 195.0 s, 18.1 s; IQR: −20.6 to 80.7 s, 22.9 s; IQR: 11.0 to 53.0 s for THI, TOI, and PbtO2, respectively). Two main patterns of responses to AP changes were identified. With preserved cerebrovascular reactivity, TOI and PbtO2 followed the direction of AP. With impaired cerebrovascular reactivity, TOI and PbtO2 decreased while AP and ICP increased. In 77% of events, the direction of TOI changes was concordant with PbtO2. Conclusions NIRS and transcranial Doppler signals reacted first to AP and ICP changes. The reaction of PbtO2 is delayed. The results imply that the analysed modalities monitor different stages of cerebral oxygenatio

The monomer-dimer problem and moment Lyapunov exponents of homogeneous Gaussian random fields

We consider an "elastic" version of the statistical mechanical monomer-dimer problem on the n-dimensional integer lattice. Our setting includes the classical "rigid" formulation as a special case and extends it by allowing each dimer to consist of particles at arbitrarily distant sites of the lattice, with the energy of interaction between the particles in a dimer depending on their relative position. We reduce the free energy of the elastic dimer-monomer (EDM) system per lattice site in the thermodynamic limit to the moment Lyapunov exponent (MLE) of a homogeneous Gaussian random field (GRF) whose mean value and covariance function are the Boltzmann factors associated with the monomer energy and dimer potential. In particular, the classical monomer-dimer problem becomes related to the MLE of a moving average GRF. We outline an approach to recursive computation of the partition function for "Manhattan" EDM systems where the dimer potential is a weighted l1-distance and the auxiliary GRF is a Markov random field of Pickard type which behaves in space like autoregressive processes do in time. For one-dimensional Manhattan EDM systems, we compute the MLE of the resulting Gaussian Markov chain as the largest eigenvalue of a compact transfer operator on a Hilbert space which is related to the annihilation and creation operators of the quantum harmonic oscillator and also recast it as the eigenvalue problem for a pantograph functional-differential equation.Comment: 24 pages, 4 figures, submitted on 14 October 2011 to a special issue of DCDS-

Synthesis, Crystal Structure, and Solid-State NMR Investigations of Heteronuclear Zn/Co Coordination Networks - A Comparative Study

Synthesis and solid-state NMR characterization of two isomorphous series of zinc and cobalt coordination networks with 1,2,4-triazolyl benzoate ligands are reported. Both series consist of 3D diamondoid networks with four-fold interpenetration. Solid-state NMR identifies the metal coordination of the ligands, and assignment of all 1H and 13C shifts was enabled by the combination of 13C editing, FSLG-HETCOR spectra, and 2D 1H–1H back-to-back (BABA) spectra with results from NMR-CASTEP calculations. The incorporation of Co2+ replacing Zn2+ ions in the MOF over the full range of concentrations has significant influences on the NMR spectra. A uniform distribution of metal ions is documented based on the analysis of 1H T1 relaxation time measurements

Identifying an avid eating profile in childhood: Associations with temperament, feeding practices and food insecurity

This study aimed to identify distinct eating behaviour profiles in young children and examine how other key predictors of children's eating behaviour, including child temperament, the experience of food insecurity, or parental feeding practices, may vary by identified profiles. An online survey was conducted with 995 parents/carers living in England and Wales (N = 995, Mage = 35.4 years, 80% female, 88% White). Participants reported on their child's eating behaviour using the Child Eating Behaviour Questionnaire and completed measures of child temperament, household food security and parental feeding practices. Latent Profile Analysis (LPA) was carried out to identify distinct eating profiles amongst the children (36–72 months, Mage = 48.8 months, 52% female). Four eating profiles emerged from the sample of children: (a) avid eating, (b) avoidant eating, (c) happy eating, and (d) typical eating. Avid eating (21.9% of children) was characterised by higher levels of food responsiveness, enjoyment of food, and emotional over-eating in combination with lower satiety responsiveness, slowness in eating and food fussiness. Children with an avid eating profile were reported to be more surgent and experienced greater food insecurity than all other eating profiles. Parents of children belonging to the avid eating profile showed significantly greater use of food for emotional regulation, varied and balanced food provision, restriction of food for health, and restriction of food for weight feeding practices than the three other eating profiles

Anomalous Aharonov--Bohm gap oscillations in carbon nanotubes

The gap oscillations caused by a magnetic flux penetrating a carbon nanotube represent one of the most spectacular observation of the Aharonov-Bohm effect at the nano--scale. Our understanding of this effect is, however, based on the assumption that the electrons are strictly confined on the tube surface, on trajectories that are not modified by curvature effects. Using an ab-initio approach based on Density Functional Theory we show that this assumption fails at the nano-scale inducing important corrections to the physics of the Aharonov-Bohm effect. Curvature effects and electronic density spilled out of the nanotube surface are shown to break the periodicity of the gap oscillations. We predict the key phenomenological features of this anomalous Aharonov-Bohm effect in semi-conductive and metallic tubes and the existence of a large metallic phase in the low flux regime of Multi-walled nanotubes, also suggesting possible experiments to validate our results.Comment: 7 figure

The Management of Bilateral Ureteric Injury following Radical Hysterectomy

Iatrogenic ureteric injury is a well-recognised complication of radical hysterectomy. Bilateral ureteric injuries are rare, but do pose a considerable reconstructive challenge. We searched a prospectively acquired departmental database of ureteric injuries to identify patients with bilateral ureteric injury following radical hysterectomy. Five patients suffered bilateral ureteric injury over a 6-year period. Initial placement of ureteric stents was attempted in all patients. Stents were placed retrogradely into 6 ureters and antegradely into 2 ureters. In 1 patient ureteric stents could not be placed and they underwent primary ureteric reimplantation. In the 4 patients in which stents were placed, 2 were managed with stents alone, 1 required ureteric reimplantation for a persistent ureterovaginal fistula, and 1 developed a recurrent stricture. No patient managed by ureteric stenting suffered deterioration in serum creatinine. We feel that ureteric stenting, when possible, offers a safe primary management of bilateral ureteric injury at radical hysterectomy