Structures, Phase Transitions and Tricritical Behavior of the Hybrid Perovskite Methyl Ammonium Lead Iodide.

We have examined the crystal structures and structural phase transitions of the deuterated, partially deuterated and hydrogenous organic-inorganic hybrid perovskite methyl ammonium lead iodide (MAPbI3) using time-of-flight neutron and synchrotron X-ray powder diffraction. Near 330 K the high temperature cubic phases transformed to a body-centered tetragonal phase. The variation of the order parameter Q for this transition scaled with temperature T as Q ∼ (Tc-T)ß, where Tc is the critical temperature and the exponent ß was close to », as predicted for a tricritical phase transition. However, we also observed coexistence of the cubic and tetragonal phases over a range of temperature in all cases, demonstrating that the phase transition was in fact first-order, although still very close to tricritical. Upon cooling further, all the tetragonal phases transformed into a low temperature orthorhombic phase around 160 K, again via a first-order phase transition. Based upon these results, we discuss the impact of the structural phase transitions upon photovoltaic performance of MAPbI3 based solar cells.

Chain conformation in polymer nanocomposites with uniformly dispersed nanoparticles.

The effect of nanoparticles (NP) on chain dimensions in polymer melts has been the source of considerable theoretical and experimental controversy. We exploit our ability to ensure a spatially uniform dispersion of 13 nm silica NPs miscible in polystyrene melts, together with neutron scattering, x-ray scattering, and transmission electron microscopy, to show that there is no measurable change in the polymer size in miscible mixtures, regardless of the relative sizes of the chains and the nanoparticles, and for NP loadings as high as 32.7 vol%. Our results provide a firm basis from which to understand the properties of polymer nanocomposites.

Retrograde nailing of femur shaft fractures through the intercondylar notch for specific indications: 10 years experience.

Closed antegrade intramedullary nailing is the current standard of care for the treatment of femoral shaft fractures. This technique has been shown to have excellent results demonstrated by a high rate of fracture union and the low incidence of infection. However, there are complications specific to antegrade intramedullary nailing, as well as specific injury patterns that make this treatment less attractive.

Safe extracapsular placement of proximal tibia transfixation pins.

OBJECTIVE: To identify the anatomic detail of the knee joint capsular insertion site on the proximal tibia, specifically as it relates to transfixation pins. DESIGN: Identification of capsular anatomy by anatomical dissection of cadaveric specimens, with radiography and arthroscopy of patients. SETTING: Cadaveric dissection. OUTCOME MEASURES: Anatomic observation of the capsular attachment site in relation to the tibial articular surface. RESULTS: The capsule inserts four to fourteen millimeters below the articular surface in a regular pattern. The anterior half of the circumference is close to the joint line (less than six millimeters). Posteromedially and posterolaterally, there are extensions distally to fourteen millimeters, occasionally communicating with the tibiofibular joint. CONCLUSION: Transfixing wires and half-pins can be placed in the proximal tibia without capsular penetration if kept more than fourteen millimeters from the subchondral line. If wire placement closer to the joint is required, wires should be placed in Zone 1 (the anterior half) and at least six millimeters from subchondral bone to avoid capsular penetration.

Design of three-element night-vision goggle objectives.

Night-vision goggle objectives are often composed of six or more conventional lens elements with spherical surfaces and homogeneous refractive indices. Special elements such as aspheres, diffractive optics, and gradient-index materials can be used to reduce the total number of lenses required to meet military design specifications. A study was performed to examine the use of various combinations of these special surfaces to determine the minimum number of elements that can be used to construct the objective system. We present and compare the best resulting designs.

Habitat linkages: the effect of intertidal saltmarshes and adjacent subtidal habitats on abundance, movement, and growth of an estuarine fish.

In this study we used pinfish (Lagodon rhomboides) in field experiments to examine linkages between intertidal saltmarsh and adjacent subtidal habitats. Pinfish are more than twice as abundant in intertidal marshes adjacent to seagrass beds than in those adjacent to the unvegetated subtidal bottom. Movement of pinfish between the marsh edge and the adjacent subtidal habitat was greater for fish captured in areas with both intertidal and subtidal vegetation than in those with intertidal vegetation and adjacent unvegetated mudflats. This movement provides an important link between habitats, allowing transfer of marsh-derived secondary production to subtidal seagrass beds and vice versa. Pinfish held in enclosures with both intertidal and subtidal vegetation were, on average, approximately 90% heavier than fish held in enclosures with intertidal vegetation and unvegetated subtidal bottom. Because saltmarshes and seagrass beds contribute to the production of living marine resources, active measures are being taken to preserve and restore these habitats. The results from this study have direct application to decisions concerning site selection and optimal spatial proximity of saltmarsh and seagrass habitats in the planning of restoration and mitigation projects. To maximize secondary production and utilization of intertidal marshes, managers may opt to restore and/or preserve marshes adjacent to subtidal seagrass beds.

Rigid endoscopic relay systems: a comparative study.

Relay systems in rigid endoscopes can compensate for aberrations in the objective and the eyepiece. Five classes of rigid relay systems are examined: conventional; glass Hopkins; plastic lens, glass-rod Hopkins; modified Hopkins; and gradient index. First-order theory, including achromatization, is developed for these systems. Design results are presented, and aberration, vignetting, and system length trade-offs are analyzed.

Oxygen Isotope Effect and Structural Phase Transitions in La2CuO4-Based Superconductors.

The oxygen isotope effect on the superconducting transition temperature (alpha(o)) varies as a function of x in La2-xSrxCuO(4) and La2-xBaxCuO(4), with the maximum alpha(o) values (alpha(o) >/= 0.5) found for x near 0.12. This unusual x dependence implies that the isotope effect is influenced by proximity to the Abma --> P4(2)/ncm structural phase transition in these systems. Synchrotron x-ray difaction measurements reveal little change in lattice parameters or orthorhombicity due to isotope exchange in strontium-doped materials where alpha(o) > 0.5, eliminating static structural distortion as a cause of the large isotope effects. The anomalous behavior of alpha(o) in both strontium- and barium-doped materials, in combination with the previously discovered Abma --> P4(2)/ncm structural phase-transition in La(1.88)B(0.12)CuO(4), suggests that an electronic contribution to the lattice instability is present and maximizes at approximately 1/8 hole per copper atom. These observations indicate a dose connection between hole doping of the Cu-O sheets, tilting instabilities of the CuO(6) octahedra, and superconductivity in La(2)CuO(4)-based superconductors.