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Pressure induced densification in a molecular arsenic sulfide glass is studied at ambient temperature using x-ray scattering, absorption and Raman spectroscopic techniques in situ in a diamond anvil cell. The relatively abrupt changes in the position of the first sharp diffraction peak, FSDP, and the pressure-volume equation of state near ∼2 GPa suggest a phase transition between low- and high-density amorphous phases characterized by different densification mechanisms and rates. Raman spectroscopic results provide clear evidence that the phase transition corresponds to a topological transformation between a low-density molecular structure and a high-density network structure via opening of the constituent As4S3 cage molecules and bond switching. Pressure induced mode softening of the high density phase suggests a low dimensional nature of the network. The phase transformation is hysteretically reversible, and therefore, reminiscent of a first-order phase transition. [ABSTRACT FROM AUTHOR]

High-resolution x-ray diffraction with diamond-anvil cells, using argon as a quasi-hydrostatic pressure medium, documents the crystal structure and equation of state of osmium to over 60 GPa at room temperature. We find the zero-pressure bulk modulus in fair agreement with other experiments as well as with relativistic electronic band-structure calculations: Osmium is the densest but not the most incompressible element at ambient conditions. We also find no evidence for anomalies in the ratio of unit-cell parameters, c/a, or in the compressibility of osmium as a function of pressure. This is in agreement with other experiments and quantum mechanical calculations but disagrees with recent claims that the electronic structure and equation of state of osmium exhibit anomalies at pressures of ∼15-25 GPa; the discrepancies are may be due to the effects of texturing. [ABSTRACT FROM AUTHOR]

The thermodynamic nature of phase stabilities and transformations are investigated in crystalline and amorphous Ge1Sb2Te4 (GST124) phase change materials as a function of pressure and temperature using high-resolution synchrotron x-ray diffraction in a diamond anvil cell. The phase transformation sequences upon compression, for cubic and hexagonal GST124 phases are found to be: cubic → amorphous → orthorhombic → bcc and hexagonal → orthorhombic → bcc. The Clapeyron slopes for melting of the hexagonal and bcc phases are negative and positive, respectively, resulting in a pressure dependent minimum in the liquidus. When taken together, the phase equilibria relations are consistent with the presence of polyamorphism in this system with the as-deposited amorphous GST phase being the low entropy low-density amorphous phase and the laser melt-quenched and high-pressure amorphized GST being the high entropy high-density amorphous phase. The metastable phase boundary between these two polyamorphic phases is expected to have a negative Clapeyron slope. [ABSTRACT FROM AUTHOR]

We studied the compressibility of monolithic fully dense electrodeposited nanocrystalline Ni (∼29 nm grain size) under both quasihydrostatic and nonhydrostatic conditions up to a nominal pressure of 50 GPa using angle-dispersive x-ray diffraction. We obtained an equation of state consistently and unambiguously from each measured reflection. The apparent bulk modulus measured under nonhydrostatic conditions is larger than that of the corresponding coarse-grained material under either type of compression, but is nearly the same when measured under quasihydrostatic conditions. These results may be consistent with a strength, but not necessarily a bulk modulus, that is enhanced in the nanomaterial relative to its coarse-grained counterparts. [ABSTRACT FROM AUTHOR]

A novel sample chamber has been developed for the study of chemical reactions involving air sensitive samples using energy-dispersive powder diffraction. The apparatus consists of a reaction vessel, automatic reactant dispensing system, heating jacket, stirrer, and gas purge system. The use of this apparatus is illustrated with data collected on station 9.7 of the UK Synchrotron Radiation Source during a study on the intercalation of cobaltocene into tin disulfide. [ABSTRACT FROM AUTHOR]

The design and implementation of a flexible and modular digital control and data acquisition system for scanned probe microscopes (SPMs) is presented. The measured performance of the system shows it to be capable of 14-bit data acquisition at a 100-kHz rate and a full 18-bit output resolution resulting in less than 0.02-Å rms position noise while maintaining a scan range in excess of 1 µm in both the X and Y dimensions. This level of performance achieves the goal of making the noise of the microscope control system an insignificant factor for most experiments. The adaptation of the system to various types of SPM experiments is discussed. Advances in audio electronics and digital signal processors have made the construction of such high performance systems possible at low cost. [ABSTRACT FROM AUTHOR]

X-ray powder diffraction measurements using synchrotron radiation are currently being carried out on three experimental stations on the SRS at Daresbury Laboratory. Station 9.1 contains a general purpose powder diffractometer for angular dispersive measurements. The instrument is sited on the wiggler beamline, which provides radiation with a critical energy of 12 keV. A Si ( 111 ) water-cooled, channel-cut monochromator gives useful flux over an energy range from 6 to 30 keV. A cryostat and furnace are available for experiments requiring special environments, while a curved position sensitive detector can be used for time-resolved measurements. The wiggler white beam is employed on Station 9.7 for energy dispersive powder diffraction experiments. Various sample stages for kinetic experiments are in use on this station, and examples are shown of recent high-pressure work. A new high resolution powder diffractometer has been built and commissioned on Station 8.3, using bending magnet radiation. The diffractometer is based on the Parrish and Hart design, in which long, parallel foils are used to define the angular divergence accepted by the detector. In this way, large synchrotron beam cross sections can be employed while maintaining high angular resolution. [ABSTRACT FROM AUTHOR]

We report on the optical investigation of single electron spins bound to fluorine donor impurities in ZnSe. Measurements of photon antibunching confirm the presence of single, isolated optical emitters, and magneto-optical studies are consistent with the presence of an exciton bound to the spin-impurity complex. The isolation of this single-donor-bound-exciton complex and its potential homogeneity offer promising prospects for a scalable semiconductor qubit with an optical interface. [ABSTRACT FROM AUTHOR]

A new data acquisition system tailored to the rapid collection of time-resolved powder diffraction data has been constructed. It is used for the study of structural phase transitions at the energy-dispersive powder diffraction facility of the UK synchrotron radiation source (SRS). The data acquisition system and some data collected during a study of the 196 °C phase transition in ammonium chloride are presented here. [ABSTRACT FROM AUTHOR]

The current status of a Unix-based VME data acquisition development project is described. It is planned to use existing Fortran data collection software to drive the existing CAMAC electronics via a VME CAMAC branch driver card and associated Daresbury Unix driving software. The first usable Unix driver has been written and produces single-action CAMAC cycles from test software. The data acquisition code has been implemented in test mode under Unix with few problems and effort is now being directed toward finalizing calls to the CAMAC-driving software and ultimate evaluation of the complete system. [ABSTRACT FROM AUTHOR]

A high-pressure synchrotron x-ray diffraction study of the phase change alloy Ge1Sb2Te4 demonstrates the existence of a polyamorphic phase transition between the 'as deposited' low density amorphous (LDA) phase and a high density amorphous (HDA) phase at ∼10 GPa. The entropy of the HDA phase is expected to be higher than that of the LDA phase resulting in a negative Clapeyron slope for this transition. These phase relations may enable the polyamorphic transition to play a role in the memory and data storage applications. [ABSTRACT FROM AUTHOR]