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At elevated pressure, PbS undergoes a first order phase transition from the NaCl or B1 structure to an orthorhombic structure. The effects of particle sizes in the nanometer range on this transition have been investigated using energy-dispersive x-ray diffraction of synchrotron produced wiggler radiation. Relative to the bulk crystals, the onset of transition pressure showed a significant increase with decreasing particle size. The results also show that compressibility increases with decreasing particle size: this increase is continuous through the phase transition.

We report a systematic investigation of dynamical fluctuation effects in the frequency-dependent microwave conductivity (45 MHz-45 GHz) of YBa 2 Cu 3 O 72d thin films for T * T c . Our measurements directly yield a dynamical critical exponent z in the range 2.3 -3.0, and the fluctuation lifetime t fl , which diverges more quickly than Gaussian theory predicts as the temperature approaches T c from above, independent of sample quality. In addition, both the temperature and the frequency dependence of the fluctuation conductivity s fl exhibit scaling behavior for temperatures 1 -2 K above T c , and can be collapsed onto the same universal curve. [S0031-9007(96)01664-X] PACS numbers: 74.25.Nf, 74.40.tk Dynamic fluctuation effects have become an intriguing subject of research in statistical and condensed matter physics, as they depend on the equations of motion of the system, and are not simply determined by the equilibrium distribution of particles at a given instant of time. Over the years, there have been both theoretical and experimental investigations of the static and dynamic fluctuations of many different types of systems. In the high T c superconductors (HTSC), measurements of fluctuation effects reveal unusual behavior, such as 3D XY critical behavior observed in the microwave penetration depth l͑T ͒ over a range of temperature 5-10 K wide below T c [1]. Heat capacity measurements are also consistent with 3D XY critical fluctuations for T T c 6 10 K [2]. In contrast to these results, temperature-dependent dc conductivity measurements have been interpreted in terms of simple 2D or 3D Gaussian fluctuation theory for T . T c [3,4]; also, low frequency penetration depth measurements have been interpreted as consistent with Gaussian fluctuations [5]. Furthermore, analysis of the nonlinear currentvoltage characteristics near T c in the HTSC yield a wide range of values for the relevant critical exponents [6,7].In this Letter we report a systematic experimental study of the microwave fluctuation conductivity s fl obtained from thin films and a single crystal of YBa 2 Cu 3 O 72d (YBCO). The study provides direct information about the fluctuation lifetime t fl , which is one of the important parameters for the identification of the universality class of the phase transition, and which describes the relevant dynamics of the fluctuations in the HTSC. We employ a swept-frequency technique (45 MHz -45 GHz) to measure both the frequency and temperature dependence of the conductivity of YBCO thin films in the vicinity of T c with nominally zero external magnetic field. The extra experimental degree of freedom afforded by access to the measured conductivity over three decades in frequency provides essential additional information about the dynamics of the system not available to temperature dependent measurements alone. Previous experiments have exploited frequency dependent techniques to verify the Gaussian dynamics of fluctuations in thin films of conventional superconductors [8,9]. More recently, frequencydependent co...

Studies of pressure induced phase transformations of ZnS nanoparticles using diamond anvil cells and synchrotron radiation were carried out to 20.0 GPa. Nanoparticles initially in the zinc-blende and wurtzite phases both transformed to the NaCl phase under the application of pressure. The zinc-blende particles, which were of 2.8 nm size, and the wurtzite particles, which were of 25.3 nm size, transformed to the NaCl phase at 19.0 and 15.0 GPa, respectively. Nanoparticles of the wurtzite phase never regained their initial wurtzite structure but returned to the zinc-blende phase upon downloading the pressure. The resultant zinc-blende nanoparticles transformed to the NaCl phase upon the reapplication of a pressure of 15.0 GPa. Nanoparticles initially in the zinc-blende phase returned to their original phase.

X-ray diffraction studies on nanometer sized lead sulfide particles reveal the presence of a compressive strain. A number of samples with particle sizes ranging from 2 to 16 nm were synthesized using the three dimensional periodic, bicontinuous cubic phase as a matrix. Samples of the larger size particles could be indexed to an fcc lattice. As the particle size decreased below 6 nm, a tetragonal distortion of the cubic lattice was observed, accompanied by a decrease in the unit cell volume.