E. F. Skelton

High pressure phase transitions in tetrahedrally coordinated semiconducting compounds

Abstract New experimental results are reported for structural transitions at high pressure in several III–V compounds and two II–VI compounds. These data, together with earlier results, are then compared with the predictions of model calculations of Van Vechten. Experimental transition pressures are often at variance with calculated values. However, his calculation assumes that the high pressure phase is metallic, with the β-Sn structure. The present results show that several compounds assume an ionic NaCl structure at high pressure, while others have neither the β-Sn nor NaCl structure.

The effect of particle size on the structural transitions in zinc sulfide

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.

Pressure induced structural transitions in nanometer size particles of PbS

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.

Evidence of strain and lattice distortion in lead sulfide nanocrystallites

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.

Synchrotron radiation x-ray fluorescence analysis

A systematic study of the minimum detection limits obtainable with synchrotron radiation continuum excitation has been made for thin samples of 10 elements distributed across the periodic table. Energy dispersion and wavelength dispersion measurements are included; the results show that both techniques achieve comparable values, ranging from 10/sup -3/ to 10/sup -1/ /sup +/g/cm/sup 2/, where line interference is absent. With the very small primary beam used for the energy dispersion measurements, this represents absolute detectability of 10/sup -12/ to 10 /sup -10/ g, in reasonable agreement with theoretical predictions. 4 figures, 5 tables.

Transport spin polarization of Ni_{x}Fe_{1-x}: Electronic kinematics and band structure

We present measurements of the transport spin polarization of Ni_xFe_{1-x} (0<x<1) using the recently-developed Point Contact Andreev Reflection technique, and compare them with our first principles calculations of the spin polarization for this system. Surpisingly, the measured spin polarization is almost composition-independent. The results clearly demonstrate that the sign of the transport spin polarization does not coincide with that of the difference of the densities of states at the Fermi level. Calculations indicate that the independence of the spin polarization of the composition is due to compensation of density of states and Fermi velocity in the s- and d- bands.

Evidence for bond strengthening in Cd1−xZnxTe (x=0.04)

Hydrostatic compression data have been obtained for Cd1−xZnxTe(x=0.04±0.01) using energy dispersive x‐ray diffraction techniques with heterochromatic, synchrotron produced radiation. The B3 to B1 structural transition pressure is observed to occur at 3.25±0.15 GPa, in agreement with published work on CdTe. The volume discontinuity at the transition is observed to be 15±1%, which is also in agreement with that found for CdTe. However, compression data for each of the two phases indicate that Cd1−xZnxTe is about 2% stiffer than CdTe, suggesting that the CdTe bond is strengthened by the addition of Zn. This is in agreement with recent bond strength calculations and provides the first microscopic, experimental evidence of this strengthening effect.

Gold as a reliable internal pressure calibrant at high temperatures

Energy‐dispersive x‐ray diffraction measurements on a mixture of powdered NaCl and Au were carried out at high temperatures up to 600 °C (at atmospheric pressure) and also at simultaneously high temperatures and high pressures (to 425 °C and 10.2 GPa). A modified diamond‐anvil high pressure cell with a mini resistance‐wire heater was used in conjunction with the synchrotron radiation at the Stanford Synchrotron Radiation Laboratory. The temperature of the sample was measured directly by a precalibrated Pt‐Pt 10% Rh thermocouple adjacent to the sample. The pressure of the sample was determined by measuring the molar volume changes in NaCl and deriving the pressure through an equation of state. The measured thermal expansion from 25 °C up to 600 °C for NaCl and Au at 1 atm can be expressed by second‐order polynomials: V/V0(NaCl)=1+(1.08±0.05)×10−4 (T−25)+(7.5±1.1)×10−8 (T−25)2 and V/V0(Au) =1+(4.26±0.85)×10−5 (T−25)+(5.0±20)×10−9 (T−25)2, where T is in °C. These results are in excellent agreement with previ...

Transparent conducting films of In2O3-ZrO2, SnO2-ZrO2 and ZnO-ZrO2

The optical transparencies and electrical conductivities of thin films of In 2 O 3 , SnO 2 and ZnO mixed with ZrO 2 have been investigated. These films were deposited on glass substrates at room temperature using pulsed-laser deposition. Indium-zirconium oxide films with a ZrO 2 content up to 15 wt.% were conducting and more than 80% transparent from 450-700 nm. As the ZrO 2 content increased from 0 to 15 wt.%, the electrical resistivities increased from 1.28 × 10 -3 to 6.48 × 10 -2 Ω cm; the carrier densities decreased from 2.14 × 10 20 to 1.0 × 10 18 cm -3 ; and the Hall mobilities decreased from 21 to 5 cm 2 V -1 s -1 , all monotonically. The electrical resistivities of tin-zirconium oxide increased from 1.65 X 10 -2 to 7.33 X 10 -2 Ω cm as the ZrO 2 content varied between 0 and 5 wt.%, whereas the resistivities of the zinc-zirconium oxide varied from 3 × 10 -2 to 3.5 × 10 -1 Ω cm for ZrO 2 contents between 0 and 10 wt.%.

Polymorphism and the crystal structures of InSb at elevated temperature and pressure

Polycrystalline x‐ray diffraction data are reported for three high‐pressure phases of InSb. InSb‐II patterns are indexed on the basis of a body‐centered tetragonal lattice with unit cell parameters a=5.810 A and c=3.136 A at P∼4.3 GPa and T≳100 °C; a unique space group has not been assigned. Possible space groups and structures have been determined for the other high‐pressure phases. Contrary to published results, InSb‐III is orthorhombic with a=5.712 A, b=5.357 A, c=3.063 A at 11.5 GPa and room temperature, the space group is either Pmmm or Pmmn, and Z=2. InSb‐IV is also orthorhombic with a=2.919 A, b=5.618 A, and c=3.066 A at P=4.4 GPa and room temperature; the space group is Pmm2, and Z=1. Atomic coordinates and appropriate bond angles are also reported.