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Cluster Sampling Method01:20

Cluster Sampling Method

Appropriate sampling methods ensure that samples are drawn without bias and accurately represent the population. Because measuring the entire population in a study is not practical, researchers use samples to represent the population of interest.
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Schottky defects arise when some lattice points in a crystal, such as those in NaCl, remain unoccupied, creating lattice vacancies without disturbing the overall electrical neutrality of the crystal. This defect is common in ionic crystals where the positive and negative ions are similar in size, as seen in sodium chloride and cesium chloride. The presence of Schottky defects enables the crystal to conduct electricity to a small extent through an ionic mechanism. Electric fields cause nearby...
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Vacancy clusters, dislocations and brown colouration in diamond.

U Bangert1, R Barnes, M H Gass

  • 1Nanostructured Materials Research Group, School of Materials, The University of Manchester, PO Box 88, Manchester M1 7HS, UK.

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Dislocations in natural diamonds absorb light, contributing to their color. High-pressure, high-temperature annealing causes vacancy clusters near dislocations to grow, reducing diamond

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Area of Science:

  • Materials Science
  • Solid State Physics
  • Geology

Background:

  • Natural diamonds exhibit brown coloration attributed to vacancy clusters.
  • Dislocations in diamonds are known to influence their optical and electronic properties.

Purpose of the Study:

  • Investigate the role of dislocations in sub-bandgap absorption in natural diamonds.
  • Understand the effect of high-pressure, high-temperature (HPHT) annealing on vacancy clusters and diamond coloration.

Main Methods:

  • Optical absorption spectroscopy on natural brown and colorless diamonds.
  • Analysis of diamond samples before and after HPHT treatment.

Main Results:

  • Dislocations exhibit sub-bandgap absorption, similar to theoretical predictions for shuffle dislocation segments.
  • HPHT annealing leads to the growth of vacancy clusters trapped in dislocation strain fields, reducing their numbers and optical absorption.
  • Observed absorption at dislocations is present in both brown and colorless diamonds, suggesting a general phenomenon.

Conclusions:

  • Dislocations contribute to sub-bandgap absorption in diamonds, but vacancy clusters are the primary cause of brown coloration.
  • HPHT annealing effectively reduces diamond coloration by altering vacancy cluster configurations at dislocations.
  • The presence of vacancies at dislocations is crucial for forming specific configurations that influence coloration.