crystal structure

Examples of crystal structure in the following topics:

• Crystal Structure: Packing Spheres

  • The unique arrangement of atoms or molecules within a crystalline solid is referred to as the crystal structure of that material.
  • A crystal structure reflects the periodic pattern of the atoms which compose a crystalline substance.
  • The crystal lattice represents the three-dimensional structure of the crystal's atomic/molecular components.
  • The most common way to describe a crystal structure is to refer to the size and shape of the material's characteristic unit cell, which is the simplest repeating unit within the crystal.
  • Each sphere that participates in a crystal structure has a coordination number, which corresponds to the number of spheres within the crystalline structure that touch the sphere that is being evaluated.

• Crystal Structure: Closest Packing

  • A crystalline material's structure is typically visualized as being composed of unit cells.
  • These cells are periodically arranged to give rise to a crystal's lattice structure.
  • The three dimensional structure of a solid crystalline material is established through the periodic patterning of the atoms that make up the crystal.
  • The packing efficiency is the fraction of volume in a crystal structure that is occupied by constituent particles, rather than empty space.
  • Discuss the two ways in which atoms/molecules pack in the most efficient way in crystals.

• Determining Atomic Structures by X-Ray Crystallography

  • X-ray crystallography is a method for determining the arrangement of atoms within a crystal structure.
  • The best x-ray crystallographic structures are derived from the purest crystal samples, meaning samples that contain only molecules of one type and as few impurities as possible.
  • The crystal is typically rotated with respect to different axes and shot again with X-rays, so that diffraction patterns from all angles of the X-rays hitting the crystal are recorded.
  • The final result is the three-dimensional structure of the molecules in the crystal.
  • An X-ray diffraction pattern of a crystallized protein molecule.

• Liquid to Solid Phase Transition

  • Most liquids freeze by crystallization, the formation of a crystalline solid from the uniform liquid.
  • Crystallization consists of two major events: nucleation and crystal growth.
  • Nucleation is the step in which the molecules start to gather into clusters (on the scale of nanometers), arranging themselves in the periodic pattern that defines the crystal structure.
  • The crystal growth is the subsequent growth of the nuclei that succeed in achieving and surpassing the critical cluster size.
  • When sugar is supersaturated in water, nucleation will occur, allowing sugar molecules to stick together and form large crystal structures.

• Crystalline Solids

  • Some decompose before melting, a few sublime, but a majority undergo repeated melting and crystallization without any change in molecular structure.
  • In some rare cases of nonpolar compounds of similar size and crystal structure, a true solid solution of one in the other, rather than a conglomerate, is formed.
  • In addition to the potential complications noted above, the simple process of taking a melting point may also be influenced by changes in crystal structure, either before or after an initial melt.
  • It has a rigid flat molecular structure, and in dilute solution has a light yellow color.
  • The crystal colors range from bright red to violet.

• Ionic Crystals

  • An ionic crystal consists of ions bound together by electrostatic attraction.
  • The arrangement of ions in a regular, geometric structure is called a crystal lattice.
  • Examples of such crystals are the alkali halides, which include:
  • Consider the structure of cesium chloride, CsCl.
  • Halite forms cubic crystals.

• Properties of Quartz and Glass

  • Quartz crystals have piezoelectric properties.
  • An early use of this property of quartz crystals was in phonograph pickups, where the mechanical movement of the stylus in the groove generates a proportional electrical voltage by creating stress within a crystal.
  • Today, a crystal oscillator is a common piezoelectric use for quartz: the vibration frequency of the crystal is used to generate an electrical signal of very precise frequency.
  • The cryptocrystalline (crystals barely visible under microscope) varieties are either translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline (large crystals identified by sight).
  • This diagram shows the crystal structure of quartz.

• Covalent Crystals

  • This means that the entire crystal is, in effect, one giant molecule.
  • Similarly, a covalent solid cannot "melt" in the usual sense, since the entire crystal is one giant molecule.
  • Its structure is very much like that of diamond, with every other carbon replaced by silicon.
  • Hexagonal boron nitride, a two-dimensional material, is similar in structure to graphite.
  • Cubic boron nitride adopts a crystal structure, which can be constructed by replacing every two carbon atoms in diamond with one boron atom and one nitrogen atom.

• Molecular Crystals

  • For example, solid phosphorus can crystallize in different allotropes called "white", "red" and "black" phosphorus.
  • White phosphorus forms molecular crystals composed of tetrahedral P4 molecules.
  • Heating white phosphorus under high (GPa) pressures converts it to black phosphorus, which has a layered, graphite-like structure.
  • Although white phosphorus is an insulator, the black allotrope, which consists of layers extending over the whole crystal, does conduct electricity.
  • However, they can convert into covalent allotropes having atomic chains extending all through the crystal.

• Metallic Crystals

  • Metallic crystals are held together by metallic bonds, electrostatic interactions between cations and delocalized electrons.
  • Several metals adopt both structures, depending on the temperature.
  • The high density of most metals is due to the tightly packed crystal lattice of the metallic structure.
  • Electrical conductivity, as well as the electrons' contribution to the heat capacity and heat conductivity of metals, can be calculated from the free electron model, which does not take the detailed structure of the ion lattice into account.