Chapter 3 - Crystalline Solids Flashcards
crystalline material
atoms are situated in a repeating or periodic array over large atomic distances
materials that form crystalline structures
all metals, many ceramic materials, certain polymers
crystal structure
the manner in which atoms, ions or molecules are spatially arranged
atomic hard-sphere model
when atoms or ions are thought of as being solid spheres having well-defined diameters, where spheres touch one another
lattice
three-dimensional array of points coinciding with atom positions
unit cells
small repeats entities in crystal structures
common crystal structures of metals (3)
face-centered cubic, body-centered cubic, hexagonal close-packed
face-centered cubic crystal structure (FCC)
unit cell of cubic geometry with atoms located at each of the corners and the centers of all the cube faces, total of four whole atoms may be assigned to a given unit cell, ex: gold, silver, copper, aluminum, APF = 0.74
Coordination number
the number of nearest-neighbor or touching atoms
atomic packing factor (APF)
the sum of the sphere volumes of all atoms within a unit cell, divided by the unit cell volume, FCC: 0.74, BCC: 0.68, HCP: 0.74
body-centered cubic crystal structure (BCC)
unit cell of cubic geometry with atoms located at each of the eight corners and a single atom at the cube center, two atoms are associated with each unit cell, ex: chromium, iron, tungsten, APF = 0.68
hexagon close-packed crystal structure (HCP)
unit cell of hexagonal geometry, equivalent of six atoms are associated with each unit cell, ex: cadmium, magnesium, titanium, zinc, APF = 0.74
polymorphism
when a metal or nonmetal has more than one crystal structure, often a change in density and other physical properties accompanies a polymorphic transformation
allotropy
when polymorphism is founding elemental solids
lattice parameters
aka interracial angles and acial lenghts
crystal systems (7)
- cubic
- tetragonal
- hexagonal
- orthorhombic
- rhombohedral
- monoclinic
- triclinic
crystal system definition
the different possible combinations of lattice parameters
cubic crystal structure
axial relationships: a = b = c
interaxial angles: α = β = γ = 90º
hexagonal crystal structure
axial relationships: a = b ≠ c
interaxial angles: α = β = 90º, γ = 120º
tetragonal crystal structure
axial relationships: a = b ≠ c
interaxial angles: α = β = γ = 90º
rhombohedral (trigonal) crystal structure
axial relationships: a = b = c
interaxial angles: α = β = γ ≠ 90º
orthorhombic crystal structure
axial relationships: a ≠ b ≠ c
interaxial angles: α = β = γ = 90º
monoclinic crystal structure
axial relationships: a ≠ b ≠ c
interaxial angles: α = γ = 90º ≠ β
triclinic crystal structure
axial relationships: a ≠ b ≠ c
interaxial angles: α ≠ β ≠ γ ≠ 90º
crystallographic direction
a line between two points, or a vector
miller-bravais
coordinate system with four axes, a1, a2, a3 are 120º apart from each other with z perpendicular to them
linear density (LD)
number of atoms centered on direction vector divided by length of the direction vector
planar density (PD)
number of atoms centered on plane divided by area of plane
single crystal
when the periodic and repeated arrangement of atoms is perfect or extends throughout the entirety of the specimen without interruption, unit cells interlock the same way with the same orientation
grains
small crystals
polycrystalline
materials made up of a collection of grains
grain boundary
the atoms mismatch where two grains meet
anisotropy
the directionality of properties, associated with the variance of atomic or ionic spacing with crystallographic direction, function of the symmetry of the crystal structure
isotropic
substances in which measured properties are independent of the direction of measurement
diffraction
occurs when a wave encounters a series of regularly spaced obstacles that are capable of scattering the wave and have spacings that are comparable in magnitude to the wavelength
diffractometer
apparatus used to determine the angles at which diffraction occurs for powdered specimens
noncrystalline
lack a systematic and regular arrangement of atoms over relatively large atomic distances
amorphous
without form, atomic structure resembles a liquid
