chap 3 - The Structure of Crystalline Solids

Flashcards on Crystalline Solids

Physical properties of solid materials depend mainly on what?

The arrangement of atoms, ions, or molecules and the bonding forces between them.

What is a Crystalline Solid?

A solid with atoms or ions arranged in a repeating three-dimensional pattern.

Give three examples of crystalline solids

Metals, Alloys, and Ceramics

What is a Unit Cell?

Small groups of atoms forming a repetitive pattern in a crystalline solid.

What does the Lattice Parameter describe?

Describes the size and shape of the unit cell, including side dimensions and angles.

What is the Coordinate Number (CN)?

The number of atoms touching a particular atom or the number of nearest neighbors.

How is density calculated?

Atomic weight (g/mol) / (Unit-cell volume * Avogadro’s number)

What is the Atomic Packing Factor (APF)?

The fraction of space occupied by atoms in a unit cell, assuming they are hard spheres.

How is APF calculated?

(number of atoms/unit cell)*(volume of each atom) / volume of unit cell

What type of atomic bonding is present in FCC structures?

Metallic

List some familiar metals having FCC structure.

Copper, Aluminum, Silver, Gold

What is the APF for FCC structures?

0.74

What is the CN for FCC structures?

12

How are the cubic edge length 'a' and atomic radius 'R' related in FCC structures?

a = 2 * R * sqrt(2)

List some familiar metals having BCC structure.

Chromium, Iron, and Tungsten

What is the APF for BCC structures?

0.68

What is the CN for BCC structures?

8

How are the unit cell length 'a' and the atomic radius 'R' related in BCC structures?

a * sqrt(3) = 4R

List some familiar metals having HCP structure.

Cadmium, Magnesium, and Zinc

What is the APF for HCP structures?

0.74

What is the CN for HCP structures?

12

What are the lattice parameters of HCP structures?

c=1.633a a=2R

How do we locate atom positions in cubic unit cells?

x, y, and z axes

How is a crystallographic direction defined?

A line between two points, or a vector.

When are directions said to be crystallographically equivalent?

The atom spacing along each direction is the same

Describe how crystallographic directions are determined

Multiply these three numbers by a factor to reduce them to the smallest integer number, and enclose the three indices are enclosed in square bracket

How are Miller Indices determined for crystallographic planes in cubic unit cells?

Choose a plane that does not pass through the origin, determine intercepts of the plane in terms of crystallographic x, y, and z axes for a unit cube, form the reciprocal of these intercepts, and clear fractions to determine the smallest set of whole numbers

How are crystal planes in HCP unit cells identified?

Identified by using four indices (Miller-Bravais indices) and are denoted by (hkil).

FCC and HCP crystal structures are described in terms of what characteristic?

Close-Packed Planes

FCC and HCP structures differ because of what?

Stacking Arrangement of Planes

What planes are close-packed in HCP unit cells?

(0001) and (0002)

What planes are close-packed in FCC structures?

{111}

How is volume density calculated?

mass/unit cell

How is Planar Atomic Density (PAD) calculated?

equiv.no.of atoms whose centers are intersected by selected area / selected area

How is Linear Atomic Density (LAD) calculated?

no.of atom diametersintersected by selected length of line in direction of interest / selected length of line

What is a Single Crystal?

A crystalline solid where the periodic arrangement of atoms is perfect throughout the entire specimen without interruption.

What are Polycrystalline Materials?

Materials composed of many small crystals or grains.

What occurs after the formation of small crystals during the solidification of a polycrystalline?

Growth of small grains

What is anisotropy?

Directionality of properties.

What are Non-crystalline Solids?

They lack a systematic and regular arrangement of atoms over relatively large distances. They are also called Amorphous.

What leads to non-crystalline structures?

Rapid cooling.

What method is used to determine crystal structures?

X-ray diffraction investigations.

X-rays used for diffraction are what kind of waves?

Electromagnetic

Where are electrons released via thermionic emission during X-ray production?

Tungsten filament of the cathode

When are X-rays given off?

Electrons strike the metal target.

What is Bragg's Law?

nλ = 2dhkl sinθ

What is a diffractometer?

An apparatus used to determine the angles at which diffractions occur for powdered specimens.

What does a diffractometer plot?

The intensity of diffracted beams as a function of 2θ

What values indicate first, second, and third order diffractions?

n=1, 2, and 3

How do you determine crystal structures from x-ray results?

Reflection Present: For BCC, (h+k+l)=even number, For FCC, (h,k,l) all odd or all even