Crystal Structure and Diffraction Lecture

Vocabulary flashcards covering key concepts related to crystal structure and diffraction, focusing on X-ray diffraction, powder diffraction, single crystal diffraction, and neutron diffraction.

X-Ray Diffraction

Atoms in a crystal act as scattering centers when illuminated by X-rays.

Bragg's Equation

The angle of the incident X-rays to a particular set of crystal planes.

X-ray Source

Generated by bombarding a metal target with high energy electrons, causing them to emit X-rays of a specific frequency when they fall back to lower energy levels.

Single Crystal XRD

Provides more crystallographic information but requires larger crystals.

Powder XRD

Used due to the difficulty in preparing large single crystals, making it a routine method for studying and analyzing materials.

Typical Powder

Contains millions of small crystalline particles in random orientations, resulting in diffraction rings.

Focusing Circle

The arrangement where the X-ray source, sample powder, and detector are placed on the circumference of a circle to focus the X-rays and increase the detected signal intensity.

Factors Determining X-Ray Pattern

Crystal class, lattice type, symmetry, unit cell parameters, and the distribution and type of atoms in the unit cell.

Identification of Unknown Materials

Used to "fingerprint" particular materials and identify them by comparing XRD patterns to a database like the JCPDS.

Phase Purity

Since different phases in a mixture can be identified and quantified, XRD is used to determine the purity of a particular phase and monitor reactions.

Indexing

Assigning Miller indices to the reflections in an XRD pattern to obtain unit cell dimensions.

Systematic Absences

Missing reflections due to destructive interference, such as in a body-centered cubic lattice where h+k+l must be an even number for a reflection to be seen.

Crystallite Size

The average size of crystals affecting the width of peaks in the XRD pattern, where smaller crystals result in wider peaks.

Rietveld Analysis

A computational technique that adjusts a "guessed" structure to fit the data and is widely used for structural determination.

Ewald Sphere

Determines which diffraction spots should be visible for a specific crystal orientation.

Atomic Scattering Factors

Related to the number of electrons around an atom and used in analysis to generate electron density maps of a crystal.

Neutron Diffraction

A technique that diffracts neutrons from atoms of a crystal to give a diffraction pattern, useful for studying materials with light atoms, atoms of similar atomic number, and magnetic properties.

Magnetic Properties

Uses neutron diffraction to study the magnetic properties of crystal structures.