Crystalline structures are like building blocks of solids, with atoms arranged in repeating patterns. These patterns form unit cells, the smallest repeating units, which come in different types like and .
helps us peek inside crystals, revealing their atomic arrangements. By bouncing X-rays off atoms and analyzing the resulting patterns, scientists can uncover the secrets of a crystal's structure and composition.
Crystalline Structures and Arrangements
Arrangement of atoms in crystals
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consist of atoms, ions, or molecules arranged in a regular, repeating pattern that extends in three dimensions
The smallest repeating unit that displays the full symmetry of the crystal structure is known as the
Unit cells are characterized by their lengths (a, b, c) and angles (α, β, γ)
Three main types of unit cells include:
Primitive cubic (P): contains atoms at each corner of the cube (e.g., polonium)
Body-centered cubic (I): features atoms at each corner and one atom in the center of the cube (e.g., sodium)
(F): has atoms at each corner and at the center of each face of the cube (e.g., copper)
Other common crystal structures:
(HCP): consists of a two-dimensional hexagonal lattice with a repeating ABABAB pattern (e.g., magnesium)
(CCP) or face-centered cubic (FCC): comprises a two-dimensional hexagonal lattice with a repeating ABCABC pattern (e.g., gold)
The , which is the number of nearest neighbors an atom or ion has in a crystal structure, varies depending on the lattice type
Crystal Lattice Properties
refers to the percentage of space occupied by atoms or ions in a crystal structure
(h, k, l) are used to describe planes and directions in crystal lattices
are the 14 unique three-dimensional that form the basis for all crystalline materials
Calculation of ionic radii
can be calculated using the edge length of the and the radius ratio of the cation and anion
For a primitive cubic unit cell with edge length a:
rcation+ranion=a
For a with edge length a:
rcation+ranion=23a
For a face-centered cubic unit cell with edge length a:
rcation+ranion=22a
X-ray Diffraction and Crystalline Structure Determination
X-ray diffraction for crystal structures
X-ray diffraction (XRD) is a powerful technique used to determine the arrangement of atoms in a crystalline solid
X-rays scatter off the electrons in the atoms of the crystal, producing a due to the interference of the scattered X-rays
###'s_law_0### describes the relationship between the wavelength of the X-rays (λ), the angle of incidence (θ), and the interplanar spacing (d) in the crystal:
nλ=2dsinθ, where n is an integer (e.g., 1, 2, 3)
The diffraction pattern provides crucial information about the crystal structure, such as the size and shape of the unit cell and the positions of atoms within the unit cell
The intensities of the diffraction peaks are determined by the types and positions of atoms in the unit cell
This information is used to generate an , which reveals the atomic structure of the crystal (e.g., the arrangement of atoms in a protein molecule)