6.1 Solid State Structures

Solid state structures are the building blocks of materials science. They determine how atoms and molecules arrange themselves in solids, influencing properties like hardness, conductivity, and melting point.

Understanding these structures is crucial for developing new materials. From everyday objects to cutting-edge tech, solid state chemistry shapes our world, making it essential for engineers and scientists alike.

Solid State Structures

Classification of Solid State Structures

Top images from around the web for Classification of Solid State Structures

• 

  Crystal - Wikipedia View original

  Is this image relevant?

• 

  The Solid State of Matter | Chemistry: Atoms First View original

  Is this image relevant?

• 

  10.6 Lattice Structures in Crystalline Solids – General Chemistry 1 & 2 View original

  Is this image relevant?

• 

  Crystal - Wikipedia View original

  Is this image relevant?

• 

  The Solid State of Matter | Chemistry: Atoms First View original

  Is this image relevant?

1 of 3

Top images from around the web for Classification of Solid State Structures

• 

  Crystal - Wikipedia View original

  Is this image relevant?

• 

  The Solid State of Matter | Chemistry: Atoms First View original

  Is this image relevant?

• 

  10.6 Lattice Structures in Crystalline Solids – General Chemistry 1 & 2 View original

  Is this image relevant?

• 

  Crystal - Wikipedia View original

  Is this image relevant?

• 

  The Solid State of Matter | Chemistry: Atoms First View original

  Is this image relevant?

1 of 3

• Solid state structures can be classified into crystalline solids, amorphous solids, and quasicrystals based on their degree of long-range order and periodic arrangement of atoms or molecules
• Crystalline solids have a highly ordered, repeating arrangement of atoms or molecules in three dimensions, resulting in a regular lattice structure with well-defined edges and angles (diamond, sodium chloride)
• Amorphous solids lack long-range order and have a random arrangement of atoms or molecules, resulting in a non-crystalline structure with no distinct lattice points or symmetry (glass, plastics)
• Quasicrystals possess a highly ordered structure but lack the translational periodicity of crystalline solids, exhibiting unique symmetrical patterns that are not found in conventional crystals (aluminum-manganese alloys)

Properties of Solid State Structures

• The characteristic properties of solid state structures, such as melting point, hardness, and electrical conductivity, are determined by the nature and strength of the chemical bonds between the constituent atoms or molecules
  • Strong covalent bonds in diamond result in high melting point and hardness
  • Metallic bonds in metals lead to high electrical and thermal conductivity
  • Weak van der Waals forces in molecular solids (ice) result in low melting points and poor electrical conductivity

Structure vs Properties of Solids

Influence of Atomic Arrangement on Properties

• The arrangement of atoms or molecules in a solid state structure directly influences its physical and chemical properties, such as mechanical strength, thermal and electrical conductivity, and optical behavior
• In crystalline solids, the regular and periodic arrangement of atoms or molecules results in unique properties, such as anisotropy (directional dependence of properties) and the presence of cleavage planes (graphite, mica)
• The type and strength of chemical bonds (ionic, covalent, metallic, or van der Waals) between the constituent atoms or molecules determine the overall stability, hardness, and melting point of the solid material
  • Ionic solids (sodium chloride) have high melting points due to strong electrostatic interactions
  • Covalent solids (diamond) exhibit high hardness and thermal stability due to strong directional bonds

Defects and Electronic Band Structure

• The presence of defects, such as vacancies, interstitials, and dislocations, can significantly alter the properties of solid materials, affecting their mechanical behavior, electrical conductivity, and chemical reactivity
  • Vacancies in ionic solids can lead to ionic conductivity (solid electrolytes)
  • Dislocations in metals can increase their strength and ductility
• The electronic band structure of a solid, which arises from the interaction of atomic orbitals, determines its electrical and optical properties, classifying materials as conductors, semiconductors, or insulators
  • Metals have partially filled valence bands, allowing for high electrical conductivity
  • Semiconductors have a small band gap between valence and conduction bands, enabling control of electrical properties through doping (silicon, germanium)
  • Insulators have a large band gap, resulting in poor electrical conductivity (diamond)

Unit Cell Parameters and Crystal Systems

Unit Cell and Lattice Parameters

• The unit cell is the smallest repeating unit that represents the entire crystal structure and is characterized by its lattice parameters (lengths and angles) and the arrangement of atoms or molecules within it
• The lattice parameters (a, b, c, α, β, γ) define the size and shape of the unit cell, with a, b, and c representing the lengths of the cell edges and α, β, and γ representing the angles between them
  • In a cubic unit cell, a = b = c and α = β = γ = 90°
  • In a hexagonal unit cell, a = b ≠ c and α = β = 90°, γ = 120°

Crystal Systems and Bravais Lattices

• The seven crystal systems (triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal, and cubic) describe the symmetry and shape of the unit cell based on the relationships between the lattice parameters
• The Bravais lattices describe the 14 unique ways in which lattice points can be arranged in three-dimensional space, taking into account the centering of atoms or molecules within the unit cell (primitive, body-centered, face-centered, or base-centered)
  • The face-centered cubic (FCC) lattice is common in metals (copper, gold)
  • The body-centered cubic (BCC) lattice is found in metals (iron, tungsten)
• Experimental techniques, such as X-ray diffraction, neutron diffraction, and electron diffraction, can be used to determine the unit cell parameters and crystal structure of a solid material

Symmetry in Crystalline Solids

Symmetry Elements and Point Groups

• Symmetry plays a fundamental role in describing the structure and properties of crystalline solids, as it governs the arrangement of atoms or molecules in the lattice and the resulting physical and chemical behavior
• The symmetry elements present in a crystal structure include rotation axes, mirror planes, inversion centers, and rotoinversion axes, which describe the various operations that leave the structure unchanged
  • A two-fold rotation axis (2) means the structure appears identical after a 180° rotation
  • A mirror plane (m) reflects the structure across a plane, resulting in an identical arrangement
• The 32 crystallographic point groups classify crystal structures based on their symmetry elements, describing the complete set of symmetry operations that can be performed on a unit cell without changing its appearance

Space Groups and Physical Properties

• The 230 space groups combine the 32 point groups with the 14 Bravais lattices to provide a comprehensive description of the symmetry and arrangement of atoms or molecules in three-dimensional crystalline solids
• The presence of certain symmetry elements can lead to specific physical properties, such as piezoelectricity (in non-centrosymmetric crystals) and optical activity (in chiral crystals), which have important technological applications
  • Quartz (SiO2) exhibits piezoelectricity due to its non-centrosymmetric structure, making it useful in pressure sensors and oscillators
  • Cinnabar (HgS) is optically active due to its chiral crystal structure, rotating plane-polarized light