💍Inorganic Chemistry II Unit 6 – Solid State Chemistry

Key Concepts and Definitions

• Solid state chemistry focuses on the synthesis, structure, and properties of solid materials
• Crystalline solids have a regular, repeating arrangement of atoms or molecules in a lattice structure
• Amorphous solids lack long-range order and have a random arrangement of atoms or molecules
• Unit cell represents the smallest repeating unit that makes up the crystal structure
• Bravais lattices describe the 14 possible arrangements of points in three-dimensional space
  • Include cubic, tetragonal, orthorhombic, hexagonal, and triclinic lattices
• Coordination number refers to the number of nearest neighbors an atom has in a crystal structure
• Packing efficiency measures how effectively atoms or molecules fill space within a crystal structure
  • Hexagonal close packing (hcp) and cubic close packing (ccp) have high packing efficiencies

Crystal Structures and Lattices

• Crystal structures are determined by the arrangement of atoms or molecules in a lattice
• Common crystal structures include simple cubic, body-centered cubic (bcc), and face-centered cubic (fcc)
  • Simple cubic has atoms at each corner of the unit cell
  • bcc has an additional atom at the center of the unit cell
  • fcc has additional atoms at the center of each face of the unit cell
• Miller indices (hkl) are used to describe planes and directions within a crystal structure
• Polymorphism occurs when a material can exist in multiple crystal structures (allotropes)
  • Examples include carbon (graphite and diamond) and titanium dioxide (rutile and anatase)
• Lattice parameters define the size and shape of the unit cell
  • Include lengths (a, b, c) and angles ($\alpha$, $\beta$, $\gamma$) between the axes
• Reciprocal lattice is a mathematical construct used to analyze diffraction patterns and electronic properties

Bonding in Solids

• Bonding in solids can be classified as ionic, covalent, metallic, or van der Waals
• Ionic bonding involves the electrostatic attraction between oppositely charged ions (NaCl)
  • Occurs when there is a large electronegativity difference between the constituent atoms
• Covalent bonding involves the sharing of electrons between atoms (diamond)
  • Results in strong, directional bonds and often leads to high hardness and melting points
• Metallic bonding arises from the delocalization of valence electrons (copper)
  • Contributes to high electrical and thermal conductivity, ductility, and malleability
• Van der Waals bonding is a weak interaction between atoms or molecules (graphite)
  • Includes dipole-dipole interactions, London dispersion forces, and hydrogen bonding
• Bond strength and character influence the physical and chemical properties of solids
• Band theory describes the electronic structure of solids based on the overlap of atomic orbitals
  • Valence band contains the highest occupied electronic states
  • Conduction band contains the lowest unoccupied electronic states

Electronic Properties of Solids

• Electronic properties of solids depend on the band structure and the presence of charge carriers
• Metals have overlapping valence and conduction bands, allowing for high electrical conductivity
• Semiconductors have a small band gap between the valence and conduction bands
  • Intrinsic semiconductors (silicon) have equal numbers of electrons and holes
  • Extrinsic semiconductors are doped with impurities to create n-type (excess electrons) or p-type (excess holes) materials
• Insulators have a large band gap, preventing the flow of electrons in the conduction band
• Fermi level represents the highest occupied electronic state at absolute zero temperature
• Charge carriers (electrons and holes) contribute to electrical and thermal conductivity
• Mobility describes the ease with which charge carriers move through a material under an applied electric field
• Optical properties, such as absorption and emission, are influenced by the electronic structure

Defects and Non-Stoichiometry

• Defects are imperfections in the crystal structure that affect the properties of solids
• Point defects include vacancies (missing atoms), interstitials (extra atoms), and substitutional impurities
  • Schottky defects involve paired cation and anion vacancies, maintaining charge neutrality
  • Frenkel defects involve an atom displaced from its lattice site to an interstitial site
• Line defects, such as dislocations, are one-dimensional imperfections
  • Edge dislocations result from an extra half-plane of atoms inserted into the crystal
  • Screw dislocations result from a spiral distortion of the crystal lattice
• Planar defects include grain boundaries, stacking faults, and twin boundaries
• Non-stoichiometry refers to a deviation from the ideal chemical composition
  • Can occur due to the presence of defects or variable oxidation states of the constituent elements
• Defect concentration and type can be controlled through doping, heat treatment, and processing conditions
• Defects can influence mechanical, electrical, and optical properties of solids

Characterization Techniques

• X-ray diffraction (XRD) is used to determine the crystal structure and lattice parameters
  • Based on the constructive interference of X-rays scattered by the periodic arrangement of atoms
• Scanning electron microscopy (SEM) provides high-resolution images of the surface morphology
  • Uses a focused electron beam to scan the sample surface and detect secondary electrons
• Transmission electron microscopy (TEM) allows for the imaging of internal structure and defects
  • Electrons are transmitted through a thin sample, providing atomic-scale resolution
• Energy-dispersive X-ray spectroscopy (EDS) is used for elemental analysis and composition mapping
  • Detects characteristic X-rays emitted by elements upon electron excitation
• Raman spectroscopy probes the vibrational modes of molecules and lattices
  • Based on the inelastic scattering of monochromatic light by phonons
• Differential scanning calorimetry (DSC) measures heat flow and phase transitions
  • Detects endothermic and exothermic events as a function of temperature
• Electron paramagnetic resonance (EPR) spectroscopy investigates paramagnetic species and defects
  • Measures the absorption of microwave radiation by unpaired electrons in an applied magnetic field

Applications in Materials Science

• Solid state chemistry plays a crucial role in the development of advanced materials
• Semiconductors (silicon, gallium arsenide) are used in electronic devices, solar cells, and light-emitting diodes (LEDs)
• Superconductors (YBa2Cu3O7) exhibit zero electrical resistance below a critical temperature
  • Applications include high-efficiency power transmission, magnetic levitation, and quantum computing
• Ferroelectric materials (BaTiO3) have a spontaneous electric polarization that can be reversed by an applied electric field
  • Used in capacitors, sensors, and memory devices
• Magnetic materials (Fe3O4, SmCo5) are used in data storage, motors, and generators
  • Ferromagnets exhibit a spontaneous magnetic moment that can be aligned by an external magnetic field
• Optical materials (TiO2, ZnO) are used in pigments, coatings, and photocatalysis
  • Exhibit unique properties such as high refractive index, transparency, and UV absorption
• Battery materials (LiCoO2, LiFePO4) are essential for energy storage in portable devices and electric vehicles
  • Intercalation compounds allow for the reversible insertion and extraction of ions during charge/discharge cycles

Emerging Trends and Research

• Nanomaterials exhibit size-dependent properties and enhanced surface area to volume ratios
  • Applications include catalysis, sensing, and drug delivery
• Perovskite solar cells (CH3NH3PbI3) have achieved high power conversion efficiencies and low-cost fabrication
  • Challenges include improving stability and reducing toxicity
• Thermoelectric materials (Bi2Te3, PbTe) convert temperature gradients into electrical energy
  • Potential for waste heat recovery and solid-state cooling
• Topological insulators (Bi2Se3) have an insulating bulk but conductive surface states
  • Promising for spintronics and quantum computing applications
• Metal-organic frameworks (MOFs) are porous crystalline materials composed of metal ions and organic linkers
  • Applications include gas storage, separation, and catalysis
• Machine learning and computational methods are increasingly used to predict and optimize material properties
  • High-throughput screening and data-driven discovery accelerate materials development
• In-situ and operando characterization techniques provide real-time insights into material behavior under operating conditions
  • Examples include in-situ XRD, TEM, and Raman spectroscopy