Solid-State Chemistry

Definition 1 (Bravais Lattice): An infinite array of discrete points generated by discrete translation operations. There are 14 Bravais lattices in 3D: 1 triclinic, 2 monoclinic, 4 orthorhombic, 2 tetragonal, 1 rhombohedral, 1 hexagonal, 3 cubic.

Simple Cubic (SC):

• Atoms at the 8 corners of a cube.
• Coordination number (CN) = 6.
• Atoms per unit cell: $\frac{8 \times 1}{8} = 1$.
• Packing fraction: $\frac{\pi}{6} \approx 52.4\%$.

Body-Centered Cubic (BCC):

• Atoms at 8 corners + 1 at the center.
• CN = 8.
• Atoms per unit cell: $\frac{8 \times 1}{8} + 1 = 2$.
• Packing fraction: $\frac{\sqrt{3}\,\pi}{8} \approx 68.0\%$.
• Examples: Fe ($\alpha$), Cr, W, Mo.

Face-Centered Cubic (FCC / Cubic Close-Packed, CCP):

• Atoms at 8 corners + 1 at the center of each face.
• CN = 12.
• Atoms per unit cell: $\frac{8 \times 1}{8} + \frac{6 \times 1}{2} = 4$.
• Packing fraction: $\frac{\pi}{3\sqrt{2}} \approx 74.0\%$ (maximum for equal spheres).
• Examples: Cu, Ag, Au, Al, Ni, Pt.

Theorem 1 (HCP Structure): ABAB stacking sequence. Each atom has CN = 12. Same packing fraction as FCC (74.0%).

Atoms per unit cell: 6 (in the conventional cell).

Examples: Mg, Zn, Ti, Co, Cd.

Example 1: Iron has a BCC structure with $a = 286.6$ pm. Calculate the atomic radius.

$r = \frac{\sqrt{3} \times 286.6}{4} = 124.1 \text{ pm}$

$\blacksquare$

• FCC arrangement of anions with cations in octahedral holes.
• CN = 6 for both ions.
• Formula: MX (1:1 stoichiometry).
• Examples: NaCl, KBr, MgO, CaO.

• Simple cubic arrangement of anions with cation at the body center.
• CN = 8 for both ions.
• Formula: MX (1:1 stoichiometry).
• Examples: CsCl, CsBr, TlCl.

• FCC arrangement of S$^{2-}$ with Zn$^{2+}$ in half the tetrahedral holes.
• CN = 4 for both ions.
• Formula: MX.
• Examples: ZnS, CuCl, GaAs.

• FCC arrangement of Ca$^{2+}$ with F$^-$ in all tetrahedral holes.
• CN: Ca$^{2+}$ = 8, F$^-$ = 4.
• Formula: MX$_2$.
• Examples: CaF$_2$, UO$_2$, ZrO$_2$.

Theorem 2 (Radius Ratio Rules): The ratio $r_+/r_-$ determines the coordination geometry:

Definition 2 (Lattice Energy, $\Delta U$): The energy released when 1 mol of an ionic solid is formed from its gaseous ions. Always exothermic.

Theorem 3 (Born-Haber Cycle): Lattice energy can be calculated thermodynamically:

$\Delta U = \Delta_f H^\circ - \Delta H_{\text{atom}} - \text{IE} - \frac{1}{2}\text{BDE} - \text{EA}$

For NaCl: $\Delta U(\text{NaCl}) = \Delta_f H^\circ(\text{NaCl}) - \Delta H_{\text{sub}}(\text{Na}) - \frac{1}{2}D(\text{Cl}_2) - \text{IE}_1(\text{Na}) - \text{EA}(\text{Cl})$

Theorem 4 (Born-Lande Equation):

$\Delta U = -\frac{N_A M z^+ z^- e^2}{4\pi\varepsilon_0 r_0}\left(1 - \frac{1}{n}\right)$

where:

• $M$ is the Madelung constant (depends on structure: NaCl = 1.748, CsCl = 1.763, ZnS = 1.638).
• $z^+$, $z^-$ are ionic charges.
• $r_0$ is the distance of closest approach.
• $n$ is the Born exponent (8–12, related to the compressibility).

Example 2: Calculate the lattice energy of NaCl with $r_0 = 282$ pm, $n = 8$, $M = 1.748$.

$\Delta U = -\frac{6.022 \times 10^{23} \times 1.748 \times 1 \times 1 \times (1.602 \times 10^{-19})^2}{4\pi \times 8.854 \times 10^{-12} \times 282 \times 10^{-12}}\left(1 - \frac{1}{8}\right)$

$= -\frac{1.748 \times (96.485 \text{ kJ/mol})}{282 \times 10^{-12}} \times 0.875 = -787 \text{ kJ/mol}$

$\blacksquare$

Definition 3 (Energy Band): When $N$ atoms are brought close together, their atomic orbitals overlap and split into $N$ closely spaced energy levels forming a continuous band.

• Valence band: Highest occupied band at $T = 0$.
• Conduction band: Lowest unoccupied band at $T = 0$.
• Band gap ($E_g$): Energy difference between the top of the valence band and the bottom of the conduction band.

Intrinsic semiconductor: Conductivity due to thermally excited electrons across the band gap:

$n_i = n_e = n_h = \sqrt{N_c N_v}\,e^{-E_g/2k_BT}$

where $n_e$ is the electron concentration, $n_h$ is the hole concentration, and $N_c$, $N_v$ are the effective density of states.

Extrinsic semiconductors:

• n-type: Doped with donors (Group 15 in Si, e.g., P, As) — extra electrons in the conduction band.
• p-type: Doped with acceptors (Group 13 in Si, e.g., B, Al) — holes in the valence band.

Theorem 5 (pn Junction): At the interface of p-type and n-type material:

• Depletion region forms (no free carriers).
• Forward bias: Current flows; reverse bias: Current blocked.
• Basis of diodes, transistors, and solar cells.

Definition 4 (Effective Mass): The curvature of the band determines the effective mass:

$m^* = \hbar^2\left(\frac{d^2E}{dk^2}\right)^{-1}$

Electrons near the bottom of the conduction band have positive $m^*$; holes near the top of the valence band have negative $m^*$ (positive effective mass in the opposite direction).

Definition 5 (Schottky Defect): A cation-anion pair vacancy. Maintains electrical neutrality and approximately constant stoichiometry.

$\text{Vacancy concentration: } n_s \approx N\,e^{-\Delta H_s/2k_BT}$

Common in NaCl, CsCl (high CN, similar ionic sizes).

Definition 6 (Frenkel Defect): An ion displaced from its lattice site to an interstitial position. Common when one ion is much smaller (e.g., AgCl, AgBr).

$\text{Frenkel concentration: } n_f \approx \sqrt{N\,N_i}\,e^{-\Delta H_f/2k_BT}$

where $N_i$ is the number of interstitial sites.

Metal excess defects:

• Anion vacancies with trapped electrons (F-centers, color centers). Examples: NaCl heated in Na vapor turns yellow (F-centers absorb blue light).
• Interstitial cations.

Metal deficiency defects:

• Cation vacancies with compensating charge (e.g., Fe$_{1-x}$O, where some Fe$^{2+}$ is replaced by Fe$^{3+}$ and vacancies maintain charge balance).

• Dislocations: Edge dislocations (extra half-plane) and screw dislocations.
• Grain boundaries: Boundaries between crystalline domains with different orientations.
• Stacking faults: Errors in the stacking sequence (e.g., ABCABABC instead of ABCABC).

Theorem 6 (Bragg’s Law): Constructive interference occurs when:

$n\lambda = 2d\sin\theta$

where $n$ is the order of reflection, $\lambda$ is the X-ray wavelength, $d$ is the interplanar spacing, and $\theta$ is the angle of incidence.

Definition 7 (Miller Indices): A set of integers $(hkl)$ that describe the orientation of a plane in a crystal lattice.

For a plane intercepting the crystallographic axes at $(a/h, b/k, c/l)$:

• $(100)$: plane perpendicular to the $a$ axis.
• $(110)$: plane bisecting $a$ and $b$ axes.
• $(111)$: plane bisecting all three axes.

For a cubic crystal:

$d_{hkl} = \frac{a}{\sqrt{h^2 + k^2 + l^2}}$

Example 3: For NaCl ($a = 564$ pm) with Cu K$\alpha$ radiation ($\lambda = 154.2$ pm), find the first-order Bragg angle for the (200) reflection.

$d_{200} = \frac{564}{\sqrt{4}} = 282 \text{ pm}$

$\sin\theta = \frac{\lambda}{2d} = \frac{154.2}{2 \times 282} = 0.2734$

$\theta = 15.9°$

$\blacksquare$

Theorem 7 (Systematic Absences): Certain reflections are absent due to the symmetry of the unit cell (glide planes, screw axes, centering).

Definition 8 (Polymorphism): The ability of a solid to exist in more than one crystal structure.

Examples:

• Carbon: diamond (cubic), graphite (hexagonal).
• Iron: $\alpha$-Fe (BCC, ferromagnetic) → $\gamma$-Fe (FCC, paramagnetic) at 912°C → $\delta$-Fe (BCC).
• Ti: $\alpha$-Ti (HCP) → $\beta$-Ti (BCC) at 882°C.

Solid solution: Atoms of different elements share the same lattice.

• Substitutional: Similar-sized atoms (e.g., Cu–Ni).
• Interstitial: Small atoms in the voids of a metal lattice (e.g., C in Fe → steel).

Theorem 8 (Quantum Confinement): When a semiconductor particle has a size comparable to the exciton Bohr radius, the band gap increases (blue shift in absorption/emission).

$E_g(\text{nanoparticle}) = E_g(\text{bulk}) + \frac{\hbar^2\pi^2}{2R^2}\left(\frac{1}{m_e^*} + \frac{1}{m_h^*}\right) - \frac{1.8e^2}{4\pi\varepsilon_0\varepsilon_r R}$

where $R$ is the particle radius.

Definition 9 (Surface-to-Volume Ratio): For a nanoparticle of radius $R$:

$\frac{A}{V} = \frac{3}{R}$

As $R \to 0$, surface atoms become a larger fraction of total atoms, leading to:

• Enhanced reactivity.
• Lower melting points ($T_m \propto 1/R$ for very small particles).
• Different mechanical properties.

Definition 10 (Zeolite): Crystalline aluminosilicates with a 3D framework of SiO$_4$ and AlO$_4$ tetrahedra, creating pores and channels of molecular dimensions.

General formula: $\text{M}_{x/n}[(\text{AlO}_2)_x(\text{SiO}_2)_y]\cdot m\text{H}_2\text{O}$

where M is the cation (e.g., Na$^+$, K$^+$, Ca$^{2+}$) balancing the negative charge from Al$^{3+}$ substitution in the framework.

• Ion exchange: Water softening (Na$^+$ replaces Ca$^{2+}$, Mg$^{2+}$).
• Molecular sieves: Size-selective adsorption based on pore dimensions.
• Catalysis: Shape-selective catalysis in petrochemical cracking.
• Gas separation: Separation of gases by molecular size.

Common zeolite structures: Linde Type A (LTA), Faujasite (FAU, includes X and Y zeolites), MFI (ZSM-5), Mordenite (MOR).

Definition 11 (Superconductor): A material with zero electrical resistance below a critical temperature $T_c$.

Theorem 9 (BCS Theory): Below $T_c$, electrons form Cooper pairs via phonon-mediated attraction:

$2\Delta = 3.53\,k_B\,T_c$

where $\Delta$ is the superconducting energy gap.

Meissner effect: Superconductors expel magnetic fields below $T_c$ and $H_c$ (critical field).

Cuprate superconductors (e.g., YBa$_2$Cu$_3$O$_7$, $T_c = 92$ K):

• Layered perovskite structures with CuO$_2$ planes.
• $T_c$ depends on oxygen stoichiometry.
• Iron-based superconductors (e.g., LaFeAsO, $T_c \sim 26$ K).

Definition 12 (Non-Stoichiometry): Many transition metal oxides have variable stoichiometry due to mixed oxidation states and defects.

Fe$_{1-x}$O: Some Fe$^{2+}$ is oxidized to Fe$^{3+}$, with vacancies maintaining charge balance.

Definition 13 (Superionic Conductor): Solids with exceptionally high ionic conductivity due to mobile ions in a rigid framework.

Examples:

• $\beta$-alumina (Na$^+$ conductivity).
• AgI above 147°C (Ag$^+$ mobility).
• Yttria-stabilized zirconia (YSZ, O$^{2-}$ conductivity — used in solid oxide fuel cells).

1. Confusing SC, BCC, and FCC packing fractions. SC = 52.4%, BCC = 68.0%, FCC/HCP = 74.0%. Fix: Calculate: $\text{SC} = \pi/6$, $\text{BCC} = \sqrt{3}\pi/8$, $\text{FCC} = \pi/(3\sqrt{2})$.
2. Wrong atoms per unit cell count. Corner atoms count as 1/8, face atoms as 1/2, edge atoms as 1/4, body atoms as 1. Fix: Always use the correct fractional count for each position.
3. Using the Born-Lande equation for covalent solids. The equation assumes purely ionic bonding. Fix: Use the Born-Haber cycle with thermodynamic data for more accurate values.
4. Confusing intrinsic and extrinsic semiconductors. Intrinsic carriers come from thermal excitation; extrinsic carriers come from dopants. Fix: At room temperature, $n_e = n_h = n_i$ (intrinsic) or $n_e \approx N_D$ (n-type), $n_h \approx N_A$ (p-type).
5. Wrong Miller indices for planes. Miller indices $(hkl)$ are the reciprocals of the axis intercepts, not the intercepts themselves. Fix: Take reciprocals, clear fractions, reduce to smallest integers.
6. Confusing Schottky and Frenkel defects. Schottky = vacancy pair (cation + anion); Frenkel = displacement (ion moves to interstitial). Fix: Schottky is favored when ions are similar in size (alkali halides); Frenkel when one ion is much smaller (AgCl).
7. Ignoring the quantum size effect for nanoparticles. The band gap depends on particle size; bulk properties don’t apply to nanomaterials. Fix: Use the quantum confinement formula or experimental data for nanoparticle-specific properties.

• Crystal structures: SC (CN=6), BCC (CN=8), FCC/HCP (CN=12); packing fractions 52%, 68%, 74%.
• Ionic structures: NaCl (6:6), CsCl (8:8), ZnS (4:4), CaF$_2$ (8:4); radius ratio rules.
• Born-Haber cycle: Lattice energy from thermodynamic cycle; Born-Lande equation.
• Band theory: Conductors ($E_g = 0$), semiconductors ($E_g \sim 0.1$–4 eV), insulators ($E_g > 4$ eV).
• Defects: Schottky (vacancy pairs), Frenkel (displacement); non-stoichiometry.
• X-ray diffraction: Bragg’s law $n\lambda = 2d\sin\theta$; Miller indices; systematic absences.
• Nanomaterials: Quantum confinement; high surface-to-volume ratio; quantum dots, nanotubes.
• Zeolites: Porous aluminosilicates; ion exchange, molecular sieves, catalysis.

Problem: Sodium chloride crystallises in a face-centred cubic structure with a = 564 pm. Calculate the density of NaCl. (M_Na = 23.0, M_Cl = 35.5 g/mol, N_A = 6.022 x 10^23). Solution: NaCl unit cell contains 4 Na+ and 4 Cl- ions (FCC arrangement). Molar mass of NaCl = 58.5 g/mol. Mass of unit cell = (4 x 58.5) / (6.022 x 10^23) = 3.886 x 10^-22 g. Volume = a^3 = (564 x 10^-10 cm)^3 = 1.795 x 10^-22 cm^3. Density = 3.886 x 10^-22 / 1.795 x 10^-22 = 2.17 g/cm^3. Literature value: 2.16 g/cm^3.

Problem: NaCl has r(Na+) = 102 pm and r(Cl-) = 181 pm. Determine the expected coordination geometry using the radius ratio rule. Solution: Radius ratio = r+/r- = 102/181 = 0.564. For 0.414 < r+/r- < 0.732, the predicted coordination number is 6 (octahedral), matching the observed NaCl (rock salt) structure. If the ratio were below 0.414, tetrahedral (ZnS) coordination would be expected. If above 0.732, cubic (CsCl) coordination.