Coordination Number for Octahedral Sites
The coordination number of an atom or site is the number of its nearest neighbors in a crystal lattice.
An octahedral site refers to a specific geometric arrangement where a central point is surrounded by six equidistant neighbors. These neighbors are located at the vertices of an octahedron.
Determining the Coordination Number
By definition, the geometry of an octahedral arrangement dictates that the central atom or site has exactly six nearest neighbors.
- In pure metals like copper, which typically adopt a Face-Centered Cubic (FCC) structure, each atom has a coordination number of 12.
- However, the question specifically asks for the coordination number associated with an octahedral site. This refers to the coordination number inherent to the octahedral geometry itself.
- Therefore, the coordination number for an octahedral site is 6.
This value (6) is characteristic of the octahedral coordination geometry, regardless of the overall crystal structure of the bulk material like pure copper.
Important Questions from Crystal Structure Density Atomic Packing Factor
- Match the crystal systems in Column I with the corresponding axial lengths (a, b, c) and interaxial angles ($\alpha$, $\beta$, $\gamma$) provided in Column II
Column I Column II (P) Tetragonal (1) $a \neq b \neq c$, $\alpha = \beta = \gamma = 90^\circ$ (Q) Rhombohedral (2) $a = b \neq c$, $\alpha = \beta = \gamma = 90^\circ$ (R) Orthorhombic (3) $a \neq b \neq c$, $\alpha = \gamma = 90^\circ \neq \beta$ (S) Monoclinic (4) $a = b = c$, $\alpha = \beta = \gamma \neq 90^\circ$ The lattice parameter of face-centered cubic iron ($\gamma$-Fe) is 0.3571 nm. The radius (in nm) of the octahedral void in $\gamma$-Fe is _______________
- For a bcc metal the ratio of the surface energy per unit area of the (100) plane to that of the (110) plane is ________
- Pure iron transforms from body centered cubic (BCC) to face centered cubic (FCC) crystal structure at $912 \text{ °C}$. If the lattice parameter of the BCC phase is $0.293 \text{ nm}$ and that of the FCC phase is $0.363 \text{ nm}$, the associated volume change is ________ (in % to one decimal place)
- If saturation magnetization of iron at room temperature is $1700 \text{ kA m}^{-1}$, the magnetic moment (in $A \text{ m}^2$) per iron atom in the crystal is: _________ $ \times 10^{-23}$
(round off to 1 decimal place).
(Given: Lattice parameter of iron at room temperature = 0.287 nm)
