Question

NaCl type crystal (with coordination no. $6:6$) can be converted into CsCl type crystal ( with coordination no.$8:8$) by applying:
(a) high temperature
(b) high pressure
(c) high temperature and high pressure
(d) low temperature and low pressure

Answer 1

Hint: The crystal NaCl having the coordination number as 6 can be converted into the crystal CsCl having the coordination number as 8 by increasing the number of the unit cells. Now identify by increasing or decreasing which of the properties i.e. the temperature and pressure, we can increase the number of unit cells.

Complete step by step solution:
First of all, let’s discuss the structures NaCl and CsCl. The structures or the lattice are formed when the smallest cell is repeated over and over again known as the unit cell. In NaCl structure, there are two face centered cubic lattices (i.e. the three atoms along the face diagonal touch each other), one is composed entirely of the sodium ions and the other one is composed of the chloride ions. In this structure, the unit cell consists of 4 sodium ions and 4 chloride ions i.e. 4 sodium chloride units , keeping in mind the contributions made by the corners, face-centers, edge centers and body-center. In this, each sodium atom is surrounded by sic chlorine ions and each chlorine is surrounded by six sodium ions. So, hence its coordination number is $6:6$.
On the other hand, in the structure of CsCl , there are two simple cubic lattices, one of the cesium atoms and the other is of the chlorine atoms which are interlocked together to give the lattice. In this structure, the unit cell consists of one cesium ion and one chloride ion i.e. one cesium chloride per unit cell. In this, each cesium ions is surrounded by eight chlorine ions and each chlorine is surrounded by eight cesium ions. So, hence its coordination number is $8:8$.
Now considering the statement;
The NaCl structure can be converted to the CsCl structure simply by just increasing the pressure. It is so because when the temperature is increased, the number of unit cells also increases and thus, it changes to CsCl lattice. On the other hand, on heating to about 760K, the CsCl structure transforms to the NaCl structure as;
$\begin{matrix}
NaCl\text{ }structure \\
6:6 \\
\end{matrix}\leftrightarrows\begin{matrix}
CsCl\text{ }structure \\
8:8 \\
\end{matrix}$
So, hence the NaCl type crystal (with coordination no. $6:6$) can be converted into CsCl type crystal ( with coordination no.$8:8$) by applying high pressure.

Thus, option (b) is correct.

Note: Coordination number may be defined as the number of the nearest neighbours of a particle and for body-centered cube; the coordination number is 8, for face-centered cube; the coordination number is 12, and for cubic-close packing ; the coordination number is 12 .