Final answer:
The interstitial position in the partition function of a monatomic crystal refers to the spaces or voids between the atoms in the crystal lattice where additional atoms or ions can be located. The size and occupation of interstitial sites can affect the crystal's stability and properties.
Step-by-step explanation:
In the partition function of a monatomic crystal, the interstitial position refers to the spaces or voids between the atoms in the crystal lattice where additional atoms or ions can be located. These interstitial atoms fill the gaps between the host atoms and can affect the crystal's stability and properties. For example, in face-centered cubic (FCC) structures, the interstitial sites include tetrahedral and octahedral holes, which foreign atoms can occupy.
In order to retain close packing, the interstitial atoms must be small enough to fit into these holes without disrupting the host lattice. When the interstitial atoms are too large, they will push the host atoms apart, causing the lattice to lose its close-packing character.
An example of an interstitial position is found in the CsCl structure, where oppositely charged ions occupy interstitial sites, but there are size restrictions to maintain stability.