In what form is the irregularly suitable phase of the atom in crystalline matter known? 

The correct answer is Point Defect.

Key Points

• A Point Defect refers to an irregularity or imperfection localized at a single lattice point within a crystalline material.
• These defects are typically due to missing atoms (vacancies), extra atoms (interstitials), or impurity atoms substituting for host atoms.
• Point defects can significantly affect the physical properties of materials, including their electrical, thermal, and mechanical properties.
• Common types of point defects include vacancies, interstitials, and substitutional atoms.
• Point defects play a crucial role in the processes such as diffusion and ionic conductivity.
• The concentration and type of point defects can be controlled through thermal treatment and alloying.