caxis

Caxis, or the c-axis, is the crystallographic axis associated with the lattice parameter c in crystal systems that include a distinct axis perpendicular to a basal plane, most often the hexagonal and tetragonal systems. It defines a specific directional orientation within the crystal structure and is used to describe stacking and anisotropic properties.

In hexagonal crystals, the lattice is described by parameters a and c, with a = b and angles such that the basal plane is defined by the a and b axes, while the c-axis runs perpendicular to that plane. The angle between a and b is 120°, and the angles between the c-axis and the basal plane are right angles. In tetragonal systems, a = b ≠ c, and the c-axis again represents the unique axis perpendicular to the equal-a directions. The c parameter thus characterizes the height of the unit cell along that vertical direction.

The c-axis is important for understanding growth, texture, and anisotropy in materials. X-ray diffraction often analyzes planes perpendicular to the c-axis, with 00l reflections giving information about interlayer spacing; for hexagonal systems, the spacing of these planes is typically d = c / l for integer l. Materials with layered architectures, such as graphite, zinc oxide, and gallium nitride, show pronounced properties along the c-axis due to the stacking of atomic planes.

Measurement of the c-axis parameter is routine in crystallography and materials science, typically via X-ray or neutron diffraction, and can reflect changes in composition, temperature, pressure, or phase transitions. The concept of the c-axis thus provides a concise way to specify directionality and periodicity in many crystalline materials.