HELP on CRYSTAL LATTICE PARAMETER a (in pm) (picometres)
There are 14 different Bravais lattices that an element could crystallise within: four orthorhombic, two tetrahedral, three cubic, one hexagonal, one triclinic, one rhombohedral and two monoclinic. The actual configuration adopted by each element depends upon its bonding arrangements, its physical diameter and the temperature. The Bravais lattice has three axes. The crystal lattice parameters a, b, and c, are the lengths of each of the three axes of the Bravais lattices.
The crystal structure of a substance represents the lowest energy arrangement of its atoms, at a particular temperature. As the temperature of a substance is increased, the atoms vibrate with greater vigour increasing the average distance between atoms. Under such circumstances, some substances will change from one crystal structure into another which has a lower energy at that temperature. This is a phase change, and occurs at the phase transition temperature. Some elements, when the temperature is raised, will go through several such phase changes before finally reaching the melting temperature. Plutonium remarkably goes through two monoclinic phases, an orthorhombic, face centred cubic, tetragonal and body centred cubic as the temperature is raised from 395 Kelvin to 753 Kelvin. Naturally, each phase has a differing density, usually less. The differing phases naturally have differing properties.
This is the crystal lattice spacing 'a' in picometres when the element is in the same crystalline state as shown in the "TABLE' "Crystalline Form".
See Crystalline Form TABLE, and Draw File 'Bravais'.
