Festzustand

Festzustand, in physics commonly translated as the solid state, denotes the state of matter in which matter exhibits rigidity and maintains a definite shape and volume. In German scientific usage it is contrasted with fluid states such as liquids and gases. In solids, particles are arranged in a condensed structure held together by strong interparticle forces, often forming a crystal lattice. Crystalline solids display long-range order and characteristic lattice parameters, while amorphous solids lack long-range order but retain rigidity and finite volume.

Key distinctions within the Festzustand include crystalline solids, which have periodic arrangements and well-defined unit cells, and amorphous solids, which lack long-range order but are not fluid. The transition from solid to other states occurs at phase transition points such as the melting point; some materials also undergo a glass transition, where a supercooled liquid becomes mechanically rigid without crystallizing.

The Festzustand encompasses a wide range of physical properties. Solids typically have high density and low compressibility, elastic responses described by moduli, and vibrational modes (phonons) that affect heat capacity and thermal conductivity. Electronic properties are described by band theory, distinguishing conductors, insulators, and semiconductors. In metals, free electrons yield high electrical conductivity, while ceramics and polymers show more diverse behavior. Phenomena such as superconductivity, magnetism, and ferroelectricity arise within the solid state.

Common solid materials include metals (e.g., iron, copper), ceramics (e.g., silicon carbide), ionic solids (e.g., sodium chloride), and amorphous solids like glass. The field of solid-state physics and its applications in materials science, electronics, and photonics study and exploit these properties. Techniques such as X-ray diffraction, electron microscopy, and spectroscopy probe the arrangement and dynamics of atoms within the Festzustand.