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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Physical crystallography before X-rays | 4/5 | https://en.wikipedia.org/wiki/Physical_crystallography_before_X-rays | reference | science, encyclopedia | 2026-05-05T16:17:31.939542+00:00 | kb-cron |
Until the 19th century crystals were regarded either as magnetic or nonmagnetic. Magnetic crystals are now called ferromagnetic to distinguish them from the several other kinds which have since been discovered. Siméon Denis Poisson (1826) put forward a theory of magnetism as applied to crystals and predicted the behaviour of crystals in a magnetic field which was verified by Julius Plücker in 1847. Plücker studied various natural crystals, such as quartz and related the reaction of the crystal to a magnetic field to its symmetry. All these crystals were repelled from a strong field, unlike ferromagnetic crystals. They were therefore called diamagnetic. In 1850 a number of investigations were carried out by Plücker and August Beer using torsion balances to measure the small forces involved in most observations. Not only were some crystals repelled from a strong field but others were slightly attracted. These were called paramagnetic. Between 1850 and 1856 John Tyndall studied diamagnetism in crystals. By the end of the 19th century the three types of crystal—ferromagnetic, diamagnetic and paramagnetic—were well established and successful theories had related diamagnetic and paramagnetic crystals to their crystal symmetry. Ferromagnetic properties were dealt with by Pierre Weiss (1896) who explained the hysteresis by assuming that the atoms have permanent magnetic poles which are normally in random positions, but arrange themselves in parallel under the influence of a magnetic field. On removing the field the mutual effect of the parallel dipoles tends to maintain the magnetized state. He further postulated that there were domains within which all the atomic dipoles were similarly orientated and that the N-S axis could be differently orientated in neighbouring domains.
=== Dielectric properties === A dielectric is an electrical insulator that can be polarised by an applied electric field. In 1851 the first experiments on the behaviour of crystals in an electric field were carried out by Hermann Knoblauch in a manner similar to that used for the study of magnetic properties. The conductivity of the crystals, both over the surface and through the body of the crystal, made these experiments unreliable. In 1876 Elihu Root avoided some of these difficulties by employing a rapidly alternating field between parallel plates. In 1893 Friedrich Pockels gave an account of the abnormally large piezoelectric constants of Rochelle salt. A brief history on the theories of dielectrics in the 19th century has been written.
== Effect of temperature change ==
=== Thermal expansion ===
In 1824 Eilhard Mitscherlich observed that the angle between the cleavage faces of calcite changed with the temperature of the crystal. Mitscherlich concluded that, on heating, calcite contracts (has a negative coefficient of thermal expansion) in a direction perpendicular to the trigonal axis while expanding (positive coefficient) along that axis. This implies that there is a cone of directions along which there is no thermal expansion. In 1864 Hippolyte Fizeau used an optical interference method to make measurements on many crystals. The measurements of the change of interfacial angle and the expansion of cut plates and bars were applied to crystals of all symmetries. Crystals with less than cubic symmetry are anisotropic and will generally have different expansion coefficients in different directions. If the crystal symmetry is monoclinic or triclinic, even the angles between the axes are subject to thermal changes. In these cases the coefficient of thermal expansion is a tensor. If the temperature T of a crystal is raised by an amount ΔT, a deformation takes place that is described by the strain tensor uij = αijΔT. The quantities αij are the coefficients of thermal expansion. Since uij is a symmetrical polar tensor of second rank and T is a scalar, αij is a symmetric tensor of second rank. The contemporary usage of the term tensor was introduced by Woldemar Voigt in 1898.
=== Thermal conduction ===
Joseph Fourier was an early researcher in thermal conduction, publishing Théorie analytique de la chaleur in 1822. The first experiments on thermal conduction in crystals were carried out by Jean-Marie Duhamel in 1832. Henri Hureau de Sénarmont conducted experiments to determine if heat would move through crystals with directional dependence. He found that, for non-cubic crystals, the isothermal envelope surrounding a point source of heat in a crystal plate had an elliptical shape whose exact form depended on the orientation of the crystal. Sénarmont's results qualitatively established that thermal conductivity is directionally dependent (thermal anisotropy), with characteristic directions related to crystallographic axes. In 1848 Duhamel provided an analysis of Sénermont's findings. George Gabriel Stokes and William Thomson provided mathematical theories to explain Sénarmont's observations. Stokes acknowledged the connection between the phenomena and the symmetry of the crystal, and showed that the number of constants of heat conductivity reduces from nine to six in the case of two planes of symmetry. The matrix of thermal conductivity components resulting from Stoke's derivation constituted a tensor. Experiments by Franz Stenger in 1884 examined the theories put forward by Stokes and Thomson and disproved some of their theoretical speculations.
=== Thermoelectricity ===
Thomas Johann Seebeck discovered the thermoelectric effect in 1821, although it has been claimed that Alessandro Volta should be given the priority. In 1844 Wilhelm Gottlieb Hankel investigated thermoelectricity in cobalt and iron sulfide crystals. Hankel showed that when certain external faces were developed the crystals were thermoelectrically positive relative to copper, whereas with other facial forms they were negative. In 1850 Jöns Svanberg used bismuth and antimony crystals to demonstrate a directional variation of the thermoelectric effect. In 1854 William Thomson put forward a mechanical theory of thermoelectric currents in crystalline solids. In 1889 Theodor Liebisch analyzed the dependence of the thermoelectric force on the crystallographic direction in anisotropic crystals.
=== Pyroelectricity ===