Physical Crystallography Before X-rays

Physical crystallography before X-rays describes how physical crystallography developed as a science up to the discovery of X-rays by Wilhelm Conrad Röntgen in 1895. In the period before X-rays, crystallography can be divided into three broad areas: geometric crystallography culminating in the discovery of the 230 space groups in 1891–4, chemical crystallography and physical crystallography.

Physical crystallography is concerned with the physical properties of crystals, such as their optical, electrical, and magnetic properties. The effect of electromagnetic radiation on crystals is covered in the following sections: double refraction, rotary polarization, conical refraction, absorption and pleochroism, luminescence, fluorescence and phosphorescence, reflection from opaque materials, and infrared optics. The effect of temperature change on crystals is covered in: thermal expansion, thermal conduction, thermoelectricity, and pyroelectricity. The effect of electricity and magnetism on crystals is covered in: electrical conduction, magnetic properties, and dielectric properties. The effect of mechanical force on crystals is covered in: photoelasticity, elastic properties, and piezoelectricity.

The study of crystals in the time before X-rays was focused more on their geometry and mathematical analysis than their physical properties. Unlike geometrical crystallography, the history of physical crystallography has no central story, but is a collection of developments in different areas.

Symmetry

During the 19th century crystallography was progressively transformed into an empirical and mathematical science by the adoption of symmetry concepts. In 1832 Franz Ernst Neumann used symmetry considerations when studying double refraction. Woldemar Voigt, who was a student of Neumann, in 1885 formalized Neumann's principle as "if a crystal is invariant with respect to certain symmetry operations, any of its physical properties must also be invariant with respect to the same symmetry operations". Neumann's principle is sometimes referred to as the Neumann–Minnigerode–Curie principle based on later work by Bernhard Minnigerode (another student of Neumann) and Pierre Curie. Curie's principle "the symmetries of the causes are to be found in the effects" is a generalization of Neumann's principle. At the end of the 19th century Voigt introduced tensor calculus to model the physical properties of anisotropic crystals.

Double refraction

Double refraction occurs when a ray of light incident upon a birefringent material, is split by polarization into two rays taking slightly different paths. The double refraction and rhomboidal cleavage of crystals of calcite, or Iceland spar, were first recorded in 1669 by Rasmus Bartholin In 1690 Christiaan Huygens analyzed double refraction in his book ''Traité de la lumière''. Huygens reasoned that the cleavage rhombohedron resulted from the stacking of spherical particles and that the peculiarities of the transmission of light can be traced to the particular asymmetry of the crystal.

In 1810 Étienne-Louis Malus determined that natural light, too, when reflected through a certain angle, behaves like one of the rays exiting a double-refracting crystal. Malus called this phenomenon polarization. In 1812 Jean-Baptiste Biot defined optically positive and negative crystals for the first time. In 1819 David Brewster found that all crystals could be classified as isotropic, uniaxial or biaxial. Augustin-Jean Fresnel was a significant researcher in the whole field of crystal optics, and published a detailed paper on double refraction in 1827 in which he described the phenomenon in terms of polarization, understanding light as a wave with field components in transverse polarization. Crystal optics was an active research area during the 19th century and comprehensive accounts of the field were published by Lazarus Fletcher (1891), Theodor Liebisch (1891) and Friedrich Pockels (1906).

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 of a crystal is raised by an amount , a deformation takes place that is described by the strain tensor = . The quantities are the coefficients of thermal expansion. Since is a symmetrical polar tensor of second rank and ''T'' is a scalar, 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 :de: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 :de: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.

Electrical conduction

The first observations on the variation of electrical conductivity with direction in a crystal (anisotropy) were made by Henri Hureau de Sénarmont in 1850 on 36 different substances. The results showed a correlation between the axes of symmetry and the directions of maximum or minimum conductivity. In 1855 Carlo Matteucci performed experiments on bismuth. In 1888, :sv:Helge Bäckström performed electrical conduction measurements on hematite, another crystal of rhombohedral symmetry.

Electrical conductivity in a crystal is now defined as a second rank symmetric tensor relating two vectors: $\mathbf_i = \boldsymbol_ \mathbf_,$ where $\mathbf_i$ is the current density, $\boldsymbol_$ is the electrical conductivity tensor, and $\mathbf_$ is the electric field intensity.

Magnetic properties

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.

Rotary polarization

In 1811 François Arago, who favoured the corpuscular theory of light, discovered the rotation of the plane of polarization of light travelling through quartz. In 1812 Jean-Baptiste Biot, who favoured the wave theory of light, enunciated the laws of rotary polarization and their application to the analysis of various substances. Biot discovered that while some crystals rotate the light to the right others rotate it to the left, and determined that the rotation is proportional to the thickness of substance traversed and to the wavelength of the light.

In 1821 John Herschel pointed out the relation between the direction of rotation and the development of faces on quartz crystals. Suspecting that rotatory polarization is an effect of a lack of symmetry, Herschel established that quartz crystals often present faces placed in such a way that those belonging to certain crystals are mirror images of the corresponding faces of other crystals. He explained the connection between this arrangement and the respective rotation of light to the right and to the left. In 1822 Augustin-Jean Fresnel explained the rotation by postulating oppositely circularly polarized beams travelling with different velocities along the optic axis. In 1831 George Biddell Airy gave an explanation of the formation of the spirals which bear his name. In 1846 Michael Faraday discovered that the plane of polarization may also be rotated when light passes through an isotropic medium when it is in a magnetic field. The corresponding Kerr effect can be observed on reflecting plane-polarized light from a polished ferromagnetic mirror when in a magnetized state.

In 1848 Louis Pasteur gave the general relation between crystal morphology and rotatory polarization. Pasteur solved the mystery of polarized light acting differently with chemically identical crystals and solutions. Pasteur discovered the phenomenon of molecular asymmetry, that is that molecules could be chiral and exist as a pair of enantiomers. Pasteur's method was to physically separate the crystals of a racemic mixture of sodium ammonium tartrate into right- and left-handed crystals, and then dissolve them to make two separate solutions which rotated polarized light in opposite directions.

In 1855 :de:Christian August Hermann Marbach discovered that crystals of sodium chlorate, sodium bromate, sodium ammonium sulfate and sodium amyl acetate have the property of rotating the polarization plane. In 1857 Alfred Des Cloizeaux advanced a general theory of rotatory polarization whilst studying cinnabar and strychnine sulphate. In 1864 Josef Stefan introduced the banded spectrum in the study of rotatory polarization. Theories of magnetic optics in ferromagnetic crystals were published in 1892 by D. A. Goldhammer, and in 1893 by Paul Drude.

Conical refraction

Conical refraction is an optical phenomenon in which a ray of light, passing through a biaxial crystal along certain directions, is refracted into a hollow cone of light. There are two possible conical refractions, one internal and one external.

In 1821-1822 Augustin-Jean Fresnel developed a theory of double refraction in both uniaxial and biaxial crystals. Fresnel derived the equation for the wavevector surface in 1823, and André-Marie Ampère rederived it in 1828. Many others investigated the wavevector surface of the biaxial crystal, but they all missed its physical implications.

William Rowan Hamilton, in his work on Hamiltonian optics, discovered the wavevector surface has four conoidal points and four tangent conics. This implies that, under certain conditions, a ray of light could be refracted into a cone of light within the crystal. He termed this phenomenon "conical refraction" and predicted two distinct types: internal and external, corresponding respectively to the conoidal points and tangent conics. Hamilton announced his discovery on 22 October 1832. He then asked Humphrey Lloyd to prove his theory experimentally. Lloyd first observed conical refraction on 14 December 1832 with a specimen of aragonite, and published his results in early 1833. In 1833 James MacCullagh claimed that Hamilton's work was a special case of a theorem he had published in 1830. Hamilton also exchanged letters with George Biddell Airy who was skeptical that conical refraction could be observed experimentally but became convinced after Lloyd's report.

Hamilton and Lloyd's discovery was a significant victory for the wave theory of light and solidified Fresnel's theory of double refraction. The discovery of conical refraction is an example of a mathematical prediction being subsequently verified by experiment.

Later theoretical work on conical refraction was published in 1860 by Robert Bellamy Clifton and in 1874 by Jules Antoine Lissajous, and experimental work in 1888 by Theodor Liebisch and in 1889 by Albrecht Schrauf.

Photoelasticity

Photoelasticity describes changes in the optical properties of a material under mechanical deformation. The photoelastic phenomenon in transparent, non-crystalline materials (gels and glasses) was first discovered by David Brewster in 1815. Brewster then detected the effect in crystals and showed that uniaxial crystals could be made biaxial. In 1822 Augustin-Jean Fresnel experimentally confirmed that the photoelastic effect was a stress-induced birefringence.

Franz Ernst Neumann investigated double refraction in stressed transparent bodies. In 1841 Neumann published his elastic equations, which describe, in differential form, the changes which polarized light experiences when travelling through a stressed body. The Neumann equations are the basis of all subsequent photoelasticity research.

The photoelastic effect was analyzed by Friedrich Pockels, who also discovered the Pockels electro-optic effect, (the production of birefringence of light on the application of an electric field). In 1889/90 Pockels produced a phenomenological theory for both of these effects for all crystal classes.

Absorption and pleochroism

In 1809 Louis Cordier discovered the phenomenon of pleochroism while investigating a new mineral that he named dichröıte. Dichröıte (cordierite) crystals showed different colors when viewed along different axes. From 1817-1819 David Brewster made a systematic study of light absorption and pleochroism in various minerals and showed that, in uniaxial crystals, the absorption is smallest in the direction of, and greatest at right angles to, the optical axis. In 1820 John Herschel studied the absorption of light in biaxial crystals and explained the interference rings first observed by David Brewster. In 1838 Jacques Babinet discovered that the greatest absorption in a crystal generally coincided with the direction of greatest refractive index. In 1845 Wilhelm Haidinger published a general account of pleochroism in crystals. In 1854 Henri Hureau de Sénarmont showed that transparent crystals stained by a dye during crystal growth became pleochroic.

In 1877 :de:Paul Glan performed photometric observations on absorption. In 1880 :de:Hugo Laspeyres pointed out the existence of absorption axes (directions of least, intermediate, and greatest absorption). He investigated certain biaxial crystals and found that the absorption axes, although subject to the symmetry of the crystal, did not necessarily coincide with the principal directions of the indicatrix. In 1888 Henri Becquerel made qualitative and quantitative observations. Woldemar Voigt (1885) and Paul Drude (1890) presented theories of the absorption of light in crystals. In 1906 Friedrich Pockels published his ''Lehrbuch der Kristalloptik'' which gave an overview of the subject.

Luminescence, fluorescence and phosphorescence

Luminescence is the non-thermal emission of visible light by a substance; an example is the emission of visible light by minerals in response to irradiation by ultraviolet light. The term luminescence was first used by Eilhard Wiedemann in 1888; he stated that luminescence was separate from thermal radiation, and he distinguished six different forms of luminescence according to their excitation, for example photoluminescence, electroluminescence, etc.

Fluorescence is luminescence which occurs during the irradiation of a substance by electromagnetic radiation; fluorescent materials generally cease to glow nearly immediately when the radiation source stops. The term fluorescence was coined by George Stokes in 1852, and was derived from the behavior of fluorite when exposed to ultraviolet light.

Phosphorescence is long-lived luminescence; phosphorescent materials continue to emit light for some time after the radiation stops. In 1857 Edmond Becquerel invented the phosphoroscope, and in a detailed study of phosphorescence and fluorescence, showed that the duration of phosphorescence varies by substance, and that phosphorescence in solids is due to the presence of finely dispersed foreign substances. Becquerel suggested that fluorescence is simply phosphorescence of a very short duration. The most prominent phosphorescent material for 130 years was ZnS doped with Cu⁺, or later Co²⁺, ions. The material was discovered in 1866 by Théodore Sidot who succeeded in growing tiny ZnS crystals by a sublimation method.

Crystalloluminescence is the emission of light during crystal growth from solution. The first observation was that of potassium sulfate which was reported by a number of researchers in the eighteenth century; other substances reported in the early literature which exhibit crystalloluminescence include strontium nitrate, cobalt sulfate, potassium hydrogen sulfate, sodium sulfate, and arsenious acid. In 1918 Harry Weiser summarised the research on crystalloluminescence up to that date. Neither the spectral distribution nor the excitation mechanisms of crystalloluminescence are understood.

Triboluminescence is the generation of light when certain materials, for example quartz, are rubbed; fractoluminescence is the emission of light from the fracture of a crystal. The first recorded observation is attributed to Francis Bacon when he recorded in his 1620 ''Novum Organum'' that sugar sparkles when broken or scraped in the dark. The scientist Robert Boyle also reported on some of his work on triboluminescence in 1664.

In 1677 Henry Oldenburg described the luminescence of fluorite, CaF₂, on heating. In 1830 Thomas Pearsall observed that colourless fluorite could be coloured by discharging sparks from a Leyden jar held against it. In 1881 luminescence excited by cathode rays was described by William Crookes. In 1885 Edmond Becquerel found that when crystals were bombarded by cathode rays they became coloured and also emitted light. In 1894 :de:Eugen Goldstein showed that ultraviolet light has the same effect as cathode rays.

Reflection from opaque materials

The study of the optical properties of opaque substances has been closely linked with the development of suitable microscopes. The first instrument adapted to reflected light was the Lieberkühn reflector attributed to Johann Nathanael Lieberkühn. The use of polished and etched surfaces for this type of study was introduced by Jöns Jacob Berzelius in 1813. A theory of the light reflected from metals was put forward by Augustin-Louis Cauchy in 1848. In 1858 Henry Clifton Sorby established the technique of cutting minerals and crystals into thin sections for examination under the polarizing microscope. In 1864 Sorby studied the microscopical structure of minerals from meteorites. In 1888 Paul Drude published work on reflection from antimony sulfide.

Infrared optics

Heinrich Rubens measured the dependence of the refractive index of quartz on wavelength, and found absorption in particular infrared wavelength ranges. By 1896 Rubens saw these bands as a potential filter that would allow him to separate out an almost monochromatic beam from the broad range of infrared radiation that his sources produced. In 1897 Rubens and his student Ernest Fox Nichols studied the '' reststrahlen'' (residual rays) obtained when infrared rays of appropriate wavelength are reflected from the surfaces of crystals.

Pyroelectricity

Pyroelectricity is the generation of a temporary voltage in a crystal when subjected to a temperature change. The appearance of electrostatic charges upon a change of temperature has been observed since ancient times, in particular with tourmaline and was described, among others, by Steno, Linnaeus, Aepinus and René Just Haüy. Aepinus published an account of his observations in 1756. Haüy made detailed investigations of pyroelectricity; he detected pyroelectricity in calamine and showed that electricity in tourmaline was strongest at the poles of the crystal and became imperceptible at the middle. Haüy published a book on electricity and magnetism in 1787. Haüy later showed that hemihedral crystals are electrified by temperature change while holohedral (symmetric) crystals are not.

Research into pyroelectricity became more quantitative in the 19th century. In 1824 David Brewster gave the effect the name it has today. In 1840 Gabriel Delafosse, Haüy's student, theorized that only molecules which are not symmetrical can be polarized electrically. Both William Thomson in 1878 and Woldemar Voigt in 1897 helped develop a theory for the processes behind pyroelectricity.

A detailed history of pyroelectricity has been written by Sidney Lang; shorter histories have also been published.

Elastic properties

Some minerals, for example mica, are highly elastic, springing back to their original shape after being bent. Others, for example talc, may be readily bent but do not return to their original form when released. The initial theory of the elasticity of solid bodies were developed in the 1820s. Augustin-Louis Cauchy and Siméon Denis Poisson published theories of the mutual action of a regular arrangement of particles for a non-cubic body in 1823 and 1829 respectively. In 1827 Claude-Louis Navier published a theory for an isotropic body. Also during the 1820s Friedrich Mohs introduced his eponymous scale of hardness. In 1834 Franz Ernst Neumann published a paper on the elasticity of homohedral crystals.

In 1828 Cauchy generalised the problem and showed that 36 independent constants were required to describe elasticity in crystals. George Green (1837) introduced the limitation that the force between any two elements of a crystal, however small, must lie along the line joining their centres. This reduced the number of constants from 36 to 21. William Thomson (1857) showed that Green’s assumption was unnecessary and that the thermodynamic requirements of a reversible process require only 21 constants, without any special assumptions. In 1874 Woldemar Voigt measured the elasticity of rock salt and G. Baumgarten measured the elasticity of calcite. In 1887 Wilhelm Röntgen and J. Schneider measured the cubic compressibility of sodium and potassium chlorides. In 1877 Lambros Koromilas measured the elasticity of gypsum and mica by twisting mineral bars.; in 1881 H. Klang carried out similar experiments with fluorites.

In the period 1874-1888 Voigt was the leading researcher on the elasticity of crystals. Voigt showed that the number of elasticity constants reduces as more symmetry is introduced into the crystal. For a triclinc crystal, which is the most general case, 21 elasticity constants are required. For a monoclinic crystal there are 13 elasticity constants, for a rhombic crystal 9, for a hexagonal crystal 7, for a tetragonal crystal 6, and finally for a cubic crystal there are only 3. A summary of developments in the field was published by W. A. Wooster.

Piezoelectricity

In 1880 Pierre and Jacques Curie discovered piezoelectricity (an electric charge that accumulates in response to applied mechanical stress) in certain crystals, including quartz, tourmaline, cane sugar and sodium chlorate. The Curies, however, did not predict the converse piezoelectric effect (the internal generation of a mechanical strain resulting from an applied electric field). The converse effect was deduced by Gabriel Lippmann in 1881. The Curies immediately confirmed the existence of the effect, and went on to obtain quantitative proof of the complete reversibility of electro-elasto-mechanical deformations in piezoelectric crystals.

In 1890 Woldemar Voigt published a phenomenological theory of the piezoelectric effect based on the symmetry of crystals without centrosymmetry.

Research community

Before the 20th century crystallography was not a well-established academic discipline. There were no academic positions specifically in crystallography. Workers in the field normally carried out their crystallographic research as an ancillary to other employment(s), or had independent means. The leading workers in the field of physical crystallography were employed as follows:

* Professors
** Mathematics or science: Airy, Arago, E. Becquerel, Biot, Curie, Drude, Hamilton, Linnaeus, Mitscherlich, Pasteur, Pockels, Plücker, Stokes, Tyndall, Thomson, Voigt
** Mineralogy: Groth, Haüy, Liebisch, Mohs, Neumann, Sénarmont
* Other employment: Bartholinus (physician), Brewster (editor), Fresnel (engineer), Hooke (municipal official), Malus (military officer)
* Independently wealthy: Herschel, Huygens

In the nineteenth century there were informal schools of physical crystallography researchers in France (Arago, E. Becquerel, Biot, Fresnel, Haüy, Sénarmont), Germany (Drude, Groth, Liebisch, Mitscherlich, Mohs, Neumann, Pockels, Voigt) and the British Isles (Airy, Brewster, Hamilton, Stokes, Thomson).

Until the founding of ''Zeitschrift für Krystallographie und Mineralogie'' by Paul Groth in 1877 there was no lead journal for the publication of crystallographic papers. The majority of crystallographic research was published in the journals of national scientific societies, or in mineralogical journals. The inauguration of Groth’s journal marked the emergence of crystallography as a mature science independent of geology.

See also

* Chemical crystallography before X-rays
* Timeline of crystallography

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{{Crystallography

Crystallography
History of physics
History of crystallography