View source for Polarizability

{{Short description|Tendency of matter subjected to an electric field to acquire an electric dipole moment}}
{{About||electromagnetic waves|Polarization (waves)|other uses|Polarization (disambiguation)}}
'''Polarizability''' usually refers to the tendency of matter, when subjected to an [[electric field]], to acquire an [[electric dipole moment]] in proportion to that applied field. It is a property of particles with an [[electric charge]]. When subject to an electric field, the negatively charged electrons and positively charged [[atomic nuclei]] are subject to opposite forces and undergo [[Electric dipole moment|charge separation]]. Polarizability is responsible for a material's [[dielectric constant]] and, at high (optical) frequencies, its [[refractive index]].

The polarizability of an atom or molecule is defined as the ratio of its induced dipole moment to the local electric field; in a crystalline solid, one considers the dipole moment per [[unit cell]].<ref name=":0">{{Cite book|title=The CRC Handbook of Chemistry and Physics|last=Lide|first=David|publisher=The Chemical Rubber Publishing Company|year=1998|pages=12–17}}</ref> Note that the local electric field seen by a molecule is generally different from the macroscopic electric field that would be measured externally. This discrepancy is taken into account by the [[Clausius–Mossotti relation]]  (below) which connects the bulk behaviour ([[polarization density]] due to an external electric field according to the [[electric susceptibility]] <math>\chi = \varepsilon_{r}-1</math>) with the molecular polarizability <math>\alpha</math> due to the local field.

[[#Magnetic polarizability|Magnetic polarizability]] likewise refers to the tendency for a [[magnetic dipole]] moment to appear in proportion to an external [[magnetic field]]. Electric and magnetic polarizabilities determine the dynamical response of a bound system (such as a molecule or crystal) to external fields, and provide insight into a molecule's internal structure.<ref name="CERN">{{cite web |author=L. Zhou |author2=F. X. Lee |author3=W. Wilcox |author4=J. Christensen  |title=Magnetic polarizability of hadrons particles from lattice QCD |url=http://cdsweb.cern.ch/record/581347/files/0209128.pdf |publisher=European Organization for Nuclear Research ([[CERN]]) |year=2002 |access-date=25 May 2010}}</ref> "Polarizability" should ''not'' be confused with the [[Permanent magnet|intrinsic magnetic]] or [[Electret|electric]] dipole moment of an atom, molecule, or bulk substance; these do not depend on the presence of an external field.

=={{anchor|Electric polarizability|Electronic polarizability}}Electric polarizability==

===Definition===

Electric polarizability is the relative tendency of a charge distribution, like the [[electron cloud]] of an [[atom]] or [[molecule]], to be distorted from its normal shape by an external [[electric field]].

The polarizability <math>\alpha</math> in [[Isotropy|isotropic]] media is defined as the ratio of the induced [[Electric dipole moment|dipole moment]] <math>\boldsymbol{p}</math> of an atom to the electric field <math>\boldsymbol{E}</math> that produces this dipole moment.<ref>Introduction to Electrodynamics (3rd Edition), D.J. Griffiths, Pearson Education, Dorling Kindersley, 2007, {{ISBN|81-7758-293-3}}</ref>

:<math>\alpha = \frac{||\boldsymbol{p}||}{||\boldsymbol{E}||}</math>

Polarizability has the [[International System of Units|SI units]] of C·m<sup>2</sup>·V<sup>−1</sup> = A<sup>2</sup>·s<sup>4</sup>·kg<sup>−1</sup> while its cgs unit is cm<sup>3</sup>. Usually it is expressed in cgs units as a so-called polarizability volume, sometimes expressed in [[Angstrom|Å]]<sup>3</sup> = 10<sup>−24</sup> cm<sup>3</sup>. One can convert from SI units (<math>\alpha</math>) to cgs units (<math>\alpha'</math>) as follows:

:<math>\alpha' (\mathrm{cm}^3) =  \frac{10^{6}}{ 4 \pi \varepsilon_0 }\alpha (\mathrm{C{\cdot}m^2{\cdot}V^{-1}}) = \frac{10^{6}}{ 4 \pi \varepsilon_0 }\alpha (\mathrm{F{\cdot}m^2}) </math> ≃ 8.988×10<sup>15</sup> × <math>\alpha  (\mathrm{F{\cdot}m^2}) </math>

where <math>\varepsilon_0 </math>, the [[permittivity|vacuum permittivity]], is ~8.854 × 10<sup>−12</sup> (F/m). If the polarizability volume in cgs units is denoted <math>\alpha'</math> the relation can be expressed generally<ref name=Atkins>{{cite book|title=Atkins' Physical Chemistry|year=2010|publisher=[[Oxford University Press]]|isbn=978-0-19-954337-3|pages=622–629|last1=Atkins|first1=Peter|last2=de Paula|first2=Julio|chapter=17}}</ref> (in SI) as <math>\alpha = 4\pi\varepsilon_0 \alpha' </math>.

The polarizability of individual particles is related to the average [[electric susceptibility]] of the medium by the [[Clausius–Mossotti relation]]: 
:<math>R={\displaystyle \left({\frac {4\pi}{3}}\right)N_\text{A}\alpha_{c}=\left({\frac {M}{p}}\right)\left({\frac {\varepsilon_\mathrm{r}-1}{\varepsilon_\mathrm{r}+2}}\right)}</math>
where ''R'' = [[molar refractivity]] , <math>N_\text{A}</math> = Avogadro constant, <math>\alpha_c</math> = electronic polarizability, ''p'' = density of molecules, ''M'' = molar mass, and <math>\varepsilon_r = \epsilon/\epsilon_0</math> is the material's relative permittivity or [[dielectric constant]] (or in optics, the square of the [[refractive index]]).

Polarizability for anisotropic or non-spherical media cannot in general be represented as a [[scalar (physics)|scalar]] quantity. Defining <math>\alpha</math> as a scalar implies both that applied electric fields can only induce polarization components parallel to the field and that the <math>x, y</math> and <math>z</math> directions respond in the same way to the applied electric field. For example, an electric field in the <math>x</math>-direction can only produce an <math>x</math> component in <math>\boldsymbol{p}</math> and if that same electric field were applied in the <math>y</math>-direction the induced polarization would be the same in magnitude but appear in the <math>y</math> component of  <math>\boldsymbol{p}</math>. Many crystalline materials have directions that are easier to polarize than others and some even become polarized in directions perpendicular to the applied electric field{{Citation needed|date=September 2020}}, and the same thing happens with non-spherical bodies. Some molecules and materials with this sort of anisotropy are [[Optical rotation|optically active]], or exhibit linear [[birefringence]] of light.

===Tensor===
To describe anisotropic media a polarizability rank two [[tensor]] or <math>3 \times 3</math> [[Matrix (mathematics)|matrix]] <math>\alpha</math> is defined,

:<math> \mathbb{\alpha} = 
\begin{bmatrix}
\alpha_{xx} & \alpha_{xy} & \alpha_{xz} \\
\alpha_{yx} & \alpha_{yy} & \alpha_{yz} \\
\alpha_{zx} & \alpha_{zy} & \alpha_{zz} \\
\end{bmatrix}
</math>

so that:

:<math>
\boldsymbol{p} = \mathbb{\alpha} \boldsymbol{E}
</math>

The elements describing the response parallel to the applied electric field are those along the diagonal. A large value of <math>\alpha_{yx}</math> here means that an electric-field applied in the <math>x</math>-direction would strongly polarize the material in the <math>y</math>-direction. Explicit expressions for <math>\alpha</math> have been given for homogeneous anisotropic ellipsoidal bodies.<ref>Electrodynamics of Continuous Media, L.D. Landau and E.M. Lifshitz, Pergamon Press, 1960, pp. 7 and 192.</ref><ref>C.E. Solivérez, ''Electrostatics and Magnetostatics of Polarized Ellipsoidal Bodies: The Depolarization Tensor Method'', Free Scientific Information, 2016 (2nd edition), {{ISBN|978-987-28304-0-3}}, pp. 20, 23, 32, 30, 33, 114 and 133.</ref>

=== Application in crystallography ===
[[File:Addition of an External Field onto a Cubic Crystal.png|thumb|Macroscopic Field Applied to a Cubic Crystal]]
The matrix above can be used with the molar refractivity equation and other data to produce density data for crystallography. Each polarizability measurement along with the refractive index associated with its direction will yield a direction specific density that can be used to develop an accurate three dimensional assessment of molecular stacking in the crystal. This relationship was first observed by Linus Pauling.<ref name=":0" />

Polarizability and molecular property are related to [[Refractive index|refractive index]] and bulk property. In crystalline structures, the interactions between molecules are considered by comparing a local field to the macroscopic field. Analyzing a cubic [[Crystal structure|crystal lattice]], we can imagine an [[Isotropy|isotropic]] spherical region to represent the entire sample. Giving the region the radius <math>a</math>, the field would be given by the volume of the sphere times the [[Electric dipole moment|dipole moment]] per unit volume <math>\overrightarrow{P}.</math>

:<math>\overrightarrow{\mu}</math> = <math>\tfrac{4 \pi a^3}{3}</math> <math>\overrightarrow{P}.</math>

We can call our local field <math>\overrightarrow{F}</math>, our macroscopic field <math>\overrightarrow{E}</math>, and the field due to matter within the sphere, <math>E_i = \tfrac{-\overrightarrow{P}}{3 \varepsilon_0}</math> <ref> 1. J. D. Jackson, Classical Electrodynamics (Wiley, New York, 1962)
</ref> We can then define the local field as the macroscopic field without the contribution of the internal field:

:<math>\overrightarrow{F}=\overrightarrow{E}-\overrightarrow{E_i}=\overrightarrow{E}+\tfrac{\overrightarrow{P}}{3 \varepsilon_0}</math>

The polarization is proportional to the macroscopic field by <math>\overrightarrow{P}=\varepsilon_0(\varepsilon_r-1)\overrightarrow{E}=\chi_{\text{e}}\varepsilon_0\overrightarrow{E}</math> where <math>\varepsilon_0</math> is the [[Vacuum permittivity|electric permittivity constant]] and <math>\chi_{\text{e}}</math> is the [[Electric susceptibility|electric susceptibility]]. Using this proportionality, we find the local field as <math>\overrightarrow{F}=\tfrac{1}{3}(\varepsilon_r+2)\overrightarrow{E}</math> which can be used in the definition of polarization 

:<math>\overrightarrow{P}=\tfrac{N\alpha}{V}\overrightarrow{F}=\tfrac{N\alpha}{3V}(\varepsilon_r+2)\overrightarrow{E}</math> 

and simplified with <math>\varepsilon_r=1+\tfrac{N\alpha}{\varepsilon_0V}</math> to get <math>\overrightarrow{P}=\varepsilon_0(\varepsilon_r-1)\overrightarrow{E}</math>. These two terms can both be set equal to the other, eliminating the <math>\overrightarrow{E}</math> term giving us <math>\tfrac{\varepsilon_r-1}{\varepsilon_r+2}=\tfrac{N\alpha}{3\varepsilon_0V}</math>. We can replace the relative permittivity <math>\varepsilon_r</math> with [[Refractive index|refractive index]] <math>n</math>, since <math>\varepsilon_r=n^2</math> for a low-pressure gas. The number density can be related to the [[molecular weight]] <math>M</math> and mass density <math>\rho</math> through <math>\tfrac{N}{V}=\tfrac{N_A\rho}{M}</math>, adjusting the final form of our equation to include molar refractivity:

:<math>R_M = \tfrac{N_A\alpha}{3\varepsilon_0} = \tfrac{M}{\rho} \tfrac{n^2-1}{n^2+2}</math>

This equation allows us to relate bulk property ([[refractive index]]) to the molecular property (polarizability) as a function of frequency.<ref> McHale, J.L. (2017). Molecular Spectroscopy (2nd ed.). CRC Press.</ref>


===Tendencies===
Generally, polarizability increases as the volume occupied by electrons increases.<ref name="anslyn"/> In atoms, this occurs because larger atoms have more loosely held electrons in contrast to smaller atoms with tightly bound electrons.<ref name="anslyn"/><ref name="Schwerdtfeger">{{cite book |last1=Schwerdtfeger |first1=Peter |title=Atomic Static Dipole Polarizabilities |publisher=[[IOS Press]] |year=2006 |editor=G. Maroulis |chapter=Computational Aspects of Electric Polarizability Calculations: Atoms, Molecules and Clusters}}[http://www.worldscibooks.com/etextbook/p464/p464_chap01.pdf]{{dead link|date=January 2018|bot=InternetArchiveBot|fix-attempted=yes}}</ref> On rows of the [[periodic table]], polarizability therefore decreases from left to right.<ref name="anslyn"/> Polarizability increases down on columns of the periodic table.<ref name="anslyn"/> Likewise, larger molecules are generally more polarizable than smaller ones.

Water is a very [[chemical polarity|polar]] molecule, but [[alkanes]] and other [[hydrophobic]] molecules are more polarizable. Water with its permanent dipole is less likely to change shape due to an external electric field. Alkanes are the most polarizable molecules.<ref name="anslyn"/> Although [[alkenes]] and [[arenes]] are expected to have larger polarizability than alkanes because of their higher reactivity compared to alkanes, alkanes are in fact more polarizable.<ref name="anslyn"/> This results because of alkene's and arene's more electronegative sp<sup>2</sup> carbons to the alkane's less electronegative sp<sup>3</sup> carbons.<ref name="anslyn"/>

Ground state electron configuration models are often inadequate in studying the polarizability of bonds because dramatic changes in molecular structure occur in a reaction.{{clarify|date=September 2020}}<ref name="anslyn">{{cite book |last1=Anslyn |first1=Eric |title=Modern Physical Organic Chemistry |last2=Dougherty |first2=Dennis |publisher=University Science |year=2006 |isbn=978-1-891389-31-3 |author-link2=Dennis A. Dougherty}}[https://books.google.com/books?id=gY-Sxijk_tMC&dq=organic+chemistry+polarizability&pg=PA25]</ref>

=={{anchor|Magnetic polarizability}}Magnetic polarizability==
Magnetic polarizability defined by [[Spin (physics)|spin]] interactions of [[nucleon]]s is an important parameter of [[deuteron]]s and [[hadron]]s.  In particular, measurement of [[tensor]] polarizabilities of nucleons yields important information about spin-dependent nuclear forces.<ref name="Silenko">{{cite journal |author=A. J. Silenko |title=Manifestation of tensor magnetic polarizability of the deuteron in storage ring experiments |journal=The European Physical Journal Special Topics |publisher=Springer Berlin / Heidelberg |date=18 Nov 2008 |volume=162 |issue=1 |pages=59–62 |doi=10.1140/epjst/e2008-00776-9 |bibcode=2008EPJST.162...59S |s2cid=122690288 }}</ref>

The method of spin amplitudes uses [[Mathematical formulation of quantum mechanics|quantum mechanics formalism]] to more easily describe spin dynamics.  Vector and tensor polarization of particle/nuclei with spin {{math|S ≥ 1}} are specified by the unit polarization vector <math>\boldsymbol{p}</math> and the polarization tensor ''P''<sub>`</sub>.  Additional tensors composed of products of three or more spin matrices are needed only for the exhaustive description of polarization of particles/nuclei with spin {{math|S ≥ {{frac|3|2}}}}.<ref name="Silenko"/>

==See also==
* [[Dielectric]]
* [[Electric susceptibility]]
* [[Polarization density]]
* [[MOSCED]], an estimation method for [[activity coefficient]]s which uses polarizability as one of its parameters

==References==
{{Reflist}}

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[[Category:Atomic physics]]
[[Category:Chemical physics]]
[[Category:Electric and magnetic fields in matter]]
[[Category:Polarization (waves)]]