What is "chemical pressure"?
External and internal pressure

To study the effect of pressure on properties of a solid, is equivalently to learn how changes in volume transform physical parameters. For external pressure at constant temperature, this relationship manifests through compressibility $\kappa$.

$$\kappa =-\frac{1}{V}\left(\frac{\partial V}{\partial P}\right)_T$$

An alteration in volume will modify the energy of a system. The relationship is internal pressure, something chemists first encounter when discussing real gases.

$$\pi_T=\left(\frac{\partial U}{\partial V}\right)_T$$

In solids, one might discover different notations. For example[1], 

$$\alpha =\left(\frac{\partial W}{\partial V}\right)_T \sim \frac{\mathrm{d}W}{\mathrm{d}V} $$

where $W$ is potential energy, and $\alpha $ is called energy–volume correlation. Disregarding possible sorption, varying external pressure is the purest option of probing internal pressure.[2] Unfortunately, this might not always be feasible.[2][3] Most actinides, for instance, are highly radioactive, thus limiting their study to very specialised laboratories.[3] Many actinides are too scarce to perform necessary large scale measurements.[3] Here is where chemical pressure comes in.

Chemical pressure

Instead of external pressure or thermodynamic internal pressure, we now discuss chemical pressure. Much akin to external pressure, scientists take interest in how energy is dependent on volume. Chemical pressure comprises of substituting in or adding chemical entities to a solid network.[2] [4] [5] [6]


External pressure presses down on the solid. Volume is modified via a compressibility relationship.[1] A more direct or pure method.[2]
Chemical pressure is induced by changing cell volumes via the introducing of new chemical entities, such as atoms or compounds, in the lattice. An indirect, less pure approach.[2]


Obviously, chemical doping modifies a system in other ways besides volume alone.[2] [5] [7] By choosing an element with similar electronic configuration, these deviations from ideality are minimised.[2] [7] Another advantage of chemical pressure over extrernal pressure is that the former may be positive and negative.[2] [8] Positive pressure implies that pressure from chemical substitution is directed inwards, and adds to external pressure. Negative pressure pushes the solid apart, hence acting against external pressure.[2]

Immunisation against terminological confusion

Condensed matter science, like other fields of study, is entitled to its own nomenclature. For chemists specialising elsewhere, this might seem a superfluous obfuscation. This is especially true for cutting-edge chemistry where terminology is fresh and perhaps not entirely settled. For this reason, synonyms are widespread, and overlap with other terms may occur.


External pressure is also called physical pressure[4] to establish a physical pressure – chemical pressure dichotomy.
Chemical pressure is often enclosed between quotation marks. More confusingly, internal pressure is used as an alternative to chemical pressure.[5] Perhaps this is to emphasise an external pressure – internal pressure dichotomy. Or, it could be, that since chemical pressure is used as a tool to measure internal pressure, the concepts have blurred lines because the distinction seemed unnecessary.


Units and experiments

Internal chemical pressure will probably have concurring units with external physical pressure. Pascals ($\ce{Pa}$) when operating in the SI framework. Very few studies explicitly mention this fact[5] because more often the changes in some concrete parameter are discussed[1] [9]. Because of what chemical pressure constitutes, we do not apply a chemical pressure of $10\ \mathrm{kPa}$. We might, however, bring about (or apply) a chemical difference in pressure ... equivalent to several $\mathrm{kbar}$ of external pressure[5]. For solids where external pressure experiments are currently unthinkable, this equivalence cannot be discussed. But the goal is not to compare chemical pressure to external pressure anyway; as before, we wish to learn more about how volume affects (potential) energy.

As far as I am aware, chemical pressure cannot be measured directly. Chemical pressure is induced inside a solid material by substituting or introducing new chemical entities. Where would one put the barometer? If it is possible to carry out external pressure measurements alongside chemical doping, we may compare one to the other. To emphasise, the effects need not match. In these and other cases, volume changes in cells are estimated or measured, and their effect on the system as well.

/(short overview/list of experiments in progress, next week)/

In my opinion, this subsection deserves a question on its own.

TL; DR

/(in progress, next week)/

Additional reading

For an overview of highly-correlated electron systems in metallic solids, I recommend C. Marini. Pressure-induced metallization process in Strongly Correlated Electron Systems [Online]; G. Stefani, G. Altarelli, P. Postorino, Eds. Università degli Studi Roma TRE: Rome, Italy, 2009–2010. Archived link: bit.ly/2yPJcCo

References and bibliography


[1] G. Borelius. 'Internal Pressure in Solids and Liquids'. Physica Scripta, 1970, 1, 2-3, pp 141–147. DOI: 10.1088/0031-8949/1/2-3/011
[2] J-M. Fournier. 'Chemical pressure in actinide systems'. Physica B: Condensed Matter, 1993, 190, 1, pp 50–54. DOI: 10.1016/0921-4526(93)90441-8
[3] U. Benedict. 'Properties of Actinide Metals Under High Pressure'. Journal de Physique Colloques, 1984, 45, C8, pp C8-145–C8-148. DOI: 10.1051/jphyscol:1984826
[4] D. M. Friedman. New Research on YBCO Superconductors; Y. Itoh, Ed. Nova Science Publishers: 2008. Chapter 1: Recent Advances of NMR and NQR Studies of $\ce{YBa2Cu3O_{7-δ}}$ and $\ce{YBa2Cu4O8}$, pp 25–69 Subsection 6: 'Chemical Pressure, Physical Pressure, and Site Disorder', pp 59–59. ISBN: 978-1-60456-084-8
[5] A. Hauser, A. Nahid, S. Delahaye, A. Sadki, S. Schenker, R. Sieber, M. Zerara. 'Chemical Pressure'. CHIMIA International Journal for Chemistry, 2002, 56, 12, pp 685–689. DOI: 10.2533/000942902777679858
[6] R. S. Liu, C. H. Shen, T. S. Chan, R. Gundakaram, S. F. Hu, J. G. Lin, C. Y. Huang. 'Chemical Pressure Induced Phase Transition in Single and  Double Perovskites with Magnetoresistance Effect'. Tamkang Journal of Science and Engineering, 2002, 5, 1, pp 59–61. Archived link: bit.ly/2fvxrsv
[7] A. Huon, D. Lee, A. Herklotz, M. R. Fitzsimmons, H. N. Lee, S. J. May. 'Effect of chemical pressure on the electronic phase transition in $\ce{Ca_{1-x}Sr_xMn7O12}$ films'. APL Materials, 2017, 096105, pp 096105-1–096105-7. DOI: 10.1063/1.4994089
[8] Masatomo Uehara, Tsuyoshi Amano, Sachiko Takano, Tatsuya K[o]ri, Takahiro Yamazaki, Yoshihide Kimishima. 'Chemical pressure effect on the superconductor $\ce{MgCNi3}$'. Physica C, 2006, 440, 1–2, pp 6–9. DOI: 10.1016/j.physc.2006.03.017
[9] M.Tropeano, C.Fanciulli, F.Canepa, M.R.Cimberle, C.Ferdeghini, G.Lamura, A.Martinelli, M.Putti, M.Vignolo, A.Palenzona. 'Effect of the chemical pressure on superconductivity and SDW in undoped and $15$% $\ce{F}$ doped $\ce{La_{1-y}Y_yFeAsO}$ compounds'. Physical Review B, 2009, 79, 174523, pp 174523-1–174523-6. DOI: 10.1103/PhysRevB.79.174523