Birch–Murnaghan equation of state

Not to be confused with Murnaghan equation of state.

The Birch–Murnaghan isothermal equation of state, published in 1947 by Francis Birch of Harvard, is a relationship between the volume of a body and the pressure to which it is subjected. This equation is named after Albert Francis Birch and Francis Dominic Murnaghan. Birch proposed this equation in a publication in 1947, based on the work of the Murnaghan of Johns Hopkins University published in 1944.

Expressions for the equation of state

The third-order Birch–Murnaghan isothermal equation of state is given by:

$P(V)=\frac{3B_0}{2} \left[\left(\frac{V_0}{V}\right)^\frac{7}{3} - \left(\frac{V_0}{V}\right)^\frac{5}{3}\right] \left\{1+\frac{3}{4}\left(B_0^\prime-4\right) \left[\left(\frac{V_0}{V}\right)^\frac{2}{3} - 1\right]\right\}.$

where P is the pressure, V₀ is the reference volume, V is the deformed volume, B₀ is the bulk modulus, and B₀' is the derivative of the bulk modulus with respect to pressure. The bulk modulus and its derivative are usually obtained from fits to experimental data and are defined as

$B_0 = -V \left(\frac{\partial P}{\partial V}\right)_{P = 0}$

and

$B_0' = \left(\frac{\partial B}{\partial P}\right)_{P = 0}$

The expression for the equation of state is obtained by expanding the free energy f in the form of a series:

$f = \frac{1}{2}\left[\left(\frac{V}{V_0}\right)^{-\frac{2}{3}} - 1\right] \,.$

The internal energy, E(V), is found by integration of the pressure:

$E(V) = E_0 + \frac{9V_0B_0}{16} \left\{ \left[\left(\frac{V_0}{V}\right)^\frac{2}{3}-1\right]^3B_0^\prime + \left[\left(\frac{V_0}{V}\right)^\frac{2}{3}-1\right]^2 \left[6-4\left(\frac{V_0}{V}\right)^\frac{2}{3}\right]\right\}.$

See also

• Albert Francis Birch
• Francis Dominic Murnaghan
• Murnaghan equation of state

References

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• ^

External links

• Equation of State Codes and Scripts This webpage provides a list of available codes and scripts used to fit energy and volume data from electronic structure calculations to equations of state such as the Birch–Murnaghan. These can be used to determine material properties such as equilibrium volume, minimum energy, and bulk modulus.