International
Tables for
Crystallography
Volume D
Physical properties of crystals
Edited by A. Authier

International Tables for Crystallography (2013). Vol. D, ch. 1.1, pp. 31-32

## Section 1.1.5. Thermodynamic functions and physical property tensors

A. Authiera*

aInstitut de Minéralogie et de Physique des Milieux Condensés, 4 Place Jussieu, 75005 Paris, France

### 1.1.5. Thermodynamic functions and physical property tensors

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[The reader may also consult Mason (1966), Nye (1985) or Sirotin & Shaskol'skaya (1982).]

#### 1.1.5.1. Isothermal study

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The energy of a system is the sum of all the forms of energy: thermal, mechanical, electrical etc. Let us consider a system whose only variables are these three. For a small variation of the associated extensive parameters, the variation of the internal energy is where Θ is the temperature and σ is the entropy; there is summation over all dummy indices; an orthonormal frame is assumed and variance is not apparent. The mechanical energy of deformation is given by (see Section 1.3.2.8 ). Let us consider the Gibbs free-energy function defined by Differentiation of gives The extensive parameters are therefore partial derivatives of the free energy: Each of these quantities may be expanded by performing a further differentiation in terms of the intensive parameters, , and Θ. We have, to the first order, To a first approximation, the partial derivatives may be considered as constants, and the above relations may be integrated: This set of equations is the equivalent of relation (1.1.1.6) of Section 1.1.1.3, which gives the coefficients of the matrix of physical properties. These coefficients are:

 (i) For the principal properties: : elastic compliances at constant temperature and field; : dielectric constant at constant temperatures and stress; : heat capacity per unit volume at constant stress and field ( is the specific mass and is the specific heat at constant stress and field). (ii) For the other properties: and are the components of the piezoelectric effect and of the converse effect. They are represented by and matrices, respectively. One may notice that which shows again that the components of two properties that are symmetric with respect to the leading diagonal of the matrix of physical properties are equal (Section 1.1.1.4) and that the corresponding matrices are transpose to one another.

In a similar way,

 (a) the matrices of the thermal expansion and of the piezocalorific effect are transpose to one another; (b) the components of the pyroelectric and of the electrocaloric effects are equal.

Remark. The piezoelectric effect, namely the existence of an electric polarization P under an applied stress, is always measured at zero applied electric field and at constant temperature. The second equation of (1.1.5.1) becomes under these circumstancesRemark. Equations (1.1.5.1) are, as has been said, first-order approximations because we have assumed the partial derivatives to be constants. Actually, this approximation is not correct, and in many cases it is necessary to take into account the higher-order terms as, for instance, in:

 (a) nonlinear elasticity (see Sections 1.3.6 and 1.3.7 ); (b) electrostriction; (c) nonlinear optics (see Chapter 1.7 ); (d) electro-optic and piezo-optic effects (see Sections 1.6.6 and 1.6.7 ).

#### 1.1.5.2. Other forms of the piezoelectric constants

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We use here another Gibbs function, the electric Gibbs function, , defined by

Differentiation of givesIt follows thatand a set of relations analogous to (1.1.5.1):where the components are the isothermal elastic stiffnesses at constant field and constant temperature, are the piezoelectric stress coefficients at constant strain and constant temperature,are the temperature-stress constants andare the components of the pyroelectric effect at constant strain.

The relations between these coefficients and the usual coefficients are easily obtained:

 (i) At constant temperature and strain: if one puts and in the first equation of (1.1.5.1) and (1.1.5.2), one obtains, respectively, from which it follows that at constant temperature and strain. (ii) At constant temperature and stress: if one puts and , one obtains in a similar way from which it follows that at constant temperature and stress.

#### 1.1.5.3. Relation between the pyroelectric coefficients at constant stress and at constant strain

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By combining relations (1.1.5.1) and (1.1.5.2), it is possible to obtain relations between the pyroelectric coefficients at constant stress, , and the pyroelectric coefficients at constant strain, , also called real pyroelectric coefficients, . Let us put and in the first equation of (1.1.5.1). For a given variation of temperature, , the observed strain is From the second equations of (1.1.5.1) and (1.1.5.2), it follows that Substituting the expression and eliminating , it follows that

This relation shows that part of the pyroelectric effect is actually due to the piezoelectric effect.