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The first justified theory of solid state was proposed by Grüneisen in the year 1912 and was based on the virial theorem. The forces of interaction between two atoms were assumed as changing with distance between them according to inverse power laws. But only virial theorem is insufficient to deduce the equation of state, so this author has introduced some relations, which are correct, when the forces linearly depend on displacement of atoms. But with such law of interaction the phase transitions cannot take place. Debye received Grüneisen equation in another way. He deduced the expression for thermocapacity, using Plank formula for energy of harmonic vibrator. Taking into account the dependence of atomic vibration frequency from distance between atoms, when the forces of interaction are anharmonic, he received the equation of state, which in classical limit turns to Grüneisen equation. The question, formulated by Debye is—How can we come to phase transitions, when Plank formula for harmonic vibrator was used? Debye solved this question not perfectly, because he was born to small anharmonicity. In the presented work a chain of atoms is considered, and their movement is analysed by means of relations, equivalent to virial theorem and theorem of Lucas (disappearing of mean force). Both are the results of variation principle of Hamilton. The Grüneisen equation for low temperature (not very low, where quantum expression for energy is essential) was obtained, and a family of isotherms and isobars are drown, which show the existence of spinodals, where phase transitions occur. So, Grüneisen equation is an equation of state for low temperatures.

The molecular-dynamical solid state equation, based on virial theorem, was derived by Sutherland [

The deduction of virial theorem from Hamilton variation principle is briefly described below. The variation of mean time Lagrange function L is transformed as follows

The first term of last expression is zero as consequence of movements equations, the second—for infinite time interval. The product of a coordinate with variable factor represents a varied coordinate:, so These variations substituted in the left side of last Equation (1) give a relation

If the kinetic energy W does not depend on coordinates (in crystals rectilinear coordinates of atoms are used), it follows the virial theorem

The represented variation procedure may be generalised. The variable coordinates are represented in the form where are variable parameters (further generalization of variation procedure is described in [

So the equations are obtained

where the theorem about uniform distribution of kinetic energy is used. Further step in calculations consists in representation of coordinates as functions of time in the form

where are the mean values of coordinates, is root mean square amplitude of vibrations, so the vibration functions satisfy the relations

If the kinetic energy depends only on velocities then (5) is represented as

and

Introducing in (6) taking into account (7) and (9) we obtain

If the system of atoms interacts with external corps, the positions of which are determined by means of coordinates, the mean forces acting on the system are determined by expressions

By shifting of the corps the forces produce a work, so the variation of energy by variation of all parameters is given by the expression

Taking into account (9), (11) a relation will be obtained

It can be seen that the quantity T which till now is only the indication for mean kinetic energy, an integrant divisor for the quantity of heat is, hence proportional to absolute temperature. The expression for entropy is the follows

We introduce a function of variables as an analogy of thermodynamic potential

(items depending on temperature only are omitted). The condition of minimum Ф by constant temperature leads to Equations (9), (11), (12) and allows to choose stable values of variables. In a crystal the mean values of displacements and amplitudes are equal for atoms belonging to the same sublattice, so the number of unknowns may be not great. Because in crystals the rectilinear coordinates are employed, the kinetic energy will not depend on coordinates.

Additional information about dynamical variation procedure is obtained through considering of a simple anharmonic vibrator with coordinate x and potential energy of the form

which is represented on

It is necessary to determine mean time potential energy, so it will be written

where is put because is an accidental function with zero mean value, so with equal probability receives positive and negative values, and in such manner behaves The mean value of is 3/2, if the function is harmonic, but is greater, if on the path is potential barrier, which reduces the speed. Later we show what value of is worthwhile to put in equations. So

The equations of state will be written as follows

It follows two solutions

The first solution corresponds to vibrations above the potential barrier, the second to vibrations in left or right potential pit. If we put = 3, then the left sides of second equalities in (25), (26) differ only through multiplier, consequently intersect the abscissa axis in the same point. For this case the Equations (25) and (26) are represented graphically on

Parts of the curves that have physical meaning, correspond to positive temperature. Heating or cooling leads to transitions from one curve to the other, which is manifested in jumps of amplitude.

Braunbek [

A serious question arises by comparing our calculations with calculations of Syrkin [

Syrkin calculated the dipole moment in electric field for charged atom by means of formula for statistical distribution, integrating over all space of the pit. It turned out that the dipole moment continuously decreases with

increasing temperature (as polarisability for thermal polarization according to Debye formula). So, a discrepancy can be marked between dynamical and statistical calculations, but really this discrepancy is illusory, because such question was considered in the literature. In the course of Frenkel [

It will be considered a chain of identical atoms situated on x-axis, where atoms interact with nearest neighbours. The extreme left atom with number 0 is fixed in the origin and on the extreme right atom a constant force f directed along the axis is acting (

Atom, which has the number k, interacts with left and right atom with numbers k – 1 and k + 1, which are placed on distances from the considered.

The energy of interaction for the whole chain is represented by the sum

If the mean time distance between neighbouring atoms is l—identical for all atoms, then the distance in moment t will be written as

where the mean square amplitude is written also as identical for all atoms. The energy of interaction is further expended in power series with respect to amplitude and the order of derivative is marked by means of Roman numerals

For taking the mean time value of (29) the relations (8) must be used and the mean time of (29) has the value

For the function determined in (16) and calculated for one atom we receive, taking into account (30), an expression

where f is a constant stretching force (hanging load). The terms depending only on temperature are omitted. The conditions of minimum have the form

If we neglect the terms containing fourth power of amplitude, an equation will be received

This is Grüneisen type equation. Now we deduce an equation with regard to fourth power of amplitude. We multiply (32) with and (33) with and subtract the second from the first, than it turns to be

The Equation (33) will be solved with respect to

The radical is taken with positive sign, because the amplitude increases with increasing temperature and which will be clear later. Expanding the radical and retaining terms quadratic in temperature, we receive.

Introducing in (35), we obtain the equation of state

Further calculations concerning the transitions in the chain will be carried out retaining only linear in temperature term.

The potential energy of two atoms, one of which is placed in the origin, other on the x-axis on the distance l will be represented by means of a usual function

(39)

which may be written in more convenient form

This expression is mathematically equivalent to given in [

The minimum energy is displaced on distance, determined through equation

The force constants may be expressed through

It follows the expression for the energy in the minimum

Further we find

As can be seen, in the vicinity of the point M it must be, because and the same is true for

These values for derivatives of interaction energy substituted in (34) give the Grüneisen equation for accepted law of interaction. We undertake now a mathematical investigation of this equation for the purpose to receive equations for isotherms and isobars and analyse their behaviour in order to discover transitions. As was noticed [

(47)

The expressions for energy and its derivatives are the follows

The Equation (36) now turns to the form

As the multiplier in front of the bracket varies slowly with the length, we put and accept following approximate equation of state

From this equation, as is usual in physical chemistry, a family of isotherms and isobars will be deduced.

The expression in brackets, including the sign in front of it, consists of two items—The first is represented as a parabola, that goes to negative infinity intersecting abscissa axis in points q = 0 and q = 1, the top of which arises above abscissa axis on 0.25. The second item is a fraction where numerator and denominator are linear functions of q and it aspires to constant negative value when q goes to infinity, by diminishing of q also diminishes till the value q = 7/13, where the fraction turns to negative infinity. So the function (53), when the temperature does not exceed definite value, is represented till this point by a curve with roots, lying between values of abscissa 0 and 1, and with top placed above the abscissa axis. By rising of the temperature the roots and the top coincide on the abscissa axis, and farther the chain can exist only under the action of negative (compressing) force. As it was noted in [

Expanding in series with respect to, we arrive at the formulated condition).

Each curve on

If this expression of temperature will be substituted in Equation (53), then the equation of the curve, connecting maxima of the isotherms (spinodal) will be obtained

The right side decreases with increasing of the abscissa from the value, where the left side has the value, till the intersection point with abscissa axis, where the top of the isotherm reaches it. As numerical calculation shows, here

The diagram (f, T) is represented in parametric form by expressions (54), (55).

Now we determine the positions of the points, where the isotherm branches intersect the abscissa axis. According to (53) one must put f = 0, then solve the equation

We shall not solve this cubic equation to express the abscissa as a function of temperature, but shall confine us with consideration of solving procedure. The right side is a curve, intersecting the abscissa axis in the points q = 7/13 and q = 1, between them has a maximum. The left side is a straight line parallel to abscissa axis, that at no high temperatures intersect the curve in two points, corresponding to roots of Equation (57). By rising of the temperature the roots converge till the straight line touches the top of the curve, then the roots and point of maximum coincide. The corresponding values of abscissa and temperature are (56).

From Equation (53) follows the expression for temperature

The right side is a product of a fraction with negative sign, whose numerator and denominator are linear functions of abscissa, and a quadratic function. The graphs of multipliers are represented on

The temperature is positive, so physical meaning has the interval on abscissa axis, where the ordinates of both curves have the same (negative) sign that means interval between points, where the fraction and parabola intersect the abscissa axis. The first point is fixed; the second depends on the value of the force and may disappear (if parabola is placed above the abscissa axis). So, the family of isobars has the form, shown in

The position of the point Q, where parabola intersects the abscissa axis is given by the expression

With increasing force this point moves toward the point with abscissa 7/13 and coincides with it by the value of the force

By this value the isobar disappears, and the system of atoms does not exist as regular structure.

For determination of maximum the expression (58) will be differentiated, and the result is

This equation has the form of Equation (55). The value of temperature maximum on the curves, shown in

and has the same mathematical form as (54). It is the equation of spinodale that joints the points of maxima on

The initial equations are (37), (50), (58) and the problem consists in excluding the quantity q. This is made as follows: Excluding the temperature leads to the equation for q (here)

where. The solution has the form

where

Introducing (63) in (37), leads to the expression for temperature

The first multiplier turns to zero, when w = 0, the last multiplier when p = 42/169 turns to zero also. When p = 0, the last multiplier is zero at w = 2/91. It can be proved directly that for p laying between these extreme values the last multiplier is zero at

So, the temperature has two roots with a maximum between them. The dependence of the temperature from the amplitude is shown in

Gilvarry states that root mean square amplitude divided through mean distance between atoms at transition point has the same value for all substances with the same structure [

With using the expression (47) for q this equality turns to

By means of equation of state (58), where is put f = 0, this equation transforms to

To receive the value of q in the transition point, one must differentiate (58) and the derivative equate to zero. Under the condition f = 0 definite value of q and consequently definite value of fraction will be obtained.

On the base of dynamic equations, that follow from variation principle of Hamilton, a thermal behavior of linear chain of atoms, interacting with nearest neighbors with forces, obeys inverse power law, and stretched with external force was considered. The mean potential energy is developed in series with respect to the amplitude of atomic vibrations and only second powers were retained. This restricts the temperature from above, and the Grüneisen type equation of state was obtained. The families of isotherms and isobars were drowing, and spinodals are marked. So, Grüneisen equation is an equation of state, which describes phase transitions. This is the answer to the question, formulated by Debye: Is the Grüneisen equation an equation of state? The original calculations contain assertions, which are strictly correct for linear dependence of interacting forces from displacements of atoms, so no transitions can occur. It is a correct reasoning, and if we put in Equations (37), (38) the numerical values (56), we come to result that linear and quadratic in temperature items are nearly equal, so at such temperature quadratic terms cannot be ignored, and Grüneisen equation is an approximation useful for lower temperature. In a discussion about Grüneisen work Nernst expressed his opinion [

“You all have received an impression, that we are so far, that can consider also the solid state from moleculartheoretical point of view, and one may hope that we soon receive a similar perfectly true theory also for solid state, that we for long time possess for gases.”

The Grüneisen equation in essential features turns into Mie equation, when in the fraction the terms, arises from attraction interaction, will be stroked out. So, the foundation for solid state equation development is the Grü- neisen equation. The equation that takes into account terms with second potent of temperature will be essentially more complicated, than Grüneisen equation.