First order transition: definition

Finally the concept of fields penults clarification of the definition of the order of transitions [22]. If one considers a space of all fields (e.g. Figure A2.5.1 but not figure A2.5.3, a first-order transition occurs where there is a discontinuity in the first derivative of one of the fields with respect to anotlier (e.g. (Sp/S 7) = -S... 

The transitions between the bottom five phases of Fig. 2 may occur close to equilibrium and can be described as thermodynamic first order transitions (Ehrenfest definition 17)). The transitions to and from the glassy states are limited to the corresponding pairs of mobile and solid phases. In a given time frame, they approach a second order transition (no heat or entropy of transition, but a jump in heat capacity, see Fig. 1). 

The simplest definition of a first-order transition is one in which heat flows into or out of the material with no change in temperature. Examples are melting and boiling and their reversals, crystallization and condensation. 

However, it is useful, to provide a thermodynamic definition of a first-order transition. Specifically, it is one in which there is a discontinuity in a first derivative of the Gibbs free energy. The advantage of this definition is the guidance it provides for the experimental study of phase transitions. A useful expression for the free energy in this regard is... 

As has aheady been indicated a large number of computer simulations have been performed to study phase transitions. In particular, some simulations have predicted the existence of the hexatic phase [256], whereas others have not [132]. Moreover, as Toxvaerd [264] has indicated simulations with a very large number of particles are needed to give a definitive conclusion about the melting order. More complex simulations, including the calculation of the angular correlation fimction [262,265] have indicated a first-order transition. 

SmA phases, and SmA and SmC phases, meet tlie line of discontinuous transitions between tire N and SmC phase. The latter transition is first order due to fluctuations of SmC order, which are continuously degenerate, being concentrated on two rings in reciprocal space ratlier tlian two points in tire case of tire N-SmA transition [18,19 and 20], Because tire NAC point corresponds to the meeting of lines of continuous and discontinuous transitions it is an example of a Lifshitz point (a precise definition of tliis critical point is provided in [18,19 and 20]). The NAC point and associated transitions between tire tliree phases are described by tire Chen-Lubensky model [97], which is able to account for tire topology of tire experimental phase diagram. In tire vicinity of tire NAC point, universal behaviour is predicted and observed experimentally [20]. 

Imaginary frequencies are listed in the output of a frequency calculation as negative numbers. By definition, a structure which has n imaginary frequencies is an n order saddle point. Thus, ordinary transition structures are usually characterized by one imaginary frequency since they are first-order saddle points. 

The martensite - austenite transition temperatures we find are for all systems in accordance with the previously published ones . Some minor deviations can be attributed to the fact that we are simulating an overheated first order phase transition. Therefore, for our limited system sizes, one cannot expect a definite transition temperature. 

Kinetic theory indicates that equation (32) should apply to this mechanism. Since the extent of protonation as well as the rate constant will vary with the acidity, the sum of protonated and unprotonated substrate concentrations, (Cs + Csh+), must be used. The observed reaction rate will be pseudo-first-order, rate constant k, since the acid medium is in vast excess compared to the substrate. The medium-independent rate constant is k(), and the activity coefficient of the transition state, /, has to be included to allow equation of concentrations and activities.145 We can use the antilogarithmic definition of h0 in equation (33) and the definition of Ksh+ in equation (34) ... 

It may not at first be obvious that the Jahn-Teller theorem applies to transition states (40). The proof rests on the fact that the matrix element of the distortion gives a first-order change in energy and hence is linear in Q. In other words there must be a non-zero slope in some direction and this is incompatible with the definition of a transition point as a saddle point on the potential energy surface. 

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