A term "phase"  is not a crystallographic term. Polytypes are good example of a controversy between crystallography and thermodynamics: one and the same phase may has different structures.

@ Volker: Surprizing to me, since the difference in a chemical or physical signal must have some reason. BTW:  Also XRD, EDS, magnetic or any other signal is only an "image" of a phase property. Who decides which signals are relevant and which aren't? Even the problem with different dopants underlines this: if it causes a change of propoerty we as scientists are actually looking for opportunities to seperate these two "phases", although in classical meaning they don't clearly vary in crystal structure as well as in chemical comppsition. 

Nevertheless: I will have a look on the recommended "phase discussion". Thanks!

@ Max: From my opinion the polytypes of a material like SiC are different phases.

@ Martin: If you observe a zoning where composition and/or lattice parameter obviously change  (but there is no "classical" phase transition) you wouldn't differenciate between these "phases"?

A pahse has it own position in a phase-diagram. All the SiC modification would lie in a one and the same point. We can not distinguish polytypes in thermodynamic. BTW that is way we can not control crystal growth in such systems.

Our definition in material science is: "The homogeneous constituents of a heterogeneous system are called "phases". Their crystal structure, composition and volume fraction is defined by the minimum of Gibbs free energy  of the whole system in thermal equilibrium." (see the book of Peter Haasen )

with best regards

Well, this seems to be more my terminalogy problem. Therefore, let's expend this question:

@ Max: Why we are talking about phase identification if it would be in for SiC more a polytype identification :-) ?

Or let's formulate this question more practical: Is there a terminology which considers both, the thermodynamic definition of phase, as well as polytypes?

BTW: is it really correct that SiC polytypes appear at the same position in phase diagram? What is the driving force for a different arrangement of stacking? (Sorry, I don't have one available at the moment, but it might be interesting to know fro many readers here as well.) 

What is with high and low-temperature modifications (ferrite-austenite, rutile-anatase, Zincblende-wurzite etc which sometimes occur simultaneously in a microstructure)?

History. We use term "phase identification" in XRD because the term "phase" was already in use. And it was assumed that phase has unique structure. Later on it was found that some compounds can form various structures. You can try to apply Gibbs rule to a system of SiC or mica (any other phyllosilicate) or ZnS, whatever. You can see that Gibbs rule does not work. To my knowledge the only exception is SiO2: tridymite and cristobalite. Polymorphs and polytypes.

A phase change can be characterize either by an abrupt change in the space group as composition is varied. When varying Temp, Pressure a phase change is apparent by a change in Enthalpy. A phase transition occurs which is either edothermic or exothermic. This is easily seen by DTA/DSC. If you have a second order phase transformation (for example order/disorder) then you will see an inflection in the DTA curve. If you take the derivative of the curve you will get a peak that indicates a phase transformation. A phase transition is strictly defined by thermodynamics and not kinetics.

@Lawrence Margulies: I'm not sure the last statement is true - to give one example, martensite is usually considered a phase even though it is metastable; thermodynamics only predicts ferrite and cementite for a low-carbon steel at room temperature.

Also, It is important to note that meta-stable phase exist which do not appear in equilibrium phase diagrams. These can be quite stable under normal conditions. A diamond, for example, is not an equilibrium phase at room temperature and atmospheric  pressure. The high Tc superconductor, Bi2Sr2Ca2Cu3Oy, is also metastable at room temperature. Most glasses are metastable, etc...

Manuel,

It seems I answered your question before I read it! Yes non-equilibrium phase diagrams are extremely important for materials like Steels. A TTT diagram is a very useful way to visualize the conditions to form non-equilibrium phases. However it is important to distinguish between meta-stable phases which were equilibrium phase under the conditions (T/P) they formed and then were quench to room T/P quickly enough to prevent them from decomposing, and non-equilibrium phases that are not thermodynamically stable under any conditions and formed due to rapid changes in conditions which "froze" in a structure that did not have enough time kinetically to form an equilibrium phase (like amorphous structures).

I see that there are different points of view which all might have their legitimisation. I was asking more from a practical point of view. If I have a material showing in SEM - e.g. in BSE mode (peridic number contrast)  - a well visible contrast, but  neither the grain orientation nor the diffraction signal does not drastically change is this not a good idea to talk about another phase? One example is already given above anthough there is no crystallographic database I know which contains martensite as phase (as I know, martensite is more a microstructure then a phase). In iron meteorites even cubic martensite has been described (lack of carbon). On the other hand, despite a cooling rate of 10K per Million years one can observe a strong concentration gradient of Ni and Fe in austenite (taenite). Another example is Wuestite/Magnetite which occurs simultaneously in corrosion layers. Who cares whether they are in equilibrium (obviously it isn't since both phases exist) since we have to describe (characterize) it?  In so far I am not sure to limit the application of the term "phase" on thermodynamically ideal conditions only. 

Gert, what do you mean by diffraction? EBSD? Have observed any difference in EDS?

Yes, EBSD. In 20ms the differences in EDS are not very pronouncing :-). But this is in general the case. The BSE signal is often clearly better, less noisy - especially if you use the EBSD patterns as BSE detectors - and the resolution fits also much better to EBSD than the EDS information.  

A change in phase is defined by Landau theory, which is thermodynamically based. This does not mean that a phase cannot exist out of thermodynamically equilibrium. If all matter were to reach equilibrium conditions the entire universe would be Iron, since it has the lowest energy of any element. Landau theory is the only precise and practical way I know of to define a structural/chemical phase and explain a phase transition. From the data you have measured, I don't think it is possible to determine if you are seeing 2 different phases, or one phase with compositional variations (non-stochiometry). x-ray or neutron diffraction would be needed to determine if there is a difference in space group.

Lawrence: Does  your last explanation actually means, that we do not have equilibrium conditions :-)? Otherwise we would have only iron? I kno this is only a philosophical question...:-)...

Unfortunately the secod part I do not get. You are certainly right that currently EBSD is definitely not the technique to determine a crystal structure, but it has this potential. We can even differentiate between the chirality of quartz. A bit tricky, but it shows that EBSD is able to display much more that crystal orientations.  

However, what do you mean with the sentence: "From the data you have measured, I don't think it is possible to determine if you are seeing 2 different phases, or one phase with compositional variations (non-stochiometry)."

Thanks, Gert

So you have two areas, which differ in contrast, but have identical or close chemical composition. EBSD shows one and the same lattice and intensity distrubution is the same for both areas. Is that correct?

exactly

Go TEM :)

TEM is certainly an option, but again: my major question was not, how to distinguish between similar phases, but  when do we have to seperate between two phases if there are distinct differences in signal, e.g. EDS (BSE is also chemical information), diffraction etc which are an expression of different chemical or physical properties.

The term phases is probably "misunderstandable" if one uses the thermodynamical definition.