For a given compound, there are two different crystal structures(both metal) reported experimentally. Will there always be a structural transformation at certain condition (at a particular temperature or pressure)? If not, then please give some reference.
1) All crystals undergo phase transitions. Which means that if we change parameters (typically T, p), one phase can become more stable than another, so the system will transform from an initial phase P stable at initial condition T,p, into another phase P' whose energy is smaller than P at T', p' final condition.
If different phases are present ( e.g. water below), it is not necessary true that is possible to transform directly from P to P' withoutpassing through a third phase P' ' .
There are different mechanism (order) how the transition can occur and put in a simple way, it depends, if the rearrangement of atoms involve the breaking of the bonds, or just a change in symmetry.
https://upload.wikimedia.org/wikipedia/commons/thumb/0/08/Phase_diagram_of_water.svg/725px-Phase_diagram_of_water.svg.png
1) All crystals undergo phase transitions. Which means that if we change parameters (typically T, p), one phase can become more stable than another, so the system will transform from an initial phase P stable at initial condition T,p, into another phase P' whose energy is smaller than P at T', p' final condition.
If different phases are present ( e.g. water below), it is not necessary true that is possible to transform directly from P to P' withoutpassing through a third phase P' ' .
There are different mechanism (order) how the transition can occur and put in a simple way, it depends, if the rearrangement of atoms involve the breaking of the bonds, or just a change in symmetry.
https://upload.wikimedia.org/wikipedia/commons/thumb/0/08/Phase_diagram_of_water.svg/725px-Phase_diagram_of_water.svg.png
It is a very interesting question, of course...
If briefly, the phase transition being allowed from thermodynamical considerations, may not occur due to kinetic reasons.
I agree with Dr. Brivio, all the materials has, at least, two phase transitions: solid to liquid and liquid to gas. Moreover, materials also show solid to solid and sometimes liquid to liquid phase transitions (as it is the case for Helium-3 and Helium-4).
As a rule, with pressure is applied to a material, at a certain value of p, a phase transformation to a denser phase occurs. Generally, at the transition an abrup change of volume is observed (for first order Landau transition).
This is a very interesting subject. You can consult (between many others):
http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/07/255/7255152.pdf
ftp://ftp.cefetes.br/cursos/EngenhariaMetalurgica/Marcelolucas/ASM%20Metals%20HandBook%20Volume%203%20%20-%20Alloy%20Phase%20Diagrams.pdf
http://nptel.ac.in/courses/113106032/9%20-%20Phase%20diagrams.pdf
http://nptel.ac.in/courses/Webcourse-contents/IISc-BANG/Material%20Science/pdf/PPTs/MTS_07_m.pdf
Best regards,
Pedro
You refer to apparent occurrence of different polymorphs of one "compound"/composition. If both structure have different crystal structures (different space groups is a sufficient but not necessary condition for that) there are, indeed, different phases. The transition can be monotropic or polytropic, or it may not be realised in practice as a pure solid-solid phase transition....
However, I wish to hint at a different aspect of polymorphism. Given that different crystal structures with e.g. different space groups have been reported, polymorphism may also be absent, in the trivial but not rare case of that one of these entries is wrong. There are e.g. many crystal structures reported in too low space group symmetry. The crystal structure features are correct but not all symmetry elements were encountered. Then there may be a crystal structure report in low symmetry and another one in high symmetry, usually a supergroup symmetry. The reported structure then are, however, not different phases in the sense of the phase rule. In other cases, there may be overlooked superstructure, and the true symmetry is lower. And another scientist found that superstructure. In such a case thinking is necessary (hopefully conducted by the authors of the later paper) whether in the earlier report really a superstructure was absent or only overlooked. A nice trick is to simulate (e.g. powder) diffraction data for both structures and to find out whether they are really that different as they seem on the first sight...
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