In molecular crystals especially hydrogen bonded systems we often see red-/blue-shift in the IR/Raman spectra when they subjected to high pressure/temperature. When IR/Raman spectra show red- to blue-shift and vice versa in the stretching frequencies, does this indicate a structural phase transition in the material? How to correlate the structural transitions based red- and blue-shift in the IR and/or Raman spectra and their corresponding intensities?
The problem is complex, report from the object you are working on. Read this article for beginners, Observation and Assignment of Silent and Higher Order Vibrations in the Infrared
Transmission of C60 Crystals
Michael C. Martin, Xiaoqun Du, John Kwon, and L. Mihaly
Department of Physics, State University of New York at Stony Brook, Stony Brook, NY 11794{3800
(Submitted to Phys. Rev. B, January 24, 1994)
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Bheema Lingam Chittari
Indian Institute of Science Education and Research Kolkata
Adding to the above answer, It is always depends on the object that you wanted to project. In case of regular crystal (as we know that the IR/RAMAN are structure properties), the blue/red shift in the IR/RAMAN frequencies (independent of intensities dynamics) renders the phase stabilization/destabilization. The red shift may be leads to phase transition (however it is not a phase transition). The occurrence of new frequencies in the spectra is an indication of new phase or phase transformation.
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Sorb Yesudhas
Iowa State University
When you apply pressure on a crystal, usually there will be volume reduction due to the reduction in interplanar spacings. The volume reduction leads to shortening of bond distances that bring about increase of vibrational frequencies. That is why, in most of the cases there will be a wavenumber (cm-1) shift to higher frequencies upon increasing the pressure. Sometimes, few modes show red shift whereas remaining modes exhibit blue shift that is because of the intrinsic properties of the materials. For example, in layered materials with Va nder waals bonding along c-axis, 'c' lattice parameter gets compressed more as compared to that of 'a'. When you heat the sample, the lattice volume gets expanded causing red shift of vibrational frequencies due to the lengthening of bond distances.
When the crystal undergoes symmetry lowering structural phase transition, the transition is accompanied with new Raman modes ensuring changes in the bonding (new bonds), provided the new phase should be Raman active. The number of peaks also depends on what kind of symmetry change and also on the type of phase transition.
If the number of peaks decreases, then the system moving towards higher symmetry structure.
The intensity change is also correlated to the band gap decrease which bring the system towards metallic state. If the high pressure phase is not Raman active, then there won’t be any Raman modes. But you can see IR modes due to the mutual exclusion principle (for centrosymmetric crystals).
If you need more clarification, please ask me
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N. Yedukondalu
Stony Brook University
Thank you all for your detailed answers which are more informative and very useful as well. The above discussions help me to understand the structural characterization and phase transitions for crystals with primary bonding under high pressure.
To specific, I am interested in understanding the behavior of crystals which are dominated by noncovalent interactions (secondary bonding). Although these interactions are weak in nature but they play a crucial role in determining the stability of molecular crystals at ambient as well as at extreme conditions. Pressure/temperature can be used as tool to tune the properties of molecular crystals since they are binding with weak noncovalent interactions.
I would like to share my idea with an example: Ammonium azide and ammonium dinitramide are ionic molecular crystals when they are subjected to high pressure, they undergoes a structural phase transition due to weakening of hydrogen bonding at the transition pressure [https://doi.org/10.1016/j.chemphys.2011.05.017; 10.1021/jp502619n; https://doi.org/10.1063/1.4959900]. There is an anti-cooperative nature between the electrostatic interactions (arises due to cation and anions present in the system) and hydrogen bonding at the transition pressure which eventually leads to structural phase transitions in these materials. From the literature, the proposed mechanism is, when a molecular crystal with hydrogen bonding is compressed, its try to form a denser closed pack structure with high density and when it is further compresssed, there is an anti-cooprative nature between two/more noncovalent interactions leads to structural phase transition in molecular crystals. For instance, consider a N-H...N hydrogen bond, when a material is compressed, the molecular entities come close together when compared to its ambient structure and hydrogen bond becomes more strengthened by reducing the distance between H...N which leads to lengthening of N-H bond, therefore the stretching frequencies corresponding to N-H bond show red-shift. When the N-H bond get stiffened it shows a blue-shift. One can see these two effects under high pressure/temperature.
Moreover, there are some studies in the literature, they made a possible correlation between red/blue-shift and intensity of IR spectra for molecules[https://pubs.acs.org/doi/abs/10.1021/ja067545z; https://pubs.acs.org/doi/abs/10.1021/ja020213j; https://pubs.acs.org/doi/abs/10.1021/jp205081r]. Is it possible to make some correlation between red-/blue-shift and structural phase transitions in molecular crystals under high pressure/temperature.
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Bheema Lingam Chittari
Indian Institute of Science Education and Research Kolkata
Dear Yedukondalu Neelam,
I appreciate your work on the given resources. As you have already notified from your publications that the subject of molecular crystal is different from the regular crystals, moreover the physical phenomenon that we look for is fundamentally unchanged. I agree that there is no direct measuring parameter that can correlate the red/blue shift in molecular crystals. However, the physical properties in the molecular crystals is direct effect of van der Waals interactions i.e. other than non-covalent interactions. So, the quantity of measuring parameter (change in the frequencies or red/blue shift) is direct effect of non-covalent interactions. The external uniform effect that fisrt modify the non-covalent interactions then later covalent interactions.
Finally, one can always make a fitting curve to the binding energy by adding explicitly a van der waals (non-covalent interaction) term so that you can always fit the curve (provided no covalent contribution is altered) by adjusting the van der Waals term for every pressure/temperature and propose a measuring quantity, based on your objective of study.
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