I would like to explain this question with the following illustrative example.
The semiconducting layered chalcogenides adopt hexagonal structure (Eg: Bi2Se3, Bi2Te3, Sb2Te3 etc.,)
They have a ionic-covalent bonded sheet of atoms in the ab plane and weak van der Waals bond along the c axis. Now I am applying hydrostatic pressure (equal pressure in all directions) to the crystal. Hence it is expected that “c” axis is more compressible than “a” axis. The slope of “a” and “c” will describe the rate at which the lattice parameter decreases (under pressure).
Similarly, in Raman point of view, A1g modes are related to interlayer vibrations along c axis and Eg modes are intralayer vibrations along ab plane. The pressure coefficient of A1g and Eg modes describes the rate at which the bond strength changes (under pressure).
In this case,
Q. Which of the following parameter will decide the anisotropy in the crystal?
1. Slope of lattice parameters “a” and “c” – High Pressure XRD measurement
2. Pressure coefficient of A1g and Eg modes - High Pressure Raman measurement
Q. What is most appropriate tool to describe the anisotropy in the crystal (under pressure)?
Dear Rajaji,
You are absolutely right when starting with difference in bonding in the ab plane and perpendicular to it. The anisotropy of any other property of the crystal (compressibility, spectra, conductivity, magnetism, diffusion coefficient, ...) will be a result of this difference. So, it is not possible to say than any physical property is more or less appropriate to describe the anisotropy: all of them will be the consequences of the same internal reason. This means the choice of the tool for description of the anisotropy should be made taking into account the real property that you are interested in. For analysis of the mechanical behaviour variation of the lattice parameters will be important, for any physics that is affected by phonons you will need Raman etc.
Dear colleague! If anisotropy means also optical anisotropy and your material is sufficiently transparent you could try using stress-induced optical birefringence. We offer measurements by means of our photoelastic microscope SIREX. Do not hesitate to ask me for more details are interested in. Wkr, M. Herms.
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