Dear Experts, I believe that there is no "arbitrary" things in science. Everything must be selected precisely with knowledge. I am trying to study the ferromagnetism in MgO rocksalt by introducing some defects Vmg and Vo using CASTEP code implemented in materials studio package. So, How can I select the position of vacancy in the supercell? Should I select all the position one by one and by comparing the formation enthalpy of each position and then choose the lowest formation enthalpy of that position. Your comments will really help me to understand more about this issue. The figure bellow is 2x2x1 supercell of 32 atoms of Mg16O16 with two vacancy of Mg to give the formula Mg14O16. I got high magnetic moment based on Mulliken population analysis and oxygen high spin and low spin is responsible for this behavior. Thanks in advance.. Mohammed
Why do you have only 2 atoms to describe in rock salt? Because of symmetry! Symmetry is defined by rotation and inversion symmetry, but also by translation symmetry. If you know how many atom positions are independent and how much bigger is your unit cell, you can exactly determine the independent positions. How much they will effect the final property is another question you should ask yourself. AND...you can again believe it that your opinion describes the result properly. If nature finally follows your believe is another question.
Short recommendation: go to the crystallographic server of Bilbao. This is the best place for subgroup relationships. I do not exactly know whether your defined cell can be immediately created by a subgroup relationship, but it might be. Your cell definition already tells you that the symmetry reduces at least to tetragonal. An alternative tool is PowderCell which also generates subgroups, but not entire Bärninghausen trees. To get a bit closer to this topic: there is a book from Ulrich Müller about subgroup relationships and the related Wyckoff position splitting, described in International Tables for Crystallography, Vol. A1. There is certainly also everything explained. Not that extensive as in Müllers book but as brief explanation it should work as well.
Dear Dr. Nolze,
Thanks for your knowledgeable answer. I removed 2 atoms arbitrarily. We synthesis MgO nanoparticles in our Lab with average coherent domain size of about 8 nm which may equal to grain size. This grain may contain about 2-4 unit cell of about 32 atoms. I think that the synthesized nanopowder may contain defects either Mg or O vacancy based on the results obtained from UV-vis spectra. Therefore, I am trying to simulate what happened in experimental work using CASTEP code. So how can find the correct procedure of selecting the atom positions to introduce these defects in the unit cell. Most of literature do not give a detail about this issue and how they select the right position.
Best Rgerads
Hello,
If you want to calculate single defect then you want each defect to separate enough distance so that they won't mutually interact. You should know the density of defect of your experimental example, usually it is not that dense to have two defect in one cell.
Dear Dr Obeid,
I don't know so much earlier works related to Vmg and Vo in MgO rock salt type structure, supporting DOS calculations for specific electronic or ferromagnetic properties. However, consideration to the isotype compounds TiN and TiC (nitride and carbides), N and C vacancy states are well known, and were analyzed in how the starting electronic properties are affected. Moreover, I think that you can point out literature related to the vacancy distribution in TiNx and TiCx , and how it was considered both experimentally and theoretically (DOS at EF).
I think that the related literature is quite considerable.
So similar approaches should be used with benefit with the MgO system.
I think as well that the answer of H. Lu is a pertinent one, in terms of probability of defects in one MgO cell.
Good luck
D. Fruchart
Dears Dr. Lu and Dr. Fruchart,
thanks for sharing your thoughts here. so it means that the number of vacancy is very few to make the system more stable according to the equ.
NV = NA × exp(-QV/kT)
NA is the total number of atoms in the solid
QV is the energy required to form a vacancy
k is Boltzmann constant,
T the temperature
and of course these numbers of defects are temperature dependence.
What I am doing is to create vacancy at ground state (zero temperature) where the number of vacancy are negligible or very very few as Dr. Lu confirm.
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