I am working on Mn-doped CH3NH3PbBr3 perovskite. I measured these samples by TEM. At the smallest amount of Mn doping, the particle size becomes bigger than the pure one. But, the size will decrease with the concentration increases. I am very confused here, the radius of Mn is smaller than Pb, shouldn't the whole particle size become smaller here.
Particle size is not the same as lattice size.
When measuring particle size by TEM, or even HRTEM, you're measuring crystals that might have grown 3, 4... hundreds unitary cells one on the side of the other.
Imagine that one unitary cell is a brick, the particles you see by TEM is a wall, depending on your sample and your magnification you'll be able to see smaller walls, or china great wall..
Jokes a part, the crystall growth depends not only on the symmetry (system, a, b, c, alfa, beta and gama constants), it will depends as well on the surface tension of crystals at the nucleation point.
Be aware as well that for high concentrations (>5%) you might have another phase growing, and instead of a substituion, you're having two phases.
My advice: go for XRD, check for the compositions of one phase. If your data is good enough, try Rietveld refinement and determine latices parameters. After that, go to microscopy and discuss about particles size.
Particle size depends on many factors. The lattice strain changes whenever you are doing external doping using another atom. So, for low doping, there is a chance of higher lattice strain which can increase the particle size slightly. However, for higher doping, there is a possibility of generation of a new phase as mentioned by Luiz Fernando de Sousa Lima
With increased level of doping, due to the different cationic sizes of the involved species, usually stress and strain in the crystallites increase with doping level, leading to a less favourable total free energy of the particles, which may limit their size, and the formation of smaller particles becomes favourable! As suggested by Luiz Fernando de Sousa Lima XRD may help to clarify this (at least if the particles are large enough to be monitored by XRD). Hope this helps, Dirk
The argument of the smaller ionic radius of a dopant concerns the lattice parameter(s) not the particle size. Doping could hinder the crystallite growth at a certain level of doping besides other factors
- Badges
- Science topic
- Similar topics
- Engineering
- Materials Engineering
- Powders