Recently obtained some nice bulk crystals of layered perovskites. These crystals are usually terminated with acute trapezoidal end facets making them look like gold bars. but when they are grown larger and larger, the end facets appear to be a hexagonal/trapezoid grafting with a boundary. This boundary resembles twin boundary at a glance but not quite.
Are these even single crystals? They don't appears to be symmetric and Friedel pairs cannot always be identified for every facet.
Your photos are absolutely stunning. These crystals do not appear to be twins in my opinion. I would characterize them as single crystals of the orthorhombic crystal system in the mm2 symmetry class.
Please take a look at the morphology of the sulfur crystal, at the bottom of the graphics on this page: https://www.pinterest.jp/pin/792703971892122481/
Measuring the Friedel pairs would be useful for determining the crystal structure.
Thank you for your comments, Armand! I also agree that they don't look like twins. Actually, the crystal structure falls into P-1 space group, which is triclinic. But they do look like the sulfur crystals. Armand Budzianowski
To elaborate more, most crystals have this kind of end facet. The cross-section of it looks like a combination of a trapezoid with an irregular hexagon. These two shapes are not parallel with each other.
I am not an expert, but if the twinning is not visible in the morphology of the crystal shape, and the morphology does not look centrosymmetric, and the crystal structure (not diffraction) shows the center of symmetry, it should be double checked. It may turn out that you actually have a twin and a different crystal structure. I'd be interested to hear what twin experts have to say about this.
So, as I wrote before. I see two mirror planes perpendicular to themselves and one two-fold axis at the intersection of these planes. However, the photos can deform crystal shape and in the real angles are varied than 90 deg., then mm2 class is not present.
Dear Wenhao Shao
Here is a short video of my understanding of the morphology of your crystals: https://youtu.be/MgYr34Dz45k
I hope this helps you, however I could be wrong.
PS. The animated "shape" was simplified due to lack of time to create and render accurately "form" of the crystal.
Thanks a lot, Armand! Traditionally mm2 usually happens in non-centrosymmetric structures. This structure is a P-1 and normally grow in Pinacoidal class. Actually if you look at the crystal at the center of the SEM image, it has only one mirror plane instead of two. But you are right, others can have 2 mirror planes.
These crystals were grew from solution via recrystallization. However, they could still have one face touching the vial wall and free-space nucleation no longer satisfied.
My current hypothesis leans toward Pinacoidal class crystals with free-space nucleation and growth, but mm2 could appear as you suggested if free-space nucleation is disrupted.
- Badges
- Science topic
- Similar topics
- Geoscience
- Remote Sensing