Questions tagged [crystallography]

For questions concerning crystalline structures and their properties.

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16
votes
2answers
986 views

Why is crystallography still used in some cases for small molecule structure determination?

I was wondering for some time why in some cases x-ray crystallography and not NMR is used for structure determination of small molecules? Does crystallization maybe require a smaller amount of the ...
14
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2answers
3k views

Are there any crystals that are theoretically possible, but haven't yet been made?

Are there any crystals (that exist at standard temperature and pressure) that are theoretically possible, but haven't yet been made? For example, the predicted requirements for temperature, pressure, ...
13
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2answers
245 views

How does the crystal know where to stop stacking the unit cells?

I tried to make different crystals in the cubic system by stacking cubes in a 3D software. However, I need to tell the software where to skip rows or stop stacking the unit cells in a specific ...
13
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1answer
160 views

Physical meaning of thermal parameters dimension (Ų)

In single-crystal x-ray crystallography both isotropic and anisotropic displacement parameters $U_{ij}$ of thermal ellipsoids have dimension of square angstrom ($Å^2$) as follows from the definition ...
12
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4answers
1k views

What is the smallest molar volume?

I wondered how small a volume a mole of material could occupy, so I started with carbon, which would need 12 grams. That's 60 carats, and there happens to be a famous 60 carat diamond. If my density/...
12
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3answers
1k views

Structure of Ca2B6O11

What is its structure? I found an image of $\ce{B6O11}$ unit, but it looks incorrect: The top left corner one. The oxygen can make max 2 bonds but atom $\mathrm{O8}$ has three! There are ...
12
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1answer
240 views

Why are snowflakes shaped as they are?

(It was the Winterbash icon that got me thinking on this.) Snow flakes take on a variety of intricate shapes, but what processes are responsible for this? (Got the image off of Wikipedia) I mean, ...
10
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1answer
997 views

Miller Indices: How to deal with some weird(-ish) cases?

I've recently been doing a spot of self-learning (Crystallography), and some of the examples provided in the Wikipedia article for Miller Indices have stumped me. I can't for the world figure out how ...
9
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1answer
307 views

Is the 'central Tin atom' in crystalline tin oxide really offset like this?

Is the position of the central Tin atom as shown in the first figure ($\ce{SnO2}$) drawn offset towards the oxygen to the right for a reason? The oxygen also seems to reciprocate and move towards the ...
9
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1answer
650 views

How many crystal forms of HCl have been identified - where to find the unit cells/lattice constants?

This blog about stomach acid shows this image of the "zig-zag" form of crystalline HCl (see also Wikipedia where it looks (to me) like five four HCl's per unit cell), but links to this cubic crystal ...
9
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1answer
285 views

Use of geometry index for the determination of coordination environment

Geometry index $\tau$ is supposed to resolve proper geometry for coordination numbers (C.N.) 4 and 5 based on its extreme values ($0$ or $1$). There is also a web app Geom which handles both cases for ...
9
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1answer
2k views

How to make pyramid-shaped NaCl salt crystals

I'm attempting to make sea salt pyramids like the one below. I understand this needs to be evaporated at a reasonable speed to stop the crystals forming a cube, as the when formed quickly, the edges ...
8
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2answers
657 views

What is the difference between crystal structures of proteins, organic and inorganic materials?

From the definition of crystal and the main differences between crystalline and amorphous material, it is known that crystal formation requires ordered bonding between atoms or molecules. How can ...
8
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3answers
788 views

How to add hydrogens to a crystal structure?

PDB protein crystal structures most of the time lack hydrogen atoms. Usually I use PyMOL (h_add) but it does not always seem to be giving the correct result. What ...
8
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2answers
2k views

Calculate the third and fourth nearest neighbours in bcc

My textbook has given data for third and fourth nearest neighbours to be 12 and 8 with distances $\sqrt{2}a$ and $\frac{\sqrt{11}a}{2}$. I have been able to calculate for the first and second ...
8
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1answer
283 views

How to dock aggregated structures comprised of “elementary” protein units like LEGO pieces?

There are proteins with intrinsic symmetries. For example: I was wondering how to use transformations such as: rotations, translation, replications to construct possible structures using 2 types of ...
7
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1answer
122 views

Validation of _atom_site_frac_*: Range outside of [0,1)?

I have the following cif file ...
6
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1answer
89 views

Are electronic descalers a scam?

It seems there are some electronic kits sold around nowadays by major retailers (e.g., Home Depot and Amazon) claimed to remove built-up scale from water pipes. But, do they actually work? If so, ...
6
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0answers
93 views

Equivalent Wyckoff position in different groups

I am not a chemist, I am a physicist, so please bare with me if what I am asking is rather obvious. If I have two symmetry groups ($P4/mmm$ and $P\!-\!1$), is there a way to find their respective ...
5
votes
1answer
215 views

What does it mean to say “the 222 space group”?

I know there are supposed to be a few rotation axes, that it is non-centrosymmetric, it belongs to the orthorhombic crystal system, and it has four equivalent positions. What do the 2s mean?
5
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1answer
132 views

What is the diagram of the roto-inversion of $\bar 6$?

Using the standard way of representation: Here we have a rotation followed by an inversion (technically called 'rotational inversion'). So as to represent it, what I have done is: 1) Both rotation ...
5
votes
1answer
190 views

Assigning absolute configuration when determining crystal structure isn't possible

When natural products are isolated, their structures are often determined by x-ray crystallography which gives a complete picture of the atom coordinates, the relative and the absolute stereochemical ...
5
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1answer
353 views

Structure of lead(II,IV) oxide

I need help sorting out the molecular structure of tri-lead tetroxide, aka lead(II,IV) oxide $\ce{Pb3O4}$. This data sheet shows this image: The Wikipedia's lead (II,IV) oxide's link to a Jmol ...
5
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0answers
54 views

Effect of spontaneous transmutation of atom in crystal

What happens to a compound when one of its atoms changes via spontaneous nuclear reaction to an atom with very different bonding? For example, beryllium-7 turns into lithium-7 with a half-life of 53 ...
5
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0answers
857 views

Can a solid be both crystalline and amorphous? [closed]

The first line in the Wikipedia article for "Glass" reads: Glass is a non-crystalline amorphous solid that is often transparent and has widespread practical, technological, and decorative usage in, ...
4
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2answers
549 views

Sulfuric acid symmetry point group

On the following page: https://cccbdb.nist.gov/pglist.asp It appears that the point symmetry group of sulphuric acid is $C_2$. I know that $C_2$ means that there is an axis of symmetry of order 2 (180 ...
4
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1answer
193 views

What is the Crystal System of MoS2?

I'm confused about the crystal system of MoS2. In the case of the monolayer MoS2, it has 3-fold rotation symmetry when viewed from the top of its unit cell (c-axis). Even in the case of bulk (AB ...
3
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2answers
78 views

Is there a piece of software that can help me see the (hkl) planes in a crystal lattice?

I am doing research with calcium carbonate structures. I need a way in which I can familiarize myself with the lattice structure of calcium carbonate polymorphs, especially calcite. I really struggle ...
3
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2answers
4k views

Second Peak in X-Ray Diffraction Pattern of bcc Metal

By Bragg’s law, $$2d\sin\theta=n\lambda$$ and $$d=\frac{a}{\sqrt{h^2+k^2+l^2}}$$ Combining the two gives: $$\sin\theta=\frac{n\lambda\sqrt{h^2+k^2+l^2}}{2a}$$ By the reflection rule for bcc, there ...
3
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1answer
94 views

Testing for asbestos using readily available chemicals and tools?

Given that it is difficult to determine who is telling the truth in the recent claim that J&J covered up finding asbestos in their powder, is there any chemical test that we can perform on our own ...
3
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1answer
72 views

Does a molecule need to be placed symmetrically in the unit cell?

I was watching an OCW by MIT (3.091X) on solid state. My previous notion of lattice point and unit cell got shattered after this example. Where are the unit cell and lattice points in this picture? ...
3
votes
1answer
91 views

Space groups: Herman-Maugin notation, diagrams and Wyckoff positions

I am currently taking an Introduction to Crystallography course. After studying and understanding the punctual symmetry groups (2mm, 4/m 2/m 2/m, 32,...), the flat groups and the diverse elements of ...
3
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1answer
86 views

Space group diagram

If you give me the name of a certain space group, for example, the P222 (orthorhombic system). How could I construct the following diagram? Image source: http://img.chem.ucl.ac.uk/sgp/large/sgp.htm ...
3
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1answer
172 views

crystallinity of seed crystals

Does the crystallinity of seed crystals affect the final crystal structure? and is it different depending on the crystallized material?
3
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0answers
33 views

What is “detwinning” as a 2D crystallography technique?

The Phys.org article Iron selenide revealed as 'garden-variety iron-based superconductor' is quoted below. Normally I would hesitate to quote so much but in this case it all seems germane. Question: ...
3
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0answers
150 views

X-ray structure analysis mirrored coordinates?

How to ensure, that X-ray crystallography structure analysis was done right to provide correct, non-mirrored atomic coordinates? Example: Following Wikipedia image, is generated from data taken from ...
3
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0answers
42 views

Computational determination of unique adosrption sites

I'm not sure it's a chemistry question or physics question or math question. Since I'm active on this site, maybe someone here can point me in the right direction. I'm trying to enumerate all the ...
3
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0answers
812 views

What is the symmetry of the cuboctahedron (FCC metal)?

The background to this is that I've recently given a tutorial wherein we had to go through the determination of point groups for atoms in various lattices (BCC, FCC/CCP and HCP). BCC and HCP, I have ...
2
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2answers
6k views

How do I calculate lattice points per unit volume? [duplicate]

My professor was explaining simple cubic, body centered cubic, and face centered cubic lattices and I didn't quite understand the calculations below: Simple Cubic: $8 \cdot 1/8 = 1$ lattice point per ...
2
votes
1answer
180 views

How to get ionic radii for coordination number 12?

I am studying $\ce{ABX3}$ perovskites, and I would like to calculate Goldschmidt tolerance factors for them. The $\ce{A}$ sites in these materials have a coordination number $12$. The Shannon's ...
2
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1answer
34 views

Why is the order of reflection simplified to n = 1?

My professor told us that for the sake of simplicity, we will assume that the order of reflection for Braggs' Law is always equal to 1 but failed to explain why. Is it because d(h,k,l) for materials ...
2
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1answer
472 views

Bravais Crystal Lattices and Voids

What is the difference between Hexagonal Close Packing (HCP) structures and the hexagonal crystal Bravais lattice? I've looked up some sites but can't seem to understand them. It would help if you ...
2
votes
1answer
49 views

Miller indices: How does one translates the orientation to the origin of the unit cell?

In a section discussing Miller indices, my textbook says the following: The rules for determining the Miller indices of a direction or an orientation in a crystal are as follows: translate the ...
2
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1answer
64 views

Crystal structure database search for space groups

I'm usually using databases like the Springer Materials site or the free Crystallography Open Database to get crystal structure data. But none of these offers the possibility to search for all entries ...
2
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0answers
48 views

Understanding lattice matching of graphene on silver (111) crystal surface and meaning of the √3 x √3 lattice?

Figure 1 in Ivor Lončarić and Vito Despoja, Phys. Rev. B 90, 075414 Benchmarking van der Waals functionals with noncontact RPA calculations on graphene-Ag(111) (available in Researchgate) shows a ...
2
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0answers
48 views

Reading Stereographs

On an old paper, I found an interesting representation of some symmetry operations: stereographs. Online I found a set of symmetry operations and related stereographs (see: http://newton.ex.ac.uk/...
2
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0answers
55 views

Rietveld (FullProf) general background function

I was reading through Eric Mittemeijer's book 'Modern Diffraction Methods' when I found the following statement in the Rietveld chapter by Robert Dinnebier. It is common practice to use orthogonal ...
2
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0answers
36 views

What system of nomenclature can I use to indicate the positions of specific kinds of atoms of a surface reconstruction?

I was reading a journal based on SiC reconstructions, and they seem to mention that every dangling bond in the $(1\times 1)$ unreconstructed Si face or the $(0001)$ direction bonds up with a Si adatom ...
2
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0answers
83 views

What is the crystal structure for a crystal where the ratio of the squares of the angle peaks is 0.825?

I followed this and it gives an example of how to identify a crystal structure using XRD data. I have an unknown alkali halide where two peaks were found at $2\theta =25.66$ and $2\theta = 28.3$. I ...
1
vote
1answer
370 views

Length of NaH bond from the edge length of unit cell

If the edge length of $\ce{NaH}$ unit cell is $\pu{488 pm}$, what is the length in $\pu{pm}$ of an $\ce{NaH}$ bond? The answer is $\pu{244 pm}$, I thought that $d = 2 \sqrt{2}\times r$ , do you not ...