Questions tagged [crystal-structure]

In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid.

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Dihedral angle of gaseous and crystalline HOOH

Why does hydrogen peroxide exhibit a dihedral angle of 111.5 degrees in the gaseous state? And a dihedral angle of 90.2 degrees in the crystalline phase? I know that in general, there is likely to ...
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Why can a face-centered cubic lattice not be redrawn as a body-centered tetragonal lattice?

A base-centered cubic lattice can be redrawn as a primitive tetragonal lattice, therefore we do not include it in the list of Bravais lattices. Why is it then that a face-centered cubic lattice cannot ...
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Are metallic/ionic bonds weaker than covalent bonds?

In mineralogy class, I was taught that metallic and ionic bonds are weaker than covalent bonds and that's why quartz and diamond have such a high hardness value. However, in organic chemistry class, I ...
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How do copper sulphate and the like change crystal structure on adding water of hydration?

Copper sulphate, in its hydrated form, is crystalline, whereas the anhydrous form is amorphous. Gypsum has a similar story-- on heating the crystalline dihydrate we get an amorphous hemihydrate. (...
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How is Persian blue rock salt made?

How is Persian blue rock salt made and what does it contain that makes it look blue?
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Why does pyrite form cubic crystals?

Pyrite ($\ce{FeS2}$) forms cubic crystals, like these ones: I know that the crystal structure is primitive cubic, but I don’t see how cubic structure on the molecular level translates to cubic ...
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Why is it that every snowflake is unique?

It's pretty hard to believe in around 4.5 billion years of this planet's existence, no snowflake pattern was repeated, and that's not including potential earth-like planets. Is there a proof to the ...
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Why is SiO2 a solid while CO2 is a gas?

I was under the impression that chemistry almost exclusively involves valence electrons because there isn't enough energy to strip off electrons located closer to the nucleus. If that is true, and ...
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What is the edge of a diamond like?

The chemical structure of a diamond is defined as an endless lattice in which each carbon atom is covalently bonded to four other carbon atoms situated at the four ends of a tetrahedron. But of ...
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What does β stand for in β-potassium zirconate?

What do the Greek letters (α, β, γ...) convey when they are prefixed to a crystalline substance? For example: β-potassium zirconate
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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 ...
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How do you determine a chemical formula given the molecule's crystal structure?

How do you determine the chemical formula of a compound given its crystal structure?
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Why polymorphs form?

We always hear about the different arrangements of molecules that lead to the formation of polymorphs, my question here is why would a polymorph form in the first place? The material was given enough ...
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Why does ZnO turn yellow when it is hot?

Why does ZnO's color become yellow when it is hot? I have read that it is because of physical properties, but I need a more elaborate explanation.
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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 ...
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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 ...
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Rock salt structure: chloride lattice or sodium lattice?

Source From this diagram of the rock salt structure ($\ce{NaCl}$) we see that both the chloride and sodium ions have the same environment. That is to say, they each have the same number of neighbours ...
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Number of atoms per unit cell

I don't understand the concept of unit cell and the number of atoms per unit cell in a cubic lattice also the calculations for the number of atoms. For example in the $\ce{fcc}$ lattice, number of ...
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What are the structures of potassium superoxide and xenon hexafluoride? [closed]

I have 2 questions in mind: 1) What is the structure of the superoxides? More specifically, what is the structure of $\ce{KO_2}$? 2) What is the structure of and dipole moment for $\ce{XeF_6}$?
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Counting body centre as lattice point when the basis consists of 2 atoms

An article in my book gives the following informatiom :- I understood part a) and b) but faced problems with c). It says that here the basis consists of 2 atoms which doesnt seem to be very ...
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How are the number of tetrahedral voids twice the number of octahedral voids in a CCP structure?

How are the number of tetrahedral voids twice the number of octahedral voids in a CCP structure? Is there some kind of mathematical proof for it? Is there some way I can understand this intuitively? ...
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Carbon atoms at the edge of a diamond

It is well known (the simplest textbook example) that a diamond has a well-defined arrangement of sp3 carbon atoms, as each atom is connected to four others in a ...
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Is a diamond a single molecule?

Assuming a perfect diamond with no impurities. Would this diamond considered to be a single large molecule? Browsing the interweb I found several opinions about this but did not find a clear Yes or No....
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How does NaCl maintain its crystalline structure?

My understanding is that $\mathrm{NaCl}$ is an ionic compound, in which $\mathrm{Cl}$ becomes (effectively) $\mathrm{Cl^-}$ and $\mathrm{Na}$ becomes $\mathrm{Na^+}$. So I understand why I would get a ...
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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 ...
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What is the reason for isotropy in amorphous solids?

In my book, it's been mentioned that crystalline solids are anisotropic whereas amorphous solids are isotropic in nature. The reason for these has been explained (with a diagram) as: Crystalline ...
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How sure can we be about the structure of purine?

While answering the question Basicities of nitrogen atoms in purine, I noticed, that there might be a rapid interchange of the hydrogen bonded to the nitrogen atoms. I have found a crystal structure ...
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Why is the buckminsterfullerene the purest form of carbon?

Other websites say that $\ce{C60}$ doesn't have surface bonds that are attracted by other atoms as in graphite and diamond. I understand that graphite may be attracted by other atoms because of its ...
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Confusion in unit cells of crystal system

Unit cells are divided into two main types Primitive Non-primitive Primitive includes simple cubic lattice whereas non-primitive includes fcc bcc end centered.. Among the seven types of crystal ...
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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 ...
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What would superionic water ice look like?

Phys.org's Giant lasers crystallize water with shockwaves, revealing the atomic structure of superionic ice links to Nature Physics: Experimental evidence for superionic water ice using shock ...
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R32 Space Group (Crystallography)

Why R32 Space Group is so named? I mean, if R stands for Rhombohedral and 3 for Threefold Axis, what 2 stands for? I don't see any Twofold Axis in the cell: http://img.chem.ucl.ac.uk/sgp/large/...
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Coordination number of carbon in methyllithium tetramer

According to J. D. Lee's Concise Inorganic Chemistry, the coordination number of carbon in $\ce{Li4(CH3)4}$ is $7$. But the structure of the tetramer is Doesn't carbon have coordination number $6$ ...
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How can I get xyz coordinates of atoms of a unit cell structure from CIF format files?

I'm trying to run DFT or other 1st principle simulation of Pyrope ($\ce{Mg3Al2Si3O12)}$ and Grossular $\ce{(Ca3Al2[SiO4]3)}$ unit cell structure. So, I'm looking for unit cell xyz periodic box ...
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Extracting “Crystal Radii” from a CIF file

I'm currently downloading .cif files from various online databases and using openbabel to convert the fractional coordinates that are in the .cif files into .xyz files. The typical output will look ...
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Represent bonds spanning a unit cell boundary plane

Is there a chemical input format that is- Supported by Open Babel (and hence supports visualization in avogadro??) Can represent a bond connecting an atom to its partner in a neighbouring unit cell, ...
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1answer
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How do we identify left handed and right handed crystals?

I was going through this article about how Louis Pasteur studied and explained the absence of optical activity in Racemic acid. Here's an extract: When Pasteur next examined crystals of the same ...
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Why alkali metals have bcc structure?

It is well known that metals are typically, well, metallic and usually favor the most dense packing, like hcp or fcc cells. This may be easily rationalized as dense packing of metal ions in a sea of ...
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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 ...
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1answer
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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 ...
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Why do we not see silicon in a structure like graphite?

Silicon exists in a structure similar to diamond, with 4 silicon atoms bonded to each other in a tetrahedron. Why do we not see it exist in a structure like graphite? Is it even possible to get ...
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Crystal structure vs melting point

Calcium oxide has a melting point of $\ce{2700^\circ C}$ and sodium chloride has a melting point of $\pu{801^\circ C}$. If they have the same crystal structure and ions are about the same distance ...
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1answer
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Why is a face-centred cubic unit cell not regarded as equivalent to a body-centred tetragonal unit cell? [duplicate]

When studying crystal lattices, I decided to learn which combinations of unit cells and lattice systems lead to the 14 Bravais lattices. I learnt that, for example, no centred triclinic unit cell ...
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Azathioprine CIF data file rendering problem in Accelrys

I have been working on a CIF data file based on experimental data [1] (free full text isn't available, but the page 2 with the experiment details can be found in the free preview provided by Springer)....
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Does crystalline glass exist?

As far as I know, glass is a non-crystalline amorphous solid. Does glass exist in crystalline forms? Or is it the definition of glass that it's non-crystalline, in which case my question is based off ...
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1answer
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Why some compounds form different crystal structure at different temperature?

Lets take an example :- Sodium superoxide has three different crystal structure, the marcasite structure at nearby 81 K, the pyrite structure between 196 K and 223 K and calcium carbide ...
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How much heat is required to dehydrate a hydrate?

If I have a hydrate, such as copper (II) sulfate pentahydrate, $\ce{CuSO_4 \cdot 5H_2O}$, what is the amount of heat to required to dehydrate it? I assume that it is equal to or greater than the ...
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Why is sand/glass polar? How does lattice structure change polarity?

The molecular geometry of sand is linear. Dipole moment is thus cancelled. But we all know from the meniscus water makes with glass that the dipole-dipole/H-O attraction between sand and water is ...
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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 ...
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How is the crystal direction important for etching silicon surfaces?

I am a student of masters study with a background only in electrical engineering, but I have studied Miller indices and crystallographic planes and directions, personally, to help myself understand ...