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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3answers
7k views

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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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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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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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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Why are there only 7 types of unit cells and 14 types of Bravais lattices?

Why are there only 7 types of unit cells and 14 types of Bravais lattices? I was reading about solid-state chemistry for the first time and this limitation made no sense to me. I tried to do the ...
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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 many Ga atoms are connected to one As atom in solid gallium arsenide?

In Boylestad and Nashelsky's Electronic Devices and Circuit Theory, the structure of GaAs is shown as follows: Arsenic is shown making five covalent bonds to neighbouring gallium atoms. However, I ...
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Why is sand, sand; and why is glass, glass; While both mainly contain Silicon Dioxide?

I know that there is the famous process of heating material and converting them to glass; but what I don't know is, what is the chemical process of the creation of glass? Is it crystallization? How ...
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Choice of doping elements in semiconductors

In a typical semiconducting material, we start with silicon (group IV) and introduce atoms of group V or group III depending on whether we are constructing an n- or p-type semiconductor respectively. ...
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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 determines how much water of crystallisation a salt has?

Is there any reason behind the number of water of crystallisation? For example, $\ce{LiCl.2H2O}$ has two waters of crystallisation, and $\ce{MgCl2.8H2O}$ has 8 waters of crystallisation. Why does $\...
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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 ...
15
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1answer
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What are these crystals on my champagne cork?

I opened up a bottle of champagne that was left in a wine cooler for approx. 8 years. To my astonishment there were small crystals at the underside of the cork. The crystals are colorless, odorless ...
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Stoichiometric defects in silver bromide

Stoichiometric defects (Schottky and Frenkel defects) in ionic crystals are formed due to irregularities in the ionic arrangement which maintain the stoichiometry and electroneutrality of the solid. ...
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When we heat a borax crystal, why does it first expand and then contract?

In the borax bead test, we take a crystal of borax and then heat it. A small loop is made in the end of a platinum or nichrome wire (as used in the flame test) and heated in a Bunsen flame until ...
14
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1answer
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Is there a theory behind selecting elements that may be successful in potential superconductors?

Looking at something like $\ce{YBa2Cu3O7}$ which was one of the first cuprate superconductors to be discovered, I'm always curious how the selection of these substances as likely superconductors comes ...
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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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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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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 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/...
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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 BCl₅ possible?

Is $\ce{BCl5}$ possible? I saw it in book. As far as we can see there is no way to get 5 chlorine atoms with boron.
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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
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How is the electron “trapped in the crystal” in irradiated NaCl?

Pennsylvania State University provides irradiated $\ce{NaCl}$ (180,000 rads of gamma radiation) to teachers for a very entertaining demonstration. The salt is orange after irradiation. When it is ...
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If salt crystals make a cube shape, what is this 'X' marking?

I am an amateur, with very little professional chemistry knowledge. I was doing a demonstration / experiment with my 6 year old child. We took very warm normal tap water, and dissolved into it a ...
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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 ...
11
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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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Why are X-rays used in crystallography?

Can any other wavelength of electromagnetic radiation be used? Why can't gamma radiation be used?
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How do you calculate a unit cell's dimensions from an XRD spectra?

My inorganic lab had us do an XRD measurement, but I've never been explained how to interpret the data. Question: Calculate the unit cell dimensions $a$, $b$, and $c$, for $\ce{YBa2Cu3O7}$ from ...
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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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How does VB theory explain the bond angles in SiO2?

Silicon dioxide has a huge variety of structures. Most of them have a tetrahedral $\ce{SiO2}$ unit cell — the $\ce{O–Si–O}$ angle is $109.5°$, accordingly. The VB/hybridization approach to this ...
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Data repository of organic compounds

Does there exist any data repository of chemical compound that gives the structure of the compounds with the help of coordinate geometry. It would be like each atom has a coordinate and the list of ...
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1answer
713 views

How thick is a snowflake?

The x, y dimensions of snowflakes get all the attention for obvious reasons, but the z dimension must be interesting too, in its own humble way. In fact, I have lived more than half a century and ...
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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 ...
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Why does Naphthalene turn into needle-shaped crystals by sublimation?

I wonder why crystals of Naphthalene are needle-shaped after sublimation. Googling the fact, I could only confirm that it is true but failed to find the reason. Could someone please shed some light as ...
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Hydrogen bonding in solid bicarbonates

Why is there two different kinds of hydrogen bonding in solid bicarbonates $\ce{NaHCO3}$ and $\ce{KHCO3}$? In both cases we consider anions of each for hydrogen bonding . The anions involved in ...
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1answer
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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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Finding large databases of sphere packings

Part of my research is investigating sphere packings and their properties.I really would like a large database of sphere packings that I could investigate using a program I have written that tells me ...
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Image of crystal structure

Is there any websites or resources where I can get a good image of a certain crystal structure? In my case I'm looking for lead(II) tungstate $\ce{PbWO4}$ and I can't find a vivid graphic on google (...
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How to calculate the height of an hcp lattice?

A hexagonal closed packing (hcp) unit cell has an ABAB type of packing. For calculating the packing fraction we require the volume of the unit cell. Volume of hcp lattice = (Base area) $\cdot$ (...
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What is this blue crystal?

I was sorting out some old boxes in one of the rooms in my house, when I found a small container with about 10 grams of this blue crystalline powder. I assume that it contains some form of copper due ...
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1answer
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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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1answer
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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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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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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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How to predict the type of intrinsic defect? [closed]

Intrinsic defects occur when a pure material crystal shows either vacancy or interstitialcy. When an ionic compound has vacancy, the imbalance in charge has to be compensated with another defect. When ...
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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 ...
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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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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. (...