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Author: Subject: Is Hg classified as crystalline at RTP?
zalamander
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[*] posted on 7-9-2013 at 01:15
Is Hg classified as crystalline at RTP?


Although it's a liquid at RTP I would expect that the metallic bond would still give it a structure that would be classified as crystalline. Does anyone know if this is true?
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DJF90
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[*] posted on 7-9-2013 at 03:31


Interesting question. Though I can't fault your logic, I suspect it does not have a crystalline structure. For comparison, does molten iron have a crystalline structure? Or any other molten metal for that matter. Crystallinity infers rigidity (in my mind at least), and in the liquid state, everything is in a state of fluidity.

Theres always the consideration that its a liquid crystal...
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turd
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[*] posted on 7-9-2013 at 03:56


No, just no.
Old definition of crystalline: A macroscopic periodic arrangement of atoms. So no.
New definition of crystalline: Gives a discrete diffraction pattern. So doubly no.
Quote: Originally posted by zalamander  
Although it's a liquid at RTP I would expect that the metallic bond would still give it a structure that would be classified as crystalline.

???
This must be one of the worst non-sequiturs ever. It is so deep in the "not even wrong" category that I have no idea where to start refuting that. :P

[Edited on 7-9-2013 by turd]
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[*] posted on 7-9-2013 at 04:49


Quote: Originally posted by turd  
No, just no.


... is the long and short of it. A liquid cannot be crystalline.

A good example is water: plenty hydrogen bonding to explain (e.g.) high BP (and other properties), yet crystallises only on solidification.




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ElectroWin
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[*] posted on 7-9-2013 at 05:56


if a liquid cannot be crystalline, then what is a "liquid crystal"?
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12AX7
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[*] posted on 7-9-2013 at 09:02


Perhaps he meant short-ranged order, in the sense that Hg2 molecules might be present, for example, in analogy to water being a liquid but made up of H2O molecules and loosely hydrogen-bonded sets.

However, crystals have long-range order, so this is not a suitable interpretation.

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[*] posted on 7-9-2013 at 09:20


Quote: Originally posted by ElectroWin  
if a liquid cannot be crystalline, then what is a "liquid crystal"?

Dictionary definition of liquid: "composed of molecules that move freely among themselves but do not tend to separate like those of gases; neither gaseous nor solid." http://dictionary.reference.com/browse/liquid

So obviously, no liquid can ever be crystalline. Never, ever, or else it is not liquid any more.

The term liquid crystal is an oxymoron which is used for liquids where molecules show some ordering tendencies (molecules can preferably orient to a certain spacial direction) under certain conditions (e.g., external electric field):
"a liquid having certain crystalline characteristics, especially different optical properties in different directions when exposed to an electric field." http://dictionary.reference.com/browse/liquid+crystal

Obviously, this does not mean that liquid crystals are crystalline. It is just a convenient oxymoron for practical use.




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12AX7
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[*] posted on 7-9-2013 at 09:41


I would assert that liquid crystals *are* crystalline, in the sense of having restricted axes of freedom. They are distinguished from liquids by having fewer degrees of freedom, and from true crystals, which have none (the molecules are fixed in place). (Freedom in positioning, that is; vibrational and rotational freedoms (depending on shape) are present in all.) Hardly an oxymoron; just a specific term. (Would you prefer "ordered liquid"? It has the same contradiction either way. :) )

Molecules which exhibit liquid crystal behavior share a high aspect ratio in one or two dimensions (i.e., long rods or thin plates), and (probably not necessary, but typical of practical ones?) have functional groups which weakly bond adjacent molecules (hydrogen bonds, or aromatic pi-stacking, for example). These features tend to align molecules parallel to one another, though they remain free to move among other axes. Long-range order is exhibited, in that distinct domains (over micrometers to milimeters) form where molecules are lined up in the same direction. Liquid crystals have two melting points: the lower conventional melting point, from crystal to liquid crystal, and the higher, liquid crystal to ordinary liquid, where the excess energy disrupts the long-range order.

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turd
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[*] posted on 7-9-2013 at 12:34


Quote: Originally posted by 12AX7  
I would assert that liquid crystals *are* crystalline, in the sense of having restricted axes of freedom.

You know what is sad? In the third post of this thread I gave the two definitions of crystalline that were valid for ca. the last 100 years[1]. They are really simple and easy to understand, yet you decided to make up your own definition out of thin air.

Crystallinity has nothing to do with degrees of freedom, cf glasses. As with every concept there are grey areas, but liquid crystals do not belong to them. Depending on their exact state you probably can apply more or less fruitfully the concepts of para-crystallinity or meso-crystallinity to them, but these are by definition not crystalline.

Just because something shows properties that you would expect from crystals, doesn't mean it's crystalline. Example: Put a hair under a microscope. Does it show birefringence? Yes. Is it crystalline? Hell, no.

[1] Actually, the basic idea of what crystals are date back to Haüy, 1784!
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watson.fawkes
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[*] posted on 7-9-2013 at 13:19


Quote: Originally posted by turd  
As with every concept there are grey areas, but liquid crystals do not belong to them.
Liquid crystals do show diffraction patterns, not in every case, but rather in certain conditions, particularly under a constant electric field. Here's a recent paper for which full text is available, where they're making a switchable optical element out of it. Switchable Bragg diffraction from liquid crystal in colloid-templated structures.
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[*] posted on 7-9-2013 at 13:59


Quote: Originally posted by watson.fawkes  
Liquid crystals do show diffraction patterns, not in every case, but rather in certain conditions, particularly under a constant electric field. Here's a recent paper for which full text is available, where they're making a switchable optical element out of it. Switchable Bragg diffraction from liquid crystal in colloid-templated structures.

You obviously did not even read that. This article says nothing about the diffraction of liquid crystals.
1) They are not talking about liquid crystals, but a composite material.
2) If I understand this correctly, the diffraction comes from the host!
Quote:
The liquid-crystal alignment
produces a diffraction pattern with the symmetry of the underlying templated structure

3) They are diffracting visible light. I thought it was so obvious that there is no need to state it - for crystallinity, the diffraction has to come from radiation with an atomic resolution wavelength. Otherwise you could simply pour a bunch of metal marbles in a beaker, diffract radio waves on it and declare it a crystal.

This article had a few face-palm moments, the worst was when they declared that they solved the crystal structure. Ouch.

Also note that I didn't say there isn't a grey area between crystals and liquid crystals or between crystals and glasses. There are second order phase transitions, after all. But if something is clearly liquid crystal it is clearly not crystalline.
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12AX7
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[*] posted on 7-9-2013 at 14:23


Quote: Originally posted by turd  

Crystallinity has nothing to do with degrees of freedom, cf glasses.


Oops! Good point. Long range *ordering* (i.e., the diffraction pattern) defines crystals; degrees of freedom define *solids*.

That said, I don't know offhand what XRD of liquid crystals does. I'd expect something, but it could be rather blurry. Which is the point.

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watson.fawkes
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[*] posted on 7-9-2013 at 15:14


Quote: Originally posted by turd  
You obviously did not even read that. This article says nothing about the diffraction of liquid crystals.
I read a number of things, because I half-remembered that the implicit claim that liquid crystal did not exhibit diffraction was just wrong. I was looking for something with full text available. The earliest I found was a 1965 paper on diffraction in liquid crystal from Nature, but only the abstract was available. There were a number of other textbook references on Google Books, but those URL's are references (amongst others, they're region locked, de facto). The short version is that yes indeed liquid crystal do have enough long range order to cause diffraction. The long range order isn't identical to that in ordinary solid crystals. There's more to it than that, for example the sharpness of the diffraction peaks can be used to estimate a characteristic order length.

Here's the Nature paper: X-ray Diffraction Patterns of Liquid Crystalline Solutions of Poly-γ-benzyl-L-glutamate
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[*] posted on 7-9-2013 at 15:28


Everything has a diffraction pattern, even gases do. Indeed, the first few years of X-ray diffraction were spent putting just about everything iton an X-ray beam.

I have no access to the Nature paper, but liquid crystals typically show the diffraction patterns of para-crystals which are not crystals, as the name implies (the Wikipedia article has a nice example). Careful with two dimensional diffraction pictures: If you cut a line with a two dimensional surface, it looks like a point (see Ewald construction). Yet, it is not a discrete diffraction spot!
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[*] posted on 7-9-2013 at 15:32


Crystalline implies that there is some sort of order to the arrangement of atoms. With liquid metals, that is not the case. Perhaps if there was some sort of hybrid between an amorphous crystalline substance and a liquid crystal... but I think that is not too likely.
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[*] posted on 8-9-2013 at 05:39


Quote: Originally posted by zalamander  
Although it's a liquid at RTP...
What the hell is "RTP"?

Also, <strong>turd</strong> is spot on. Consider my signature to be part of this reply.




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[*] posted on 8-9-2013 at 07:42


RTP: I suspect he meant 'room temperature' but I agree it's not very clear. Too many damn abbreviations. Maybe he meant to write 'STP' (Standard Temperature and Pressure)?


[Edited on 8-9-2013 by blogfast25]




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[*] posted on 8-9-2013 at 09:19


STP 0C and 1atm. RTP 20C and 1atm. I don't know if the latter is defined officially but it's very common in ideal gas problems. 22.4 L/mol versus 24.0 L/mol.
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