Melting point of graphite and diamond?

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Discussion Overview

The discussion revolves around the melting points of graphite and diamond, exploring the discrepancies in reported values and the nature of these materials under varying conditions. Participants examine the concept of melting versus sublimation and the challenges in defining melting points for carbon allotropes, particularly under normal pressure conditions.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant notes that different sources provide varying melting points for graphite, with values ranging significantly, leading to confusion.
  • Another participant highlights that diamond's melting point is also inconsistently reported, with estimates varying from 3,000°C to nearly 4,000°C.
  • It is mentioned that diamond burns in air at around 400°C, complicating the determination of its melting point due to potential chemical reactions before melting occurs.
  • Some participants discuss the phases of carbon, including graphite, graphene, diamond, and liquid carbon, suggesting that at atmospheric pressure, direct phase transitions may not occur.
  • A question is raised about the freezing point of steam, drawing a parallel to the complexities of defining phase transitions in different contexts.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the melting points of both graphite and diamond, with no consensus reached on definitive values. Multiple competing views and interpretations of the data remain present throughout the discussion.

Contextual Notes

The discussion highlights limitations in the available data, including the dependence on pressure conditions and the lack of universally accepted definitions for melting points in the context of carbon allotropes.

Warp
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TL;DR
It's strangely difficult to find these temperatures online. Different sources give wildly different values. What are the actual temperatures?
I was conversing with ChatGPT when I asked which material has the highest melting point, and it answered "tungsten" (giving its melting temperature). It so happened that in the previous question the melting point of graphite had come up, and it was listed as higher than that. When I asked which one of the two has the higher melting point, it insisted that it's tungsten, even though in that very response it listed a higher temperature for graphite. ("Graphite has a melting point of approximately 3,550°C (6,422°F), while tungsten has a melting point of approximately 3,422°C (6,192°F). Tungsten has the higher melting point between the two materials.")

In fact, other similar AIs also give wildly different answers (and most of them incorrect by the numbers).

This seemed to be an AI hallucination, so I googled what the melting point of graphite is, and to my surprise I found wildly different answers from different sources, varying by, like, a thousand degrees.

(Yes, I know that graphite, and diamond, do not melt per se, they sublimate. However, by "melting point" I mean "the temperature at which it stops being a solid.)

Turns out that also the melting point of diamond seems equally vague. Googling for it gives wildly different results:

"Diamond has a very high melting point (almost 4000°C)"
"Thus, we infer that diamond melts at about 9,000 K and between 0.60 and 1.05 TPa along the Hugoniot."
"In case of Diamond, the melting point is 3550 degrees Celsius"
"However, the melting point of diamond is estimated to be around 3,000-3,500°C (5,432-6,332°F)."
"Using carbon's theoretical phase diagram below (from Wikimedia), "liquid diamond" could be achieved at about 10 GPa (99 thousand atmospheres) and 5000 K (4700 °C)."
"Diamond has the highest melting point (3820 degrees Kelvin)"
"The ultimate melting point of the diamond is around 4,027° Celsius (7,280° Fahrenheit)"

No wonder the AIs are confused about it. So am I.

In comparison, the melting point of tungsten seems to be quite universally agreed upon (and likewise all AIs give a very consistent value for it).

What is the melting (well, sublimation) point of graphite and diamond? (I suppose I should add, under normal pressure conditions.)
 
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Warp said:
Turns out that also the melting point of diamond seems equally vague.
A diamond will burn in the air at about 400°C. That makes melting diamond difficult, as it will often react chemically before it melts.
 
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Graphite, grapheme. diamond and liquid carbon are all phases. At atmospheric pressure, there may not be a direct phase transition from one phase to another. What is the freezing point of steam?
 
Vanadium 50 said:
Graphite, grapheme. diamond and liquid carbon are all phases. At atmospheric pressure, there may not be a direct phase transition from one phase to another. Where is the freezing point of steam?
It doesn't freeze here, but it does in Oymyakon. So is it somewhere near Omsk?
 

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