Chemistry Reference
In-Depth Information
Magne´ li phases, Ti
2
O
2n
1
(n
4-10) [35] between Ti
2
O
3
and TiO
2
(a natural
mineral with three polymorphs) where the liquidus topology [36] constrains
the formation of a deep metastable eutectic. Random combinations of
several Magne´ li phases co-exist depending on kinetic factors either during
quenching or subsequent solid-state heat treatment under laboratory or
natural conditions.
A deep metastable eutectic allows the formation of an amorphous
compound with an intermediate composition between the eutectics by rapid
liquid quenching and thus avoiding the formation of crystalline equilibrium
compounds [37]. The deep eutectic temperature has to be below T
g
at which
the system is configurationally frozen [37] where it will remain amorphous
albeit with detectable ordering. Deep eutectic behavior was also observed
for co-existing melts in a supercooled liquid above T
g
at ambient pressure
[38]. The eutectics defining the deep metastable eutectics refer to the melt
freezing point rather than the vapor sublimation temperature and com-
position. Another deep metastable eutectic in the ''region of carbyne'' is
defined by the metastable extension of the diamond liquidus and the triple
¼
point (
Figure 16.1
)
. This solution, being restricted to carbynes, is not really
satisfactory but perhaps this solution describes the highly kinetic behavior
of elemental carbons during carbon vapor condensation.
In another revision of the high-T/low-P region in the carbon phase dia-
gram, with the carbyne vaporus shifted from
2600-4000K (see Figure
16.1) to
4000-5000K, the ''region of carbynes'' is replaced by a low-P field
of electrically nonconduction and conducting carbon liquids with a criti-
cal point estimated around 0.2GPa at 6800K [13]. While carbynes are
de-emphasized because their existence is controversial [13], the carbon phase
diagram should constrain potential metastable phases such as carbynes and
C
60
fullerene. A field for an intermediate state of ''extreme-disordered
amorphous carbon'' (EDAC) could be a solution to carbon inter-relation-
ships. This EDAC field replaces the low-P fields of carbon liquids [13] and
the ''region of carbynes'' [27]. The formation of an EDAC field in the
carbon phase diagram would have to be constrained by low-pressure
diamond and superheated graphite ''melting''. This particular solution of
metastable carbyne formation would describe less-extreme kinetic behavior
compared to vapor condensation in natural environments.
First-order solid-state amorphization occurs due to an entropy catas-
trophe [39] causing ''melting'' of superheated graphite and decompressed
diamond below T
g
when the entropy of the ordered crystal would exceed the
entropy of the disordered liquid. This condition is resolved with the occur-
rence of a ''kinetic transition'' to a (supercooled) glass whereby the exact
kinetic conditions during carbon transformation will be critically T
g
-depen-
dent [39]. It is important to consider the crystal to liquid transition and the
effect of a superheated crystal whereof the ultimate stability is determined
by the equality of crystal and liquid entropies [40]. When this condition is
met, a solid below its T
g
will ''melt'' to an amorphous solid, particularly
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