Фазовая диаграмма системы Ca-O
К оглавлению: Другие диаграммы (Others phase diargams)
Ca-O (Calcium-Oxygen)
H.A. Wriedt
The established equilibrium solid phases of the Ca-O system are (1) the
terminal fcc solid solution, (aCa); (2) the terminal bcc solid solution, (bCa);
(3) the fcc oxide CaO; and (4) the bct peroxide CaO2. At high oxygen (O2)
fugacity, the oxides CaO4 (superoxide) and CaO6 (ozonide) are considered to
exist, although they have not been prepared in the pure state. These two
compounds are possibly stable phases of the system, but their stability
relative to other oxides is not established.
Because the phase equilibria at higher O concentrations are not known, the
assessed diagram, based primarily on the work of [59Bev], is restricted to the
range 0 to 50 at.% O. In this composition range at high temperatures, two
immiscible liquids (Ca-rich L1 and O-rich L2) may occur.
The solid solutions (aCa) and (bCa) saturate with respect to CaO at very small
O concentrations, which have not been determined. The boundaries of the (aCa)
and (bCa) fields corresponding to coexistence of these phases, except where
they terminate at 443 C in pure Ca, and the (bCa) solidus are also
undetermined. The nature of the (aCa) + (bCa) + CaO equilibrium reaction is
unknown.
For Ca-rich L1, in equilibrium with (bCa), the liquidus extends from the
melting point of pure bCa at 842 C to the eutectic point at 839 C and 0.3 at.
% O. For L1, in equilibrium with CaO, no experimental data between the
eutectic point and 1350 C or above 1350 C have been reported. [59Bev]
located this liquidus at about 17 at.% O at 1350 C and speculated that the
liquidus might terminate either at the congruent melting point of CaO or at a
monotectic reaction where the Ca-rich L1 is in equilibrium with CaO and O-rich
L2 containing less than 50 at.% O. This question is unresolved. There are no
data on the compositions of L2 in equilibrium with L1 or with CaO at
temperatures above this speculative monotectic point.
The CaO phase is stable at O2 fugacities intermediate between those at which
it is in equilibrium with (aCa), (bCa), or L1 (or possibly L2) and those at
which it is in equilibrium with CaO2 or liquid with more than 50 at.% O. At 0.
1 MPa hydrostatic pressure, only the fcc form of CaO has been observed.
In examination of the Ca-rich boundary of CaO, [59Bev] and [66Bev] detected no
measurable deviation from the stoichiometric composition at temperatures up to
1400 C; the O-rich boundary of this phase has not been investigated. Pure CaO
melts congruently at 2613 C.
A metastable form of CaO was reported in a layer at the interface of stable
CaO and decomposing calcite [76Sea].
When it is Ca rich, the peroxide CaO2 is in equilibrium with CaO and, when O
rich, probably with CaO4. The CaO-CaO2 equilibrium is difficult to establish
and the latter equilibrium has not been realized. The range of deviations from
the stoichiometric composition in CaO2 is unknown; the melting point is also
undetermined.
The superoxide CaO4 is probably a stable phase of the condensed system, that
is, stable relative to a mixture of CaO2 and CaO6. Pure CaO4 has never been
prepared, so that the structure, melting point, and range of compositions are
undetermined. Theoretical examination of CaO4 has suggested that it may have a
rutile-type lattice [79Sad].
The ozonide CaO6, which is unstable at ambient conditions, is the highest
known oxide of Ca. Its structure, melting point, and range of compositions are
unknown. Its equilibrium coexistence with CaO4 has not been observed.
59Bev: D.J.M. Bevan and F.D. Richardson, Proc. Aust. At. Energy Symp. on the
Peaceful Uses of Atomic Energy 1958, Melbourne Univ. Press, 586-587 (1959).
66Bev: D.J.M. Bevan, F.J. Lincoln, and F.D. Richardson, Aust. J. Chem., 19,
725-739 (1966).
76Sea: A.W. Searcy and D. Beruto, J. Phys. Chem., 80(4), 425-429 (1976).
79Sad: P. Sadhukhan and A.T. Bell, J. Solid State Chem., 29, 97-100 (1979).
Published in Bull. Alloy Phase Diagrams, 6(4), Aug 1985. Complete evaluation
contains 1 figure, 3 tables, and 77 references.
Special Points of the Ca-O System
