6 Nonstoichiometry in Fluorite-Type Oxides
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L. E. J. ROBERTS Atomic Energy Research Establishment,
Harwell,
Berkshire,
England
The ionic defects characteristic of the fluorite lattice are interstitial anions and anion vacancies, and
the
actinide
Thermodynamic
dioxides
data
for
provide the
examples.
uranium
oxides
show wide ranges of nonstoichiometry at high temperatures and the formation of ordered compounds at low temperatures.
Analogous ordered
structures are found in the Pa-O system, but not in the Np-O or Pu-O systems. compounds
Nonstoichiometric
exist between PuO
high temperatures, but
no
2
and
PuO
1.6
intermediate
pounds exist at room temperature.
at
com-
The inter-
action of defects with each other and with metallic ions in the lattice is discussed.
I n "anomalous" solid solution, i n w h i c h lattice points are occupied b y ions of unusual charge, is closely analogous to a nonstoichiometric ionic solid, i n w h i c h the same condition must hold as the crystal departs from the " i d e a l " formula associated w i t h the structural type. M a n y studies of solid solutions of oxides, fluorides, and oxyfluorides having the fluorite ( C a F ) lattice have established that the defects characteristic of this structure are interstitial anions and anion vacancies. Some of the systems that have been studied are summarized i n Table I; i n most cases, the densities of the solid solutions have been measured a n d have agreed closely w i t h values predicted on the assumption of a complete cation sublattice, sometimes containing cations of two types. It can be seen that the fluorite structure can tolerate large concentrations of anion defects; so long as the x-ray evidence establishes that the structure remains fluorite, the cations must be statistically distributed on the f.c.c. cation sublattice. 2
Table I. Fluorite Phase GaF LaOF 2
Th0
2
Uo.5Tho.5O2
Th0 Th0 UOO
2 2
Anomalous Solid Solutions Having Fluorite Structure Lattice Const., A.
Dissolved Phase
5.468 5.756 5.586 5.524
ThF LaF ThF
5.586 5.586 5.457
Y2O3
o
Defect Type
Ref.
Laoo.55Fl.88 ~ThOi.6F .8 M0 .32
5.588 5.816 5.663 5.510
Interstitial Interstitial Interstitial Interstitial
(29) (19) (12) (1)
MOL.75 MOL. 4 MOL. 5
5.558 5.647 5.400
Vacancy Vacancy Vacancy
(18) (17) (13)
MF .48
4
2
3
4
0
2
2
La 0 2
Y O 2
Lattice Const., A.
Compn. of Limiting Solid Solution
3
3
7
7
66
Ward; Nonstoichiometric Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1963.
6. ROBERTS
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Uranium
67
Fluorite-Type Oxides
Oxides
A l l the elements of the actinide series from T h to C m form fluorite-type dioxides, and the series affords an opportunity of studying the nonstoichiometry and ordered "defect" phases characteristic of the structure. T o date, the most complete set of results refers to uranium dioxide, and these are discussed first. U r a n i u m dioxide can absorb additional oxygen at low temperatures, but the true solubility of oxygen i n the lattice is low below 300° C . T h e phase diagram shown in Figure 1 is based upon a number of x-ray studies at room temperature (3, 9, 15), one extended to 970° (14), and three studies of the equilibrium pressures of oxygen over the oxides at high temperatures. A t low temperatures, the stable phases are U 0 , the pseudocubic U 0 phase, two (or more) tetragonal phases w h i c h can be represented as slight distortions of the fluorite phase, having c/a ratios of 1.016 and 1.030, and the orthorhombic U 0 phase. The densities of U 0 and of the two tetragonal phases are higher than that of U 0 ; the additional oxygen is i n interstitial positions, as w o u l d be expected. Above 300° C , there is a genuine solubility of oxygen i n the U 0 structure, the unit cell contracting linearly as the oxygen concentration increases. T h e contraction is undoubtedly due to the fact that the U ( V ) or U ( V I ) ion is smaller than the U ( I V ) ion. L y n d s and L i b o w i t z (20) give evidence that the nonstoichiometric U 0 , phase also contains interstitial oxygen, a result expected from the behavior of solid solutions 2
4
9
a
4
8
9
2
2
2 + a
Composition, O/U
Figure
1.
Uranium-oxygen phase diagram between and U 0 3
U0
2
8
Reprinted by permission of the editor from Quarterly Reviews, 15,442 (1961) Thermodynamic
Data for Uranium
Oxides
Oxygen pressures have been measured from 950° to 1150° b y an effusion method (10), from 500° to 1100° by a high-temperature e.m.f. method (5, 22), and from 1000° to 1450° by direct tensiometric means (26). T h e agreement between the results obtained by these very different methods is, i n general, remarkably good. The tensiometric results used to construct the phase diagram from 1000° to 1200° are those shown i n Figure 3 (26), w h i c h show the existence of two nonstoichiometric ranges of composition, U 0 . and U 0 _ , and of two ranges 2 + a
4
9
î /
Ward; Nonstoichiometric Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1963.
68
ADVANCES IN CHEMISTRY SERIES
where constancy of oxygen pressure with composition proves the coexistence of two solid phases-the U 0 — U 0 _ region and the U 0 — U 0 . region below 1123°, more properly described as the U 0 — U0 region at higher temperatures. T h e resulting phase diagram [Figure 1, and Figure 4 of (26)] c a n be plotted w i t h considerable confidence; the phase boundaries deduced from our tensiometric results agree w e l l w i t h those deduced b y Blackburn from effusion ex periments w i t h the exception of the O-rich limit of the U 0 phase; w e placed this at U 0 ( U 0 ) , since the equilibrium pressures for three independent compositions analyzed as U O , U0 , and U 0 all fell on the log ρ vs. 1/T plot for the U 0 - U 0 two-phase region below 1200° C . (22, 26). I n a truly nonstoichiometric region, the activity of any component must be a continuous function of composition. This condition was shown to hold for the U0 . phase b y a plot of the partial molal free energy of oxygen ( G = RT In p) against x; values of C were calculated both from tensiometric measurements (26) a n d from e.m.f. measurements ( 5 ) , w i t h excellent agreement between the two sets of results (22). The region between U 0 and U 0 has recently been investigated at H a r w e l l a n d G shown to fall very rapidly as χ —> 0, as w o u l d be expected from the l o w pressure of oxygen i n equilibrium w i t h U 0 a n d uranium metal ( 1 0 atm. at 1396° K ) (21). T h e equilibrium oxygen pressures were at tained very rapidly above 1000° and the pressures measured were independent of the thermal history of the solid. There was no hysteresis; the same values were ob tained on heating, on cooling, and on reheating, provided the composition of the sample d i d not change, as tended to occur at the highest temperatures because of loss of U 0 gas. 2 + a r
4
9
? y
4
2 + i r
4
2 2 5
4
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2
6
9
9
2 2 5 0
4
9
2 6
9
2
2 2 5 7
2 2 6 2
6
2 + a
0 2
0i
2 0 1
2 0 0
( ) 2
2
- 3 2
3
Values of the partial molal enthalpies (H) and entropies (S) were calculated from the temperature coefficient of G creased w i t h χ i n U 0
( 0 z )
.
T h e values of (— S) consistently i n
and tended to 0 as χ —> 0; much higher values of (— S)
2 + ; r
were obtained for the U 0 _ 4
9
2 /
phase, increasing as y increased.
Values obtained
for the entropy change, AS, for the reaction UOz+x +
^-(0.25
-
0.25 y -
x)0
2
=
^
U O 4
f l
_
! /
200,
Ο JU ο Ε
Q.
ΙΟΟ
3
if)
