Structural and Thermodynamic Characterization of the Perovskite-Related Ba1+yUO3+xand (Ba, Sr)1+yUO3+xPhases

Article abstract of DOI:10.1006/jssc.1997.7392

The perovskite-type BaUO₃structure has been investigated by X-ray and neutron diffraction. The Ba/U ratio, the (Ba, Sr)/U ratio, and the oxygen stoichiometry in Ba_1+yUO_3+xwere varied, and the integral enthalpies of formation were determined by solution calorimetry. In addition, equilibrium oxygen partial pressures were measured using a reversible EMF cell. The chemical defect mechanism is discussed, and it is shown that the continuous series BaUO₃-Ba_1+yUO_3+x-Ba₃UO ₆exists in which uranium vacancies are gradually filled with barium ions whereas uranium is oxidized via the pentavalent to the hexavalent state in Ba₃UO₆(=Ba₂(Ba,U)O₆).

Full text of DOI:10.1006/jssc.1997.7392

JOURNAL OF SOLID STATE CHEMISTRY 131, 341—349 (1997)
ARTICLE NO. SC977392
Structural and Thermodynamic Characterization
of the Perovskite-Related Ba UO and (Ba, Sr) UO
Phases
1
؉y
3؉x
1؉y
3؉x
E. H. P. Cordfunke, A. S. Booij, V. Smit-Groen, and P. van Vlaanderen
Netherlands Energy Research Foundation ECN, P. O. Box 1, 1755 ZG Petten, The Netherlands
and
D. J. W. IJdo
Gorlaeus Laboratories, Leiden Institute of Chemistry, Leiden University, P. O. Box 9502, 2300 RA Leiden, The Netherlands
Received November 18, 1996; in revised form March 10, 1997; accepted March 12, 1997
paper we reported a study of the perovskite-type strontium
uranate (4). Instead of SrUO , a new phase with the formal
The perovskite-type BaUO structure has been investigated by
3
ꢁ
X-ray and neutron diffraction. The Ba/U ratio, the (Ba, Sr)/U composition Sr UO was found with a statistical distribu-
ꢀ
ꢂꢃꢄ
ratio, and the oxygen stoichiometry in Ba1؉yUO3؉x were varied, tion of Sr and Uꢄ> on one of the octahedral positions and
and the integral enthalpies of formation were determined by one Uꢄ> on the other position, leading to the formula
solution calorimetry. In addition, equilibrium oxygen partial
Sr (Sr
U
)UO . Ternary oxides ABO (A"alkaline
ꢀ
ꢀꢅꢁ ꢆꢅꢁ ꢇ ꢁ
pressures were measured using a reversible EMF cell. The chem-
ical defect mechanism is discussed, and it is shown that the
continuous series BaUO –Ba1؉yUO3؉x–Ba UO exists in which
earth metal and B"actinide metal) have been studied by
many authors (5—8), and the various aspects related to
thermal stability and defect mechanisms have been dis-
3
3
6
uranium vacancies are gradually filled with barium ions whereas
uranium is oxidized via the pentavalent to the hexavalent state in
Ba UO (؍Ba (Ba, U)O ). ( 1997 Academic Press
cussed. In the present study the BaUO structure was inves-
ꢁ
tigated in which both the Ba/U ratio and the oxygen
3
6
2
6
stoichiometry were varied. In addition, partial replacement
of barium by strontium was studied with the aim of clarifi-
cation of the difference in behavior of barium and strontium
in these perovskite-type structures.
1
. INTRODUCTION
Complex actinide oxides with one or more metal ions in
addition to the actinide ions are of interest because they can
be formed during fission in nuclear fuel. Thus, the so-called
2. EXPERIMENTAL
The starting materials for the preparation of BaUO were
ꢁ
‘‘grey phase’’ is formed as inclusions in highly irradiated BaO and UO . The latter compound was prepared by
ꢀ
UO or (U, Pu)O fuel. This phase has the perovskite-type reduction of U O in hydrogen at 700°C, and BaO was
ꢁ ꢈ
ABO structure and contains, besides the cations on the prepared by decomposition of BaCO (purified from stron-
ꢀ
ꢀ
ꢁ
ꢁ
A sites (barium and strontium), on the B sites uranium and tium by recrystallization of the starting material Ba(NO ) ,
ꢁ
ꢀ
elements such as plutonium and zirconium. Its general for- Baker p.a.) in a gold boat in high vacuum at 1000°C. SrO
mula is (Ba, Sr, Cs) (U, Pu, Zr, Mo,)O (1). was prepared by decomposition of SrCO (Baker, p.a.)
ꢁ
ꢁ
The fission products strontium and barium exhibit differ- in high vacuum at temperatures which were gradually in-
ent behavior in irradiated fuels. Whereas the oxide of stron- creased to 1050°C. The alkaline earth uranates were pre-
tium is predominantly dissolved in UO , the majority of pared in X/U ratios (X"Ba or (Ba#Sr)) that varied from
ꢀ
barium is precipitated in the grey phase (2, 3). A knowledge 1.0 to 3.0 by heating the purified oxides in an (argon#hy-
of the chemical state of these fission products in irradiated drogen) atmosphere at a temperature of 950°C followed by
fuels is of importance for an understanding of the fuel heating the samples at 1300°C. Since the BaO in the samples
performance and for their behavior under accident circum- reacts with both gold and platinum, it was necessary to heat
stances. To this purpose a series of investigations was car- them as pellets on already reacted pellets in crucibles of
ried out in our laboratories to characterize the ABO -type stabilized ZrO . After the heatings the samples were stored
ꢁ
ꢀ
perovskites, with A"Ba and Sr and B"U. In a previous in a glove box filled with dried argon.
3
41
0
022-4596/97 $25.00
Copyright ( 1997 by Academic Press
All rights of reproduction in any form reserved.

3
42
CORDFUNKE ET AL.
The chemical analyses were carried out after dissolution of the calorimetric measurements have been described
of the sample in HCl and separation of the alkaline earths elsewhere (13).
by an ion exchanger. Total U was determined titrimetrically
The electromotive force (EMF) measurements were car-
with dichromate. In the case of only Ba the content of the ried out, using a Keithley 617 electrometer, with a zirconia
alkaline earth was determined complexometrically with tube as the electrolyte, and separated electrode compart-
EDTA; in the case of two different alkaline earth metals the ments. Temperature measurements were done with calib-
analyses were done with AES. All handlings of the samples. rated Pt/Pt, Rh(10%) thermocouples.
including the weighing, were carried out in an argon-filled
CO -free dry glove box. The oxygen content of BaUO
was determined from the weight increase by ignition of the
3. RESULTS
ꢀ
ꢁ>V
Synthesis of Ba UO
sample in oxygen to BaUO ; Ba
from the analyzed Ba/U ratio, and the weight increase was
obtained by oxidizing the samples in oxygen at 1000°C to were unsuccessful. In all cases a trace of UO was found as
UO
was determined
ꢆ>W ꢁ>V
ꢂ
ꢆ>W
ꢁ>V
Attempts to prepare BaUO
with a Ba/U ratio of 1.0
ꢁ
>V
ꢀ
Ba UO #BaUO .
a secondary phase. A monophasic barium uranate was
ꢁ
ꢇ
ꢂ
Density measurements were done picnometrically with obtained at Ba/U ratios'1, and it appeared that a solubil-
double-distilled CCl . To achieve a satisfactory accuracy, ity limit for barium in excess was obtained at Ba/U ratios
ꢂ
large samples (&5 g) were used in the determinations in '2.0 under the conditions of the synthesis (low oxygen
a picnometer with a content of 25 cmꢁ. To avoid trapped pressures; see experimental). At higher ratios BaO was pres-
voids in the polycrystalline samples, the picnometer was ent as a secondary phase. As expected, the volume of the
filled in vacuum; the measurements were done in triplicate. unit cell increases with increasing Ba content until the
The X-ray diffraction patterns were made on single- solubility limit is reached. Figure 1 shows the relationship
coated film with a focusing Guinier camera (FR 552, Enraf between the volume of the pseudo-cubic ‘‘BaUO ’’ cell and
ꢁ
Nonius, Delft, The Netherlands) using CuKa radiation the Ba/U ratio. The results are given in Table 1 with the
ꢆ
(
j"1.5405981(3) A
s
) with a-SiO (hexagonal, a"4.9133(2), O/U ratio of the samples.
ꢀ
c"5.4053(4) A) as an internal standard.
s
The neutron diffraction measurements were taken on the
powder diffractometer at the HFR in Petten. Neutrons with
Synthesis of (Ba, Sr)
UO
ꢆ>W
ꢁ>V
j"2.57176(3) A
s
were obtained using the beam reflected
To study the influence of the replacement of Ba by Sr ions
from the hkl (111) planes of a single crystal of copper, on the BaUO structure, samples containing (Ba#Sr)/U
ꢁ
reducing the j/n contamination to less than 0.1% by means ratios up to 2.0 were prepared by heating mixtures of BaO,
of a pyrolitic graphite filter. Soller slits with a horizontal SrO, and UO under the same conditions as described for
ꢀ
divergence of 30ꢀ were placed between the reactor and the BaUO . It appeared to be possible to dissolve a little less
ꢁ
monochromator and in front of the four ꢁHe counters. The than 1.0 mol SrO in the BaUO structure, which resulted in
ꢁ
sample holder (diameter 1.43 cm) consisted of a V tube a slight decrease of the cell volume (Fig. 1). At the ratio
closed with Cu plugs fitted with O rings. The diffraction (Ba#Sr)/U"2.0 some SrO was present as the second
pattern was taken at 300 K and analyzed by means of phase. As discussed in a previous paper (4), it is not possible
Rietveld’s profile refinement technique (9). Absorption cor- to obtain SrUO ; the compound Sr UO
is formed in-
ꢁ
ꢀ
ꢂꢃꢄ
rections were applied according to Weber (10). For the stead. It appeared possible to dissolve BaO in this phase
coherent scattering lengths we used the values for O 5.805, (Table 2); it is to be expected that the larger Ba ions will be
Ba 5.07, Sr 7.02, and U 8.417 fm (11). For the refinement of situated on the A sites, where more space is available than
the neutron diffraction data the program DBW 9006, ver- on the B sites.
sion 8.491, was used (12). The variables include a scale
factor, five background parameters, three half-width para-
meters defining the Gaussian-like peak shape, the counter
Structure Determinations
zero, an asymmetry parameter, the unit cell dimensions,
atomic position parameters, and thermal parameters. group Pbnm, analogous to BaPuO (14). The agreement
A scale factor for the small contribution of the V sample with the observed and calculated profiles is shown in
The structure of BaUO
could be refined with space
ꢁ
ꢃꢉꢄ
ꢁ
holder was also refined. Three samples were studied by Fig. 2a. The data given in Tables 3a and 3b exhibit regular
neutron diffraction, namely BaUO
, Ba
UO
,
UO octahedra, as in BaPuO , which is, however, in dis-
ꢁ
ꢃꢉꢄ ꢆꢃꢄꢄꢁ
ꢁꢃꢈꢇꢇ
ꢇ
ꢁ
and (Ba
ꢉꢃꢊꢀ ꢉꢃꢀꢁ
The enthalpies of formation of BaUO
Sr
)UO
.
agreement with the earlier results of Barrett et al. (6). Barrett
and et al. assumed Ba vacancies to be present but from our
ꢁꢃꢂꢄ
ꢁ
>V
Ba UO
at 298.15K were obtained from their enthal- neutron diffraction data it is not possible to discriminate
ꢆ>W
ꢁ>V
pies of solution in 0.77 M HCl(#FeCl ) as measured in an between uranium and barium vacancies. The X-ray powder
isoperibol solution calorimeter at 298.15K. The details diffraction data of a second sample Ba
ꢁ
UO
ꢆꢃꢄꢄꢁ
ꢁꢃꢈꢇꢇ

CHARACTERIZATION OF Ba
UO
AND (Ba, Sr)
UO
343
ꢆ>W
ꢁ>V
ꢆ>W
ꢁ>V
FIG 1. The unit cell volume of Ba UO
ꢆ>W
and (Ba, Sr) UO
as a function of the Ba/U ratio (᭹) or the (Ba, Sr)/U ratio (᭡).
ꢁ>V
ꢆ>W
ꢁ>V
(
"Ba(Ba
ꢉꢃꢀꢉꢋ ꢉꢃꢋꢋꢋ
centered unit cell (a"8.8 A
U
)O ) indicate a nearly cubic face-
site by U and the other by (U#Ba) introduces a consider-
able disorder in the structure which cannot easily be
modelled. As a consequence, only an average structure can
be determined. A significant improvement was obtained
when the statistic disorder was represented by anisotropic
oxygen thermal parameters. It should be noted that the
transition from a pseudo-cubic to a rhombohedral structure
is only evident from the neutron diffraction work. The
ꢁ
s
), but refinement of the neutron
powder diffraction data with space group Fm3m, with
U and (U#Ba) in octahedral positions, failed. Refinement
in space group Pnma analogous to BaUO also failed, but
refinement in space group R3
and 4b). The simultaneous occupancy of one octahedral
ꢁ
ꢁ
gave better results (Tables 4a
structure of (Ba
ꢉꢃꢊꢀ ꢉꢃꢀꢁ
space group Pbnm analogous to BaUO
Sr
)UO
could be refined with
ꢁꢃꢂꢄ
TABLE 1
(Tables 5a and
Oxygen Contents and Cell Volumes of Ba1؉yUO3؉x Phases
ꢁ>V
U
5
b). It has been assumed that in the compound, which can
also be described as Ba
Sr
(Sr
)O , no va-
(
(
Ba/U ratio)
1#y)
x
Phases
(P, perovskite)
Cell volume (A
s
ꢁ)?
ꢉꢃꢈꢉ ꢉꢃꢉꢈ ꢉꢃꢆꢁ ꢉꢃꢈꢋ
ꢁ
cancies are present in the A positions and in the O lattice;
this is confirmed by density measurements (see Discussion).
The agreement with the observed and calculated profiles is
shown in Fig. 2b.
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
.033
.038
.040
.065
.086
.182
.238
.371
.400
.499
.553
.631
.766
.809
.140
.635
0.135—0.133
0.163—0.181
P
P
685.0
685.3
P#tr.UOꢀ
P
P
P
P
P
P
P
P
P
P
P
P
686.9
685.8
689.5
693.1
694.6
697.4
697.8
699.6
700.5
701.9
703.1
706.3
706.9
TABLE 2
(
Ba, Sr)UO3؉x Compositions and Cell Volumes of Equilibrium
Phases
0.417—0.397
0.595—0.613
0.864—0.867
Sample
Phase
s
Cell volume (Aꢁ)
BaSr
UO
UO
UO
UO
BaUO
BaUO
BaUO
682.1
680.9
677.9
677.4
ꢉꢃꢀꢄ
ꢁ>V
ꢁ>V
ꢁ>V
ꢁ>V
ꢁ
ꢁ
ꢁ
BaSr
BaSr
BaSr
ꢉꢃꢄꢉ
ꢉꢃꢋꢄ
ꢆꢃꢉꢉ
BaUO #SrO
ꢁ
P#BaO
Sr (Sr
U
)UO
U
Sr UO
640.7
653.7
657.9
ꢀ
ꢉꢃꢇꢋ ꢉꢃꢁꢁ
ꢇ
ꢀ
ꢂꢃꢄ
ꢂꢃꢄ
Sr
Sr
Ba (Sr
)ꢉꢃꢆ)UOꢇ Sr UO
ꢆꢃꢄꢉ ꢉꢃꢄ ꢉꢃꢄ ꢉꢃꢂ
ꢀ
?
Cell volume calculated with a cubic cell with 2a , obtained from the
Ba
(Sr
U
)UO
Sr UO #SrO
ꢉ
ꢆꢃꢁꢁ ꢉꢃꢇꢋ ꢉꢃꢇꢋ ꢉꢃꢁꢁ
ꢇ
ꢀ
ꢂꢃꢄ
Guinier measurements.

3
44
CORDFUNKE ET AL.
TABLE 3a
The Oxygen Potential of BaUO
ꢁ>V
Fractional Atomic Coordinates and Isotropic Thermal
The thermodynamic stability of BaUO
termined by measuring the oxygen potential of a reversible
Occupation electromotive cell of the type
has been de-
Parameters of Ba0.99U0.99O (Pbnm)
3
ꢁ>V
x
y
z
B (A
s
ꢀ)
Ba
U
O(1)
O(2)
0.9975(14)
0
0.0699(10)
0.7266(9)
0.0224(10)
0.5
0.4966(12)
0.2735(9)
0.25
0
0.25
1.25(10)
0.25(5)
0.92(15)
0.988(6)
0.989(5)
1.0
(!)(Pt)BaUOꢁ>V
"ZrO "O (Pt)(#)
ꢀ ꢀ
0.0389(5) 1.93(11)
1.0
in which ZrO is a calcia-stabilized ZrO tube which is
ꢀ
ꢀ
closed at one side. The EMF cell has separated electrodes,
Note. a"6.2590(7) A, b"6.2397(5) A, c"8.8123(8) A, »"344.16(6) Aꢁ,
o#!ꢀ#"8.087 g/cmꢁ, R "3.37, R "4.57, S"1.71, D—wD"0.91.
with the reference electrode O at a fixed pressure (air,
ꢀ
ꢁ
ꢂꢁ
pO "0.2 atm) and a sintered pellet of Ba
UO
in an
ꢀ
ꢆ>W
ꢁ>V
argon/CO (20%) atmosphere as the second electrode. Since
the latter electrode is monophasic, the equilibrium
The Enthalpies of Formation of Ba UO
ꢆ>W ꢁ>V
The enthalpies of formation of five different barium
uranate compositions were calculated from the enthalpy-of-
solution data listed in Table 6, according to the reaction
scheme given for one of the compositions, Ba
in Table 7. The reference data for reactions 2 through 8 have
ꢆO (g)#2e"Oꢀ\
ꢀ
ꢀ
is a function of x in Ba UO
ꢆ>W
at a constant temperature.
This can be measured either by coulometric titration or by
analysis of the equilibrium composition at a certain temper-
ꢁ>V
UO
,
ꢆꢃꢉꢁꢁ
ꢁꢃꢆꢁꢂ
been taken from recent publications by Cordfunke et al. (15,
ature. Because the oxygen potential of BaUO
is very low
ꢁ
>V
1
6). In Table 8 the enthalpies of formation of all samples are
near the stoichiometric composition, slight oxidation of the
uranate via the gas phase always occurs, resulting in a slow
drift of the EMF. For that reason we preferred to measure
listed with data for Ba U O and Ba UO (17). A recent
third-law evaluation of mass-spectrometric measurements
of the dissociation pressures of BaUO
and UO (18), yields for the enthalpy of formation
ꢀ
ꢀ ꢋ
ꢁ
ꢇ
, to give BaO(g)
the EMF of the cell as a function of x in BaUO
temperature. After each equilibrium measurement the com-
position was analyzed, as described before; the composition
at a fixed
ꢁ
ꢃꢆꢀ
ꢁ
>V
ꢀ
*
H°(298.15K)"! 1742.5$16.5 kJ mol\ꢆ in perfect
&
agreement with the calorimetric results.
Ba
UO
was taken as the starting material. When the
ꢆꢃꢉꢁꢁ
ꢁꢃꢆꢁ
As expected, there is a linear correlation between the
enthalpy of formation and the Ba/U ratio (Fig. 3), from
which it is possible to extrapolate the enthalpy of formation
equilibrium composition was reached, the cell was rapidly
cooled and the composition determined by chemical analy-
sis. It was assumed that during cooling the composition did
not change. It was checked that the results of the EMF
measurements at a certain composition were reversible and
of ‘‘BaUO ’’ with a Ba/U ratio of 1.0 and x"0:
ꢁ
*
H°(298.15K)"! (1680$10) kJ mol\ꢆ. This value is in
&
good agreement with previous determinations (5). As will be
reproducible. To that purpose a sample of Ba
UO
ꢆꢃꢉꢁꢁ
ꢁ>V
discussed later in this paper, the correlation is valid over the
whole range from BaUO , in which uranium has the formal
valency of 4#, to Ba UO , in which uranium is 6#.
was measured as a function of temperature increase and
decrease, and the composition was determined after the
measurements. The equilibrium was always attained rap-
idly. The results of the EMF measurements at three different
temperatures are collected in Table 9. We thus obtain
at 1060 K E(mV)"1595.9!1138.2 · x and at 1090 K
ꢁ
ꢁ
ꢇ
TABLE 3b
Atomic Distances (A
s
) and Angles (°) in Ba0.99U0.99O at Room E(mV)"1494.6!874.8 · x. The partial oxygen pressures
3
Temperature
for the various x values calculated from these EMF values
are listed in Table 9 and shown in Fig. 4.
Ba—O(1)
Ba—O(1)
Ba—O(1)
Ba—O(2)
2.712(11)
2.993(10)
3.312(10)
U—O(1)
—O(2)
—O(2)
O(1)—U—O(2)
O(1)—U—O(2)
O(2)—U—O(2)
U—O(1)—U
U—O(2)—U
2.246(1)
2.245(6)
2.246(6)
90.4(2)
91.1(2)
91.5(2)
4
. DISCUSSION
2.800(8) 2;
2
3
.965(8) 2;
.187(7) 2;
Attempts to prepare stoichiometric BaUO
ways been unsuccesful; such samples always contain a very
have al-
ꢁ
ꢃꢉꢉ
157.5(3)
159.5(2)
small amount of UO as a secondary phase. This has been
ꢀ
found before (5, 6, 8) and is confirmed by the results of
&
&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& && &&&&&"
FIG. 2. Observed (dots) and calculated (full line) neutron diffraction profiles of (a) BaUO and (b) (Ba Sr )UO [Ba Sr
ꢁꢃꢉꢄ
ꢉꢃꢊꢀ ꢉꢃꢀꢁ ꢉꢃꢊꢀ ꢉꢃꢉꢈ
]O ]. Tick marks below the profiles indicate the positions of the Bragg reflections. Difference (observed!calculated) curves appear at the
ꢉꢃꢆꢁ ꢉꢃꢈꢋ
bottom of the plots. Two theta in degrees.
ꢁꢃꢂꢄ
[
Sr
U
ꢁ

CHARACTERIZATION OF Ba
UO
AND (Ba, Sr)
UO
345
ꢆ>W
ꢁ>V
ꢆ>W
ꢁ>V

3
46
CORDFUNKE ET AL.
TABLE 4a
Fractional Atomic Coordinates and Isotropic Thermal
TABLE 5a
Fractional Atomic Coordinates and Isotropic Thermal
Parameters of Ba (Ba0.414U0.554)UO (R3
ꢁ
)
Parameters of Ba0.92Sr0.08(Sr0.13U0.87)O (Pbnm)
2
6
3
Atom
x
y
z
B(A
s
ꢀ)
Occupation
x
y
z
s
B(Aꢀ)
Ba(1)
U(1)
Ba(2)
U(2)
0.2456(22)
0
0.5
0.5
x
x
2.10(13)
1.13(7)
1.13
1.0
1.0
0.414
0.554
Ba/Sr
U/Sr
O(1)
0.9968(24)
0
0.0753(19)
0.7242(16)
0.0248(3)
0.5
0.4897(18)
0.2747(17)
0.25
0
0.25
0.0370(7)
1.30(10)
0.65(6)
2.50(6)
2.50(6)
0
0
0.5
0.5
0.5
0.5
1.13
O(2)
O(1) !0.2418(25)
0.1839(9) 0.2931(7) 3.2
1.0
Note. a"6.2344(11) A
s
,
b"6.2245(8) A
s
,
c"8.7949(13) A
s
,
»"
3
41.30(11) Aꢁ, o "7.780 g/cmꢁ, o "7.77 g/cmꢁ, R "3.87, R "5.00,
Note. a"6.2713(6) A
s
, a"60.165(8), R "4.24, Rꢂꢁ"5.37, S"3.41,
#!ꢀ# %ꢃꢁ ꢂꢁ
S"1.92, D—wD"0.77.
ꢁ
ꢁ
D—wD"0.26, o "7.563 g/cmꢁ, o "7.55 g/cmꢁ.
#
!ꢀ# %ꢃꢁ
the present study. The equilibrium oxygen potential of
stoichiometric BaUO
Whereas the Sr UO phase has a discrete composition,
ꢂꢃꢄ
ꢀ
(&!615 kJ mol\ꢆ; see Table 9 the situation in ‘‘BaUO ’’ is totally different. Again, stabiliz-
ꢁ
ꢃꢉꢉ
ꢁ
and Fig. 4) is not only low enough to cause some dissocia- ation of BaUO occurs by oxidation of Uꢂ> ions via the
ꢁ
tion of BaO to Ba(g), but also too low to be maintained formation of metal vacancies rather than oxygen inter-
during the preparation, and uptake of oxygen into the stitials, but now Ba is gradually placed in the U vacancies on
lattice will take place, either during the synthesis or during the B site. In the Kro
¨
ger—Vink notation,
handling in the glove box. Thus, the phase BaUO (Table
ꢁ
ꢃꢉꢈ
8
) can be described as Ba
U
O , in which 2.6% of
x
•
ꢉꢃꢊꢋꢂ ꢉꢃꢊꢋꢂ ꢁ
BaUO # O "Ba )ꢀꢀ
U" U Baꢀꢀ)ꢀꢀꢀ
O
the uranium positions are empty, and as having an equiva-
lent number of Ba and U vacancies. By addition of BaO
ꢁ
2
ꢀ
ꢆ\W
V
>WA ꢆ\ꢀV ꢀV
Baꢀꢀ )ꢀꢀꢀꢀ
ꢁ>V ꢁ>V ꢁ>V ꢁ>V
W
V
\WB ꢁ>V
ꢁ
ꢁ
3
#x
3
the vacancies are gradually filled. In Ba
ꢆꢃꢉꢁꢁ
O ) the uranium vacancies amount to
ꢉꢃꢊꢊ ꢉꢃꢊꢄꢋ ꢁ
.3%. In agreement here are the densities of BaUO
UO
;
•
"
Ba
C
)ꢀꢀ
U"
U
O
B D
ꢁꢃꢆꢁꢂ
ꢁ
\ꢁW V>ꢁW
A
ꢁ\ꢇV ꢇV
ꢁW V\ꢁW
ꢁ
(
4
"Ba
U
ꢁ>V ꢁ>V
ꢁ
ꢃꢉꢈ
or
(
Ba
ꢉꢃꢊꢋꢂ ꢉꢃꢊꢋꢂ ꢁ
O ) for which we calculate from the X-ray data the values
U
O ) and Ba
UO
("Ba
U
ꢆꢃꢄꢄꢁ
ꢁꢃꢈꢇꢇ
ꢆꢃꢀꢉꢄ ꢉꢃꢋꢋꢇ
ꢁ
3
8
.135 and 7.563 g/cmꢁ, respectively, which are to be com-
pared with the experimentally determined values (8.08$
.03) and (7.55$0.03) g/cmꢁ, respectively.
Baꢀ># O (g)#6Uꢂ>"Baꢀ>#2) #6Uꢄ>#3Oꢀ\.
2
ꢀ
3
!
0
The perovskite-type barium uranate ‘‘BaUO ’’ and its Although other defect mechanisms are possible, our neutron
ꢁ
solid solutions with strontium differ considerably from the diffraction analysis and those by Barrett et al. (6), as well as
perovskite-type strontium uranate. In the latter case, the lattice minimization calculations by Ball (20), suggest that
‘
‘SrUO ’’ phase was shown to be not stable (4), and stabiliz- the defect chemistry is dominated by anti-site disorder and
ꢁ
ation of the uranate is only possible in the presence of that the process of moving a lattice A ion into uranium
oxygen by oxidation of Uꢂ> to Uꢄ> via defect chemical vacancies is energetically favorable.
reactions to give a phase with the discrete composition
From the experiments listed in Table 1 it follows that
Sr (Sr
U
)UO (or Sr UO ), which is structurally re- BaUO is able to take BaO into solid solution up to a
ꢂꢃꢄ
lated to Sr UO ("Sr (Sr)UO ) (19), however, with only
ꢀ
ꢀꢅꢁ ꢆꢅꢁ
ꢇ
ꢀ
ꢁ
ꢁ
ꢇ
ꢀ
ꢇ
2
/3 Sr per unit cell on a B site. At the same time a slight
tilting of the UO octahedra occurs which reduces the size
of the cavity which is occupied by the alkaline-earth metal
ions.
TABLE 5b
) and Angles (°) in Ba0.92Sr0.08(Sr0.13U0.87)O3
at Room Temperature
ꢇ
Atomic Distances (A
s
Ba/Sr—O(1)
O(1)
—O(1)
2.677(19)
2.935(14)
3.367(14)
2.798(12) 2;
2.969(12) 2;
3.153(18) 2;
U—O(1)
—O(2)
—O(2)
O(1)—U—O(2)
O(1)—U—O(2)
O(2)—U—O(2)
U—O(1)—U
U—O(2)—U
2.249(3)
2.232(9)
2.243(9)
90.6(33)
90.0(33)
91.3(32)
155.7(6)
159.7(4)
TABLE 4b
s
) and Angles (°) in Ba (Ba0.414U0.554)UO
2 6
at Room Temperature
—
Atomic Distances (A
—
—
O(2)
O(2)
Ba—O(1)
Ba—O(1)
Ba—O(1)
2.806(10) 3;
3.121(28) 3;
3.190(23) 3;
U—O(1)
2.146(5) 6;
2.347(5) 6;
—O(2)
Ba(2)/U(2)—O(1)

CHARACTERIZATION OF Ba
UO
AND (Ba, Sr)
UO
347
ꢆ>W
ꢁ>V
ꢆ>W
ꢁ>V
TABLE 6
TABLE 8
The Enthalpies of Formation at 298.15 K of Barium Uranates
Molar Enthalpy of Solution of a Mass m of Ba1؉yUO3؉x
3
in 250 cm (HCl؉0.0400FeCl ؉70.68H O) at 298.15K
3
2
Compound
Molar ratio
* H°(298.15 K)
Reference
&
m(g)
e# (J)
*#
*ꢄꢅꢀHꢆ (kJ mol\ꢆ)
Ba/U ratio
Uꢄ>/U
(kJ mol\ꢆ)
a. Ba
UO
BaUO
BaUO
0.10
0.16
0.20
0.21
0.34
0.41
0.63
1.0
!1700.4$3.1
!1710.0$3.0
!1741.4
ꢆꢃꢉꢁꢁ
ꢁꢃꢆꢁꢂ
ꢁ
ꢁ
ꢃꢉꢄ
ꢃꢉꢈ
0
0
0
.10635
.10390
.10497
120.5281
118.7923
119.7415
0.5426
0.5372
0.5409
mean:
!264.441
!264.124
Ba
UO
UO
UO
UO
UO
!265.335
ꢆꢃꢉꢁꢁ
Ba
ꢁꢃꢆꢁꢂ
ꢁꢃꢆꢋꢀ
ꢁꢃꢂꢉꢋ
ꢁꢃꢇꢉꢂ
ꢁꢃꢈꢇꢇ
!1754.4
!264.63$0.73
ꢆꢃꢉꢇꢄ
Ba
!1902.0
ꢆꢃꢀꢁꢈ
Ba
b. Ba
UO
UO
UO
!2023.6
ꢆꢃꢉꢇꢄ
ꢁꢃꢆꢋꢀ
ꢆꢃꢂꢉꢉ
Ba
0.11443
0.11512
0.10784
119.4770
122.4086
120.7086
0.6156
!279.618
!2182.2
ꢆꢃꢄꢄꢁ
0.6076
0.5752
mean:
!281.061
Ba U O
!3739.8
15
17
!280.091
ꢀ ꢀ ꢋ
ꢇ
Ba UO
—
!3210.7
!280.26$0.85
ꢁ
c. Ba
ꢆꢃꢀꢁꢈ
ꢁꢃꢂꢉꢋ
0.10541
0.10572
0.10398
120.4779
119.8305
118.9405
0.5656
!299.015
0.5733
0.5627
mean:
!300.573
!297.725
maximum composition of approximately Ba/U'2.0. As
shown in Fig. 1 the volume of the unit cell gradually in-
creases with the BaO content until maximum solubility is
reached. Then BaO is present as the secondary phase, and
no indications for the oxide Ba UO , as suggested by Char-
villat et al. (7), have been found. The saturation concentra-
tion of BaO in the BaUO structure depends on the oxygen
pressure which governs the uranium vacancies. Finally, with
increasing oxygen potential, e.g., in a H O/H atmosphere,
the pentavalent uranate Ba U O will be formed, in agree-
ment with observations by Braun et al. (8).
The solution of BaO into BaUO can occur either substi-
tutionally or interstitially with charge compensation
through either vacancy or interstials, and a variety of defect
schemes can be formulated. Again, lattice minimization
calculations clearly show that the preferred method of solu-
tion involves the substitution of Ba into the barium and
uranium sites in the BaUO structure (20), with charge
compensation through anion vacancies (half an oxygen va-
!299.10$1.65
d. Ba
ꢆꢃꢂꢉꢉ
ꢁꢃꢇꢉꢂ
0.10780
0.10486
0.10364
119.4450
118.7794
120.6702
0.5985
!323.584
0.5896
0.5707
mean:
!325.884
!324.231
!324.57$1.37
ꢁ
ꢄ
e. Ba
UO
ꢆꢃꢄꢄꢁ ꢁꢃꢈꢇꢇ
0.10573
0.10356
0.10594
120.1681
119.9638
119.6386
0.5751
0.5659
0.5778
mean:
!335.412
ꢁ
!336.390
!334.838
ꢀ
ꢀ
!335.55$0.91
ꢀ
ꢀ ꢋ
f. BaCl UCl
ꢀ
ꢂ
0.05168
0.06473
0.04337
0.07101
0.06871
0.08342
0.05269
0.05261
0.08435
0.08982
0.07431
0.09512
0.06791
0.07404
0.06108
0.07607
0.06294
0.06665
119.6323
120.2788
117.3773
118.3587
119.2078
120.5713
121.0748
117.6739
117.3116
0.3911
0.3309
0.4177
0.3119
0.3339
0.2841
0.3317
0.2872
0.3146
!197.863
!203.441
!195.784
!206.482
!204.200
!213.018
!200.533
!203.957
!210.329
1.0495
1.5890
0.8317
1.9074
1.6928
2.4913
1.2635
1.5247
2.3085
ꢁ
ꢁ
cancy per AO formule unit dissolved), ultimately resulting
in a phase of composition Ba (Ba, U)O . Stabilization of
ꢀ
ꢇ
TABLE 7
Reaction Scheme for the Standard Molar Enthalpy of Formation of Ba1.033UO3.134(s) at the Temperature 298.15K
Reaction
*ꢄꢅꢀH° (kJ mol\ꢆ)
ꢆ
1
2
3
4
5
6
7
8
. Ba
UO
(s)#(2.268HCl#1.798FeCl )(sln)
"(1.033BaCl #UO Cl #1.798FeCl #1.134H O)(sln)
!264.63$0.73
ꢆꢃꢉꢁꢁ
ꢁꢃꢆꢁꢂ
ꢁ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
. 1.033BaCl (s)#0.899UCl (s)#(1.798 FeCl #1.798H O)(sln) "(1.033BaCl #0.899UO Cl #1.798FeCl #3.596HCl)(sln) !199.13$0.24
ꢀ
ꢂ
ꢁ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
. 0.101UO Cl (s)#sln
"0.101UO Cl (sln)
!10.32$0.06
!883.37$1.79
!125.59$0.13
!915.90$2.25
!966.33$0.53
!838.09$0.12
ꢀ
ꢀ
ꢀ ꢀ
. 1.033Ba(s)#1.033Cl (g)
"1.033BaCl (s)
ꢀ
ꢀ
. 0.101U(s)#0.101O (g)#0.101Cl (g)
"0.101UO Cl (s)
ꢀ
. 0.899U(s)#1.798Cl (g)
ꢀ
ꢀ
ꢀ ꢀ
ꢂ
"0.899UCl (s)
. 2.932H (g)#2.932Cl (g)#sln
"5.864HCl(sln)
ꢀ
ꢀ
. 2.932H (g)#1.466O (g)#sln
"2.932H O(sln)
ꢀ
ꢀ
ꢀ
9
. 1.033Ba(s)#U(s)#1.567O (g)
"BaꢆꢃꢉꢁꢁUOꢁꢃꢆꢁꢂ(s)
!1741.44$3.03
ꢀ
Note. *H9"! * H 1 # * H 2 # * H 3 # * H # * H 5 # * H 6 ! * H 7 # * H 8. (sln) refers to (HCl#0.040FeCl #70.68 H O).
ꢂ
ꢁ
ꢀ

3
48
CORDFUNKE ET AL.
FIG. 3. The relationship between enthalpy of formation and the Ba/U ratio in Ba UO
ꢆ>W
at 298.15K.
ꢁ>V
the BaUO -phase that occurs via oxidation of Uꢂ> to Uꢄ>, room temperature but above '405 K it transforms to
ꢁ
the creation of vacancies on the U sites in the lattice, and the a rhombohedral structure. When additional barium atoms
simultaneous placement of Ba ions in the U vacancies, are introduced into the structure, substitution of Ba for
which ultimately leads to the Ba UO
structure Bi(III) occurs to give a rhombohedral structure.
ꢁ
ꢇ
(
"Ba (Ba, U)O ), in which U is in the hexavalent state, or It is interesting to note that SrO also dissolves in the
ꢀ
ꢇ
Ba SrUO (21). The system Ba (Ba U )UO is analog- ‘‘BaUO ’’ structure, however, with a small decrease in the
ous to the system Ba (Ba Bi
x"0 (Ba Bi(III)Bi(V)O ) has a monoclinic structure at sizes in the 12 coordination [Baꢀ> 1.75 A
ꢀ
ꢇ
ꢀ
V ꢆ\V
ꢇ
ꢁ
)BiO (22). The end number cell volume (Fig. 1 and Table 2), which follows from the ion
ꢀ
V
ꢆ\V
ꢇ
s
, Srꢀ> 1.44 A
s
]
ꢀ
ꢇ
FIG. 4. Variation of oxygen potential with x in Ba
UO
ꢆꢃꢉꢁꢁ
at two different temperatures.
ꢁ>V

CHARACTERIZATION OF Ba
UO
AND (Ba, Sr)
UO
349
ꢆ>W
ꢁ>V
ꢆ>W
ꢁ>V
TABLE 9
EMF Measurements vs Composition for Ba1.033UO3؉x
whereas uranium is oxidized via the pentavalent to the
hexavalent state in Ba UO .
ꢁ
ꢇ
Temperature (K)
Composition
x in Ba UO
EMF
(mV)
log pO (atm)
ꢀ
REFERENCES
(
)
ꢆꢃꢉꢁꢁ
ꢁ>V
1060
1060
1060
1060
1060
0.154
0.204
0.228
0.265
0.335
1407.0
1368.3
1346.8
1304.7
1202.6
!27.44
!26.73
!26.32
!25.53
!23.59
1. H. Kleykamp, J. O. Paschoal, R. Pesja, and F. Thu
¨ mmler, J. Nucl.
Mater. 130, 426 (1985).
2. H. Kleykamp, J. Nucl. Mater. 131, 221 (1985).
3. H. Kleykamp, J. Nucl. Mater. 206, 82 (1993).
4. E. H. P. Cordfunke and D. J. W. IJdo, J. Solid State Chem. 109, 272
1994).
. C. W. Williams, L. R. Morss, and In-Kyu Choi, ‘‘ACS Symposium
Series,’’ No. 246, p. 323. Am. Chem. Soc., Washington, DC, 1984.
6. S. A. Barrett, A. J. Jacobson, B. C. Tofield, and B. E. F. Fender, Acta
Crystallogr. B 38, 2775 (1982).
(
1
084
0.238
1293.5
!24.76
5
1
1
1
090
090
090
0.145
0.185
0.261
1371.1
1327.6
1268.0
!26.1
!25.3
!24.1
7
8
. J. P. Charvillat, G. Baud, and J. P. Besse, Mater Res. Bull. 5, 933 (1970).
. R. Braun, S. Kemmler-Sack, H. Roller, I. Seemann, and I. Wall,
Z. Anorg. Allg. Chem. 415, 133 (1975).
9
. H. M. Rietveld, J. Appl. Crystallogr. 2, 65 (1969).
1
0. K. Weber, Acta Crystallogr. 23, 720 (1967).
compared with Uꢄ>(0.76 A
s
) (23). Again, its solubility de- 11. F. Sears, Neutron News 3, 26 (1992).
pends on the oxygen potential.
12. D. B. Wiles and R. A. Young, J. Appl. Crystallogr. 14, 149 (1981).
1
3. E. H. P. Cordfunke, W. Ouweltjes, and G. Prins, J. Chem. ¹hermo-
dynam. 7, 1137 (1975).
When the enthalpies of formation at 298.15 K of
Ba
UO
(Table 8) are plotted as a function of y, a
ꢆ>W
ꢁ>V
14. G. G. Cristoph, A. C. Larson, P. G. Eller, J. D. Purson, J. D. Zahrt,
R. A. Penneman, and G. H. Rinnehart, Acta Crystallogr. B 44, 575—580
(1988).
linear relationship is obtained (Fig. 3). As expected,
*
H°(Ba UO ) is a point on this line, being a member of the
perovskite series BaUO —Ba UO —Ba UO . In con- 15. E. H. P. Cordfunke and W. Ouweltjes, J. Chem. ¹hermodynam. 20,
&
ꢁ
ꢇ
ꢁ
ꢆ>W
ꢁ>V
ꢁ
ꢇ
2
35—238 (1988).
16. E. H. P. Cordfunke, R. J. M. Konings, and W. Ouweltjes, J. Chem.
hermodynam. 22, 991 (1990).
trast, Ba U O , in which U is pentavalent, is not a member
of this series, having a different crystal structure (24).
ꢀ
ꢀ ꢋ
¹
1
7. ‘‘Thermochemical Data for Reactor Materials and Fission Products,’’
5
. CONCLUSIONS
E. H. P. Cordfunke and R. J. M. Konings (Eds.), p. 72—73. Amsterdam,
1
990.
1
8. M. Yamawaki, J. Huang, K. Yamaguchi, M. Yasumoto, H. Sakurai,
and Y. Susuki, J. Nucl. Mater. 231, 199—203 (1996).
The perovskite-type BaUO structure has been investi-
gated by X-ray and neutron diffraction, using samples in
which the Ba/U ratio, (Ba, Sr)/U ratio, and the oxygen
ꢁ
1
9. D. J. W. IJdo, Acta Crystallogr. Sect. C 49, 650 (1993).
0. R. G. J. Ball, J. Mater. Chem. 2, 641 (1992).
2
stoichiometry in Ba UO
were varied. In addition, 21. W. A. Groen and D. J. W. IJdo, Acta Crystallogr. C 43, 1033—1036
ꢆ>W
ꢁ>V
thermodynamic properties of these samples, the integral
enthalpies of formation and the equilibrium oxygen partial
pressure, were measured. It is concluded that the continuous
(1987).
2
2. K. P. Reis, A. J. Jacobson, and J. M. Nichol, J. Solid State Comm. 107,
4
28—443 (1993).
2
2
3. R. D. Shannon, Acta Crystallogr. A 32, 751 (1976).
4. E. H. P. Cordfunke and D. J. W. IJdo, J. Phys. Chem. Solids 49,
551—554 (1988).
series BaUO —Ba
uranium vacancies are gradually filled with barium ions
UO —Ba UO exists in which the
ꢁ
ꢆ>W
ꢁ>V
ꢁ
ꢇ
.