3
116
Journal of the American Ceramic Society—Navi et al.
Vol. 94, No. 9
2
1
31
substitution levels of Ca by Y in the CaTiO
could therefore lead to formation of a separated Y
with a molar volume misfit of 15%, due to the molar volume
3
host matrix
Ti O phase,
2 7
2
3
differences between CaTiO
(
3
(33.7 cm /moL) and Y
2 2 7
Ti O
3
38.7 cm /moL for YTiO3.5). However, due to the rapid decay
9
0
31
90 21
(half life of 64.1 h), relative to that of Sr (half life of
of
Y
8.8 yr), the yttrium concentration of the intermediate at secular
2
equilibrium is expected to be far o1%. Thus, no significant
9
0 31
Y
sequently the current study suggests that the CaTiO host matrix
build-up is expected to occur in the waste form and con-
3
9
0 31
could potentially accommodate the small amounts of
produced in a waste mixture. Because Zr is the final decay
Y
9
0
41
9
0
21
product of the Sr , its concentration will increase, changing
41
the overall charge balance and stoichiometry. Though Zr
could be soluble in both perovskite and pyrochlore phases, as
41
a substitute for Ti , the presence of other phases, such as
ZrTiO , due to stoichiometry changes, should be considered.
Therefore additional work on phase equilibria in the MO–TiO2–
–ZrO system is desirable.
Fig. 6. XRD diffraction patterns of the single divalent cation Ca–Y–
Ti–O xerogel samples that were heated in air to 8001C for 3 h (bottom),
4
8001C for 3 h plus 11001C for 2 h (middle), 8001C for 3 h plus 14001C for
h (top). Asterisks represent CaTiO , plus signs represent Y Ti
2
3
2
2 7
O .
Y O
2 3
2
formed from the initially homogeneous xerogel accommodates a
metastable larger amount of Ca due to slow diffusion at low
temperature. When temperature increases, the Ca component
IV. Conclusions
This study shows that in MO (M5 Ca, Sr, Ba or a combination
thereof)–TiO –Y O systems there is only very minor substitu-
tion of Y in the perovskite phase and of M in the pyrochlore
phase under oxidizing conditions. Yet, because the decay of Sr
apparently leaves the Y
phase, inducing an increase in the CaTiO
and an increase in the Y Ti O molar volume due to the increase
Ti O
2 2 7
to form more CaTiO
3
perovskite
2
2
3
3
/Y Ti
2
2 7
O mole ratios
9
0
2
2
7
9
0
of oxygen concentration that is needed for the charge balance
produces Y as a short living transient intermediate with a very
low concentration, it probably can be incorporated into the
perovskite waste form. Because Zr is the final decay product
12
13
compensation between Ca and Y . Additionally, from Fig. 7
it seen that the CaTiO molar volume does not change signifi-
cantly with temperature. This indicates that the substitution of
9
0
41
3
9
0
21
of Sr further studies should be addressed to explore the
2
1
31
14
12
Ca by Y in the perovskite CaTiO
in the initial low-temperature product from xerogel synthesis.
Potential ceramic waste forms containing CaTiO perovskite
could accommodate Sr as (Sr Ca )TiO solid solution. Over
3
is apparently small even
substitution limits of Zr with M in multicomponent MTiO3
potential waste forms.
3
x
1ꢀx
3
9
0
90
the years the Sr will decay and produce Zr through an in-
Acknowledgments
9
0
termediate short lived2 Y isotope. This sequence will change
cation charge from Sr to Y to Zr , with the surrounding
environment a source/sink for electrons. The present study in-
dicates a low substitution level of Ca by Y in the CaTiO
host matrix and therefore points to a tendency of the decay
The authors would thank Dr. Sarah Roeske (UCD) and Dr. Sean. R. Mulcahy
(UCD) for microprobe analysis. Dr. Elena Goncharov (BGU) is acknowledged for
xerogel synthesis.
1
31
41
21
31
3
9
0
31
Y
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x
x
3
12
(
¨
2
2
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7
&