Below 2000°C rare earth sesquioxides (RESOX)
have three crystal structures: hexagonal, cubic
and monoclinic, designated as A, C and B
respectively [1-3]. Early studies, based on low
temperature (LT) synthesis, suggested that RESOX
phase stability versus temperature is a function
of the metallic ion radii (MIR). La2
O3
Ce2
O3
and
Nd2
O3
with the highest MIR are A-type, while
for Sm2
O3
, Eu2
O3
and Gd2
O3
with intermediate
MIR the structure is C-type at LT and B-type at
high temperature (HT) [1-3]. All other RESOX
including Y2
O3
and Sc2
O3
were assumed to be cubic
(C-type) at all temperatures below 2000°C. The
transformation from LT cubic to high temperature
(HT) monoclinic structure in Sm2
O3
, Eu2
O3
and
Gd2
O3
is unusual and therefore Brauer [4] and
Yokogawa et al. [5] suggested that the stable phase
is monoclinic at all temperatures below 2000°C.
To resolve the controversy, we have demonstrated
that slowing down grain growth of Sm2
O3
and
Gd2
O3
[9] prevented transition from C to B-types
in the expected temperatures (1100 and 1300°C
respectively). Hence, we suggest that the surface
energy plays an important role in determining the
structure of nanomaterials [6,7]. The monoclinic
Sm2
O3
, Eu2
O3
and Gd2
O3
is the stable structure at all
temperatures below 2000 °C when the grain size is
large in the nanoscale. However, for smaller nanocrystals the stable structure is cubic since it has a
lower surface energy than the monoclinic phase.
In addition, Kimmel et al. [9] suggested that for all
RESOX with MIR lower than Gd3+ (except Sc2
O3
)
obtained by HT synthesis [10-17] or under high
pressure [18-20] the monoclinic phase is the stable
phase also at LT. Figure 1 shows the transition from
LT monoclinic to the HT cubic phase according to
Sato et al. [17]. Figue 2 shows the suggested RESOX
stability diagram as function of temperature versus
MIR. In sol-gel production the formation of C-type
structures is due to the formation of nano-crystals.
Subsequent firing at high temperatures yields the HT
cubic phase. Thus. the assumption of a continuous
cubic structure at all temperatures is wrong. As
seen in Figure 1, in Sc2
O3
the monoclinic to cubic
transition is below room temperature, in agreement
with the fact that HT synthesis yields cubic Sc2
O3
[17]. (The ion radius of Sc3+ is 0.087 nm [21,22])
Phase Stability of Rare-Earth Sesquioxides - New Approach
