Phase transformations and thermal properties of erbium trifluoride at high temperatures

Article abstract of DOI:10.1023/A:1026654303585

The density of ErF₃ was measured with a gamma densimeter at temperatures from below the α-β phase transition to above the melting point. The density and volume expansivity of liquid ErF₃ just above the melting point were found to be 6092 ±30 kg/m³ and (15.1 ± 0.5) x 10^-5 K^-1. The relative changes in density accompanying the melting and polymorphic transformation of ErF₃ are 0.7±0.1 and 16.9± 1.3%, respectively. The results, together with earlier data, were used to reveal general trends in the variation of thermal properties across the ErF₃ to LuF₃ series.

Full text of DOI:10.1023/A:1026654303585

Inorganic Materials, Vol. 36, No. 12, 2000, pp. 1289–1292. Translated from Neorganicheskie Materialy, Vol. 36, No. 12, 2000, pp. 1523–1526.
Original Russian Text Copyright © 2000 by Khairulin, Stankus, Lyapunov.
Phase Transformations and Thermal Properties
of Erbium Trifluoride at High Temperatures
R. A. Khairulin, S. V. Stankus, and K. M. Lyapunov
Institute of Thermal Physics, Siberian Division, Russian Academy of Sciences,
pr. Akademika Lavrent’eva 1, Novosibirsk, 630090 Russia
Received April 14, 1999
Abstract—The density of ErF3 was measured with a gamma densimeter at temperatures from below the α–β
phase transition to above the melting point. The density and volume expansivity of liquid ErF3 just above the
melting point were found to be 6092 ± 30 kg/m3 and (15.1 ± 0.5) × 10–5 K–1. The relative changes in density
accompanying the melting and polymorphic transformation of ErF3 are 0.75 ± 0.1 and 16.9 ± 1.3%, respec-
tively. The results, together with earlier data, were used to reveal general trends in the variation of thermal prop-
erties across the ErF3 to LuF3 series.
INTRODUCTION
with bidistilled water, air-dried, remelted in a vacuum
furnace, and finally distilled in vacuum to remove rare-
earth oxides and oxyfluorides. The procedure and facil-
ities employed to purify the product were described in
more detail in [4, 5].
ErF3, along with other trifluorides of heavy rare
earths (TmF3, YbF3, and LuF3), undergoes, on heating,
a polymorphic transformation from the β-YF3 to the
α-YF3 structure [1, 2]. Earlier, we carried out high-tem-
perature measurements of the density, thermal expan-
sion coefficient, and volume change at the phase transi-
tion forYbF3, LuF3, andYF3 [3–5] and revealed a num-
ber of common features in their thermal properties and
some correlations with the parameters of the constitu-
ent cations [3, 4]. In particular, it was shown that the
transition to the α-YF3 structure is accompanied by a
significant decrease in density (Ӎ16.5%), whereas the
density change upon the melting of YF3, YbF3, and
LuF3 (1.6–4.2%) is substantially smaller than that for
DyF3 and HoF3 (15.5–15.8%), which have the β-YF3
structure near the melting point [6, 7]. Measurements
on YF3 and LuF3 [8] show that the anionic conductivi-
ties of their melts and high-temperature phases differ
very little and far exceed (by a factor of more than 10)
those of the low-temperature forms. All this confirms
the conclusion that the α-phases are strongly disor-
dered [2] and also suggests that the α-phases and melts
of the fluorides in question are similar in short-range
order. Detailed analysis of their thermal behavior is
however impeded by the lack of data on ErF3, which
led us to study its thermal properties.
The thermal properties of ErF3 were studied in a
gamma densimeter [9, 10] by passing a narrow beam of
gamma radiation through the sample (~200 g) enclosed
in a Mo cell. The sample temperature was measured to
an accuracy of 3 K or better with tungsten–rhenium
thermocouples enclosed in thin-wall Mo cases. The cal-
ibration of the thermocouples was checked against the
solidification points of pure Cu and Ni both before and
after each run. The first heating was performed in a vac-
uum of ~10–2 Pa. After melting of the sample, the cell
was filled with pure argon to a pressure of 0.1 MPa.
RESULTS AND DISCUSSION
Preliminary data on the thermal behavior of ErF3
around the phase transition and melting point are dis-
played in Fig. 1 and Table 1. The melt density, mea-
sured during both cooling and heating up to 250 K
above the melting point, was found to vary linearly with
temperature:
ρ(T) = (6092 ± 30)
– (0.918 ± 0.030)(T – 1414), kg/m3.
EXPERIMENTAL
Erbium trifluoride was prepared as described else-
where [4]. ErM-1 erbium was additionally purified by
double vacuum distillation and then dissolved in hydro-
chloric acid. Hydrated ErF3 was precipitated by the
addition of hydrofluoric acid (both acids were of extra-
Both melting and solidification occurred within nar-
row temperature intervals, ∆TLS ≤ 1.6 K. Solidification
began after a slight supercooling (~1 K). After the onset
pure grade). The precipitate was repeatedly washed of solidification, the sample temperature increased to
0020-1685/00/3612-1289$25.00 © 2000 MAIK “Nauka/Interperiodica”

1290
ρ, kg/m3
KHAIRULIN et al.
Because of the narrow temperature stability range of
α-ErF3 (Ӎ42 K), its volume expansivity was deter-
mined with a fairly low accuracy, βα = (3.1 ± 2.0) ×
10−5 K–1. This value is close to the βα of ErF3,YbF3, and
1
2
3
7200
6800
6400
6000
LuF3 [3–5]. The density of α-ErF3 near the α
β
phase transition was determined to be 6147 ± 30 kg/m3.
This value differs substantially from the 6422 kg/m3
calculated from the lattice parameters of α-ErF3 at
Ӎ1360 K [1]. Since our earlier results on the density of
α-YbF3 and α-LuF3 [3, 4] agreed within the experimen-
tal error with the data reported by Thoma and Brunton
[1], it seems likely that their data for ErF3 were
obtained with a substantial error.
D
C
S
The α
β phase transition of ErF3 began after a
B
supercooling of ~10 K and was accompanied by a con-
siderable increase in density. The onset of the transfor-
mation was observed at Ttr = 1372 ± 5 K (the literature
values are 1348 and 1390 K [1, 11]). The increase in
density resulted in cracking, which led to underestimat-
ing the density change at the polymorphic transforma-
tion, δρtr = (ρβ – ρα)/ρβ, where ρβ and ρα are, respec-
tively, the densities of β- and α-ErF3 near the phase
transition. To more accurately determine ρβ and ρtr,
additional measurements were made on a β-ErF3 sam-
ple sintered below Ttr. Since the sintered sample was
porous, those measurements yielded only the
ρβ(T)/ρβ(293 K) ratio, which was found to decrease by
5.4 ± 1.1% between 293 and 1372 K. To determine
ρβ(1372 K), we used the value ρβ(293 K) = 7818 kg/m3
derived from the room-temperature lattice parameters
of the β-form [12]. In this way, we obtained
ρβ(1372 K) = 7396 ± 85 kg/m3 and δρtr = 16.9 ± 1.3%.
L
A
1300
1400
1500
1600
1700
T, K
Fig. 1. Density of ErF as a function of temperature around
the phase transitions: (1, 2) data obtained in two successive
cooling cycles, (3) data for the sintered sample of β-ErF .
AL, melt; LS, solidification; SB, α-phase; BC, polymorphic
transformation; CD, β-phase. The irreproducibility of the
data for β-ErF is due to sample cracking at the α
3
3
β
3
transformation.
1414 ± 3 K, which is close to the reported melting point
of ErF3, Tm = 1413 or 1419 K [1, 11]. The melting of
ErF3 was accompanied by a minor decrease in density.
Table 1. Thermal properties of ErF3 near the melting point
The thermal properties of ErF3, YbF3 [3], and LuF3
[4] are summarized in Table 2, together with the results
for HoF3 [7] andYF3 [5] (HoF3 undergoes no first-order
phase transitions below Tm and has the β-YF3 structure;
YF3 is close in cation size to the lanthanide trifluorides
in question). It can be seen that Er to Lu trifluorides
have many features in common. In particular, the ther-
mal expansivities of their high-temperature forms coin-
cide to within the experimental error. A noteworthy fea-
ture is that the thermal expansion of the α-phases is
small compared to the phases with the β-YF3 and LaF3
structures [6, 7, 14] at high temperatures. Er to Lu trif-
luorides are also close in the relative change in density
at the β–α transformation. The other parameters—Ttr,
Tm, δρm, and βL—are monotonic functions of Rc. By
way of example, Figs. 2 and 3 show the δρm(Rc) and
βL(Rc) data. Note that the βL of HoF3 also falls on the
plot obtained for the melts of Er to Lu trifluorides. The
revealed correlations can be used to assess certain ther-
mal properties of TmF3, for which experimental data
are missing (Figs. 2, 3; Table 2).
Measurement
conditions
ρL, kg/m3 βL × 105, K–1 δρm, %
First heating
6088 ± 11 14.3 ± 1.8
–
–
–
Second heating
Third heating
First cooling
6095 ± 6
6089 ± 3
6096 ± 3
17.3 ± 2.2
15.0 ± 0.3
15.5 ± 0.5 0.69 ± 0.10
Second cooling
Third cooling
Weighted average
6088 ± 10 14.5 ± 4.5 0.84 ± 0.20
6093 ± 3 15.0 ± 0.3 0.79 ± 0.09
6092 ± 30 15.1 ± 0.5 0.75 ± 0.1
Note: ρ and β are, respectively, the density and volume expansiv-
L
L
ity of the ErF melt just above the melting point; δρ = (ρ –
3
m
S
ρ ) × 100/ρ is the percentage change in density on melting
L
S
(ρ is the density of solid ErF just below the melting point).
S
3
For each run, the random error of an individual measurement
is given; for weighted averages, we give the 95% confidence
interval.
INORGANIC MATERIALS Vol. 36 No. 12 2000

PHASE TRANSFORMATIONS AND THERMAL PROPERTIES OF ERBIUM TRIFLUORIDE
1291
βL × 105, K–1
δρm, %
HoF3
4
LuF3
16
14
12
ErF3
3
2
1
0
YbF3
TmF3
(estimate)
YbF3
LuF3
TmF3
(estimate)
ErF3
0.112
0.114
0.116
0.112
0.113
0.114 Rc, nm
Rc, nm
Fig. 2. Percentage change in density on melting as a func-
tion of ionic radius for RF (R = Er–Lu).
Fig. 3. Volume expansivity of the melt as a function of ionic
radius for RF (R = Ho–Lu).
3
3
The Y3+ ion is in many respects substantially differ- in the ErF3 to LuF3 series. These correlations would
include the data for YF3 if Y3+ were close in ionic
ent from the heavy rare-earth ions: it is considerably
lighter and has no 4f electrons. Nevertheless, as seen
from Table 2, Er to Lu and Y trifluorides are closely
similar in properties. This supports the conclusion
drawn earlier [3] that the cation size plays a key role in
determining the density and structural characteristics of
these compounds. At the same time, the δρm , Ttr, and
radius to Tm3+. Actually, the ionic radius of Y3+ is even
larger than that of Ho3+ and, as mentioned above, HoF3
undergoes no first-order phase transitions below Tm.
These observations suggest that the electronic configu-
ration of the rare-earth ion (partially filled 4f shell close
in energy to the valence electrons) has a significant
Tm of YF3 do not fit the property–Rc correlations found effect on the structure of liquid and solid fluorides.
Table 2. Thermal properties of RF3 (R = Ho–Lu, Y) [3–7]
Fluoride
Rc, nm
Ttr, K
δρtr, %
βα × 105, K–1
Tm, K
δρm, %
βL × 105, K–1
HoF3
ErF3
TmF3
YbF3
LuF3
YF3
0.1155
0.1144
0.1134
0.1125
0.1117
0.1159
–
–
–
1418
15.8 ± 0.2
0.75 ± 0.1
1.5*
16.1 ± 0.6
15.1 ± 0.5
14.5*
1372
16.9 ± 1.3
16.8*
3.1 ± 2.0
3.0*
1414
1326 [11]
1256
1431 [11]
1440
16.6 ± 1.4
16.4 ± 1.5
16.7 ± 2.0
2.9 ± 0.6
3.0 ± 0.6
2.6 ± 0.15
4.2 ± 0.15
1.6 ± 0.15
14.7 ± 0.9
12.5 ± 0.6
15.3 ± 0.9
1220
1461
1343
3.7 ± 1.4
1430
3+
–
Note: R is the eightfold-coordinated radius of the R cation in the R(F ) = 0.119 nm system [13].
c
* Estimated value.
INORGANIC MATERIALS Vol. 36
No. 12 2000

1292
KHAIRULIN et al.
Temperatures, Teplofiz. Vys. Temp., 2000, vol. 38, no. 1,
ACKNOWLEDGMENTS
pp. 154–156.
This work was supported through the National Inte-
gratsiya Program (Basic Research in the Field of Phys-
ics and High Technology, 1997–2000), project no. 274.
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No. 12 2000