Composition of saturated vapor over ytterbium bromides

Article abstract of DOI:10.1134/S0036024411050062

The vaporization process of ytterbium di- and tribromide was studied using high-temperature mass spectrometry over the temperature range of 850 to 1300 K. It was ascertained that, at the early vaporization stages, the vapor contained molecules YbBr₃, YbBr₂, YbBr, Br₂, Yb ₂Br₂, Yb₂Br₃, Yb₂Br ₄, Yb₂Br₅, Yb₂Br₆, and atoms Yb and Br. The partial pressures of all components of saturated vapor were calculated. It was found that vapor composition reflected the course of the reactions of decomposition of tribromide and disproportionation of dibromide in the condensed phase. It was concluded that vaporization of di- and tribromide was incongruent at the initial stages; vaporization of both agents acquired a congruent character with the Yb: Br = 1.0: 1.9_±0.2 ratio with time.

Full text of DOI:10.1134/S0036024411050062

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ISSN 0036ꢀ0244, Russian Journal of Physical Chemistry A, 2011, Vol. 85, No. 5, pp. 751–759. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © M.F. Butman, V.B. Motalov, D.N. Sergeev, L.S. Kudin, K.W. Krämer, 2011, published in Zhurnal Fizicheskoi Khimii, 2011, Vol. 85, No. 5, pp. 838–846.
CHEMICAL THERMODYNAMICS
AND THERMOCHEMISTRY
Composition of Saturated Vapor over Ytterbium Bromides
M. F. Butmana, V. B. Motalova, D. N. Sergeeva, L. S. Kudina, and K. W. Krämerb
a Ivanovo State University of Chemistry and Technology, pr. Engelsa 7, Ivanovo, 153000 Russia
eꢀmail: butman@isuct.ru
b Department of Chemistry, University of Bern, Bern, Switzerland
eꢀmail: karl.kraemer@iac.unibe.ch
Received April 23, 2010
Abstract—The vaporization process of ytterbium diꢀ and tribromide was studied using highꢀtemperature
mass spectrometry over the temperature range of 850 to 1300 K. It was ascertained that, at the early vaporizaꢀ
tion stages, the vapor contained molecules YbBr3, YbBr2, YbBr, Br2, Yb2Br2, Yb2Br3, Yb2Br4, Yb2Br5,
Yb2Br6, and atoms Yb and Br. The partial pressures of all components of saturated vapor were calculated. It
was found that vapor composition reflected the course of the reactions of decomposition of tribromide and
disproportionation of dibromide in the condensed phase. It was concluded that vaporization of diꢀ and triꢀ
bromide was incongruent at the initial stages; vaporization of both agents acquired a congruent character with
the Yb : Br = 1.0 : 1.9 0.2 ratio with time.
Keywords: highꢀtemperature mass spectrometry, ytterbium bromide, saturated vapor, vaporization, partial
pressure.
DOI: 10.1134/S0036024411050062
INTRODUCTION
It should be noted that reactions (1) and (2) are
mutually concurrent in a certain sense, since the lanꢀ
thanide trihalogenide released in reaction (2) at such
high temperatures must decompose via reaction (1),
particularly in the presence of a metal. Thus far, no
attention has been given to this in the literature,
although this concurrence can result in subtle chemiꢀ
cal effects associated with the highꢀtemperature
valence transformation of a lanthanide. In turn, this
circumstance considerably complicates the investigaꢀ
tion of vaporization regularities of individual comꢀ
pounds LnX2 and LnX3 with the valenceꢀinstable state
of lanthanide. Since the composition of saturated
vapor is complex and susceptible to serious changes,
this type of study can be carried out only using the difꢀ
ferential tensimetry methods, highꢀtemperature mass
spectrometry in particular [13].
Lanthanide atoms are known to most likely exist in
halogen compounds in stable trivalent state. The therꢀ
modynamics of vaporization of LnX3 has recently
been studied fairly completely [1–4]. Europium,
ytterbium, and samarium are exceptions for which
reliable thermodynamic characteristics of the vaporꢀ
ization process have virtually not been published. This
primarily accounts for the incongruent character of
evaporation [5, 6] and the valence transformation
Ln(III)
→
Ln(II) in these compounds at high temperꢀ
atures, which is in accord with the general tendency
toward decreasing stability of the trivalent state in the
lanthanide series [7, 8]: La, Lu, Gd, Ce, Tb, Pr, Er,
Nd, Ho, Pm, Dy, Tm, Sm, Yb, and Eu. Their thermal
decomposition occurs due to the decreased stability of
the state of Ln(III) in trihalogenide compounds [1, 9]:
In this work, the mass spectrometric investigation
of regularities in vaporization of ytterbium triꢀ and
dibromide was performed in order to determine the
qualitative and quantitative composition of saturated
vapor.
2LnX3(s)
→
2LnX2(cr) + X2(g).
(1)
On the other hand, it was noted in [7, 10] that lanꢀ
thanide dihalogenides disproportionate at high temꢀ
perature via the reaction
3LnX2(s)
→
Ln.(cr) + 2LnX3(cr).
(2)
EXPERIMENTAL
Unfortunately, no detailed information on the conꢀ
ditions of reaction (2) has been published, with the
The experiment was conducted on an MI 1201
exception of data for LnCl2 compounds, which disꢀ serial magnetic mass spectrometer (
proportionate under vacuum at 1273 K [11, 12]. radius of 200 mm) reequipped for highꢀtemperature
∠
90
°
, curvature
T
≥
The type of reaction (2) was determined mainly by effusion measurements from a Knudsen cell (materiꢀ
analyzing the composition of the condensed phase, als: graphite, molybdenum). The molecular and ion
whereas the composition of the gas phase during this beams were located coaxially. The instrument was
reaction has not been investigated.
described in more detail in [14]. Mass spectra were
751

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752
BUTMAN et al.
(a)
Transition
0
τ
Stage
I
Stage II
1.6
1.2
0.8
0.4
1.6
1.39
1.2
0.8
0.4
0.91
0.42
1
2
3
4
~
~
~
~
0.18
0.18
0.2
0.1
0
0.2
0.1
0
0.02
800
900
1000
1100
1200
(b)
T
, K
1100
0.02
1050
1000
0.02
1000 K
1273 K
1100 K
0.2
0.01
0
0.01
0
0.1
0
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
Fig. 1. Temperature dependences of the mass spectra and their change over time upon the vaporization of YbBr ; (a) ion i: (1)
3
+
+
+
+
+
+
+
+
+
Yb ; (
2
) YbBr ; (
3)
YbBr3 ; (
4)
Br2 ; (b) ion i: (
1)
Yb Br ; (
2)
Yb2Br2 ; (
3)
Yb2Br3 ; (
4)
Yb2Br4 ; (
5)
Yb2Br5
; τ is the duration
2
of the experiment.
The YbBr3 sample was synthesized from Yb2O3
(Fluka, 99.9%) using the NH4Br procedure [15, 16],
which includes the following stages: dissolution of
ytterbium oxide in concentrated (47%) HBr solution;
introduction of ammonium bromide in Yb : NH4Br
ratio of 1 : 3.5, followed by vaporization of the soluꢀ
recorded at an energy of ionizing electrons
E
e = 30 eV,
e 1 mA.
and an emission current from the cathode of
I
A secondary electron multiplier combined with a Keiꢀ
thley digital picoampermeter allowed us to attain a
sensitivity of the registration system of 10–17 A in the
direct current measurement mode. The temperature
of the effusion cell was measured using a standard
tungsten–rhenium thermocouple calibrated for the
melting points of pure CsI and Ag. The sensitivity conꢀ
stant of the instrument was calibrated according to the
saturated vapor pressure over metallic silver. A softꢀ
ware module designed in our laboratory allowed autoꢀ
matic registering of the ion current, cell temperature,
and energy of the ionizing electrons in the course of
measuring.
tion; grinding of the residue and its heating to 150
in argon flow and to 450 under vacuum. The brutto
reactions of synthesis are described by equations:
°С
°С
Yb2O3 + 6HBr + 6NH4Br
→
2(NH4)3YbBr6 + 3H2O,
(3)
(NH4)3YbBr6
→
YbBr3 + 3NH4Br.
(4)
For further purification, the dry YbBr3 powder was
sublimed in an airtight quartz reactor at 950°C under
vacuum.
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COMPOSITION OF SATURATED VAPOR OVER YTTERBIUM BROMIDES
753
I, rel. units
Stage
I
Stage II
30
25
20
15
10
5
30
25
20
15
10
5
1
2
3
4
0
0
10
16
22
28
10
16
22
28
, eV
E
+
+
+
+
Fig. 2. Ionization efficiency curves at I and II stages of vaporization of YbBr for ions (
1)
Yb ; (
2)
YbBr ; (
3
)
YbBr2 ; (
4)
YbBr3
.
3
The YbBr2 sample was obtained by reducing YbBr3
As can be seen in Fig. 1, the mass spectra of vapor
with metallic Yb (Metall Rare Earth Ltd., 99.99%) in differ considerably at different stages, this distinction
a tantalum container sealed by arc welding in a helium comprising not only the qualitative change in the
+
atmosphere and enclosed in a quartz ampoule. The
ratios of ion currents but the complete vanishing of
Br2
metal was used in excess YbBr3 : Yb ratio of 2 : 1.05.
The temperature was raised to 980 and kept there
for 24 h, then lowered to 800 C and kept for 24 h folꢀ
and Br+ ions from the mass spectrum at stage II as well.
We note that at both stages, the mass spectra differ
from those of lanthanide tribromides [17–21]. The
phenomena that stand out in particular are a very high
fraction of YbBr+ ion and a wide variety of ions conꢀ
°C
°
lowed by slow cooling to room temperature over 72 h.
The resultant pale yellowishꢀwhite powder was identiꢀ
fied via Xꢀray diffraction as phase pure YbBr2
.
taining two ytterbium atoms. The presence of Br+ and
+
ions at stage I indicates the release of atomic and
Br2
2
RESULTS AND DISCUSSION
molecular bromine due to the thermal decomposiꢀ
tion of the sample via reaction (1).
Mass Spectra of YbBr3 Vaporization Products
The following ions were registered in the mass
Analysis of Ionization Efficiency Curves
spectrum upon the vaporization of YbBr3 over the
temperature range of 850 to 1300 K: Yb+, YbBr+,
In order to determine the molecular precursors of
ions at each vaporization stage, we recorded the ionꢀ
ization efficiency curves (IECs) (Fig. 2), which are the
dependences of mass spectra on energy of ionizing
electrons. The energy scale in Fig. 2 was calibrated with
+
1
,
+
+
+
+
+
,
,
, Br+
Yb Br+,
,
,
YbBr2 YbBr3 Br2
Yb2Br2 Yb2Br3
2
+
+
, and
.
Yb2Br4
Yb2Br5
In the course of measurements, two main stages
were observed; these are denoted by Roman numerals I
respect to the
ion, since it forms only as a result
YbBr3
and II in Fig. 1. Note that these and all subsequent of the ionization of YbBr3 molecules (by analogy with
data were obtained using a graphite cell, stage I being
more prolonged than when a molybdenum cell is used,
which ensures greater reliability in measuring the
ratios between the ion currents in a mass spectrum.
other lanthanide tribromides [17–21]) and correꢀ
sponds to a soꢀcalled “pure line” in the mass spectrum.
+
The ion appearance energy AE
(
, YbBr3) = 9.5
YbBr3
0.5 eV, which is the same as the ionization energy I0 of
1
+
The measurements of ion current of Br were complicated by
2
considerable background noise from the instrument at m/e =
79, 81 and were performed only at several individual temperaꢀ
ture values (not shown in Fig. 1). The atomic bromine pressure
was, however, taken into account in calculating the relative conꢀ
tent of Br/Yb atomic feractions in the vapor (see below).
According to the assumption in [22] on possibility of ignoring
the dissociative ionization of Br , the ratio between ion currents
2
+
+
of Br and Br2 corresponds to the equilibrium constant of reacꢀ
tion Br = 2Br.
2
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754
BUTMAN et al.
Fragmentation coefficients of YbBr, YbBr2, and YbBr3
molecules
The observations described above and our interpreꢀ
tation of them suggest that the vapor composition at
which up to four ytterbiumꢀcontaining gas compoꢀ
nents (YbBr3, YbBr2, YbBr, and Yb) can simultaꢀ
neously coexist is complex. The next stage of processꢀ
ing the primary data therefore involved separating the
contributions to ion currents from different molecular
precursors.
Coefficient
(Yb+,YbBr)/ (YbBr+,YbBr)
Value
f01
f02
f03
f12
f13
f23
=
=
=
=
=
=
I
I
I
I
I
I
I
0.05 0.02
0.19 0.03
0.11 0.03
0.90 0.11
0.35 0.05
+
(Yb+,YbBr2)/
(Yb+,YbBr3)/
I
(
,YbBr2)
YbBr2
+
I(
,YbBr3)
YbBr3
Attribution of Ion Currents to Molecular Precursors
+
(YbBr+,YbBr2)/
(YbBr+,YbBr3)/
I
(
(
I
,YbBr2)
,YbBr3)
YbBr2
Let us introduce the concept of fragmentation
coefficient
+
I
YbBr3
+
(YbBr2+,YbBr3)/
(
,YbBr3)
1.24 0.14
fij
=
Iij/Ijj,
(5)
YbBr3
which determines the ratio between fragmentary
+
+
Note: I(Yb , YbBr) is the intensity of the current of Yb ions formed
+
+
from YbBr molecule. The same is true for the other cases.
and molecular
ion currents formed
). Let us express ion curꢀ
YbBri
from the YbBrj molecule (
rents I03 I13
I23 of fragmentary ions Yb+, YbBr+, and
YbBrj
i < j
,
,
the YbBr3 molecule, was determined by the extrapoꢀ
lated difference method [23] using I0(Br2) = 10.53
+
, the products of ionization of YbBr3 molecule,
in terms of ion current I33 of the pure line of YbBr3
and the corresponding fragmentation coefficient:
YbBr2
+
0.01 eV [12] as a standard. It should be noted that the
AE values for the other ions were not determined preꢀ
cisely in this work, such determination being compliꢀ
cated under the superposing of spectra of individual
molecules due to the distortion of IEC shapes. In
interpreting the mass spectra, we therefore relied on
our analysis of shapes and relative shifts of the IECs
obtained in one experiment.
Ii3
=
fi3I33
,
(i
= 0, 1, 2).
(6)
Likewise, for YbBr2 and YbBr molecules we obtain the
expressions
Ii2
=
fi2I22
,
(i
= 0, 1);
(7)
(8)
Ii1
=
fi1I11
, (i = 0);
As can be seen in Fig. 2, the curve of YbBr+ ion is
shifted leftward at stage I and yields an AE value of
approximately 9 eV. Note that it is the lowest value,
compared to the other ions. Moreover, it is considerꢀ
ably lower than the values of ~15–16 and ~10–11 eV,
predicted for AE(YbBr+, YbBr3) and AE(YbBr+,
YbBr2). This demonstrates that at stage I, not only
YbBr3 and YbBr2 but also YbBr molecules contribute
respectively. The task of attribution of ion currents to
molecular precursors is thus reduced to determining
the coefficients f03, f13, f23, f02, f12, and f01. With this in
mind, we considered the balance equations of ion curꢀ
rents measured upon vaporization of YbBr3 at stage I,
where the content of Yb atoms in vapor could be
ignored (see above):
to the ion current of YbBr+
.
I01
I11
=
=
I0
I1
–
–
f03 I3
f13I3
–
f02
(
I2
–
f23I3),
(9)
In moving to stage II, the IEC of YbBr+ shifts rightꢀ
ward and curves in more to mask its break, while the
lower region of the curve can still be extrapolated into
the low energy range (<9 eV). Such behavior correꢀ
sponds to a relatively lower contribution to the total
current of YbBr+ from ytterbium monobromide at
stage II. The AE(Yb+) value on the order of 15–16 eV
(Fig. 2, stage I), along with the IEC shape for Yb+ ion,
demonstrates that it prefers to form at stage I from
YbBr3 and YbBr2 molecules. At stage II, the break of
–
f12(I2
–
f23I3),
(10)
where Ii is the measured ion current.
Substituting (9) and (10) into (8), we express
I0 − f03I3 − f02(I2 − f23I3)
.
(11)
f01
=
I1 − f13I3 − f12(I2 − f23I3)
Since f01 is independent of gas phase composition and
temperature (as we assumed), expression (11) is valid
for each of the experimental points at stage I. It can
therefore be written for two points as
the IEC of Yb+ becomes more pronounced, and a long
region extending to the low energy range emerges in
the curve, attesting to the emergence of a contribution
'
'
'
'
I0 − f03I3 − f02(I2 − f23I3)
'
'
'
'
I1 − f13I3 − f12(I2 − f23I3)
to current of Yb+ from Yb atoms. In the case of the
(12)
+
'' '' '' ''
I0 − f03I3 − f02(I2 − f23I3 )
ion, the changes in IEC at stages I and II
(Fig. 2) are pronounced to a lesser extent and are
manifested as a small shift of the curve (~0.5 eV) leftꢀ
YbBr2
=
,
''
''
''
''
I1 − f13I3 − f12(I2 − f23I3 )
'
''
ward, indicating a relative increase in the contribution where and
Ii
are the ion currents for the first and
Ii
second random points. Using the experimental data
to this ion from dibromide molecules at stage II.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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No. 5 2011