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Library of Congress Cataloging-in-Publication Data

Molten salts chemistry and technology / edited by Marcelle Gaune-Escard and Geir Martin Haarberg.

pages cm

Includes index.

ISBN 978-1-118-44873-1 (cloth)

1. Fused salts. I. Gaune-Escard, Marcelle. II. Haarberg, Geir Martin.

TP230.M655 2014

546′.34 – dc23

2013035011

A catalogue record for this book is available from the British Library.

ISBN: 9781118448731

Typeset in 10/12 Times by Laserwords Private Limited, Chennai, India.

1 2014

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3.7

Molecular and Ionic Species in Vapor overMolten Ytterbium Bromides

M. F. Butman,1 D. N. Sergeev,1 V. B. Motalov,1 L. S. Kudin,1 L. Rycerz2 and M. Gaune-Escard3

1Ivanovo State University of Chemistry and Technology, Russia2Chemical Metallurgy Group, Faculty of Chemistry, Wroclaw University of Technology, Poland3Aix-Marseille Université, CNRS IUSTI UMR 7343, Technopole de Château-Gombert, France

3.7.1 Introduction

Lanthanide atoms are known to most likely exist in halogen compounds in a stable trivalent state. The ther-

modynamics of vaporization of LnX3 was recently studied fairly completely [1–4]. Europium, ytterbium,

and samarium are exceptions for which reliable thermodynamic characteristics of the vaporization 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 temperatures, 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]:

2LnX3(s) → 2LnX2(s) + X2(g). (3.7.1)

On the other hand, it was noted in [7] that lanthanide dihalogenides disproportionate at high temperature

via the reaction:

3LnX2(s) → Ln(s) + 2LnX3(s). (3.7.2)

Unfortunately, no detailed information on the conditions of the reaction in Equation 3.7.2 has been pub-

lished, with the exception of data for LnCl2 compounds, which disproportionate under vacuum at T≥ 1273 K

[9, 10]. The type of reaction in Equation 3.7.2 was determined mainly by analyzing the composition of the

condensed phase, whereas the composition of the gas phase during this reaction was not investigated.

Molten Salts Chemistry and Technology, First Edition. Edited by Marcelle Gaune-Escard and Geir Martin Haarberg.© 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.

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204 Molten Salts Chemistry and Technology

It should be noted that the reactions in Equation 3.7.1 and 3.7.2 are mutually concurrent in a certain sense,

since the lanthanide trihalogenide released in Equation 3.7.2 at such high temperatures must decompose via

Equation 3.7.1, particularly in the presence of a metal. Thus far, no attention has been given to this in the lit-

erature, although this concurrence can result in subtle chemical effects associated with the high-temperature

valence transformation of a lanthanide. In turn, this circumstance considerably complicates the investigation

of vaporization regularities of individual compounds LnX2 and LnX3 with the valence-instable state of lan-

thanide. Since the composition of saturated vapor is complex and susceptible to serious changes, this type of

study can be carried out only using differential tensimetry methods, and high-temperature mass spectrometry

in particular [11].

In this work, the mass spectrometric investigation of regularities during the vaporization of ytterbium tri-

and dibromide was performed in order to determine the qualitative and quantitative composition of saturated

vapor.

3.7.2 Experimental

A MI1201 magnetic sector-type (angle of 90∘, curvature radius of 200 mm) mass spectrometer modiied for

high-temperature studies was used. A special ion source allowed us to perform measurements under electron

ionization (EI) and thermal ion emission (TE) regimes and study both neutral and charged vapor components.

In EI regime, the molecular composition of equilibrium vapor over condensed phase was analyzed. The mass

spectra of a molecular beam from the Knudsen effusion cell were recorded at an ionizing electron energy

of 50 eV, and an emission current from the cathode of 1 mA. A movable molecular beam shutter, interposed

between the effusion cell and the ionization chamber, made it possible to distinguish the species effusing

from the cell from those of the background. In TE regime, the charged species were identiied. In this case the

ions generated inside the effusion cell at high temperature were drawn out from it by a weak (104 –105 V/m)

electric ield. The voltage applied to the cell was negative with respect to the ground to detect the emission of

negative ions. The system for registration of ion currents included a secondary electron multiplier combined

with a Keithley picoamperemeter. The sensitivity of the registration system on direct current was 10−17 A. The

sample under investigation was loaded into graphite and molybdenum cells. The ratio of the cross-sectional

area of the cell to the area of the effusion oriice (0.16 mm2) was about 300. The cell was heated by a resistance

oven. The temperature of the cell was measured by a standard tungsten–rhenium thermocouple calibrated to

the melting points of pure NaBr and Ag. The accuracy of the temperature measurement was estimated to

be within ± 5 K. Instrument calibration was performed according to the internal standard procedure using

metallic silver as a reference. A program module [12] permitted to record automatically the ion current, the

temperature of the cell, and the energy of ionizing electrons. A more detailed description of the unit and

experimental procedure was given in [13].

The YbBr3 sample was synthesized from Yb2O3 (Fluka, 99.9%) using the NH4Br procedure [14, 15], 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 solution, grinding of the

residue and its heating to 150 ∘C in argon low and to 450 ∘C under vacuum. The brutto reactions of synthesis

are described by Equations 3.7.3 and 3.7.4:

Yb2O3 + 6HBr + 6NH4Br → 2(NH4)3YbBr6 + 3H2O, (3.7.3)

(NH4)3YbBr6 → YbBr3 + 3NH4Br. (3.7.4)

For further puriication, the dry YbBr3 powder was sublimed in an airtight quartz reactor at 950 ∘C under

vacuum.

Molecular and Ionic Species in Vapor over Molten Ytterbium Bromides 205

The YbBr2 sample was obtained by reducing YbBr3 with metallic Yb (99.99%; Metall Rare Earth Ltd) in

a tantalum container sealed by arc welding in a helium atmosphere and enclosed in a quartz ampoule. The

metal was used in an excess YbBr3:Yb ratio of 2.0 : 1.05. The temperature was raised to 980 ∘C and kept there

for 24 h, then lowered to 800 ∘C and kept for 24 h, followed by slow cooling to room temperature over 72 h.

The resultant pale yellowish-white powder was identiied by X-ray diffraction as phase pure YbBr2.

3.7.3 Results and discussion

3.7.3.1 Mass spectra and ionization eficiency curves

3.7.3.1.1 Ytterbium tribromide

The following ions were registered in the mass spectrum upon the vaporization of YbBr3 over the tem-

perature range 850–1150 K: Yb+, YbBr+, YbBr2+, YbBr3

+, Br2+, Br+, Yb2Br3

+, Yb2Br4+, and Yb2Br5

+

(Figure 3.7.1). Measurements of the ion current of Br+ was complicated by considerable background noise

from the instrument at m/e= 79, 81 and thus performed only at several individual temperature values.

In the course of measurements, two main stages were observed; these are denoted by the Roman numerals

I and II in Figure 3.7.1. Note that these and all subsequent 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.

As can be seen in Figure 3.7.1, the mass spectra of vapor differ considerably at different stages, this distinc-

tion comprising not only the qualitative change in the ratios of ion currents but the complete vanishing of Br2+

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 [16–20]. The phenomena that stand out in particular are a very high

fraction of YbBr+ ion and a wide variety of ions containing two ytterbium atoms. The presence of Br+ and

Br2+ ions at stage I indicates the release of atomic and molecular bromine due to the thermal decomposition

of the sample via the reaction in Equation 3.7.1.

In order to determine the molecular precursors of ions at each vaporization stage, we recorded the

ionization eficiency curves (IECs), which are the dependences of the mass spectra on the energy of the

ionizing electrons. IECs for the Yb+, YbBr+, YbBr2+, YbBr3

+, and Br2+ ions measured in the course

of one experiment and normalized at the ixed electron energy (4–5 eV above a threshold) are shown in

Figure 3.7.2. The energy scale in Figure 3.7.2 was calibrated using the ionization energy of molecular

bromine; I0(Br2)= 10.53± 0.01 eV [10]. It should be noted that the ion appearance energies were not

determined precisely in this work, such determination being complicated under the superposing of the

spectra of individual molecules. In interpreting the mass spectra, we therefore relied on the analysis of the

IEC shapes. It can be seen from Figure 3.7.2 that the IEC shapes of Yb+ and YbBr2+ differ substantially in

the two temperature ranges 877–988 and 1067–1174 K that correspond to the different stages of vaporization

(Figure 3.7.1). It is easy to draw a conclusion that, at stage I (877–988 K), these ions are mainly formed

from YbBr3 molecules. Besides, the low energy tails extending to about 15 eV (Yb+) and 9 eV (YbBr2+)

demonstrate the presence of YbBr2 molecules. The latter dominate in vapor at stage II (1067–1174 K).

At this stage the intensities of YbBr3+ ions were too low to perform measurements of the IECs. Note that

the YbBr3+ ions can be formed exclusively from YbBr3 molecules by analogy with other LnBr3 [16–20].

The IEC shape for YbBr+ changes insigniicantly at stage II as compared with Yb+ and YbBr2+. This

observation can be explained if the YbBr2 molecules are the main precursors for the YbBr+ ions at any stage

of vaporization. In addition the low energy tail, which appears on the YbBr+ IEC at 1067–1174 K, indicates

some contribution from the YbBr molecules.

206 Molten Salts Chemistry and Technology

Yb2Br3+

Yb2Br3+

Yb2Br3+

Yb2Br4+

Yb2Br4+

Yb2Br4+

Yb2Br5+

Yb2Br5+

I(i)

/I(Y

bB

r 2+)

0.00932 K

0.05

0.10

0.15

0.00980 K

0.01

0.02

0.001176 K

0.01

0.02

0,00850 900 950 1000 1000 1050 1100 1150

0,05

0,10

0,15

0,20

0,4

0,6

0,8

1,0 0.91

0.42

0.18

Br2+

YbBr3+

Yb+

YbBr+

Stage I Stage II

1,2

1,4

1,6

0,00

0,05

0,10

0,15

0,20

0,4

0,6

0,8

1,0

1,2

1,4

1,6

0.02

0.18

1.39

T, K

I(i)

/I(Y

bB

r 2+)

(a)

(b)

Figure 3.7.1 Temperature dependences of the mass spectra and their change over time upon the vaporizationof YbBr3

The variation of fractions of ions containing two Yb atoms at the different vaporization stages (Figure 3.7.1)

correlates with that of the YbBr3 and YbBr2 contributions to the YbBr2+ ion current. This observation allows

attributing the Yb2Br3+, Yb2Br4

+, and Yb2Br5+ ion currents to the Yb2Br4, Yb2Br5, and Yb2Br6 molecular

associates, which are combinations of the corresponding molecules.

3.7.3.1.2 Ytterbium dibromide

The Yb+, YbBr+, YbBr2+, YbBr3

+, Yb2Br3+, and Yb2Br4

+ ions were recorded in the mass spectra from

heating YbBr2 over the temperature range 960–1300 K. The ion current ratios and the IECs are shown in

Figures 3.7.3 and 3.7.4, respectively. It can be seen from Figure 3.7.3 that the I(Yb+)/I(YbBr2+) ratio changes

with temperature non-monotonically. Such behavior relects the competition of different contributions to the

ion current of Yb+ and follows the change in shape of the IEC for Yb+ (Figure 3.7.4) with temperature. These

contributions are predominately from Yb atoms and YbBr2 molecules. There are also two contributions to

the YbBr+ ion current. The main one comes from YbBr2 and the minor one from YbBr. The fraction of YbBr

Molecular and Ionic Species in Vapor over Molten Ytterbium Bromides 207

0

5 6 7 8 9 10 11 12 13

Energy of ionizing electrons, eV

Ion c

urr

ent,

arb

.units

14 15 16 17 18 19 20 21

0

00

10

20

30

40

50

60

70

80

90

100

100

100100 YbBr3

+

YbBr2+

YbBr+

Yb+

Br2+

Stage I

877 K932 K988 K

1067 K1113 K1174 K

Stage II

Figure 3.7.2 Ionization efficiency curves at stages I and II of the vaporization of YbBr3

in vapor is estimated to be about 0.5%. The situation is similar in the case of YbBr2+ ions that are formed

mainly from YbBr2 and to a much lesser extent from YbBr3, which fraction is estimated to be about 4%.

It is noteworthy that the presence of atomic Yb in the vapor over ytterbium dibromide is evidence of the

occurrence of the disproportionation reaction of the type in Equation 3.7.2.

3.7.3.1.3 Vapor composition

The observations described above and our interpretation of them suggest that the vapor composition at which

up to three ytterbium-containing components (YbBr3, YbBr2, YbBr, and Yb) can simultaneously coexist is

complex. The next stage of processing the primary data therefore involved separating the contributions to ion

currents from different molecular precursors.

Let us introduce the concept of the fragmentation coeficient:

fij = Iij∕Ijj, (3.7.5)

which determines the ratio between fragmentary YbBri+ and molecular YbBrj

+ ion currents formed from

the YbBrj molecule (i< j). Let us express the ion currents I03, I13, I23 of the fragmentary ions Yb+, YbBr+,

and YbBr2+, the products of ionization of the YbBr3 molecule, in terms of ion current I33 of the pure line of

YbBr3+ and the corresponding fragmentation coeficient:

Ii3 = fi3I33, (i = 0, 1, 2). (3.7.6)

Likewise, for YbBr2 molecules we obtain the expressions:

Ii2 = fi2I22, (i = 0, 1), (3.7.7)

208 Molten Salts Chemistry and Technology

0.0

0.1

0.2

1.0

1.2

1.4

1.6

1.8

2.0

900 1000 1100 1200 1300

1.26

0.19

0.02

YbBr3+

Yb+

YbBr+

T, K

I(i)

/I(Y

bB

r 2+)

Figure 3.7.3 Temperature dependence of the mass spectra upon the vaporization of YbBr2

respectively. The task of attribution of ion currents to molecular precursors is thus reduced to determining the

coeficients f03, f13, f23, f02, and f12. With this in mind, we considered the balance equations of ion currents

measured upon the vaporization of YbBr3 at stage I:

I2 = I22 + f23I3, (3.7.8)

I1 = f13I3 + f12I22, (3.7.9)

I0 = f03I3 + f02I22, (3.7.10)

where Ii is the measured ion current.

The fragmentation coeficients for YbBr2 molecules can be expressed as:

f12 =I1 − f13I3

I2 − f23I3

, (3.7.11)

f02 =I0 − f03I3

I2 − f23I3

. (3.7.12)

Equations 3.7.9 and 3.7.10 are valid for each of the experimental points at stage I if one neglects the

temperature dependence of f02 and f12. It can therefore be written for two points as:

f02(T1) = f02(T2), (3.7.13)

f12(T1) = f12(T2). (3.7.14)

Then:

I′1− f13I

′3

I′2− f23I

′3

=I′′1− f13I

′′3

I′′2− f23I

′′3

, (3.7.15)

I′0− f03I

′3

I′2− f23I

′3

=I′′0− f03I

′′3

I′′2− f23I

′′3

, (3.7.16)

Molecular and Ionic Species in Vapor over Molten Ytterbium Bromides 209

0

5 6 7 8 9 10 11 12 13

Energy of ionizing electrons, eV

Ion c

urr

ent,

arb

.units

14 15 16 17 18

0

10

20

30

40

50

60

70

80

90

100

100

0

100

971 K

1015 K

1055 K

1118 K

1191 K

1235 K

YbBr2+

YbBr+

Yb+

Figure 3.7.4 Ionization efficiency curves upon the vaporization of YbBr2

Table 3.7.1 Fragmentation coefficients of YbBr2and YbBr3 molecules

Coefficient Value

f02 = I(Yb+,YbBr2)/I(YbBr2+,YbBr2) 0.2± 0.1

f12 = I(YbBr+,YbBr2)/I(YbBr2+,YbBr2) 1.2± 0.2f03 = I(Yb+,YbBr3)/I(YbBr3

+,YbBr3) 0.4± 0.1f13 = I(YbBr+,YbBr3)/I(YbBr3+,YbBr3) 1.2± 0.3f23 = I(YbBr2+,YbBr3)/I(YbBr3+,YbBr3) 2.0± 0.3

Note: I(Yb+, YbBr2) is the intensity of the current of Yb+ ions formed

from YbBr molecule. The same is true for the other cases.

where Ii′ and Ii

′′ are the ion currents for the irst and second random points.

A system of the type shown in Equations 3.7.15 and 3.7.16 was devised for the experimental data on ion

currents measured at different temperatures. Its solution resulted in the f03, f13, f23, f02, and f12 values given

in Table 3.7.1. These coeficients were used for calculating the total ion currents from the YbBr3 and YbBr2

molecules:∑

I(YbBr3) = I3 ⋅ 4.60±0.44 (3.7.17)∑

I(YbBr2) = (I2 –I3 ⋅ 2±0.3) ⋅ 2.40±0.21 (3.7.18)

The contributions of YbBr molecules and Yb atoms to the total currents of YbBr+ and Yb+ were estimated

using IECs.

210 Molten Salts Chemistry and Technology

Table 3.7.2 The composition of vapor over YbBr3 and YbBr2

YbBr3 YbBr2(1190K)Stage I (932K) Stage II (1176K)

Br2 15 – –Br 90 – –Yb – – 1YbBr – 0.5 0.5YbBr2 30 100 100YbBr3 100 4 4Yb2Br4 1.5 0.5 0.5Yb2Br5 11 0.02 0.02Yb2Br6 8 – –

The fractions of vapor species (Table 3.7.2) were calculated using the relationship:

pj ∼T

�j

∑

i

Iij

�iai

, (3.7.19)

where pj is the partial pressure, T is the cell temperature, �jmol is the total ionization cross section of the jth

molecule with the working energy of ionizing electrons (calculated on the basis of ionization cross sections

�nat of atoms n [21] using the expression �mol

j= 0.75

∑

n

�atn [22]),

∑

i

Iij

�iai

is the total ion current of ions i of all

types formed from molecule j (calculated on the basis of the resultant fragmentation coeficients; Table 3.7.1),

ai is the coeficient taking into account the natural abundance of isotopes of the measured ion, and � i is the

coeficient of ion-electron conversion (it is assumed that � i ∼Mi−1/2 [23], where Mi is the molecular mass

of ion).

3.7.3.1.4 Negative ions in the vapor of ytterbium bromides

Ions of YbBr4− were discovered at the stage I of vaporization of the YbBr3 preparation at temperatures near

900 K. Starting from a temperature of 950 K, YbBr3− ions were also recorded in the mass spectrum, the

content of which was nearly 10−4. Upon the transition to stage II, the concentrations of YbBr3− and YbBr4

−

ions become comparable, as was observed upon vaporization of the preparation of YbBr2 under study. In

addition Br−, Yb2Br5−, Yb2Br7

− ions were registered.

Even though there were molecules of ytterbium monobromide in the vapor, none of experiments with YbBr3

and YbBr2 revealed the presence of YbBr2− ions. These ions were not recorded in an additional experiment

with the YbBr2-Yb system.

Acknowledgments

This study was supported by the Russian Foundation for Basic Research, project no. 09-03-97536.

Molecular and Ionic Species in Vapor over Molten Ytterbium Bromides 211

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