Chlorination of Pr2O3 and Pr6O11. Crystal structure, magnetic and spectroscopic properties of praseodymium oxychloride

Article abstract of DOI:10.1016/j.jallcom.2018.10.329

The reactions of Pr₂O₃ and Pr₆O₁₁ with Cl₂(g) were studied thermodynamically and experimentally, and proposed in the systems Pr₂O₃/Cl₂(g) and Pr₆O₁₁/Cl₂(g). A kinetic analysis was performed on Pr₆O₁₁ chlorination, which differs from with other light rare earth oxide chlorinations. The praseodymium oxychlorides obtained by full chlorination of Pr₆O₁₁ at 425 °C and 800 °C are different, being well characterized by X-ray diffraction, scanning electron microscopy, magnetic susceptibility measurements and X-ray photoelectron spectroscopy. The crystal structures of the oxychlorides were refined with the Rietveld method. The comparison between the theoretical magnetic moment of Pr³⁺ and the estimated magnetic moment in the praseodymium oxychloride obtained at 425 °C revealed the presence of Pr³⁺ and Pr⁴⁺, whereas no significant difference was observed between the theoretical and the experimental values of Pr³⁺ obtained for the praseodymium oxychloride prepared at 800 °C. These observations are consistent with the analyses in the X-ray photoelectron spectra for both oxychlorides. Finally, a general formula of PrO_1+xCl is proposed for the oxychlorides synthesized at T < 800 °C.

Full text of DOI:10.1016/j.jallcom.2018.10.329

Accepted Manuscript
Chlorination of Pr O and Pr O . Crystal structure, magnetic and spectroscopic
2 3
6 11
properties of praseodymium oxychloride
Federico J. Pomiro, Juan P. Gaviría, Gastón G. Fouga, Leonardo D. Vega, Ana E.
Bohé
PII:
S0925-8388(18)34021-0
DOI:
https://doi.org/10.1016/j.jallcom.2018.10.329
JALCOM 48147
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 15 August 2018
Revised Date: 23 October 2018
Accepted Date: 25 October 2018
Please cite this article as: F.J. Pomiro, J.P. Gaviría, Gastó.G. Fouga, L.D. Vega, A.E. Bohé, Chlorination
of Pr O and Pr O . Crystal structure, magnetic and spectroscopic properties of praseodymium
2 3
6 11
oxychloride, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.10.329.
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ACCEPTED MANUSCRIPT
Chlorination of Pr2O3 and Pr6O11. Crystal Structure, magnetic and
spectroscopic properties of Praseodymium Oxychloride
Federico J. Pomiro(1*), Juan P. Gaviría(1,2), Gastón G. Fouga(1,2), Leonardo D. Vega(1), Ana E.
Bohé(1,2,3)
(1)
Departamento de Fisicoquímica y Control de Calidad. Complejo Tecnológico Pilcaniyeu.
Centro Atómico Bariloche. Comisión Nacional de Energía Atómica. Av. Bustillo 9500, 8400
S.C. de Bariloche, Río Negro, Argentina.
(2) Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina
(3) Centro Regional Universitario Bariloche. Universidad Nacional del Comahue. 8400 S.C. de
Bariloche, Río Negro, Argentina.
*e-mail: pomiro@cab.cnea.gov.ar
Tel / Fax: +54 02 94 444 5293
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ABSTRACT
The reactions of Pr2O3 and Pr6O11 with Cl2(g) were studied thermodynamically and
experimentally, and proposed in the systems Pr2O3/Cl2(g) and Pr6O11/Cl2(g). A kinetic
analysis was performed on Pr6O11 chlorination, which differs from with other light rare earth
oxide chlorinations. The praseodymium oxychlorides obtained by full chlorination of Pr6O11 at
425 °C and 800 °C are different, being well charact erized by X-ray diffraction, scanning
electron microscopy, magnetic susceptibility measurements and X-ray photoelectron
spectroscopy. The crystal structures of the oxychlorides were refined with the Rietveld
method. The comparison between the theoretical magnetic moment of Pr3+ and the
estimated magnetic moment in the praseodymium oxychloride obtained at 425 °C revealed
the presence of Pr3+ and Pr4+, whereas no significant difference was observed between the
theoretical and the experimental values of Pr3+ obtained for the praseodymium oxychloride
prepared at 800 °C. These observations are consiste nt with the analyses in the X-ray
photoelectron spectra for both oxychlorides. Finally, a general formula of PrO1+xCl is
proposed for the oxychlorides synthesized at T<800 °C.
KEYWORDS: Praseodymium; oxychloride; chlorination; kinetics; light rare earth.
1. INTRODUCTION
Lanthanides are the elements spanning atomic number 57 to 71. They all occur in nature,
although promethium, the rarest, only occurs in trace quantities in natural materials, as it is
not long-lived and has no stable isotopes. Rare earths are usually divided into lower atomic
weight elements, lanthanum to europium, referred to as the light rare earth elements (LREE),
and heavy rare earth elements (HREE) gadolinium to lutetium and yttrium. The division is
sometimes arbitrary and the term middle REE (MREE) is sometimes used to refer to those
elements between europium to dysprosium. Lanthanum, cerium, praseodymium and
neodymium are the most abundant rare earths [1]. It is known that Pr can be consumed in
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Nd-Fe-B magnet alloys and in other alloys such as magnesium and aluminum alloys,
improving some mechanical properties and even increasing resistance to the corrosion. Pr is
used as a bright yellow pigment in praseodymium-doped zirconia and is added as a green
colorant for decorative glass (ZrSiO4:Pr4+). Several studies have been conducted on its
optical properties for use in amplification of telecommunication systems, including its use as
a doping agent in fluoride fibers. It is also used in scintillator for medical CAT scans [1,2].
One of the methods for obtaining these REEs is the high-temperature chlorination process,
which enables anhydrous rare earth chlorides or oxychlorides to be obtain directly. This
method has been widely explored as a way of extracting no just REEs but also other metals
from different ores [3]. Dry chlorination of several LREE oxides using Cl2(g) as chlorinating
agent have been studied [4-9]. Praseodymium oxide behaves differently from these oxides in
that it has two oxidation states (3+ and 4+), enabling praseodymium oxide with Pr3+ to be
oxidized during the chlorination reaction. The Pr-O system shows a number of discrete
phases, with all the REE oxides being the most complex system [10,11].
Few papers have been published on the chlorination of Pr oxides. Mendil et. al. [12] studied
the reaction between Pr6O11 and NH4Cl in the temperature range of 250-450 ºC under Ar
atmosphere, obtaining PrCl3 as a product. They report the influence of various synthesis
parameters (temperature, contact time and chemical composition) on the reaction yield.
Holsa et. al. [13] analyzed the paramagnetic susceptibility of some rare earth oxychlorides.
They synthesized oxychlorides through the reaction between the REE2O3 and NH4Cl at
temperatures between 900 and 1050 ºC under N2(g) atmosphere.
The present paper analyzes Pr2O3 and Pr6O11 chlorination reactions by means of
thermodynamic studies and characterized the products formed at different temperatures. A
kinetic study was performed for the chlorination of Pr6O11. The results will contribute to the
understanding of the chemistry of praseodymium oxides under different condition and the
products obtained in the Pr-O-Cl system.
2. EXPERIMENTAL SECTION
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2.1 Materials and sample preparation
The starting praseodymium oxide is composed of a mixture of Pr oxides, Pr(OH)3 and
Pr2O2SO4 (oxysulphate content is lower than 1%). Three samples were prepared with this
starting oxide to perform the different studies, as follows:
a) Chlorination of Pr2O3: the starting oxide was treated in Ar-5 % H2(g) atmosphere at 970 ºC
for 4 h to transform it into the form Pr2O3. Powder X-ray diffraction (PXRD) showed the
typical profile of Pr2O3 (>97%) and minor impurities of the phases Pr7O12 (<1%) and
praseodymium oxysulphite (<1%).
b) Chlorination of Pr6O11: the starting oxide was treated in O2(g) at 900 °C for 4 h to convert
the Pr(OH)3 and obtain the crystalline phase Pr6O11, which was confirmed by PXRD. The
praseodymium oxysulphate is still present in the sample, but does not undergo any change
during the chlorinations.
c) Characterization of Praseodymium Oxychloride: in order to obtain Pr6O11 and eliminate the
praseodymium oxysulphate in the initial samples, the starting praseodymium oxide was
treated in a furnace in simulated air (20 % O2(g) and 80 % N2(g)) at 1400 °C. The PXRD
pattern of the product shows high crystallinity and high purity of Pr6O11 without trace phases.
2.2 Equipment and procedure
A silica glass fixed-bed reactor was used in chlorination reactions. It consisted of a
horizontally mounted tube 30 mm in diameter located inside an electric furnace where the
sample was introduced using a silica glass crucible (area = 1 cm2, height = 0.5 cm). The
reactant gas was introduced when the system reached the working temperature. The
experimental conditions in non-isothermal reactions were: PCl2(g) = 1 atm; Cl2(g) flow = 3 l
h-1; m0 ≈ 5 mg; ∆T ∆t-1= 4 °C min -1. To end each run, the reactant gas was cut off and a
nitrogen stream was introduced to purge the reactor.
Samples were characterized by X-ray diffraction, Scanning Electron Microscopy (FEI Inspect
S50). PXRD were collected using a D8 Advance diffractometer (Bruker AXS, Germany) with
Bragg-Brentano θ/θ geometry and Cu Kα radiation (40 kV, 30 mA). The diffraction angles
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were measured from 10 to 100°of 2 θ in 0.02°steps with a counting time of 2.5 s per s tep.
The X-ray diagrams were refined and the phases present were quantified by the Rietveld
method using DIFFRACplus TOPAS 4.2 software [14]. The following parameters were
refined: scale factor, background coefficients (Chebychev polynomial of 7 order), crystallite
size, zero-shift and sample displacement, peak shape parameters (using Fundamental
Parameters FP), atomic positions and unit-cell parameters.
The magnetic measurements were performed using a Quantum Design SQUID
magnetometer model MPMS2 on powdered samples, between 2 and 200 K with an external
magnetic field of 1000 Oe.
Surface compositions were analyzed by means of X-ray photoelectron spectroscopy (XPS)
using a standard Al/Mg twin-anode X-ray gun and a hemispherical electrostatic electron
energy analyzer. The measurements were carried out in high vacuum conditions (HV), with a
base pressure of ∼10−9 Torr. For the analysis, the powdered samples were spread on
adhesive graphitic carbon tape.
3. RESULTS AND DISCUSSION
3.1 Thermodynamic study
Figure 1 shows the Kellogg stability field diagrams of the system Pr-O-Cl at 300 and 500 ºC.
These diagrams were made with HSC 6.0 software [15]. The proposed condensed phases in
the calculations were Pr, PrCl3, PrOCl, Pr2O3, Pr7O12, Pr6O11 and PrO2. It can be seen that all
of the proposed species are thermodynamically feasible to be formed in the ranges selected
of Cl2(g) and O2(g) pressures, except Pr6O11. The diagrams also show a feature of the Pr
oxides: the most stable oxide is PrO2 in the diagram at 300 ºC under oxygen pressure of 1
bar, whereas Pr7O12 is the most stable oxide at 500 ºC (not the most oxidized oxide).
The phase stability diagrams show that there is not thermodynamic equilibrium between
praseodymium oxides and PrCl3 phases except in the PrO2 chlorination. Therefore, PrOCl
has to be formed prior to the formation of PrCl3. This behavior is typical in the chlorination of
REE2O3 oxides [4-9, 16].
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The following experimental conditions were used: PCl2 ~ 1 atm and PO2: 10-4 atm (indicated
with an asterisk in Figure 1). According to the stability diagrams, PrCl3 is the most stable
species under these conditions, being very close to the stability field of PrOCl.
Ellingham diagrams (Standard Gibbs free energy vs. temperature) were also prepared. The
following chlorination reactions of the praseodymium oxides were considered for this analysis
(the ∆Gsº were fitted with linear functions between 250 and 700 °C):
Pr2O3 + Cl2(g) = 2PrOCl + 1/2O2(g)
2/7Pr7O12 + Cl2(g) = 2PrOCl + 5/7O2(g)
1/3Pr6O11 + Cl2(g) = 2PrOCl + 5/6O2(g)
2PrO2 + Cl2(g) = 2PrOCl + O2(g)
∆Gº(kJ/mol) = 0.577 T(ºC)-517.8
∆Gº(kJ/mol) = 0.227 T(ºC)-138.6
∆Gº(kJ/mol) = 0.187 T(ºC)-169.6
∆Gº(kJ/mol) = 0.184 T(ºC)-117.7
(1)
(2)
(3)
(4)
Praseodymium oxychloride can be chlorinated according to reaction 5:
PrOCl + Cl2(g) = PrCl3 + 1/2 O2(g)
∆Gº(kJ/mol) = -0.039 T(ºC)-7.6
(5)
The Ellingham diagrams of the reactions 1 a 5 are shown in the Figure 2. It can be seen that
all of the reactions with PrOCl formation are feasible thermodynamically in the temperature
range of 250 to 600 ºC. The Pr2O3 chlorination has the lowest ∆Gº, and Pr7O12 and PrO2
chlorinations have ∆Gº > 0 at temperatures higher than 600 ºC. The PrO2 chlorination
reaction has ∆Gº=0 at 640 ºC and ∆Gº=19 kJ molCl2-1 at 700 ºC.
The PrOCl chlorination (reaction 5) has ∆Gº close to zero in the temperature range analyzed
(-21.7 and -35.9 kJ molCl2-1 at 250 and 700 ºC, respectively).
Calculations were also made for the oxidation reactions of the following type:
PrOx + (y-x)/2 O2(g) = PrOy (y > x)
(6)
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All oxidation reactions of Pr oxides to produce a more oxidized Pr oxide have ∆Gº<0 except
Pr7O12 oxidation with formation of Pr6O11 (∆Gº(kJ/mol) = 0.02 T(ºC)+15.9), whereas if the
product is PrO2, ∆Gº<0 at temperatures lower than 500 ºC (∆Gº(kJ/mol) = 0.022 T(ºC)-10.9),
-1
being -5.5 and 4.4 kJ molCl2 at 250 and 700 ºC, respectively. Pr6O11 oxidation with
formation of PrO2 has ∆Gº<0 in the temperature range analyzed (∆Gº(kJ/mol) = 0.003 T(ºC)-
26.7).
3.2 Non-isothermal chlorinations
Figure 3 shows the percentage ratio between the mass change and initial sample mass as a
function of temperature for Pr2O3 and Pr6O11 chlorinations (and its error bar). Rietveld
analyses were performed on the final products to quantify the crystalline phases, and were
consistent with the mass changes measured. The chlorination of Pr2O3 begins at
approximately 260 °C, whereas the chlorination of P r6O11 begins at 320 °C, showing a final
mass increase of 14.3 % and 18.7 % in the Pr6O11 and Pr2O3 chlorinations, respectively.
The Pr2O3 chlorination begins with formation of praseodymium oxychloride and Pr6O11, and
the PrO2 phase is observed in addition to these phases at 330 °C. When the temperature is
higher than 340 °C, the only praseodymium oxide det erminate by PXRD is PrO2. Finally, the
only product of the chlorination is PrOCl. Therefore, the following reactions may occur during
the non-isothermal chlorinations:
Pr2O3 + Cl2(g) = 2PrOCl + 1/2O2(g)
Pr2O3 + 1/3O2(g) = 1/3Pr6O11
(1)
(7)
(8)
(3)
(4)
Pr6O11 + 1/2Cl2(g) = 5PrO2 + PrOCl
1/3Pr6O11 + Cl2(g) = 2PrOCl + 5/6O2(g)
2PrO2 + Cl2(g) = 2PrOCl + O2(g)
Initially, Pr2O3 is chlorinated, producing PrOCl and oxygen. Then, oxygen can form Pr6O11 by
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reacting with Pr2O3 (this reaction has been verified experimentally at the same temperature
range and reported in other works) [11]. Subsequently, Pr6O11 can be chlorinated, producing
PrOCl, PrO2 and oxygen through reactions 8 and 3 (both reactions have ∆Gº<0 at reaction
temperature). The last reaction of the overall process is the chlorination of the recently
formed PrO2, shown by a change in the slope in the non-isothermal curve. The oxidation of
Pr6O11 to produce PrO2 is not proposed because calcinations of Pr6O11 were carried out and
no change was observed. During the chlorination reactions, oxysulphite transforms into
oxysulphate, a reaction which is negligible with respect to the other reactions involved.
The reactions involved in the system Pr2O3/Cl2(g) can be expressed by the following general
equation:
(x+y+z)/2Pr2O3 + x/2Cl2(g) = xPrOCl + yPrO2 + z/6Pr6O11 + (1/4x-1/4y-1/6z)O2(g)
(9)
The global reaction in the full (complete) Pr2O3 chlorination has the stoichiometry of reaction
(1), however the final %∆M/mi observed (18.7 %) is higher than the expected value for this
reaction (16.65 %).
Pr6O11 chlorination begins at about 320 ºC with formation of the praseodymium oxychloride.
Pr6O11 is not observed in the initial phase at temperature higher than 375 °C, so equation 4
can be considered the only reaction to occur above this temperature. The reactions involved
in Pr6O11 chlorination could thus be 8, 3 and 4. The global reaction can be expressed as
reaction 3, in which ∆M/mi = 12.99 %. However, a higher value (14.1 %) is observed. This
behavior is analogous to that observed in the Pr2O3 chlorination which will be explained later.
The reactions involved in the system Pr6O11/Cl2(g) can be expressed by the following general
equation:
(x+y)/6Pr6O11 + x/2Cl2(g) = xPrOCl + yPrO2 + (5/12x-1/12y)O2(g)
3.3 Kinetics of Pr6O11 chlorination
(10)
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The experimental conditions under which mass-transfer resistance is absent were evaluated
by performing chlorinations at 425 °C and 1 atm of Cl2(g) for 400 s. No difference in the
%∆M/mi of reaction was observed for gaseous flow rates from 2 to 8 l h-1 and 12 mg of
sample mass (Figure 4). This suggests that the reaction does not occur under conditions of
gas reactant starvation. When the sample mass is 5 to 15 mg, there is no effect on the rate
of reaction obtained (inset in Figure 4). Consequently, it can be concluded that temperatures
below 425 °C, gas flow rate of 2 l h -1 (or higher) and sample mass of 12 mg (or lower) is
enough to eliminate mass-transfer resistance.
A chlorine diffusion rate through crucible top of 10-2 mol Cl2 s-1 for PCl2 = 1 atm, Cl2(g) flow =
6 l h-1, mo = 12 mg was calculated at 425 °C according to the R anz & Marshall correlation
[17,18], using the procedure described in the paper “Kinetic Study of Europium Oxide
Chlorination” [8]. The experimental rate at 425 °C calculated using the mass change and
Rietveld data was 4.0 x 10-5 mol Cl2 s-1. Thus, the value of the experimental rate is two or
more orders of magnitude smaller than the calculated rate and it can be assumed that at 425
°C the convective mass transfer through the boundar y layer does not affect the kinetic
regime.
3.3.1 Effect of Temperature. Activation Energy calculation
Figure 5 outlines the effect of reaction time on Pr6O11 conversion degree (α) for chlorination
at 425 °C and the Rietveld results in the products obtained. Alpha was determined from the
weight change after long chlorination times when no further increase in sample weight was
ꢂ ꢄ
ꢁ ꢃ ꢅꢁꢂꢃꢆꢇꢄ
detected, ꢀ = ꢁ ꢃ→ꢈ ꢅꢁꢂꢃꢆꢉꢄ. With increasing reaction time, the PXRD analyses indicate that
ꢂ
ꢄ
the beginning of the reaction promoted the chlorination of Pr6O11 with formation of PrO2 and
PrOCl. For times longer than 500 seconds, the chlorination of intermediate PrO2 to form
PrOCl is observed according to reaction 4. Based on this observation, the overall chlorination
Pr6O11 is further divided into two parts. In other LREE oxides chlorinations, the isothermal
curves exhibit a sigmoidal (S-shaped) response upon increase in time, indicating that the
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reaction proceeds according to a nucleation and growth model (JMA model) [4,9]. However,
this shape is not observed in the Pr6O11 chlorination due to the presence of different
reactions in the overall reaction.
The effect of temperature in the kinetic parameters of Pr6O11 chlorination was determinate
through experiments performed at different temperatures between 300 and 425 °C, with 1
atm of Cl2(g), 12 mg samples and a flow rate of 6 l h-1. Activation energy (Ea) can be
calculated by applying a “model-free” method [19-20] from the slope of the plot ln Timei vs T-
1, where Timei is the time necessary to reach a given degree of conversion value. Apparent
activation energies of 105 ± 3 kJ mol-1 and 123 ± 2 kJ mol-1 were determined for α 0.42 and
0.90, respectively. The graphical representations of these results are shown in the Figure 6.
The nonconstant value of Ea with α suggests that the rate controlling step changes during
the total reaction. This can be understood by considering the formation of PrO2 and its
subsequent chlorination such as was explained in the non-isothermal reaction. Reactions 8
and 4 can occur at the beginning of the reaction, whereas the main reaction is 4 to higher
conversion degrees, so the mechanism changes with the conversion degree. Consequently,
the apparent activation energy also changes.
3.4 Characterization of Praseodymium oxychloride
Chlorination reactions using high purity Pr6O11 as reactant were done between 425 °C and
800 °C with 6 l h -1 and PCl2 = 1 atm, during 2 (425 °C) and 1 h. The products o btained in
these reactions are named PrOCl-T, where T is the reaction temperature. The first difference
observed was color, PrOCl-425 is light grayish orange and the oxychlorides turn light green
when the temperature increases. The colors of some praseodymium compounds have been
reported. Many Pr3+ compounds such as Pr(NO3)3, PrBr3, PrF3 and PrCl3 are green, the same
color observed in PrOCl-800, whereas if traces of Pr4+ are presents in solids, the color will be
darker [21,22].
The percentage ratio between the mass change and initial sample mass was determined in
the reactions by using a more sensible balance (Sartorius Cubis Ultra Micro Balance). The
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%∆M/mi = 13.000 ± 0.002 was calculated for the reaction at 800 °C, which agrees with the
change expected in reaction 3. However, %∆M/mi = 14.004 ± 0.002 was observed in the
reaction at 425 °C, which suggests that the product obtained is not PrOCl. Percentage mass
changes between these values were calculated for reactions at 500, 600 and 700 °C (13.95,
13.64 and 13.09, respectively). Moreover, PrOCl-425 was heated at 800 °C in N 2(g)
atmosphere. This sample suffered a loss of mass, achieving %∆M/mi ≈ 13 (referred to the
initial mass of the starting Pr6O11) and the color of the final product is light green. This
observation agrees with the formation of PrOCl after this treatment (PXRD is similar to
PrOCl-800).
Calcinations reactions of praseodymium oxychlorides were performed at 950 °C in air for 4 h.
Pr6O11 was formed in both PrOCl-425 and PrOCl-800 calcinations (confirmed by PXRD). This
result disagrees with Yang et. al. [23], who proposed the conversion of PrOCl into PrO2
under various oxygen partial pressures at temperatures higher than 827 °C.
The PXRD patterns for praseodymium oxychlorides are shown in Figure 7. As it can be seen
in these patterns, there are notable changes in the peak's shape in the samples. The
material presents a more amorphous state (with wide peaks) in the reaction at 425 °C and
starts crystallizing at higher temperatures observing the intensities increases. It can be
considered that the PrOCl has high crystallinity when was prepared at 800 °C, while the
oxychloride prepared at 425 °C has wide peaks and t herefore smaller crystallite sizes. The
structures were refined with tetragonal symmetry and P4/nmm (#129) space group. The
crystallites sizes (between 21.8 and 423.9 nm) calculated by Rietveld method increase when
the reaction temperature increases (between 425 and 800 °C).
Figure 8 shows SEM images of the Pr6O11 and the product after full chlorinations at 425 °C
and 800 °C. Figure 8 a) shows that the starting rea ctant, which is formed for particles with
average sizes of 10 µm, these particles are formed for agglomerates of smaller particles with
non-uniform plate shape of around 2 µm. Figure 8 b) and c) shows that the praseodymium
oxychlorides are formed by particles with microstructures more heterogeneous than
microstructures of Pr6O11. No significant morphological difference between the oxychlorides
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is observed. EDS analysis performed of the oxychlorides (prepared between 425 °C and 800
°C) indicated that they present 1:1 Pr:Cl molar rel ation.
The magnetic susceptibility of praseodymium oxychlorides was measured for PrOCl-425 and
PrOCl-800. The experimental data were fitted with a function described by Curie-Weiss law:
χ = C/(T-θ), where χ is the magnetic susceptibility, C is the Curie constant, T is the absolute
temperature, and θ is the Curie temperature measured in Kelvin. Figure 9 shows the
measured data and fits of the χ-1 vs. T curves with Curie-Weiss law between 50 and 200 K.
The parameters obtained in the oxychlorides were: θ = -20.2 ± 0.1 K, C = 1.4641 ± 10-4 emu
K mol-1, and estimated effective magnetic moment µeff = 3.42 µB for the PrOCl-425 and θ = -
24.2 ± 0.1 K, C = 1.5719 ± 10-4 emu K mol-1, and estimated effective magnetic moment µeff =
3.55 µB for the PrOCl-800. The theoretical values of magnetic moment for Pr3+ in Pr2O3 and
Pr4+ in PrO2 are 3.59 and 2.51 µB, respectively [25,26]. The difference between the
theoretical magnetic moment of Pr3+ and the estimated magnetic moment in the PrOCl-425
agrees with the presence of Pr3+ and Pr4+. However, no significant difference is observed
between the Pr3+ theoretical value and the experimental value obtained for PrOCl-800.
Figure 10 shows the XPS spectrum of samples of PrOCl-425 and PrOCl-800. Peaks of Pr3d,
O1s, Pr4s, C1s, Cl2s, Pr4p, Cl2p, Pr4d, Pr5s and Pr5p can be distinguished in both spectra.
Charging effects are observed in the peaks of C1s and O1s (especially in PrOCl-800);
however, the peaks of Pr3d have the proper form of the oxide type Pr3+ and Pr4+. The spectra
in detail of the Pr3d peaks are shown in Figure 11 a). The references of Pr3+ corresponding
to the compound Pr2O3 in BE = 932.9 eV and of Pr4+ corresponding to the compound PrO2 in
BE = 935 eV are represented with vertical lines [27,28]. The main peak of PrOCl-425 is
compatible with the presence of Pr3+. Spin orbit splitting is 20.5 eV for the oxide with Pr3+ and
17.8 eV for the oxide with Pr4+. The shapes of the two spectra are compared in Figure 11 b),
for which the PrOCl-800 spectrum is corrected horizontally in 1.4 eV to the right. The shapes
of the peaks are very similar in both spectra. When the spectra are superimposed, they have
the same spin-orbit-splitting of 20.5 eV, compatible with the presence of Pr3+ and a small shift
is observed to lower energies in PrOCl-425 which could correspond to the presence of Pr4+.
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The energy scale was calibrated taking references from the Fermi level.
According to the results and considering a formula of type PrO1+xCl for the PrOCl-425, the
mass balance in the chlorination reaction agrees with a value of x = 0.108, whereas PrOCl
formation was observed in the chlorination at 800 °C. The molecular formula PrO1.108Cl
agrees with the results obtained in magnetic susceptibility measurement. For the
chlorinations between 425 °C and 800 °C, oxychlorid es of type PrO1+xCl are formed
according to the mass balance, with values of 0<x<0.108. When the PrO1+xCl is treated at
800 ºC in N2(g) atmosphere, PrOCl is obtained according to reaction 11:
PrO1+xCl = PrOCl + x/2O2(g)
(11)
3.5 Comparison between RE2O3 chlorinations with Cl2(g)
Dry chlorinations of LREE oxides using Cl2(g) as chlorinating agent were studied using
similar methodology. In the present work, it was observed that the chlorination of
praseodymium oxide has different behaviors due to the fact that it has two oxidation
numbers, 3+ and 4+, so of all the REE oxides, this system is the most complex. The starting
temperatures of LREE2O3 chlorinations are between 250 and 260 °C [4, 6-8], except Ce2O3
chlorination, which was not studied. This temperature change for REE≥Gd, where it was
determined that the reaction between Gd2O3 and Cl2(g) begins at 327 °C [9]. This
observation agrees with the consideration that the gadolinium presents different
characteristics from LREEs. It was also shown that LRE2O3 chlorination reaction begins with
the formation of REOCl, whereas in praseodymium oxides chlorinations, PrO2 is observed as
intermediate in the reaction and the oxychloride formed presents a general formula of
PrO1+xCl with presence of Pr3+ and Pr4+. Similar behavior was observed in EuOCl, where the
presence of Eu2+ impurity was determined [29].
4. CONCLUSION
The chlorination reactions between praseodymium oxides (Pr6O11 and Pr2O3) with Cl2(g) as
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chlorinating agent were studied. Thermodynamic analysis of praseodymium oxide
chlorination was conducted and the results showed that the formation of PrOCl is feasible
and that Pr oxides can undergo oxidation during the process.
The non-isothermal reactions were analyzed by mass change and Rietveld analysis of PXRD
data. Pr2O3 chlorination begins at 260 ºC and Pr6O11 chlorination begins at 320 ºC. In both
reactions, it was observed that PrO2 was formed as the intermediate product and
praseodymium oxychloride as the final product. This behavior is also observed in the kinetic
analysis carried out in the Pr6O11 chlorination, where it was determined that the apparent
activation energy changes during the reaction. It was established that the praseodymium
oxychloride formed by full chlorination of Pr6O11 at 425 ºC has the general formula PrO1+xCl
with presence of Pr3+ and Pr4+, where x = 0.108 considering the mass balance in the
chlorination reaction, Whereas the formation of PrOCl is observed in the full chlorination at
800 ºC. Oxychlorides with values of 0<x<0.108 are formed at temperatures between 425 and
800 °C.
Acknowledgments
This work was supported by the Comisión Nacional de Energía Atómica (CNEA), Consejo
Nacional de Investigaciones Científicas y Técnicas (CONICET) and Ministerio de Ciencia,
Tecnología e Innovación Productiva, Agencia Nacional de Promoción Científica y
Tecnológica. The authors wish to thank Dr. Silvina Bengió from Física de Superficies
Division of Centro Atómico Bariloche for measuring the XPS spectrum and for the data
interpretation and Dr. Marcelo Vasquez from the Resonancias Magnéticas Laboratory of
Centro Atómico Bariloche for the magnetic measurements and for encouraging discussions.
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FIGURE CAPTIONS
Figure 1. Phase stability diagrams of Pr-O-Cl system at 300 and 500 ºC. Experimental
conditions are indicated in the graphics.
Figure 2. Standard Gibbs free energy (per mole of chlorine) vs. temperature for reactions
involved in the PrOx-Cl2(g) system.
Figure 3. Non-isothermal curves of Pr2O3 and Pr6O11 chlorinations.
Figure 4. Effect of the flow rate of Cl2(g) and (inset) effect of the initial mass on the
chlorination of Pr6O11.
Figure 5. Isothermal chlorination of Pr6O11 at 425 °C.
Figure 6. Fits of the ln Timei vs T-1 at α = 0.42 and 0.90 to calculate the apparent activation
energies.
Figure 7. PXRD of praseodymium oxychlorides formed by full chlorination at temperatures
between 425 and 800 °C.
Figure 8. SEM images of: a) Pr6O11, b) PrOCl-425 and c) PrOCl-800.
Figure 9. Fits of the χ-1 vs. T curves with Curie-Weiss law of PrOCl-425 and PrOCl-800.
Scatter graphs: experimental curves; line graphs: calculated curves.
Figure 10. Spectra of PrOCl-425 and PrOCl-800. The Al-Kα line was used as source of
excitation. The dotted line boxes indicate the peaks that were subsequently measured in
detail.
Figure 11. Experimental spectra of the Pr3d level (a) and modifying the horizontal scale of
PrOCl-800 in 1.4 eV (b).
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Table 1. Unit-cell parameters, crystallite size, and discrepancy factors obtained by the
Rietveld refinement from PXRD data of PrOCl-425 and PrOCl-800.
Space
group
Crystallite
Size(nm)
Colour
a(Å)
b(Å)
GOF
2.1
Rwp(%)
6.2
Rp(%)
4.9
RBragg(%)
2.1
PrOCl-425
PrOCl-800
P4/nmm
P4/nmm
20.8
4.0510
6.8077
418.5
4.0514
6.8001
1.8
5.4
4.1
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