Study of Ce1-xPrxO2 pigments

Article abstract of DOI:10.1016/S0040-6031(02)00218-6

The synthesis of new compounds based on CeO₂, which can be used as pigments for colouring of ceramic glazes, is investigated in our laboratory. The optimum conditions for the syntheses of these compounds have been estimated and the pigments pre

Full text of DOI:10.1016/S0040-6031(02)00218-6

Thermochimica Acta 395 (2003) 251–255
Study of Ce1ꢀxPrxO2 pigments
*
Petra Sulcova , Miroslav Trojan
ˇ
´
Faculty of Chemical Technology, Department of Inorganic Technology, University of Pardubice,
Legions Sq. 565, 53210 Pardubice, Czech Republic
Received 9 January 2002; received in revised form 9 April 2002; accepted 11 April 2002
Abstract
The synthesis of new compounds based on CeO2, which can be used as pigments for colouring of ceramic glazes, is
investigated in our laboratory. The optimum conditions for the syntheses of these compounds have been estimated and the
pigments prepared have been evaluated from the standpoint of their structure. The first information about the temperature region
of the formation of the pigments investigated is provided by thermal analysis. The synthesis of these compounds is followed by
thermal analysis using STA 449C Jupiter (NETZSCH, Germany).
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Ceramic pigments; Optical properties; Solid solutions CeO2–PrO2; Thermal analysis
1. Introduction
and ceramics, they are sometimes referred to as col-
ours or stains; when used to colour paints and plastics,
they are known as pigments.
The pigments on the base of CeO2 belong to
special inorganic pigments with high-temperature
stability, which represent only a small, but an impor-
tant part of the entire family of the inorganic pig-
ments [1]. This type of pigments also represents
mixed metal oxides.
Each pigment has a defined crystal structure that is
determined by the host lattice. Other oxides interdif-
fuse at high temperatures into the host lattice structure
forming either a solid-state solution or a new com-
pound. Most of mixed metal oxide pigments contain
metal cations balanced by oxygen anions with struc-
tures similar to naturally occurring minerals. Their
commercial significance is in their thermal, chemical
and light stability, combined with their low toxicity.
When they are employed for colouring glass enamels
The colour of mixed metal oxide pigments results
from the incorporation of cation of transition metals,
the so-called chromofores, into the structure of stable
host oxides. The host can be a single oxide (e.g. SnO2,
TiO2) or a mixed oxide (e.g. ZrSiO4, MgAl2O4).
Typical examples of the chromofores are transition
metal ions (Fe, Cr, Mn, Ni, Co, Cu, V, etc.) and rare-
earth elements (Ce, Pr, Nd).
The pigments of the CeO2–PrO2 system give inter-
esting colour hues in ceramic glazes which are based
on the incorporation of praseodymium ions (as chro-
mofore) into the host lattice of CeO2. These pigments
are formed by a solid solution Ce1ꢀxPrxO2 with the
fluorite structure of CeO2. This type of pigments is
prepared by high-temperature calcination of the basic
starting oxides CeO2 and Pr6O11 [2]. Their hues of
colour in ceramic glazes are very interesting. They
give various pink–orange hues, which represent only a
* Corresponding author. Fax: þ420-40-603-7275.
ˇ
E-mail address: petra.sulcova@upce.cz (P. Sulcova).
´
0040-6031/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 ( 0 2 ) 0 0 2 1 8 - 6

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P. Sulcova, M. Trojan / Thermochimica Acta 395 (2003) 251–255
252
small, but an important part of the entire family of the
inorganic pigments.
angular range of 2W < 35ꢁ and Ka1 (l ¼
0:154051 nm) for the range of 2W > 35ꢁ, employing
a nickel filter for attenuation of the Kb radiation. A
proportional detector was used.
2. Experimental
As a starting material for the preparation of the
Ce1ꢀxPrxO2 pigments, we have used commercial
CeO2 of 95% purity and Pr6O11 of 90% purity (Indian
Rare Earths Ltd., India).
3. Results and discussion
The main aim was to found such a pigment, which
would give the intensive pink–orange colour and at the
same time being of the low content of praseodymium.
The effect of the praseodymium content in the starting
mixtures on the colour hue of the pigments was studied.
The effect of the praseodymium content in the
mixtures on the colour hue of these pigments applied
in glaze is demonstrated in Table 1. The increasing
content of praseodymium decreases the red hue (coor-
dinate aÃ) and yellow hue (coordinate bÃ) of these
pigments. The intensive pink–orange colour with the
acceptablelow content of praseodymium was attributed
to the pigment containing 10 mol% of praseodymium.
This pigment can be described by the formula
Ce0.90Pr0.10O2. When 50 mol% of praseodymium is
used, the colour hue of the pigment is shifted to red–
brown hue. Higher content of praseodymium increases
value Là and aà and shifts colour to brown–yellow
(Fig. 1). This effect corresponds with the fact that these
samples are heterogeneous because free PrO2 exists
next solid solution of both oxides (PrO2 in CeO2).
The structure of the pigments of Ce1ꢀxPrxO2 type
was also investigated. The samples with the increasing
content of praseodymium (x ¼ 0:05, 0.10, 0.20, 0.30,
0.40, 0.50, 0.60, 0.70, 0.80 and 0.90) were studied by
The starting mixtures containing basic oxides (CeO2
and Pr6O11) with the increasing content of praseody-
mium (x ¼ 0:05, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70,
0.80 and 0.90) were homogenised in an agate mortar.
The mixtures were then calcinated in corundum cruci-
bles in an electric resistance furnace (the increase of the
temperature 10 K/min). The calcination temperature
was 1350 8C for the duration of 1 h. The pigments
prepared were applied to a middle-temperature glaze
in amounts of 10% (mass/mass) with a glazing tem-
perature of 1050 8C for 15 min [2]. The final glazes
were evaluated with regard to their colour hues by
measurements of spectral reflectance in the visible
region of light using a MiniScan (HunterLab, USA).
The colour properties are described in terms of CIE
LÃaÃbà system (1976). The values aà (the axis red–
green) and bà (the axis yellow–blue) indicate the colour
hue. Thevalue LÃ represents the lightness or darkness of
thecolourasrelatedtoa neutral greyscale. IntheLÃaÃbÃ
system, it is described by numbers from zero (black) to
100 (white).
The formation of these types of pigments was
followed by thermal analysis using STA 449C Jupiter
(NETZSCH, Germany) which allows the evaluation of
data and simultaneous registration of the thermoana-
lytical curves TG and DTA. The starting raw material
and the some prepared starting mixtures were studied
by thermal analysis in ceramic crucible in air in
temperature region from 30 to 1500 8C. The increase
of temperature was 10 K/min. a-Al2O3 was used as
reference material.
Table 1
The effect of Pr content on the colour properties of the Ce1ꢀxPrxO2
pigments applied to glaze G07091
Formula
LÃ
aÃ
bÃ
Ce0.95Pr0.05O2
Ce0.90Pr0.10O2
Ce0.80Pr0.20O2
Ce0.70Pr0.30O2
Ce0.60Pr0.40O2
Ce0.50Pr0.50O2
Ce0.40Pr0.60O2
Ce0.30Pr0.70O2
Ce0.20Pr0.80O2
Ce0.10Pr0.90O2
62.71
60.21
56.82
56.36
55.27
55.02
50.51
56.72
61.35
62.21
26.96
23.53
22.62
18.51
16.06
14.02
8.64
33.17
33.21
32.61
29.61
27.06
26.01
21.62
25.85
30.21
39.21
The powder pigments were studied by X-ray dif-
fraction analysis. The X-ray diffractograms in the
range 20–608 2W of the samples were obtained using
a vertical X-ray diffractometer HZG-4B (Freiberger
4.96
¨
Prazisionsmechanik, Germany) equipped with a goni-
4.21
ometer of 25 cm diameter in the range 20–608 2W. Cu
Ka (l ¼ 0:154178 nm) radiation was used for the
3.52

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253
Fig. 1. The effect of Pr content on the colour properties of the Ce1ꢀxPrxO2 pigments applied to glaze G07091.
X-ray diffraction analyses. The observed diffraction
lines corresponded with characteristic lines of fluorite
structure of CeO2. The samples with the highest content
of praseodymium (x ¼ 0:70, 0.80 and 0.90) exhibited
additional peaks, which have been assigned to PrO2.
PrO2 dissolves in CeO2 forming thus Ce1ꢀxPrxO2 solid
solutions up to x ¼ 0:6.
defects is associated with the decrease of the volume
of the elementary cell of CeO2. Praseodymium enters
into CeO2 as substitutional defects instead of cerium
because the tetravalent praseodymium ion (r½Pr4þŠ ¼
0:092 nm) has a smaller radius than the tetravalent
cerium ion (r½Ce4þŠ ¼ 0:101 nm).
The raw material for the preparation of the
Ce1ꢀxPrxO2 pigments was mixed oxide Pr6O11. Pra-
seodymium ions are available in two oxidation states
in this mixed oxide Pr6O11 (4PrO2ÁPr2O3). In the
temperature range from 250 to 400 8C, mixed oxide
Pr6O11 is reduced to Pr2O3 [3]. On the base of results
of thermal analysis (Fig. 2), it follows that process of
reduction is represented by endothermal effect on the
DTA curve (350 8C).
The values of lattice parameters of cerium dioxide
(Table 2) show that the parameter a decreases with the
increasing content of praseodymium. Praseodymium
atoms substitute cerium atoms in their crystal lattice
x
forming uncharged substitutional defects PrCe in the
solid solution Ce1ꢀxPrxO2. The formation of these
Table 2
Structural parameters of samples of Ce1ꢀxPrxO2 pigments and
CeO2
Pr6O11 ! 3Pr2O3 þ O2
(1)
Crystal lattice of Pr2O3 is characterised by excess of
oxygen. From DTA curve (Fig. 2), it follows that the
content of oxygen is decreased with increasing tem-
perature. This loss of oxygen is represented by several
endothermal effects on DTA curve (Fig. 2). The for-
mula of praseodymium oxides was evaluated on the
base of mass loss. The sequence of these changes is
described by following schemes (2)–(5):
Formula
a (nm)
V (nm3)
D2ua
CeO2
0.54221(6)
0.54205(2)
0.54178(2)
0.54163(4)
0.54145(5)
0.54137(3)
0.54122(1)
0.15941(6)
0.15927(2)
0.15902(2)
0.15889(4)
0.15873(1)
0.15866(2)
0.15853(1)
0.002
0.007
0.003
0.005
0.004
0.005
0.002
Ce0.90Pr0.10O2
Ce0.80Pr0.20O2
Ce0.70Pr0.30O2
Ce0.60Pr0.40O2
Ce0.50Pr0.50O2
Ce0.40Pr0.60O2
a D2u ¼ Nꢀ1ð2uexp ꢀ 2ucalcÞ, where 2uexp is the experimental
diffraction angle, 2ucalc the angle calculated from lattice parameters
and N the number of investigated diffraction lines.
Pr2O3 ! Pr2O2:75 þ 0:125O2 ð751 ꢁCÞ
Pr2O2:75 ! Pr2O2:60 þ 0:075O2 ð1000 ꢁCÞ
(2)
(3)

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254
Fig. 2. TG and DTA curves of mixed oxide Pr6O11 (mass of sample: 246.40 mg; atmosphere: air; heating rate: 10 K/min).
Fig. 3. TG and DTA curves of mixture for synthesis Ce0.90Pr0.10O2 (mass of sample: 256.30 mg; atmosphere: air; heating rate: 10 K/min).

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255
Pr2O2:60 ! Pr2O2:35 þ 0:125O2 ð1246 ꢁCÞ
Pr2O2:35 þ 0:825O2 ! 2PrO2 ð1432 ꢁCÞ
From the DTA curve (Fig. 2) it follows that the last
peak, which is exothermal, belongs to the oxidation
of trivalent praseodymium ions to tetravalent ions.
This effect is not characterised by an increasing of
sample weight, because the content of oxygen is still
decreased. For oxide PrO2 (scheme (5)), it can be used
better formula PrO2ꢀd, where parameter d charac-
terises loss of oxygen [4].
The mixture from starting oxides (CeO2 and
Pr6O11) containing 10 mol% of Pr was homogenised
in agate mortar. This mixture was also studied by
DTA. From DTA curve of mixture for synthesis
of Ce0.9Pr0.1O2 (Fig. 3), it follows that the endother-
mal effect of reduction of Pr6O11 is shifted to
lower temperature (313 8C). Exothermal effect above
1400 8C attributes to oxidation of trivalent praseody-
mium ions. Last peak on DTA curve belongs to
dissolving of PrO2 into CeO2. This fact corresponds
with the results of X-ray diffraction analyses. It means
that the pigment crystals are formed during the high-
temperature reaction between CeO2 and PrO2. PrO2
dissolves in CeO2 during the heat treatment of the
starting mixtures forming the solid solution of both
oxides Ce1ꢀxPrxO2. The dissolving of PrO2 in CeO2 is
characterised by endothermic effect on the DTA curve
of mixture for synthesis of Ce0.9Pr0.1O2 with mini-
mum at 1445 8C. The formation of solid solution is
described by the following scheme:
(4)
(5)
4. Conclusion
The compounds based on CeO2 are heat- and
chemical-resistant. They have intensive colour and
great hiding power. Due to their high resistance to the
attack of molten glass in glazes and enamels, these
pigments belong to high-temperature pigments. They
are suitable for all types of ceramic glazes. The
pigments are insoluble in concentrated H2SO4 and
HCl [4]. This property reflects the strength of the
crystal lattice of the pigments prepared. These pig-
ments are environmentally friendly and therefore
very progressive too. In addition, the colour of these
pigments is pleasant and interesting. They could
complete the basic assortment of colours of ceramic
pigments.
Acknowledgements
The study of this type of pigments was supported by
the grant No. 104/02/1443 from Grant Agency of
Czech Republic.
References
[1] S.O. Tumanov, V.A. Pavlova, Pigmenty systemy CeO2–PrO2,
Steklo i Keramika 2 (1974) 28.
ˇ
ˇ
´
[2] P. Sulcova, M. Trojan, Z. Solc, Dyes Pigments 37 (1998)
65.
ˇ
´
[3] P. Sulcova, M. Trojan, J. Therm. Anal. Cal. 65 (2001) 399.
[4] S.T. Aruna, S. Glosh, K.C. Patil, Int. J. Inorg. Mater. 3 (2001)
387.
ð1 ꢀ xÞCeO2 þ xPrO2 ! Ce1ꢀxPrxO2
(6)