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Journal of the European Ceramic Society 35 (2015) 357–376
Synthesis, stability and coloring properties of yellow–orange pigments based
on Ni-doped karrooite (Ni,Mg)Ti
2
O
5
M. Llusar
∗
, E. García, M.T. García, C. Gargori, J.A. Badenes, G. Monrós
Departamento de Química Inorgánica y Orgánica, Universitat Jaume I, Avda. Sos Baynat s/n, 12071, Castellón, Spain
Received 2 May 2014; received in revised form 4 August 2014; accepted 9 August 2014
Available online 1 September 2014
Abstract
Solids solutions of Ni in MgTi
2
O
5
pseudobrookite (karrooite) were investigated to develop new yellowish ceramic pigments or dyes. Ni-Karrooite
samples (Mg
1-x
Ni
x
Ti
2
O
5
) were prepared by ceramic method and through a citrate-gel metalorganic decomposition route, using rapid firing
conditions (800–1500
◦
C). Fired pigments were characterized by XRD, SEM/EDX, UV–vis–NIR and color measurement (CIE-L*a*b*) techniques.
The entropy-stabilization of MgTi
2
O
5
with respect to ilmenite and rutile decreased with Ni doping, and Ni solid-solubility increased with temperature
(40 mol% at 1200
◦
C, 60 mol% at 1400–1500
◦
C). Optical absorptions of Ni
2+
in karrooite octahedral sites (mainly M1 sites) produced intense
yellowish-orange colors, increasing saturation and red/yellow hues with Ni doping (up to x = 0.4–0.5). The pigments showed a limited stability within
low-temperature (1000–1050
◦
C) ceramic glazes, although glazed samples developed intense yellowish colorations. Noteworthy, metalorganic
decomposition route enabled the formation of Ni-karrooite at lower temperatures (1000
◦
C, 20 mol% Ni) and produced finer-grained powders,
although the colors of powders and enameled samples were similar.
© 2014 Elsevier Ltd. All rights reserved.
Keywords: Pseudobrookite; Nickel; Ceramic pigments; Yellow pigments; Citrates route
1. Introduction
Classical and recent research tendencies in the field of
ceramic pigments are based on the search of new crys-
talline host lattices, the utilization of new chromophores and/or
the employment of new synthesis routes.
1
The main pur-
pose of such investigations is to develop new pigments of
improved technological performance
2
and lower toxicity.
3–5
Within this context, different titanate-based structures doped
with transition metals have been postulated as new or
emergent families of ceramic pigments, such as alkaline
earth Ti perovskites,
6–8
Y
2
Ti
2
O
7
pyrochlores,
9–11
Zr titanate-
stannate Zr(Ti,Sn)O
4
srilankites,
12–14
or M
2
TiO
5
–MTi
2
O
5
pseudobrookites.
15–17
∗
Corresponding author. Tel.: +34 964 728244; fax: +34 964 728214.
E-mail address: mllusar@qio.uji.es (M. Llusar).
Titanium pseudobrookites AB
2
O
5
(i.e. FeTi
2
O
5
, MgTi
2
O
5
,
Fe
2
TiO
5
, Al
2
TiO
5
, Cr
2
TiO
5
, Ti
3
O
5
. . .)
18–20
are isostructural
phases with orthorhombic symmetry (Cmcm spatial group)
21
and an extraordinary structure flexibility to accommodate many
distinct metals in their two different and distorted octahedral
cationic sites, M1 or A (4c) and M2 or B (8f). In this structure,
the strongly distorted octahedra share edges to form trioctahe-
dral units, which are linked into infinite double chains extending
along the longest axis (c).
18,22–25
In M
3+
2
Ti
4+
O
5
pseudo-
brookites (M
3+
= Fe, Al, Ga, Ti. . .) Ti is accommodated mainly
in M1 (4c) positions, which are less distorted and have a smaller
polyhedral volume,
22
whereas in M
2+
Ti
4+
2
O
5
pseudobrookites
(M
2+
= Mg, Fe, Co. . .) Ti occupies mainly M2 (8f) positions, the
less distorted in this case. However, this ideal ordered occupation
is uncommon due to the existence of an important cationic disor-
der between both positions (M1 and M2), resulting in a number
of solid solutions with general formula M
3+
2(1-x)
M
2+
x
Ti
1+x
O
5
or [M
3+
,M
2+
,Ti]
M1
[Ti,M
2+
,M
3+
]
M2
2
O
5
.
26
Therefore, pseudo-
brookites are entropy-stabilized phases
24,27,28
: the cationic
http://dx.doi.org/10.1016/j.jeurceramsoc.2014.08.010
0955-2219/© 2014 Elsevier Ltd. All rights reserved.