Isovalent Doping of Structure Type Inverse Spinel: an
Analysis of the Structural Effect of Cr(III) Insertion at
High Levels Via Rietveld Method
J. A. Sobrinho1, A. C. S. Silva2, C. Polini1, S. Lanfredi1, M. A. L. Nobre1, A. M. Pires1
1
FCT, Univ Estadual Paulista, UNESP, Presidente Prudente, SP, Brazil
2
IQAr, Univ Estadual Paulista, UNESP, Araraquara, SP, Brazil
Zn7Sb2O12 crystallizes in the cubic structure of type inverse spinel. This host structure can
compose solid solutions with a large quantities and broad variety of cations. As an example,
cations chromium are one of the most influential cations on the microstructural evolution of
ZnO varistors [1]. In specific, the insertion of Cr(III) at Zn7Sb2O12 is of great interest, since
Cr(III) exhibits 3d electrons providing strong interaction with crystalline lattice via crystalline
field. The aim of this work is to apply the Rietveld method refinement using the program
FullProf as a further analysis tool of the influence of the doping with Cr(III). The crystalline
stability of the spinel under doping with 2 to 10% of Cr(III) was evaluated. Solid solutions
were prepared from Pechini modified method [2]. Experimental XDR patterns for the samples
with different doping levels were analyzed taking into account original parameters listed in
JCPDS card 74-1858. The refinement procedure was carried out for the unit cell parameters
and FWHM (full-width-at-half-maximum). Both RWP and REXP parameters indicate a good
agreement between observed and calculated profiles. There is a decreasing of cell parameters
and by consequence the volume cell, as a function of increase of doping fraction suggesting a
retraction of the unit cell, which may be related to the effect size of cation and/or stabilizing
champ. Furthermore, from the curves of FWHM versus POSN (position of 2 for each
diffraction line) graphics, shows in Fig. 1, the 2% doped sample, with no minor phase, seems
has the smallest crystallite size. At both doping levels with 8 and 10%, the ZnSb2O6 phase
was also observed, have similar profiles,
0,20
Cr(III) 2 at. %
Cr(III) 4 at. %
being that one doped with 10% have smaller
0,18
Cr(III) 8 at. %
Cr(III) 10 at. %
crystallite size. The undoped sample and
0,16
Zn Sb O
sample doped with 2% show similar curve
0,14
shape, but the doped one having the smallest
0,12
crystallite size and the undoped one presenting
0,10
secondary phase formation. Summarizing,
0,08
XRD confirmed that powders exhibit two
0,06
types of symmetry. Zn7Sb2O12 phase with
0
20
40
60
80
100
POSN (2 - degree)
cubic symmetry occurs for all samples,
Fig.1:
Graphs
of HW(full-width-at-half-maximum)
(half-width) versus POSNversus
(hkl positions)
Fig.1:
Graphs
of FWHM
POSN (hkl positions) however
the undoped one shows the
formation of β-Zn7Sb2O12 orthorhombic phase as well and the solid solutions doped with 8
and 10% also have the ZnSb2O6 one, both as minor phases. The last phase, ZnSb2O6, seems
an effect of the occupation of octahedral Zn(II) sites. On the other hand, the ones doped with
2 at% and 4 at% show exclusively Zn7Sb2O12 phase. The addition of Cr(III) actuates on the
non-isotropic contraction of the unit cell that trigger change of symmetry.
2
12
FWHM
7
Keywords: Chromium(III) , Inverse Spinel, Rietveld Refinement.
Work supported by FAPESP, CAPES and CNPq.
[1] Inada, M. Japonese Journal of Applied Physics, 1978, 17, 1.
[2] Silva, A. C. S. Souza, G. G. Nobre, M. A. L. Pires, A. M. Journal of Material Science. 45, 4216, (2010).
E-mail: [email protected]. Address: P. O. Box 467, 19060-900, Presidente Prudente, SP.