rhombohedral angle which is 59.971° whereas in BiFeO 3 the angle is 59.348° and
for cubic, in the rhombohedral coordinate system the angle is 60°. If we assume that
the distortion is only caused by the lone pairs, it means that the A-site substitutions
lead to a weakening of the lone pair mechanism. Similarly like in BiFeO 3 in the
material, there are three Bi–O and Pb–O bonds which are considerably shorter than
the rest. Ab initio calculations confirm the highly covalent character of this shorter
bonds which is a driving force of the rhombohedral distortion. The Pb–O bond
covalency is lower than the Bi–O and in this way, the distortive force is reduced and
as a result, the distortion is smaller [12].
B-Site Disorder
Mössbauer spectroscopy can be used to check whether the arrangement of ions at
the B-site is random or ordered. In PFN, the answer to this problem is not so clear,
and there are measurements which confirmed both of the possibilities [41, 59, 60,
62]. In PFN, the B-sites are equally occupied by Nb
5+ and Fe
3+ cations. This system
is different; here the magnetic sublattice is only partially diluted by non-magnetic
ions and most (75%) of the ions are magnetic.
Mössbauer spectroscopy is sensitive to fine changes of the local environment
caused by the first and even second neighbors of tested iron atoms. In crystalline
compounds, every crystallographic iron site could be observed as one component of
the spectrum. In case of disordered systems, there are many different local iron
environments and every one of them is described by a different set of hyperfine
interaction parameters. As a consequence, the collected spectrum is a composition
of many individual components. In the studied solid solution, although iron could
be present only in one inequivalent site, the Fe
3+ /Nb
5+ substitution lead to disorder
in the B-sublattice. Every change in the local B-site order should reflect the
observed effect and the hyperfine interaction parameters. In the case of disordered
systems, one method for processing the spectra is an Extended Voigtian-Based
analysis (xVBF) [72, 73]. In this method, every iron atom site is given by the
Gaussian distribution of independently isomer shift (IS) , quadrupole splitting (QS),
and hyperfine field (B hf ) due to some randomness in the local environment of iron.
In the considered system, there are at least two sources of the randomness, first
are local environment changes, but the second can be fluctuations of the iron
magnetic moments. So to check the local environmental disorder, one needs to do
the measurements in conditions in which magnetic moments will not change or the
changes will be negligible. This can be done by measuring the sample in a magnetic
saturation at a low temperature far away from magnetic ordering point. The Néel
temperature of Bi 0.5 Pb 0.5 (Fe 0.75 Nb 0.25 )O 3 is about 460 K. So one can assume that at
liquid nitrogen (77 K), the sample is in a magnetic saturation. The
57 Fe Mössbauer
effect spectrum (Fig. 9.5a) is Zeeman splitted sextet and at this temperature, all iron
ions are in the magnetically ordered state. Due to one iron site in the crystal
structure, only one component was used in which IS, QS, and B hf parameters were
9 Mössbauer Spectroscopy of Magnetoelectric Perovskite Oxides
291
for cubic, in the rhombohedral coordinate system the angle is 60°. If we assume that
the distortion is only caused by the lone pairs, it means that the A-site substitutions
lead to a weakening of the lone pair mechanism. Similarly like in BiFeO 3 in the
material, there are three Bi–O and Pb–O bonds which are considerably shorter than
the rest. Ab initio calculations confirm the highly covalent character of this shorter
bonds which is a driving force of the rhombohedral distortion. The Pb–O bond
covalency is lower than the Bi–O and in this way, the distortive force is reduced and
as a result, the distortion is smaller [12].
B-Site Disorder
Mössbauer spectroscopy can be used to check whether the arrangement of ions at
the B-site is random or ordered. In PFN, the answer to this problem is not so clear,
and there are measurements which confirmed both of the possibilities [41, 59, 60,
62]. In PFN, the B-sites are equally occupied by Nb
5+ and Fe
3+ cations. This system
is different; here the magnetic sublattice is only partially diluted by non-magnetic
ions and most (75%) of the ions are magnetic.
Mössbauer spectroscopy is sensitive to fine changes of the local environment
caused by the first and even second neighbors of tested iron atoms. In crystalline
compounds, every crystallographic iron site could be observed as one component of
the spectrum. In case of disordered systems, there are many different local iron
environments and every one of them is described by a different set of hyperfine
interaction parameters. As a consequence, the collected spectrum is a composition
of many individual components. In the studied solid solution, although iron could
be present only in one inequivalent site, the Fe
3+ /Nb
5+ substitution lead to disorder
in the B-sublattice. Every change in the local B-site order should reflect the
observed effect and the hyperfine interaction parameters. In the case of disordered
systems, one method for processing the spectra is an Extended Voigtian-Based
analysis (xVBF) [72, 73]. In this method, every iron atom site is given by the
Gaussian distribution of independently isomer shift (IS) , quadrupole splitting (QS),
and hyperfine field (B hf ) due to some randomness in the local environment of iron.
In the considered system, there are at least two sources of the randomness, first
are local environment changes, but the second can be fluctuations of the iron
magnetic moments. So to check the local environmental disorder, one needs to do
the measurements in conditions in which magnetic moments will not change or the
changes will be negligible. This can be done by measuring the sample in a magnetic
saturation at a low temperature far away from magnetic ordering point. The Néel
temperature of Bi 0.5 Pb 0.5 (Fe 0.75 Nb 0.25 )O 3 is about 460 K. So one can assume that at
liquid nitrogen (77 K), the sample is in a magnetic saturation. The
57 Fe Mössbauer
effect spectrum (Fig. 9.5a) is Zeeman splitted sextet and at this temperature, all iron
ions are in the magnetically ordered state. Due to one iron site in the crystal
structure, only one component was used in which IS, QS, and B hf parameters were
9 Mössbauer Spectroscopy of Magnetoelectric Perovskite Oxides
291