considered as a fully disordered in B-site sublattice of perovskite [41, 60]. On the
other hand, large scattering of magnetic ordering temperature could be explained by
local compositional ordering and formation of Nb-poor-Fe-rich and
Nb-rich-Fe-poor regions as postulated in [59, 61, 62].
We should also mention that in PFN system Pb
2+ cations have also stereochemical active 6 s
2+ lone pairs which similarly like in BFO can stabilize the
non-centrosymmetric distortion of the lattice [12].
Room temperature Mössbauer spectrum is a doublet, which is a consequence of
the disorder in PFN which produces strongly inhomogeneous electric field. This is
related to substitution of Fe
3+ by Nb
5+ which differs considerably the formal
charges. The mean IS value is 0.41 mm/s, and mean QS is about 0.42 mm/s [63].
This implies that all iron is present as +3 in octahedral symmetry. The distribution
of quadrupole splitting at 300 K forms Gauss-like curve, indicating that arrangement of Fe
3+ cations is randomly distributed. At temperatures below Néel point,
Mössbauer effect spectra are composed of a number of Zeeman splitted sextets
which correspond to several iron environments. In PFN crystal structure, there is
only one inequivalent iron position. Therefore, the distribution of the magnetic
hyperfine field is due to differences in iron coordination related to disorder in
B-sites. Substitution of Fe
3+ by Nb
5+ in the nearest B-site surrounding influences
the superexchange interaction what in turn influences the iron magnetic moment.
Thus, the magnetic hyperfine field is disturbed. In this indirect way, the Mössbauer
spectroscopy can confirm the random distribution of cations in B-sites [12, 63]. On
the other hand, ab initio calculations of different configurations of cations in B-sites
of PFN showed the possibility of iron ions clustering in PFN [61].
9.3.1.3 Bi 0.5 Pb 0.5 (Fe 0.75 Nb 0.25 )O 3
Bismuth ferrite easily forms solid solutions with different magnetoelectric and
ferroelectric perovskite oxides [64–71]. This is mostly done to tune electrical or
magnetic properties of the final material and also from the fundamental point of
view. One of the examples of such solid solutions can be 0.5BiFeO 3 –0.5Pb
(Fe 0.5 Nb 0.5 )O 3 . This is a mixture of two magnetoelectrics in which the multiferroicity is realized in two different ways. From one side is BiFeO 3 where structural
distortion and the properties are governed by Bi
3+ 6s
2 lone pairs, ferroelectricity and
magnetism are related to different sublattices. On the other side is PFN were the
ferroelectricity and structural distortion is related to Nb
5+ empty d-shell which
influence is weakened by substitution of magnetic Fe
3+ . In this case, the magnetism
and ferroelectricity are related to the same sublattice. In solid solution, there are
substitutions of Bi
3+ by Pb
2+ cations in A-sites. Both of the cations have 6 s
2 lone
pairs which are stereochemically active and are able to stabilize the rhombohedral
distortion. The second substitution is realized in B-sites where magnetism related to
Fe
3+ is diluted by Nb
5+ cations which should increase the electric properties.
Bi 0.5 Pb 0.5 (Fe 0.75 Nb 0.25 )O 3 crystal structure is rhombohedral R3c, the same as
BiFeO 3 but the level of the distortion is smaller. It can be easily compared by a
290
P. Stoch and A. Stoch
other hand, large scattering of magnetic ordering temperature could be explained by
local compositional ordering and formation of Nb-poor-Fe-rich and
Nb-rich-Fe-poor regions as postulated in [59, 61, 62].
We should also mention that in PFN system Pb
2+ cations have also stereochemical active 6 s
2+ lone pairs which similarly like in BFO can stabilize the
non-centrosymmetric distortion of the lattice [12].
Room temperature Mössbauer spectrum is a doublet, which is a consequence of
the disorder in PFN which produces strongly inhomogeneous electric field. This is
related to substitution of Fe
3+ by Nb
5+ which differs considerably the formal
charges. The mean IS value is 0.41 mm/s, and mean QS is about 0.42 mm/s [63].
This implies that all iron is present as +3 in octahedral symmetry. The distribution
of quadrupole splitting at 300 K forms Gauss-like curve, indicating that arrangement of Fe
3+ cations is randomly distributed. At temperatures below Néel point,
Mössbauer effect spectra are composed of a number of Zeeman splitted sextets
which correspond to several iron environments. In PFN crystal structure, there is
only one inequivalent iron position. Therefore, the distribution of the magnetic
hyperfine field is due to differences in iron coordination related to disorder in
B-sites. Substitution of Fe
3+ by Nb
5+ in the nearest B-site surrounding influences
the superexchange interaction what in turn influences the iron magnetic moment.
Thus, the magnetic hyperfine field is disturbed. In this indirect way, the Mössbauer
spectroscopy can confirm the random distribution of cations in B-sites [12, 63]. On
the other hand, ab initio calculations of different configurations of cations in B-sites
of PFN showed the possibility of iron ions clustering in PFN [61].
9.3.1.3 Bi 0.5 Pb 0.5 (Fe 0.75 Nb 0.25 )O 3
Bismuth ferrite easily forms solid solutions with different magnetoelectric and
ferroelectric perovskite oxides [64–71]. This is mostly done to tune electrical or
magnetic properties of the final material and also from the fundamental point of
view. One of the examples of such solid solutions can be 0.5BiFeO 3 –0.5Pb
(Fe 0.5 Nb 0.5 )O 3 . This is a mixture of two magnetoelectrics in which the multiferroicity is realized in two different ways. From one side is BiFeO 3 where structural
distortion and the properties are governed by Bi
3+ 6s
2 lone pairs, ferroelectricity and
magnetism are related to different sublattices. On the other side is PFN were the
ferroelectricity and structural distortion is related to Nb
5+ empty d-shell which
influence is weakened by substitution of magnetic Fe
3+ . In this case, the magnetism
and ferroelectricity are related to the same sublattice. In solid solution, there are
substitutions of Bi
3+ by Pb
2+ cations in A-sites. Both of the cations have 6 s
2 lone
pairs which are stereochemically active and are able to stabilize the rhombohedral
distortion. The second substitution is realized in B-sites where magnetism related to
Fe
3+ is diluted by Nb
5+ cations which should increase the electric properties.
Bi 0.5 Pb 0.5 (Fe 0.75 Nb 0.25 )O 3 crystal structure is rhombohedral R3c, the same as
BiFeO 3 but the level of the distortion is smaller. It can be easily compared by a
290
P. Stoch and A. Stoch