using the Brillouin function for S = 5/2. The isomer shift value at room temperature
is 0.38 mm/s [50] which is a value characteristic for high spin Fe
3+ . More precise
measurements have shown that although in BiFeO 3 structure there is only one iron
position, the spectrum can be fitted with the application of two Zeeman sextets. This
means that there are two different iron positions which give two sets of hyperfine
interaction parameters. These two positions are characterized by the same values of
magnetic hyperfine field and isomer shift parameter but different values of the
quadruple split. The origin of this behavior is not clear up to now but it could be due
to a distribution in the direction of the magnetic moments relative to the crystal axis
caused by the canting spin structure of BFO [50].
9.3.1.2 Pb(Fe 0.5 Nb 0.5 )O 3
Lead iron niobate Pb(Fe 0.5 Nb 0.5 )O 3 (PFN) is another type of magnetoelectric
material which belongs to a group of the independent multiferroics system, in
which magnetoelectric properties are realized by mixing magnetically and ferroelectric active cations in the same sublattice.
PFN belongs to relaxor ferroelectrics which are a large group of ferroelectric
perovskites with a general formula Pb(B 1 B 2 )O 3 in which B 1 can be Mg
2+ , Ni
2+ ,
Zn
2+ , Fe
3+ , Sc
3+ , and B 2 is Nb
5+ , Ta
5+ , W
6+ . All of them have mixed valence
cations at the B-sites. The relaxor ferroelectrics differ from the conventional ferroelectric in that the relative permittivity shows a wide diffuse peak which position
is frequency dependent on temperature [51], whereas non-relaxor ferroelectric are
characterized by a sharp transition of the relative permittivity at Curie temperature.
Contrary to the normal ferroelectrics, the crystal structure does not change significantly in the ferroelectric–paraelectric transition region. In case of PFN, this region
is in the temperature range of 370–380 K [52].
At room temperature, PFN adopts ferroelectric trigonal R3 m structure which
has a rhombohedral unit cell in which Fe
3+ and Nb
5+ atoms occupy 3a sites and are
distributed randomly. In hexagonal representation of the unit cell, the cell parameters are a = b = 5.6729 Å and c = 6.9493 Å [53, 54]. The spontaneous polarization lies along the pseudocubic [1 1 1] direction. In this phase, there is no
octahedral tilt. On the other hand, it should be noted that according to some authors
the PFN unit cell at room temperature is rather monoclinic [29, 55, 56].
Magnetic properties of PFN are characterized by two diffuse transitions at 150 K
and around 10 K [54, 57, 58]. Below 150 K PFN is similarly like BFO a G-type
antiferromagnetic in which the magnetic moments of Fe
3+ have aligned normally
the [1 1 1] direction [53, 57, 59]. Less clear is the origin of the low-temperature
magnetic anomaly which is believed to be a magnetic glass phase transition [52,
54]. In the unit cell, the Fe–O–Fe angle is 180° which leads to a strong superexchange interaction. The magnetic properties are dependent on the number of Fe–O–
Fe linkages in a crystal lattice [32]. The number of the linkages can be governed by
changing the Nb
5+ /Fe
3+ ratio or/and degree of Fe
3+ and Nb
5+ cations ordering.
However, no such ordering have been observed and Pb(Fe 0.5 Nb 0.5 )O 3 could be
9 Mössbauer Spectroscopy of Magnetoelectric Perovskite Oxides
289
is 0.38 mm/s [50] which is a value characteristic for high spin Fe
3+ . More precise
measurements have shown that although in BiFeO 3 structure there is only one iron
position, the spectrum can be fitted with the application of two Zeeman sextets. This
means that there are two different iron positions which give two sets of hyperfine
interaction parameters. These two positions are characterized by the same values of
magnetic hyperfine field and isomer shift parameter but different values of the
quadruple split. The origin of this behavior is not clear up to now but it could be due
to a distribution in the direction of the magnetic moments relative to the crystal axis
caused by the canting spin structure of BFO [50].
9.3.1.2 Pb(Fe 0.5 Nb 0.5 )O 3
Lead iron niobate Pb(Fe 0.5 Nb 0.5 )O 3 (PFN) is another type of magnetoelectric
material which belongs to a group of the independent multiferroics system, in
which magnetoelectric properties are realized by mixing magnetically and ferroelectric active cations in the same sublattice.
PFN belongs to relaxor ferroelectrics which are a large group of ferroelectric
perovskites with a general formula Pb(B 1 B 2 )O 3 in which B 1 can be Mg
2+ , Ni
2+ ,
Zn
2+ , Fe
3+ , Sc
3+ , and B 2 is Nb
5+ , Ta
5+ , W
6+ . All of them have mixed valence
cations at the B-sites. The relaxor ferroelectrics differ from the conventional ferroelectric in that the relative permittivity shows a wide diffuse peak which position
is frequency dependent on temperature [51], whereas non-relaxor ferroelectric are
characterized by a sharp transition of the relative permittivity at Curie temperature.
Contrary to the normal ferroelectrics, the crystal structure does not change significantly in the ferroelectric–paraelectric transition region. In case of PFN, this region
is in the temperature range of 370–380 K [52].
At room temperature, PFN adopts ferroelectric trigonal R3 m structure which
has a rhombohedral unit cell in which Fe
3+ and Nb
5+ atoms occupy 3a sites and are
distributed randomly. In hexagonal representation of the unit cell, the cell parameters are a = b = 5.6729 Å and c = 6.9493 Å [53, 54]. The spontaneous polarization lies along the pseudocubic [1 1 1] direction. In this phase, there is no
octahedral tilt. On the other hand, it should be noted that according to some authors
the PFN unit cell at room temperature is rather monoclinic [29, 55, 56].
Magnetic properties of PFN are characterized by two diffuse transitions at 150 K
and around 10 K [54, 57, 58]. Below 150 K PFN is similarly like BFO a G-type
antiferromagnetic in which the magnetic moments of Fe
3+ have aligned normally
the [1 1 1] direction [53, 57, 59]. Less clear is the origin of the low-temperature
magnetic anomaly which is believed to be a magnetic glass phase transition [52,
54]. In the unit cell, the Fe–O–Fe angle is 180° which leads to a strong superexchange interaction. The magnetic properties are dependent on the number of Fe–O–
Fe linkages in a crystal lattice [32]. The number of the linkages can be governed by
changing the Nb
5+ /Fe
3+ ratio or/and degree of Fe
3+ and Nb
5+ cations ordering.
However, no such ordering have been observed and Pb(Fe 0.5 Nb 0.5 )O 3 could be
9 Mössbauer Spectroscopy of Magnetoelectric Perovskite Oxides
289