Dielectric, Structural and Raman Studies on (1-x)
Na0.5Bi0.5TiO3-xBiCrO3 Ceramic
Local Structure and Raman Spectrum of Na0.5Bi0.5TiO3M. K.
SinghLaser Materials Development and Devices DivisionRaja Ramanna
Centre for Advanced Technology Indore 452 013 (INDIA)1Sodium
Bismuth Titanate (Na0.5Bi0.5TiO3)Structure: PerovskiteCrystal
Class: RhombohedralSpace Group: R3c (at 300K)Lattice Parameter:
a=5.48, c=13.42 (R3cH)Structural Transitions in NBT: Pm3m P4bm
(P4/mbm) R3c (>540oC) (540-500oC) (500oC- 400oC) (400-300oC)
(< 300oC)Dielectric Anomalies in NBT: Tm = 300oC, TRT = 275oC,
Td = 180oC, G. O. Jones and P. A. Thomas, Acta Cryst. (2002). B58,
168-178Y. Hiruma, H. Nagata, and T. Takenaka, J. Appl. Phys. 104
(2008) 124106 2Literature GroupMethod SystemSpace groupJ. A.
Zvirgzds (1982)X-ray diffractionRhom.R3mS. B. Vakhrushev
(1983)Neutron diffractionRhom.R3cSoukhojak et al. (2000)Electron
diffractionTetra.-----Jones et al (2000)Neutron
diffractionTetra.P4bmJones et al (2002)Neutron
diffractionRhom.R3cA. Pietraszko et al (2006)X-ray
diffractionRhom.R3cBalagurov, A. M. (2006)Neutron
diffractionRhom.+Tetra.----------E. Aksel et al (2011)X-ray
diffraction
MonoclinicCc3GroupSpace groupRpRwp
Jones et al (2000)P4bm
5.016.47Jones et al (2002)R3c5.907.80E. Aksel
(2011)Cc5.878.09Our resultR3c9.3011.40
Ambiguity in structure, calculated from X-ray/Neutron
DiffractionAb-initio calculation will provide better understanding
of crystal structure4Objective Calculation of local structure of
NBT using ab-initio method and comparison with Raman and XRD
data5Structure modeling of NBT
Na0.5Bi0.50NBT constitutes a complex perovskite structure with
two different ions Na+ and Bi3+ occupy at A site.In R3c structure,
we argued that two different ions at A-site cant occupy the
symmetrically equivalent position. It suggests that the structure
should be lower that R3c. Ti6Computational DetailsFirst-principles
calculations have been performed within the generalized gradient
approximation (GGA) using PW91 exchange-correlation functional to
density functional theory as implemented in CRYSTAL09 code.Tight
convergence criteria are used to obtain fully relaxed total energy
calculations. We used following norm-conversing pseudo-potentials
for Bi, Na , Ti and O.Bi: 5d10 6s2 6p3, Na:2p6 3s1, Ti: 3s2 3p6 4s2
3d2 and O:2s2 2p4An 8x8x8 Monkhorst-Pack k-point mesh for the
primitive cell is used for Brillouin zone integration. In all the
calculation plane-wave energy cutoff is set to be 770 eV resulting
in a numerical error of less than 1meV/atom.7
In R3 structureIn R3m structure
We converted the structure in P1 symmetry and the resulting
configuration is structurally optimized. R3 or R3m structures are
obtained from the optimized structure.8Raman scattering
intensitiesRaman scattering is the inelastic scattering of photons.
The monochromatic laser light interacts with molecular vibrations
(i.e. phonons), resulting in the energy of the laser photons being
shifted up or down. This shift of energy gives crucial information
about the vibrational modes in the system.
Using Raman intensity calculation, it is possible to identify
the symmetry and chemical structure of the system.9Calculation of
Raman scattering intensities of NBT in R3 and R3m structures
Vibration modes exist between 80 930cm-110
Calculation of Raman scattering intensities for R3c structureNa+
0.5Bi3+ 0.50.5x1+0.5x3=2Ba2+ 1.0Na0.5Bi0.5TiO3
For BiFeO3 having R3c symmetry the vibration modes predicted
between 95 590cm-1 H. M. Ttnca and G. P. Srivastava, JAP, 1 03
(2008) 083712Vibration modes exist between 120 500cm-111
Raman spectrum of NBT12
13Vibrational modes at higher frequency
14
Bi-O bond length2.273 In R3c structure Bi-O bond lengths all
greater than 2.53 . However the experimentally measured shortest
bond length from EXAFS study is 2.2 . Our calculated value of the
shortest bond length (Bi-O=2.25 ) in R3 structure is in good
agreement with the experimental value.150.85NBT-0.15BiCrO3
16ConclusionLocal structure of NBT is R3/R3m or lower
symmetry.The high frequency modes in Raman spectrum are intrinsic
feature of NBT.17Thank You18
