Phenomena And Mechanism On Phase Transformation Twinning In Nanocrystalline BaTiO 3
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1256-N06-49
Phenomena And Mechanism On Phase Transformation Twinning In Nanocrystalline BaTiO3 Sujata Mazumder & Jiten Ghosh Analytical Facility Division, Central Glass & Ceramic Research Institute CSIR, Kolkata ,West Bengal,India
ABSTRACT The detailed structure of nanocrystalline BaTiO3 powder during ball milling has been studied using XRD & TEM. The study illustrates important advances in understanding atomic scale properties of this material. Ferroelectric BaTiO3 powder undergoes phase transformation along the sequence Cubic(Pm3m)-tetragonal(P4mm)-orthohombic (Amm2)-rhombohedral(R3m) structure when pressureless sintered samples are cooled from high temperature to low temperature. The high to low symmetry phases are not related to group subgroup symmetry as transformation is discontinuous and first order in nature and the twin relationship in the low symmetry is forbidden by Landau theory. In case of ball milled BaTiO3 powder a continuous and diffusionless phase transition occur via second order to and from a metastable intermediate phase. In this pathway crystallites in the aggregation are twinned and the twin structure is related to crystal point group m3m which in the present case is illustrated as having 6mm symmetry formed under low driving force. The unit cell evolution due to phase transition and the crystallographic relationship are established.The phase transformation, coalescence and twin structure of thermally annealed BaTiO3 nanocrystals under high vacuum has been investigated using in situ high temperature XRD. The structure analysis is performed with the use of the method of computer modelling of disorder structure and simulation of corresponding diffraction pattern. INTRODUCTION BaTiO3 is a well-known perovskite structure belonging to displacive category[ 1 ] and depending on temperature it undergoes phase transformation along the sequence Cubic(Pm3m)tetragonal(P4mm)-orthohombic(Amm2)-rhombohedral(R3m)phase. Displacive transformation, in which there occurs a coordinated motion of atoms , the transformation is diffusionless. The dominant process in diffusionless transformation of one structure into other causes enormous strain and in order to avoid these intolerable strain, the martensite either slip or undergoes transformation twinning. Phase transformation induced twinning in perovskites eg CaTiO3 [2,3], La GaO3 [4,5], BaTiO3 [6] are well characterised. Twinning may occur as a consequence of a phase transition from high to low symmetry phase and twinning produced from symmetry change are derived on the basis of symmetry element loss of phase that are not related crystallographically by group subgroup symmetry [7]. The Landau theory [8,9] forbids transformation domain in lower symmetry phases that exhibit twin relationship when structural coherence is lost upon first order transition. According to Christy [10 ] for successful bridging two nongroup subgroup related phases without jeopardizing the restriction, the phase transformation can still be modelled thermodynamically using Landau formalism. A
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