Computer simulation of ferroelastic phase transition in LaNbO 4
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Computer simulation of ferroelastic phase transition in LaNbO4 K. Parlinskia) Institute for Materials Research, Tohoku University, Sendai 980-77, Japan
Y. Hashi Research and Development Center, Hitachi Tohoku Software, Ltd., Sendai 980, Japan
S. Tsunekawa and Y. Kawazoe Institute for Materials Research, Tohoku University, Sendai 980-77, Japan (Received 12 February 1996; accepted 24 March 1997)
A model of lanthanum orthoniobate which possesses a ferroelastic tetragonalmonoclinic phase transition is proposed. It contains only one particle per unit cell, but it is constructed consistently with symmetry changes at the phase transition. The model parameters are chosen to reproduce the bare soft mode, degree of deformation of the tetragonal unit cell to a monoclinic one, and the phase transition temperature. The ferroelastic system with free boundary conditions was simulated by the molecular dynamics technique, and the second order phase transition was reproduced. The studied annealing process shows formation of the stripe lenticular domain pattern, which has been interrupted by the appearance of a temporary band of perpendicularly oriented lenticular domains. The maps contain W′-type domain walls whose orientations are fixed only by interplay of potential parameters and not by symmetry elements. The simulated domain pattern has the same features as those observed by transmission electron microscopy.
I. INTRODUCTION
Many rare-earth orthoniobates are isostructural with BiVO4 and mineral fergusonite YbNbO4 . One of them is the lanthanum orthoniobate, LaNbO4 , which undergoes a ferroelastic phase transition at Tc 768 K. Above Tc the crystal belongs to tetragonal body-centered structure 6 described by the nonsymmetric space group I41ya sC4h d with two molecules per elementary unit cell sZ 2d,1 as shown in Fig. 1(a). Below Tc the crystal belongs 6 to monoclinic space group I2ya sC2h d sZ 2d with approximately the same volume of the unit cell. The phase transition of LaNbO4 is considered to be a proper ferroelastic one with the order parameter being the elastic strain. The reduction of symmetry from the space group I41ya to I2ya occurs through the Bg irreducible representation labeled by the wave vector from the center of the Brillouin zone.2 The x 2 2 y 2 and xy shear components transform according to Bg representation. The list of symmetry elements of the high-symmetry space group I41ya does not contain any mirror plane that is parallel to the fourfold symmetry axis of the tetragonal structure. Therefore, this phase transition, according to Sapriel’s theory,3 could produce in the monoclinic phase only two sets of domain walls of type W 0 , which orientation is not fixed by any mirror plane. a)
On leave from the Institute of Nuclear Physics, ul. Radzikowskiego 152, 31-342 Cracow, Poland.
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http://journals.cambridge.org
J. Mater. Res., Vol. 12, No. 9, Sep 1997
Downloaded: 12 Mar 2015
In the unstrained lattice, two domain wall sets of W 0 would be perpendicular to
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