First principles based simulation of the ferroelectric to relaxor

First principles based simulation of the ferroelectric to relaxor

transition as a function of pressure in Pb(Sc_1/2Nb_1/2)O₃

S. Tinte, B.P. Burton, E. Cockayne¹,

U.V. Waghmare²

¹Ceramics Division, Materials Science and Engineering Laboratory,

NIST, Gaithersburg, MD 20899-8520, USA

²J. Nehru Theoretical Sciences Unit, JNCASR, Jakkur, Bangalore, 560 064, INDIA

In general, increasing pressure depresses the transition temperature of ferroelectric transitions T_FE, and sufficient pressure yields a ferroelectric → paraelectric transition. A more complicated behavior is observed for disordered Pb(Sc_1/2Nb_1/2)O₃ or PSN.

At atmospheric pressure, PSN exhibits relaxor ferroelectric (RFE) properties within a small temperature range before transforming to a FE phase at low temperatures. Pressure of approximately 1.5 to 2.5 GPa induces a low-temperature RFE state without RFE/FE phase boundary.

To investigate the effects of pressure on phase transitions in PSN, we perform molecular dynamics simulations using a first-principles based effective Hamiltonian. This model combines an effective Hamiltonian for a normal FE with a local electric field term that includes the fields at the Pb-sites arising from the charge difference between the Sc³⁺ and Nb⁵⁺ions. In this treatment, pressure acts on the strain coupling term to reduce the stability of the FE phase, and therefore, increases the relative importance of the local electric field term, which is essentially pressure independent. Thus, as pressure is increased in an incipient relaxor such as PSN, the system is driven away from a normal FE and towards a relaxor state.

Our molecular dynamics simulations clearly show the prediction of a transition from a system with a normal FE ground state to one with full relaxor properties when pressure is increased. The results presented here qualitatively confirm the transition observed experimentally. Quantitatively however, the model predicts a too high transition pressure compared to the experimental one.

Silvia Tinte

Mentor: Benjamin Burton

Mail Stop 8500

A317 / Bldg 223

Phone 301-975-5647/5716

Fax 301-975-5012

silvia.tinte@nist.gov

Sigma Xi member: no

Category: Materials