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2 Apr 2012

Volume 100, Issue 14, Articles (14xxxx)

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Appl. Phys. Lett. 100, 144101 (2012); http://dx.doi.org/10.1063/1.3697983 (4 pages)

H. Xu (徐涵), Wei Yu (余玮), M. Y. Yu (郁明阳), A. Y. Wong (黄燿煇), Z. M. Sheng (盛政明), M. Murakami (村上匡且), and J. Zhang (张杰)
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High-temperature phonon spectra of multiferroic BiFeO3 from inelastic neutron spectroscopy

Mohamed Zbiri, Helmut Schober, Narayani Choudhury, Ranjan Mittal, Samrath L. Chaplot, Sadequa J. Patwe, Srungarpu N. Achary, and Avesh K. Tyagi

Appl. Phys. Lett. 100, 142901 (2012); http://dx.doi.org/10.1063/1.3700745 (5 pages) | Cited 1 time

Online Publication Date: 4 April 2012

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We report inelastic neutron scattering measurements of the phonon spectra in a pure powder sample of the multiferroic material BiFeO3. A high-temperature range was covered to unravel the changes in the phonon dynamics across the Néel (TN ∼ 650 K) and Curie (TC ∼ 1100 K) temperatures. Experimental results are accompanied by ab initio lattice dynamical simulations of phonon density of states to enable microscopic interpretations of the observed data. The calculations reproduce well the observed vibrational features and provide the partial atomic vibrational components. Our results reveal clearly the signature of three different phase transitions both in the diffraction patterns and phonon spectra. The phonon modes are found to be most affected by the transition at the TC. The spectroscopic evidence for the existence of a different structural modification just below the decomposition limit (TD ∼ 1240 K) is unambiguous indicating strong structural changes that may be related to oxygen vacancies and concomitant Fe3+ → Fe2+ reduction and spin transition.
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63.20.-e Phonons in crystal lattices
77.80.-e Ferroelectricity and antiferroelectricity
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
61.72.jd Vacancies
71.20.-b Electron density of states and band structure of crystalline solids

Surface polar states and pyroelectricity in ferroelastics induced by flexo-roto field

A. N. Morozovska, E. A. Eliseev, S. V. Kalinin, Long Qing Chen, and Venkatraman Gopalan

Appl. Phys. Lett. 100, 142902 (2012); http://dx.doi.org/10.1063/1.3701152 (5 pages) | Cited 9 times

Online Publication Date: 4 April 2012

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Theoretical analysis based on the Landau-Ginzburg-Devonshire theory is used to show that the joint action of flexoelectric effect and rotostriction leads to a large spontaneous in-plane polarization (∼1-5 μC/cm2) and pyroelectric coefficient (∼10−3 C/m2K) in the vicinity of surfaces of otherwise non-ferroelectric ferroelastics, such as SrTiO3, with static octahedral rotations. The origin of the improper polarization and pyroelectricity is an electric field we name flexo-roto field whose strength is proportional to the convolution of the flexoelectric and rotostriction tensors with octahedral tilts and their gradients. Flexo-roto field should exist at surfaces and interfaces in all structures with static octahedral rotations, and thus, it can induce surface polar states and pyroelectricity in a large class of otherwise nonpolar materials.
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77.70.+a Pyroelectric and electrocaloric effects
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
73.20.At Surface states, band structure, electron density of states
77.22.Ej Polarization and depolarization
77.65.-j Piezoelectricity and electromechanical effects
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