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Phase transitions in nanocrystalline BaTiO3 and BaTiO3@SiO2 core-shell
nanostructures: NMR and EPR study
V.V. Laguta1*, C. Elissalde2, M. Maglione2, A.M. Artemenko2, V. Chlan3, H. Štěpánková3
1
Institute of Physics, AS CR
Cukrovarnicka 10, Prague, Czech Republic, 16253
2
Institute of Condensed Matter Chemistry, CNRS, University Bordeaux 1
Av. Dr. Schweitzer 87, Bordeaux, France, F – 33608 PESSAC
3
Faculty of Mathematics and Physics, Charles University
V Holesovickach 2, Prague, Czech Republic, 18000
*e-mail: [email protected]
The continuous trend in miniaturization in micro/nanoelectronics and telecommunications needs
ferroelectric materials at nanometric scale. Nanomaterials often show novel phenomena which can be
explored in novel device concepts. In this relation, BaTiO3 and its composites have attracted great interest
due to their excellent dielectric, piezoelectric and ferroelectric properties. Currently, there are numerous
studies of the size-dependent phenomena in BaTiO3 nanoparticles and nanograin ceramics. It was shown
that in general the temperature of the ferroelectric phase transition decreases from 394 K down to room
temperature at the particles critical size from about 80 to 120 nm. This size dependent phenomenon was
explained by increasing influence of mechanical surface stresses and depolarization fields on stability of
polar phase. Recently, BaTiO3 core-shell nanostructures were introduced in order to produce nanograin
ceramics with reduced dielectric losses [1]. An additional stresses from the dielectric shell is expected in
such nanostructure which can much stronger change the ferroelectric properties of nanoparticle. Such
objects are very suitable for investigation of crystal structure transformations induced by surface stresses.
We have investigated the phase transition sequence and crystallite size dependent phenomena in BaTiO3
nanoparticles and BaTiO3@SiO2 core-shall nanostructured ceramics with particle sizes from about 500 to
300 nm, where the ferroelectric phase transitions are still sharp and well seen. We used nuclear magnetic
and electron paramagnetic resonances as a sensitive probes of the local structure of nanoparticles. Both
methods do not need the long range atomic ordering and therefore they allow relatively easy to distinguish
different structural phases in small particles and their changes with the change of temperature and particles
size. Our main finding is that even in relatively large particles of BaTiO3, with average size of 300 nm,
there is marked influence of mechanical surface stresses on lattice parameters in vicinity of phase
transitions, especially at the low-temperatures [2]. The influence of the surface stresses is essentially
stronger in the core-shell nanostructured ceramics as compared to single particles. There is a coexistence of
adjacent ferroelectric phases in wide temperature regions. The most stable is the orthorhombic-like phase
which coexists with other polar phases in the core-shell nanostructured ceramics up to the Curie
temperature. Our results suggest that besides compressive surface tension, anisotropic stresses like shear
stress, can be dominated in the core-shell nanostructures which increase also global stability of polar state.
We summarize our results in a phase diagram.
Acknowledgements: the research was in part supported by the GA CR under project No. 13-11473S.
1. High-Frequency Dielectric Spectroscopy of BaTiO3 Core – Silica Shell Nanocomposites: Problem of
Interdiffusion, D. Nuzhnyy et al., Journal of Advanced Dielectrics, 1, 309 (2011).
2. Crystal structure transformations induced by surface stresses in BaTiO3 and BaTiO3@SiO2 nanoparticles
and ceramics, V.V. Laguta et al., Phase Transitions, http://dx.doi.org/10.1080/01411594.2014.996852.
Curriculum Vitae
Valentin Laguta obtained the MS degree from Kiev State University, then received the CSc and DrSc
degrees (Physics and Mathematics) from the Institute for Problems of Material Science of the
Ukrainian National Academy of Science in 1988 and 2006, respectively. Since 2006, he is leading
senior scientist of the Institute of Physics of the AS CR, Prague, head of EPR and NMR spectroscopy
group in the department of Optical materials. His recent research focuses on physical properties of
ferroelectric and magnetoelectric materials, defects and radiation-induced phenomena in solids.
Author of 170 original papers in refereed international journals. The publications received more than
2000 citations in the international literature.
19.05.2015 / Springer-Verlag, Physical Sciences & Engineering / Dr. W. Skolaut