By changing the size and the shape of ferroelectric nanoparticles, one can govern their polar properties including their improvements in comparison with the bulk prototypes. At that the shift of the ferroelectric transition temperature can reach as much as hundreds of Kelvins. Phenomenological description of these effects was proposed in the framework of Landau-Ginsburg-Devonshire (LGD) theory using the conceptions of surface tension and surface bond contraction. However, this description contains a series of poorly defined parameters, which physical nature is ambiguous. It is appeared that the size and shape dependences of the phase transition temperature along with all polar properties are defined by the nature of the size effect.Existing LGD-type models do not take into account that defects concentration strongly increases near the particle surface. In order to develop an adequate phenomenological description of size effects in ferroelectric nanoparticles, one should consider Vegard strains (local lattice deformations) originated from defects accumulation the near surface.In the paper we propose a theoretical model that takes into account Vegard strains and perform a detailed quantitative comparison of the theoretical results with experimental ones for quasi-spherical nanoparticles, which reveal the essential (about 100 K) increase of the transition temperature in spherical nanoparticles in comparison with bulk crystals. The average radius of nanoparticles was about 25 nm, they consist of KTa 1х Nb х O 3 solid solution, where KTaO 3 is a quantum paraelectric, while KNbO 3 is a ferroelectric. From the comparison between the theory and experiment we unambiguously established the leading contribution of Vegard strains into the extrinsic size effect in ferroelectric nanoparticles. We determined the dependence of * Corresponding author 1: anna.n.morozovska@gmail.com † Corresponding author 2: isgolovina@ukr.net 1 Vegard strains on the content of Nb and reconstructed the Curie temperature dependence on the content of Nb using this dependence. Appeared that the dependence of the Curie temperature on the Nb content becomes non-monotonic one for the small (< 20 nm) elongated KTa 1-х Nb х O 3 nanoparticles. We established that the accumulation of intrinsic and extrinsic defects near the surface can play the key role in the physical origin of extrinsic size effect in ferroelecric nanoparticles and govern its main features.

The paper reports on thorough Raman-scattering study of newly synthesized nanoscale powders of potassium niobate (KNbO3) aimed at evaluating the possible modification of the phase transition temperatures versus the corresponding bulk values. We register a significant expansion of the temperature ranges of all phase transitions, which is attributed to a wide distribution of particle sizes. An average temperature of each phase transition shifts to the high-temperature region differently, namely by 10°, 25°, and 40°, as compared with corresponding transition temperature in bulk crystals of KNbO3 and amounts 0, 248, and 475 °C, respectively. In the range from 100 to 110 °C, we also find the features, which could be associated with an additional rearrangement of the structure.

Newly synthesized undoped and iron-doped nanoscale powders of KNbO3 are investigated using magnetic resonance and static magnetization methods in order to determine how the crystal size and doping affect the structure of magnetic defects and material properties. Although the bulk crystals of KNbO3 are nonmagnetic, the undoped KNbO3 powder with average particle size of 80 nm exhibits magnetic properties. The ferromagnetic resonance signal and the magnetization curve registered on the powder are thoroughly analyzed. It is concluded that the appearance of the defect driven ferromagnetism in the undoped powder is due to the nano-size of the particles. This effect disappears in the iron-doped KNbO3 powder with particle sizes above 300 nm. In case of low doping (&lt;1 mol. % Fe), a new electron paramagnetic resonance signal with geff = 4.21 is found out in the KNbO3:Fe powder. Such a signal has not been observed in the bulk crystals of KNbO3:Fe. We suppose that this signal corresponds to individual paramagnetic Fe3+ ions having rhombic symmetry.

Magnetic defects in non‐doped and iron‐doped nanopowders of KTaO3 were studied by resonance and static magnetic measurements. The resonance spectra and hysteresis loops of KTaO3:Fe nanocrystalline powders were observed and investigated for the first time. Unlike bulk crystals, both non‐doped and doped nanopowders exhibit two types of resonance lines, paramagnetic ones from the isolated Fe3+ ions and a ferromagnetic signal from ions involved in clusters. Theoretical description and full identification of the electron paramagnetic resonance (EPR) spectra were made. It was established that low doping (up to 0.4 mol%) increases the number of paramagnetic Fe3+ centers with axial and rhombic symmetries, and has no effect on the centers with cubic symmetry and magnetic clusters. Analysis of the temperature dependence of the spectra showed that the cluster's resonance line follows the Bloch law “T3/2”, which indicates a ferromagnetic nature of the cluster. A partial collapse of the ferromagnetic subsystem in the temperature range of 100–400 K was also found. Magnetization and other characteristics of ferromagnetic and paramagnetic subsystems were determined. An average cluster size was estimated to be equal to a length of 6.2 nm and the fractional volume of the ferromagnetic cluster system was found.

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