ABSTRACT
Sodium niobate (NaNbO3, or NNO) is known to be antiferroelectric at temperatures between 45 and 753 K. Here we show experimentally the presence of the ferroelectric phase at temperatures between 100 and 830 K in the NNO crystals obtained by top-seeded solution growth. The ferroelectric phase and new phase transitions are evidenced using a combination of thermo-optical studies by variable angle spectroscopic ellipsometry, Raman spectroscopy analysis, and photoelectron emission microscopy. The possibility for strain-induced ferroelectricity in NNO is suggested.
Subject(s)
Crystallization/methods , Magnetic Fields , Niobium/chemistry , Sodium/chemistry , Materials Testing , Molecular Conformation , Phase TransitionABSTRACT
The lattice dynamics of La(1/2)Na(1/2)TiO(3) single crystal have been investigated by far-infrared and Raman scattering spectroscopy. Far-IR reflectivity spectra revealed Slater, Last and Axe modes near 140, 190 and 555 cm(-1), respectively. The frequency of the Slater mode decreases by about 10% on cooling, which is enough to account for the observed increase of the low-frequency permittivity. The results allowed us to discuss the reasons for the suppressed ferroelectricity of the material. Temperature evolution of the Raman scattering intensity of an oxygen octahedra bending mode near 455 cm(-1) suggests a second-order phase transition towards the perovskite aristotype phase near 870 K.
Subject(s)
Calcium Compounds/chemistry , Lanthanum/chemistry , Oxides/chemistry , Sodium/chemistry , Spectrophotometry, Infrared , Spectrum Analysis, Raman , Titanium/chemistry , Models, Molecular , Molecular Structure , Phase Transition , Temperature , ThermodynamicsABSTRACT
Dielectric properties of Eu(0.5)Ba(0.5)TiO(3) ceramics were investigated between 10 and 300 K in the frequency range of 1 MHz-100 THz. Permittivity exhibits a strong peak near the ferroelectric phase transition at 215 K. This is mainly due to softening of the lowest frequency polar phonon revealed in THz and infrared spectra. Dielectric relaxation was observed also below the ferroelectric soft mode frequency in the whole investigated temperature region, but it is probably caused by some defects such as Eu(3 + ) cations or oxygen vacancies. This implies that the ferroelectric phase transition has predominantly a displacive character. Raman scattering spectra revealed a lowering of crystal symmetry in the ferroelectric phase and XRD analysis indicated orthorhombic A2mm symmetry below 215 K. The magnetic measurements performed at various frequencies in the field cooled and field heating regime after cooling in zero magnetic fields excluded spin glass behavior and proved an antiferromagnetic order below 1.9 K in Eu(0.5)Ba(0.5)TiO(3).
