Synthesis and Investigation on Phase Transition of BaTiO3 and Cr3+-Doped BaTiO3 Nanocrystals
Various sizes of BaTiO3 and Cr3+-doped BaTiO3 nanocrystals were synthesized through hydrothermal and solvothermal methods. The applied solvents water, ethanol and benzyl alcohol lead to nanoparticles with average sizes of 200, 10 and 5 nm, respectively. The nanocrystals were treated with trioctylphosphine oxide to remove surface-bound dopant ions, and colloidal free-standing nanocrystals smaller than 10 nm were obtained by using oleic acid as a dispersant surfactant. The tetragonal-to-cubic phase transition at room temperature of undoped nanocrystalline BaTiO3 has been investigated by powder X-ray diffraction (XRD) and Raman spectroscopy. The size effect of nanoscale BaTiO3 is observed that the tetragonal phase becomes unstable with decreasing particle size. However, we found that ferroelectric tetragonal structure persists to some extent even for particles at 5 nm. The successful substitution of Ti4+ with Cr3+ in the host BaTiO3 lattice for all three sizes was achieved at different Cr3+/Ti4+ molar ratios. The dopant is found to significantly promote the phase transition, even dominate over the size effect. Ligand-field electronic absorption spectroscopy suggests a subtle change of the octahedral coordinated Cr3+ environments between particles at 5 and 10 nm, confirming the structural differences. Preliminary magnetic measurement indicates Cr3+ as isolated paramagnetic ions without any chromium clusters or oxides. The ability to rationally manipulate the ferroelectric properties of BaTiO3 by size and dopants, in combination with possible ferromagnetism induced by incorporating paramagnetic transition metal ions, opens up new opportunities for modern multiferroic materials in information storage technology.