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We studied the temperature and pressure structural stability of 5 nm BaTiO3 particles by using high resolution synchrotron X-ray diffraction, high pressure Raman spectroscopy, and high pressure impedance technique. A coexistence of the tetragonal and orthorhombic phases is observed in 5 nm Barium titanate BaTiO3 particles with weight fractions 67(6.6)% and 33(8.0)% at ambient condition, respectively. In the temperature range from -123 °C to 177 °C, the phase boundaries of 5 nm BaTiO3 are diffusive and several phases coexist. Pressure dependent Raman spectra of 5 nm BaTiO3 indicates that there exist two phase transitions: the first is at 7.5 GPa when the coexisting phases of orthorhombic and tetragonal undergoes transition to tetragonal phase; the second is around 17.3 GPa when the tetragonal transforms to cubic phase. Observed transitions were confirmed by the high pressure impedance property measurement. [ABSTRACT FROM AUTHOR]