Pressure-Dependent Raman Scattering Spectrum of Piezoelectric (Li,Na,K)NbO₃ Lead-Free Ceramics

High-pressure Raman scattering experiments have been carried out for (Na 0.5 K 0.5 )NbO 3 and Li x (Na 0.5 K 0.5 ) 1- x NbO 3 (abbreviated as LNKN-100 x ) ceramics using a diamond-anvil cell (DAC) in the backscattering geometry. Appreciable spectral changes occurred between 2.04 and 3.41 GPa in (Na 0.5 K 0.5 )NbO 3 ceramics, which was a sign of the orthorhombic (O) to tetragonal (T) phase transition. The Raman modes of T-phase LNKN-6 and LNKN-7 were less affected by increasing pressure, indicating them to be particularly stable against hydrostatic pressure. In LNKN-5.5, however, a pressure-induced phase transition from a polymorphic O–T phase mixture to a single O phase, similar to the case of LNKN-3, was observed at around 2.84 GPa, since the hydrostatic compression might favor the dense O phase rather than the T phase. Accompanying this result, it was newly found that a further important role of Li substitution in LNKN ceramics is the introduction of tensile stress in the crystal structure.