Fig. 4: Second harmonic generation (SHG) study of the phase transitions.

The temperature dependences of the SHG signal for a Ba₄Nd₂Ti₄Nb₆O₃₀, b Ba₄Sm₂Ti₄Nb₆O₃₀, and c Ba₄Eu₂Ti₄Nb₆O₃₀, indicating the phase transitions from the high-temperature centrosymmetric nonpolar phase to the a centric polar phases for all the compositions