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The ferroelectric to paraelectric phase transition of multiferroicCaMnTi$_2$O$_6$ has been investigated at high pressures and ambient temperatureby second harmonic generation (SHG), Raman spectroscopy, and powder andsingle-crystal x-ray diffraction. We have found that CaMnTi$_2$O$_6$ undergoesa pressure-induced structural phase transition ($P4_2mc \rightarrow P4_2/nmc$)at $\sim$7 GPa to the same paraelectric structure found at ambient pressure and$T_c$ = 630 K. The continuous linear decrease of the SHG intensity thatdisappears at 7 GPa and the existence of a Raman active mode at 244 cm$^{-1}$that first softens up to 7 GPa and then hardens with pressure, are used todiscuss the nature of the phase transition of CaMnTi$_2$O$_6$ for which ad$T_c$/d$P = -48$ K/GPa has been found. Neither a volume contraction nor achange of the normalized pressure on the eulerian strain are observed acrossthe phase transition with all the unit-cell volume data following a secondorder Birch-Murnaghan equation of state with a bulk modulus of $B_0$ =182.95(2) GPa.

Ambient temperature high-pressure-induced ferroelectric phase transition in CaMnTi$_2$O$_6$