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

Abstract

The ferroelectric to paraelectric phase transition of multiferroic CaMnTi${}_2$O${}_6$ has been investigated at high pressures and ambient temperature by second harmonic generation (SHG), Raman spectroscopy, and powder and single-crystal x-ray diffraction. We have found that CaMnTi${}_2$O${}_6$ undergoes a pressure-induced structural phase transition ($P{4}_{2}mc\to P{4}_2/nmc$) 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 that disappears 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 to discuss the nature of the phase transition of CaMnTi${}_2$O${}_6$ for which a d$T_c$/d$P=-48$ K/GPa has been found. Neither a volume contraction nor a change of the normalized pressure on the eulerian strain are observed across the phase transition with all the unit-cell volume data following a second order Birch-Murnaghan equation of state with a bulk modulus of $B_0$ = 182.95(2) GPa.

7 pages, 7 figures