Ambient-temperature high-pressure-induced ferroelectric phase transition in CaMnTi2O6

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 → P 4 2 /nmc) at ∼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 dT c /dP = −48 K/GPa has been found. Neither a volume contraction nor a change in the normalized pressure on the Eulerian strain is 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.

Rise of A-site columnar-ordered A2A′A′′B4O12 quadruple perovskites with intrinsic triple order

“…The number of reported A 2 A′A″B 4 O 12 perovskites is so limited at the moment that they all can be simply listed here (for example, in the chronological order of discovery and in the short formulae): CaFeTi 2 O 6 (space group P4 2 /nmc), 9,10,19,20,27 CaMnTi 2 O 6 (space group P4 2 mc), 21,28,29 RMnMnSbO 6 (R = La, Pr, Nd and Sm) (space group P4 2 /n), 22 RMnGaTiO 6 (R = Sm and Gd) (space group P4 2 /nmc), 18 means a B-site order. 30 (We note that it was not possible to precisely determine the degree of Cu 2+ and Mn 2+ ordering and Mn 3+ and Ga 3+ ordering in the last two compounds with X-ray powder diffraction).…”

“…29 The P4 2 mc-to-P4 2 /nmc ferroelectric transition was also found at room temperature at about 7 GPa. 28 Without the assistance from the B sublattice, the interaction among magnetic cations at the A′ and A″ sites is usually very weak (at least, among Mn 2+ ): CaMnTi 2 O 6 has the Néel temperature (T A,N ) of about 10 K, 21 while SmMnGaTiO 6 was reported to order at T A,N = 3 K, and no ordering was found in GdMnGaTiO 6 . 18 In the case of SmMnGaTiO 6 and GdMnGaTiO 6 , even small disordering of Ga 3+ and Mn 2+ between the A″ and B sites could suppress a long-range magnetic ordering.…”

Magnetic and electronic properties of the ferroelectric-photovoltaic ordered double perovskiteCaMnTi2O6investigated by x-ray absorption spectroscopies

“…Synchrotron powder XRD experiments (λ = 0.29036Å) were also carried out as a function of pressure at the Extreme Conditions P2.2 beamline at PETRA III (Germany). High-pressure characterization by means of second-harmonic generation (SHG), Raman spectroscopy and single-crystal XRD (SXRD) experiments were carried out using Boehler-Almax diamond-anvil cells (DACs) [6].…”

“…1(a)]. In contrast to the centrosymmetric DP structure adopted by CaFeTi 2 O 6 (P 4 2 /nmc) [6], Ti 4+ ions in CaMnTi 2 O 6 are shifted from the basal plane of the octahedron along the c axis, losing spatial inversion [5]. For its part, Mn 2+ ions with square-planar coordination suffer a similar displacement [see the shift of Mn2 atoms along z direction in Fig.…”

“…The FE phase of CMTO persists at ambient pressure up to T C = 630 K [5] or at ambient temperature up to ∼7 GPa [6] when a second-order structural phase transition suppresses the distortion associated to the − 3 irrep, and the P 4 2 mc symmetry gives way to its supergroup P 4 2 /nmc and the paraelectric (PE) phase. When the system enters the PE phase (P 4 2 /nmc) either by increasing the temperature or hydrostatic pressure the offcenter atomic displacement of Ti completely vanishes, while the shift of Mn2 cations from the center of the square-plane gets reduced to half its original size with 50% of the Mn2 ions being shifted up and the other 50% down.…”