Abstract
The structural phase transition path from the low-temperature polar structure up to the highest symmetric phase in the hybrid improper ferroelectric ${\mathrm{Ca}}_{3}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ compound is here investigated at atomic scale. Measurements using the perturbed angular correlation local probe technique are combined with ab initio electronic structure calculations to observe the evolution of the electric field gradient parameters at the $\mathrm{Ca}$ site within the 10--1200 K temperature range. The results show that polar-phase clusters persist at temperatures as high as 500 K. In addition, evidence is given for a structural phase transition occurring above 1150 K. The high-temperature symmetry is here confirmed to be $I4/mmm$.
Highlights
Layered perovskites such as the RuddlesdenPopper (RP) phases, with general formula An+1BnO3n+1 (A = rare-earth or alkaline-earth metal and B = transition metal) are studied as feasible routes to develop and design novel functional materials, ranging from superconductors, multiferroics, to materials that display negative thermal expansion [1,2,3,4,5,6,7,8]
The electric field gradient (EFG) measured at the highest temperatures, labeled as F3 environment and identified as the red dots in Figure 3, presents a |Vzz| similar to the F2 one, but with a near axially symmetric local environment, η ≈ 0.11, which is identified in the Fourier transforms (FTs) of Fig. 2(a) by the red shaded equidistant frequency triplet: ω1, ω2, and ω3 with ω2 ∼ 2ω1 and ω3 ∼ 3ω1
The measurement of the EFG combined with ab initio density functional theory (DFT) calculations has shown to be a valuable tool to probe the MnO6 octahedral rotations that lie behind the Ca3Mn2O7 multiferroic and anisotropic thermal expansion properties
Summary
Layered perovskites such as the RuddlesdenPopper (RP) phases, with general formula An+1BnO3n+1 (A = rare-earth or alkaline-earth metal and B = transition metal) are studied as feasible routes to develop and design novel functional materials, ranging from superconductors, multiferroics, to materials that display negative thermal expansion [1,2,3,4,5,6,7,8]. Senn et al [7] showed experimentally that Ca3Mn2O7 exhibits, for a certain temperature range, a uniaxial negative thermal expansion (UNTE), presenting a UNTE coefficient of ≈ −3.6 ppm/K [7]. The polar distortion zone center mode ( 5−), involving the antiferroelectric displacement of Ca-site ions along the [100] direction which is represented in the inset of Fig. 1(a), acts as a secondary order parameter
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