High-Pressure Synthesis of A-Site Ordered Double Perovskite

We successfully synthesized a novel ferroelectric A-site-ordered double perovskite CaMnTi2O6 under high-pressure and investigated its structure, ferro...
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High-Pressure Synthesis of A‑Site Ordered Double Perovskite CaMnTi2O6 and Ferroelectricity Driven by Coupling of A‑Site Ordering and the Second-Order Jahn−Teller Effect Akihisa Aimi,† Daisuke Mori,† Ko-ichi Hiraki,‡ Toshihiro Takahashi,‡ Yue Jin Shan,§ Yuichi Shirako,⊥,∥ Jianshi Zhou,⊥ and Yoshiyuki Inaguma*,† †

Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan Department of Physics, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan § Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University, 7-1-2 Yoto, Utsunomiya, Tochigi 321-8585, Japan ⊥ Texas Materials Institute, University of Texas at Austin, 204 E. Dean Keeton, C2200, Austin, Texas 78712, United States ‡

S Supporting Information *

ABSTRACT: We successfully synthesized a novel ferroelectric A-site-ordered double perovskite CaMnTi2O6 under high-pressure and investigated its structure, ferroelectric, magnetic and dielectric properties, and high-temperature phase transition behavior. Optical second harmonic generation signal, by frequency doubling 1064 nm radiation to 532 nm, was observed and its efficiency is about 9 times as much as that of SiO2 (α-quartz). This compound possesses a tetragonal polar structure with space group P42mc. P-E hysteresis measurement demonstrated that CaMnTi2O6 is also ferroelectric. A spontaneous polarization calculated by use of point charge model and the observed remnant polarization are 24 and 3.5 μC/cm2, respectively. CaMnTi2O6 undergoes a ferroelectric−paraelectric order−disorder-type phase transition at 630 K. The structural analysis implies that both the ordering of shift of Mn2+ from the square-planar and the off-center displacement of Ti4+ in TiO6 octahedra are responsible for ferroelectricity. CaMnTi2O6 belongs to a new class of ferroelectrics in which A-site ordering and second-order Jahn−Teller distortion are cooperatively coupled. The finding gave us a new concept for the design of ferroelectric materials.

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perovskite-type oxides such as BaTiO3 and PbTiO3,4,12 which have a net dipole along [001] direction, are the mainstreams of ferroelectrics. Recently, LiNbO3-type oxides synthesized under high-pressure conditions such as ZnBO3 (B = Ti, Sn),13,14 PbNiO3,15 and MnBO3 (B = Ti, Sn)16 whose cations all shift to the pseudocubic [111] direction, have been shown to be novel ferroelectrics possessing exceptionally large spontaneous polarization.17 However, until now, ferroelectricity has not been observed in these LiNbO3-type bulk materials due to their relatively large leakage current and coercive field. Our group studied MnTiO3−CaTiO3 solid solution in order to enhance the dielectric properties for LiNbO3-type MnTiO3. As a result, we found the novel A-site-ordered tetragonal perovskite CaMnTi2O6 exhibiting ferroelectric behavior. A-site-ordered perovskites have been widely investigated due to their attractive properties, such as ionic conductivity of La2/3‑xLi3xTiO3,18 colossal dielectric permittivity of Ca-

INTRODUCTION

Ferroelectric compounds are the most beneficial materials in industrial manufacturing due to their high potential for use in actuators, sensors, and memory storage devices. Large numbers of ferroelectric materials are found in perovskite-type oxides such as BaTiO3, PbTiO3, Pb(Ti, Zr)O3, and BiFeO3.1−5 These compounds have second-order Jahn−Teller (SOJT) active cations 6 in common; these cations have an electron configuration of d0 (Ti4+, Zr4+, Nb5+) or s2 (Pb2+, Bi3+). SOJT active cations have been shown to induce structural distortion by anisotropic covalent bonding with ligands. However, not all compounds containing SOJT active cations possess ferroelectricity. For example, in cubic perovskite-type SrTiO3,7,8 TiO6 octahedra do not have macroscopic distortion; therefore, this compound does not have a polar structure. In Asite ordered orthorhombic perovskite-type La1/3NbO3,9−11 the NbO6 polyhedra are heavily distorted but local dipoles are arrayed in an antiparallel alignment, resulting in a net polarization of zero. The structural distortion arising from the SOJT effect should be aligned in almost the same direction to develop ferroelectricity. At the present day, the tetragonal © 2014 American Chemical Society

Received: January 3, 2014 Revised: March 21, 2014 Published: April 7, 2014 2601

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Cu3Ti4O12,19 and negative thermal expansion of SrCu3Fe4O12.20 However, there has been no report of an A-site-ordered perovskite exhibiting ferroelectric behavior. The majority of the previously synthesized A-site-ordered perovskites lacked a polar structure, because the A-site ordering arrays SOJT distortion in an antiparallel manner.21 In contrast, the CaMnTi2O6 found in this study belongs to an unprecedented class of ferroelectric materials in which the A-site ordering and SOJT distortion are coupled ferroelectrically. In this report, the synthesis of CaMnTi2O6, and its crystal structure and magnetic and dielectric properties are investigated, and the origin of its ferroelectricity is discussed.

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SQUID magnetometer. Heat capacity measurements were performed using a Quantum Design PPMS system from 2 to 300 K. For dielectric property measurements, the polycrystalline sample was polished to a thickness of 0.29 mm for permittivity and 0.18 mm for the P-E hysteresis loop, and Au electrodes were then formed on both sides of the pellet by a dc sputtering method. Dielectric permittivity measurements were performed using an Agilent 4284 precision LCR meter for frequencies of 1 kHz to 1 MHz in the temperature range of 300−900 K. The ferroelectric property was evaluated by the P-E hysteresis loop measurement at 200 Hz using an aixACCT Easy Check and TF Analyzer 2000 ferroelectric tester at room temperature.

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RESULTS AND DISCUSSION Synthesis and Structural Analysis. The color of the assynthesized CaMnTi2O6 powder under 7 GPa and 1200 °C was orange, similar to that of LiNbO3-type MnTiO3.16 The relative sintered density of the sample was greater than 90%. The powder XRD patterns for CaMnTi2O6 and the starting materials are shown in Figure 1. The high-pressure phase of

EXPERIMENTAL PROCEDURES

First, ilmenite-type MnTiO3 and perovskite-type CaTiO3 were synthesized as the precursors for the high-pressure experiments. Ilmenite-type MnTiO3 was synthesized by heating an equimolar mixture of MnCO3(Sigma-Aldrich, 99.9%) and rutile-type TiO2 (Rare Metallic, 99.9%) in a N2 atmosphere at 1300 °C for 5 h. Perovskitetype CaTiO3 was synthesized by heating an equimolar mixture of CaCO3 (Rare Metallic, 99.9%) and rutile-type TiO2 in air at 1200 °C for 8 h. Both single-crystal and polycrystalline samples of A-siteordered double perovskite CaMnTi2O6 were synthesized by solid-state reaction under high pressure and high temperature using a TRY cubic multianvil-type high-pressure apparatus (NAMO 2001). An equimolar mixture of MnTiO3 and CaTiO3 was ground under ethanol and then sealed in gold and platinum capsules for synthesis of the polycrystalline and single-crystal samples, respectively. A pyrophyllite cube block and cylindrical graphite were used as a pressure medium and a heater, respectively. A NaCl and MgO sleeve was used as an insulator for the polycrystalline and single-crystal samples, respectively. Samples were allowed to react at 7 GPa and 1200−1700 °C for 0.5 h. After the reaction, the samples were quenched to room temperature followed by a release of pressure. As-synthesized CaMnTi2O6 under high pressure was then annealed at 500 °C for 6 h in an N2 atmosphere in order to remove the internal stress generated during high-pressure synthesis. Phase identification was performed by the X-ray powder diffraction (XRD) method using a Rigaku RINT2100 diffractometer (Cu Kα radiation). The crystal structure of CaMnTi2O6 at room temperature was determined by the single-crystal X-ray diffraction method. A single crystal with dimensions of about 45 × 45 × 135 μm was selected for the measurements. Intensity data were collected by using the Mo Kα line in the 2θ range up to 56.92° (d > 0.75 Å) on a Bruker AXS Smart APEX2 diffractometer, equipped with a charge couple device (CCD) area detector. 603 reflections were unique (Rint = 0.0203). Multiscan absorption correction and extinction correction were performed using the SADABS22 and SHELXL97 programs,23 respectively. The atomic positions were determined by the direct method using the SHELXS97 program and refined against F2 by the full matrix least-squares technique using the SHELXL97 program23 with anisotropic displacement parameters for all atoms. All calculations were performed by using the Crystal Structure Crystallographic software package WinGX.24 The program VESTA was used for drawing the crystal structures.25 Synchrotron powder XRD data for structural analyses were collected in a temperature range of 300−900 K on a Debye− Scherrer-type powder diffractometer with an imaging-plate-type detector installed in beamline BL02B2 at SPring-8, Hyogo in Japan. The scan step size was 0.01° in 2θ at wavelength of λ = 0.52075 Å. The sample was packed into a quartz glass capillary with a diameter of 0.2 mm. The structural parameters were refined by Rietveld analysis using the program RIETAN-FP.26 The optical second harmonic generation (SHG) response was measured for ungraded powders in the temperature range of 300−700 K using a Continuum Minilite YAG:Nd laser (λ = 1064 nm). Differential scanning calorimetry (DSC) was carried out using a RIGAKU EVO II DSC8230 from 300 to 773 K at a heating rate of 10 K/min in a N2 atmosphere. The magnetic properties were measured between 5 and 300 K in an applied magnetic field of up to 10000 Oe by using a Quantum Design MPMS

Figure 1. Powder XRD patterns for CaMnTi2O6 and starting materials, MnTiO3 and CaTiO3.

CaMnTi2O6 crystallizes in a tetragonal structure with lattice parameters a = 7.5376(7) Å and c = 7.6002(12) Å, unlike both of the end-members LiNbO3-type MnTiO3 and perovskite-type CaTiO3. A small amount of unknown impurity was observed. No change in CaMnTi2O6 was observed after annealing at 500 °C for 6 h in an N2 atmosphere. The XRD pattern of CaMnTi2O6 was similar to that previously reported for the Asite-ordered double perovskite CaFeTi2O6 with a centrosymmetric space group P42/nmc [No. 137].27 In CaMnTi2O6, the SHG signal with its efficiency was 9 times as much as that observed for SiO2 (α-quartz), indicating that CaMnTi2O6 adopts a noncentrosymmetric space group. Furthermore, CaMnTi2O6 did not match any previously known structures. The structure was then solved using single-crystal X-ray diffraction. The details of the structure analysis, structural parameters, anisotropic displacement parameters, and selected bond lengths and angles of CaMnTi2O6 are summarized in Tables 1, 2, S1, and S2 of the Supporting Information, respectively. It is revealed that CaMnTi2O6 adopts the polar space group P42mc [No. 105], which is a subgroup of P42/nmc. To the best of our knowledge, this compound is the first example of an oxide adopting a space group P42mc. The crystal structure of CaMnTi2O6 is shown in Figure 2a. This structure has two Mn2+ sites, two Ca2+ sites, one Ti4+ site, and five O2− sites. Each 2602

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coordinate prism site is 1.70, slightly greater than that of the Mn with a square-planar coordinate. However, the secondneighbor Mn−O bond length 2.46 Å is much longer than the length of 2.24 Å expected from the ionic radii31 of the 6coordinated Mn2+ and 4-coordinated O2‑. Therefore, we regard this Mn2+ site as square-planar. The unusual small values of the BVSs for Mn in CaMnTi2O6 in contrast with LiNbO3-type MnTiO3 are not indicative of a reduction of Mn, but rather of an enlarged coordination environment of Mn2+. In comparison, a similar environment was also found in CaFeTi2O6,27 where the BVSs of the square-planar and tetrahedral Fe sites were 1.47 and 1.55, respectively. The local moments of square-planar coordinated Mn2+ and Ti4+ are relatively large, indicating that in CaMnTi2O6, the shifts of square-planar coordinated Mn2+ and Ti4+ are responsible to spontaneous polarization. The Ca1 and Ca2 do not really contribute to spontaneous polarization because of their antiparallel shift. The calculated distortion of the TiO6 octahedron ΔTi, Ti shift along the c-axis from the center of octahedron and spontaneous polarization P for CaMnTi2O6 and other titanates16,27,32−34 are summarized in Table 4. ΔTi is estimated by the following equation:35

Table 1. Structure Refinement for CaMnTi2O6 formula

CaMnTi2O6

molecular weight crystal color crystal system lattice parameter

Z space group measurement temperature measured/unique reflections scan mode index ranges

2θ max Rint refinement method goodness-of-fit residuals: R(F)/wR(F2) Flack parameter

286.746 g/mol orange tetragonal a = 7.5376(7) Å c = 7.6002(12) Å V = 431.81(9) Å3 4 P42mc (No. 105) 293 K 4975/603 ω −9 ≤ h ≤ 10 −9 ≤ k ≤ 9 −9 ≤ l ≤ 10 56.92° 0.0203 full-matrix least-squares of F2 1.261 0.0197/0.0703 0.49(5)

cation is completely ordering in their site. The coordination polyhedra of each cation for CaMnTi2O6 and, for comparison, CaFeTi2O6, are shown in Figure 2b. Similar to the structure of CaFeTi2O6, both Ca2+ are at the 10-coordinate site and one of the Mn2+ sites is a tetrahedral site and the other is a squareplanar site. However, unlike in CaFeTi2O6, square-planar coordinated Mn2+ shifts along the c-axis and the Ti4+ ion is displaced toward one of the O2− on the 8f site. These distortions result in a polar structure. Similar to that of CaFeTi2O6, the structure of CaMnTi2O6 is based on a framework of corner-sharing of TiO6 octahedra with a+a+c− tilting in Glazer’s notation28,29 and the Mn2+ and Ca2+ are ordered into columns directed along the c-axis. The bond valence sums (BVS)30 and local moments of each cation for CaMnTi2O6 and LiNbO3-type MnTiO316 are summarized in Table 3. The local moments are calculated by multiplying each cation shift along the c-axis and formal charge. A distance between each cation and center of polyhedron is used as cation shift. The BVSs of Ca and Ti for CaMnTi2O6 are closely matched with their nominal charges; but the Mn ions at the square-planar and tetrahedral sites have the BVSs of 1.38 and 1.60, respectively. Taking into account the second-neighbor bonds in CaMnTi2O6, the square-planar site translates into a 6coordinate regular trigonal prism site. The BVS of Mn in the 6-

ΔTi =

1 6

⎧ (d i − dave) ⎫2 ⎬ dave ⎩ ⎭

∑⎨ i

(1)

where di and dave are the interatomic distances of Ti−O and its average value, respectively. P is estimated from the structural data by vector difference between the center of anion and cation. The ΔTi varies in magnitude for listed titanates, though all the compounds contain SOJT active Ti4+ as the central metal. CaMnTi2O6 is relatively high ΔTi attributed to Ti shift and tilting of TiO6 octahedra. CaMnTi2O6 exhibits greater spontaneous polarization than BaTiO3, though the Ti shift along the c-axis is smaller, indicating that the A-site cation of Mn2+ contributes to spontaneous polarization as well as Ti4+in CaMnTi2O6. In the A-site ordered perovskite, various A-site ordering manners can be seen in rock salt, columnar and layered-type structures for double perovskite AA′B2O6, and quadruple perovskite AA′3B4O12. Though the layered-type double perovskites and quadruple perovskites have been extensively investigated, such as in (La2/3□1/3)(□)B2O6 (□ = vacancy, B = Nb, Ta),9−11 RBaCo2O6−δ (R = rare earth),36,37 (Na 0.88 Th 0.12 )(Na 0.45 Th 0.55 )Ti 2 O 6 , 38 ,3 9 (Nd 0.75 Ag 0.25 )(Nd0.25Ag0.75)Ti2O640 and ACu3Ti4O12 (A = Ca, Sr),19,41 rock salt and columnar-type ordering are found in (Na0.1Ba0.9)-

Table 2. Structural Parameters for CaMnTi2O6 at Room Temperature Derived from Single-Crystal X-ray Diffraction Structural Analysis atom

site

g

x

y

z

B (Å2)

Ca1 Ca2 Mn1 Mn2 Ti O1 O2 O3 O4 O5

2a 2b 2c 2c 8f 4e 4d 4d 4e 8f

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

0 1/2 0 0 0.25653(9) 0.3025(5) 0.2952(5) 0.1976(5) 0.2111(5) 0.2005(4)

0 1/2 1/2 1/2 0.24524(9) 1/2 0 0 1/2 0.2992(4)

0 0.0494(3) 0.5209(3) 0.0740(4) 0.2821(4) 0.3015(7) 0.8246(6) 0.2318(7) 0.7040(6) 0.0158(6)

0.77(5) 0.51(4) 0.81(3) 0.79(3) 0.362(16) 0.62(6) 0.53(6) 0.56(7) 0.56(6) 0.66(4)

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Figure 2. Crystal structure of CaMnTi2O6. (A) CaMnTi2O6 unit cell at room temperature. (B) Coordination polyhedral of each cations for CaMnTi2O6 and CaFeTi2O6. Mn(tet) and Mn(sq) represent tetrahedral and square-planar coordinated Mn2+, respectively.

Table 3. Bond Valence Sums and Local Moment of Each Cation for CaMnTi2O6 and LiNbO3-type MnTiO3a CaMnTi2O6 BVS moment/μC Å a

MnTiO3

Mn1(tet)

Mn2(sq)

Ca1

Ca2

Ti

Mn

Ti

1.60 0.099

1.38 0.884

2.22 0.548

2.12 −0.275

4.12 0.502

2.02 1.241

3.98 1.063

Mn1(tet) and Mn2(sq) indicate Mn in the tetrahedral and square-planar coordinated site, respectively.

Table 4. Octahedron Distortions, ΔTi, Shift of Ti from Center of Octahedron, and Spontaneous Polarization, P, of CaMnTi2O6 and Other Titanates −5

ΔTi/ 10 Ti shift/Å P/μC cm−2

CaMnTi2O6

CaFeTi2O6

MnTiO3 (LiNbO3-phase)

CaTiO3

BaTiO3

PbTiO3

168 0.1255 24

4.7 0 0

380 0.2638 69

0.32 0 0

99 0.1515 18

795 0.2989 54

(Na0.9Ba0.1)(LiNi)2F642 and CaFeTi2O6,27 respectively. With respect to polar structures, to our knowledge, the quadruple perovskite BiMn3Mn4O1243 (but controversial44), and both A and B site ordered perovskite AA′BB′O6 series such as NaRMnWO6 (R = La, Nd, and Tb)21,45 are all A-site-ordered perovskites having spontaneous polarization. The CaMnTi2O6 discovered in this study is the first compound shown to be a “polar” columnar-type A-site-ordered perovskite. The reason that there are so few polar materials among the A-site-ordered perovskites is that antiparallel SOJT distortion is promoted by the layered-type A-site-ordering to stabilize the layered structure. Almost all the layered A-site-ordered perovskites contain two different valent ions or vacancies in their A-site. The SOJT active B-site cation shifts toward one layer which contains lower valent cations to relieve the bonding instability of underbonded anions.21 These cooperative distortions result in the formation of an antipolar centrosymmetric structure. In AA′BB′O6, with a polar structure induced by cation ordering and octahedral tilting, e.g., NaRMnWO6,21,45 the SOJT active B-site cation is displaced antiparallelly, similar to other layered A-site-ordered perovskite-type oxides. In contrast, in CaMnTi2O6, whose Ca2+ and Mn2+ are ordered one-dimensionally, all of the Ti4+ and Mn2+ ions shift in almost the same direction, resulting in the ferroelectric coordination of one-dimensional A-site ordering and SOJT distortion. Therefore, CaMnTi2O6 is expected to be suitable for use as a ferroelectric material.

Ferroelectric and Dielectric Properties. Figure 3 shows the P-E hysteresis loop for CaMnTi2O6. As the figure shows,

Figure 3. P-E hysteresis loop for CaMnTi2O6 at room temperature. A sample thickness is 0.18 mm and measurement frequency is 200 Hz.

typical ferroelectric behavior was observed in CaMnTi2O6, though not in LiNbO3-type MnTiO3. The coercive field and remnant polarization are 53 kV/cm and 3.5 μC/cm2, respectively. The P-E hysteresis loop shown in Figure 3 was not saturated due to the leakage current of the sample. Therefore, measured remnant polarization is much smaller than spontaneous polarization calculated using the point charge model, 24 μC/cm2. To the best of our knowledge, there has 2604

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been no report of polarization reversal in a Mn2+-containing oxide except in the case of MnWO4 below 12 K.46 Moreover, CaMnTi2O6 is the first example of a ferroelectric A-site-ordered perovskite oxide. A-site-ordered perovskites are advantageous for the design of new multifunctional compounds such as multiferroics due to their compositional variety. Therefore, it is significant that CaMnTi2O6 exhibits polarization reversal at room temperature. This is probably attributable to the enlarged Mn2+ coordination environment, especially the square-planar site, in CaMnTi2O6. As will be discussed in the later section on high-temperature phase transitions, square-planar Mn ions should move to the opposite position through the oxygen basal plane when polarization is reversed by applying an electric field. Mn−O bond lengths in CaMnTi2O6 are longer than expected from the bond valence (see Table 2), which leads us to expect weak binding for Mn2+, resulting in easy polarization reversal. In the denser sample, greater remnant polarization and a less coercive field were expected due to suppression of the leakage current attributable to high porosity. The temperature dependences of the dielectric permittivity ε and tan δ of CaMnTi2O6 at a few selected frequencies on cooling are shown in Figures 4 and S1 in the Supporting

Figure 5. Magnetic susceptibilities and specific heat for CaMnTi2O6. (A) Temperature dependencies of ZFC and FC magnetic susceptibilities at 10 Oe. (B) Temperature dependencies of total specific heat. The solid line represents the phonon contributions estimated by extrapolation from the data. (C) Magnetic specific heat divided by temperature CmagT−1. The solid line represents the magnetic entropy ΔS.

the impurity phase. The magnetic specific heat (hereafter abbreviated as Cmag) was estimated by subtracting the lattice specific heat from total specific heat. The lattice contribution was estimated by extrapolation from the total specific heat data at high temperatures. The temperature dependences of the magnetic specific heat divided by temperature Cmag/T and the magnetic entropy (hereafter abbreviated as ΔS) are shown in Figure 5c. The resulting entropy was 8.7 J mol−1 K−1, which was less than that expected for a Mn2+ ion (14.9 J mol−1 K−1) from the following equation: ΔS = Rln(2S + 1), where R is the gas constant and S is the spin angular momentum quantum number. This finding suggests the presence of a short-range spin ordering well above the magnetic transition temperature. High-Temperature Phase Transition. SHG, synchrotron XRD, and DSC measurements at high temperatures were performed in order to clarify the ferroelectric phase transition. The temperature dependences of the dielectric permittivity and SHG signal on cooling are shown in Figure 6a,b, respectively. The SHG signal was absent above 630 K, indicating the extinction of spontaneous polarization. Figure 6c shows the DSC measurement on cooling for CaMnTi2O6. A change of heat capacity was observed at 630 K, which can be attributed to a second-order phase transition. The temperature dependences of lattice constants calculated from synchrotron XRD data on heating are shown in Figure 6d. A local minimum in the c-axis length was observed at 700 K though no discontinuity in the lattice volume change was observed, indicating that this phase transition is second-order. These results reveals that CaMnTi2O6 undergoes a second-order ferroelectric phase transition and its ferroelectric Curie temperature Tc is 630 K. Structural refinements were performed at high temperature for the paraelectric phase. The two structural models for the paraelectric phase were based on the displacive (isomorphic with CaFeTi2O6) and order−disorder phase transition. Both the model structures have space group P42/nmc, which is one of the nonisomorphic supergroups of P42mc. The fit obtained by

Figure 4. Temperature dependences of dielectric permittivity for CaMnTi2O6 at different frequencies.

Information, respectively. Clear dielectric peaks were observed around 630 K, which corresponds to the ferroelectric phase transition. The transition temperature is independent of measurement frequency, indicating that CaMnTi2O6 is a not relaxer-type ferroelectric material. Magnetic Properties. Figure 5a shows the temperature dependences of magnetic susceptibilities χ in an applied magnetic field of 10 Oe in both zero field cool (ZFC) and field cool (FC) for CaMnTi2O6. Clear cusps were observed at 10 K, implying an antiferromagnetic spin ordering. The effective Bohr magneton number was calculated to be 5.83 from the data obtained in the temperature range 50−300 K, which is close to the spin-only value of 5.92 expected from Mn2+ (S = 5/2). The derived Weiss temperature is −32 K, indicating that the magnetic interaction is antiferromagnetic. The isothermal magnetizations measured at 5 and 20 K are shown in Figure S2 in the Supporting Information. No hysteresis was observed, which confirms the antiferromagnetic state below TN. Consequently, CaMnTi2O6 is regarded as multiferroics below 10 K in which ferroelectricity and magnetic ordering coexist. Figure 5b shows the temperature dependences of the total specific heat C for CaMnTi2O6. Typical λ-type transition was observed at the same temperature as the peak in magnetic susceptibility, indicating that the observed magnetic anomaly corresponds to that for the bulk material and not to 2605

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and 700 K, respectively. The refined structural parameters are listed in Tables S3−10 in the Supporting Information. The temperature dependences of the spontaneous polarization, the order parameter ζ, the shifts of the square-planar and tetrahedral-coordinated Mn2+ and the octahedral Ti4+, and the average Ti−O−Ti angle are shown in Figure 8. The order

Figure 6. Temperature dependencies of dielectric permittivity (A), SHG response (B), DSC curve (C), and lattice constants and volume (D) for CaMnTi2O6. The dotted line represents a ferroelectric phase transition temperature.

using the displacive phase transition model was not good, whereas the order−disorder transition model in which only the square-planar Mn2+ was split achieved a good fitting. This result corresponds to the second-order ferroelectric phase transition character revealed by the lattice volume change and DSC measurement. The crystal structure of paraelectric phase of CaMnTi2O6 is shown in Figure 7. In the paraelectric phase,

Figure 8. Temperature dependencies of spontaneous polarization (A), square-planar Mn2+ order parameter ζ (B), shift distances for Mn (C) and Ti (D), and Ti−O−Ti bond angle (E) for CaMnTi2O6. Mn(tet) and Mn(sq) represent tetrahedral and square-planar coordinate Mn2+, respectively.

parameter ζ is the difference between the occupancy of the square-planar Mn2+ displacing in the +z and −z directions. The shifts of Mn2+ and Ti4+ are the displacement distance of cations along the c-axis from the center of their polyhedra. With temperature increases, the spontaneous polarization tends to diminish, especially near the transition temperature, and it disappears in the paraelectric phase. Similar trends were found in the order parameter and the shift of Ti4+, indicating that the ordering of the shift of square-planar Mn2+ and shift of Ti4+ significantly contributes to the spontaneous polarization. The shift of square-planar Mn2+ decreased with the increase in temperature but remained even in the paraelectric phase. The average Ti−O−Ti bond angle increased with temperature, but the change was not significant, and the tilting of TiO6 octahedra was unchanged on crossing Tc, indicating that octahedral tilting was not directly responsible for the polar structure. These findings indicate that in CaMnTi2O6, the origin of spontaneous polarization was the cooperative effect of an ordered shift of square-planar Mn2+ and the off-center displacement of Ti4+ along the c-axis. The reason that CaFeTi2O6, which has the same cation ordering manner and TiO6 tilt system with CaMnTi2O6, does

Figure 7. Structure of paraelectric phase of CaMnTi2O6.

Ti4+ and tetrahedral coordinated Mn2+ are placed at the center of their polyhedra. Ca2+ antiparallel shift each other; therefore, there is no net dipole moment. Structural refinements of both the ferroelectric and paraelectric phase for synchrotron powder XRD data were performed by taking into account a partial splitting of the square-planar Mn2+. Figures S3 and S4 in the Supporting Information show the calculated, observed, and difference synchrotron XRD patterns for CaMnTi2O6 at 300 2606

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approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2013A1697). This work was supported by JSPS KAKENHI Grant Numbers 21360325 and 24360275. J.S.Z. was supported by NSF (DMR 1122603) in the United States.

not possess a polar structure may be that the electron configuration differs between Mn2+ and Fe2+. The electron configuration of Fe2+ is [Ar]3d6. Therefore, Fe2+ favors the square-planar coordination attributable to the gain in the crystal field stabilization energy.47,48 In CaFeTi2O6, the centered square-planar Fe2+ suppressed the shift of Ti4+ because of the electrostatic repulsion between cations, though Ti4+ fundamentally prefers a distorted coordination due to the SOJT effect. In the case of Mn2+ ions, with an electron coordination of [Ar]3d5, the lack of stabilization from the crystal field effect results in high coordination flexibility and thus causes a shift of Mn2+ from the square-planar oxygen basal plane along the caxis. Therefore, in CaMnTi 2 O 6 , Mn 2+ and Ti 4+ shift cooperatively in order to avoid the cation−cation repulsion, resulting in ferroelectric distortion.

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CONCLUSION In this study, a novel ferroelectric oxide CaMnTi2O6 was synthesized under high pressure and temperature. CaMnTi2O6 has a novel structure of polar columnar A-site-ordered double perovskite-type with space group P42mc. CaMnTi2O6 is the first example of an A-site-ordered perovskite-type oxide exhibiting ferroelectricity. The cooperative coupling of the ordered shift of Mn ions along the c-axis from the square-planar basal plane and off-center displacement of Ti ions in TiO6 octahedra is responsible for the spontaneous polarization. Because of a high Tc, CaMnTi2O6 is a good candidate for a Pb-free ferroelectric material. Moreover, due to their compositional variety, A-site-ordered double perovskites are promising compounds for use in the design of new multifunctional materials. This finding opens a new avenue in the search for ferroelectric materials.

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ASSOCIATED CONTENT

S Supporting Information *

Isothermal magnetizations and synchrotron XRD patterns at 300 and 700 K for CaMnTi2O6. Anisotropic displacement parameters, selected bond lengths and angles, structural parameters at 300−900 K for CaMnTi2O6, and cif file. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Yoshiyuki Inaguma. E-mail: [email protected]. jp. Present Address ∥

Yuichi Shirako. Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Mr. H. Yamazaki and Y. Habe of Utsunomiya University for their help with the single-crystal structure analysis, Mr. Y. Masuda of Rigaku Co., Ltd. for performing the DSC measurements, Dr. H. Arii of Gakushuin University for helpful advice on the single-crystal X-ray diffraction measurement and Prof. T. Katsumata of Tokai University for valuable discussions. The synchrotron XRD experiment was conducted at BL02B2 of SPring-8 with the 2607

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