Reversible Structural Transformation between Polar Polymorphs of

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Reversible Structural Transformation between Polar Polymorphs of Li2GeTeO6 Mei-Huan Zhao,† Wei Wang,‡,§ Yifeng Han,† Xueli Xu,∥ Zhigao Sheng,∥ Yaojin Wang,⊥ Meixia Wu,† Christoph P. Grams,# Joachim Hemberger,# David Walker,¶ Martha Greenblatt,*,§ and Man-Rong Li*,†

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†

Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry and ‡School of Materials, Sun Yat-Sen University, Guangzhou 510275, China § Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, New Jersey 08854, United States ∥ High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei, Anhui 230031, China ⊥ School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, China # II. Physikalisches Institut, Universität zu Köln, D-50937 Köln, Germany ¶ Lamont-Doherty Earth Observatory, Columbia University 61 Route 9W, Palisades, New York 10964, United States S Supporting Information *

ABSTRACT: Li2GeTeO6 prepared at ambient pressure adopts the corundum derivative ordered ilmenite structure (rhombohedral R3). When heated at 1073 K and 3−5 GPa, the as-made Li2GeTeO6 can convert into a LiSbO3-derived Li2TiTeO6-type phase (orthorhombic Pnn2), which is the third LiSbO3-derived double A2BB′O6 phase in addition to Li2TiTeO6 and Li2SnTeO6. This Pnn2 Li2GeTeO6 phase spontaneously reverts to the R3 phase if annealed up to 1023 K at ambient pressure. Although the crystal structural analyses and second harmonic generation measurements clearly demonstrate the polar nature of both the R3 and Pnn2 phases, P(E) and dielectric measurements do not show any convincing ferroelectric response. Given the large estimated spontaneous polarization (17 and 80 μC/cm2), the absence of ferroelectric behavior could be attributed to the random domain distribution and leakage due to Li-ion migration.

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(LTTO, Pnn2)27-type structures. The crystal structure type of A2BB′O6 with small A-site cations can transform between the above polymorphs at different experimental conditions.28 In some cases, the high-pressure phase can encounter irreversible phase transition with thermal treatment at ambient pressure (AP). For instance, the high-pressure Ni3TeO6 (NTO)-type Mn2FeMoO6 (R3) undergoes a phase transition to an ordered ilmenite (OIL) structure upon heating around 400 K at AP;20,29 similarly, the double perovskite Mn2CrSbO6 prepared at 8 GPa (P21/n) can change to ilmenite R-3 when heated to 973 K at AP.30 In contrast, the low-pressure phase can transform into a higher-density phase with pressure, such as the ilmenite (R-3, at 3 GPa) to double perovskite (P21/n, at 5.5 GPa) conversion of Mn2FeSbO6.31,32 These structure variations offer an inviting playground for structural modulation and resultant desired physical properties. For example, Mn2FeSbO6 is a ferrimagnetic−antiferroelectric multiferroic

INTRODUCTION Polar oxides are of fundamental and technological interest due to their symmetry-dependent properties, such as piezoelectricity, pyroelectricity, ferroelectricity, second harmonic generation (SHG) effect,1−6 even multiferroic or magnetoelectric behavior if magnetic cations are incorporated,7,8 or ferroelectric photovoltaic behavior given a proper band gap.9−11 Among these, A2BB′O6-type double perovskites are one of the most intensively studied families,7,12−14 particularly, exotic double-perovskite-related oxides with small A-site cations (usually the effective ionic radius is no larger than that of the high-spin (HS) Mn2+) have drawn increasing attention. Their structures not only allow the incorporation of transition-metal cations at both the A- and B-sites for robust magnetic interactions15,16 but also provide a high degree of freedom to achieve structural distortion for large spontaneous polarization (PS).17−21 High-pressure synthesis is usually required to stabilize these exotic perovskite phases over competition with other polymorphs, such as the corundum derivatives,21−23 bixbyite,24−26 and LiSbO3-derived Li2TiTeO6 © XXXX American Chemical Society

Received: November 5, 2018

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DOI: 10.1021/acs.inorgchem.8b03114 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. Comparison of the LiSbO3 and LTTO-type crystal structures. Edge-shared octahedral chain of (a) SbO6 in LiSbO3 and (d) TiO6−TeO6 in LTTO viewed along the b-axis. Polyhedral view of the supercell (2 × 2 × 2) crystal structure of (b) LiSbO3 and (e) LTTO viewed along the aaxis; the orange dashed line highlights the edge-sharing octahedral chain in (a) and (d). Perspective wireframe structure of (c) LiSbO3 and (f) LTTO viewed along the b-axis, where the topological zigzag array circled by a cerulean dashed line demonstrates the Li−Li arrangements highlighted by the same color dashed line in (b) and (e). SbO6 and TiO6, cyan; TeO6, khaki; Li, green spheres; O, red spheres. The wire color in (c) and (f) corresponds to that of the octahedra and atoms in (b) and (e), respectively.

around 270 K in an ilmenite structure (R-3)31 but an antiferromagnetic insulator in the higher-pressure perovskite (P21/n) phase.29 So far, pressure-induced phase transitions have been observed between bixbyite−corundum−perovskite polymorphs in a few compounds.33,34 The perovskite tolerance factor (t = (rA + r0)/√2(rB + r0), where rA, rB, and rO are the effective ionic radii of the A, B, and O ions in perovskite structures, respectively) also determines the crystal structure along with synthesis pressure, as exemplified by the AP-made Li2MTeO6, which crystallizes in OIL for M = Ge (ionic radius in octahedral coordination, VI r(Ge4+) = 0.53 Å, t = 0.88), LTTO for M = Ti, Sn (VIr(Ti4+) = 0.605 Å, t = 0.86; VIr(Sn4+) = 0.69 Å, t = 0.84), and NTO for M = Zr, Hf (VIr(Zr4+) = 0.72 Å, t = 0.84; VIr(Hf4+) = 0.71 Å, t = 0.84).35,36 The OIL- and LTTO-type structural compounds are rare compared with other small A-site A2BB′O6. To the best of our knowledge, only two OIL- and two LTTO-type compounds have been reported to date: the OIL-type Li2GeTeO6 (LGTO)37 and Mn2FeMoO629 and the LTTOtype Li2TiTeO6 and Li2SnTeO6.27 The rhombohedral OIL (R3) is a corundum derivative with face-sharing octahedral pairs along the c-axis and edge-sharing octahedral honeycomb layers in the ab-plane, as described in our previous work.21,28 The LTTO structure is derived from orthorhombic LiSbO3 (Pnna) when Sb is replaced by two ordered heteroatoms. Figure 1a presents the edge-sharing SbO6 octahedral chains in the LiSbO3 structure; these chains are connected via cornersharing (Figure 1b) to form four-membered ring channels with zigzag Li atoms running along those channels (Figure 1c). The LTTO-type structure (orthorhombic Pnn2, Figure 1d−f) shows a network similar to that of the LiSbO3 matrix but with a more distinguishable and distorted Li-site array (Figure 1d), due to the existence of alternative neighboring octahedral M4+O6/Te6+O6 (Figure 1d). Presumably, the OIL and LTTO

structures can transfer into each other at certain conditions given their very comparable t. However, this has not been reported so far. In this work, we observe a reversible structure transformation between OIL- and LTTO-type structures in LGTO. The AP-made LGTO (rhombohedral R3, hereafter denoted as R-LGTO) was found to convert into LTTO-type phase (orthorhombic Pnn2, hereafter labeled as O-LGTO) at high pressure (HP), whereas the O-LTGO transforms back to RLGTO upon heating at AP. The preparation, crystal structure, ferroelectric, dielectric, and SHG properties were studied, as well, and are reported below.

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EXPERIMENTAL SECTION

Synthesis and Phase Analysis. Polycrystalline R-LGTO was prepared via conventional solid-state reaction at AP, as reported in the literature.37 A stoichiometric mixture of GeO2 (99.999%, Strem Chemicals), TeO2 (99.99%, J&K Chemicals), and Li2CO3 (99.999%, J&K Chemicals) was pelletized and heated in air at 923 K for 36 h and then furnace cooled to room temperature to entirely oxidize Te4+ into Te6+, followed by 973 K for 24 h (cooled at 20 K/h from 973 to 773 K and 40 K/h from 773 to 573 K before furnace cooling) and 1023 K for 12 h (cooled at 20 K/h from 1023 to 823 K and at 40 K/h from 823 to 623 K) with intermediate grinding and pelletizing.37 The heating rate was set at 200 K/h unless otherwise specified. The asmade R-LGTO phase was subsequently heated at 1070 K at 1.5, 3, 4, and 5 GPa for 0.5 h to observe phase evolution in a Walker-type multianvil press, as reported in previous work.18−21,38 Powder X-ray diffraction (PXD) measurements were carried out with a PANalytical Empyrean diffractometer (Cu Kα, λ = 1.5148 Å) between 10 and 120° (step size 0.02°, 10 s per step). Rietveld refinements were performed with the TOPAS software package.39 Physical Properties Measurements. The ferroelectric hysteresis (P−E) loops and leakage current of specimens were characterized by the radiant precision multiferroic test system at 1000 Hz. Pt electrodes were sputtered on the specimens by magnetron sputtering B


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Inorganic Chemistry at room temperature. In addition, the frequency-dependent dielectric permittivity was measured employing a high-impedance frequency response analyzer (NOVOCONTROL) with a low stimulus of about 1 V/mm in a frequency range of 10 Hz < ν < 10 kHz. The polarizations of both the R- and O-LGTO were measured by polarized second SHG technique with a 90° reflection geometry, as schematically shown in Figure S1 of the Supporting Information. The fundamental light was supplied by a Ti:sapphire oscillator with 80 MHz repetition rate and 100 fs bandwidth. The generated SH (∼400 nm) from samples was filtered by color filters and a monochromator and then detected by a photomultiplier tube. During the measurements, the polarization of both incident and output light beams can be manipulated by polarizer components, such as a Glan prism. All of the above measurements were carried out on dense ceramic pellets. The R-LGTO phase dense pellets (74.2% of theoretical density 4.97 g/cm3) were obtained from a reaction-sintering process at 1023 K after a cold isostatic pressing at 300 MPa. The as-made O-LGTO pellets (∼91% of theoretical density 5.23 g/cm3) are dense enough to perform the above measurements.

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RESULTS AND DISCUSSION Synthesis and Crystal Structure. PXD patterns measured at AP on LGTO prepared at different conditions are shown in Figure 2. The AP-made R-LGTO phase (Figure 2a)

Figure 3. Rietveld refinements from the PXD data of LGTO prepared at (a) AP in the OIL structure and (b) 5 GPa in the LTTO structure. Insets show the perspective polyhedral view unit cell structure along (a) [110] and (b) [010] directions. GeO6, cyan; TeO6, khaki; O, red spheres; LiO6, green in (a), and Li, green spheres in (b).

(∼7(1)%), as shown in Table 1, probably owing to the cooling rate difference during synthesis. Figures S2 and 3b display the refinements of data from the samples prepared at 4 and 5 GPa, respectively, in LTTO-type O-LGTO structure (Pnn2, a = 4.9944(1) Å, b = 4.8064(1) Å, c = 8.2092(1) Å, V = 197.06(4) Å3, Z = 2, Rp/Rwp = 5.48/7.66% for the 5 GPa phase). Structural parameters of the O-LGTO prepared at 4 and 5 GPa are almost identical considering the estimated standard deviation (Table 1). The crystal structure of O-LGTO is shown in the inset of Figure 3b. Compared with that of LiSbO6, the ordering of Ge4+ and Te6+ over the B-site in O-LGTO renders two crystallographically distinguished Li sites and more anisotropic LiO6 coordination, as can be seen from the metal−oxygen distance evolution in Table 2. The Li−O bond lengths range from 2.01 to 2.07 Å in LiSbO3 but 1.76 to 2.60 Å in O-LGTO. These values are 1.78−2.62 Å in the isostructural Li2TiTeO6.27 The structural distortion in O-LGTO gives PS ∼ 80 μC/cm2 compared to that of ∼17 μC/cm2 in R-LGTO, as estimated from point-charge model calculations.40,41 The unit cell volume of the O-LGTO is well in line with the other LiSbO3-type compounds considering the average B-site ionic radius ⟨B⟩ evolution, as shown in Figure S3, in which Li2SnTeO6 is an exception because its unit cell volume is smaller than expected, probably due to its smaller t (∼0.84), almost identical with those for the corundum derivatives: NTO-type Li2ZrTeO6 and Li2HfTeO6 in R3.27 Presumably, Li2SnTeO6 is near the phase boundary and might be able to

Figure 2. PXD patterns of LGTO synthesized at (a) AP (R-LGTO); (b) 1.5 GPa (R-and O-LGTO); (c) 3 GPa (O-LGTO); (d) 4 GPa (O-LGTO); and (e) 5 GPa (O-LGTO) measured at AP.

adopts the OIL structure.37 After being heated at 1073 K and 1.5 GPa for 30 min, the R-LGTO is still there but slightly deformed, as reflected by the broadened peaks and weakened intensity in Figure 2b. The R-LGTO phase disappears when the pressure is further increased to 3 GPa (Figure 2c), where the PXD patterns can be well indexed by a LTTO-type OLGTO phase. There is no obvious evolution at higher pressure to 5 GPa (Figure 2d,e). Subsequent Rietveld refinements were carried out on the AP (Figure 3a) and 4−5 GPa (Figures S2 and 3b) PXD data in OIL and LTTO structure types, respectively. The crystallographic parameters and agreement factors from refinements on the AP, 4, and 5 GPa data at room temperature are listed in Table 1. The crystal structure of the AP-made OIL-type R-LGTO phase (R3, a = 5.0038(1) Å, c = 14.3269(5) Å, V = 310.66(20) Å3, Z = 3, Rp/Rwp = 4.95/ 6.36%) is shown in the inset of Figure 3a. Detailed description of the crystal structure of R-LGTO is reported in the literature,37 with 18(1)% Ge/Te antisite disordering and a structural formula of Li2(Ge0.82(1)Te0.18(1))(Ge0.18(1)Te0.82(1))O6. In this work, we observed less Ge/Te antisite disordering C


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Inorganic Chemistry

expansion, mainly due to the (∼6.10%) elongated bond distance in the R-phase (Table 2). Physical Properties. Crystal structure analyses corroborate the polar nature of both the R- and O-LGTO phases and considerable high PS (∼17 and 80 μC/cm2 for the R- and OLGTO, respectively). Although both samples are white and highly insulating, as examined by a multimeter, the P−E loop measurements on polycrystalline dense pellets of both the APand 5-GPa-made phases do not show any convincing ferroelectric switching, as the leakage current is as high as 10−6 A under a field of 10 kV/cm (Figure S4). In addition to these measurements at room temperature, we performed temperature-dependent linear dielectric spectroscopy down to 2 K. The results of 5-GPa-made O-LGTO are shown in Figure 5. At temperatures above 140 K, the dielectric constant ε′ is dominated by a dispersive increase with increasing temperature; the dielectric loss, ε″, shows similar features (Figure 5). Such a type of behavior can be explained by the formation of capacitive contact contributions at the grain boundaries and/or electrodes. As already evidenced by the P(E) loops and the leakage measurements discussed above, the material possesses finite residual conductivity at elevated temperatures. Together with the capacitive contact contributions, this residual resistivity forms RC elements that give rise to frequency-dependent Maxwell−Wagner-type relaxations.45 However, for lower temperatures (and higher frequencies), the characteristic time constant, τ = RC, increases due to the exponential increase of the thermally activated resistivity and nonintrinsic contact features are short-cut. Thus, below 140 K, the loss ε″ drops to zero and dispersion in the real part of the permittivity vanishes, reproducing an intrinsic value ε′ ∼ 10.8 at lowest T. The slight rise of the intrinsic permittivity with temperature toward ε′(150 K) ∼ 11 can be explained by the typical slight softening of the phonon spectrum due to thermal expansion and cannot be understood as positive evidence for a diverging permittivity at higher temperatures due to a possible ferroelectric phase transition. Nevertheless, the polar nature of the structure already present at higher T also cannot be ruled out from these experimental findings. Measurements on APmade R-LGTO (Figure S5) show qualitatively the same behavior. When comparing the absolute values at low temperatures, R-LGTO shows a significantly smaller value of ε′ ∼ 6.9 compared to that of O-LGTO. To further explore the polar nature of Li2GeTeO6, polarized SHG measurements with the 90° reflection geometry were carried out, due to the method’s high sensitivity to the noncentrosymmetry in electronic and magnetic crystals.46−48 Figure 6 presents typical SH polarization analysis diagrams with p-polarization incidence (Pin) for both the AP-made RLGTO and 5 GPa product O-LGTO. Both samples have obvious SH output signals, which indicates the presence of the electric polarization in those samples. When an electric field (about 5 kV/cm) was applied on the O-LGTO (Figure 6b), the SH signals show negligible change, which implies that the electric field is unable to modulate the polar state. Apparently, both the P−E and SHG measurements confirm experimentally the polar structure features of both the R and O phases but do not evidence any distinctive ferroelectric switching. The random polarization domain distribution in polycrystalline specimens could be partially responsible for the absence of ferroelectric switching, whereas the current leakage may stem from possible local Li-ion migration. Thus, it is still too early to draw a conclusion on the existence of

Table 1. Thermal, Positional, and Agreement Factors for AP (Z = 3) and HP (Z = 2) Li2GeTeO6 after PXD Rietveld Refinements AP (R3) a/Å b/Å c/Å V/Å3 Li1 (0 0 z) z B/Å2 Li2 (0 0 z) z B/Å2 Ge1/Te1 (0 y z) y z occ. B/Å2 Ge2/Te2 (0 0 z) Z occ. B/Å2 O1 (x y z) x y z B/Å2 O2 (x y z) x y z B/Å2 O3 (x y z) x y z B/Å2 χ2 RP (%) Rwp (%)

5.0038(20)

5 GPa (Pnn2)

14.3269(5) 310.66(20)

4.9940(2) 4.8064(1) 8.2077(2) 197.01(2)

4.9944(1) 4.8064(1) 8.2092(1) 197.06(4)

0.131(10) 1

0.599(10) 2.88(14)

0.599(10) 0.66(12)

0.858(20) 1

0.742(10) 2.88(14)

0.742(10) 0.66(12)

0 0.660(10) 0.93(1)/0.07(1) 3.99(8)

1/2 0.3066(10) 1 2.88(14)

1/2 0.3071(2) 1 0.66(12)

0.340(10) 0.07(1)/0.93(1) 3.99(8)

0 1 2.88(14)

0 1 0.66(12)

0.312(10) −0.026(5) 0.258(3) 2.36(20)

0.152(17) 0.240(5) 0.157(3) 2.88(14)

0.152(15) 0.241(25) 0.157(14) 2.85(8)

0.723(3) 0.038(3) 0.740(10) 2.36(20)

0.683(24) 0.209(25) 0.972(17) 2.88(14)

0.682(15) 0.209(13) 0.972(14) 2.85(8)

1.25 4.95 6.36

0.825(4) 0.789(4) 0.815(23) 2.88(14) 16.5 5.12 7.15

0.825(25) 0.790(20) 0.819(19) 2.85(8) 2.70 5.48 7.66

transfer into the NTO-type structure at high pressure, which will be investigated in the future. Reversible Phase Transition. Generally, high-pressure synthesis can enhance steric atomic interactions and induce transformation to denser structures.42−44 Sometimes this kind of phase transition can occur at temperatures as low as 473 K as in the NTO-to-OIL transition in Mn2FeMoO629 or up to 973 K as in the monoclinic perovskite (P21/n) to ilmenite (R3) conversion of Mn2CrSbO6.34 Thus, it is always essential to examine the thermal persistence of the high-pressure phase. Figure 4 shows the room-temperature PXD patterns of the OLGTO (4 GPa product) after it was annealed at different temperatures in air. It can be seen that the O-LGTO persists up to 843 K (Figure 4a,b), and peaks of the R-LGTO appear at 873 K (Figure 4c) and grow much stronger at 923 K (Figure 4d). At 1023 K, the O-LGTO is fully converted to R-LGTO, as shown in Figure 4e,f. Accordingly, the theoretical density of the high-pressure O-LGTO (5.23 g·cm−1) decreases to 4.97 g· cm−1 of the R-LGTO phase, attributed to the structural D


Article

Inorganic Chemistry Table 2. Bond Length and Bond Angle of AP and HP Li2GeTeO6 AP LiO6 Li1−O1 Li1−O2 Li1−O3 Li2−O1 Li2−O2 Li2−O3 (Ge/Te)O6 (Ge1/Te1)−O1 (Ge1/Te1)−O2 (Ge1/Te1)−O3 (Ge2/Te2)−O1 (Ge2/Te2)−O2 (Ge2/Te2)−O3 angles around O (Ge1/Te1)−O1−(Ge2/Te2) (Ge1/Te1)−O2−(Ge2/Te2) (Ge1/Te1)−O3−(Ge2/Te2) angles around Ge/Te O1−(Ge1/Te1)−O1 O2−(Ge1/Te1)−O2 O3−(Ge1/Te1)−O3 O1−(Ge2/Te2)−O1 O2−(Ge2/Te2)−O2 O3−(Ge2/Te2)−O3 angles around Li O1−Li1−O1 O2−Li1−O2 O3−Li1−O3 O1−Li2−O1 O2−Li2−O2 O3−Li2−O3

4 GPa

2.44(4) (×3) 2.08(17) (×3)

2.19(16) 1.97(13) 2.22(20) 2.13(16) 2.20(17) 2.60(15) 1.75(19) 2.18(17)

2.26(29) 2.03(3) (×3) 2.25(4) (×3) 2.14(3) 1.87(27) (×3) 1.88(18) (×3)

(×2) (×2) (×2) (×2) (×2) (×2)

5 GPa 2.19(10) (×2) 1.96(6) (×2) 2.25(16) (×2) 2.13(29) 2.20(10) (×2) 2.60(9) (×2) 1.76(12) (×2) 2.19(4)

1.91(24) (×2) 1.92(13) (×2) 1.92(19) (×2) 1.92(19) 1.89(24) (×2) 1.89 (12) (×2) 2.02(19) (×2) 1.93(18)

1.91(11) (×2) 1.91(8) (×2) 1.91(12) (×2) 1.91(10) 1.89(11) (×2) 1.89(16) (×2) 2.00(14) (×2) 1.93(14)

96.3(6) 100.2(7)

132.7(12) 101.9(6) 97.3(8)

132.7(7) 101.9(4) 98.2(6)

95.0(7) 86.5(6)

100.0(9) 89.9(5) 176.0(7) 94.0(9) 166.1(5) 82.6(7)

99.8(4) 90.1(4) 174.1(5) 94.0(5) 165.9(3) 83.9(5)

154.9(8) 116.1(6) 74.1(7) 143.0(8) 62.9(4) 140.0(9)

154.8(3) 115.9(28) 72.9(5) 143.1(3) 62.8(9) 138.0(5)

1.88(23) 2.01(28) (×3) 1.90(17) (×3) 1.96(23)

89.0(9) 97.0(6)

70.5(9) 105.4(6) 99.5(7) 70.0(5)

Figure 4. Room-temperature PXD patterns of (a) LGTO prepared at 4 GPa compared with those of the 4 GPa sample after being annealed in air for 1 h at (b) 843 K, (c) 873 K, (d) 923 K, (e) 1023 K, and (f) LGTO prepared at AP. Figure 5. Measurements of the temperature-dependent complex dielectric permittivity ε* for frequencies from 10 Hz to 3.3 kHz employing a small stimulus of 5 V/mm.

ferroelectricity in the two polymorphs of LGTO. Further measurements on single crystal or epitaxial film samples together with theoretical calculations are required. E


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Inorganic Chemistry

Figure 6. Polarization angle dependence of SH signal with p-polarization incidence geometry for both AP-made R-LGTO sample (a) and 5 GPa product O-LGTO sample (b).

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Accession Codes

CONCLUSIONS We have prepared the ordered ilmenite structural Li2GeTeO6 (rhombohedral R3) at ambient pressure. Crystal structure analysis indicates partial antisite disordering over the Ge and Te sites, giving the structural formula of Li2(Ge0.93(1)Te0.07(1))(Te0.93(1)Ge0.07(1))O6. When the sample is heated at 1073 K at 3−5 GPa, the R3 phase converts to a LiSbO3-derived orthorhombic symmetry (Pnn2) isostructural with Li2TiTeO6 and Li2SnTeO6. Compared with LiSbO3, the orthorhombic high-pressure Li2GeTeO6 shows ordered Ge/Te over the Sb site and accordingly two crystallographically independent Li atoms at the A-sites. When annealed up to 1023 K at ambient pressure, the orthorhombic high-pressure Li2GeTeO6 undergoes a phase transition back to the ambient-pressure rhombohedral phase. The polar natures of both the R3 and Pnn2 phases are established by structural analyses and second harmonic generation measurements. Although point-charge model calculations give spontaneous polarization constants of 17 and 80 μC/cm2 for the rhombohedral and orthorhombic phase, respectively, P(E) and dielectric measurements show no distinctive ferroelectric response in the polycrystalline samples. The random domain distribution and leakage from possible Liion migration might be responsible for the absence of switchable polarization. Further research on single domain thin film or single crystal samples is required to confirm the switchability. Theoretical calculations would also be useful to shed light on the possible switching mechanism. These findings show that pressure- and temperature-induced reversible phase transitions can occur between corundum derivatives and LiSbO3-derived double system, thus enriching the phase diagram of A2BB′O6 with small A-site cations.

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CCDC 1877141 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Martha Greenblatt: 0000-0002-1806-2766 Man-Rong Li: 0000-0001-8424-9134 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Fund of China (NSFC-21875287), and NSF-DMR-1507252 grant of USA. X.L.X. and Z.G.S. are supported by the National Key R&D Program of China (Grant No. 017YFA0303603) and the National Natural Science Foundation of China (NSFC-U1532155 and 11574316). Y.J.W. is supported by the National Natural Science Foundation of China (NSFC11874032). C.P.G. and J.H. are supported by the German Research Foundation through CRC 1238 (Project B02). A portion of this work was performed on the Steady High Magnetic Field Facilities, High Magnetic Field Laboratory, CAS.

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ASSOCIATED CONTENT

S Supporting Information *

REFERENCES

(1) Arévalo-López, A. M.; McNally, G. M.; Attfield, J. P. Large Magnetization and Frustration Switching of Magnetoresistance in the Double-Perovskite Ferrimagnet Mn2FeReO6. Angew. Chem., Int. Ed. 2015, 54 (41), 12074−12077. (2) Bazuev, G. V.; Golovkin, B. G.; Zubkov, V. G.; Tyutyunnik, A. S. Synthesis, Crystal Structure, and Magnetic Properties of Complex Oxides Cu2BSbO6 (B = Mn, Fe, Ga) with a Bixbyite Structure. J. Solid State Chem. 1994, 113 (1), 132−137. (3) Becker, R.; Berger, H. Cu2CoTeO6. Acta Crystallogr., Sect. E: Struct. Rep. Online 2006, 62 (12), i261−i262. (4) Cai, G.-H.; Greenblatt, M.; Li, M.-R. Polar Magnets in Double Corundum Oxides. Chem. Mater. 2017, 29 (13), 5447−5457.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03114. Schematic of SHG measurements; Rietveld refinements of the PXD data for the 4 GPa sample; unit cell evolution of LiSbO3 and derived phases; P(E) loops and leaking current of both the ambient pressure and 5 GPa samples; dielectric properties of the AP-made R-LGTO (PDF) F


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