Connection between Unusual Lattice Thermal Expansion and

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

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Connection between Unusual Lattice Thermal Expansion and Cooperative Jahn−Teller Effect in Double Perovskites LaPbMSbO6 (M = Mn, Co, Ni) Yijia Bai,*,†,‡ Lin Han,§ Jian Meng,*,§ Volodymyr Baran,∥ Jianmin Hao,†,‡ and Limin Han†,‡

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†

Chemical Engineering College, Inner Mongolia University of Technology, 49Aimin Street, Hohhot 010051, People’s Republic of China ‡ Inner Mongolia Engineering Research Center for CO2 Capture and Utilization, 49 Aimin Street, Hohhot 010051, People’s Republic of China § State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, People’s Republic of China ∥ Forschungsneutronenquelle Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universität München, Lichtenbergestrasse 1, D-85747 Garching bei München, Germany S Supporting Information *

ABSTRACT: Lattice thermal expansion (LTE) has been investigated in double perovskites LaPbMSbO6 (M = Mn, Co, Ni). Ordinary LTE behavior with good thermal stability is identified for the Mn sample, whereas unusual LTE with a preferably expanded interplanar distance of (040) is revealed for Co and Ni samples. Temperature-dependent X-ray diffraction patterns (T-XRD), Raman spectra (T-Raman), and specific heat capacities (T-Cp) consistently indicate that a rare isostructural displacive phase transition (IDPT) with a second-order phase transition nature is predominant near the critical temperature. Refinements of neutron powder diffraction (NPD) and in situ T-XRD data present temperature-sensitive bond parameters which are relevant to planar oxygen O1. X-ray photoelectron spectra (XPS) further confirm the Jahn−Teller (J-T) activated Co2+ (HS) or Ni3+ (HS/LS) cations at the B-site sublattice. This unusual LTE behavior could be understood by the cooperative J-T effect contributed by a Pb2+ ion and Co2+/Ni3+ ion from A- and B-site sublattices, respectively. The importance of 6s(Pb)-2p(O)-3d(Co/Ni) extended orbital hybridization on affecting thermal expansion behavior is highlighted on the basis of temperature-induced phonon mode softening. This study presents a microscopic description of connection between anisotropic thermal expansion and a cooperative J-T effect, which inspired exploration of thermal−mechanical coupled functional materials based on LaPbMSbO6 double perovskites. Sr2CoSbO6 perovskite in the 1960s,4,13 a great deal of interest have been focused on understanding the geometric magnetic frustration behavior in antimony-based DPOs.8,9,14,15 Recently, the effect of ferroelectric relaxation was discovered in the analogues of LnPbMSbO6 (Ln = La, Ce, Pr, Nd; M = Mn, Co, Ni, Mg).16−20 It is revealed that, in the periodic ion arrangement denoted as “···B−O−Sb−O−B···”, the role of the Sb5+ ion is as a blocker, which suppresses the 180° spin interaction/electron transportation between the next-nearestneighboring B2+ ions.9 However, a 90° electron correlation route along “···B−O−O−B···” is established, owing to a strongly tilted and distorted B/SbO6 octahedron.17,21 Consequently, an antimony-containing DPO with a rock salt type ordering structure could be considered as a superior model for

1. INTRODUCTION Double perovskite oxides (DPOs) with a rock salt type B-site ordering structure have been widely investigated for their many interesting physical properties and potential technology applications.1−3 Generally, for the purpose of designing novel functional DPO materials with ferromagnetism, ferroelectricity, and other ferroic properties, the B-site elements are always selected among d-block transition-metal ions.4−6 However, in order to realize the band structure manipulation within the visible light range, DPOs containing p-block elements are often considered as a new option.7−12 Most p-block ions, e.g. Sn4+, Sb5+, and Te6+, possess full-shell electron configurations with higher oxidation states and smaller ionic sizes. Thus, a more distorted B-site sublattice is preferably formed where the natures of crystal chemistry (bond covalency) and semiconductor physics (band structures) are capable of being considerably modified. Since the first report by Blasse on © XXXX American Chemical Society

Received: December 25, 2018

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DOI: 10.1021/acs.inorgchem.8b03595 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. NPD patterns of LaPbCoSbO6 (a) and LaPbNiSbO6 (b) with Rietveld refinements at room temperature. Illustrations of local crystal structures with corner-shared BO6 octahedra along the c axis (c) are presented.

Oxygen-related structural parameters are obtained from the refinement of neutron powder diffraction (NPD) data. Multiple temperature-dependent in situ characterizations, such as X-ray diffraction, Raman, specific heat capacity, and bulk thermal expansion coefficients have been performed to deeply investigate the unusual lattice thermal expansion (LTE) phenomenon found in samples of M = Co, Ni. Finally, the mechanism of the unusual LTE is discussed, and a cooperative Jahn−Teller effect contributed by a Pb2+ ion and a Co2+/Ni3+ ion from both A-site and B-site sublattices is proposed. The importance of 6s(Pb)-2p(O)-3d(Co/Ni) extended orbital hybridization in affecting thermal expansion behavior is highlighted on the basis of temperature-induced phonon mode softening.

investigation of the valence electron behavior under the potential field of spin/geometry frustration. It is well-known that many lead-containing perovskites possess piezoelectric, ferroelectric, and pyroelectric properties.22 These intriguing features can be understood through the noncentrosymmetric lattice distortion originating from the stereochemical effect of Pb2+, a typical second-order Jahn− Teller (SOJT) ion with 6s2 lone-pair electrons. In addition, Taylor et al.23 and Chen et al.24,25 discovered that an unusual negative thermal expansion (NTE) phenomenon begins to emerge when the Pb2+ ion moves toward to the center of a Pb−O coordinated polyhedron with a drastic lowering of the c/a ratio in the Pb-based perovskites. This temperaturedependent lattice contraction can be ascribed to the significant suppression of asymmetric orbital hybridization (AOH) between 6s (Pb2+) and 2p (O2−) by thermal activation.26 As the result of theoretical research on AOH, a BO6 octahedral rotation induced novel ferroelectric polarization was first proposed by Rondinelli, Fennie, et al. in the DPOs with layer-ordered A/A′ arrangement.27,28 Theoretically, the La−Ba layer ordered structure helps to differentiate the chemical environment of axial oxygen from that of the planar structure, which breaks the center symmetry and renders the compound ferroelectric. The authors ascribed this ferroelectricity to the out-of-phase BO6 octahedral rotation correlated with improper orbital hybridization via enhanced covalent A− O bonds.29 Inspired by their studies, we believe the concept of improper hybridization is also suitable to the senario of orbital hybridization between 6s and 2p in the Pb-based perovskites. Consequently, for the double perovskite series of LnPbMSbO6 (Ln = La, Pr, Nd; M = Co, Ni, Mn, Mg), questions about whether similar thermal expansion anomalies would appear and how the mechanism of action works is worth exploring. In this work, a series of antimony-based double perovskites LaPbMSbO6 (M = Mn, Co, Ni) have been synthesized.

2. EXPERIMENTAL METHOD The polycrystalline samples of LaPbMSbO6 (M = Mn, Co, Ni) were prepared by the modified two-step method,17,20 although different synthesis steps of B-site precursors were applied. Specifically, the sol− gel route was used in the samples M = Co, Ni while the solid-state synthesis method was preferable in the sample with M = Mn for suppressing oxidation of MnO. In addition, all of the heating procedures of LaPbMnSbO6 were performed under a nitrogen atmosphere to maintain the divalent state of manganese. In situ temperature-dependent XRD characterization was carried out using a Rigaku D/Max-2500 X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å) equipped with an RINT2000 vertical goniometer and multipurpose high-temperature attachment operated at 50 kV and 200 mA. The elevating temperature program was initiated at room temperature, and diffraction data were collected at nine temperatures (35, 100, 150, 200, 250, 300, 350, 400, and 500 °C) for M = Co, Ni samples and at four temperatures (35, 100, 300, 500 °C) for the M = Mn sample, respectively. All of the diffraction patterns were obtained between 10 and 80° in a step width of 0.02°/ step with a continuous scanning rate of 4°/min. The heating rate was 4°/min and the equilibrium time was 5 min at each temperature point before collecting diffraction data. Good reversibility of observations B


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Inorganic Chemistry

Table 1. Crystallographic Parameters and Selected Bond Lengths (Å) and Angles (deg) of LaPbCoSbO6 and LaPbNiSbO6 Cell Parameters LaPbCoSbO6

LaPbNiSbO6 β = 89.998(7)° V = 256.593(7) Å3

a = 5.6487(1) Å b = 5.6743(1) Å c = 8.0054(2) Å

LaPbNiSbO6

LaPbCoSbO6 La(Pb)−O1

La(Pb)−O2

La(Pb)−O3

⟨A−O⟩

2.696(5) 2.513(6) 2.923(5) 3.244(6) 2.509(6) 2.811(5) 2.799(6) 3.248(6) 2.977(3) 2.735(3) 2.470(3) 3.212(3) 2.845(6)

β = 89.980(4)° V = 253.863(5) Å3

a = 5.6261(1) Å b = 5.6624(1) Å c = 7.9688(1) Å Bond Lengths (Å) and Angles (°)

Co−O1 × 2 Co−O2 × 2 Co−O3 × 2 ⟨Co−O⟩ Sb−O1 × 2 Sb−O2 × 2 Sb−O3 × 2 ⟨Sb−O⟩ Co−O1−Sb Co−O2−Sb Co−O3−Sb Tilting angle along c axis

2.085(5) 2.076(4) 2.112(5) 2.091(5) 1.987(4) 1.988(4) 1.969(5) 1.981(4) 158.8(3) 160.2(3) 157.6(1) 11.2

La(Pb)−O1

La(Pb)−O2

La(Pb)−O3

⟨A−O⟩

2.674(6) 2.514(5) 2.933(6) 3.201(5) 2.539(4) 2.751(5) 2.850(5) 3.172(4) 2.940(3) 2.741(3) 2.502 (2) 3.165(2) 2.832(4)

Ni−O1 × 2 Ni−O2 × 2 Ni−O3 × 2 ⟨Ni−O⟩ Sb−O1 × 2 Sb−O2 × 2 Sb−O3 × 2 ⟨Sb−O⟩ Ni−O1−Sb Ni−O2−Sb Ni−O3−Sb Tilting angle along c axis

2.067(3) 2.046(3) 2.082(4) 2.065(3) 1.985(3) 1.991(3) 1.967(4) 1.981(3) 160.2(2) 162.6(2) 159.4(1) 10.3

expansion mode was selected with an air flow rate of 50 mL/min and a temperature rate increase of 3 K/min.

without degradation of the sample was examined by applying a new temperature scanning experiment. The neutron powder diffraction (NPD) data of LaPbMSbO6 (M = Co, Ni) were collected by a high-resolution powder diffractometer (SPODI) at the research reactor FRM-II at Heinz Maier-Leibnitz Zentrum (MLZ).30 The monochromator was Ge (551) wafer stack crystals with the wavelength λ = 1.5483 Å. The powder samples (ca. 4 g) were loaded in vanadium cans. The diffraction patterns were obtained between 5 and 150° in a step width of 0.05°. The GSASEXPGUI program package31,32 was used for performing Rietveld refinements. Lattice parameters, zero-point errors, scale factors, and the background were refined. A suitable pseudo-Voigt function was selected, and corresponding profile parameters (i.e. GU, GV, GW, GP, LX, and LY) were refined next. Structural parameters, including atomic coordinates (x, y, z) and isotropic atomic displacement parameters (Uiso), were refined. Necessary constraints on atomic coordinates of La/Pb and Uiso values of oxygen atoms were applied. Temperature-dependent Raman spectra were examined by a Renishaw inVia Reflex laser Raman spectrometer equipped with a Linkam heating−freezing stage. A633 nm He−Ne laser excitation with a power of 1 mW was used, and the laser beam was focused on a spot with a diameter of about 2 μm using a Leica DM2700 M microscope. The spectra were measured within the temperature range 25−500 °C. The heating rate was 20 K/min, and the dwelling time was 5 min at each temperature point. The Raman signal was collected with an exposure time of 30 × 3 s. An Ar atmosphere with a flow rate of 40 mL/min was applied to avoid oxidation of divalent B-site cations at high temperature. The Raman shift was corrected by a standard single-crystal Si plate at 520.8 cm−1 before measurements. Homogeneous spectra were guaranteed by performing examination at multiple spots. Differential scanning calorimetry (DSC) was performed using a NETZSCH STA 449F3 instrument with samples sealed in platinum pans. The heating/cooling speed was set as 10 K/min with a nitrogen flow rate of 50 mL/min. The temperature-dependent specific heat capacity (Cp) data were obtained with reference to a standard sapphire crystal and plotted using the software NETZSCH proteus. Bulk thermal expansion coefficients (αbulk) were measured using both NETZSCH DIL 402C and NETZSCH DIL 402SU instrument within the temperature range of 25−800 °C. The standard thermal

3. RESULTS 3.1. Crystal Structure. NPD patterns of LaPbMSbO6 (M = Co, Ni) with Rietveld refinements at room temperature are shown in Figure 1a,b. Good fitting results can be obtained by selecting a proper space group of monoclinic P21/n (No. 14) which coincides well with the previously reported literature.16−18,20 Thanks to the large neutron scattering intensity for oxygen atoms, accurate bond parameters correlated with oxygen are capable of being determined by neutron diffraction techniques. The cell parameters as well as the selected bond parameters are given in Table 1. In comparison to the bond parameters obtained from the Rietveld refinement of XRD patterns,17 it is interesting to find that smaller isotropic displacement parameters for oxygen atoms (see Table S1) and closer bond lengths between B−O1 and B−O2 (see Table 1) are identified in this work. Particularly, the M−O1−Sb bond angles, measured to be 158.8° (Co) and 160.2° (Ni), are much greater than the values of ∼150° reported from the XRD pattern refinements.17 This large discrepancy in crystal structure can be ascribed to the inaccurate assignment of the atomic positions of oxygen using X-ray diffraction techniques. Additionally, for the M = Co, Ni samples, refinement results further reveal that the Co/Ni−O−Sb bond angles correlated with the apical oxygen O3 are extremely small (see in Table 1). Figure 1c depicts the corner-shared BO6 octahedra along the c axis. The corresponding bond parameters related with oxygen O3 are labeled. On the basis of the observations above, a significantly large octahedral tilting can be identified with tilting angles of 11.2 and 10.3° for Co and Ni, respectively. Generally, a perovskite with a highly distorted crystal structure always possesses a large energy stemming from the intrinsic lattice stress and becomes thermodynamically unstable. However, a flexible coordinated structure, formed by the C


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Inorganic Chemistry strengthened orbital overlapping between 3d (Co2+/Ni3+) and 2p (O2−), is necessary to stabilize the monoclinic structure. Furthermore, anisotropic displacement parameters (Uani) for all atoms have been refined (see Tables S1 and S2 in the Supporting Information). In particular, necessary constraints are applied on values of Uani for B-site cations Co, Ni, and Sb on consideration of their physical meaning. It can be inferred that the Uani values of oxygen atoms are more sensitive to temperature than those of heavy atoms: i.e., La/Pb, Co, Ni, and Sb. Therefore, in the Co sample, abnormally large U33, U12, and U11 values are obtained for the two planar coordinated oxygen atoms O1 and O2, respectively, in comparison to the magnitude of U11 for the axially coordinated oxygen O3. In the Ni sample, however, considerably large U33, U12, and U12 values are noted for O1 and O3. This difference in Uani values of oxygen is thought to be related to the bonding environment within the B-site sublattice, which possibly indicates a different orbital hybridization path along 3d t2g (Co2+ HS)−2p (O2−) or 3d eg (Ni3+ LS)−2p (O2−). As a consequence, the unusual LTE might be explained by the preferable thermal displacements of O1 and O2 for the Co sample and O1 and O3 for the Ni sample. 3.2. Temperature-Dependent X-ray Diffraction Analysis. In situ temperature-dependent XRD (T-XRD) measurements are widely used to monitor lattice thermal expansion behavior for various negative thermal expansion (NTE) or near zero thermal expansion (ZTE) materials. In Figure 2, superior structural stability against heating is presented for each sample. Moreover, the B-site cation ordering sublattice is also retained at temperatures as high as 500 °C. All diffraction peaks have regular and positive thermal expansion effects except for the (040) and (400) peaks in Co and Ni samples. Consequently, more attention has been paid to deeply investigate the unusual thermal expansion behavior of these two samples. According to the magnified graphs on the right of Figure 2a,b, the whole thermal expansion process could be divided into three stages. (i) Within the low-temperature range (labeled as a white region), the separation between Bragg reflections (040) and (400) keeps getting wider with an increase in temperature until 200 °C for both Co and Ni samples. However, in Figure 2c, no significant temperature dependence on peak separation is observed in the Mn sample. These observations provide solid proof of anisotropic lattice expansion and thus reveal a thermally activated unusual LTE behavior for M = Co, Ni samples. (ii) Within the middle temperature range (labeled as a light yellow region), an obvious enhancement could be found in the intensity of the (040) peak for both Co and Ni samples after the first stage is completed. This process corresponds to the formation of the crystallographic plane of (040) and indicates a thermally activated atomic displacement. (iii) Within the high-temperature range (labeled as a white region), the two (040) and (400) peaks, exhibiting equivalent diffraction intensity, keep moving toward the low-angle direction with a smaller shifting rate vs temperature in comparison to that which appears in the first stage. For the M = Mn sample, without any unusual phenomenon for (040) diffraction being noted, a normal thermal expansion with a constant shifting rate against temperature is shown. In Figure 1a,b, the space group is confirmed to be monoclinic P21/n by NPD refinements at room temperature. Further characterization of T-XRD gives evidence that the space group will not be increased to higher symmetry at high temperature (see Figure S1 and Table S3 in

Figure 2. In situ temperature-dependent X-ray diffraction patterns of LaPbCoSbO6 (a), LaPbNiSbO6 (b), and LaPbMnSbO6 (c). The right graphs show the magnified patterns noted with (040) and (400) reflections. The asterisks label the major diffractions of Pt metal (PDF# 65-2868) from the high-temperature sample holder.

the Supporting Information). Likewise, research on pressureand temperature-induced structural evolution in β-Bi2O333 and Sr2CuWO634 indicates that the crystallographic symmetry will not change under the applied influence of either thermal vibration or compressive stress. The authors studied the correlation between this particular phase transition and Jahn− Teller distortion and concluded that it is an isostructural displacive phase transition (IDPT). In our research, an irregular lattice thermal expansion is observed especially for the Co and Ni samples, wherein the chemical states of B-site elements are identified to be dominantly divalent for cobalt and mixed valence with minor trivalence for nickel by careful analysis of the X-ray photoelectron spectra (see Figures S3 and S4 in the Supporting Information) on the basis of our previous work.17 Hence, the Jahn−Teller (J-T) effect at the B-site sublattice, caused by the degenerate electron occupations of Co2+ 35−37 and Ni3+,38−40 is revealed in the samples of Co and Ni and is considered as the major difference from that in the Mn sample (half-filled d5 configuration without J-T effect). As a result, a cooperative J-T distortion brought on by the A-site Pb2+ ion and B-site Co2+/Ni3+ ions is considered to be highly correlated with the process of IDPT as well as the behavior of anisotropic LTE. A careful discussion will be elaborated on below. D


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Inorganic Chemistry

Figure 3. Temperature dependence of lattice parameters and primitive cell volumes of LaPbCoSbO6 (a, b), LaPbNiSbO6 (c, d), and LaPbMnSbO6 (e, f), respectively.

(d) the distinct αbulk kink at ∼300 °C (Figure 6b). Considering the relatively small content of J-T active Ni3+ ion existing in the Ni sample (see Figure S4), such inconsistent temperatures might be correlated to the inconsistent directions of orbital hybridization paths between 6s(Pb2+)-2p(O2−) and 3d(Ni3+)2p(O2−). This will suppress the NiO6 octahedral tilting (see Table S3) and partially offset the effect of cooperative J-T distortion. In addition, for both Co (Figure 3a) and Ni samples (Figure 3c), substantial decreases have been confirmed for the lattice parameters of a and c when the temperature is raised above 400 °C. This phenomenon can effectively support the anisotropic LTE behavior. Consequently, the interplanar distance perpendicular to the b axis is more sensitive to the thermal-activated atomic displacement in comparison to that of the other axes during the process of IDPT. On the basis of the investigation of in situ T-XRD characterizations, the unusual shift in the (040) plane is seen to be highly connected with anisotropic LTE behavior in Co and Ni samples. For further exploration of the temperature-dependent crystallographic symmetry and structural parameters quantitatively, high-resolution in situ XRD patterns were collected at various temperatures. Then Rietveld refinements were performed (see

It is also confirmed from the variations in lattice parameters that the unusual LTE behavior is not merely limited to the anomalies of (040) diffraction. In Figure 3, the evolutions of lattice parameters and primitive cell volumes have been plotted as a function of temperature. It can be seen that the overall correlation between cell volume and temperature is identified to be positive for the Mn sample (Figure 3f), whereas intriguing humps are noted in Co (Figure 3b) and Ni samples (Figure 3d) with maximum temperatures of 350 and 400 °C, respectively. Above these maxima, shrinkage of cell volume with negative thermal expansion coefficient can be observed. For the Co sample, four specific temperatures, namely the anomaly in T-XRD curves, the onset of phonon mode softening from Raman spectra, the peak of specific heat capacity (noted as Cp), and the kink of bulk thermal expansion coefficient (noted as αbulk), consistently point to the positions around 350 °C. However, for the Ni sample , temperatures obtained from the above characterizations seem to be inconsistent, specifically (a) the turning point of the cell volume curve at ∼400 °C (Figure 3d), (b) the onset of mode softening at ∼150 °C from Raman spectra (Figure 4b), (c) the maximum of small the Cp hump at 234 °C (Figure 5b), and E


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Inorganic Chemistry

Figure 4. Temperature dependence of Raman spectra of LaPbCoSbO6 (a), LaPbNiSbO6 (b), and LaPbMnSbO6 (c) plotted as contour maps using laser excitation of 633 nm. Graphs on the illustrate the fitted Raman spectra of LaPbCoSbO6 (d), LaPbNiSbO6 (e), and LaPbMnSbO6 (f) at 25 and 500 °C, respectively.

structure can be obtained from the parent space group of Fm3̅m by rotations of BO6 octahedra around the [110]c direction.41 Group theoretical analysis reveals that, for the symmetry of Pm3̅m with ideal simple perovskite, all Γ-point phonons are Raman forbidden. However, in the case of an ideal ordered perovskite structure with the space group of Fm3̅m, four Γ-point phonons, noted as irreducible representations (A1g + Eg+ 2F2g), are Raman active.42 Under the lattice distortion coming from the different BO6 octahedra rotating and tilting, the degeneracy of Eg and F2g are lifted on lowering the symmetry to P21/n and more vibration modes become Raman active. As is shown in Figure 4, major signals of Raman spectra are appear intensely within the scattering bands of 300−800 and 100−200 cm−1. For the ideal monoclinic double perovskite A2BB′O6, the Raman spectrum could be deconvoluted into 24 Raman-active modes (12 Ag + 12 Bg) theoretically,42,43 whereas only 10−11 vibration modes can be identified experimentally for LaPbMSbO6 DPOs. The complexities in Raman spectra can be ascribed to the B-site ordered structure with a different environment of the BO6 octahedron skeleton vibration and have been well explained by Bull et al.44 According to previous studies,43,45,46 major acoustic modes in the objective compounds can correspond

Figure S1) and the corresponding crystallographic parameters are given in Table S3 in the Supporting Information. The refinement results indicate that no sign of symmetry change is present at high temperature, although an apparently low-angleshifted (040) reflection is noted. In addition, for samples of both Co and Ni, the cell parameter b undergoes a great enhancement with temperature while the corresponding values of a and c increase moderately. This result accords well with the anisotropic thermal expansion observed in Figure 3. More interestingly, remarkable variations in M−O1 bond lengths and M−O1−Sb bond angles are noted for both Co and Ni samples which possibly result from the cooperative lattice distortion between A sites and B sites. On the basis of the combined crystallographic studies on NPD and in situ XRD, it can be concluded that the anisotropic evolution of bond parameters plays a key role in triggering this unusual LTE phenomenon. 3.3. Temperature-Dependent Raman Spectra. For a better insight into the structure changes with development of temperature, the Raman spectra, plotted as contour maps, are shown for Co, Ni, and Mn samples (see Figure 4a−c). In addition, the spectra fitted at 25 and 500 °C, respectively, are also provided for comparison (see Figure 4d−f). The experimentally confirmed M-Sb ordered monoclinic (P21/n) F


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Inorganic Chemistry

slight peak broadening with indistinct shifting is noticed around 690 cm−1 (assigned as Ag mode), which indicates a considerably high degree of stability of the Mn−Sb ordering structure to heating. In addition, considerable phonon mode softening can be seen at ∼460 and ∼570 cm−1 in Figure 4c. In particular, intriguing envelopes in Raman line width around 580 and 700 cm−1 are shown simultaneously at ∼250 and ∼350 °C in the Co sample. In the case of the Ni sample, similar but faint peak broadening around 700 cm−1 is noted at ∼150 °C. These observations are quite consistent with the onset temperatures of stage 2 and stage 3 from T-XRD characterizations in Figure 2a,b and further evidence a close correlation between unusual LTE and phonon soft mode. ́ Martin-Carró n et al.47 systematically studied the Raman spectra of RMnO3 perovskites and claimed that the stretching mode frequencies correlate to Mn−O bond lengths while the R−O bond and MnO6 octahedron tilt angles dominate the frequencies of bending and tilting modes. Considering the J-T effect of Co2+ and Ni3+, it is not difficult to understand that the extent of phonon softening for the Bg mode around 580 cm−1 is more profound than that for the Ag mode around 700 cm−1 at temperatures of both 250 and 350 °C. With respect to the low-wavenumber region below 200 cm −1 , tremendous enhancements in Raman scattering intensity which str highly related to off-centered Pb2+ ion displacements48 are notid for both Co and Ni samples in Figure 4d,e at 500 °C. This phenomenon indicates a very sensitive lattice response to temperature under the cooperative effect of J-T distortion over A-site and B-site sublattices. In addition, an intense softening of the Bg mode around 140 cm−1 is observed in the Co sample, which is commonly believed to drive a displacive structural phase transition in the Pb-based perovskites49,50 and might contribute to the unusual LTE behavior. 3.4. Calorimetry. Figure 5 exhibits the temperature dependence of heat capacities (Cp) with fitted baselines. For a nonlinear fit to the baseline data, the higher approximated equation of the Debye model is

Figure 5. Temperature dependence of specific heat capacities of LaPbCoSbO6 (a), LaPbNiSbO6 (b), and LaPbMnSbO6 (c). The baselines are fitted using both the Debye model (red dashed line) and Einstein model (blue dashed line).

ij Θ 2 1 yz C V = 3nR jjj1 − D 2 zzz j 20 T z{ k

to an symmetric B−O stretching vibration (denoted as Ag within 650−710 cm−1), an antisymmetric B−O stretching vibration (denoted as 3Bg within 550−650 cm−1), an O−B−O bending vibration (denoted as Bg within 450−550 cm−1), and an A-site ion correlated lattice vibration (within 50−150 cm−1). From Figure 4a,b, it is clear to see that remarkable peak broadening with noticeable low-wavenumber shifting is present for all visible Raman signals in either the Co or Ni sample with an increase in temperature. In contrast, for the Mn sample,

(1)

The parameters n and R denote the atomic number for each primitive cell and the mole gas constant, respectively. The Debye temperatures, symbolized by ΘD, signify the bonding strength of the lattice and are calculated to be 350 K for Co, 347 K for Ni, and 321 K for Mn. These values, which are somewhat smaller than those for LaMnO3 (427 K)51 and

Figure 6. Temperature-dependent bulk thermal expansion coefficients (red circles) of LaPbCoSbO6 (a) and LaPbNiSbO6 (b) measured from 100 to 800 °C. G


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Inorganic Chemistry

reaching 11.8 × 10−6 K−1. Within the high-temperature region starting from 370 °C, αbulk keeps rising linearly until it reaches the measured limit of 800 °C. Beyond that point, a diffusion of data together with a deviation from linearity could be seen for the αbulk evolution. For the Ni sample, however, there are three regions, which can be readily divided by two αbulk kinks. After that, a slow increase of αbulk from 11.5 × 10−6 to 12.9 × 10−6 K−1 is observed across a wide temperature range of 500 °C. To evidence that this unusual thermal expansion behavior was intrinsic, repeated measurements were performed on extra parallel samples using two types of thermal expansion instruments. According to the results shown in Figure S5, abnormal αbulk elevation always appeared within the narrow temperature ranges of 280−370 °C (for the Co sample) and 200−320 °C (for the Ni sample), respectively. These temperature ranges have good repeatability and coincide well with those of unusual lattice parameters (Figure 3) and of Ag phonon mode softening from T-Raman (Figure 4). It should be emphasized that the αbulk kink discrepancy together with the inconsistent abnormal temperatures determined by different methods could be partially explained by extrinsic factors: i.e., chemical inhomogeneity, poor compactness, and microcracks.

La0.8Ca0.2MnO3 (460 K),52 are well consistent with the onset temperatures of the soft mode from Raman measurments as well as octahedral tilting from XRD. The equation of the Einstein model is exp(ΘE /T ) iΘ y Cp = 3nR jjj E zzz + αT 2 k T { [exp(ΘE /T ) − 1] 2

(2)

The parameters of ΘE and α denote the Einstein temperature and free factor. For LaPbCoSbO6, it can be seen from Figure 5a that a group of continuous thermal anomalies which can be deconvoluted by two major Cp peaks (260 and 352 °C) are observed within the temperature range of 200−400 °C. Different from the case of the Co sample, only one small humplike Cp anomaly (Figure 5b), accompanied by a weak exothermal peak in the DSC results (Figure S6), is noted at 234 °C in the Ni sample. These aforementioned Cp anomalies accompanied by an the exothermal signal from the DSC curve are suggested to be typical features of a secondorder phase transition.52,53 For the M = Mn sample (Figure 5c), a good fitting without noticeable Cp peaks is applied over the whole temperature range, although subtle deviation from the Einstein model can be seen above 380 °C. This observation indicates a high degree of structural stability against an increase in temperature. It should be noted that the critical temperatures wherein the Cp anomalies are located for Co and Ni samples are well consistent with the emerging temperatures of anisotropic LTE effects (labeled as red XRD patterns of 250 and 350 °C, respectively, in Figure 2): i.e., the transition temperatures of the second-order IDPT. In addition, the enthalpy changes (ΔHm) of the second-order phase transitions in Co and Ni samples can be estimated from the integrated area of Cp peaks. The corresponding thermodynamic parameters are calculated as ΔHm(1) = 216 J mol−1 and ΔHm(2) = 196 J mol−1 for the M = Co sample and ΔHm = 128 J mol−1 for the M = Ni sample. All the values of ΔHm are of hundreds of joules per mole, which are much smaller than the first-order structural phase transition enthalpy (3.36 kJ mol−1) of LaMnO350 but close to the magnitude of the magnetic phase transition enthalpy of Ln1−xAexMO3 (Ln = Pr, Nd, Sm; Ae = Ca, Sr; M = Cr, Mn) with second-order nature.53,54 The magnitude of ΔHm indicates the energetic barrier of the phase transition which helps to understand the thermodynamic stability during the process of IDPT. Consequently, it can be inferred by a comparison of ΔHm that the high-temperature structural phase of the Co sample exhibits a more stable thermodynamic state in comparison to that of the Ni sample. 3.5. Bulk Thermal Expansion Coefficient Measurements. In order to deeply investigate the unusual LTE behaviors found in Co- and Ni-based samples, the bulk thermal expansion coefficients αbulk, defined by the derivative function of ΔL/L0 to temperature T, are measured as a function of temperature ranging from 100 to 800 °C. On the basis of the slope changes of the linear αbulk curves, the processes of thermal expansion can be recognized as multiple regions. For the Co sample, three regions are divided by two αbulk kinks where the first is located at ∼280 °C and the second is located at ∼370 °C, which not only are close to the temperatures of Cp anomalies (260 and 352 °C) but are also consistent with the onset temperatures of IDPT (250 and 350 °C) from T-XRD and T-Raman. Within the white region below 280 °C, slight enhancement of αbulk is shown from 10.0 × 10−6 to 10.1 × 10−6 K−1. Then a steep steplike increase is followed with αbulk

4. DISCUSSION Figure 7 shows the schematic crystal structure of a Co or Ni sample observed along the [001] direction. The (040) plane

Figure 7. Schematic picture of a supercell crystal structure plotted within the ab plane. The dashed lines denote crystal planes of (040).

labeled with a yellow parallelogram is shown to be formed by two planar coordinated oxygen atoms: i.e., O1 and O2. Therefore, the unusual shift of (040) diffraction peaks in Figure 2a,b could be attributed primarily to the factors of BO6 tilting along the c axis and the variation of bond lengths correlated with O1 and O2. Reminiscent of the first stage of the LTE process in Co and Ni samples, the character of dramatic (040) shifting is addressed while the correlated diffraction intensity is shown to be unchanged. This result can be well explained by the thermal-activated stretching of highly distorted Co/NiO6−SbO6 octahedral chains along the c axis. With a continuous elevation of temperature, the B−O bond lengths in connection with O1 and O2, which exhibit noticeable divergence at room temperature in Table 1, become convergent to almost the same values. During this stage, an abrupt growth of (040) diffraction intensity with slight peak shifting can be observed. After the second stage, the thermal fluctuation of planar oxygen comes to an end and quite regular H


Inorganic Chemistry

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arranged (040) planes are formed. Then, a traditional thermal expansion mechanism predominates again in the third stage at high temperature. It should be stressed that the Pb2+ ion with second-order Jahn−Teller (SOJT) activation is widely known to introduce asymmetric lattice distortion as well as an abnormal LTE phenomenon. However, for the sample LaPbMnSbO 6 , where the Mn 2+ ion with half-filled d 5 configuration does not have a J-T effect, a normal LTE behavior results. Accordingly, we propose that a cooperative Jahn−Teller effect, formed through the extended orbital hybridization of 6s(Pb)-2p(O)-3d(Co/Ni) between A- and B-site sublattice, might be suspected as the major cause of this anisotropic LTE phenomenon. In addition, several small potential heats, i.e. phase transition enthalpies, identified in Figure 5a,b for samples of both Co and Ni correspond to the typical features of a second-order phase transition, whereas no hint of a structural phase transition with lifted symmetry is confirmed. Buerger et al.55,56 recognized two major structural groups of phase transitions, which include a reconstructive type and a displacive or reversible type. They demonstrated that, within the crystal structure evolution of the lateral type, the secondary bonds may be broken but the primary bonds are still unaffected. Then many researchers57,58 have further investigated the origin of pure displacive phase transitions (DPTs) and clarified the importance of polyhedral tilting in triggering DPT. For a structural phase transition governed by polyhedral tilting, no symmetry change or small phase transition enthalpy is expected, which is often explained by a soft phonon mode.59,60 Consequently, the concept of isostructural displacive phase transition (IDPT) is well described. Unfortunately, few studies related to the investigations of IDPT have been reported. Until recently, some compounds with quasi-IDPT features, such as Ca-rich pigeonite61 and Pb-contained lawsonite,62 were found to have different space groups but unchanged point groups at different temperatures, which may shed light on the understanding of this interesting behavior.

Article

ASSOCIATED CONTENT

S Supporting Information *

are provided in the text. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03595. Additional structural information, XRD patterns with Rietveld refinements at various temperatures, Raman spectra, fitted X-ray emitted photoelectron spectra of Co 2p and Ni 2p core levels, temperature-dependent bulk thermal expansion coefficients of three parallel ceramic samples, and DSC results (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Y.B.: tel, +86-471-6575722; fax, +86-471-6575722; e-mail, [email protected]. *J.M.: e-mail, [email protected]. ORCID

Yijia Bai: 0000-0002-7948-5332 Jianmin Hao: 0000-0001-9414-1741 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21661025), the Open Project of State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Science (Grant No. RERU2014001), and the Key Project of Natural Science Foundation of Inner Mongolia University of Technology (Grant No. ZD201401). The NPD measurement was performed at the SPODI instrument operated by TUM and KIT at the Heinz Maier-Leibnitz Zentrum (MLZ), Garching, Germany. The authors thank Dr. Yuanding Huang and Dr. Weimin Gan from MLZ for the help with data collection.

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5. CONCLUSIONS In summary, we have synthesized a series of antimony-based double perovskites LaPbMSbO6 (M = Mn, Co, Ni). Multiple in situ characterizations, such as T-XRD, T-Raman, T-Cp, and T-αbulk, were performed to get an insight into the unusual LTE phenomenon found in M = Co, Ni samples. It is suggested that the anisotropic LTE behavior, accompanied by a the secondorder isostructural displacive phase transition, can be well understood by a cooperative J-T distortion induced by the extended orbital hybridization via 6s(Pb)-2p(O)-3d(Co/Ni). Accordingly, interesting modifications on lattice thermal expansion could be realized in the LaPbMSbO6 perovskites through introducing Jahn−Teller distortion at both A- and Bsite sublattices. In addition, with regard to the normal LTE behavior seen in the Mn sample, good thermal stability of the Mn−Sb ordering structure has been evidenced even under at a high temperature of 500 °C. As a result, for the LaPbMSbO6 perovskite ceramics, superior structural stability to high temperature might be expected by introducing rigidly bonded ions with a half or fully occupied electron configuration at Bsite. This study has built a connection between anisotropic thermal expansion and cooperative J-T effects. Intense investigations focusing on the derivatives of the LaPbMSbO6 prototype will benefit the exploration of novel thermal− mechanical coupled functional materials.

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Connection between Unusual Lattice Thermal Expansion and

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