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Apr 13, 2017 - from other NTE materials like ferroelectric PbTiO3-based compounds,3 charge-transfer LaCu3Fe4O12. 4 and BiNiO3,5 magnetic Invar alloys,...
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Large Positive Thermal Expansion and Small Band Gap in DoubleReO3‑Type Compound NaSbF6 C. Yang,†,‡ B. Y. Qu,§ S. S. Pan,† L. Zhang,∥ R. R. Zhang,∥ P. Tong,*,† R. C. Xiao,†,‡ J. C. Lin,† X. G. Guo,†,‡ K. Zhang,†,‡ H. Y. Tong,†,‡ W. J. Lu,† Y. Wu,† S. Lin,† W. H. Song,† and Y. P. Sun*,†,∥,⊥ †

Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China ‡ University of Science and Technology of China, Hefei 230026, People’s Republic of China § Laboratory of Amorphous Materials, School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, People’s Republic of China ∥ High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China ⊥ Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China S Supporting Information *

ABSTRACT: Double-ReO3-type structure compound NaSbF6 undergoes a lowtemperature rhombohedral to high-temperature cubic phase between 303 and 323 K, as revealed by temperature-dependent X-ray diffractions. Although many double-ReO3-type fluorides exhibit either low thermal expansion or negative thermal expansion (NTE), NaSbF6 exhibits positive thermal expansion (PTE) with a large volumetric coefficient of thermal expansion, αv = 62 ppm/K, in its cubic phase. Raman spectroscopy reveals that the low-frequency transverse vibration of fluorine atoms is stiffened in NaSbF6, compared with the typical NTE compound CaZrF6 with the same structure. The related weak contraction associated with the polyhedral rocking would be overcome by the notable elongation of the Na−F bond length on heating, thus leading to the large volumetric PTE. Unlike ScF3 and CaZrF6 which are insulators with a wide band gap, a relative small band gap of 3.76 eV was observed in NaSbF6. The small band gap can be attributed to the hybridization between the Sb 5s and F 2p orbitals.

1. INTRODUCTION Recent years have witnessed more and more metal fluorides being reported for their special physical properties arising from the highly ionic character of the M−F bonding.1,2 Among all of them, metal fluorides with a ReO3-type structure have attracted extraordinary attentions since the observation of pronounced negative thermal expansion (NTE) in cubic ScF3.2 Different from other NTE materials like ferroelectric PbTiO3-based compounds,3 charge-transfer LaCu3Fe4O124 and BiNiO3,5 magnetic Invar alloys,6 and Mn-based antiperovskite compounds,7−12 such phonon related NTE usually emerges over a quite wide temperature range.2,13,14 Compared with other similar open-framework compounds showing NTE property such as ZrW2O813 and Ag3[Co(CN)6],14 the ReO3-type structure is so simple that the rigid unit models (RUMs) associated with the low-frequency transverse vibration, which is considered responsible for the NTE, could be clearly illustrated.2,15,16 Except for ScF3, however, almost all the metal trifluorides adopt a low-symmetry disordered structure at room temperature, which usually present strong positive thermal expansion (PTE).17−20 Furthermore, the number of such ReO3-type metal trifluorides is limited. Recent attentions are paid to the ABF6-structure compounds which also adopt a © XXXX American Chemical Society

cation-ordered ReO3-type structure, built up by corner-linked AF6 and BF6 octahedra (namely, the so-called double-ReO3type structure).21,22 The dynamic rotations of the octahedra in cubic phase could always provide possibilities for the emergence of NTE. For example, cubic CaZrF6 and CaHfF6 have recently been reported to display a larger magnitude of NTE coefficient than that of ScF3.21,22 Meanwhile, many double-ReO3-type metal fluorides often undergo a rhombohedral (R3̅) to a cubic (Fm3̅m) phase transition.21,22 For those compounds, the LiSbF6-type usually refers to the rhombohedral structure and NaSbF6-type to the cubic structure,23 which is always considered to be necessary for the NTE showing up. Nevertheless, the coefficient of thermal expansion (CTE) of NaSbF6 is lacking in the literature and the temperature point of phase transition is still controversial.23,24 Here, we demonstrate that the rhombohedral to cubic transformation occurs at 303−323 K in NaSbF6. The hightemperature cubic phase exhibits a large volumetric CTE of αv ∼ 62 ppm/K. Temperature-dependent Raman spectroscopy shows a remarkable stiffness of the fluorine transverse Received: January 4, 2017

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DOI: 10.1021/acs.inorgchem.7b00002 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Between 303 and 323 K, both phases coexist, which is typically shown in the XRD pattern at 303 K. In the past,

vibrations in PTE compound NaSbF6, compared with that observed in CaZrF6.25 Moreover, analyses of the temperaturedependent X-ray diffraction (XRD) data suggest that, in the cubic phase, the Na−F bond extends remarkably with the increasing temperature while the Sb−F bond shrinks. However, the elongation of Na−F bond lengths is larger than the reduction of the Sb−F bond, which absolutely counteracts the contraction related to the polyhedral rocking. Moreover, UV− vis absorption measurement shows that NaSbF6 exhibits a noticeable visible-light absorption (∼400−700 nm). A relatively small band gap (∼3.76 eV) was observed in NaSbF6. The probable reason for this is also discussed.

2. EXPERIMENTAL AND CALCULATION DETAILS The polycrystalline NaSbF6 sample (3 N) was obtained commercially from the Xiya Reagent Company (China). Variable temperature XRDs were performed on a Philips X’ pert PRO X-ray diffractometer, operating at 40 kV and 40 mA with Cu Kα radiations (Kα1 = 1.5406 Å, Kα2 = 1.5418 Å). Temperature-dependent lattice parameters were obtained from the standard Rietveld refinement. UV−vis−NIR spectra were obtained by a diffused reflection method with a spectrometer (UV-3600, Shimadzu Ltd.) using BaSO4 as the standard background. The temperature-dependent Raman vibrational spectra ranged from 150 to 370 K and were recorded from a Raman Spectrometer equipped with a Torus 532 nm solid laser (Horiba Jobin Yvon T64000). The first-principles calculations based on density functional theory (DFT) were carried out using the QUANTUM-ESPRESSO package.26 The ultrasoft pseudo-potentials and the general gradient approximation (GGA) according to the PBE functional were used. The energy cutoff for the plane-wave (charge density) basis was set to 60 Ry (1500 Ry). The Brillouin zone was sampled with a 8 × 8 × 8 mesh of k-points. The lattice constants and ions were optimized using the Broyden−Fletcher−Goldfarb−Shanno (BFGS) quasi-Newton algorithm. The phonon frequencies at the Γ-point were calculated using density functional perturbation theory (DFPT),27 and the symmetries of the Γ phonons were analyzed using the IR Raman and HyperRaman Modes in Bilbao Crystallographic Server.28

Figure 2. Standard Rietveld refinements of the XRD patterns of the rhombohedral phase (a) and the cubic phase (b) of NaSbF6. Insets show the two crystal structures.

Fukushima investigated the site symmetry at the antimony in NaSbF6 by means of nuclear magnetic resonance. Very small deformation from Oh symmetry of the surrounding atoms was observed at 293 K, which was just regarded as the phase transition temperature.23 Sowa reported press-induced Fm3̅m to R3̅ phase transition in NaSbF6 and established that cubic single-crystal NaSbF6 could stably exist at room temperature, thus at least meaning the low-symmetry distortion could not destroying the cubic crystal around room temperature.24 However, neither work provided the exact phase transition temperature of NaSbF6, which could be clearly seen in our present study. The crystal structure of the R3̅ phase is plotted in the inset of Figure 2a, where the cations are ordered, but the corner-linked NaF6 and SbF6 octahedra are rotated relatively. In the hightemperature cubic phase as shown in the inset of Figure 2b, such rotations are absent. Lattice parameters of NaSbF6 at different temperatures were obtained from the Rietveld refinements of the XRDs. Lattice parameters in a hexagonal setting, ah and ch, are reduced to their cubic equivalents as shown in Figure 3a. In the rhombohedral phase (a = b ≠ c, α = β = 90°, γ = 120°), the lattice parameter a increases while c decreases with the rise of temperature. In the temperature zone with phase coexistence, a and c change rapidly to an intermediate value, corresponding to the “unfolding” of the tortile Na−F−Sb bond. Above 323 K, the lattice constant of the cubic phase continuously increases upon heating, leading to a

3. RESULTS AND DISCUSSION Temperature-dependent XRDs are shown in Figure 1. Bragg reflections for the high-temperature (>323 K) phase of NaSbF6 can be well indexed with the double-ReO3-type structure (space group, Fm3m ̅ ), whereas the low-temperature (320 K), the stiffening of the F2g mode is consistent with that observed for CaZrF6.24 It is rather C

DOI: 10.1021/acs.inorgchem.7b00002 Inorg. Chem. XXXX, XXX, XXX−XXX

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elements such as Sc, Zr. Therefore, the weak ionic Sb−F bonding in NaSbF6 should be responsible for the cubic to rhombhedral phase transition at room temperature. UV−vis absorption deduced from the corresponding UV− vis−NIR diffuse reflection spectra measurement was carried out for NaSbF6, and the result is shown in Figure 6. A slightly

abnormal in CaZrF6 because the transverse vibrations (namely, the F2g mode), responsible for the NTE property, should display phonon softening with the contracting crystal lattice.24 However, it is reasonable here, since NaSbF6 shows PTE in its cubic phase. A recent investigation on the NTE mechanism of ScF3 reveals that the Sc−F nearest-neighbor distance strongly expands with increasing temperature, while the Sc−Sc nextnearest-neighbor distance contracts.29 It is the large transverse vibrations of fluorine atoms perpendicular to the Sc-F-Sc chain that give rise to the pronounced NTE.29 For CaZrF6, Hancock et al. reported that, when the temperature is above 350 K, the apparent Zr−F bond lengths decrease on heating while those for Ca−F start to increase.21 As shown in Figure S2, analyses of the XRD data show that the apparent Na−F bond stretches remarkably with increasing temperature while the Sb−F bond turns short. Obviously, the elongation of the Na−F bond length is larger in magnitude than the drop of the Sb−F bond length. The overall thermal expansion behavior is a combination of all lattice vibration contributions. At present, it is not clear the exact contributions from each vibration modes in NaSbF6. However, according to the Raman spectroscopy discussed above, the stiffness of the transverse vibration of F atoms indicates a much weakened negative Grüneisen contribution to the overall thermal expansion. The rotation of NaF6 and SbF6 octahedra is restrained strongly, so the contraction related to polyhedral rocking could not overcome the normal PTE due to the remarkable elongation of the Na−F bond length at all. Consequently, the large PTE was observed in NaSbF6. In double-ReO3-type fluorides, the NTE comes from the transverse vibrations of the F atom.21,22 For a large ionic radius, a weak bond strength can be expected, which would lead to softening of the structural framework and thus to enhanced transverse vibrations of fluorides. In contrast, a small ionic radius will stiffen the structural framework and thus degrade the NTE.22 For example, as reported by Hu et al., in MZrF6 (M = Ca, Mn, Fe, Co, Ni, and Zn), the NTE is suppressed as the radius of M2+ decreases. In those compounds, NiZrF6 displays the largest PTE as Ni2+ has the smallest ionic radius, r(Ni2+) = 0.69 Å.22 In NaSbF6, the ionic radius for Sb5+ and Na1+ is 0.6 and 1.02 Å, respectively. The F1− anion locates closer to Sb5+ rather than to Na1+ because of the distinct cation sizes. The vibration of the F atom should be more influenced by Sb5+ relative to Na1+ along the Na-F-Sb link. Such a small size of Sb5+ may strongly restrict the vibrations of F vertical to the SbF-Na link, which gives rise to the well stiffened F2g phonon mode as revealed by our Raman spectroscopy. Therefore, it is understandable that the PTE of NaSbF6 is a little stronger than that of NiZrF6.22 As a result of the strongest electronegativity of fluorine atoms, the chemical bonds are usually highly ionic when consisting of reactive metal cations and fluorine anions, such as ScF3,2 CaZrF6, and CaHfF6.21 Most ionic bonds are relatively stable because of the strong Coulomb electrostatic interaction. It is exactly why ScF3 and CaZrF6 maintain cubic symmetry to extremely low temperature, which is necessary for strong NTE emerging.21 As proposed in ref 21, the highly ionic bonding in CaZrF6 plays an important role in stabilizing its cubic structure against distortion down to 10 K. Here in NaSbF6, the element Sb just lies right at the boundary line between metallic elements and nonmetallic elements. The metallicity of element Sb is much weakened relative to Sc, Ca, and Zr. Consequently, the Sb−F bond is less ionic than those consisting of F and metallic

Figure 6. UV−vis−NIR optical absorption spectra at 323 K of NaSbF6. Inset shows the plot of (Ahv)2 against photon energy for NaSbF6, indicating the band gap (Eg) of ∼3.76 eV.

noticeable visible-light absorption (400−700 nm) was observed. By Tauc plotting, the direct optical band gap of the NaSbF6 is determined to be 3.76 eV at the intercept of the linear part of the curve with (αhυ)2 = 0, as shown in the inset of Figure 6. Such a band gap is relatively narrow compared with that of the strong NTE material ScF3 (Eg ≈ 5.76 eV).30 The previous work clarified that (Sc, Fe)F3 could exhibit semiconductivity with rather narrow band gaps.30 Here, we also performed theoretical calculations of the electronic band dispersion, density of states (DOS), and partial density of states (PDOS) of the cubic NaSbF6. As shown in Figure 7, the

Figure 7. Computed electronic band dispersion, density of states (DOS), and partial density of states (PDOS) of the cubic NaSbF6. The Fermi level is set to 0 eV.

high valence bands are mainly composed by the 2p states of F. The antibonding orbitals of the Sb−F bonds contribute to the low conduction bands. From the PDOS shown in Figure 7, we can see that the lower conduction bands are constructed by the 5s state of Sb and 2p states of F. It is considerable that the Sb 5s states have chances to hybridize with the F 2p states. Therefore, compared with ScF3, enhanced orbital overlapping due to the closer separation distance between Sb and F may result in a smaller band gap in NaSbF6. D

DOI: 10.1021/acs.inorgchem.7b00002 Inorg. Chem. XXXX, XXX, XXX−XXX

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under Contract U1632158, and the Key Research Program of Frontier Sciences, CAS (QYZDB-SSW-SLH015).

Recently, the research on ScF3 opens a new avenue for realizing magnetic semiconductors in fluorides.30 For example, Hu et al. reported that Fe-doped scandium fluoride (Sc,Fe)F3 can be functionalized to exhibit high-Curie-temperature ferromagnetism and narrow-gap semiconductor feature (Eg = 1.87 eV for Sc0.9Fe0.1F3).30 As shown in Figure S1, NaSbF6 is quite stable in air. Only a very small amount of impurity (NaSbF6·nH2O) appears after a 6 h exposure in air. Upon doping with magnetic elements, the band gap is expected to be narrowed,30 which might also improve the stability in air. Therefore, NaSbF6 compound with a direct band gap of 3.76 eV, which is smaller than that of ScF3 (5.76 eV),30 could be a suitable matrix for revealing a diluted magnetic semiconductor.

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4. CONCLUSIONS In summary, we reported the phase transition and thermal expansion of the NaSbF6 sample by variable temperature X-ray diffractions. It is revealed that NaSbF6 adopts rhombohedral symmetry below 303 K and cubic symmetry above 323 K. In between 303 and 323 K, both cubic and rhombohedral phases coexist. Unlike most double-ReO3-type metal fluorides, NaSbF6 exhibits large PTE (αv = 62 ppm/K) in its cubic phase. The result of Raman study suggests much stiffened transverse vibration mode of fluorine atoms, compared with the NTE compound CaZrF6. By analyzing the XRD data, it is revealed that the remarkable elongation of the Na−F bond length compensates for any contraction associated with polyhedral rocking, thus playing an important role in the emerging large PTE of NaSbF6. Moreover, a relatively small band gap of ∼3.76 eV was observed in NaSbF6. The hybridization between the Sb 5s states and the F 2p states, due to enhanced orbital overlapping, might be responsible for the smaller band gap of NaSbF6 than that of ScF3.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00002. XRD patterns of the NaSbF6 sample, variations of the apparent Na−F and Sb−F bond lengths, and calculated vibrational frequencies of NaSbF6 at the Γ-point of the Brillouin zone (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Tel: +86-551-6559-2785. Fax: +86-551-6559-1434. E-mail: [email protected] (P.T.). *Tel: +86-551-6559-2757. Fax: +86-551-6559-1434. E-mail: [email protected] (Y.P.S.). ORCID

C. Yang: 0000-0002-2497-7014 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (Nos. 51322105, 51371005, 51301167, 51301165), the Joint Funds of the National Natural Science Foundation of China and the Chinese Academy of Sciences’ Large-Scale Scientific Facility E

DOI: 10.1021/acs.inorgchem.7b00002 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry (20) Morelock, C. R.; Gallington, L. C.; Wilkinson, A. P. Solid solubility, phase transitions, thermal expansion, and compressibility in Sc1−xAlxF3. J. Solid State Chem. 2015, 222, 96. (21) Hancock, J. C.; Chapman, K. W.; Halder, G. J.; Morelock, C. R.; Kaplan, B. S.; Gallington, L. C.; Bongiorno, A.; Han, C.; Zhou, S.; Wilkinson, A. P. Large negative thermal expansion and anomalous behavior on compression in cubic ReO3-type AIIBIVF6: CaZrF6 and CaHfF6. Chem. Mater. 2015, 27, 3912. (22) Hu, L.; Chen, J.; Xu, J. L.; Wang, N.; Han, F.; Ren, Y.; Pan, Z.; Rong, Y. C.; Huang, R. J.; Deng, J. X.; Li, L. F.; Xing, X. R. Atomic Linkage Flexibility Tuned Isotropic Negative, Zero, and Positive Thermal Expansion in MZrF6 (M = Ca, Mn, Fe, Co, Ni, and Zn). J. Am. Chem. Soc. 2016, 138, 14530. (23) Fukushima, E. Nuclear magnetic resonance study of the NaSbF6 structure. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1971, A27, 65. (24) Sowa, H. Pressure-Induced Fm-3m to R-3 Phase Transition in NaSbF6. Acta Crystallogr., Sect. B: Struct. Sci. 1997, B53, 25. (25) Sanson, A.; Giarola, M.; Mariotto, G.; Hu, L.; Chen, J.; Xing, X. R. Lattice dynamics and anharmonicity of CaZrF6 from Raman spectroscopy and ab initio calculations. Mater. Chem. Phys. 2016, 180, 213. (26) Giannozzi, P.; et al. QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials. J. Phys.: Condens. Matter 2009, 21, 395502. (27) Baroni, S.; de Gironcoli, S.; Dal Corso, A.; Giannozzi, P. Phonons and related crystal properties from density-functional perturbation theory. Rev. Mod. Phys. 2001, 73, 515. (28) Aroyo, M. I.; Orobengoa, D.; de la Flor, G.; Tasci, E. S.; PerezMato, J. M.; Wondratschek, H. Brillouin-zone database on the Bilbao Crystallographic Server. Acta Crystallogr., Sect. A: Found. Adv. 2014, A70, 126. (29) Hu, L.; Chen, J.; Sanson, A.; Wu, H.; Rodriguez, C. G.; Olivi, L.; Ren, Y.; Fan, L. L.; Deng, J. X.; Xing, X. R. New insights into the negative thermal expansion: direct experimental evidence for the “Guitar-String” effect in cubic ScF3. J. Am. Chem. Soc. 2016, 138, 8320. (30) Hu, L.; Chen, J.; Fan, L. L.; Ren, Y.; Huang, Q. Z.; Sanson, A.; Jiang, Z.; Zhou, M.; Rong, Y. C.; Wang, Y.; Deng, J. X.; Xing, X. R. High-Curie-Temperature Ferromagnetism in (Sc, Fe)F3 Fluorides and its Dependence on Chemical Valence. Adv. Mater. 2015, 27, 4592.

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DOI: 10.1021/acs.inorgchem.7b00002 Inorg. Chem. XXXX, XXX, XXX−XXX