[H₃N(CH₂)₇NH₃]₈(CH₃NH₃)₂Sn(IV)Sn(II)₁₂I₄₆ – a mixed-valent hybrid compound with a uniquely templated defect-perovskite structure

Abstract

The incorporation of H₃N(CH₂)₇NH₃ with CH₃NH₃SnI₃ resulted in the formation of a mixed-valent and semiconducting (Eg = 0.84 eV) organic-based perovskite, [H₃N(CH₂)₇NH₃]₈(CH₃NH₃)₂Sn(IV)Sn(II)₁₂I₄₆, with a unique 3D defect-perovskite structure with ordered vacancies at the Sn and I sites.

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The electronic delocalization properties of mixed-valent perovskites continue to be of great interest, in part because of their relevance to studies on high-Tc superconductivity¹ as well as catalytic, dielectric, optical, transport and magnetic properties of transition metal perovskites.^2,3 Introduction of mixed-valency in perovskite compounds is usually accompanied by significant structural distortions, substitutions and/or formation of vacancies and defects within the perovskite framework. The important structural flexibility of perovskites (ABX₃) is partly due to the ability of the B cations to adopt a variety of coordination numbers, and robustness of the B–X network in accommodating vacancies in the cation (A, B) and anion (X) sites.^3–5 Most defect perovskites (metal oxides and halides) exhibit vacancies at the cation A- (CN = 12) and anion X- (e.g. O, F, Cl, Br, I) sites. However, occurrences of vacancies at the B-sites are rare and have been ascribed to thermodynamic instability of vacancies due to the higher charge and smaller size of the B-cations.⁵

Organic-based metal halide perovskites make up an important class of tunable materials.⁶ In particular, the tunable electronic properties of the hybrid Sn and Pb halides arise from the systematic modular combinations of the organic and inorganic components.⁷ They also exhibit a unique structural flexibility accompanied by a modulation of their electronic properties. This is exemplified by the transition from semiconducting to metallic behavior, with increasing dimensionality, in the different families of layered tin(II) iodide perovskites.^6,7 Their structures are significantly influenced by the nature and molecular conformations of the organic components, as effectively shown in the families of layered perovskites, and in the polymorphism of α- and β-(NH₃(CH₂)₅NH₃)SnI₄.⁸ The formation of metal defects in organic-based layered perovskites has been templated by organic molecules, albeit in a hybrid Bi(III) iodide.⁹

Efforts to introduce mixed-valency in organic-based Sn halide perovskites have so far been unsuccessful. This important question relates to the metallic cubic perovskites, CsSnI₃ and CH₃NH₃SnI₃, which exhibit high mobilities and low carrier (hole) densities.¹⁰ Yet to date, efforts to increase the carrier (hole) densities by doping (oxidation) have been unsuccessful. Herein we report the synthesis of a mixed-valent organic-based tin halide defect-perovskite, [H₃N(CH₂)₇NH₃]₈(CH₃NH₃)₂Sn(IV)Sn(II)₁₂I₄₆ (1).

The title compound was prepared by reacting stoichiometric amounts of SnI₂ (0.2 mmol) and the corresponding organic ammonium iodide salts in concentrated HI solution (3 ml). The resulting HI solution was heated to 130 °C and then slowly cooled to room temperature. Formation of very dark-red brick crystals was observed during slow cooling. The crystals were moderately air stable at room temperature, in that the crystals were unchanged over 2 days. As a general precaution, all reactions and experimental manipulations were carried out under nitrogen atmosphere, and solvents were degassed before use. The typical reaction yield was 95%.

Compound 1 crystallizes in the tetragonal space group P4₂/mnm and its crystal structure is shown in Fig. 1.¹¹ The complex 3D perovskite structure can be described using three structural components: a) perovskite columns of corner-shared octahedra, CH₃NH₃SnI_{(4 = 4/2+2/1)}; b) chains of trans corner-shared square pyramids, SnI_{(4 = 2/2+3/1)}; c) isolated SnI₆ octahedra. The columns are normal perovskite units with CH₃NH₃, Sn and I at the A-, B- and X-sites, respectively. The Sn–I bonding distances (d_ave = 3.144 Å) within the columns are similar to those observed in CH₃NH₃SnI₃ and their Sn(II) layered derivatives.^6–10 The columns are further linked through their iodine-vertices, along the a–b plane, with the unshared basal vertices of the square pyramidal SnI₄ chains. The Sn–I distances (d_ave = 3.11 Å) in the SnI₅ square pyramids are also comparable to those in low-dimensional Sn(II)–I networks. The remaining A-sites are occupied by the terminal -NH₃ of the diamine cations, H₃N(CH₂)₇NH₃. The cations template cross-like cavities within the 3D perovskite network with vacancies occuring at the B- (Sn) and X- (I) sites. One type of empty B-sites, collinear with the Sn and the apical I of the square pyramids, are occupied by Sn which is further coordinated by six I atoms that do not coordinate to any other tin atom. However, two iodine atoms of the isolated octahedral SnI₆ units can be derived from a nominal octahedral coordination around the neighboring square pyramidal Sn. The square pyramidal SnI₅ and isolated SnI₆ units can be derived from a breathing mode distortion (contraction and elongation) of the Sn–I distances around adjacent SnI₆ octahedra with a concomitant formation of stereochemical lone pairs and isolation of the other SnI₆ octahedra. Inspection of the Sn–I distances within the different Sn–I polyhedra indicate the isolated SnI₆ octahedra exhibit significantly shorter Sn–I distances (d_ave = 2.862(1) Å) than those observed in other Sn(II) iodide perovskites. The observed Sn–I distances are comparable to Sn⁴⁺–I distances in Cs₂Sn⁴⁺I₆ (Sn–I (CN = 6): 2.864(1) Å) and in tin(IV) iodide organometallic complexes (Sn–I (CN = 6): 2.78–82 Å).¹² Assignment of the tetravalent oxidation state to the corresponding Sn atom yields the proper ionic charge balance: [H₃N(CH₂)₇NH₃]²⁺₈(CH₃NH₃)⁺₂[Sn(IV)Sn(II)₁₂I₄₆]¹⁸⁻.

Fig. 1 A [001] view of the crystal structure of 1. The [Sn(II)I₆] and [Sn(IV)I₆] octahedra are represented as dark-yellow and green polyhedra, respectively; [Sn(II)I₅] square pyramids are shown as light yellow pyramids; C and N are represented as black and blue spheres, respectively. The unit cell is outlined.

The defect structure of 1 from the perovskite can be traced to two B-site (Sn) vacancies in the structure. These are shown in Fig. 2. The first type (A) is surrounded by three iodines from neighboring SnI₆ octahedral units. Three other missing iodines (2/2 + 1) are associated with this vacancy resulting in a vacant “SnI₂” unit. Similarly, the second vacancy (B) has 4 neighboring I from the neighboring Sn–I polyhedra (2 SnI₆ + 2 SnI₅). Two other missing iodines are part of a missing “SnI₂” unit in vacancy A. The occurrence of Sn and I vacancies allows the alkyldiamine cations to occupy the A-sites of the perovskite framework, and the voids created by the vacancies are effectively compensated by the alkyl backbone of the cations. The ratio of A to B sites is 4 and the vacancies account for “Sn₅I₈” in the idealized perovskite; i.e. “A₁₈Sn₁₈I₅₄” − “Sn₅I₈” = “A₁₈Sn₁₃I₄₆”. The unit cell relationships also confirm the assignment, and a superstructure (1_cell ≈ 3√2a_p × 3√2a_p × 2a_p) of the perovskite cell (p) results from the ordered vacancies.

Fig. 2 The B-site vacancies, A and B, viewed along 001 and 110, respectively. Dashed lines indicate I coordination around each vacancy. The [Sn(II)I₆] and [Sn(IV)I₆] octahedra are shaded dark-yellow and green, respectively. The [Sn(II)I₅] square pyramids are shaded light-yellow. The C and N atoms are represented as black and blue spheres, respectively.

A crucial factor in the formation of the structure is that the distance between nearest neighbor A-sites (∼6.3 Å) and the syn-conformation length of the alkyl chain in the cation (∼6.9 Å) are essentially matched, and the -CH₂NH₃ ends of the diamine cations correspond to the CH₃NH₃ of the parent perovskite. Furthermore, incorporation of other diamine cations NH₃(CH₂)_nNH₃, n ≠ 7 results in the expected non-defect layered perovskites.¹³ Thus, the critical templating role of the organic cation is convincingly exhibited in the novel structure. The end member of this new family of 3-D defect-perovskite structures, [NH₃(CH₂)₇NH₃]₂Sn₃I₁₀, has also been prepared.¹³

UV-Vis-NIR diffuse reflectance measurements indicated an optical band gap of 0.84 eV.¹⁴ The observed crystal structure and narrow-gap semiconducting behavior are consistent with a mixed-valent assignment, in accord with the Robin–Day model.¹⁵ This is surprising considering the cubic Sn(II) iodide perovskites are metallic and conducting due to delocalized Sn 5s² electrons. Introducing ‘electronic holes’ in the form of Sn(IV) states apparently results in localization of the carriers (holes) at the Sn(IV)I₆ octahedra. In addition, some Sn 5s states in the perovskite network are further localized at the Sn(II)I₅ square pyramids as evidenced by the appearance of stereochemically active lone pairs. However, the remaining perovskite columns may still exhibit electronic character more associated with the conducting parent CH₃NH₃SnI₃. Interestingly, high-pressures may be needed in inducing the delocalization of the localized states (holes and lone-pairs), as in the mixed-valent Au(I/III) perovskites.¹⁶ More intriguing is that the hole concentration in 1 is 8% which closely approaches the hole concentrations in the superconducting perovskites.¹

Notes and references

1. (a) K. A. Muller and J. G. Bednorz, Science, 1987, 237, 1113; (b) A. W. Sleight, Science, 1988, 242, 1519; (c) R. J. Cava, Science, 1990, 247, 656; (d) A. W. Sleight, Acc. Chem. Res., 1995, 28, 103.
2. L. J. Tejuca and J. L. G. Fierro, Eds., Properties and Applications of Perovskite Type Oxides, Marcel Dekker, New York, 1993.
3. C. N. R. Rao and B. Raveau, Transition Metal Oxides, VCH Publishers, Inc: Weinheim, Germany, 1995.
4. (a) M. T. Anderson, J. T. Vaughey and K. R. Poeppelmeier, Chem. Mater., 1993, 5, 151; (b) D. M. Smyth, Annu. Rev. Mater. Sci., 1985, 15, 329.
5. M. A. Pena and J. L. G. Fierro, Chem. Rev., 2001, 101, 1981.
6. (a) D. B. Mitzi, Prog. Inorg. Chem., 1999, 48, 1; (b) G. C. Papavassiliou, Prog. Solid State Chem., 1997, 25, 125.
7. (a) D. B. Mitzi, C. A. Field, W. T. A. Harrison and A. M. Guloy, Nature, 1994, 369, 467; (b) D. B. Mitzi, S. Wang, C. A. Field, C. A. Chess and A. M. Guloy, Science, 1995, 267, 1473.
8. J. Guan, Z. Tang and A. M. Guloy, Chem. Commun., 1999, 1833.
9. D. B. Mitzi, Inorg. Chem., 2000, 39, 6107.
10. (a) D. Z. Weber, Z. Naturforsch., Teil B, 1978, 33, 862; (b) D. B. Mitzi, C. A. Field, Z. Schlesinger and R. B. Laibowitz, J. Solid State Chem., 1995, 114, 159.
11. Crystal data: tetragonal, space group P4₂/mnm; a = 26.9174(13) Å, c = 12.7329(9) Å, V = 9225.6(9) ų, Z = 2; μ = 9.455 mm⁻¹; diffractometer, Siemens SMART; T = 223 K; 47031 reflections measured, 4411 unique (R_int = 0.0393), 3523 reflections with I/σ(I) > 2, R1 = 0.0527, wR2 = 0.1102; R1 = 0.0689 and wR2 = 0.1320 (all data); Crystallographic Software: SHELXTL. Thermal parameters of non-hydrogen atoms were treated anisotropically. Idealized positions of H atoms were calculated (C–H = 0.96 Å, N–H = 0.90 Å). CCDC 246164. See http://www.rsc.org/suppdata/cc/b4/b411322e/ for crystallographic data in .cif or other electronic format.
12. D. Tudela, A. J. Sanchez-Herencia, M. Diaz, R. Fernandez-Ruiz, N. Menendez and J. D. Tornero, J. Chem. Soc., Dalton Trans., 1999, 4019.
13. J. Guan, PhD Dissertation, University of Houston, 2001.
14. (a) W. W. Wendlandt and H. G. Hecht, Reflectance Spectroscopy, Interscience Publishers, NY, 1966; (b) G. Kotiim, Reflectance Spectroscopy, Springer Verlag, New York, 1969.
15. M. B. Robin and P. Day, Adv. Inorg. Chem. Radiochem., 1967, 10, 247.
16. N. Kojima, M. Hasegawa, H. Kitagawa, T. Kikegawa and O. Shimomura, J. Am. Chem. Soc., 1994, 116, 11368.

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Footnotes

† Electronic supplementary information (ESI) available: crystallographic tables and diffuse reflectance spectrum for 1. See http://www.rsc.org/suppdata/cc/b4/b411322e/

‡ This work was partially supported by the R. A. Welch Foundation, the Petroleum Research Fund and the NSF.

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