Synthesis, crystal structure, magnetic and electronic properties of the caesium-based transition metal halide Cs₃Fe₂Br₉

Abstract

The diversity of halide materials related to important solar energy systems such as CsPbX₃ (X = Cl, Br, I) is explored by introducing the transition metal element Fe. In particular a new compound, Cs₃Fe₂Br₉ (space group P6₃/mmc with a = 7.5427(8) and c = 18.5849(13) Å), has been synthesized and found to contain 0D face-sharing Fe₂Br₉ octahedral dimers. Unlike its isomorph, Cs₃Bi₂I₉, it is black in color, has a low optical bandgap of 1.65 eV and exhibits antiferromagnetic behavior below T_N = 13 K. Density functional theory calculations shed further light on these properties and also predict that the material should have anisotropic transport characteristics.

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1. Introduction

In the past few years, lead halide perovskites such as APbI₃ (A = methylammonium, MA, and cesium) have attracted much attention as photovoltaic materials because of their remarkable photo-conversion efficiency in solar cell devices.^1,2 Due to the toxicity of lead and the intrinsic moisture sensitivity of the lead(II) compounds, a search for environmentally friendly alternatives has been undertaken.³ Several perovskite-related families have been proposed, such as double perovskites where Pb²⁺ is replaced by isoelectronic Bi/In/Sb³⁺ and a monovalent cation, e.g. (MA)₂KBiCl₆, (MA)₂TlBiBr₆, (MA)₂AgBiBr₆ and the inorganic phases Cs₂AgBX₆ (X = Cl, Br and B = Bi, In).^4–11 Another popular candidate family is A₃M₂I₉, where A = K⁺, Rb⁺, NH₄⁺, MA⁺, Cs⁺etc., M = Bi³⁺ and Sb³⁺, consisting of either corner- and edge-sharing MI₆ octahedral layers or face-sharing MI₆ dimers.^12–16 All of the above systems exhibit very interesting optoelectronic properties.

Transition metals have attracted our attention as a method of tuning the optoelectronic properties. For example, using Fe³⁺ to replace Bi³⁺ can reduce the bandgap: Cs₂NaFeCl₆, which adopts a double perovskite architecture (Fig. S1, ESI†) is red, while its Cl analogues with other trivalent cations show much lighter colours. For instance, Cs₂NaBiCl₆ is yellow¹⁷ while the Cs₂NaLnCl₆ (Ln = Lanthanide) phases are mostly white.¹⁸ A much darker color is expected for the hypothetical Cs₂NaFeBr₆, but our attempts to synthesize this compound yielded black octahedral crystals of composition Cs₂FeBr₅·H₂O (Fig. S2, ESI†), crystallizing in space group Pnma. This material consists of 0D FeBr₅O octahedral monomers in which the oxygen is part of a water molecule, as in the known Cs₂FeCl₅·H₂O.¹⁹ The dimensionality indicates the degree of connectivity of the octahedra. In this case the octahedra are discrete.

Incorporating Fe into the A₃Bi₂X₉ (X = Cl, Br and I) family turns out to have a long history. Cs₃Fe₂Cl₉, which is dark red in color, was reported to form two polymorphs: a 2D layered system with P3̄m1 symmetry and 0D dimeric system in space group P6₃/mmc,^20,21 In the latter, both intradimer and interdimer magnetic interactions are present, and the two competing interactions lead to very interesting magnetic properties. In the present work, we report a new compound, Cs₃Fe₂Br₉ (CCDC 1575068), which is isostructural with Cs₃Bi₂I₉ (red)¹³ and (MA)₃Bi₂I₉ (red),¹⁴ yet is black in color. Its variable temperature behavior, thermal stability, optical and magnetic properties are investigated in combination with density functional theory (DFT) calculations.

2. Experimental and computational methods and results

2.1 Synthesis

A two-step synthesis method was used, involving both hydrothermal and room temperature crystallization. 2 mmol CsBr (99.9%, Sigma Aldrich), 1 mmol FeCl₃·6H₂O (>99%, Sigma Aldrich) together with 1.5 ml HBr acid (47 wt%) were placed in a 23 ml stainless steel Parr autoclave and heated at 160 °C for 3 days. Intermediate products of brown needle shaped crystals of CsFeBr₄ (Fig. S3, ESI†) were formed. The Teflon autoclave was then left in the fume hood at room temperature (>15 °C) and black crystals formed after one week. The following chemical reactions take place during the synthesis:

CsBr + FeCl₃·6H₂O + 3HBr → CsFeBr₄ + 3HCl + 6H₂O (1)

CsBr + 2CsFeBr₄ → Cs₃Fe₂Br₉ (2)

During the hydrothermal process, reaction (1) dominates and almost no black Cs₃Fe₂Br₉ is formed. Even using exact stoichiometric ratios of the starting reagents does not result in the target material. However, black octahedral crystals of Cs₃Fe₂Br₉, ∼0.5 mm in size, can be collected after standing at room temperature for 3 weeks. The sample is soluble in most polar solvents, including water, ethanol and acetone.

2.2 Crystallographic studies

Cs₃Fe₂Br₉ crystallizes in the hexagonal space group P6₃/mmc (a = 7.5427(8) and c = 18.5849(13) Å). It consists of face-sharing Fe₂Br₉ octahedral dimers with Cs serving as bridging atoms between the dimers (Fig. 1a and b). The octahedra are slightly distorted, with two sets of Fe–Br bonds (2.427(1) Å and 2.701(2) Å) and distorted Br–Fe–Br angles (80.76(6)°, 90.55(3)° and 97.01(7)°), compared to the nominal octahedral angle of 90°. Due to the Coulombic repulsive force between the cations within the dimer (Fe–Fe distance = 3.585(3) Å), the Fe³⁺ ions are displaced outwards with respect to the shared face. Therefore, the smallest octahedral angles and longer Fe–Br bonds are found with the shared Br⁻ ions (Fig. 1) and the largest angles and shorter Fe–Br distances are from the unshared ones. According to the interatomic distances, the bond strengths between Fe³⁺ and unshared Br⁻ are stronger than those with shared Br⁻ ions. Moreover, the angular distortion of the Br_shared–Fe–Br_unshared angle is minor (90.55°). The shortest distance between Cs and Br is 3.762(1) Å.

Fig. 1 (a) Crystal structure of Cs₃Fe₂Br₉ viewed along the c-axis, (b) view along the b-axis showing of the Fe₂Br₉ dimers. The angles and bond lengths illustrate the distortion of the octahedra. The subscripts s and u indicate shared and unshared Br anions respectively and the arrows indicate the direction of Coulombic repulsion between cations, (c) cell volume and (d) lattice parameters as a function of temperature measured using single crystal X-ray diffraction.

Variable temperature single crystal diffraction suggests no phase transition down to 120 K. The thermal expansion coefficients are approximately linear with α_a = 45.3 MK⁻¹, α_c = 39.6 MK⁻¹, giving α_v = 131.2 MK⁻¹. The repulsion between the Fe³⁺ ions in the dimeric unit decreases upon cooling, as shown by the less distorted octahedral dimer and reduced interatomic Fe⋯Fe distances (Fig. S4, ESI†). As a result, negative expansion occurs for the shorter bonds and positive thermal expansion is found for the longer bonds. A similar phenomenon is observed for the octahedral angles: on cooling, the smaller angles tend to increase, while the larger angles decrease.

2.3 Thermal analysis

Thermal stability was investigated using an SDT (simultaneous differential scanning calorimetry (DSC) – thermogravimetric analysis (TGA)) Q600 instrument. Powder samples were heated from room temperature to 1123 K at 10 K min⁻¹ under an air flow of 100 ml min⁻¹. Cs₃Fe₂Br₉ is stable until 537.5 K and then experiences a two-step decomposition process (Fig. 2). When the sample is heated, moisture and residual HBr at the particle surfaces start to evaporate, resulting in a small weight loss (∼3.6%) at the beginning of the curve. For comparison, the thermal stability of its bismuth analogues Cs₃Bi₂I₉ and MA₃Bi₂I₉ were also measured; the former decomposed at 636.4 K, while the latter was stable until 529.3 K (Fig. S5 and S6, ESI†).

Fig. 2 Thermogravimetric analysis curve; the inset shows a photo of the sample (small crystals).

2.4 Optical characterization

The optical bandgap was measured on a PerkinElmer Lambda 750 UV-Visible spectrometer in the absorption mode with a 2 nm slit width. The scan interval was 1 nm and the scan range was between 500 and 1100 nm. The absorption edge is observed at ∼800 nm (i.e. 1.55 eV). In accordance with our DFT calculation (see below), we deduced a direct optical bandgap of ∼1.65 eV from the Tauc plot derived from the reflectance spectrum (Fig. 3). Note that the analogous A₃Bi₂I₉ phases (A = Cs and MA) were reported to have indirect bandgaps which are larger in the range 1.9 eV to 2.2 eV.^22,23

Fig. 3 (a) Absorption spectrum and (b) Tauc plot for indirect and direct bandgaps. (c) Band structure (non-magnetic case) calculated using the HSE06 exchange–correlation functional. (d) Charge density isosurfaces (antiferromagnetic case) calculated using the PBEsol exchange–correlation functional and viewed along the b-axis. The top and bottom panels show the Highest Occupied Crystal Orbital (HOCO) and Lowest Unoccupied Crystal Orbital (LUCO) respectively. The charge is displayed using a threshold of 0.001 e Bohr⁻³. The different spin channels are shown in blue and green. Atom colors are the same as in Fig. 1.

2.5 Density functional calculations

The DFT calculations were performed using the projector augmented wave (PAW) method as implemented in VASP.²⁴ The experimental structure obtained at room temperature was fully optimized using the PBEsol exchange–correlation functional²⁵ which reduced the atomic forces below 1 meV Å⁻¹ at effectively zero Kelvin (see ESI† for further computational details). The resulting atomic positions are given in Table S1 (ESI†). The presence of Fe in the material suggests that it could exhibit magnetic ordering due to unpaired 3d electrons. To examine this possibility, spin-polarized calculations were performed on the optimized structure in the ferromagnetic (FM) state and three possible antiferromagnetic (AFM) states. It was found that one of the AFM states in which neighboring Fe atoms have opposite spin orientation is significantly lower in energy than either the FM or non-magnetic states, by 80 meV f.u.⁻¹ and 335 meV f.u.⁻¹ respectively (see Table S2 (ESI†) for details). The calculations therefore predict that at very low temperatures Cs₃Fe₂Br₉ prefers to be antiferromagnetic. The calculated magnetic moment on each Fe atom is 3.38 μ_B. This value is lower than the value of 5.79 μ_B obtained from analysis of the magnetic susceptibility data in the higher temperature paramagnetic region (see below). There are several reasons for this, including the well-known reduction in spin in magnetically ordered structures due to covalency. For example, neutron scattering measurements on FeCl₃ show that the spin is reduced to 4.7(3) μ_B in the antiferromagnetic phase.²⁶Fig. 3 shows charge density isosurfaces corresponding to the HOCO and LUCO for the lowest energy AFM state.

In order to determine an improved band structure for Cs₃Fe₂Br₉ the HSE06 hybrid exchange–correlation functional was used,²⁷ although it is acknowledged that GW would normally be the preferred method. The calculation was performed on the non-magnetic state to contain the cost of the calculation and because previous work has indicated that, while HSE06 provides a reasonable band structure, it may not be adequate for magnetic properties.²⁸ The material is found to have a 2.254 eV direct band gap which occurs at the Γ point with a relatively flat band structure (Fig. 3). At the band edge it is possible to calculate the effective masses in the parabolic approximation (Table 1). The values indicate a high anisotropy with reduced transport along the c-direction (Γ → A). The Fe atoms have been described with 3p⁶3d⁷4s¹ as valence electrons, while other core states have been substituted by the pseudopotential. The valence band maximum (VBM) contains Fe 3d and Br 4p states, whereas the conduction band minimum (CBM) contains mostly Fe 3d, Fe 4s and Br 4p states.

Table 1 Calculated effective masses (relative to the rest mass m₀)

	Γ → M	Γ → K	Γ → A
m h*/mo	−0.11	−0.06	−1.02
m e*/mo	0.25	0.16	13.87

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2.6 Magnetic measurements

Magnetic susceptibility measurements, χ(T) = M(T)/H, were conducted using a Quantum Design Magnetic Properties Measurement System (MPMS3) with a superconducting interference device (SQUID) magnetometer. Measurements were made after cooling in zero field (ZFC) and in a measuring field (FC) of μ₀H = 0.01 T over the temperature range 2 ≤ T ≤ 300 K. Cs₃Fe₂Br₉ shows antiferromagnetic behavior with a Néel temperature T_N = 13 K (Fig. 4), higher than that of analogous Cs₃Fe₂Cl₉ which also exhibits an antiferromagnetic long range order at T_N = 5.3 K.²⁹ The results are in good agreement with the DFT calculations.

Fig. 4 Magnetic susceptibility as a function of temperature, blue: experimental, red square: fitted curve using dimer model. The inset illustrates the definition of J, J_p and J_c. Cs atoms are not included for clarity.

Appling the Curie–Weiss law in the paramagnetic region (from 50 K to 300 K), a negative Weiss constant of −36.10(3) K was obtained, as expected for an antiferromagnetic compound (Fig. S7, ESI†) and the calculated effective magnetic moment μ_eff was 5.79(4) μ_B. In order to obtain a comprehensive χ fitting from 2 K to 300 K, a weakly coupled dimer model was applied (see ESI† for fitting formula).³⁰ This system contains Fe³⁺ ions as dimeric units Fe₂Br₉, with three distinct Fe–Fe distances (3.585(3), 7.179(2) and 7.543(1) Å) corresponding to intradimer (J) and interdimer (J_c + J_p) interactions, respectively. The dominant intradimer exchange yields J = −8.2 K, while the weak interdimer interactions between the Fe³⁺ of neighbouring dimers are given as J_p + J_c = −3.4 K. The intradimer interactions are weaker than those in the Cr counterpart, Cs₃Cr₂Br₉, which was reported to have J = −10.3 K, corresponding to a shorter Cr–Cr distance ∼3.317(4) Å. On the other hand, the interdimer interactions of Cs₃Cr₂Br₉ are weaker with J_p + J_c = −1.1 K due to the longer distances (7.420(1) and 7.507(1) Å).³⁰ In the case of Cs₃Fe₂Cl₉, both intra- and interdimer interactions are smaller (J = −2.4 K and J_p + J_c = −1.2 K for single crystals).²⁹

3. Conclusions

We have synthesized black crystals of Cs₃Fe₂Br₉ and determined its crystal structure. Cs₃Fe₂Br₉ crystallizes in the hexagonal space group P6₃/mmc and the structure contains Fe₂Br₉ face-sharing octahedral dimers. The Fe–Br bond strengths differ between the shared and unshared faces, and the distorted octahedra tend to become more symmetrical upon cooling due to the reduced cation–cation Coulombic repulsion. The compound is thermally stable up to 537.5 K, and has an optical bandgap of 1.65 eV. DFT calculations indicate that the band gap is direct and also predict reduced transport along the c-direction. Magnetic susceptibility measurements show antiferromagnetic behavior, with T_N = 13 K, and can be fitted with a weakly coupled dimer model. The spin polarized DFT calculations agree with this behavior at low temperatures and predict which antiferromagnetic configuration is preferred.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

F. Wei is a holder of an A*STAR international fellowship granted by the Agency for Science, Technology and Research, Singapore. A. K. Cheetham and Y. Wu thank the Ras al Khaimah Center for Advanced Materials for financial support. The calculations were performed at the Cambridge HPCS and the UK National Supercomputing Service, ARCHER. Access to the latter was obtained via the Materials Chemistry Consortium and funded by EPSRC under Grant Number EP/L000202/1.

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Footnote

† Electronic supplementary information (ESI) available: Additional information, including CIF files, experimental methods and further information on the DFT calculations. CCDC 1575067–1575070. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7tc04798c

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