Synthesis, crystal structure, magnetic and electronic properties of the caesium-based transition metal halide Cs 3 Fe 2 Br 9 †

The diversity of halide materials related to important solar energy systems such as CsPbX 3 (X = Cl, Br, I) is explored by introducing the transition metal element Fe. In particular a new compound, Cs 3 Fe 2 Br 9 (space group P 6 3 / mmc with a = 7.5427(8) and c = 18.5849(13) Å), has been synthesized and found to contain 0D face-sharing Fe 2 Br 9 octahedral dimers. Unlike its isomorph, Cs 3 Bi 2 I 9 , 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.


Introduction
In the past few years, lead halide perovskites such as APbI 3 (A = methylammonium, MA, and cesium) have attracted much attention as photovoltaic materials because of their remarkable photo-conversion efficiency in solar cell devices. 1,2Due to the toxicity of lead and the intrinsic moisture sensitivity of the lead(II) compounds, a search for environmentally friendly alternatives has been undertaken. 3Several perovskite-related families have been proposed, such as double perovskites where Pb 2+ is replaced by isoelectronic Bi/In/Sb 3+ and a monovalent cation, e.g.3][14][15][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 Fe 3+ to replace Bi 3+ can reduce the bandgap: Cs 2 NaFeCl 6 , 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, Cs 2 NaBiCl 6 is yellow 17 while the Cs 2 NaLnCl 6 (Ln = Lanthanide) phases are mostly white. 18A much darker color is expected for the hypothetical Cs 2 NaFeBr 6 , but our attempts to synthesize this compound yielded black octahedral crystals of composition Cs 2 FeBr 5 ÁH 2 O (Fig. S2, ESI †), crystallizing in space group Pnma.This material consists of 0D FeBr 5 O octahedral monomers in which the oxygen is part of a water molecule, as in the known Cs 2 FeCl 5 ÁH 2 O. 19 The dimensionality indicates the degree of connectivity of the octahedra.In this case the octahedra are discrete.
Incorporating Fe into the A 3 Bi 2 X 9 (X = Cl, Br and I) family turns out to have a long history.Cs 3 Fe 2 Cl 9 , which is dark red in color, was reported to form two polymorphs: a 2D layered system with P% 3m1 symmetry and 0D dimeric system in space group P6 3 /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, Cs 3 Fe 2 Br 9 (CCDC 1575068), which is isostructural with Cs 3 Bi 2 I 9 (red) 13 and (MA) 3 Bi 2 I 9 (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.

Synthesis
A two-step synthesis method was used, involving both hydrothermal and room temperature crystallization. 2 mmol CsBr (99.9%,Sigma Aldrich), 1 mmol FeCl 3 Á6H 2 O (499%, 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 1C for 3 days.Intermediate products of brown needle shaped crystals of CsFeBr 4 (Fig. S3, ESI †) were formed.The Teflon autoclave was then left in the fume hood at room temperature (415 1C) and black crystals formed after one week.The following chemical reactions take place during the synthesis: CsBr + 2CsFeBr 4 -Cs 3 Fe 2 Br 9 During the hydrothermal process, reaction (1) dominates and almost no black Cs 3 Fe 2 Br 9 is formed.Even using exact stoichiometric ratios of the starting reagents does not result in the target material.However, black octahedral crystals of Cs 3 Fe 2 Br 9 , B0.5 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.

Crystallographic studies
Cs 3 Fe 2 Br 9 crystallizes in the hexagonal space group P6 3 /mmc (a = 7.5427( 8) and c = 18.5849(13)Å).It consists of face-sharing Fe 2 Br 9 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)1, 90.55(3)1 and 97.01(7)1), compared to the nominal octahedral angle of 901.Due to the Coulombic repulsive force between the cations within the dimer (Fe-Fe distance = 3.585(3) Å), the Fe 3+ 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 À 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 Fe 3+ and unshared Br À are stronger than those with shared Br À ions.Moreover, the angular distortion of the Br shared -Fe-Br unshared angle is minor (90.551).The shortest distance between Cs and Br is 3.762(1) Å.
Variable temperature single crystal diffraction suggests no phase transition down to 120 K.The thermal expansion coefficients are approximately linear with a a = 45.3MK À1 , a c = 39.6 MK À1 , giving a v = 131.2MK À1 .The repulsion between the Fe 3+ ions in the dimeric unit decreases upon cooling, as shown by the less distorted octahedral dimer and reduced interatomic FeÁ Á ÁFe 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.

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 À1 under an air flow of 100 ml min À1 .Cs 3 Fe 2 Br 9 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 (B3.6%) at the beginning of the curve.For comparison, the thermal stability of its bismuth analogues Cs 3 Bi 2 I 9 and MA 3 Bi 2 I 9 were also measured; the former decomposed at 636.4 K, while the latter was stable until 529.3K (Fig. S5 and S6, ESI †).

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 B800 nm (i.e.1.55 eV).In accordance with our DFT calculation (see below), we deduced a direct optical bandgap of B1.65 eV from the Tauc plot derived from the reflectance spectrum (Fig. 3).Note that the analogous A 3 Bi 2 I 9 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

Density functional calculations
The DFT calculations were performed using the projector augmented wave (PAW) method as implemented in VASP. 24The experimental structure obtained at room temperature was fully optimized using the PBEsol exchange-correlation functional 25 which reduced the atomic forces below 1 meV Å À1 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.À1 and 335 meV f.u.À1 respectively (see Table S2 (ESI †) for details).The calculations therefore predict that at very low temperatures Cs 3 Fe 2 Br 9 prefers to be antiferromagnetic.The calculated magnetic moment on each Fe atom is 3.38 m B .This value is lower than the value of 5.79 m 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 FeCl 3 show that the spin is reduced to 4.7(3) m 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 Cs 3 Fe 2 Br 9 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. 28The material is found to have a 2.254 eV direct band gap which occurs at the G 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 (G -A).The Fe atoms have been described with 3p 6 3d 7 4s 1 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.

Magnetic measurements
Magnetic susceptibility measurements, w(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 m 0 H = 0.01 T over the temperature range 2 r T r 300 K. Cs 3 Fe 2 Br 9 shows antiferromagnetic behavior with a Ne ´el temperature T N = 13 K (Fig. 4), higher than that of analogous Cs 3 Fe 2 Cl 9 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. 1
Fig. 1 (a) Crystal structure of Cs 3 Fe 2 Br 9 viewed along the c-axis, (b) view along the b-axis showing of the Fe 2 Br 9 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.

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

Fig. 3
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 À3 .The different spin channels are shown in blue and green.Atom colors are the same as in Fig. 1.

Table 1
Calculated effective masses (relative to the rest mass m 0 )