Molybdenum chloride double perovskites: dimensionality control of optical and magnetic properties

Abstract

Halide double perovskites are a promising class of semiconducting materials for applications in solar cells and other optoelectronic devices. Recently, there has been a surge of interest in these materials to study phenomena beyond optoelectronics, especially magnetism. Here, we report three new Mo³⁺ (4d³) based chloride double perovskites: a 3-D rock-salt ordered Cs₂NaMoCl₆, a 1-D chain (MA)₂AgMoCl₆ and a Dion–Jacobson type 2-D layered (1,4-BDA)₂AgMoCl₈ (MA = methylammonium; 1,4-BDA = 1,4-butanediammonium). Their structures and dimensionalities can be tuned by means of the A-cation. The measured bandgaps are relatively narrow (2.0–2.1 eV) which show a blueshift on reducing the dimensionality. At low temperatures, we observe antiferromagnetic coupling between the nearest-neighbour Mo³⁺ ions in all these systems. Cs₂NaMoCl₆ shows stronger coupling with a frustration index f of 5 which we attribute to the geometrically frustrating fcc lattice of Mo³⁺ ions. This work expands the scope of halide double perovskites beyond main group metals and beyond optoelectronics, and we hope that it will lead to future developments in magnetic halide perovskites.

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1. Introduction

Halide double perovskites (HDPs) (also known by their alternative name, elpasolite) are typically a class of quaternary compounds of composition A₂B^IB^IIIX₆ (A = A-site monocation; B^I = B-site monovalent metal; B^III = B-site trivalent metal; X = halide). At the six-coordinated B-site, B^I and B^III metal ions show rock-salt ordering, which is often driven by their charge difference.¹ In some cases, such as Cs₂Hg^IIM^IICl₆ (M^II = Cu or Pd), coordination geometry can drive the B-site metal ordering even though the charge difference is zero.² The inorganic lattice of A₂B^IB^IIIX₆ could be constructed by transmutation of two divalent metal ions (B^II) of a AB^IIX₃ single perovskite into one B^I ion and one B^III ion, i.e. 2B^II → B^I + B^III.³ The interest in HDPs began to surge in 2016, when three research groups independently reported the synthesis and the optoelectronic properties of Cs₂AgBiX₆ (X = Cl, Br).^1,4,5 Since then, many such materials have been realized, thanks to the rich degree of freedom in selecting B^I and B^III metals. Most of the HDPs of current interest are based on post-transition metals with a d¹⁰s² configuration (such as Sb⁶ and Bi⁷) or d¹⁰s⁰ configuration (such as In⁸) at the B^III site and any size-matching monovalent metals (such as Ag, Tl, Na, and K) at the B^I site, and have been widely reported for use in photovoltaics,⁹ X-ray detectors,¹⁰ photodetectors,¹¹ and humidity sensors.¹² Unfortunately, they have their own limitations mainly due to large band gaps and large effective carrier masses.^13,14

During the 1970s to the 1990s, a number of fluoride double perovskites of trivalent 3d metal ions,^15–19 and some 4d metal ions such as Mo,^20,21 and Rh,²² were reported. Rare-earth metals were also incorporated in fluoride based double perovskites.^23,24 At that time, particular interest was focused on synthesis and structures, and in some cases magnetism. In light of the recent surge of interest in semiconducting halide perovskites, there have been reports on incorporating transition metals at the perovskite B-site with the intention to develop novel perovskites with high potential to be used in emerging applications such as spin-electronics, where exchange interaction between unpaired spins is important.^25–28 Fe^III(3d⁵),²⁹ and Ru^III (4d⁵)³⁰ have been alloyed into the benchmark double perovskite Cs₂AgBiBr₆ to decrease its bandgap by introducing an intermediate band inside the pristine bandgaps. Recently, Ning et al. reported interesting magnetic exchange properties in Fe-alloyed Cs₂AgBiBr₆ and suggested that this material has potential for use in spintronics.³¹ Fe^III based 3D perovskites, Cs₂M^IFe^IIICl₆ (M^I = Na or Ag) have been reported as narrow bandgap semiconductors (E_g = ∼1.5 eV),³² as well as geometrically frustrating antiferromagnetic systems.³³ Ferromagnetic Cu^I–Cu^II–In^III based layered halide double perovskites have been recently reported.³⁴ Moving to 4d metals increases the importance of spin–orbit coupling (SOC) in exotic physics of unpaired spins such as magnetic frustration in layered α-RuCl₃,^35,36 α-MoCl₃,³⁷ and other heavy transition metal halides.³⁸ High SOC metal ions such as Ru^III, and Rh^III have been incorporated in 1-D chain,^39,40 and 2-D layered double perovskites.⁴¹ The effect of SOC on the magnetic properties has been studied in these low dimensional systems. The Ru^III systems display unusual magnetic properties, particularly, deviation from the Kotani model for a low spin d⁵ metal. By using the Kotani model, single ion magnetic properties of vacancy ordered double perovskites of Ru^IV,⁴² and Os^IV,⁴³ both low spin d⁴ ions, have been recently studied. These recent reports indeed suggest that transition metal based HDPs are of potential interest, which researchers have begun to explore.

In this work, we sought to introduce Mo^III (4d³) as the magnetic ion into three new chloride based double perovskites: a 3-D rock-salt ordered Cs₂NaMoCl₆, a 1-D chain (MA)₂AgMoCl₆ and a 2-D layered (1,4-BDA)₂AgMoCl₈ (where MA = methylammonium; 1,4-BDA = 1,4-butanediammonium). To our knowledge, Cs₂NaMoCl₆ is the first example of a chloride based 3-D HDP in which an open-shell 4d metal is incorporated at the B-site. Structurally, it is similar to prototype compound Sr₂FeMoO₆ which is one of the heavily discussed oxide perovskites in the condensed matter physics community in terms of unusual magnetic and electronic properties arising from the rock salt ordered sub-lattices of Fe^III (d⁵) and Mo^V (d¹) ions.^44,45 (MA)₂AgMoCl₆ and (1,4-BDA)₂AgMoCl₈ present rare examples of 1-D chain and 2-D layered hybrid double perovskites, respectively, in which both B^I and B^III metals are transition metal ions, i.e. Ag and Mo. We find that their structures are tuneable with different A-cations, i.e., Cs, MA, and 1,4-BDA. Their optical and magnetic properties can be controlled with both chemical composition and structural dimensionality. Their X-ray structures, optical absorption, and magnetism are discussed below.

2. Results and discussion

2.1. Synthesis

The synthetic protocol for the compounds reported here is similar to the method reported in the literature by some of us (see the ESI for the method†).³⁹ Typically, to obtain Cs₂NaMoCl₆ and (MA)₂AgMoCl₆, a mixture of ACl (A = Cs or MA), B^ICl (B^I = Na or Ag) and MoCl₃ was heated hydrothermally in aqueous hydrochloric acid as the solvent. Crystals were obtained on slow cooling of the reaction autoclaves. Similarly, (1,4-BDA)₂AgMoCl₈ was obtained from the reaction of 1,4-butanediamine, AgCl and MoCl₃. Initially in all the reactions, we observed the formation of a green solution likely stemming from the formation of higher valence Mo ions in aqueous HCl. A stoichiometric amount of H₃PO₂ (reducing agent) was used to control the oxidation. The structures of the as synthesized compounds were determined from single-crystal X-ray diffraction (SCXRD) data. Selected unit-cell parameters are given in Table 1 and the details in Tables S1–S5.† The selected bond parameters are given in Table S6.† The simulated powder X-ray diffraction (PXRD) patterns from the SCXRD data show a good match with the experimental PXRD patterns, confirming that all these compounds can be prepared as phase pure (Fig. S1–S3†).

Table 1 Unit-cell parameters for Cs₂NaMoCl₆, (MA)₂AgMoCl₆, (1,4-BDA)₂AgMoCl₈ and (MA)₄MoCl₆·Cl

	Cs2NaMoCl6	(MA)2AgMoCl6	(1,4-BDA)2AgMoCl8	(1,4-BDA)2AgMoCl8_460 K	(MA)4MoCl6·Cl
Space group	Cubic, Fm3̄m	Trigonal, P3̄m1	Triclinic, P1̄	Triclinic, P1̄	Monoclinic, P2/n
a (Å)	10.41290(10)	7.3359(2)	7.6668(15)	7.636(2)	16.0647(9)
b (Å)	10.41290(10)	7.3359(2)	7.7072(14)	7.671(2)	7.3477(4)
c (Å)	10.41290(10)	6.8017(2)	9.4490(14)	9.789(3)	16.1264(8)
α (°)	90	90	102.229(8)	80.191(12)	90
β (°)	90	90	91.183(6)	88.082(12)	103.599(2)
γ (°)	90	120	90.127(6)	89.402(12)	90
Volume (Å^3)	1129.05(3)	317.00(2)	545.54(17)	564.7(3)	1850.17(17)
CCDC number	2234517	2234518	2234519	2234520	2234521

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2.2. Structure of Cs₂NaMoCl₆

As shown in Fig. 1, Cs₂NaMoCl₆ crystallizes in a cubic Fm3̄m space group, the 3D inorganic framework of which is composed of corner-shared [NaCl₆]⁵⁻ and [MoCl₆]³⁻ units at the B-site with rock-salt ordering. Locally, the Na⁺ and Mo³⁺ ions form ideal octahedral units which are interconnected infinitely through Cl bridges along all three axes. The Cs⁺ cation occupies the A-site interstitials. The Na–Cl (2.756(3) Å) and the Mo–Cl (2.450(3) Å) bond lengths are slightly shorter than those calculated (Na–Cl = 2.813 Å; Mo–Cl = 2.476 Å) from Shannon radii,⁴⁶ which is due to the increased covalency in the metal–halide bonds. Furthermore, a small displacement parameter (B_eq) of 2.16 in the case of a Cs cation, Cs⁺ size to A-cavity size ratio of 0.99 and tolerance factor of 0.97 collectively suggest that the Cs⁺ ion fits well in the A-site cavity. Perhaps, due to this reason, the compound shows high thermal and ambient stabilities (Fig. S4 and S5†).

Fig. 1 SCXRD structure of Cs₂NaMoCl₆, and the ball and stick model of the corresponding perovskite frameworks is also provided to show corner-sharing connectivity of [NaCl₆]⁵⁻ and [MoCl₆]³⁻ units.

2.3. Structure of (MA)₂AgMoCl₆

We, recently reported that a double perovskite most likely forms a corner-shared 3-D framework when the tolerance factor (where r_A, r_x, r_M^I, and r_M^III are the radii of the A-cation, halide, monovalent metal and trivalent metal, respectively) is in the 0.8–1.0 range, and a face-shared 1-D chain when the TF is slightly more than 1.0.⁴⁷ Here, we replaced Cs⁺ with MA and Na⁺ with Ag⁺ to obtain (MA)₂AgMoCl₆ (Fig. 2a and b) which has a TF of 1.03. It crystallizes in a hexagonal phase, similar to previously reported Ag^I–Ru^III halide perovskites.³⁹ The [AgCl₆]⁵⁻ and [MoCl₆]³⁻ octahedra are alternatively connected by face-sharing of two opposite faces in 1-D chain running along the c-axis. The MA cation occupies the interchain space and forms N⋯H⋯Cl hydrogen bonds with chlorides (the shortest N⋯Cl distance of 3.37 Å). It is worthwhile to note that both [MoCl₆]³⁻ and [AgCl₆]⁵⁻ octahedra are trigonally distorted, with the latter being heavily distorted as suggested by the bond angle variance (σ²) (Table 2).

Fig. 2 SCXRD structure of (MA)₂AgMoCl₆: (a) side view and (b) top view. Ball and stick model of the corresponding perovskite framework is also provided to show face-sharing connectivity of [AgCl₆]⁵⁻ and [MoCl₆]³⁻ units.

Table 2 Bond distortion level (Δd) and bond angle variance (σ²)

Compounds	Δd	σ ^2
Cs2NaMoCl6	[MoCl6] = 0	[MoCl6] = 0
	[NaCl6] = 0	[NaCl6] = 0
(MA)2AgMoCl6	[MoCl6] = 0	[MoCl6] = 0.6136
	[AgCl6] = 0	[AgCl6] = 217.49
(1,4-BDA)2AgMoCl8	[MoCl6] = 1.03 × 10^−6	[MoCl6] = 0.5047
	[AgCl6] = 9.85 × 10^−3	[AgCl6] = 6.448
(MA)4MoCl6·Cl	[MoCl6] = 1.59 × 10^−6	[MoCl6] = 0.1609
	[MoCl6] = 1.48 × 10^−7	[MoCl6] = 0.0979

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2.4. Structure of (1,4-BDA)₂AgMoCl₈

Layers of 2-D perovskite can be thought of as derived from slicing (conceptually) the 3-D analogue along a specific crystallographic plane and stacking the slices over each other when large organic ammonium is used as the A-cation.⁴⁸ We used 1,4-BDA to obtain (1,4-BDA)₂AgMoCl₈ which crystallizes in a triclinic P1̄ space group as a (100)-oriented Dion–Jacobson (DJ) type double perovskite (Fig. 3a). The perovskite layer is composed of orderly arranged [AgCl₆]⁵⁻ and [MoCl₆]³⁻ in the ab-plane which are inter-connected through four equatorial chlorides, while two axial chlorides remain dangling in the interlayer space (Fig. 3b). The [0,0] type of layer stacking pattern is clearly distinguishable without stacking-fault or without mixed stacking of Ag^I/Ag^I and Ag^I/Mo^III phases.⁴⁹ The [AgCl₆]⁵⁻ polyhedra are highly distorted which is commonly ascribed to preferential formation of linear coordination geometry due to the mixing of filled Ag 4d orbitals with empty Ag 5s orbitals.⁵⁰ The 1,4-BDA spacer adopts a gauche conformation (Fig. 3c). Our previous studies have revealed that the gauche conformation of this spacer is stable in layered Ag–Ru–Br perovskite, (1,4-BDA)₂AgRuBr₈ and the anti conformation, in layered Ag–Ru–Cl perovskite, (1,4-BDA)₂AgRuCl₈ at room temperature.⁴¹ Here, we show that this spacer can exhibit both gauche and anti conformations in the same compound, but at different temperatures. The calorimetric measurements suggest a reversible phase transition (Fig. 3d) and the SCXRD structure confirms that the phase transition is driven by a gauche ⇋ anti conformational change. In a 460 K structure (Fig. 3e), half the number of 1,4-BDA cations have changed from the gauche (Fig. 3f) form to the anti (Fig. 3g) form without changing the overall lattice symmetry and the compound formula can therefore be written as [(gauche-1,4-BDA)(anti-1,4-BDA)]AgMoCl₈. There is a slight volume expansion due to the increase in the interlayer distance. The octahedral tilting in the perovskite layer is slightly reduced (Fig. 3h). As shown in Fig. S6 and S7,† the perovskite layer has cavities of two different sizes and two crystallographically different 1,4-BDA cations. The BDA-2 which forms N–H⋯Cl hydrogen bonds with the chlorides of the larger cavity is more disordered with larger thermal parameters in the room temperature as well as in the high temperature structures. Furthermore, BDA-2 is found to be longer with more penetration depth below the plane of axial chlorides. We believe that BDA-2 changes the conformation from gauche to anti during the phase transition, whilst BDA-1 remains quiet. We did not observe significant changes in the N⋯Cl distances as a consequence of the change in the 1,4-BDA conformation (Table S7†).

Fig. 3 (a) Room temperature SCXRD structure of (1,4-BDA)₂AgMoCl₈. (b) Corresponding ball and stick models for the perovskite layer showing octahedral connectivity between [AgCl₆]⁵⁻ and [MoCl₆]³⁻ units. (c) 1,4-BDA spacer in a gauche conformation. (d) DSC curve of (1,4-BDA)₂AgMoCl₈. (e) SCXRD structure of (1,4-BDA)₂AgMoCl₈ at 460 K. (f and g) 1,4-BDA spacer in gauche and anti conformations. (h) Ball and stick models for the perovskite layer in the 460 K structure, showing reduced octahedral tilting than that of the room temperature structure.

2.5. XPS

It is important to note that Mo is susceptible to oxidation and can therefore exist in multiple oxidation states under ambient conditions. We measured X-ray photoelectron spectroscopy (XPS) spectra to examine the chemical composition as well as the valence state of Mo ions (Fig. S9–S14 and Table S8†). The spectra show the presence of the desired elements. The core level Mo 3d spectra exhibit characteristic doublet peaks at around 229 eV and 232 eV with a peak area ratio of 3 : 2, corresponding to the Mo 3d_5/2 and Mo 3d_3/2 states (Fig. 4). These binding energies and the peak area ratios clearly suggest that the Mo is in the trivalent state in all these compounds.⁵¹ Furthermore, we did not observe any Mo 3d peaks at higher binding energies, confirming the absence of higher valence states.

Fig. 4 Core level XPS spectra of Mo 3d states.

2.6. UV-visible absorption spectra

Fig. 5a shows the UV-visible absorption spectra of these double perovskites which were recorded in the reflectance mode and converted to absorbance by using the Kubelka–Munk formula [α/S = (1 − R)²/(2R)] (where α, S, and R represent the absorption coefficient, scattering coefficient, and absolute reflectance, respectively). The spectra suggest that the optical bandgap increases in the order of Cs₂NaMoCl₆ (E_g = 2.0 eV), (1,4-BDA)₂AgMoCl₈ (E_g = 2.04 eV) and (MA)₂AgMoCl₆ (E_g = 2.12 eV). We attribute this bandgap blueshift to the reduced orbital overlap caused by both reduced dimensionality and increased structural distortion (Fig. 5b).¹⁴ However, the bandgap shift is minute between the 3-D and the 2-D compounds, which is likely due to the increased bandgap of the former compound as a consequence of poor contribution of Na to the electronic band near the extrema when compared to that of Ag.⁵² As listed in Table 2, (MA)₂AgMoCl₆ has the highest bond angle variance (σ²),⁵³ indicating a high octahedral distortion which correlates well with its higher value of bandgap. Furthermore, the dimensionality control of the bandgap shift is undoubtedly clear in the case of the 2-D and the 1-D phases, wherein the chemical constituents of the perovskite frameworks are identical, e.g., Ag and Mo at the B-site. However, the magnitude of the bandgap shift is smaller than those observed in post-transition-metal analogues. Furthermore, the bandgaps are smaller than those of heavily explored halide double perovskites Cs₂AgBiBr₆ (E_g = 2.17 eV),⁴ Cs₂AgBiCl₆ (E_g = 2.77 eV),⁴ and Cs₂AgInCl₆ (E_g = 3.2 eV).⁵⁴ We also observed a weak band at low energy in each spectrum which is consistent with the spin forbidden ⁴A_2g → ⁴T_1g transitions in Mo^III chloride.⁵⁵

Fig. 5 (a) Optical absorption spectra. (b) Schematic illustration of reduced orbital overlapping between [M^ICl₆]⁵⁻ and [Mo^IIICl₆]³⁻ octahedra due to lattice distortion.

2.7. Magnetism

We measured magnetic susceptibility (χ) for all the three perovskites in the temperature range of 2 K to 300 K by using a superconducting quantum interference device-vibrating sample magnetometer (SQUID-VSM). The susceptibility data were fitted to the modified Curie–Weiss law equation [χ = C/(T − θ_CW) + χ₀], where C is the Curie constant which is proportional to the square of the effective magnetic moment , T is the temperature, θ_CW is the Curie–Weiss temperature which is a measure of the strength and the nature of magnetic exchange interaction between the nearest-neighbors, and χ₀ is the temperature-independent diamagnetic contribution. Fig. 6a and b show the molar susceptibility curves. We shall first discuss the magnetic properties of Cs₂NaMoCl₆ which exhibits long range antiferromagnetic ordering at a Néel temperature (T_N) of 3.12 K, and there is no divergence in zero field cooled (ZFC) and field cooled (FC) curves (Fig. S16†). Above T_N, the paramagnetic susceptibility shows an excellent fit to the Curie–Weiss equation with a negative Curie–Weiss temperature (θ_CW) of −15.8 K, and Curie constant (C) of 1.472 emu Oe⁻¹ K mol⁻¹ (Fig. 6c). The effective magnetic moment µ_β is calculated to be 3.43 µ_B f.u.⁻¹ which is slightly less than the spin only moment of 3.87 µ_B f.u.⁻¹ for a d³ ion [, where n = number of unpaired electrons, n = 3 in this case]. Fig. 6d shows the fcc sub-lattice composed of magnetic Mo ions in Cs₂NaMoCl₆ which clarifies the edge-shared tetrahedral arrangement of Mo³⁺ metal ions in a geometrically frustrating system. The tetrahedron is dimensionally symmetric with a nearest-neighbor Mo⋯Mo separation of 7.36 Å. The calculated magnetic frustration index f of 5.0 indeed suggests that the system has weak magnetic frustration, and it warrants further studies.^56,57 The 1-D compound (MA)₂AgMoCl₆ features an antiferromagnetic transition temperature T_N of 2.35 K and a negative θ_CW of −2.9 K with an effective magnetic moment of 3.48 µ_B f.u.⁻¹ The calculated frustration index (f) of 1.2 indicates that this 1-D system is magnetically non-frustrating.

Fig. 6 (a and b) Temperature dependent magnetic susceptibility curves. (c) Curie–Weiss fitting of the susceptibility data shown in panel (a). (d) Geometrically frustrating tetrahedron based on Mo³⁺ ions in Cs₂NaMoCl₆. (e) Square lattice based on Mo³⁺ ions in (1,4-BDA)₂AgMoCl₈. (f) Thermal dependence of the χ_MT products.

In the case of (1,4-BDA)₂AgMoCl₈, we did not observe magnetic ordering down to the lowest measurement temperature of 2 K, but the system obeys the Curie–Weiss law. Only a weak antiferromagnetic exchange was observed with a θ_CW of −1.7 K and an effective moment of 3.74 µ_B f.u.⁻¹ We attribute the weakening of AFM exchange to the increased distance between the nearest-neighbor spins (Mo⋯Mo = 7.667 Å) compared to that in Cs₂NaMoCl₆ (7.363 Å) and (MA)₂AgMoCl₆ (6.802 Å). The geometric frustration has also significantly relaxed in the 2D system due to the formation of a square lattice (Fig. 6e). The isothermal field dependent M–H curve (Fig. S20c†) of (1,4-BDA)₂AgMoCl₈ at 2 K adopts a non-linear shape, but with no hysteresis (neither remanence nor coercivity), suggesting paramagnetic behaviour.⁵⁸ The M–H loops (Fig. S20a and b†) of Cs₂NaMoCl₆ and (MA)₂AgMoCl₆ are linear which is characteristic of an antiferromagnetic nature. Additionally, we synthesized (MA)₄MoCl₆·Cl (Fig. S21†), a molecular compound, with a nearest-neighbor Mo⋯Mo distance of 7.35 Å which is close to that found in Cs₂NaMoCl₆ (7.363 Å). Also, it shows a T_N of 3.36 K, close to that of Cs₂NaMoCl₆, but the strength of AFM coupling is much weaker as indicated by a θ_CW of −1.2 K. This suggests that the magnetic coupling in these systems primarily takes place through the –Mo^III–Cl–M^I–Cl–Mo^III– pathway, the strength of which depends not only on the nearest-neighbor Mo⋯Mo distance, but also on the geometry of the magnetic sub-lattice. Fig. 6f shows that on lowering the temperature, χ_MT values decrease which indicates antiferromagnetic interactions in all these systems.

3. Conclusions

In summary, we have synthesized and structurally characterized three new chloride double perovskites with Na⁺/Ag⁺ and Mo³⁺ metal ions ordering at the perovskite B-site. The X-ray structures suggest that octahedral connectivity and dimensionality of the inorganic sub-lattice can readily be modulated with the choice of the A-site cation. The measured bandgaps are relatively narrow (2.0–2.1 eV) which are even lower than that of the benchmark double perovskite Cs₂AgBiBr₆ (2.2 eV), and are tuneable with structural dimensionality. Magnetic measurements suggest nearest-neighbor antiferromagnetic coupling at low temperatures which occurs due to super-exchange through the –Mo^III–Cl–M^I–Cl–Mo^III– (M^I = Na or Ag) pathway. The magnetic ordering parameters, the Curie–Weiss temperature (θ_CW) and the Néel temperature (T_N) can be tuned with the dimensionality of the magnetic sub-lattice. In the case of 3-D perovskite, |θ_CW| ≫ T_N suggests magnetic frustration as a consequence of the fcc arrangement of Mo³⁺ (4d³) ion, whilst the 1-D and 2-D analogues do not exhibit such an arrangement. These materials expand the scope of halide double perovskites beyond optoelectronics, and we hope that these results will trigger future developments in magnetic optoelectronic halide perovskites by making use of other A-site cations, B-site metals and halides.

Data availability

Crystallographic information files have been deposited in CCDC (2234517–2234521). All necessary supporting data are provided in the ESI.†

Author contributions

DCB: carried out the experiments, data analysis (except magnetic data), wrote the initial draft of manuscript. PPM and DPP: magnetic data collection and analysis. PV: project design, conceptualization, methodology, data curation, manuscript writing and revision, project administration, and funding acquisition.

Conflicts of interest

Authors declare no conflict of interest.

Acknowledgements

The authors thank Prof. C. N. R. Rao (FRS) for his kind support and guidance. This work was financially supported by the Science & Engineering Research Board (SERB) of the Govt. of India through a Ramanujan Fellowship (Grant No. RJF/2020/000106), and a Core Research Grant (Grant No. CRG/2022/009230) to PV. We thank the SERB for these grants and the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bangalore, for the research infrastructure. PV also thanks the Sheikh Saqr Laboratory (SSL) for financial support. DCB thanks JNCASR and SSL for fellowships. DPP thanks CSIR for fellowship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthesis, characterization methods, crystallographic information files (CIFs), PXRD patterns, XPS data, and additional figures of magnetic data. CCDC 2234517–2234521. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3sc00132f

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