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. 2023 Feb 13;62(8):3629–3636. doi: 10.1021/acs.inorgchem.2c04433

Enhanced Photoluminescence and Reduced Dimensionality via Vacancy Ordering in a 10H Halide Perovskite

Hang Liu †, Hassan Hafeez ‡, David B Cordes †, Alexandra M Z Slawin †, Gavin Peters †, Stephen L Lee §, Ifor D W Samuel ‡, Finlay D Morrison †,*
PMCID: PMC9976281  PMID: 36780272

Abstract

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Vacancy-ordered halide perovskites have received great interest in optoelectronic applications. In this work, we report the novel inorganic halide Cs10MnSb6Cl30 with a distinctive 10H (10-layer hexagonal) perovskite polytype structure with (hcccc)2 stacking. Cs10MnSb6Cl30 has 30% B-site vacancies ordered at both corner- and face-sharing sites, resulting in [MnSb6Cl30]10–n columns, i.e., a reduction of octahedral connectivity to 1D. This results in enhanced photoluminescence in comparison to the previously reported 25% vacancy-ordered 3C polytype Cs4MnSb2Cl12 with 2D connectivity. This demonstrates not only the existence of the 10H perovskite structure in halides but also demonstrates the degree of B-site deficiency and stacking sequence variation as a direction to tune the optical properties of perovskite polytypes via vacancy rearrangements.

Short abstract

The compound Cs10MnSb6Cl30 is a novel vacancy-ordered halide perovskite with a 10H polytype structure and (hcccc)2 stacking. The B-site vacancies order on both face- and corner-sharing sites reducing the octahedral connectivity to 1D, resulting in enhanced photoluminescence compared to related 2D compositions.

Introduction

In the past decade, both inorganic and organic–inorganic hybrid halide perovskites have generated immense research interest for application in optoelectronic, photocatalytic, and light-emitting diode devices because of their favorable electronic structure, structural tunability, and low cost of fabrication.1 For the stoichiometric perovskite with the formula ABX3, the B-site cation occupies one quarter of the interlayer octahedral voids formed by the close-packed AX3 layers. Several perovskite polytypes exist depending on the close packing sequence, which can be cubic (ABC, e.g., in the prototypical perovskite mineral CaTiO32 or the halide CsPbCl33), hexagonal (ABA, e.g., BaNiO34 or CsNiCl35), or a mixture of both. The latter two cases demonstrate the hexagonal perovskite polytypes.6

The various perovskite polytypes can be informatively described using a combination of Jagodzinski and Ramsdell notations.7 The Jagodzinski notation describes the repeat stacking sequence of cubic (c) and hexagonal (h) close-packed layers, whereas the Ramsdell notation takes the form nM where n describes the number of layers in the aristotype unit cell and M = C, H, or R indicating cubic, hexagonal, or rhombohedral parent symmetry, respectively. For example, a conventional perovskite formed by three cubic (ccc, i.e., ABC) packed AX3 layers is denoted as 3C (e.g., SrTiO38), a perovskite with only hexagonal packing (hh, i.e., AB) is denoted as 2H (e.g., BaNiO3 and CsNiCl3), the hexagonal polytype with four cubic and two hexagonal AX3 layers, (cch)2, in a hexagonal primitive cell is denoted as 6H (e.g., RbMgF39 or BaFeO310), and so on. To specifically define each hexagonal polytype, a combination of Ramsdell and Jagodzinski notations is unambiguous, e.g., 10H (hhccc)2,11 10H (hhhcc)2,12 and 10H (hcccc)2.13

A key feature of these perovskite polytypes is the connectivity of the BX6 units, which can be either corner-sharing octahedra (CSO) formed in cubic close-packed stacking or face-sharing octahedra (FSO) generated by hexagonal close-packed stacking. For stoichiometric perovskites, CSO in 3C materials results in three-dimensional (3D) octahedral connectivity, but the 2H (hh) polytype with only FSO exhibits one-dimensional (1D) chains of octahedra. For other hexagonal polytypes, varying degrees of CSO and FSO exist while retaining a 3D network, and this variation of connectivity influences the band structure due to the different B–X orbital interactions in the FSO.14 Increasing degrees of FSO generally leads to an increase of the band gap.15 For inorganic perovskites, substitution of higher valence B cations is a common strategy to adjust the octahedral connectivity by the introduction of compensating B-site vacancies to maintain electroneutrality.16 The preferred occupancy (ordering) of vacancies at specific sites disrupts the connectivity of the 3D octahedral framework, resulting in a reduction of dimensionality to 2D, 1D, or 0D, depending on the relative amount and position of the vacancies. In comparison with 3D counterparts, lower dimensional perovskites have been proven to be important materials with highly efficient light emitting performance and improved moisture stability.17

In the recent years, the B-site-deficient halide perovskites A3B(III)2X9 (A = K, Rb, Cs; B = Sb, Bi, Tl, etc.; and X = Cl–, Br–, I–) have been revisited to investigate the relationship between the crystal and electronic structure (specifically band gap).18−21 It is typical for these A3B2X9 compounds that AX3 forms close-packed layers with the B cations occupying 2/3 of the octahedral sites with the remaining 1/3 vacant. The most well-defined A3B2X9 structures can be classified into four aristotype structures in relation to the packing sequence of the fully occupied perovskite equivalent and the resulting vacancy ordering as shown in Figure 1. Cs3Tl2Cl9 is derived from the 2H parent with only hexagonal (h)2 stacking but with 1/3 B-site vacancies ordered in each FSO layer, resulting in the 0D connectivity of B2X9 “dimers,” Figure 1a.22 In comparison, Cs3Bi2Br9 is based on conventional cubic close-packed 3C (c)3 perovskites with vacancies ordered in every third CSO layer to generate a 2D layered structure, Figure 1b.23 Both Cs3Bi2I9 (also denoted as Cs3Cr2Cl9 type)24 and Cs3Bi2Cl925 have 6H (hcc)2 stacking, but the different vacancy ordering results in differing dimensionality of the octahedral connectivity. In Cs3Bi2I9, the vacancies occur only in the corner-sharing octahedral (CSO) sites, resulting in a 0D dimer structure, Figure 1c, which is similar to Cs3Tl2Cl9 but with layering of the dimers in the close packing plane.26 In contrast, Cs3Bi2Cl9 has vacancy ordering at the FSO sites with 50% occupancy, instead resulting in 1D connective columns of four octahedra running perpendicular to the closed-packing direction, Figure 1d. These examples neatly demonstrate the opportunity for structural tuning through vacancy ordering.

Figure 1.

Figure 1

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Polyhedral representation of the structures of A3B2X9 B-site-deficient perovskites: (a) 0D 2H (h)3 Cs3Tl2Cl9, (b) 2D 3C (c)3 Cs3Bi2Br9, (c) 0D 6H (hcc) Cs3Bi2I9, (d) 1D 6H (hcc)2 Cs3Bi2Cl9 type, and (e) 2D 3C (c)4 Cs4MnSb2Cl12. For ease of comparison, all structures are viewed along the close packing layers.

Recently, a new series of B-site-deficient perovskites with 25% vacancies was reported by Vargas et al. and can be described by the formula A4B′(II)B(III)2X12 (A = Cs+, Rb+; B′ = Mn2+, Cd2+, Cu2+; B = Sb3+, Bi3+, In3+; and X = Cl–, Br–).27−30 These compounds have an entirely cubic close-packed AX3 structure (the same as Cs3Bi2Br9, Figure 1b), but the B-site vacancies order such that every fourth CSO layer is empty rather than every third, Figure 1e. From a general view, both A3B2X9 and A4B′B2X12 belong to special cases of B-site-deficient halide perovskite with the general formula A(□nB′1–3nB2n)X3 with n = 0.33 and 0.25, respectively (n represents the fraction of vacancies per perovskite formula unit). By varying n, a series of potential new compounds and structures with varying octahedral connectivity can be generated, which is dependent on the extent and configuration of vacancy distribution as well as the AX3 packing sequence of the structure.

In this study, we report the synthesis of a novel 1D halide derivative 10H perovskite Cs10MnSb6Cl30 (n = 0.3) with ordered vacancies at both CSO and FSO sites, which displays enhanced photoluminescence compared to the related 2D Cs4MnSb2Cl12 compound. The discovery of this vacancy-ordered 10H halide demonstrates the strategy to tailor the structure and properties of B-cation-deficient perovskites through a combination of vacancy ordering and stacking sequence variation.

Experimental Methods

Synthesis

Raw materials, CsCl (99%), MnCl2 (97%), SbCl3 (99%), MgCl2 (99%), and hydrochloric acid in water (37%), were purchased from Alfa Aesar and used without further purification. All other solvents were obtained from commercial routes and used as-received.

Cs10MnSb6Cl30 single crystals (SCs) were prepared by a hydrothermal reaction in a 40 mL stainless steel autoclave. To obtain the required high chloride ion concentration during synthesis while minimizing the amount of HCl used, an excess amount of MgCl2 (10 mmol, 0.9521 g) was dissolved in 5 mL of dilute 20% HCl to obtain a transparent solution. Stoichiometric amounts of MnCl2 (1 mmol, 0.1258 g) and SbCl3 (6 mmol, 1.3688 g) were then dissolved in the solution. To prevent the formation of the secondary phase Cs3Sb2Cl9, a smaller amount of CsCl (1 mmol, 0.1684 g) was finally added to the solution. The pressure vessel was then heated in an oven at 150 °C for 24 h before cooling to room temperature in air. The product was filtered by vacuum filtration, washed with ethanol several times, and dried in a vacuum desiccator overnight.

Cs10MnSb6Cl30 powder was also prepared by solid-state synthesis. Stoichiometric amounts of CsCl (10 mmol, 1.6836 g), MnCl2 (1 mmol, 0.1258 g), and SbCl3 (6 mmol, 1.3688 g) were mixed and ground together in a Fritsch Pulverisette planetary ball mill at 600 rpm for 2 h using 60 cm3 Teflon pots and high-wear-resistant zirconia media. The mixture was then uniaxially pressed into a 10 mm diameter pellet using a stainless steel pellet die under a load of 1 ton and heated at 220 °C in a furnace for 2 h.

Cs4MnSb2Cl12 powder was prepared by a conventional solution route.27 Stoichiometric amounts of MnCl2 (1 mmol, 0.1258 g) and SbCl3 (2 mmol, 0.4562 g) were dissolved in 3 mL of conc. 37% HCl on a hot plate at 90 °C with continuous magnetic stirring. CsCl (1 mmol, 0.1684 g) was then dissolved in 2 mL of conc. 37% HCl with stirring. Cs4MnSb2Cl12 powder was obtained by adding the precursor CsCl solution dropwise to the MnCl2 and SbCl3 solution. The resulting powder was washed with ethanol and dried under vacuum for 12 h.

Characterization

Single crystals prepared by hydrothermal reactions and powders synthesized by either a precipitation or solid-state method were characterized by single-crystal and powder X-ray diffraction (SCXRD and PXRD), respectively. X-ray diffraction data for Cs10MnSb6Cl30 were collected using a Rigaku FR-X Ultrahigh Brilliance Microfocus RA generator/confocal optics with an XtaLAB P200 diffractometer [Mo Kα radiation (λ = 0.71073 Å)]. Intensity data were collected using ω steps accumulating area detector images spanning at least a hemisphere of reciprocal space. Details of structural solution are provided in the Supporting Information. PXRD data were obtained using a PANalytical Empyrean diffractometer with Cu Kα1 (λ = 1.5406 Å) and 2θ angle ranging from 3 to 70° at 298 K. Basic Rietveld refinements of PXRD data using GSASII31 were performed to confirm phase purity and for the determination of lattice parameters. The structures of all samples were refined using their corresponding structural model obtained from single-crystal data.27

Morphology observation and elemental analysis based on energy-dispersive X-ray spectroscopy (EDS) were performed on the single crystals to confirm the composition and stoichiometry and to exclude the presence of magnesium using a JEOL JSM-IT200 scanning electron microscope (SEM) with an accelerating voltage of 15 kV.

Pseudoabsorbances were calculated from steady-state UV–vis diffuse reflectance data recorded using a JASCO-V650 double-beam spectrophotometer based on the Kubelka–Munk transformation.32 Tauc plots assuming both direct and indirect transitions were applied to the pseudoabsorbance spectra to estimate the band gap of Cs10MnSb6Cl30. Powders from ground Cs10MnSb6Cl30 SCs and solution-prepared Cs4MnSb2Cl12 were used in the UV–vis diffuse reflectance measurements (see the Supporting Information for more details).

Steady-state emission and excitation spectra of the powder samples were recorded at 298 K using a spectrofluorometer (Edinburgh Instruments FLS980) with a 150 W ozone-free xenon arc lamp as the excitation source. The photoluminescence quantum yield measurement was conducted using an absolute quantum yield spectrometer model C9920-02 (Hamamatsu, Japan) equipped with a xenon light source. A quartz substrate was used to obtain the blank measurement shown in Figures S6 and S7. All PL, PLE, and PLQY measurements were performed on the solid-state synthesized Cs10MnSb6Cl30 and solution-prepared Cs4MnSb2Cl12 powders.

Dielectric measurements and thermogravimetric and differential scanning calorimetry (TG-DSC) analysis were performed to examine the thermal stability and possible phase transition in Cs10MnSb6Cl30 (see the Supporting Information for details).

The magnetic susceptibility of ground SCs Cs10MnSb6Cl30 was measured using a Quantum Design MPMS3 SQUID magnetometer. For magnetic measurements, single crystals were favored over solid-state powders to avoid any risk of signals from impurities picked up from the stainless-steel pellet die during synthesis; note that small Cs3Sb2Cl9 crystals were only present in negligible amounts in the SC samples and in any case is diamagnetic. Approximately 58 mg of samples were enclosed within in a polycarbonate capsule that was then located inside a plastic straw attached to the MPMS3 sample rod. Field-cooled (FC) measurements were applied in the temperature ranging from 2 to 300 K under a DC magnetic field of 100 Oe. Diamagnetic correction was performed on the collected data using Pascal’s constant.33

Results and Discussion

Chemical and Structural Characterization

SEM of as-synthesized Cs10MnSb6Cl30 SCs indicated imperfect transparent hexagons with evidence of twin boundaries at the surface as demonstrated in Figure 2. EDS analyses were performed at nine sites on three single crystals of Cs10MnSb6Cl30, confirming the stoichiometry of the crystals. A few smaller single crystals of different morphologies with a clear phase boundary from the main phase Cs10MnSb6Cl30 were observed, Figure 2d; EDS confirms these as Cs3Sb2Cl9, Table 1. It is important to note that these Cs3Sb2Cl9 crystals are present in negligible amounts and were only rarely detected due to the different morphologies; the scarcity of these crystals in samples prepared under hydrothermal conditions means that they were undetectable by all other techniques used in this study. Due to the high Sb3+ concentration utilized in the hydrothermal reaction, the cocrystallization of both Cs3Sb2Cl9 and Cs10MnSb6Cl30 is very difficult to inhibit; however, the former was present in only very small amounts as to be undetectable by XRD. Although an excess of MgCl2 was added to achieve the required chloride ion concentration, no Mg could be detected in the single crystals by EDS. EDS mapping was also performed at four sites on Cs10MnSb6Cl30 powder prepared by solid-state synthesis as demonstrated in Table 1 and Figure S1.

Figure 2.

Figure 2

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SEM morphologies (secondary electron image, a) and EDS analysis sites (back-scattered electron images, b–d) of Cs10MnSb6Cl30 and secondary phase Cs3Sb2Cl9 single crystals (far left crystal in panel (d), indicated by the Spc_005 data point). The scale bar in panel (a) is 50 μm.

Table 1. Compositional Analysis Using EDS for Cs10MnSb6Cl30 Single Crystals, Secondary (Cs3Sb2Cl9) Phase Crystals, and Cs10MnSb6Cl30 Powder Prepared by Solid-State Synthesis.

  Cs (atom %) Mn (atom %) Sb (atom %) Cl (%)
SCs Cs10MnSb6Cl30 21.71 ± 0.12 1.95 ± 0.06 13.90 ± 0.09 62.43 ± 0.19
secondary phase Cs3Sb2Cl9 20.63 ± 0.3 NA 17.18 ± 0.25 62.18 ± 0.46
powder Cs10MnSb6Cl30 22.44 ± 0.12 2.02 ± 0.06 14.04 ± 0.09 61.51 ± 0.19
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The crystal structure of Cs10MnSb6Cl30 was determined at 173 K by single-crystal X-ray diffraction (SCXRD), Table 2. Based on the SCXRD data, Cs10MnSb6Cl30 is found as an orthorhombic phase in the space group Pnnm (space group #58). Additional data collections were also undertaken at room temperature and 100 K, both of which showed isostructural unit cells. Phase purity and lattice parameters of samples prepared by the various routes were also confirmed by powder X-ray diffraction (PXRD), Figures 3 and S2. Initial PXRD data from crushed single crystals of Cs10MnSb6Cl30 showed systematic intensity variations associated with preferred orientation, and Rietveld refinements were performed after regrinding the single crystals into finer particles. To correlate the preferred orientation in ground single crystals, spherical harmonic functions were employed to refine the texture in the sample as shown in Figure 3a.34 The increased intensity of (h00)-indexed peaks indicates that the PXRD pattern obtained from ground SCs exhibits crystallographic preferred orientation along the closed-packing (100) direction and is consistent with the plate-like hexagonal-shaped single crystals obtained. There was no evidence of preferred orientation in either the solid-state synthesized Cs10MnSb6Cl30 sample or solution-prepared Cs4MnSb2Cl12. All Cs10MnSb6Cl30 samples appear single phase despite the observation of Cs3Sb2Cl9 single crystals during SEM and EDS analyses; this confirms that this secondary phase is present in only very small amounts as to be undetectable by XRD. The refinements of both solid-state and crushed SC Cs10MnSb6Cl30 samples yield similar lattice parameters.

Table 2. Selected Crystallographic Data for Cs10MnSb6Cl30.

formula Cs10MnSb6Cl30
formula weight (g/mol) 3178.04
crystal description colorless plate
crystal size (mm) 0.09 × 0.06 × 0.01
crystal system orthorhombic
space group Pnnm (no. 58)
a (Å) 30.7287(7)
b (Å) 12.99555(3)
c (Å) 7.5200(3)
volume (Å3) 2993.75(13)
Z 2
density (calc., g/cm3) 3.526
absorption coefficient (mm–1) 10.213
F(000) 2782
θ range (deg) 1.706–29.752
no. of reflections collected 52 396
no. of unique reflections (Rint) 8078 (0.0356)
parameters, restraints 129, 0
goodness of fit on F2 1.015
R1, wR2 [I > 2σ(I)] 0.0297, 0.0825
R1, wR2 (all data) 0.0409, 0.0863
largest difference peak/hole [e/Å3] 2.438, −0.724
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Figure 3.

Figure 3

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Rietveld refinement profiles for powder X-ray diffraction data of (a) ground single crystals of Cs10MnSb6Cl30 and (b) solid-state synthesized Cs10MnSb6Cl30, confirming the formation of 10H phase as determined from single-crystal XRD.

From the TGA analysis, it can be observed that Cs10MnSb6Cl30 is thermally stable until 545 K in an inert atmosphere as demonstrated in Figure S5a. From DSC analysis and dielectric spectroscopy data shown in Figures S4 and S5b, there is no evidence of any phase transitions between 50 and 545 K.

As illustrated in Figure 4, Cs10MnSb6Cl30 is a 10H perovskite with (hcccc)2 stacking but with ordered vacancies in both FSO and CSO octahedral sites. Initially ignoring the vacancies for clarity, the 10H structure can be described by a successive stacking of FSO layers separated by CSO blocks three octahedra deep. The Mn cations selectively occupy only the middle CSO layer within these blocks. Hence, a complete sequence can be viewed as the succession of three CSO blocks and two FSO blocks. An important feature is the vacancy distribution among the five octahedral layers, which leads to a dimensionality reduction from 3D to 1D. Four crystallographic octahedral sites are labeled in Figure 4, where Sb(01) and Sb(02) fully occupy the outer layers of the CSO block and Sb(03) is distributed in an ordered fashion across 50% of the octahedral sites in the FSO block. This distribution results in a distorted “checkerboard” ordering within each layer but with an offset between the layers such that there are no face-sharing Sb2Cl9 dimers. The Mn2+, located in the middle CSO layer, also occupies 50% of the octahedral sites in this layer in a similar checkerboard ordered fashion. From Figure 4, it can be observed that the vacancy ordering in Cs10MnSb6Cl30 can be described by a combination of checkerboard ordering of vacancies in both the half-occupied FSO and Mn-containing CSO layers, and which disrupt the connectivity, resulting in 1D [MnSb6Cl30]10–n columns running along the c-axis of the Pnnm structure (i.e., orthogonal to the AX3 stacking direction).

Figure 4.

Figure 4

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Structure of the vacancy-ordered (hcccc)2 type 10H layered Cs10MnSb6Cl30 perovskite viewed along the c-axis (in the packing layer plane). To emphasize the underlying 10H parent structure, vacant polyhedra are outlined, with occupied polyhedra shown as shaded.

Vacancy-ordered structures appear to be energetically favorable during aliovalent doping for both close-packed hexagonal halide and oxide perovskites.16,35,36 In A3B2X9 halides, the preferred vacant octahedral site is mainly determined by the relative ionic size and electron configuration of B3+. For example, in β-Cs3Sb2Cl9,37 vacancies tend to occupy the FSO to form a similar “checkerboard” distribution as in Cs10MnSb6Cl30. This is to avoid B–B repulsion in FSO dimers due to the close distance (about 2.943 Å) between two hexagonal packed SbCl6 and the poor screening due to the relatively low polarizability of the chloride ion, resulting in the formation of a 1D 6H (hcc)2 structure.38 However, in Cs3Sb2I9, vacancies prefer to occupy the CSO sequence because of the relatively larger distance (ca. 3.97 Å) and weaker influence of lone pair electrons between two hexagonal packed layers and better screening provided by I–, hence yielding a 0D 6H (hcc)2 structure. If a cation smaller than Sb is incorporated, such as Cs3Cr2Cl9,39 a preferred distribution of vacancies at CSO sites is observed. In comparison with deficient halides, vacancies in analogous B-site-deficient oxides normally preferentially appear in the FSO. For instance, in the similar deficient 10H (hcccc)2 Ba10Mg0.25Ta7.9O30, the long-range ordering of vacancies at FSO blocks was determined by TEM, although the periodicity of the ordered vacancies is more complex than the zig-zag chains of Cs10MnSb6Cl30 and results in retention of 3D octahedral connectivity.40 Compared to 10H oxide analogues with only FSO vacancies, the 10H halide Cs10MnSb6Cl30 contains a higher vacancy ratio of 3/10 with both FSO and CSO vacancies, resulting in a further loss of connectivity to form a 1D structure. Such coexistence of both CSO and FSO vacancies to form isolated 1D columns is rarely observed in either oxide or halide perovskites.

Magnetic Properties

To investigate any magnetic coupling between Mn2+ in Cs10MnSb6Cl30, magnetic susceptibility was collected in the temperature range 2–300 K. Despite some noise in the data, presumably due to the low field used and possibly due to some settling of the plate-like single crystals used in the measurement, a linear fitting from 40 to 300 K using the Curie–Weiss law, Figure 5, yields a Weiss constant of −9.43(8) K and an effective magnetic moment of 5.494(5) μB. Although a little low due to data quality as mentioned above, the magnitude of the moment is consistent with the theoretical value 5.916 μB for spin-only moment of high spin state d5 Mn2+ and further corroborates the inclusion of Mn2+. The negative Weiss constant indicates weak antiferromagnetic coupling between Mn2+. Such weak superexchange was also predicted and observed in both 2D Cs4MnSb2Cl12 and Cs4MnBi2Cl12, which is attributed to the four bond Mn–Cl–(Sb/Bi)–Cl–Mn exchange pathway.27,41 The product of magnetic susceptibility and temperature is only slowly varying over most of the high temperature range, further supporting the existence of weak antiferromagnetic interactions and a constant effective moment per site.

Figure 5.

Figure 5

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Temperature dependence of magnetic properties for Cs10MnSb6Cl30.

Optical Properties

UV–vis diffuse reflectance of Cs10MnSb6Cl30 was measured and converted into pseudoabsorbance using the Kubelka–Munk (KM) transformation α = (1 – R)2/2R, where α is the absorption coefficient and R is the reflectance.32 Equivalent data for Cs4MnSb2Cl12 were collected to allow comparison and are shown in Figure 6a. The spectra demonstrate that Cs10MnSb6Cl30 shows similar behavior to Cs4MnSb2Cl12 with the main absorption onset at around 450 nm. Both compounds exhibit weak absorption at about 520 nm, which is assigned to the forbidden 6A1(S) → 4T1(G) d–d transition of high spin Mn2+.41,42 For Cs4MnSb2Cl12, two additional strong absorption peaks are observed at ca. 350 and 380 nm and are ascribed to the partially allowed 1S0 → 3P1 transition of Sb3+ split by a second-order Jahn–Teller distortion.43−45 In comparison with the 3C Cs4MnSb2Cl12 polytype, only one strong, well-defined absorption peak is observed at 380 nm for Cs10MnSb6Cl30, which is also likely due to the 1S0 → 3P1 transition of Sb3+. Tauc plots of Cs10MnSb6Cl30 were obtained assuming both a direct and indirect band gap, giving values of 2.98 and 2.69 eV, respectively (see the Supporting Information for further details). While indirect band gaps are typically expected for perovskites with adjacent occupied FSO, the combination of the checkerboard ordering in-plane and the zig-zag arrangement between FSO results in no Sb–Sb FSO in this case, and so, the electronic structure is likely to be dominated by CSO orbital interactions. To verify this, DFT calculations need to be performed; these are underway but are complex and time-consuming due to the heavy ions involved and also large supercell volumes associated with the various permutations of the antiferromagnetically ordered ground state. However, considering the structural and compositional similarity to Cs4MnSb2Cl12, which also has no 90° Sb–Cl–Sb interactions, a 2.98 eV direct band gap may be tentatively assumed for Cs10MnSb6Cl30.

Figure 6.

Figure 6

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(a) Steady-state absorbance spectra for Cs4MnSb2Cl12 and Cs10MnSb6Cl30; (b) steady-state PL (excited at 340 nm) and PLE (detected at 620 nm) spectra for Cs4MnSb2Cl12 and Cs10MnSb6Cl30 (note the data are offset on the y-axis for clarity); (c) PL spectra excited at 340 nm for Cs4MnSb2Cl12 and Cs10MnSb6Cl30; and (d) PL spectra of Cs10MnSb6Cl30 for excitation at 340, 370, and 440 nm.

Steady-state photoluminescence emission (PL), photoluminescence excitation (PLE), and photoluminescence quantum yield (PLQY) data were collected from both Cs4MnSb2Cl12 and Cs10MnSb6Cl30 powders at room temperature. The PL spectra show wide peaks centered at 620 nm with a full width at half-maximum (FWHM) of ca. 88 nm as illustrated in Figure 6b,c, indicating that the same PL mechanism attributed to the octahedrally coordinated Mn2+ transition from 4T1 to 6A1 for both Cs10MnSb6Cl30 and the previously reported Cs4MnSb2Cl12.27 In addition to a peak at 340 nm (which we attribute to the onset of 1S0 → 3P1 transition of Sb3+), the PLE data of Cs10MnSb6Cl30 show a strong peak at 440 nm, which corresponds to the 6A1(S) → 4T2(G) transitions of high spin Mn2+.45 However, the emission is independent of excitation wavelength (Figure 6d), indicating that the same species is likely to be responsible for the emission under excitation wavelengths from 340 to 440 nm. In Cs4MnSb2Cl12, the 440 nm feature in PLE is smaller and the emission spectrum is the same for all excitation wavelengths (Figures 6b and S8). The Cs10MnSb6Cl30 samples exhibit larger PLQYs than Cs4MnSb2Cl12. The PLQY of Cs10MnSb6Cl30 was 3.5±1% for excitation at 340 nm and 12.5 ± 1% for excitation at 440 nm (Table 3). For Cs4MnSb2Cl12, the PLQY was 1.3 ± 1% for both excitation wavelengths. The higher PLE and PLQY of Cs10MnSb6Cl30 may suggest that the forbidden d–d transition (4T1 to 6A1) may be increasingly allowed due to a symmetry reduction of the MnCl6 octahedra as a result of a local distortion; however, further experiments are required to clarify this and, in particular, the reason for the higher PLQY under 440 nm excitation.

Table 3. Average Quantum Yield Excited at 340 and 440 nm.

  average PLQY (%)
excitation wavelength (nm) 340 nm 440 nm
Cs4MnSb2Cl12 1.3 ± 1 1.3 ± 1
Cs10MnSb6Cl30 3.5 ± 1 12.5 ± 1
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Conclusions

In conclusion, a novel one-dimensional perovskite Cs10MnSb6Cl30 with a unique vacancy-ordered 10H structure has been prepared and is the first reported 10H halide perovskite. In comparison to the vacancy-ordered 3C Cs4MnSb2Cl12 phase, which displays 2D (layered) octahedral connectivity, the specific vacancy ordering in the 10H Cs10MnSb6Cl30 compound further reduces this dimensionality to 1D (columnar). As a result, the 10H-type Cs10MnSb6Cl30 exhibits stronger photoluminescence and improved PLQY compared to the reported 3C-type Cs4MnSb2Cl12. The current study highlights structural tuning via vacancy ordering as a mechanism to obtain low-dimensional halide perovskites with enhanced optical properties.

Acknowledgments

H.L. would like to thank the University of St. Andrews for financial support via a St. Leonard’s scholarship. The authors acknowledge facilities access made possible through support from the EPSRC Light Element Analysis facility Grant EP/T019298/1 and the EPSRC Strategic Equipment Resource Grant EP/R023751/1.

Data Availability Statement

The research data underpinning this publication can be accessed at https://doi.org/10.17630/51767b58-94a9-4ab2-9f93-c4c7edc4fdd0 [ref (46)].

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.2c04433.

  • Materials characterization methods and data including compositional analysis; single-crystal and powder X-ray diffraction; UV–VIS, TGA–DCS, and dielectric data; photoluminescence and quantum yield data (PDF)

The authors declare no competing financial interest.

Supplementary Material

ic2c04433_si_001.pdf (1.8MB, pdf)

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Data Availability Statement

The research data underpinning this publication can be accessed at https://doi.org/10.17630/51767b58-94a9-4ab2-9f93-c4c7edc4fdd0 [ref (46)].

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