Layered perovskite-like nitrates A₂Ca(NO₃)₂Cl₂ (A = Rb, Cs) as ultraviolet birefringent optical materials

Abstract

The progress of all-inorganic Ruddlesden–Popper (RP) perovskites broadens the selection of materials suitable for optoelectronic applications, especially in improving stability and performance. Two new alkali metal calcium halide nitrate-based layered RP perovskite variants, Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂, have been synthesized through a facile solution evaporation method. Remarkably, the title compounds exhibit a unique RP perovskite-like nitrate layered structure with significant birefringence (0.099–0.104@546 nm) and short ultraviolet (UV) absorption wavelengths (220–225 nm). In-depth structural investigation and theoretical analyses indicate that the birefringence substantially arises from the planar triangular [NO₃] groups. This study not only enhances the structural diversity of layered perovskites but also provides an effective strategy for the continued investigation of all-inorganic birefringent optical materials in the UV region.

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Introduction

Ultraviolet (UV) birefringent crystals are becoming increasingly significant in modern technology and scientific applications, such as polarimetry, optical communication, and scientific instrumentation.^1,2 Owing to their remarkable ability to modify light polarization, birefringent materials have garnered significant interest as important optical materials.^3–5 Numerous UV birefringent crystals, including CaCO₃,⁶ MgF₂,⁷ and α-BBO,⁸ have been developed and commercialized over the past few decades. Though the drawbacks related to these materials, such as the contaminants and crystal growth challenges of CaCO₃, the limited birefringence of MgF₂, and the crystal growth crack caused by phase transformation for α-BBO, have severely restricted their practical applications. Therefore, it is critical to create efficient plans for discovering new, high-performing UV birefringent materials.

A metal cation polyhedron in an extremely deformed structure is advantageous for the formation of large polarizability anisotropy, leading to a notable increase in birefringence:⁹ as an illustration, consider K₂SbMoO₂F₇,¹⁰ an ordered d⁰ transition metal fluoroantimonite with a significant birefringence of 0.220@550 nm or a metal cation deformed polyhedron, such as PbO₄Br₄ in Cs₂Pb(NO₃)₂Br₂ ⁵ (Δn = 0.147@546 nm) and SnO₇Cl₂ in Sn₂B₅O₉Cl ¹¹ (Δn = 0.168@546 nm), caused by the action of stereochemically active lone pairs (SCALP).^12–17 Unfortunately, the incorporation of SCALP metal cations or d⁰ transition metal cations leads to a significant red shift in the UV cutoff edge, rendering this method unsuitable for exploring new good birefringent materials in the UV range. As a result, alkali metals and alkaline-earth metals are typically selected to develop novel UV optical materials, achieving high transmittance in the UV range. Consequently, the advancement of innovative UV birefringent crystals will critically depend on how to effectively employ the anionic groups to optimize their birefringence contribution.

In the context of anion frameworks, achieving high birefringence is favored by two key factors: (1) anionic groups with large anisotropy, such as planar groups, [BO₃], [NO₃], [CO₃], and [C₃N₃O₃],^18–29 and (2) their arrangement, particularly when these planar units are organized in a layered structure. This is attributed to the optimized alignment of microscopic polarizability anisotropy. A prominent class of layered materials with anisotropic anionic groups is the family of layered perovskites (A_n+1B_nX_3n+1; where X = Cl⁻, NO₃⁻).^30–34 For example, in Cs₂Pb(NO₃)₂Cl₂ ³⁵ and Cs₂Pb(NO₃)₂Br₂,⁵ the layered arrangement of π-conjugated [NO₃]⁻ anion groups and SCALP-active Pb²⁺ cations exert a synergistic effect to enhance the material's birefringence. Herein, we proposed to strategically replace Pb²⁺ with Ca²⁺ for the targeted design and development of new birefringent materials effective in the shorter UV range. Additionally, Rb and Cs cations were chosen as counter ions to ensure charge equilibrium.

Two new alkali metal calcium halide nitrates, A₂Ca(NO₃)₂Cl₂, (A = Rb, Cs), were successfully obtained. Notably, both compounds exhibit a unique layered structure composed of consecutive two-dimensional (2D) perovskite slabs that resemble Ruddlesden–Popper perovskite,³⁵ [Ca(NO₃)₂Cl₂]²⁻_∞ anion layers, and interlayer Rb⁺ and Cs⁺ cations. This structure endows them with excellent multifunctional optical properties, including considerable birefringence values and short UV absorption wavelengths.

Experimental

Reagents and crystal growth

All starting materials were utilized without additional purification: CsNO₃ (AR 99.5%, Macklin Chemicals), RbCl (AR 99.5%, Macklin Chemicals), Ca(NO₃)₂·4H₂O (AR 99.5%, Aladdin Chemicals) and CaCl₂ (AR 99.9%, Aladdin Chemicals). Single crystals of A₂Ca(NO₃)₂Cl₂ (A = Rb, Cs) were prepared via a room-temperature water evaporation method. Reagents were weighted as follows: for Rb₂Ca(NO₃)₂Cl₂, RbCl (0.605 g, 5 mmol) and Ca(NO₃)₂·4H₂O (1.181 g, 5 mmol); for Cs₂Ca(NO₃)₂Cl₂, CsNO₃ (0.975 g, 5 mmol) and CaCl₂ (0.555 g, 5 mmol). The weighed samples were placed in 100 mL beakers, respectively, and each beaker was filled with 20 mL of water. The measured pH values of the aqueous solutions were found to be in the range of 5–7. The opening of each beaker was covered with a stretch film punctured with a few holes to allow a very slow water evaporation rate in the air. Eventually, colourless bulk crystals (Fig. S1) were obtained with yields of 60–70% (based on Ca). Transparent, high-quality single crystals of A₂Ca(NO₃)₂Cl₂ (A = Rb, Cs) were carefully selected under an optical microscope for further structural characterization. Polycrystalline powder samples for characterization were prepared by grinding the bulk crystals.

Single-crystal X-ray diffraction

Single-crystal X-ray diffraction analysis was conducted with a Bruker SMART APEX II CCD diffractometer, utilizing a Mo Kα radiation source with λ = 0.71073 Å.³⁶ The collected data were subsequently processed with SAINT software. After data collection and integration, the crystal structures were solved and refined using direct methods from SHELXTL software suite.³⁷ The full-matrix least-squares refinement provided accurate atomic coordinates for all atoms in the structures. The PLATON program and the CIF check tool (https://checkcif.iucr.org) were used to verify the crystal structures.³⁸Table 1 presents the structural refinement data for both crystals.

Table 1 Refined structure and crystallographic data for Rb₂CaN₂O₆Cl₂ and Cs₂CaN₂O₆Cl₂

a F o ^2 > 2σ(Fo^2), R1 = ∑||Fo| − |Fc||/∑|Fo| and wR2 = [∑w(Fo^2 − Fc^2)^2/∑wFo^4]^1/2.
Molecular formula	Rb2CaN2O6Cl2	Cs2CaN2O6Cl2
Formula weight	405.94	500.82
Temperature/K	298.0	298.0
Radiation	Mo Kα, λ = 0.71073 Å	Mo Kα, λ = 0.71073 Å
Crystal system, space group	Tetragonal, I41/amd	Tetragonal, I41/amd
a (Å)	5.4268(2)	5.5603(5)
b (Å)	5.4268(2)	5.5603(5)
c (Å)	31.877(2)	32.482(3)
β (Å)	90	90
Volume (Å^3)	938.77(9)	1004.2(2)
Z	4	4
ρ calc. (g cm^−3)	2.872	3.313
Index ranges	−7 ≤ h ≤ 7, −6 ≤ k ≤ 7, −40 ≤ l ≤ 39	−7 ≤ h ≤ 7, −7 ≤ k ≤ 7, −39 ≤ l ≤ 39
μ (mm^−1)	11.539	8.301
F (000)	760.0	904.0
2θ range	5.112 to 54.974	5.016 to 54.954
Collected reflections	7509	3343
Independent reflections	328 [Rint = 0.0506]	335 [Rint = 0.0591]
Completeness (%)	100	96
Goodness-of-fit on F^2	1.063	1.232
Final R indices [I > 2sigma(I)]^a	R 1 = 0.0147, wR2 = 0.0354	R 1 = 0.0238, wR2 = 0.0507
Data/restraints/parameters	328/0/26	335/0/26

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Elemental analysis

Elemental composition of the samples was determined using energy-dispersive X-ray spectroscopy (EDS) integrated with a JEOL JSM-7610F Plus field-emission scanning electron microscope (FE-SEM) operating at an accelerating voltage of 5 kV.

Powder X-ray diffraction

Powder X-ray diffraction (PXRD) analysis was completed utilizing Cu Kα (λ = 1.54056 Å) radiation on a Bruker D2 PHASER diffractometer. The experiment was conducted at ambient temperature with a 2θ angle range of 5–70°, using a scan step width of 0.01° and a fixed duration of 0.2 s.

Optical characteristics

IR spectra of Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂ were tested using a Shimadzu IR Affinity-1 Fourier transform infrared spectrometer at a resolution of 1 cm⁻¹, over a spectral range of 400–4000 cm⁻¹ and utilizing a KBr disc as a reference (1 : 100 as sample : KBr ratio). The UV-vis-NIR transmittance spectra were recorded at room temperature using a Shimadzu SolidSpec-3700DUV spectrophotometer and polished crystal wafers (≈0.3 mm thick) over the spectral window of 200–1600 nm. UV-vis-NIR diffuse reflection data were further collected. The measurements extended across wavelengths from 200 nm to 2600 nm. The Kubelka–Munk function, K/S = (1 − R)²/2R, was used to extract absorption data from diffuse reflection data. K and S stand for the absorption and scattering coefficients, respectively.^39–42 By extending the absorption edge to the baseline on the K/S versus energy plot, the band gap value can be determined.

Thermal analysis

To assess the thermal stability of the compounds, we conducted concurrent differential scanning calorimetry and thermogravimetric analysis using a Netzsch STA 449 F3 analyzer. A controlled heating program at a rate of 5 °C min⁻¹ was utilized to evaluate the thermal stability of A₂Ca(NO₃)₂Cl₂ (A = Rb, Cs) in powder form. Each sample was seated inside an alumina crucible and subjected to a controlled heating procedure from ambient temperature (25 °C) to 1000 °C.

Refractive index difference measurements

The birefringence of Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂ was measured using a cross-polarizing microscope (ZEISS Axio Scope. 5) equipped with a Berek compensator. The wavelength of the light source is 546 nm. The relative error was minimized due to the clear visibility of the first-, second-, and third-order interference color boundaries. To enhance the accuracy of the birefringence measurements, small, transparent single crystals of Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂ were utilized. The birefringence (Δn) was calculated using the formula: R = |N_g − N_p| × T = Δn × T, where R represents the difference in the optical path, N_g and N_p are the refractive indices of fast and slow light, and T is the thickness of the crystal sample.⁴³

Theoretical calculations

The electronic structure calculations were carried out by first-principles methods with the CASTEP package, employing norm-conserving pseudopotentials (NCPs).⁴⁴ The exchange–correlation potential was described using the Perdew–Burke–Ernzerhof (PBE) functional, a representative of the generalized gradient approximation (GGA).⁴⁵ The following valence electron configurations were considered for the norm-conserving pseudopotentials: Rb-4s²4p⁶5s¹; Cs-5s²5p⁶6s¹; Ca-3s²3p⁶4s²; N-2s²2p³; Cl-3s²3p⁵; O-2s²2p⁴. The kinetic energy cutoff for the plane-wave basis was set to 750 eV for both Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂.⁴⁶ The Brillouin zone was sampled using Monkhorst–Pack k-point meshes of 5 × 5 × 1 and 6 × 6 × 1 for Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂, respectively. Owing to the inherent discontinuity in the exchange–correlation energy, the band gaps calculated with GGA-PBE are generally underestimated. To align the calculated band gaps with the experimentally measured values, a scissor operator was applied to rigidly shift all conduction bands upward.

Results and discussion

Crystal structure

A₂Ca(NO₃)₂Cl₂ (A = Rb, Cs) crystallize in the tetragonal space group I4₁/amd (no. 141). As A₂Ca(NO₃)₂Cl₂ (A = Rb, Cs) are isostructural, the structure of Cs₂Ca(NO₃)₂Cl₂ is presented in detail as a representative example. The asymmetric unit has one independent Cs, one Ca, one Cl, one N, and two O atoms. According to Fig. 1, the structure consists of CaO₄Cl₄ polyhedra and [NO₃] trigonal planar groups. A CaO₄Cl₄ polyhedron with Ca–Cl and Ca–O bond lengths of 2.801(4) and 2.582(4) Å, respectively, is formed when the Ca atom coordinates with four Cl and four O atoms (Fig. 1a). Cl–Ca–Cl and O–Ca–O bond angles are 90.845(8)–166.05(7)° and 48.90(18)–145.97(12)°, respectively. Through two apical bidentate nitrate groups with N–O bond lengths of 1.227–1.257 Å, the CaO₄Cl₄ polyhedron shares four oxygen atoms. What's interesting to note is that the two trans [NO₃] groups positioned at right angles to one another are coordinated to the calcium central atom (Fig. 1b). Compared to the uncoordinated N–O bond length (1.227(8) Å), the two bond lengths of N–O utilized for the Ca²⁺ coordination are comparatively longer (1.257(5) Å). Adjacent eight-coordinate distorted CaO₄Cl₄ polyhedra form a 2D [Ca(NO₃)₂Cl₂]_∞²⁻ anion layer in the ab-plane (Fig. 1d) by sharing Cl atoms in the equatorial plane, with Cs⁺ cations positioned between the layers to maintain charge balance (Fig. 1c). The layer shows eight-membered square rings (Fig. 1d). The square rings have approximate dimensions of 5.560(6) Å × 5.560(6) Å. The special Cs⁺ cation is in 12-fold coordination geometry with eight Cs–O connections that range from 3.164(3) to 3.807(4) Å and four Cs–Cl links that range from 3.442(11) to 3.882(13) Å. Moreover, we examined the angular variations between the Cs₃Ca₂Cl₇ ⁴⁷ and Cs₂Ca(NO₃)₂Cl₂ structures. All the Cl–Ca–Cl angles are nearly 90°, as seen in panels a and b of Fig. S4a and b, leading to the almost flat layers made up of CaCl₄ squares for Cs₃Ca₂Cl₇. Nevertheless, the N–Ca–Cl angles in Cs₂Ca(NO₃)₂Cl₂ (Fig. S4c and d) are 96.97 and 83.03°, respectively, indicating that the CaCl₄ square layers are corrugated because of the steric barrier effect of the mutually perpendicular [NO₃] groups. Therefore, the degree of deformation of the Ca-centred polyhedron is significantly increased when the [NO₃] groups replace the apical halogen atoms.

Fig. 1 (a) Ball-and-stick models depicting the [NO₃] unit and the CaO₄Cl₄ polyhedron. (b) Crystal structure of [Ca(NO₃)₂Cl₂]²⁻; observe that the two trans [NO₃] groups are aligned perpendicularly to one another. (c) The bc-plane view of the crystal structure of Cs₂Ca(NO₃)₂Cl₂. (d) A single [Ca(NO₃)₂Cl₂]²⁻_∞ layer consisting of NO₃ groups and CaO₄Cl₄ polyhedra, as seen in the ab-plane.

The structural similarity of Cs₂Ca(NO₃)₂Cl₂ to layered perovskites (A_n+1B_nO_3n+1) is one of its most intriguing structural characteristics. Layered perovskites, in general, consist of endless 2D ABO₃-type structural layers separated by a motif. One of the main families of layered perovskites that has been categorized according to the motif that divides the layers, and their offsetting methods, is Ruddlesden–Popper (RP) compounds. Fig. 2a shows a typical structure of the n = 1 Ruddlesden–Popper family, Sr₂RuO₄.⁴⁸ The corner-sharing RuO₆ octahedra produce endless 2D slabs that are separated by Sr cations. Furthermore, a (1/2, 1/2) translation is applied to the layers in the [100] and [010] directions to offset them. On the other hand, slabs of corner-shared CaO₄Cl₄ polyhedra are exhibited by Cs₂Ca(NO₃)₂Cl₂ through chlorine atoms, and the Cs⁺ cations are located at interlayer sites (Fig. 2b). The Cs₂Ca(NO₃)₂Cl₂ and the n = 1 layered perovskites share a similar structural motif, despite the B site having an 8-coordination. Furthermore, as can be shown in Fig. 2b, the CaO₄Cl₄ polyhedra include [NO₃]⁻ apical ligands, which are rarely seen in any perovskite family. A closer glance at the structure exhibits that the coordinated [NO₃]⁻ ligands are crucial for the offset of the layers in Cs₂Ca(NO₃)₂Cl₂. The offset between slabs 2 and 3 corresponds to a 1/2 translation along the [100] direction (Fig. 2b). However, no offset along [100] was observed between slabs 1 and 2, or between slabs 3 and 4. Nevertheless, between slabs 1 and 2 or 3 and 4, there are offsets corresponding to a 1/2 translation in the [010] direction. The reason behind all these offsets appears to be the planar [NO₃]⁻ groups that are coordinated perpendicularly to avoid the layers making undesirable direct interactions. Therefore, by fitting slabs 1 and 2 (or slabs 3 and 4) with the [NO₃]⁻ “studs” aligned along the a-direction and then further interlocking those fitted double slabs in the b-direction, the framework structure of Cs₂Ca(NO₃)₂Cl₂ can be constructed.

Fig. 2 (a) The polyhedral depiction of the Ruddlesden–Popper layered perovskite with n = 1, specifically Sr₂RuO₄. The structure is composed of infinite layers of RuO₆ octahedra that share corners. Sr²⁺ cations completely occupy the interlayer sites. (b) Polyhedral depiction of Cs₂CaCl₂(NO₃)₂ highlights the offset of the slabs in the bc-plane due to the coordinated [NO₃]⁻ units. Cs⁺ cations separate the corner-sharing CaO₄Cl₄ polyhedra's 2D layers with apical [NO₃]⁻ ligands.

The calculated values of bond valence sums (BVS) of Cs₂Ca(NO₃)₂Cl 1.166, 2.007, 1.073, 2.006–2.020 and 4.950 are in accordance with the oxidation states of Cs⁺, Ca²⁺, Cl⁻, O²⁻ and N⁵⁺, respectively (Table S4). Elemental analysis using energy dispersive X-ray spectroscopy (EDS) was conducted, as depicted in Fig. S6. The ratios of Ca : Rb : Cl and Ca : Cs : Cl were found to be approximately, 1.00 : 1.98 : 1.85 and 1.00 : 2.29 : 1.84, respectively, which align with the single-crystal structure analysis for Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂.

Phase analysis

The PXRD patterns for Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂ match well with the XRD patterns that were computed using the single-crystal models (Fig. 3a and d). The matching of the XRD patterns confirms that the samples are adequately pure for the subsequent performance tests.

Fig. 3 Experimental and calculated powder X-ray diffraction patterns of Rb₂Ca(NO₃)₂Cl₂ (a) and Cs₂Ca(NO₃)₂Cl₂ (d). TG-DSC curves of Rb₂Ca(NO₃)₂Cl₂ (b) and Cs₂Ca(NO₃)₂Cl₂ (e). UV-vis-NIR transmittance spectrum of Rb₂Ca(NO₃)₂Cl₂ (c) and Cs₂Ca(NO₃)₂Cl₂ (f). An inset shows a polished crystal approximately 1 mm thick, which was used for the transmittance measurement.

Thermal analysis

The differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) curves indicate that Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂ can remain stable up to approximately 490–525 °C, suggesting that both compounds exhibit high thermodynamic stability. The DSC curves show endothermic peaks at around 302 °C and 296 °C, corresponding to the melting points of Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂, respectively. The TG curves for the title compounds exhibit two steps of weight loss for Rb₂Ca(NO₃)₂Cl₂ (Fig. 3b). The first step occurs between 490 and 625 °C, featuring a weight loss of 25.70%, attributed to the evaporation of one N₂O₅ (calculated value: 26.61%). The second step, occurring from 625 to 850 °C, relates to the decomposition of one Rb₂O and one Cl₂, resulting in a weight loss of 61.80% (calculated value: 63.51%). On the other hand, Cs₂Ca(NO₃)₂Cl₂ (Fig. 3e) exhibits a single weight loss step between 525 and 970 °C, with a weight loss of 88.50% (calculated value: 92.00%) due to the evaporation of one N₂O₅, one Cl₂, and one Cs₂O.

Optical properties

As shown in Fig. S5a and b, the infrared spectra of Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂ indicate that the bands at 1431, 1380, and 1045 cm⁻¹, along with those at 1465, 1384, and 1044 cm⁻¹, correspond to N–O stretching vibrations in the [NO₃] triangles. Additionally, the bands at 825 and 730 cm⁻¹ for Rb₂Ca(NO₃)₂Cl₂ and 822 and 727 cm⁻¹ for Cs₂Ca(NO₃)₂Cl₂ are attributed to the nonplanar bending vibrations of the [NO₃] planar groups. These findings are consistent with previously published studies.^49–51 The UV-vis-NIR transmittance spectra of Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂ reveal that the preliminarily polished single-crystal plates exhibit short UV cutoff edges at 225 and 220 nm, corresponding to band gaps of 5.51 eV and 5.64 eV, respectively (Fig. 3c and f). In addition, diffuse reflectance spectra of Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂ have been studied (Fig. S5c and d). The overall band gaps of the materials may be related to the extent of distortions associated with the CaO₄Cl₄ polyhedra. These large band gap values are notably higher than those of Cs₂Pb(NO₃)₂Cl₂ and Cs₂Pb(NO₃)₂Br₂ (3.5 and 3.01 eV) due to the incorporation of calcium atoms with no d–d or f–f transitions, implying that our title compounds may have potential applications as short UV birefringent materials. The birefringence of A₂Ca(NO₃)₂Cl₂ (A = Rb, Cs) was evaluated using a cross-polarizing microscope (ZEISS Axio Scope. 5). Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂ display significant birefringence values of 0.099 and 0.104@546 nm, respectively (Table S7 and Fig. 4). These values are comparable to those of the commercially available birefringent crystal, α-BBO with the birefringence of 0.122@546 nm,⁸ and KSrCO₃F with the birefringence of 0.112@532 nm.⁵² It is widely recognized that the presence of a planar π-conjugated anionic group [NO₃] can significantly enhance birefringence. When comparing our compounds to several reported nitrate compounds of alkali and alkaline earth metals, we observe that the birefringence values of our compounds are greater than those of Ba₂B₅O₈(OH)₂(NO₃)(H₂O)₃ (calculated, 0.033@1064 nm),⁵³ Rb(NO₃)(SO₃NH₃) (experimental, 0.070@546 nm),⁵⁴ Sr(NH₂SO₃)-(NO₃)(H₂O) (experimental, 0.077@546.1 nm),⁵⁵ Ba₂(OH)₃(NO₃) (experimental, 0.080@589.3 nm),⁴⁹ KRb₂(NO₃)₂Cl (calculated, 0.084@1064 nm),⁵⁶ and Na₃K₆(CO₃)(NO₃)₂(H₂O)₆ (experimental, 0.094@546 nm).⁵⁷ However, the birefringence values for our compounds are lower than that of Cs₂Pb(NO₃)₂Br₂ (0.147@546 nm). Summary of some recently reported UV/visible halide, nitrate-halide-based, and hybrid perovskites is mentioned in Table S8.

Fig. 4 Birefringence measurements of Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂ using a polarizing microscope. Panels (a and b) and (d and e) display the views of the crystals at extinction during negative and positive rotations of the compensator for Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂, respectively; panels (c) and (f) show the thickness of the crystals used for measurement.

Theoretical calculations

To acquire a deeper understanding of the relationship between microstructure and optical properties in Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂, we employed systematic theoretical calculations using density functional theory (DFT).^45,58,59 As illustrated in Fig. 5a and e, calculations of band structures indicate that Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂ possess indirect band gaps of 3.495 and 3.555 eV, respectively. The calculated values are lower than the experimental range (5.51–5.64 eV) due to the typical tendency of the DFT method to underestimate the band gap.^60,61 Consequently, a scissors operator was applied as the difference between the experimentally observed and the calculated GGA band gap.

Fig. 5 Calculated band structures of Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂ (a and e), DOS and PDOS of Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂ (b and f), wavelength-dependent refractive indices for Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂ (c and g), and electron localization functions of Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂ (d and h).

The electron clouds at the top of the valence band (VB) and the bottom of the conduction band (CB), which are close to the Fermi level, describe the optical properties of the material. Fig. 5b and f present the total and partial densities of states (DOS and PDOS) for Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂. Near the Fermi level, O-2p and Cl-3p orbitals actively interact within the valence band. The conduction band is primarily formed from O-2p and N-2p states, indicating the presence of interactions consistent with N–O bonding characteristics. Based on the information presented, we can conclude that the linear optical properties of Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂ are primarily the result of the interaction between the [NO₃] groups and the CaO₄Cl₄ polyhedra.

It is widely recognized that the polarizability anisotropy of the [NO₃] group is highest among the planar triangular anion groups, which also include [BO₃] and [CO₃].^49–51 Therefore, it can be inferred that the material may display significant birefringence when the [NO₃] groups are arranged in a stacked manner, creating an ideal layered structure. The computed linear optical properties of Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂ indicate that n_z > n_x = n_y, or equivalently, n_e > n_o, suggesting that these compounds are positive uniaxial optical crystals. The calculated birefringence values are 0.086@1064 nm for Rb₂Ca(NO₃)₂Cl₂ (Fig. 5c) and 0.080@1064 nm for Cs₂Ca(NO₃)₂Cl₂ (Fig. 5g), which are close to the experimental values. To illustrate the influence of different groups on birefringence, the techniques of electron localization function (ELF) and response electron distribution anisotropy⁶² (REDA) are employed. ELF diagrams of Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂ show a small impact of Ca²⁺ cations on the optical birefringence due to their almost spherical electron distribution, while the [NO₃] units display distinct asymmetric lobes (Fig. 5d and h). The contributions from the [Rb(1)O₆Cl₄], [Ca(1)O₄Cl₄], and [NO₃] groups in Rb₂Ca(NO₃)₂Cl₂ are calculated to be 4.99, 25.40, and 69.61 percent, respectively. Meanwhile, the contributions from the [Cs(1)O₈Cl₄], [Ca(1)O₄Cl₄], and [NO₃] groups in Cs₂Ca(NO₃)₂Cl₂ are found to be 4.36, 22.78, and 72.86 percent, respectively, based on the calculated bonding electron density difference (Δρ) values of the individual building units (Fig. S7a and b). This suggests that the [NO₃] groups work together to enhance the anisotropy of the compounds, thereby affecting their birefringence values.

Conclusions

In conclusion, novel variants of layered Ruddlesden–Popper perovskites, Rb₂Ca(NO₃)₂Cl₂ and Cs₂Ca(NO₃)₂Cl₂, have been successfully synthesized for the first time by incorporating Ca²⁺ cations into the nitrate and chloride system. The integration of CaO₄Cl₄ polyhedra within a unique layered structure imparts tunable optical properties, including favourable short UV absorption edges (220–225 nm) and significant birefringence (0.099–0.104@546 nm). Title compounds also demonstrate high thermodynamic stability at temperatures ranging from 490 to 525 °C. Theoretical calculations indicate that the π-conjugated [NO₃] anionic group significantly influences their optical anisotropy. This research not only expands the structural diversity of perovskite variants but also offers valuable insights for the continued development of high-performance multifunctional optical materials, particularly in the UV region.

Author contributions

Fangfang Zhang supervised research and the manuscript revisions. Hussin Elkik designed and conducted the experiments, wrote the initial draft, and made revisions. Aqsa Munawar performed the theoretical calculations with assistance from Zhihua Yang. Muhammad Mujahid Iqbal contributed to the experimental work. Shilie Pan provided insights and guidance. All authors were involved in the discussions.

Conflicts of interest

The authors state that there are no conflicts of interest to disclose.

Data availability

The authors declare that the main data that support the findings of this study are available within the article and the supplementary information (SI). Other relevant data are available from the authors upon reasonable request.

Supplementary information: experimental related figures and tables. See DOI: https://doi.org/10.1039/d6qi00086j.

CCDC 2412905 (1) and 2412906 (2) contain the supplementary crystallographic data for this paper.^63a,b

Acknowledgements

This work was supported by the Tianshan Innovation Team Foundation (2022TSYCTD0005), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB0880000) and the CAS Youth Interdisciplinary Team Foundation (JCTD-2022-19).

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63. (a) CCDC 2412905: Experimental Crystal Structure Determination, 2026,  DOI:10.25505/fiz.icsd.cc2lztn2; (b) CCDC 2412906: Experimental Crystal Structure Determination, 2026,  DOI:10.25505/fiz.icsd.cc2lztp3.

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