1 Introduction

Metal-halide hybrid organic perovskite (HOP) belong to an interesting class of compounds with general formula (R-NH3)2 MX4, where R is an organic group, X a halogen atom (X = I, Br, Cl) and M a transition metal (Cu, Mn, Co, Zn). They have received increasing attention in the last decade due to their crystal structure diversity and their important optical properties [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19]. They crystallize from zero- to three-dimensional structures. The bulk materials find considerable application in fabrication of hybrid solar cells in photovoltaic area with 20% efficiency and in thin-film field-effect transistor [20,21,22,23,24,25,26,27]. The 2D organic-inorganic perovskites have attracted increasing attention because of their great environmental stability compared with 3D perovskites. The 2D HOP have drawn attention for their large band gap which can be used as an absorber in solar cells [28,29,30,31]. Previous studies show that the 2D hybrid-based solar cells are fit to hold tight 60% of their first power conversion efficiency under lighting after 2250 h and display more moisture tolerance in comparison to their 3D-based solar cells [32]. In addition, the HOP based on manganese atoms find a certain interest for their optical, magnetic, and solid-solid phase transition properties [33, 34]. The correlation between the electronic structures such as gap energy, density of states, and macroscopic properties can benefit from predictive modeling of these materials using density functional theory (DFT+U), since the DFT is a known weakness methodology of d localized electrons. Recently, the DFT+U method was introduced in computational calculation which consists of the correlation between electronic and Hubbard-type model for a subset of states in the system [35]. This implementation enhances the calculation of energetic, electronic, and magnetic properties of metals and semi-conducting and insulating materials with d delocalized electron [36,37,38,39,40].

In this work, we report the synthesis, crystal structure, optical properties from the UV-vis spectroscopy, and electronic properties using DFT+U calculations in order to perform structure-properties correlation of the hybrid PEA-MnCl4.

2 Experimental

2.1 Synthesis of PEA-MnCl4

Under ambient conditions, PEA-MnCl4 single crystals have been successfully synthesized by the reaction between MnCl2 and ((C6H5C2H4NH2)HCl) giving (C6 H5C2H4NH3)2MnCl4. Firstly, phenylethylamonium (C6H5C2H4NH2) was protonated by HCl (37%) in 5 ml of water/ethanol (1:1 in the ratio) solution. The solvent was evaporated until a white crystal powder is precipitated as (C6H5C2H4NH2)HCl. Then a saturated solution of the ammonium salts and MnCl2 powder was prepared. The mixture between these solutions was carried out at room temperature in a glass tube. Pink plate-shaped crystals are obtained after a few weeks.

2.2 Characterization of PEA-MnCl4

2.2.1 Single-crystal X-ray diffraction data collection

A PEA-MnCl4 single crystal of size 0.3 × 0.5 × 0.03 mm3 was selected under a polarizing microscope. The measurements were carried out, at 293 K, on a Nonius Kappa CCD diffractometer using Mo-Kα radiation (λ = 0.71073 Å) from graphite monochromator. The collection data was made using a mixture of Φ and Ω scan modes. The crystal to detector distance was 35.30 mm. The structure was solved by the direct method using SIR97 [41] program refined by the fully matrix least squares technique F2 using SHELX97 [42]. The non-hydrogen atoms were refined anisotropically and the hydrogen atoms were placed theoretically. All above programs were used within the WINGX package [43] and the drawings were made with Diamond [44]. The crystallographic data and experimental parameters for the intensity collection are summarized in Table 1.

Table 1 Crystallographic and structure refinement data for PEA-MnCl4
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2.2.2 X-ray powder diffraction

The powder X-ray diffraction pattern of the compound was performed on a LabX XRD-6100 Shimadzu powder X-ray diffractometer using graphite monochromator Cu–Kα radiation. The scanning step was 0.05 in the 2θ angle ranging from 5° to 50°. The experimental and simulated X-ray diffraction patterns represented in Fig. 1 are in good, which confirms the crystalline purity of the prepared compound.

Fig. 1
figure 1

Experimental and simulated X-ray diffraction patterns of PEA-MnCl4

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2.2.3 Morphology SEM-EDX

The morphology of the sample was observed by a JSM-6400 electron microscope (JEOL, Japan) with an acceleration voltage of 40 kV. The SEM image (Fig. 2) analysis shows good sample reactivity and good dispersion of the elements at the micrometric scale, as well as the surface of the hybrid appears in the form of the sheets for this compound and the crystallization is carried out without secondary phase training. The EDX microanalysis associated with the SEM, carried out on the zones of high contrast, shows the presence of characteristic carbon signal (Kα = 0.277 keV) and chlorine (Kα = 2.621 keV), characteristic signal of Mn (Kα = 5.894 keV and Lα = 0.637 keV). Note also the absence of impurities in the studied phase and the conformity and homogeneity of the analyzed composition with that desired.

Fig. 2
figure 2

SEM image (top) and EDX spectrum (bottom) of PEA-MnCl4

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2.2.4 UV-vis analysis

The UV-vis diffuse reflectance spectrum of PEA-MnCl4 crystal was carried out on a Jasco v-570 spectrophotometer in the range 200–1200 nm. A barium sulfate (BaSO4) plate was used as the standard (100% reflectance) on which the finely ground sample from the crystal was coated. The optical density was calculated from the reflectance spectrum, while the absorption spectrum was calculated using the Kubelka–Munk function [45].

3 Computational details

All the calculations have been performed using the projector augmented wave (PAW) method with the generalized (GGA-PBE) [46] gradient approximation using U-Hubbard (GGA+U) implemented in the ABINIT code [47]. The Brillouin-zone integration (4 × 4 × 1) was performed using special k points sampled within the Monkhorst–Pack scheme [48]. Khon–Sham orbitals were expanded using a plane wave basis up to a kinetic energy cutoff equal to 23 Ha. Those experimental lattice parameters and atomic positions were used as a starting point for the optimization of atomic positions. The atomic positions within the unit cell relaxed until the forces were less than 0.01 eV/A. The displacement parameters ∆ (∆xx, ∆yy, ∆zz) along the tree axis between experimental and optimized atomic positions are shown in Table 2.

Table 2 Atomic positions displacement (A) of Mn, Cl, N, C atoms for PEA-MnCl4
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4 Results and discussion

4.1 Crystal structure description

The crystal structure has been redetermined in order to gain more accurate values, for bonds and angles within [MnCl4]2− anion and intermolecular interactions. The compound crystallizes in the orthorhombic system Pbca (n°61) space group with the cell parameters a = 7.202(5) Å, b = 7.293(5) Å, c = 39.368(5) Å, and Z = 8 at 293(2) K. These parameters are in good agreement with those of previous study [34]. The asymmetric unit contains one half of [MnCl4]2− anion and one (C6H5C2H4NH3)2+ cation as shown in Fig. 3a.

Fig. 3
figure 3

Overall crystal structure: a asymmetric unit cell and b projection along a-axis (2D structure)

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The overall structure consists of alternating organic and inorganic layers, where the inorganic layer form 2D network, interplayed by the (C6H5C2H4NH3) bilayer ions (Fig. 3b). The octahedral manganese (II) center is coordinated by six terminal Cl− ions. The Mn-Cl distances are 2.5754(14) Å and 2.4816(7) Å for equatorial ligands and 2.5744 Å for the axial ones. The bridges angle (Mn-Cl-Mn) is 168.701(28)° which is not linear. The obtained values are in the same order to those of the previous study [34], which are respectively 2.577 Å and 2.484 Å for bond length and 168.66° for angle bridges. The Baur distortion [49] indices (ID) were calculated for PEA-MnCl4, using the formalism of Baur. The obtained indices are ID(d) = 1.66 × 10−2 for distances and ID(φ) = 6.2 × 10−3 for angles. These values are lower than those obtained for the compound (C6H9N2)2HgCl4 [50] (ID(d) = 1.75 × 10−2; ID((φ)) = 3.95 × 10−2) indicating a significant higher symmetry in the [MnCl4]2− entity. Figure 4 displays the hydrogen bond connectivity N-H…Cl between the [MnCl6] octahedra and the (C6H5C2H4NH3)+ cation. H bond is mainly involved in the cohesion of the crystal (Fig. 4). The N…Cl distances are in the range 3.295(3) Å to 3.488(3) Å. Another kind of interactions exists between the inorganic layers assured by van der Waals interactions. In the phenylethylamonium cation, the C-C distances are in the range 1.333(6) Å to 1.379(5) Å for benzene rings and in average of 1.498(4)°A for the alkyl ammonium when the C-C-C average angle is about 118.9(3)° and the torsion angle C-C-C-N is about 172.3(2)°.

Fig. 4
figure 4

Hydrogen interaction

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4.2 Optical properties

Figure 5 shows the optical reflectance spectrum of PEA-MnCl4 recorded in the range 200 to 1400 nm at room temperature. As it can be seen from the figure, a high reflectance is observed in the range 280–1400 nm (up than 70%) with the appearing of some anomalies in the range of 300–580 nm due to absorption phenomena.

Fig. 5
figure 5

Reflectance spectrum of PEA-MnCl4

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The study of optical properties, such as optical transitions and electronic band structure, near the absorption edge in the UV-Vis region is of major importance. The optical density spectrum of PEA-MnCl4 in the visible and near infrared range, measured at room temperature, is shown in Fig. 6, which shows also decomposition in Gaussian peaks labeled A, B, C, D, E, and F centered at 530, 460, 431, 381, 356, and 326 nm, respectively. These bands can be assigned to the transitions of Mn2+ ions in octahedral symmetry [51].

Fig. 6
figure 6

Optical density of PEA-MnCl4 (CT, charge transfer; d-d: internal transitions between d orbitals)

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Using Tanabe–Sugano diagram [52] for d5 configuration and from the literature [53,54,55], we can attributed the bands A and B to the transition from ground state 6A1g(S) to 4T1g(G) and to 4T2g(G), respectively, where the C and D are assigned to the transition from 6A1g(S) to the excited 4A1g(G) and 4Eg(G) states and from 6A1g(S) to the excited 4T2g(D) states, respectively. The bands E and F are attributed to the transitions from ground state to the excited 4Eg(D) and 4T1g(P) states, respectively.

The optical band gap was determined to inquire the conductivity of PEA-MnCl4 as the intersection point between the energy axis and the line extrapolated from the linear portion of the absorption edge in a plot of Kubelka–Munk function F(R) shown in Fig. 7. According to the previous equation, we found an optical energy gap of about 2.14 eV characteristic of semiconductor materials with a wide band gap.

Fig. 7
figure 7

Kubelka–Munk function versus photon energy for PEA-MnCl4

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4.3 Electronic properties

The total energies of different magnetic state configurations (Fig. 8) are gathered in the Table 3.

Fig. 8
figure 8

Different magnetic configurations

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Table 3 Total energies of different magnetic configurations
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By comparing the total energy in different magnetic configurations (FM, ferromagnetic; AFM, antiferromagnetic), it is found that the studied hybrid keeps the antiferromagnetic state according to the experimental results [34], although the antiferromagnetic configuration type (C) presents the stable ground state than the configurations AFM type (A) and AFM type (B).

The total density of states (TDOS) for the spin up and spin down of PEA-MnCl4 in antiferromagnetic states type (C) with Hubbard parameter (Ueff = 4 eV) is shown in Fig. 9. The analysis along energy axis reveals that the studied hybrid exhibits a semiconductor behavior with a band gap energy of about 2.07 eV, which is slightly smaller than the experimental value (2.14 eV). This underestimation of the band gap energy is explained by an usual artifact of DFT computations. Both experimental and computed values are in the same order than the similar compound NH3(CH2)5NH3MnCl4 [56].

Fig. 9
figure 9

Total density states

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To gain more insight into the bonding between all atoms, we have calculated the partial density of states (PDOS) of each atom (Fig. 10). The analysis of the density contribution in the valence band (VB) and the conduction band (CB) reveals that the maximum valence band (MVB) consists mainly of the orbital contribution Mn-d for spin up and spin down. Low contributions of Mn-s, Mn-p, N-p, and Cl-p orbitals are shown too. While the minimum of the conduction band (MCB) consists of the d-orbital up-down spin of Mn, N-2p, and C-2p orbitals. We can also show that the orbitals C-2s, C-2p, N-2s, and N-2p contribute around − 20 eV. However, the contribution of the various components (Mn-s, Mn-p, Mn-d, Cl-s, Cl-p, N-s, N-p, C-s, and C-p) is observed in the energy range [− 15, 0] eV.

Fig. 10
figure 10

Calculated PDOS diagram

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5 Conclusion

In this work, we have synthesized the organic-inorganic hybrid [C6H5C2H4NH3]2MnCl4 single crystals by slow evaporation at room temperature. The crystal structure, morphology and the purity of the compound were checked using single-crystal X-ray diffraction, SEM-EDX, and powder XRD analysis. The crystal structure consists of infinite chains forming a 2D framework. The optical density spectrum recorded in UV-vis shows six peaks assigned to the Mn2+ ions d-d transitions in the octahedral symmetry. The electronic properties were investigated by the high-throughput performed DFT+U calculation using PAW method, The different magnetic configurations calculated reveal the antiferromagnetic behavior of the studied material. The hybrid has an indirect band gap materiel, with a band gap energy of about 2.07 eV, which is slightly lower than the experimental value (2.14 eV). The analysis of the partial densities of states (PDOS) of each atom reveals the contribution of different atomic orbitals, to the maximum of the valence band (MVB) and the minimum of the conduction band (MCB).