Understanding the crystal structure of a material is essential for gaining insight into its physical properties. XRD spectra were recorded to determine the crystal structure and detect the presence of any impurity phases in the as-prepared samples. Figures 1(a) - (c) present the XRD results of Ni-substituted La₂Cr_1−xNi_xMnO₆ (x = 0.00, 0.50, 1.00) double perovskites recorded at room temperature. All the diffraction peaks were indexed to the Pbnm space group (No. 62), and their exact match with previously reported patterns [15] [17] [19] . This indicates that the original structure of La₂CrMnO₆ remains intact and does not undergo any phase transformation or structural instability upon Ni doping. The crystallite size of Ni-substituted La₂Cr_1−xNi_xMnO₆ (x = 0.00, 0.50, 1.00) double perovskites was calculated using Scherrer’s equation [19] , yielding values of 36 nm for LCMO, 25 nm for LCMO-Ni50, and 20 nm for LNMO. These results exhibit a decreasing trend with increasing Ni substitution at the Cr site.

Figures 1(d) - (f) present an enlarged view of the X-ray diffraction pattern in the range of 2θ = 46˚ - 47.5˚, showing a peak shift to higher angles with Ni substitution at the Cr-site, as observed in the XRD spectra of the LCMO-Ni50 sample. The peak shift suggests slight lattice shrinkage, likely due to the variation in the ionic radii of Ni and Cr, consistent with explanations and observations reported in other Ni-substituted compounds [21] [23] [24] [29] and similar findings in materials with a tetragonal tungsten bronze (TTB) structure type [30] .

Nickel (Ni) doping in oxide-based perovskite structures significantly impacts the crystal structure. These effects are influenced by factors such as the concentration of Ni dopants, the ionic radii of the dopant and host ions, and the oxidation state of Ni [23] [29] [31] . XRD patterns, along with the XPS survey (discussed in Section 4.4 below), confirm the presence of Cr, Ni, and Mn in their ionic states (ionic radius): Cr³⁺ (0.615 Å), Ni²⁺ (0.69 Å)/Ni³⁺ (0.60 Å), and Mn³⁺ (0.65 Å)/Mn⁴⁺ (0.53 Å), respectively, which aligns with those reported in previous studies [23] [24] [29] [31] . Ni²⁺ ions, being larger than Ni³⁺ ions, cause greater lattice expansion or distortion, while Ni³⁺ ions, with a smaller radius, contribute to lattice contraction, balancing structural changes in Ni-substituted La₂Cr_1−xNi_xMnO₆ samples [29] . The substitution of Ni²⁺ at the Cr³⁺ site results in changes to particle size and grain size (discussed in Section 4.2), which could potentially enable tuning of the optical and electrical properties in the La₂CrMnO₆ system, similar to modifications observed in compounds with a tetragonal tungsten bronze (TTB) structure type [32] [33] .

Field Emission Scanning Electron Microscopy (FESEM), shown in Figures 2(a) - (c) , and Energy-Dispersive X-ray Analysis (EDX), shown in Figures 3(a) - (c) , were conducted to examine the surface morphology and elemental composition of the as-synthesized samples. As shown in Figures 2(a) - (c) , the SEM images reveal a lack of discernible grain boundary formation, with most grains exhibiting independent growth, similar to the behavior observed in Bi-substituted La₂CoMnO₆

compounds [28] . Figures 3(a) - (c) present the EDX analysis of the structural and chemical properties of LCMO, LCMO-Ni50, and LNMO samples, corresponding to the SEM images shown previously. The EDX spectra indicate that all samples (LCMO, LCMO-Ni50, and LNMO) have retained their key elements, particularly Ni, as evidenced by the corresponding energy peaks and their concentration levels [18] [19] [34] . The EDX spectra also confirm the absence of any foreign elements, thereby justifying the phase purity in the LCMO, LCMO-Ni50, and LNMO samples. Grain size, average particle size, and area were determined using ImageJ Software, with the corresponding data provided in Table 1 . The average particle size, as determined from the SEM micrographs, is 2 μm for LCMO, 1.56 μm for LCMO-Ni50, and 1.32 μm for LNMO, indicating a gradual decrease in particle size with increasing nickel doping. The mean particle size of the materials, analyzed using SEM micrographs, shows a good correlation with the XRD patterns. The average grain size reported for LCMO, LCMO-Ni50, and LNMO is in the

micrometer range, while the crystallite size is in the nanometer range, confirming the polycrystalline nature of all samples with relatively large grain sizes, consistent with observations in Ni-substituted Ca₂FeNbO₆ double perovskite compounds [35] .

As shown in Table 1 , the grain size progressively decreases with increasing Ni concentration, which is consistent with findings reported in previous studies [28] . The distribution of chemical components in La₂Cr_1−xNi_xMnO₆ (x = 0.00, 0.50, 1.00) confirms that it follows the intended stoichiometric ratio, as expected. The intriguing SEM analysis data, including the reduction in grain size and average particle size with Ni substitution at the Cr site, highlights the potential for further research to enhance the electrical properties of Ni-substituted La₂CrMnO₆ double perovskite.

The optical bandgap properties were meticulously studied to obtain a deeper understanding of the electronic band structure in Ni-substituted La₂CrMnO₆ double perovskites. The diffuse reflectance UV-Vis spectra of the samples were recorded over the wavelength range of 200 - 800 nm, as illustrated in Figure 4 . Figures 4(a) - (c)

depicts the relationship between [T(R)hν]^1/2 and E(hν) for an indirect band gap, determined by converting the DRS UV-Vis spectra into the Kubelka-Munk function, following the methodology established in prior studies [28] .

The sharp edge of each curve was extrapolated to intersect the energy axis. The intercept at this edge provides the energy band gap values, which were calculated as 1.12 eV, 0.98 eV, and 0.95 eV for LCMO, LCMO-Ni50, and LNMO samples. These findings are summarized in Table 1 . As the quantity of Ni doping increases, all the samples show a progressive drop in bandgap values. The reduction in the band gap can be attributed to the substitution of Ni²⁺ ions at Cr³⁺ ions, which may introduce additional electronic states near the Fermi level, thereby narrowing the band gap [35] - [38] . Furthermore, the difference in ionic radii between Ni²⁺ (0.69 Å) and Cr³⁺ (0.615 Å), along with the reduction in crystallite size, may induce structural distortions, as indicated by the peak shifts in the XRD patterns, thereby affecting the electronic band structure [35] [38] . Ni substitution at the Cr site in La₂CrMnO₆ strongly correlates structural distortions and crystallite size reduction with optical properties. This makes these materials promising for advanced technologies requiring precise band gap engineering.

The magnetic exchange interactions of most compounds are very responsive to variations in their oxidation states. An extensive examination of the oxidation states of Cr and Ni ions in the La₂Cr_1−xNi_xMnO₆ series using X-ray photoelectron spectroscopy is essential for a thorough comprehension of its magnetic characteristics. To examine the chemical valence states of La, Cr, Ni, and Mn in La₂Cr_1−xNi_xMnO₆ for (x = 0.00, 0.50, 1.00), XPS investigation was conducted at room temperature. Figure 5 illustrates the XPS survey spectra of the synthesized samples, showcasing the photoelectron lines corresponding to chromium, oxygen, lanthanum, manganese, and nickel. The observed peak positions are consistent

with those reported in previous studies [20] [34] [39] . Figure 6 presents the XPS spectra for the O 1s and C 1s core-level regions. The C 1s line with a binding energy of 284.6 eV is apparent in the spectrum, presumably due to the presence of this element on the powder’s surface, as shown in Figure 6(a) . The core level XPS spectra of O 1s, Cr 2p, Mn 2p and Ni 2p are obtained to elucidate their chemical valences. The asymmetric profile of the O 1s XPS high-resolution spectra for all synthesized samples, shown in Figure 6(b) , reveals a distinct shoulder on the high binding energy side, with a prominent peak around ~529 eV, which is

consistent with previous reports [18] [20] [31] [34] . The chemical states of Ni, Cr, and Mn in the LCMO, LCMO-Ni50, and LNMO samples were investigated through XPS analysis, as depicted in Figure 7 . The XPS spectrum of chromium shows a clear splitting of the Cr 2p peak into two spectral lines, 2p_3/2 and 2p_1/2, at approximately ~575 eV and ~585 eV for both LCMO and LCMO-Ni50, as shown in Figure 7(a) [1] [17] . The splitting of the Cr 2p core-level spectrum at ~575 eV and ~585 eV is attributed to spin-orbit coupling and corresponds to the Cr³⁺ ionic state, as validated by previously reported data [18] [39] [40] .

The XPS spectrum of nickel for both LCMO-Ni50 and LNMO, shown in Figure 7(b) , reveals a mixed valence state of Ni²⁺ and Ni³⁺, with the binding energy peak positions consistent with previously reported data [20] [31] [34] . The satellite features of Ni 2p_3/2 and La 3d_3/2 overlap with one another, as observed in previous studies [31] [39] . Figure 7(c) illustrates the XPS survey of the Mn 2p core-level peaks, revealing two primary features: Mn 2p_3/2 peak at ~642 eV and Mn 2p_1/2 peak at ~654 eV. These peaks arise due to spin-orbit coupling. The observed peak positions of Mn 2p_3/2 and Mn 2p_1/2 align precisely with values reported in the literature, confirming the presence of Mn in mixed oxidation states, Mn³⁺ and Mn⁴⁺ [17] [18] [20] [31] [34] . The XPS results confirm the presence of mixed valence states for both Ni and Mn ions in Ni-substituted La₂CrMnO₆ double perovskites.