**3. Results and Discussion**

Figure 1 shows the morphology and composition analysis of the CSO: 2Mn4+ sample. The FE-SEM image shown in Figure 1a depicts that the particles formed were of submicrometer size. The particles have irregular shapes with a smooth surface. The elemental analysis of the as-prepared sample was performed using the EDX attachment of FE-SEM. The EDX spectrum of the sample is shown in Figure 1b and contains all elements, i.e., Ca, Sb, O, and Mn. The Ca, O, and Mn occupy K-shell and show peaks at 3.7, 0.52, and 5.9 eV, respectively. Sb and Mn occupy the M-shell and show peaks at 0.73 and 0.64 eV, respectively, and Mn also occupies the K shell with a peak at 5.9 eV. Elemental mapping of a small area is also presented in the figure which shows the elemental distribution in the sample. From Figure 1c–f it can be said that the elements Ca, Sb, O, and Mn were uniformly distributed in the sample. A layered image of all the elements present in the sample for the measured area is shown in the inset of Figure 1b.

**Figure 1.** (**a**) FE-SEM image, (**b**) EDX spectrum, and elemental mapping of elements (**c**) Ca, (**d**) Sb, (**e**) O, (**f**) Mn. Inset of (**b**) shows a layered image of all the elements present in the sample.

The XRD pattern of the CSO: 2Mn4+ sample is shown in Figure 2a. The XRD pattern shows sharp diffraction peaks with major peaks at 2θ values of 17.62◦, 19.53◦, 26.44◦, 34.17◦, 38.71◦, and 50.27◦ which corresponds to (001), (100), (101), (110), (2-11), and (2-12) planes, respectively, with other minor peaks at higher 2θ values. All the diffraction peaks of the sample match well with the standard JCPDS card #46-1496. The results show the sample crystallized in a hexagonal system with a space group of P -3 1 m (162). The lattice parameters of the sample area=b (Å): 5.24, c (Å): 5.02, and V (Å3): 119.42 [24].

The crystal structure of the sample was drawn using diamond software and is shown in Figure 2b. The crystal structure shown in the figure contains alternating layers of CaO6 octahedra and SbO6 octahedra. Each of the SbO6 octahedra share corners with six CaO6 octahedra, on the other hand, each CaO6 octahedra shares corners with twelve SbO6 octahedra. The length of all Ca–O bonds is 2.43 Å and the lengths of all Sb–O bonds are 2.02 Å. Generally, Mn4+ occupies and stabilizes in an octahedral site with 6-fold coordination [25] but, in this structure, both Ca and Sb form octahedra coordination, but the ionic radio of Ca is 1.00 Å and Sb is 0.6 Å, while it is 0.53 Å for Mn. Usually, the difference of ionic radii between the dopant and the host atom should not be more than 30% which suggests that Mn will be doped in the Sb site rather than the Ca site. The oxidation states of the elements present in CSO: 2Mn4+ sample were studied using the results obtained from XPS. The survey scan spectrum and high resolution Ca 2p, Sb 3d, O 1s, and Mn 2p were shown in Figure 2c. The survey scan spectrum taken in the binding energy range of 0–1300 eV shows peaks of all elements at respective binding energy values. The extrinsic hydrocarbon environment during the XPS measurement results in the visibility of C 1s peak around 280 eV in the survey scan spectrum. High resolution spectrum of Ca 2p shows two peaks at 349.5 and 346.03 which correspond to Ca 2p1/2 and Ca 2p3/2, respectively. Similarly, the high resolution spectrum of Sb 3d shows three peaks at binding energy values of 538.65, 531.89, and 529.3 eV. The peaks at 538.65 and 529.3 correspond to Sb 3d3/2 and Sb 3d5/2, respectively, and the peak at 531.89 eV belongs to O 1s. The high resolution spectrum of Mn 2p in the binding energy range of 660–630 eV shows high noise to signal ratio as the doping concentration of Mn in the host lattice is nominal.

**Figure 2.** (**a**) XRD spectrum, (**b**) crystal structure, and (**c**) XPS results of CSO: 2Mn4+.

The room temperature photoluminescence excitation (PLE) spectrum of the CSO: 2Mn4+ is shown in Figure 3a.

**Figure 3.** (**a**) PLE, (**b**) PL, (**c**,**d**) Gaussian fitting results and (**e**) 3D luminescence of CSO: 2Mn4+. Inset of (**b**) shows a graph of intensity variation with different concentrations of Mn4+.

The PLE spectrum shows broadband ranging from 200 to 400 nm with peak maxima located at 340 nm. The broadband is attributed to the spin-allowed transitions which have large electron-phonon coupling, i.e., 4A2g → 4T1g and Mn4+-O2- charge transfer. As can be seen from the Gaussian fitting results (Figure 3c), the broad band can be de-convoluted into two peaks, of which the charge transfer band is dominant, which is centered at 305 nm. Weak bands arising from the 4A2g → 2T2g and 4A2g → 4T2g electronic transitions are observed in the longer wavelength region (magnified in Figure 3a) and the results are in accordance with previous reports [21,26]. These results confirm that the Mn is in +4 oxidation state in the host material. On the other hand, the photoluminescence (PL) spectrum of the sample is shown in Figure 3b. The Mn4+-doped sample when excited at 340 nm shows a bright far-red emission band ranging from 575 to 800 nm with a peak maximum at 642 nm. The band observed is due to the spin-forbidden 2Eg → 4A2g electronic transitions in Mn4+ ions. However, the broadband splits into two peaks when fitted with a Gaussian function (Figure 3d) with peak maxima at 636 and 688 nm corresponding to the 4T1g → 4A2g and 2Eg → 4A2g electronic transitions, respectively. The results are consistent with the previous reports of Mn-doped oxide materials [26–29]. To determine the optimal doping concentration of Mn4+ ions, the host sample was doped with different concentrations and the PL spectra was measured. As the concentration of Mn4+ increases, the PL intensity increases and reaches the maximum at a concentration of 2 mol% and then decreases with further increase in concentration (inset of Figure 3b). This observed decrease in concentration is due to the quenching effect caused by the energy migration between neighboring Mn4+ activator ions. There are several reasons for the concentration quenching effect; as the PLE and PL spectra do not overlap, the re-absorption of radiation will not be

one among them. So, to further evaluate, the critical distance (Rc) between the two Mn4+ ions was calculated. If the Rc value is less than 5 Å, the exchange interaction is dominant and the multipole-multipole interaction will be dominant if the value is greater than 5 Å. The ionic radii of Sb and Mn are almost similar, and very dilute concentrations were doped into the host lattice. Therefore, this does not affect the volume of the unit cell much, and the critical concentration is Xc = 0.02. The critical distance calculated was found to be 15.6 Å which depicts that the multipole-multipole interaction is dominant in the concentration quenching effect [30,31]. On the other hand, the three-dimensional (3D) spectra of the CSO: 2Mn4+ sample were obtained to confirm the broad excitation and narrow emission of the sample. The contour line and 3D surface plots were shown in Figure 3e. The 3D surface plot was measured in the wavelength range of 400–800 nm for each nanometer excitation wavelength from 200 to 500 nm. From the contour line and 3D surface plots we can say that emission is only observed with lower excitation wavelengths, and at higher excitation wavelengths, i.e., above 380 nm, there is no emission observed. Strong red emission is observed in the excitation wavelengths range of 320–350 nm.

Figure 4 shows the Tanabe-Sugano energy level diagram of Mn4+ which is generally used to examine the luminescence mechanism. In general, the 3d3 electronic configuration of Mn4+ ions make them sensitive to the surrounding crystal field. To explain the effect of the crystal field strength on the PL properties of the CSO: Mn4+ phosphors, the Racah parameters and crystal field strength (Dq) are estimated. The Dq value can be estimated with the support of the 4A2g → 4T2g peak energy (23,753 cm<sup>−</sup>1) according to the following calculation:

$$D\_q = E \frac{\left(^{4}\text{A}\_{2\text{g}} \to \,^{4}\text{T}\_{2\text{g}}\right)}{10} \tag{1}$$

Then, the Racah parameter can be evaluated by using the peak energy difference between the 4A2g → 4T1g and 4A2g → 4T2g transitions by the following equations:

$$\frac{D\_{\!\!\!=1}}{B} = \frac{15(\text{x} - 8)}{(\text{x}^2 - 10\text{x})} \tag{2}$$

$$\mathbf{x} = E\left(^{4}\mathbf{A\_{2g}} \to \,^{4}\mathbf{T\_{1g}}\right) - E\frac{\left(^{4}\mathbf{A\_{2g}} \to \,^{4}\mathbf{T\_{2g}}\right)}{D\_{\mathbf{q}}}\tag{3}$$

The Racah parameter *<sup>C</sup>* can be estimated by the peak energy of the 2Eg → 4A2g emission transition using

$$E\frac{\left(^{2}E\text{g}\rightarrow\,^{4}A\_{2\text{g}}\right)}{B} = \frac{3.05\text{C}}{B} - \frac{1.8B}{D\_{q}} + 7.9\tag{4}$$

According to Equations (1)–(4), the values of *Dq*, *B*, and *C* were estimated to be 2375.3, 502.96, and 3525.67 cm<sup>−</sup>1, respectively. The *Dq*/*B* value estimation will decide the presence of Mn4+ in strong or weak crystal filed [32]. The calculated *Dq*/*B* was found to be 4.72. The obtained result shows that the Mn4+ ions are placed in a strong crystal field in CSO host lattice. Furthermore, the nephelauxetic ratio (*β*1) shows a substantial effect on the position of 2Eg energy level in a different host which can be obtained using the equation as shown below:

$$
\beta 1 = \sqrt{\left(\frac{B}{B\_o}\right)^2 / \left(\frac{C}{C\_o}\right)^2} \tag{5}
$$

*Bo* and *Co* are the Racah parameters of Mn4+ free-ions, which are 1160 and 4303 cm−1, respectively. The *β*1 value calculated for CSO: 2Mn4+ was found to be 0.927.

Figure 5 shows the comparison of the emission spectrum of CSO: 2Mn4+ and the absorption spectra of chlorophyll a and chlorophyll b.

**Figure 4.** Tanabe-Sugano energy level diagram of Mn4+.

**Figure 5.** (**a**) Comparison of PL spectrum of CSO: 2Mn4+ and absorption spectra of chlorophyll a and chlorophyll b. (**b**) PLE and PL spectra of Na+-doped CSO: 2Mn4+.

When analyzing the figure, we can say that there is a significant overlap between the emission spectrum of CSO: 2Mn4+ and the absorption spectra of chlorophyll a and chlorophyll b in the red region. Generally, chlorophyll is the green pigment of plants that absorbs the incident light and converts it into usable energy through a process called photosynthesis. On the other hand, the emission spectrum also overlaps with the absorption spectra of phytochrome Pr and phytochrome Pfr. Pr is the inactive form and converts to the active Pfr form on the absorption of red light and Pfr converts back to the inactive Pr form on absorption of far-red light or in the absence of light [33]. The results indicate that the CSO: 2Mn4+ sample, which has the potential to produce red light, can tune and accelerate the plant growth cycle and the carbohydrate yield. Furthermore, the effect of a charge compensator on the luminescence property of the CSO: Mn4+ sample was studied by doping Na+ ions in the host lattice. Usually, the ionic radii of the charge compensator should be more than the dopant ion, otherwise the charge compensator easily occupies the Sb site more than the actual dopant ion and decreases the concentration of Mn4+ at the Sb site [34,35]. The ionic radio of Na+ and Mn4+ are 1.02 and 0.53 Å, respectively. So, when Na+ was doped along with Mn4+ ions at the Sb site, an increase in the intensity i.e., both in PL and PLE spectra, was observed.

Figure 6a shows the decay profiles of the CSO: 2Mn4+ and CSO: 2Mn4+, Na+ samples for excitation and emission wavelengths of 340 and 642 nm, respectively. The Mn4+ single doped sample best fit with the single exponential function as shown below:

$$I(t) = I\_o + Ae^{\frac{-t}{\tau}} \tag{6}$$

where *Io* and *I*(*t*) are initial luminescence intensity and luminescence intensity at time t, A is fitting constant and *τ* is decay time. The CSO: 2Mn4+ sample shows a decay time of 2111.51 μs. Similarly, CSO: 2Mn4+, Na+ sample was fitted for a double exponential function as shown below:

$$I(t) = I\_o + A\_1 e^{\frac{-t}{\overline{\tau}\_1}} + A\_2 e^{\frac{-t}{\overline{\tau}\_2}} \tag{7}$$

where *A*<sup>1</sup> and *A*<sup>2</sup> are fitting constants and τ<sup>1</sup> and τ<sup>2</sup> are short and long decay times. The obtained *τ*<sup>1</sup> and *τ*<sup>2</sup> values are 431.47 and 1968.78 μs, respectively. The short and long life times indicate the presence of Mn4+ ions near and far from the charge compensation defects, respectively [36]. Figure 6b shows the CIE chromaticity diagram where the CIE values were plotted. The CIE values were calculated from the emission data (400–800 nm) of CSO: 2Mn4+ and CSO: 2Mn4+, Na+ samples obtained at the excitation and emission wavelengths of 340 and 642 nm, respectively. The calculated color coordinates for Mn4+ (0.6107, 0.3313) and Mn4+- and Na+- (0.6338, 0.341) doped samples, respectively. The inset of Figure 6b shows photographic images of powder samples under 365 nm ultraviolet light.

**Figure 6.** (**a**) Decay curves of CSO: 2Mn4+ and Na+-doped CSO: 2Mn4+, (**b**) CIE chromaticity diagram for the prepared phosphors. The inset of (**b**) shows the powder images under 365 nm ultraviolet lamp.

### **4. Conclusions**

In summary, the CSO: Mn4+-doped phosphors were prepared using a conventional solid-state reaction method. The powders were characterized using different techniques, and the results were analyzed. The samples crystallized in a hexagonal system with a space group of P -3 1 m (162). The diffraction peaks matched well with the standard values, and no other impurities or peaks related to other phases were observed. SEM and EDX analysis showed that the particles are in the sub-micrometer range and the elements were uniformly distributed throughout the sample. The PLE spectrum displays that the sample can be effectively excited at 340 nm wavelength. The strong red emission centered at 642 nm is mainly due to the spin-forbidden 2Eg → 4A2g electronic transitions in Mn4+ ions. The optimum doping concentration is found to be 2 mol%, and thereafter concentration quenching was observed, which is due to the multipole-multipole interaction. In addition, a charge compensator (Na+) doping increased the emission intensity by 35%. The emission results match well with the absorption range of chlorophyll a, chlorophyll b, Pr, and Pfr, indicating that the materials are suitable candidates for plant growth LED applications.

**Author Contributions:** L.K.B., H.P. and J.S.Y. conceived, designed and directed the project. L.K.B. and H.P. synthesized and characterized the phosphor powders. S.V.G. directed the fabrication process and calculations. L.K.B., A.S. and J.S.Y. co-wrote the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by a grant of the Ministry of Science and Higher Education of the Russian Federation for large scientific projects in priority areas of scientific and technological development (grant number 075-15-2020-774).

**Institutional Review Board Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
