Structural, Optical, and Magnetic Studies of the Metallic Lead Effect on MnO2-Pb-PbO2 Vitroceramics

MnO2-lead materials have attracted attention in their applications as electrodes. This work reports a detailed spectroscopic study of the compositional variation of MnO2-xLead vitroceramic materials with varied Pb contents. The concentration variation of lead and manganese ions issystematically characterized throughthe analysis of X-ray diffraction (XRD), Fourier transform infrared (FTIR), ultraviolet–visible (UV–Vis), and electron paramagnetic resonance (EPR) spectroscopy.The MnO2-xLead samples consist of a vitroceramic structure with Pb, PbO, PbO2,and Mn3O4 crystalline phases. The introduction of higher Pb content in the host vitroceramic reveals the [PbO6]→[PbOn] conversion, where n = 3, 4, and the formation of distorted [MnO6] octahedral units. The UV–Vis data of the samples possess the intense bands between 300 and 500 nm, which are due to the presence of divalent lead ions (320 nm) and divalent and trivalent manganese ions (420 and 490 nm, respectively) in the structure of glass ceramics. The EPR data show resonance lines located around g ~ 8 and 4.3, and a sextet hyperfine structure at g ~ 2, which isascribed to the Mn+3 and Mn+2 ions.

The manganese ions may enhance the chemical resistance of the lead network due to its unique chemistry. Different manganese ions, such as Mn +2 and Mn +3 ions, are known as paramagnetic ions. In glass and vitroceramics, the amount of these species depends on the mobility of the cations, field strength, condition of melting, concentrationof manganese in the host matrix, glassy modifiers, and formers [9].
Manganese oxide materials, namely MnO 2 and Mn 3 O 4 , are considered some of the most promising electrode materials because they are inexpensive, eco-friendly, and have a highly specific capacitance and cycling stability [10].
Oxide glass and vitroceramics doped with transition metal ions, such as MnO 2 , are technologically important because theycontain semiconduction and photo-conduction properties, switchingbehavior, and good optical absorption [11]. The tetrahedral or octahedral geometries of the manganese ions incorporated into the host network by varying the composition of glass or vitroceramics can be performed in the EPR and UV-Vis spectra.
In order to better understand manganese-lead glass and vitroceramics from an application point of view, the study of structure is necessary. The MnO 2 -Pb-PbO 2 vitroceramics can be used as electrodes for alead acid battery [8]. The present paper evidences structural information on the MnO 2 -xLead vitroceramic materials with x = 0-100 mol% Pb. The molar percentage of manganese dioxide was constantly 15 mol% MnO 2 . The effect of the substitution of lead dioxide by metallic lead on the host matrix was investigated through the present paper evidences structural information on the MnO2-xLead vitroceramic materials with x = 0-100 mol% Pb. The molar percentage of manganese dioxide was constantly 15 mol% MnO2. The effect of the substitution of lead dioxide by metallic lead on the host matrix was investigated through the analysis of XRD, IR, and UV-Vis spectroscopy.In this context, the variation of manganese ions'valence states as a function of Pb concentration was also studied using EPR spectroscopy. The role of the lead level on the structural aspects of MnO2-xLead vitroceramics materials using a systematic study on spectroscopic characteristics wasalso investigated.

Experimental Procedure
The MnO2-xLead samples with x = 0-100 mol% Pb were prepared from MnO2, PbO2,and metallic Pb powders. The fine mixtures of substances were melted in a sintered alumina crucible using an electric furnace set to 850 °C for 10 min. The melt was rapidly cooled on a steel plate.
The flow diagram of the synthesis and the Fujifilm images of the obtained samples are shown in Figure 1. The samples have a metallic appearance and are black in color. The composition of the MnO2-xLead prepared samples is listed in Table 1.  X-ray diffractograms were obtained at room temperature witha Shimadzu XRD-6000 diffractometer using the fine powder sample and an increment of 0.02 o ·s −1 .
IR absorption spectra were recorded using the JASCO 6200 FTIR spectrometer. The FTIR measurements were carried out at a resolution of 4 cm −1 .
UV-Visible spectra of the samples were recorded using a Perkin-Elmer Lambda 45 UV/VIS spectrometer with an integrated sphere and a resolution of 2 nm. The sampleto-KBr ratio was 1:150. Electron paramagnetic resonance (EPR) spectroscopy measurements were performed using a Bruker ELEXSYS 500 spectrometer in X-band. The samples were introduced in the glass tube using equal amounts of samples.

Structural Investigations by IR Spectroscopy
The MnO2-xLead vitroceramic materials, where x = 0-100 mol% Pb, were investigated by IR spectroscopy in order to obtain information concerning the compositional evolution of the structural units with the addition of the Pb content in the vitroceramic matrix. The obtained IR spectra are shown in Figure 3. Wavenumbers and the assignment

Structural Investigations by IR Spectroscopy
The MnO 2 -xLead vitroceramic materials, where x = 0-100 mol% Pb, were investigated by IR spectroscopy in order to obtain information concerning the compositional evolution of the structural units with the addition of the Pb content in the vitroceramic matrix. The obtained IR spectra are shown in Figure 3. Wavenumbers and the assignment of the IR bands characteristic of different structural units of the vitroceramic matrix are listed in Table 2.

Structural Investigations by IR Spectroscopy
The MnO2-xLead vitroceramic materials, where x = 0-100 mol% Pb, were investigated by IR spectroscopy in order to obtain information concerning the compositional evolution of the structural units with the addition of the Pb content in the vitroceramic matrix. The obtained IR spectra are shown in Figure 3. Wavenumbers and the assignment of the IR bands characteristic of different structural units of the vitroceramic matrix are listed in Table 2.      For the validation of the results, a deconvolution procedure of IR spectra was applied using a Gaussian-type function with fit multi-peaks of the Origin software. The deconvoluted IR spectrum of the sample with 30 mol% Pbis shown in the Figure 3c. The parameters of deconvolution, such as location of the center, C and integral intensity, A of detected IR peaks of deconvolution, are summarized in Table 3 3 and 4), which act as a network modifier and remain in the structure as Pb +2 ions. Table 3. Parameters (location of the center, C and integral intensity, A of IR peak) of deconvolution of the FTIR spectra of the MnO 2 -xLead vitroceramic materials, where x = 0-100 mol% Pb.

Samples
Positions and Integral Intensity of the IR Peaks

Structural Investigation by UV-Vis Spectroscopy
The UV-Vis spectra of the MnO 2 -xLead samples are shown in Figure 4. The first region of UV-Vis bands located between 200 and 300 nm is due to n-π * transitions of the Pb = O bond in the [PbO 3 ] structural units. The second region of UV-Vis bands located between 300 and 500 nm is due to the presence of Pb +2 (320 nm), Mn +2 (420 nm) [13,14], and Mn +3 (490 nm) ions [15] in the structure of glass ceramics. The band situated at about 420 nm is attributed to the transition of 6 A 1 (S) to 4 A 1 of Mn +2 ions [16].
By increasing the Pb content over 70 mol%, the intensity of UV-Vis bands located in the range between 300 and 440 nm was enriched. This indicates an increase in the fractions of Pb +2 and Mn +2 ions by increasing the dopant level.

Optical Band Gap Energy
The variation of (αhν) 1/2 and (αhν) 2 as functions of photon energy, hν, and the dependence of the x mol% Pb on the optical band gap energy, Eg, for the MnO 2 -xLead samples are plotted in Figures 5 and 6. The value of Eg was determined by extrapolating the linear domain of the (αhν) 1/2 and (αhν) 2 graphs as a function of hν at αhν→ 0 [17]. The E g values are situated between 2.47 and 2.65 eV for indirect transitions (with n = 2) and between 2.2-2.49 eV for direct transitions (with n = 1/2). This indicates a semiconductor behavior (gap energy, Eg < 3 eV) for all samples. tween 300 and 500 nm is due to the presence of Pb +2 (320 nm), Mn +2 (420 nm) [13,14], and Mn +3 (490 nm) ions [15] in the structure of glass ceramics. The band situated at about 420 nm is attributed to the transition of 6 A1(S) to 4 A1 of Mn +2 ions [16].
By increasing the Pb content over 70 mol%, the intensity of UV-Vis bands located in the range between 300 and 440 nm was enriched. This indicates an increase in the fractions of Pb +2 and Mn +2 ions by increasing the dopant level.

Structural Investigation by EPR Spectroscopy
The local geometry of the two manganese paramagnetic ions Mn +2 and Mn +3 was characterized by EPR spectroscopy. The EPR spectra of the MnO 2 -xLead vitroceramic materials are shown in Figure 7. Our EPR data indicate that the local geometry around the paramagnetic ions depends on the Pb content of the host vitroceramic. The EPR spectra show three resonance lines:two absorption signals centered at g~2 and g~4.3 corresponding to the Mn +2 ions [18][19][20], and the resonance line located at g~8 is attributed to Mn +3 ions.
The resonance line centered at g~2 was assigned to isolated Mn +2 ions located in sites with distorted octahedral geometry, as well as to those involved in dipole and/or superexchange magnetic interactions. Through the addition of metallic Pb up to x ≤ 60 mol% in vitroceramics, this resonance line consisted of a poorly hyperfine structure superposed over a wider resonance line corresponding to the clustered Mn +2 ions. For x ≥ 70 mol% Pb, this line became broader and more intense, becoming predominant in the EPR spectrum. This fact highlights the clustered nature of the main fraction of Mn +2 ions.
The Mn +2 ions at low manganese concentration had a hyperfine structure consistingof six resonance lines, which result from the dipole-dipole interaction between the magnetic moment of the 55 Mn nucleus and the electronic moment of the paramagnetic Mn +2 ion [16].
For the samples with x ≤ 60 mol%, the gradual disappearance of the hyperfine structure in the vitroceramics by doping with Pb content suggested the improvement of dipole-dipole interactions. The addition of higher Pb concentrations (x ≥ 70 mol% Pb) indicates an increase in magnetic exchange interaction.
The resonance signal located at g~4.3 corresponded to a rhombic distorted geometry of isolated Mn +2 ions. The intensity of this line decreased slightly with the addition of metallic Pb up to x ≤ 70 mol%, after which it disappeared. For higher dopant concentrationsabove 70 mol% Pb, the Mn +2 ions situated in isolated rhombic or distorted octahedral positions became clustered manganese ions. At higher metallic lead contents, x ≥ 80 mol%, the structural disorder around the Mn +2 ions increased, and these ions were present only as clustered Mn +2 species.
The resonance line located at g~8 was given by the Mn +3 ions (at~800G in the X band) [20]. Its intensity decreasedwith the introduction of Pb levels up to x ≤ 70 mol%, and for higher concentrations, the resonance line was not detected. This structural evolution is explained through the Mn +3 →Mn +2 conversion (according to the UV-Vis data) by increasing the Pb content over x ≥ 70 mol%.

Optical Band Gap Energy
The variation of (αhν) 1/2 and (αhν) 2 as functions of photon energy, hν, and the dependence of the x mol% Pb on the optical band gap energy, Eg,for the MnO2-xLead samples are plotted in Figures 5 and 6. The value ofEg was determined by extrapolating the linear domain of the (αhν) 1/2 and (αhν) 2 graphs as a function of hν at αhν→ 0 [17]. The Eg values are situated between 2.47 and 2.65 eV for indirect transitions (with n = 2) and between 2.2-2.49 eV for direct transitions (with n = 1/2). This indicates a semiconductor behavior (gap energy, Eg< 3eV) for all samples.

Structural Investigation by EPR Spectroscopy
The local geometry of the two manganese paramagnetic ions Mn +2 and Mn +3 was characterized by EPR spectroscopy. The EPR spectra of the MnO2-xLead vitroceramic materials are shown in Figure 7. Our EPR data indicate that the local geometry around the paramagnetic ions depends on the Pb content of the host vitroceramic. The EPR  spectra show three resonance lines:two absorption signals centered at g ~ 2 and g ~ 4.3 corresponding to the Mn +2 ions [18][19][20], and the resonance line located at g ~ 8 is attributed to Mn +3 ions. The resonance line centered at g ~ 2 was assigned to isolated Mn +2 ions located in sites with distorted octahedral geometry, as well as to those involved in dipole and/or superexchange magnetic interactions. Through the addition of metallic Pb up to x ≤ 60 mol% in vitroceramics, this resonance line consisted of a poorly hyperfine structure superposed over a wider resonance line corresponding to the clustered Mn +2 ions. For x ≥ 70 In conclusion, the effect of the Pb content on the host vitroceramic can be presented by two mechanisms: (i) For the vitroceramics with x ≤ 70 mol% Pb, manganese ions are present in two valence states, Mn +2 and Mn +3 . Mn +3 ions are obtained during synthesis, upon sudden cooling of the melt. There is a local ordering process of the oxygen atoms around the Mn +2 ion; (ii) For the vitroceramics with x ≥ 80 mol% Pb, only clustered Mn +2 ions predominated in the studied vitroceramics.

Conclusions
In this paper, the structural properties of the MnO 2 -xLead vitroceramic materials were investigated in order to find the role of lead content in the host matrix. The structural investigations of the prepared samples were realized by XRD, FTIR, UV-Vis, and EPR spectroscopy. The structure of the vitroceramic materials containingthe 0.15MnO 2 ·0.85Pb, 0.15MnO 2 ·0.85PbO 2 , and 0.15MnO 2 ·0.85[(1−x)PbO 2 ·xPb] compositions depends on the lead content.
XRD data indicatedvitroceramic structures for MnO 2 -xLead samples. The analysis of IR data evidenced that the structures are mainly composed of [PbO n ] and [MnO n ] structural units. The presence of the lead and manganese ions were evidenced in the UV-Vis spectra.
The EPR data indicated three resonance lines. The resonance lines located at g~2 and 4.3 are due to Mn +2 ions, while the last resonance signal corresponds to the Mn +3 ions.