The investigation of structural and vibrational properties and optical behavior of Ti-doped La0.67Ba0.25Ca0.08Mn(1−x)TixO3 (x = 0.00, 0.05 and 0.10) manganites

The influence of Ti4+ ions incorporated into the B site on the structural, vibrational and optical properties of La0.67Ba0.25Ca0.08Mn(1−x)TixO3 (LBCM(1−x)Tx), a polycrystalline compound prepared by a molten salt method, was discussed. The X-ray diffraction (XRD) studies confirmed that at room temperature these compounds crystallize in the rhombohedral phases of R3̄c. Rietveld refinement indicated that the octahedron (Mn/Ti)O6 underwent a slight deformation and the θ(Mn/Ti)–O–(Mn/Ti) bond angles decreased with the increase in the Ti content. Furthermore, Raman spectra were recorded at room temperature for the LBCM(1−x)Tx ceramics to investigate the influence of incorporated Ti4+ ions in LBCM(1−x)Tx. Moreover, we controlled the frequency and damping of the optic modes based on Ti incorporation. The infrared (IR) absorption spectrum (FTIR) analysis in the span of 420–750 cm−1 supports the XRD results. The diffuse reflectance data at room temperature verified that both transition levels (5Eg → 5T5g) and (4A2g → 4T2g) correspond to the Mn3+ and Mn4+ ions. The optical band gap (Eg) values decreased from 2.90 eV to 2.70 eV with the increase in the Ti4+ content, implying that our samples could be good candidates for some applications in luminescent devices, such as ultrafast optoelectronic devices. Moreover, the photoluminescence spectra (PL) features at room temperature decreased for all samples. CIE were estimated for all the concentrations of Ti4+ ions. The results indicated that are a shifts in the CIE values of the compounds.


Introduction
Perovskite manganese oxides of Ln 1Àx A x MnO 3 (A ¼ Ca, Sr, and Ba) have been an important topic in scientic studies and potential technological applications due to their interesting physical properties, resulting from the orbital degree of freedom, lattice charge and spin coupling. 1,2 These properties can be manipulated due to the exibility of the lattice deformation, 3,4 the lling of the one-electron level, the internal strain and its width. In fact, manganites, owing to their physical and chemical properties, have attracted much interest because of their novel magnetic, electronic and optical properties. Colossal magneto-resistance (CMR) has recently been a source of great interest for these manganites for their potential applications in magnetic storage systems, magnetic eld sensors and spintronics. [5][6][7][8][9][10] Recently, the exceptional properties of bariumdoped lanthanum manganites (La 1Àx Ba x MnO 3 , LBM) have secured a prominent position in many industrial technologies, such as magnetic sensors. [11][12][13][14] These manganites demonstrate a transition from metal behavior to insulator behavior, followed by a ferro-paramagnetic transition, i.e., the Curie temperature (T C ), which is characterized by high magnetic entropy. 15,16 The substitution of bivalent cations for the rare-earth cations in manganites leads to converting twice as many Mn 3+ into Mn 4+ ions to donate information to the double exchange interactions, which are the origin of the ferromagnetic behavior. 17,18 Moreover, the replacement of manganese by titanium ions has been the subject of diverse studies examining the magnetic, magneto-caloric and electrical behaviors of La 0.7 Sr 0.25 Na 0.05 -Mn (1Àx) Ti x O 3 , 19 La 0.85 Ba 0.15 Mn (1Àx) Ti x O 3 (ref. 20) and La 0.57 Nd 0.1 Pb 0.33 Mn 1Àx Ti x O 3 . 21 Kossi et al. 22 investigated the impact of the incorporation of Ti 4+ ions on the morphological and various electrical properties of a La 0.7 Sr 0.25 Na 0.05 Mn 0.9 Ti 0.1 O 3 manganese perovskite. They conrmed that the lattice effects on the physical properties can be ignored since Mn 4+ ions are replaced directly with Ti 4+ ions. However, new insight into the physical properties with respect to the optical properties has not been established in manganites that either show insulator behavior (large band gap; typically, >4 eV) or metallic behavior (no band gap), which makes them less important for optical studies. Kumar et al. 23 conrmed that the (La 0.6 Pr 0.4 ) 0.65 Ca 0.35 MnO 3 ceramic is a potential candidate for optical applications.
A detailed literature survey shows that not much work has been done on the vibrational and optical properties in the simultaneous substitution of Ba 2+ /Ca 2+ ions and transition metal Ti 4+ ions in manganese perovskites. For this reason, we report the effect of Ti 4+ ions incorporated in the Mn sites on the structural, vibrational and optical properties of the polycrystalline LBCM (1Àx) T x . This polycrystalline material is synthesized by the molten salt method. The LBCM (1Àx) T x compounds were synthesized via a ux method. The metal precursors with high purity were La 2 O 3 , BaCO 3 , CaCO 3 MnO 2 and TiO 2 . Then, these precursors were mixed well for 2 h in an agate mortar and 2 h in alcohol. The resultant blends were heated at 800 C for 24 h in an alumina melting-pot, and then cooled at room temperature. Aer the completion of the reaction, the rest were crushed and washed repeatedly with distilled water to remove the salts. Aer slowly drying at 100 C in air, it was pressed into pellets (e ¼ 1 mm and d ¼ 10 mm) and sintered at 1000 C for 24 h. For clarity, Fig. 1 shows the steps of the synthesis process.

Characterization
The crystalline phases of LBCM (1Àx) T x were checked by XRD using the "PANalytical X'Pert Pro" diffractometer (l Cu-Ka ¼ 1.5406 A). The spectra were noted in the 2q region of 10 -60 with a pitch of 0.02 . The structural analysis was carried out using the FullProf program. 24,25 The infrared spectra were measured by transmittance mode in the range 450 to 4000 cm À1 at room temperature on a Perki-nElmer spectrum 100 spectrophotometer.
The Raman measurements were registered in the frequency region from 50 to 1000 cm À1 utilizing a LabRAM HR800. The compounds were excited using a 488 nm laser.
The UV-visible reectance spectra were measured on a Shimadzu UV-3101PC spectrophotometer.  The photoluminescence (PL) spectra were collected at 300 K employing a iHR320 monochromator. The LBCM (1Àx) T x were excited using a 300 nm source. The performance of XRD data renement is evaluated by the adjustment sign, such as the weighted pattern R wp , pattern R p , and the goodness of t c 2 . Good agreement was observed between the calculated and experimental XRD patterns. The results aer tting are illustrated in Table 1. In our case, it is worth remarking that the Ti 4+ ionic radius (r Ti

X-ray diffraction
Therefore, the incorporation of Ti 4+ into the Mn 4+ site induces a deformation of the hexagonal phase by an elongation along both the a and c axes and, consequently, an increase of the cell volume.
The increase of the cell parameters is caused by the increase of the Mn-O bond length (hd Mn/Ti-O-Mn i). Table 1 Refined structure parameters at room temperature for LBCM (1Àx) T x (x ¼ 0.00, 0.05 and 0.10) Cell parameters a (Å) 5.530 (7)    The structure and the (Mn/Ti)O 6 octahedron for the LBCM 0.95 T 0.05 sample (as an example) were plotted by the "Diamond" program, which, based on the rened atomic positions, is depicted graphically in Fig. 3. From this gure, the lattice deformation can be seen. Therefore, lattice effects may impact the vibrational properties and optical behaviors in these compounds.
To better understand the inuence of cation incorporation, Raman and infrared spectroscopy are useful techniques for examining the structure and identifying the functional groups in these compounds.

Raman spectroscopy investigation
Raman spectroscopy was carried out for all the samples in order to better understand the modication changes in the structure. In addition, the observed Raman spectra allow us to study the inuence of the deformation introduced by the incorporation of Ti 4+ ions at the B site and correlate between the structural details. Fig. 4 shows the Raman spectra in the range of 80-1000 cm À1 for the LBCM (1Àx) T x (x ¼ 0.00, 0.05 and 0.10) manganite at room temperature.
We notice that the LBCM (1Àx) T x ceramics Raman spectra are very similar in position and prole to the data of the pure LBCM ceramic, which is characterized by the rhombohedral phase (D 3d ). These results are similar to other literature reports. [28][29][30][31] In addition, we noticed the peak positions are shied to higher frequencies and the intensity increases as the rate of Ti increases in our compounds. This behavior may be due to disorder in our compounds, which is followed by the Ti 4+ substitution at the B site for the LBCM (1Àx) T x ceramic. Meanwhile, an important modication of the local vibrational dynamics generated by the structural distortion and slight modication in the local symmetry of the LBCM manganite (for x ¼ 0.00) can be observed.
In our case, for the compound LBCM (x ¼ 0.00), we observed ve Raman modes at 110, 180, 309, 486 and 609 cm À1 . This is in agreement with that reported for La 0.65 Eu 0.05 Sr 0.3Àx MnO 3 by Bellouz et al. 32 The band at low frequency occurring around 111 cm À1 is dominant since it corresponds to the distortions in the A-site cations (La/Ba/Ca). The peak at 180 cm À1 is assigned as A 1g . The mode near 300-400 cm À1 corresponds to the E g (1) mode, which are the bending, rotational and stretching  vibrations of the MnO 6 octahedra. [33][34][35] We ascribe the E g(2) mode (at 609 and 486 cm À1 ) to the Mn-O and O-Mn-O vibrations, respectively. Fig. 5 illustrates the ionic pattern associated with the A and E vibration in a rhombohedral of LBCM 0.95 T 0.05 , as an example.
For LBCM (1Àx) T x (x ¼ 0.05 and 0.10) compounds, they are similar to the compound where x ¼ 0.00, i.e. there are no new peaks during incorporation. The Ti ions do not substitute exactly for Mn ions, and a displacement of the smaller Ti with respect to the Mn site is responsible for a modied local symmetry, which can result in the activation of phonons out of the Brillouin zone center.
In addition, the frequency and damping of the three modes 110, 490 and 605 cm À1 for the LBCM (1Àx) T x samples is listed in Table 2. We conrm that the band wavenumber shis slightly to a higher value. This shi, due to the incorporation of partial Ti ions, leads to the deformation of the octahedral (MnO 6 ) of our perovskite, and so allows for a modication in the Mn-O-Mn bond angle. These vibration modes in manganites arise from the Jahn-Teller (J-T) distortions. Furthermore, we underscore that the (E g ) mode (z609 cm À1 ) originates from the symmetric stretching vibration of oxygen in MnO 6 octahedra.
So, the damping of the phonons is also much more important in the LBCM 0.9 T 0.10 sample than in pure LBCM and the LBCM 0.95 T 0.05 sample. The contrast is most obvious with the highest frequency mode E g (2) , whose damping and frequency are unaffected.
This can be related to the fact that as the (Ca, Ba) ion radius is notably smaller than that of the substituted La ion, it can participate easily in A-site and A 1g motions. On the contrary, the E g (2) mode is controlled by the introduction of Ti ions into the B site of LaMnO 3 ceramics. Fig. 6 shows the Raman response that has been rectied by the population factor n(u) ¼ (e ħu/kT À 1) À1 . It allows us to eliminate supplementary bands as artifacts within the tting process. We remark that the factor n(u) + 1 is related to rstorder Stokes scattering.
At low frequencies, we notice a disappearance in the modes when comparing between the original spectra and aer dividing the Raman response by the factor n(u) + 1. This results have inuenced on the optical response at room temperature.

Fourier transform infrared spectroscopy
At room temperature, the Fourier transform infrared absorption spectra (FTIR) are illustrated in Fig. 7. We notice our samples  The second band is due to the stretching mode, n S , at about 600 cm À1 , implying that the internal movement of the Mn 4+ ion is opposed to the (Mn/Ti)O 6 octahedron resulting from the Jahn-Teller (J-T) effect. 30,31 The transmission wave number (n) of the Mn/Ti-O bond vibration is given by Hooke's relation: where k is the average force constant of the Mn/Ti-O bond (hd Mn/Ti-O-Mn i), c is the velocity of light, and m is the effective mass of the Mn/Ti-O bond expressed by the following relation: where M 0 is the atomic weight of O, and M B is the atomic mass of the B site ions given by the relation: where M Ti and M Mn are the atomic weights of Ti and Mn, respectively. The force constant can be connected to the average Mn/Ti-O bond length (r) by the following expression: Based on eqn (1)-(4), the effective mass value, Mn/Ti-O bond lengths and the force constant for all compounds were determined from the FTIR spectra and are summarized in Table 3. We can remark that the calculated Mn/Ti-O bond lengths from the FTIR spectra are very close to the results obtained from the Rietveld renement. [36][37][38] From Fig. 7, it is observed that a remarkable change in the FTIR occurs, in which the transmission bands associated with the Mn/Ti-O stretching vibration shi from 577.57 cm À1 to 585.49 cm À1 with the increasing value of x and increase in Mn/ Ti-O bond lengths (i.e., decrease in force constant (k)).
So, the results establish by FTIR, whither the frequency shi is expected by the deformation structure (see Fig. 4), a more disordered structure will created by a shi of the E g mode in Raman spectra too.

Optical properties
3.4.1 UV-visible diffuse reectance. At room temperature, the UV-visible diffuse reectance data of the LBCM (1Àx) T x (x ¼ 0.00, 0.05 and 0.10) ceramic samples are represented in Fig. 8.
The bands at 500 nm for these compounds are ascribed to dd transitions of ( 5 E g / 5 T 5g ) and ( 4 A 2g / 4 T 2g ) for the Mn 3+ and Mn 4+ ions, respectively. 39 From the UV-visible reectance data, the optical band gap energy (E g ) was determined by the Kubelka and Munk method. 40 This method is usually employed to study the diffuse reectance measurement acquired from faintly absorbing compounds. In our case, the Kubelka-Munk equation for any wavelength is described by: where t is the thickness of the LBCM (1Àx) T x compound which approximately equals 1 mm.
The optical band gap (E g ) values of our ceramics were then calculated by the following equation: 43 where hn is the photon energy, A is an energy independent constant and n is a constant associated with the different types  of electronic transitions (n ¼ 1/2 for the direct allowed, n ¼ 2 for the indirect allowed, n ¼ 3/2 for the direct forbidden and n ¼ 3 for the indirect forbidden). However, it is likely that only direct transitions will occur in the perovskite.
Consequently, eqn (8) becomes: We have plotted the variation of [F(R)hn] 1/2 versus (hn) for our compounds. The values of the indirect optical band gap (E g ) for the LBCM (1Àx) T x (x ¼ 0.00, 0.05 and 0.10) were obtained, as represented in Fig. 8, by adjustment of the linear part of these plots at [F(R)hn] 1/2 ¼ 0. The E g values are presented in Table 4.
The values of the optical band gap are 2.90, 2.80 and 2.70 eV for x ¼ 0.00, 0.05 and 0.10 respectively.
We notice that the E g values decreased with the increase of Ti 4+ rate in the LBCM (1Àx) T  Table 1).
Further, E g is connected with W as follows: E g ¼ D À W, where D is the charge-transfer energy. 45 In our case, all these structural changes are explained in a clean decrease of the one electron bandwidth (W). So, the substitution of Ti into Mn in the A site results in a decrease in the optical band gap.
In addition, the width of the defect bands existing in the optical band gap correspond to the Urbach energy, E u . 46 It can be described by the equation: The Urbach energy, E u , was determined by plotting ln (a) vs. hv (Fig. 9).
The E u values are also regrouped in Table 4. A slight increase in the Urbach energy values is observed with the addition of Ti 4+ ions.
For our sample, the LBCM (1Àx) T x ceramics, the optical band gaps span over 2.72-2.89 eV in the UV range, allowing for carrier excitation with femtosecond laser pulses, which makes these ceramics viable for applications in ultrafast optoelectronic devices (Fig. 10). 47 3.4.2 Photoluminescence study. The emission response of LBCM (1Àx) T x O 3 (x ¼ 0.00, 0.05 and 0.10) at room temperature is illustrated in Fig. 11. We notice that the proles of the PL spectra are similar to the pure sample.
Furthermore, we note that when titanium is introduced in the LBCM (1Àx) T x ceramic, the PL response intensity increases. This is demonstrated by the increase of defects in our compound.
The existence of (Ca 2+ , Ba 2+ ) for La 3+ in the A site and Ti 4+ for Mn 3+ in the B site of the LaMnO 3 lattice results in a distortion of the lattice. In our case, the Ti 4+ ions approach each other, which reinforces their interactions and results in quenched emission by non-radiative pathways (i.e. lattice vibrations). The experimental results show that photoluminescence is connected to the structural disorder and deformation.
The same result was reported by Zhu et al. 48 for La 0.67 -Ca 0.33 MnO 3 compounds and Chen et al. 49 for La 0.825 Sr 0.175 MnO 3 compounds.
In addition, this comportment can be produced by oxygen vacancies for random common "perovskites" and/or disorder coupled to the "tilt" of a [Mn/TiO 6 ]-[Mn/TiO 6 ] complex cluster.
The CIE chromaticity of LBCM (1Àx) T x (x ¼ 0.00, 0.05 and 0.10) is illustrated in Fig. 12. The CIE diagram shows that the estimated coordinates are associated in the blue region and tuned towards the pure blue region. The calculated values of the CIE coordinates of the LBCM (1Àx) T x ceramic are tabulated in Table 5. The CIE coordinates vary from (0.1752, 0.1626) to (0.1773, 0.1688) upon the incorporation of Ti ions in the LBCM (1Àx) T x ceramic.
Increasing the Ti 4+ ion rate in the host lattice led to signicant modications of the optical behavior, which can be attributed to the structural deformation. However, our investigation of the structural, vibrational and optical properties of the polycrystalline LBCM (1Àx) T x demonstrate that their optical sensitivity places them as good candidates for many practical applications in luminescent devices.

Conclusion
Polycrystalline LBCM (1Àx) T x (x ¼ 0.00, 0.05 and 0.10) ceramics were synthesized by the molten salt process. The XRD analysis indicated that the ceramic possesses a rhombohedral phase structure R 3c. The XRD analysis, along with the Raman and FTIR measurements, verify the incorporation of Ti into the Mnsite and the ensuing chemical disorder in these compounds. The optical band gap (E g ) of our samples, as measured by UVvisible reectance, decreases from 2.90 eV to 2.70 eV with the increase in the Ti content. The photoluminescence spectra (PL) features at room temperature are smaller for all samples. This behavior is explained by oxygen vacancies. The CIE coordinate varies from (0.1752, 0.1626) to (0.1773, 0.1688) upon the incorporation of Ti ions at Mn ion sites in the LBCM (1Àx) T x ceramic.
The investigation of the structural, vibrational and optical properties for the LBCM (1Àx) T x manganites shows that our compounds can be a good candidate for applications in luminescent devices, such as ultrafast optoelectronics.

Conflicts of interest
There are no conicts to declare.