Unveiling the Structural and Optical Properties of Samarium Titanates Pyrochlore, Sm2Ti2O7

A magnetic system whose geometry prevents all pairwise spin interactions from being fulfilled simultaneously to minimize the energy is said to be frustrated. These systems are prevalent throughout all disciplines of physical and chemical study. This article reports a detailed structural and optical analysis of such frustrated Sm2Ti2O7 (STO), a pyrochlore structure. The ordered pyrochlore, STO of cubic symmetry, space group Fd3¯m [no. 227], has been synthesized using the standard solid-state reaction method. The Tauc’s plot reveals a direct band gap value of 3.8 eV from ultraviolet-visible reflectance data. Photoluminescence studies for STO using 280 nm excitation wavelength show the most intense emission peak at 395 nm, 468 nm, and other weak emissions at 368 nm, 412 nm, 430 nm, 449 nm, and 514 nm corresponding to the respective electronic transitions, indicates that the materials show photoluminescence properties. However, the unique characteristics of Sm2Ti2O7, including its wide band gap, photoluminescence, and high refractive index, make it a promising candidate for various magneto-optical, solid oxide fuel cell device fabrication applications.


Introduction
The cubic-structured titanate oxides, RE2Ti2O7 (RE: Rare-Earth) pyrochlore are noteworthy materials due to their unique ground states, including spin ice, spin glass, cluster glass, spin liquid, and Griffiths phase-like behaviour resulting from magnetic frustration and coupling among various ordered parameters.The interplay of various ordered parameters makes them promising for diverse applications such as optical device manufacturing, solid oxide fuel cells, water purification, nuclear waste containment, and spintronics [1][2][3][4][5] .Geometrically frustrated rare-earth pyrochlore magnets have been intensely studied in recent years, rich in various unconventional ground states and quantum phenomena due to their special structural character at low temperatures 6 .The frustrated pyrochlore structure is extremely susceptible to various external impacts, such as crystal field, pressure, magnetic field, frequency, temperature, composition, and chemical pressure [7][8][9] .Only a few research works are concerned with the structural, optical, electrical, and magnetic properties of samarium-based pyrochlores.Most of these related to pure and doped oxides as Sm2X2O7, X=Ti, Zr 10-12 , ising antiferromagnetic in Sm2Ti2O7 pyrochlore 5 , ternary oxides Ln2CrTaO7, Ln = Y, Sm, Gd 13 , Sm2Ti2O7/SmCrO3 heterojunction photocatalyst 14,15 .The idealized chemical formula A2B2X6X' describes the pyrochlore structure, where the A-site element belongs to the rare earth metal family, and the B-site element is typically from the transition metal class.And pyrochlore family can be more accurately represented as A2B2X6X' 6 .These rare earth titanate pyrochlores, RE2Ti2O7 create an endless 3-dimensional corner-sharing tetrahedral network with a very high potential for frustration.The pyrochlore compound Sm2Ti2O7 exhibits ordered atomic arrangements and possesses a face-centred cubic (FCC) crystal structure.It can be described by the stoichiometry A2B2X6X' classified under the space group Fd3 ̅ m [no.227] shown in Figure 1, in which Sm 3+ 3p and Ti 4+ 4þ ions reside at 16d and 16c sites, respectively.In the structure, 16d crystallographic site is occupied by larger Sm 3p cations, coordinated with eight oxygen anions (6X and 2X') positioned inside a distorted cubic polyhedron formed by eight oxygen atoms.While 16c crystallographic site is occupied by smaller Ti 4p cations coordinated with six oxygen anions (6X) and placed inside distorted octahedron formed by six oxygen atoms, BX6.The 48f site, with a 2mm symmetry, is occupied by oxygen anions, and each is coordinated with two Ti and two Sm cations.As for the X' anions, they are positioned at the 8a site, exhibiting 43m symmetry, and are tetrahedrally coordinated with four A cations 5,11 .The present article aims to detail the synthesis and characterizations (structural and optical) of Sm2Ti2O7 pyrochlore material with Sm +3 4f 5 and Ti +4 3d 0 ions on an fcc lattice of edge-shared network of tetrahedra.The ordered pyrochlore exhibits cubic symmetry, used to perform optical properties and band gap value using Wood and Tauc's equation of reflectance data.It is one of the most concise structural and optical studies of an ordered frustrated samarium titanate pyrochlore system.

Synthesis method
The ordered polycrystalline samples of Sm2Ti2O7 pyrochlore oxide material were prepared by firing and sintering stoichiometric quantities of Sm2O3 (~99.999%,Sigma Aldrich), and TiO2 (~98%, Sigma Aldrich) under open air for 20 hours at 1200, 1400 and 1475°C in alumina, Al2O3 crucibles.The stepwise synthesis of samarium titanate pyrochlore oxide by standard solid-state reaction synthesis process is detailed in the flow chart, as shown in Figure 2. The resultant product was found to be impure, so subsequent regrinding, pelletization, and sintering stages were needed to get a pure single-phase product.The expected synthesis reaction of rare earth titanate oxide is given as,

Characterizations
The crystallographic phase analysis (crystal structure) of the resultant product was confirmed using a powder x-ray diffraction and high-resolution x-ray diffraction, HRXRD (Smart Lab 3 kW and 9 kW, Rigaku, Japan) with Cu Kα radiation, which was initially standardized by a high-purity Si powder sample.Photoluminescence (PL) emission spectra for the STO powder sample were recorded at CIF, IIT(BHU) on the F-4600 spectrometer, Hitachi, Japan, equipped with a Xenon lamp with a scan rate of 30 nm −1 at room temperature.The PL emission spectra were measured in the wave number range from 300 to 3700 cm -1 .Absorption spectra (at room temperature) were recorded using a UV-visible (doublebeam) spectrophotometer of SHIMADZU-2450, and reflectance data were collected in the 200-1200 nm range.A standard BaSO4 (Barium sulphate) standard sample was initially used as a reference sample.

Powder XRD and HRXRD
Powder XRD and high-resolution x-ray diffraction (HRXRD) pattern of STO polycrystalline powder samples is illustrated in Figure 3 (a) and (b).To eliminate unwanted phase impurities and ensure pure cubic phase evolution, the firing temperature was optimized at different calcination temperatures.After the initial firing, the product sample consisted of a few unwanted reflection indices of very small intensity up to 1400 o C temperature.The calcination process optimization was done by recording successive XRD patterns.Regrinding and sintering pellets under open air yielded only the cubic phase as the resultant product.The determined space group of the final product is Fd3 ̅ m [227] consistent with the Sm2Ti2O7 phase reported by Subramanian e. al. and Kun Yang et al. 16,17 where the most prominent reflection peak indexed in diffraction pattern as [222] with the International Centre for Diffraction Data (ICDD) file No. 73-1699.Moreover, all reflection indices for STO obtained from their high-resolution powder x-ray diffractions agree with the previously reported rare earth titanate pyrochlores.Also, similar cubic structures have been presented before by Geioushy et al. and Surbhi et al. 18,19 for STO prepared by the sol-gel method of synthesis and the related compounds RE2Ti2O7 analogues to samarium titanate pyrochlore extensively reported in hundreds of research articles 3,6,20,21 .

Photoluminescence Spectroscopy
The absorption and emission properties of RE2Ti2O7 have been widely investigated in previous experiments and theoretical studies 12,22 .Figure 4 depicts the emission spectrum at room temperature of a samarium sample that had undergone sintering at 1475°C for 20 hours, and recorded in the range 350-580 nm when excited with 280 nm wavelength.This spectrum was recorded within the wavelength range of 350 to 580 nm when excited with light at 280 nm.The STO pyrochlore sample's PL (emission) spectra consist of numerous emission peaks corresponding to several transitions from the CB (conduction band) edge to the ground state.The higher wavelength emission edges from 449 nm correspond to the charge transfer transition (CTT) within Sm states 23 , as depicted in Figure 4 (a).PL emission phenomenon was further assured by the chromatic response (CIE) chromaticity colour model, where the schematic depicts a 2-dimensional (x, y) CIE chromaticity colour model of STO pyrochlore under an excitation wavelength of 280 nm depicted in Figure 4 (b).The CIE chromaticity (x, y) coordinate comes out to be (0.193, 0.169), which is close to light blue colour emission.This outcome provides additional evidence supporting the idea that the emission spectra are being suppressed in the visible and near-infrared (IR) range due to the arrangement of oxygen vacancies surrounding the B site ion.

Ultraviolet-Visible Spectroscopy
Further, Figure 5 presents the room temperature ultraviolet (UV) -visible absorption spectra recorded for the polycrystalline samarium titanate oxide sample sintered at 1475°C for 20 hours in the 200-700 nm wavelength range.The STO sample's absorption spectrum contains high and low-intensity absorption edges, which agrees with the previous work for rare earth titanates pyrochlore 18,23 .In addition, the absorption edges centred at 302 nm in the STO powder sample is associated with the ligandto-metal charge transfer (LMCT) process.This involves the transition of electrons from the O-2p orbital in the valence band to the Ti-3d orbital in the conduction band.The optical band gap has been determined from UV-visible absorption data employing Wood and Tauc's equation and by extrapolating a graph of (αhν) 2 vs. hν considering the direct transition 19 , as depicted in the inset of Figure 4.The Wood and Tauc's equation is written as (αhν) n = A (hν -Eg) where it consists of α, which is an absorption coefficient, A is a material-dependent constant parameter, hν is an incident photon energy, and here, the value of n decides the nature of the transition (2corresponds to direct).For determining the direct optical band gap, the linear portion of the curve was extrapolated to the x-axis yielding a value of 3.8 eV for the STO sample.It corresponds to the electronic transition of the O -2p to the Ti -3d orbital and it is equated to the absorption edge value at 302 nm, i.e. 1240/300 = 4.1 eV.We can estimate the conduction band (CB) and valence band (VB) edge energies using the following equations, ECB = χ − E C − 1 2 Eg and EVB = ECB + Eg Where χ is the absolute electronegativity of STO, band gap energy Eg, and E C is the energy of the free electrons on the hydrogen scale about (~4.5 eV).The computed position of the conduction band edge (ECB) for STO sample is -1.14 eV, with the valence band edge (EVB) situated at 2.66 eV.

Conclusion
In summary, using the well-established method of synthesis and characterization, we discovered the evolution of crystalline Sm2Ti2O7 pyrochlore and structure upon calcination of initially mixed oxides by a complex of powder XRD and HRXRD.Further, the direct band gap of STO powder sample is estimated using Tauc's plot that is comparable to the theoretically and experimentally reported value.Moreover, the chromaticity colour model CIE diagram reveals emission edge colour coordinates towards the blue part of the spectrum, implying that the ordering of oxygen vacancies has a significant role in tuning the optical characteristics of rare earth titanate oxides.These findings introduce a IOP Publishing doi:10.1088/1757-899X/1300/1/0120426 promising approach to manipulating the absorption and emission properties, particularly in the visible to near-infrared (IR) spectrum, which is important for magneto-optical device fabrication.

Figure 1 .
Figure 1.Schematic Sm2Ti2O7 pyrochlore crystal structure with oxygen crystal field of rare earth atom, Sm at A site and transition metal atom, Ti at B site.

Figure 2 .
Figure 2. Flow chart showing the detailed synthesis of Sm2Ti2O7 pyrochlore by solid-state synthesis method.

Figure 3 .
Figure 3. (a) XRD patterns for the illustration of optimization of sintering temperature of Sm2Ti2O7 pyrochlore at different temperatures for 20 hours.(b) HRXRD pattern of STO pyrochlore sintered at 1475 o C for 20 hours showing pure cubic pyrochlore phase with reflection indices.

Figure 5 .
Figure 5. (a) Room temperature UV-vis spectrum of Sm2Ti2O7 pyrochlore oxide in the wavelength range 200-700 nm.(Inset shows Tauc's plot [(αhν) 2 vs hν] for band gap).(b) An energy band model shows the maxima of VB and the minima of CB in E-k space.