Structural, electronic and optical properties of monoclinic Na{sub 2}Ti{sub 3}O{sub 7} from density functional theory calculations: A comparison with XRD and optical absorption measurements
- Departamento de Física, Universidade Federal do Ceará, Caixa Postal 6030, 60455-760 Fortaleza, CE (Brazil)
- Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte, MG 31270-901 (Brazil)
Powder samples of bulk monoclinic sodium trititanate Na{sub 2}Ti{sub 3}O{sub 7} were prepared carefully by solid state reaction, and its monoclinic P2{sub 1}/m crystal structure and morphology were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM), respectively. Moreover, the sodium trititanate main energy band gap was estimated as E{sub g}=3.51±0.01 eV employing UV–Vis spectroscopy, which is smaller than the measured 3.70 eV energy gap published previously by other authors. Aiming to achieve a better understanding of the experimental data, density functional theory (DFT) computations were performed within the local density and generalized gradient approximations (LDA and GGA, respectively) taking into account dispersion effects through the scheme of Tkatchenko and Scheffler (GGA+TS). Optimal lattice parameters, with deviations relative to measurements Δa=−0.06 Å, Δb=0.02 Å, and Δc=−0.09 Å, were obtained at the GGA level, which was then used to simulate the sodium trititanate electronic and optical properties. Indirect band transitions have led to a theoretical gap energy value of about 3.25 eV. Our results, however, differ from pioneer DFT results with respect to the specific Brillouin zone vectors for which the indirect transition with smallest energy value occurs. Effective masses for electrons and holes were also estimated along a set of directions in reciprocal space. Lastly, our calculations revealed a relatively large degree of optical isotropy for the Na{sub 2}Ti{sub 3}O{sub 7} optical absorption and complex dielectric function. - Graphical abstract: Monoclinic sodium trititanate Na2Ti3O7 was characterized by experiment and dispersion-corrected DFT calculations. An indirect gap of 3.5 eV is predicted, with heavy electrons and anisotropic holes ruling its conductivity. - Highlights: • Monoclinic Na2Ti3O7 was characterized by experiment (XRD, SEM, UV–Vis spectroscopy). • DFT GGA+TS optimized geometry and optoelectronic properties were obtained. • An experimental (theoretical) indirect gap of 3.5 (3.25) eV is predicted. • Heavy electrons and anisotropic holes rule the conductivity. • Ti-O bond lengths and charge states probably cause oxygen reactivity variations.
- OSTI ID:
- 22658289
- Journal Information:
- Journal of Solid State Chemistry, Journal Name: Journal of Solid State Chemistry Vol. 250; ISSN 0022-4596; ISSN JSSCBI
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ABSORPTION
BOND LENGTHS
BRILLOUIN ZONES
COMPUTER CALCULATIONS
DENSITY FUNCTIONAL METHOD
DIELECTRIC MATERIALS
ENERGY GAP
EXPERIMENTAL DATA
LATTICE PARAMETERS
MONOCLINIC LATTICES
OPTICAL PROPERTIES
SCANNING ELECTRON MICROSCOPY
SODIUM COMPOUNDS
TITANATES
X-RAY DIFFRACTION