TlInP2Se6 single crystal: Electronic, optical and vibrational properties

We report on successful synthesis by the Bridgman technique of a large TlInP₂Se₆ single crystal and studies of its electronic, optical and vibrational properties. In particular, centimeter size dimensions of the TlInP₂Se₆ crystal allow its practical use in optical devices. The crystal was characterized by DTA, SEM, EDS, XPS, XES techniques which reveal its high optical quality, single-phase composition and stoichiometry. The present refinements of the crystal structure of TlInP₂Se₆ bring the following unit-cell constants: a = 6.4494(4) Å, b = 7.5423(5) Å, c = 12.1669(9) Å, α = 100.784(4)°, β = 93.622(4)°, γ = 113.331(3)°, and V = 527.8(2) ų. The experimental optical reflection spectrum of the crystal was measured in the far-infrared spectral range 30–500 cm⁻¹ using the synchrotron radiation. Furthermore, we employed first-principles computations within a density functional theory (DFT) formalism to elucidate peculiarities of filling the valence-band and conduction-band regions and the nature of semiconducting gap. The DFT calculations confirm the existing measurements that the TlInP₂Se₆ crystal is an indirect semiconductor (maximum of the valence band and minimum of the conduction band are positioned at high-symmetry F and Z points of the Brillouin zone, respectively), and the principal contributors to the valence band are Se 4p states filling mainly its top and also the bottom of the conduction band. The DFT calculations explain observed highly anisotropic properties (layered structure) by strong differentiation of calculated real and imaginary parts of complex dielectric function in the y direction, with relation to x and z directions for the TlInP₂Se₆ compound. The vibrational properties of the TlInP₂Se₆ crystal were also investigated both experimentally by Raman measurements and theoretically using the DFPT method.