Nanoscale physics

Structure and vibrational study of the Ag8SnSe6 argyrodite
Semkiv I.V.1, Ilchuk H.A.1, Petrus R.Yu.1, Kashuba A.I.1
1 Lviv Polytechnic National University, 12, S. Bandera Str., Lviv-79013, Ukraine. 
E-mail: Semkiv.Igor.5@gmail.com

The aim of this study was to provide further insight into the structure of the β phase of Ag8SnSe6 argyrodite by X-ray diffraction (XRD), Raman spectroscopy and density functional theory calculations.
X-ray powder diffraction measurements were conducting with a DRON-3M powder diffractometer with monochromatized Cu Kα radiation. The crystal structure was determined using the FullProf Suite (3.0) program. Raman studies were performed using Renisaw InVia 3000 Spectrometer. In measurements 514 nm wavelength of Ar+ laser was applied for excitation. Density functional theory calculations of the Raman scattering for Ag8SnSe6 of the argyrodite structural modification were performed within the generalized gradient approximation using the Perdew–Burke–Ernzerhof local functional.
According to the X-ray phase analysis the synthesized argyrodite is practically a single-phase sample of the Ag8SnSe6 compound, β-modification, space group Pmn21, stable at normal conditions. On the XRD pattern there are no peaks related to SnSe2, Ag2Se binary and Ag, Sn, Se elemental phases. The calculated lattice parameters are next: a = 7.91299(17), b = 7.81808(18), c = 11.0422(3) Å, V = 683.12(3) Å3.
Zone-center phonon modes of argyrodite β-Ag8SnSe6 with orthorhombic structure and Pmn21 (C2υ) space group on the basis of the factor group analysis are classified into 26A1 + 26B1 + 19A2 + 19B2, where 3 modes (A1 + B1 + B2) are acoustic and 87 modes (25A1 + 25B1 + 19A2 + 18B2) are optical one. All of the 90 modes are active in Raman scattering spectra. In infrared spectra all modes except the A2 are active. 
Experimental Ag8SnSe6 Raman spectra curve exhibit one intensive peak at 214.3 cm–1 and broad peak with less intensity in the 100–200 cm–1 region which can be a combination of several modes. Observed peaks correspond to A1 and B1 and B2 modes. These modes can correspond to modulation of SnSe4 tetrahedra including Sn–Se stretching and bending, demonstrate highest intensity caused by high structure unit polarizability. Other low intensity modes can be attributed to the Ag–Se vibration modes.