The second order phase transition in Sn_2P_2S_6 crystals: anharmonic oscillator model

Abstract

Statistical theory for ferroelectrics based on triple well anharmonic potential was used for the case of structural second order phase transition in Sn 2 P 2 S 6 crystals. Parameters of effective Hamiltonian of the model were estimated using available experimental data. These findings confirm the assumption that the phase transition in these crystals is located in crossover region between order-disorder and displacive type, and very closely to tricritical point.

Tailoring Negative Thermal Expansion in Ferroelectric Sn2P2S6 by Lone-Pair Cations

“…As shown in Figure a,b, a distinctive Raman peak exhibits rapid and nonlinear softening from 120 K (78.7 cm –1 ) to T C (68.3 cm –1 ). Moreover, the softening presents a similar curve to the decay of the P S and stops abruptly at T C . The line width of this mode also stops increasing at the T C .…”

“…(a) Temperature-dependent Raman shift of modes between 50 and 125 cm –1 in Sn 2 P 2 S 6 from 120 to 400 K; the red star indicates the mode showing distinctive softening below T C . (b) The softening route of the studied mode and the temperature-dependent normalized P S (ref ), the fitting error of Raman shift is about 0.1 cm –1 , smaller than the data icon. The inset shows the line width of the mode.…”

Acoustic attenuation in ferroelectric Sn2P2S6 crystals

“…This transition was described in an anharmonic oscillator model [8,9] with a triple well potential in ground state. The crossover character of the PT and their proximity to the tricritical point were explained by such an approach [10].…”

Optical second harmonic generation and its photoinduced dynamics in ferroelectric semiconductor Sn2P2S6

“…The SPS is an intriguing crystal from the researcher's point of view, a wide-bandgap semiconducting ferroelectric with unique nonlinear optical properties and a wide optical radiation transmission bandwidth in the range from 530 nm to 8 μm. At room temperature, the SPS crystal is a ferroelectric with monoclinic point symmetry group m and undergoes a second-order phase transition into a paraelectric phase with point symmetry group 2/m at a temperature about 337 K [24]. At the same time, this material behaves as a wide-bandgap semiconductor with a band-gap energy of E g = 2.31 eV.…”