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Journal of Physical Studies 19(3), Article 3703 [9 pages] (2015)
DOI: https://doi.org/10.30970/jps.19.3703 THE INFLUENCE OF HYDROSTATIC PRESSURE ph AND SHEAR STRESS σ5 ON THE PHASE TRANSITIONS AND THERMODYNAMICS OF THE RbHSO4 FERROELECTRICSI. R. Zachek1, R. R. Levitsky2, A. S. Vdovych2
1Lviv Polytechnic National University |
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We use a four-sublattice model of the RbHSO$_{4}$ crystal modified by taking into account the piezoelectric coupling to the strains $\varepsilon_i$, $\varepsilon_j$. Within the mean field approximation we calculate the electric thermodynamic potential of the system. From the conditions of the thermodynamic equilibrium we obtain equations for the strains and the pseudospin mean values, from where we derive the expressions for the components of the polarization vector and static dielectric permittivity tensor of mechanically clamped and free crystals as well as their piezoelectric characteristics.
We study the influence of the hydrostatic pressure and shear stress on the phase transition and physical characteristics of the crystal. The adopted values of the model parameters provide a good quantitative description of the experimental data for $T_c(p)$. We conclude that the transition temperature in RHS is increased by the pressure $p_{h}$ with the slope $\frac{dT_c}{dp}$=120 K/GPa and by the stress $\sigma_{5}$ with the slope $\frac{dT_c}{dp}$= 277 K/GPa.
It is established that the pressure $p_{h}$ and the shear stress $\sigma_{5}$ change the strains $\varepsilon_i$, $\varepsilon_j$; the influence of the shear stress is more pronounced. Both $p_{h}$ and $\sigma_{5}$ alter the character of the phase transition. While in the absence of external stresses the RHS crystals undergo the second order phase transition in the presence of $p_{h}$ and $\sigma_{5}$ the transition is of the first order; with the increasing stresses the degree of the ``first-orderness'' is increased.
Other theoretical results for the dependences of physical characteristics on the hydrostatic pressure $p_{h}$ are in agreement with experimental data as well. With increasing pressure and shear stress the dielectric permittivities $\varepsilon_{33}$ increase in the ferroelectric phase and decrease in the paraelectric phase. The transverse static dielectric permittivities of RHS are decreased by the pressure and shear stress, whereas the permittivity maximum shifts to higher temperatures.
PACS number(s): 77.84.-s, 64.60.Cn, 77.22.-d, 77.80.-e, 77.80.Bh, 77.65.Bn