Phase diagrams of incommensurate ferroelectric NH₄HSeO₄: phenomenological treatment

The phenomenological analysis of the pressure-temperature (P −T ) phase diagram of NH₄HSeO₄ crystals is presented. It is shown that the disagreement between the experimental results and the theory may be removed assuming that the coefficient of the Landau free-energy expansion κ at the gradient term (dq/dz)(dq∗/dz) changes the sign in the experimental range of pressures. According to the present model the triple point observed in NH₄HSeO₄ at PK ≈ 455 MPa, TK ≈ 236 K may be considered as artificial points which result from the limitation of experimental resolution. Therefore, even above PK there still exist two very close (unresolved) lines of the incommensurate phase transitions.Представлений феноменологічний аналіз фазової діаграми тисктемпература (P − T ) кристалів NH₄HSeO₄. Показано, що неузгодження між експериментальними результатами і теорією може бути зняте, припускаючи, що коефіцієнт κ при градієнтному члені (dq/dz)(dq∗/dz) в розкладi вільної енергії Ландау змінює знак в області прикладеного тиску. Відповідно до представленої моделі, потрійна точка, спостережена в NH₄HSeO₄ при PK≈ 455 MPa, TK≈ 236 K, може розглядатись як штучна точка, яка є результатом експериментального обмеження. Отже, навіть вище PK ще існують дві дуже близькі лінії неспівмірних фазових переходів

Ferroelectric AgNa(NO2)2 crystals as novel highly efficient nonlinear optical material: Phase matched second harmonic generation driven by a spontaneous and electric field induced polarizations

Paper reports the second harmonic generation(SHG) in ferroelectric AgNa ( NO 2 ) 2 crystals being driven by the spontaneous and electric field induced polarizations. Obtained results are interpreted within the phenomenological theory which considers the free energy describing the interaction between the spontaneous or electric field induced polarizations and spatially inhomogeneous electric polarizations resulted from propagating optical waves. Relatively high magnitudes of the effective second order nonlinear optical (NLO) susceptibilities in these crystals are combined with several phase matching geometries which allows to consider them as high-performance materials for potential NLO applications, such as parametric generation and amplification, frequency doubling, or other applications that require high-efficient frequency conversion. In addition, an anomalously large response of NLO susceptibilities with respect to an applied electric field has been found in the vicinity of the Curie point. This may also have a number of applications, especially in those devices where an efficient tunable control of SHG intensity is demanded

Complete sets of elastic constants and photoelastic coefficients of pure and MgO-doped lithium niobate crystals at room temperature

This paper presents the results of ultrasonic measurements of LiNbO3 and LiNbO3:MgO crystals. The tensors of piezoelectric coefficients, elastic stiffness constants, and elastic compliances are determined for both crystals at room temperature. Combining these data with the results of piezo-optical measurements, a complete set of photoelastic tensor coefficients is also calculated. Doping of LiNbO3 crystals by MgO does not lead to a considerable modification of their elastic and photoelastic properties. However, LiNbO3:MgO is characterized by a considerably higher resistance with respect to powerful light radiation, making it promising for future application in acousto-optic devices that deal with superpowerful laser radiation. Presented here are the complete tensor sets of elastic constants and photoelastic coefficients of LiNbO3 and LiNbO3:MgO crystals that may be used for a geometry optimization of acousto-optical interaction providing the best diffraction efficiency of acousto-optical cells made of these materials

PHASE DIAGRAMS OF INCOMMENSURATE FERROELECTRIC NH_4HSeO_4: PHENOMENOLOGICAL TREATMENT

The phenomenological analysis of the pressure-temperature (P −T ) phase diagram of NH₄HSeO₄ crystals is presented. It is shown that the disagreement between the experimental results and the theory may be removed assuming that the coefficient of the Landau free-energy expansion κ at the gradient term (dq/dz)(dq∗/dz) changes the sign in the experimental range of pressures. According to the present model the triple point observed in NH₄HSeO₄ at PK ≈ 455 MPa, TK ≈ 236 K may be considered as artificial points which result from the limitation of experimental resolution. Therefore, even above PK there still exist two very close (unresolved) lines of the incommensurate phase transitions.Представлений феноменологічний аналіз фазової діаграми тисктемпература (P − T ) кристалів NH₄HSeO₄. Показано, що неузгодження між експериментальними результатами і теорією може бути зняте, припускаючи, що коефіцієнт κ при градієнтному члені (dq/dz)(dq∗/dz) в розкладi вільної енергії Ландау змінює знак в області прикладеного тиску. Відповідно до представленої моделі, потрійна точка, спостережена в NH₄HSeO₄ при PK≈ 455 MPa, TK≈ 236 K, може розглядатись як штучна точка, яка є результатом експериментального обмеження. Отже, навіть вище PK ще існують дві дуже близькі лінії неспівмірних фазових переходів

Aging and memory effects in a clathrate

The out-of-equilibrium low-frequency complex susceptibility of the orientational glass methanol(73%)-β-hydroquinone-clathrate is studied using temperature-stop protocols in aging experiments. Although the material does not have a sharp glass transition aging effects including rejuvenation and memory similar to the effects in spin glasses are found at low temperatures

Lattice dynamics of a monoclinic CsH₂PO₄ crystal

The phonon dispersion relations in a CsH₂PO₄ crystal in a paraelectric phase by means of a rigid molecular-ion model are calculated. The phonon spectra in the quasi-harmonic approximation at various values of temperature and hydrostatic pressure are obtained. The condensation of Au external phonon branch in the centre and at the boundary of the Brillouin zone is obtained.В рамках моделі жорстких іонів обчислюються фононні дисперсійні співвідношення в параелектричній фазі кристала CsH₂PO₄. Фононні спектри отримані в квазігармонічному наближенні для різних значень температури і гідростатичного тиску. Виявлено конденсацію зовнішньої фононної гілки Au в центрі і на границі зони Бріллюена