Thermal expansion of Sn2P2S6 crystals

We present the results for thermal expansion coefficients of Sn₂P₂S₆ crystals determined both in the crystallographic system and the system based on eigenvectors of thermal expansion tensor. Peculiarities of temperature evolution of the indicative surface of thermal expansion tensor for Sn₂P₂S₆ are discussed, including the region of their ferroelectric phase transition.     

New phase-transformation line and intermediate phase in Sn2P2S6

A study is made of the effect of temperature and hydrostatic pressure on the permittivity, forbidden-band width, electrical conductivity, and photoconductivity of crystals of Sn₂P₂S₆. The empirical results obtained indicate that a new phase-transformation line exists on the p-T diagram of Sn₂P₂S₆ p > 0.17 GPa. This phase transformation is evidently a second-order transition, separating a paraelectric, proportionate phase from an intermediate, possibly nonproportionate phase. The results also indicate similarity of the p-T diagram of Sn₂P₂S₆ to the x-T diagrams of ferroelectric solid solutions of Sn₂P₂(Se_xS_1–x)₆.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 57–60, September, 1985.     

Critical behaviour of Sn2P2S6 and Sn2P2(Se0.28S0.72)6 crystals under high hydrostatic pressures

Basing on the temperature dependences of optical birefringence for Sn₂P₂S₆ and Sn₂P₂(Se_0.28S_0.72)₆ crystals subjected to hydrostatic pressures, we prove unambiguously that Sn₂P₂S₆ reveals a tricritical point on its (p, T)-phase diagram with the coordinates (p, T) = (4.3 kbar, 259 K), so that the second-order phase transition transforms into the first-order one whenever the pressure increases above 4.3 kbar. We also find that increasing hydrostatic pressure applied to Sn₂P₂(Se_0.28S_0.72)₆ leads to the change in the phase transition character from tricritical to first order. Further increase in the pressure up to ~2.5 kbar imposes splitting of the first-order paraelectric-to-ferroelectric phase transition into two phase transitions, a second-order paraelectric-to-incommensurate one and a first-order incommensurate-to-ferroelectric transition.     

Baric dependence of the fundamental absorption edge in the region of phase transitions of Sn2P2S6-type intrinsic ferroelectrics

The energy position of the fundamental absorption edge in the neighborhood of phase transitions in Sn₂P₂S₆ and (Pb_0.1Sn_0.9)₂P₂S₆ ferroelectric crystals is studied as a function of the temperature and baric dependence on the basis of direct measurements of the isoabsorption relations. Thep, T phase diagram of Sn₂P₂S₆ crystals is constructed. the temperature-dependent variation of the band gap in those crystals is due mainly to electron-phonon interaction. Uzhgorod State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 82–85, August, 1997.     

Mutually pumped phase conjugate mirror using cross polarized photorefractive coupling

This paper presents a new proposal for a mutually pumped phase conjugate mirror (MPPCM) using cross polarization. In the cross polarized MPPCM, two light beams, whose polarizations are orthogonal to each other, incident on a photorefractive crystal, causing photorefractive interaction to generate phase conjugate waves with an orthogonal polarization to an incident beam. The use of Ar⁺ laser and BaTiO₃ crystal in an experiment for the cross polarized MPPCM found that diffraction efficiency for an extraordinary beam is about 20%. As opposed to a conventional parallel polarized MPPCM in which the use of a beam splitter is required to separate a diffraction beam, the cross polarized MPPCM can efficiently extract phase conjugate waves by means of a polarized beam splitter, demonstrating the advantage of substantially improved efficiency.     

Study on Optimum Incident Angles of Cross Polarized Four-Wave Mixing with a Photorefractive Crystal

We consider the optimum incident angles of cross polarized four-wave mixing with a photorefractive crystal. First, the phase matching angles are obtained for two operation modes: one in which the writing beams are ordinary rays and the reading beam is an extraordinary ray, we call this Mode 1. The other in which the writing beams are extraordinary rays and the reading beam is an ordinary ray, Mode 2. Next, practicable ranges of the incident angles are calculated considering of the total internal reflection of the phase conjugate beam in cases where the two writing beams illuminate the same surface of the BaTiO₃ crystal and where they illuminate the neighboring right-angled surfaces, respectively. Then, the coupling coefficient is determined as a function of the incident angle, and the optimum incident angles for effective beam interaction in Mode 1 and Mode 2 are estimated.     

Splitting of the phase transition in ferroelectric solid solutions Sn2P2(SexS1−x)6 at high pressures

The influence of the temperature and hydrostatic pressure on the forbidden band width and dielectric permittivity of the ferroelectric solid solutions Sn₂P₂(Se_xS_1–x)₆ (x=0.04, 0.20, 0.30) is investigated. A change in the species and splitting of ferroelectric phase transition lines are detected for p = 0.140 and 0.026 GPa, respectively, for the solid solutions Sn₂P₂(Se_0.04S_0.96)₆ and Sn₂P₂(Se_0.20S_{0.80 6}, which is due to the existence of a Lifshits critical point separating the transitions into codimensional and noncodimensional phases on the p, T diagram. Shift coefficients with the pressure of the phase transition temperature are found for the solid solutions investigated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 28–32, February, 1988.     

Wave mixing in nominally undoped Sn<Subscript>2</Subscript>P<Subscript>2</Subscript>S<Subscript>6</Subscript> at high light intensities

The intensity dependence of the photorefractive response of Sn₂P₂S₆ is studied for the Kr⁺-laser wavelength of 647 nm and pump-beam intensities of up to 10 W/cm². A considerable enhancement of the two-beam coupling gain factor with increasing intensity at a grating spacing of ≃1 μm is attributed to a light-induced increase of the effective trap density. The large gain reached at high intensities is applied for the build up of a double phase conjugate mirror with a sub-millisecond switch-on time.     

Acoustic attenuation in ferroelectric Sn<Subscript>2</Subscript>P<Subscript>2</Subscript>S<Subscript>6</Subscript> crystals

The temperature and frequency dependencies of sound attenuation for the proper uniaxial ferroelectric Sn₂P₂S₆, which has a strong nonlinear interaction of the polar soft optic and fully symmetrical optic modes that is related to the triple well potential, were studied by Brillouin spectroscopy. It was found that the sound velocity anomaly is described in the Landau-Khalatnikov approximation with one relaxation time. For explanation of the observed temperature and frequency dependencies of the sound attenuation in the ferroelectrric phase, the accounting of several relaxation times is needed and, for quantitative calculations, the mode Gruneisen coefficients are more appropriate as interacting parameters than are the electrostrictive coefficients. Relaxational sound attenuation by domain walls also appears in the ferroelectric phase of Sn₂P₂S₆ crystals.     

Transient short-circuit photocurrent in ferroelectric semiconductor Sn2P2S6 films

Transient short-circuit photocurrents in films of the ferroelectric semiconductor Sn₂P₂S₆ are studied in a temperature range that includes the phase transition point. The influence of an external electric field and white-light illumination on the photoelectric response of samples is discussed.     

On the tricritical point on the (p,T)-phase diagram of Sn2P2S6 crystals

Using studies of the temperature dependence of the acoustic wave velocity at high hydrostatic pressures, a tricritical point where second-order phase transitions change to first-order ones was experimentally revealed on the pressure–temperature phase diagram of Sn₂P₂S₆ crystals. The behavior of the acoustic wave velocity is explained in the framework of mean-field theory.     

Acoustic wave velocities and elastic properties of Sn2P2(Se0.28S0.72)6 solid solutions

An experimental study and analysis is presented of the acoustic properties of Sn₂P₂(Se_0.28S_0.72)₆ ferroelectric crystals. Complete matrices of elastic stiffness and compliance as well as characteristic acoustic slowness surfaces have been obtained. The directions of propagation and polarisation of the slowest acoustic waves were determined. It was found that the elastic properties and the acoustic wave velocities for Sn₂P₂(Se_0.28S_0.72)₆ and Sn₂P₂S₆ crystals are almost the same at room temperature.     

Temperature behavior of the photovoltaic and pyroelectric responses of Sn2P2S6 semiconductor ferroelectric films

The electric response of Sn₂P₂S₆ semiconductor ferroelectric films to focused laser radiation (λ = 6328 Å) with a modulation frequency of 24 Hz is studied experimentally. It is shown that the signal involves an initial spike of current (an unsteady component) followed by the relaxation decrease to a certain value (a quasisteady component). The most significant changes in the shape and amplitude of the film response is observed near the Sn₂P₂S₆ monocrystal phase transition point of T _C = 66°C. The behavior of the unsteady component of the response is related to the behavior of the spontaneous polarization. This component decreases with increasing temperature. The quasi-steady component reaches its maximum at T = C _C. It is revealed that the behavior of relaxation processes corresponding to the decreasing response undergoes a change near the phase transition temperature.     

Dielectric properties of Sn2P2S6 crystals as a function of their growth conditions

The behavior of the permittivity near a phase transition in Sn₂P₂S₆ crystals of different technological quality is studied. It is established that, in high-resistance crystals, where an internal electric field is formed by the screening of spontaneous polarization in the polar phase, long-time relaxation of ɛ is observed in a temperature range ∼2 K above T _max. This relaxation and change in the form of the maximum of ɛ′(T) at a phase transition are attributed to an internal electric field induced by the volume space charge formed in regions near the surface. It is established that the existing differences in the properties of Sn₂P₂S₆ crystals are due to deviations from stoichiometry, arising during growth and synthesis of the crystals. Fiz. Tverd. Tela (St. Petersburg) 41, 1456–1461 (August 1999)     

Thermal expansion of the Sn2P2(Se0.28S0.72)6 solid solutions

In the present work, we have experimentally studied and analysed the dilatometric properties of Sn₂P₂(Se_0.28S_0.72)₆ crystals during ferroelectric phase transition. A complete matrix of thermal expansion tensors as well as the characteristic surface of the thermal expansion tensor has been obtained. The critical exponent for the volume thermal expansion coefficient in Sn₂P₂(Se_0.28S_0.72)₆ crystals has been determined to be equal to 0.50 ± 0.06, which is peculiar to tricritical behaviour. Non-agreement in the values of the critical exponents evaluated from the temperature dependencies of relative elongation (0.17 ± 0.01) and volume thermal expansion coefficient (0.50 ± 0.06) is shown to be due to the possible temperature rotation of the spontaneous polarisation vector.     

Optical second harmonic generation and its photoinduced dynamics in ferroelectric semiconductor Sn<Subscript>2</Subscript>P<Subscript>2</Subscript>S<Subscript>6</Subscript>

By means of optical pump–probe technique, the ultrafast dynamics of nonlinear optical response of the ferroelectric semiconductor Sn₂P₂S₆ crystal excited with a femtosecond laser pulse has been investigated. It has been shown that, under the action of femtosecond pulses, change in optical second harmonic generation occurs in the sample, which can be due to screening of existing electric polarization.     

Electronic and spatial structure of nanoclusters of Sn2P2S6 ferroelectric crystals

The results obtained from ab initio calculations of the optimized configuration and electronic structure of the cluster models of Sn₂P₂S₆ ferroelectric crystals are presented. The calculations have been performed using the spin-restricted Hartree-Fock method and the density functional theory in the DZVP basis set. It has been shown that the clusters are stable and retain the topology of the simulated crystal. The influence of the cluster sizes on the properties under investigation has been analyzed.     

Concept of Photorefractive Connection Module by Photorefractive Four-Wave Mixing with Extraordinary Writing Beams and an Ordinary Reading Beam

We propose a new optical network device photorefractive connection module (PRCM) which operates as optical switch, amplifier and signal distributor controlled by parallel optical signals. Simple optical control bus systems can be realized by cascade connection of PRCMs. PRCM branches off a desired channel from the spatial multiplexed optical bus line by appropriate setting of the control beam pattern. PRCM uses cross polarized four wave mixing (CPFWM) with extraordinary polarized writing beams and an ordinary polarized reading beam to achieve a high connection gain to the next PRCM stage. We analyze the phase matching angle of CPFWM in which the optical paths of two pump beams are slightly different. The phase conjugate reflectivity indicating a branching ratio of optical signal is derived and calculated in consideration of the phase mismatching Δk. The optimum pump ratio and the grating vector orientation for the largest phase conjugate reflectivity and signal amplification factor are discussed for optical design of PRCM. Since the measured signal beam power after passing through the BaTiO₃ crystal is three or four times higher than its incident power, PRCM has a sufficient connection gain for optical bus and interconnection systems.     

Electric-field enhancement of beam coupling in Sn2P2S6

The influence of an external field on photorefractive recording in Sn₂P₂S₆ (SPS) crystals is studied. A large gain factor of more then 15 cm^-1 is achieved for a grating spacing of 12 μm at λ=0.9 μm. For an applied field exceeding ±200 V/cm a switching of the beam coupling direction is detected, exhibiting a pronounced hysteresis. Received: 25 October 2000 / Revised version: 18 January 2001 / Published online: 21 March 2001     

Microwave study of the soft ferroelectric mode in Sn2P2S6 crystals

This paper presents the results of the investigation of dielectric dispersion in semiconductive ferroelectric Sn₂P₂S₆ crystals over the frequency range 1 kHz to 78.5 GHz. The main dielectric dispersion in Sn₂P₂S₆ is caused by the soft ferroelectric B_u mode and in the vicinity of the Curie point it occurs in the millimeter region. The frequency of the soft mode in the paraelectric phase varies according to v_s = 35 (T-T_c )^½ GHz on approaching the Curie point. The soft mode is strongly overdamped. Close to T_c the relative damping is y/v_s = 14. The dielectric contribution of the soft mode is equal to the static dielectric permittivity and it explains its whole temperature-dependence.