A. K. Bain

Determination of the photoelastic coefficients in lithium heptagermanate crystals

Abstract The photoelastic (piezo-optic) property of the ferroelectric crystals lithium heptagermanate Li2Ge7O15 (LGO) were studied in para- and ferro-electric phases for different orientations. The value of stress optical coefficient Cijk (in Brewsters) along a, b, c-axes were determined at RT (291 K) and 278 K and were compared with some other ferroelectric systems like Rochelle-salt (RS), potassium dihydrogen phosphate (KDP) and ammonium dihydrogen phosphate (ADP). Large photoelastic coefficients and other properties like good mechanical strength, or transition temperature close to RT and stability in ambient environment favour LGO as a potential candidate for photoelastic applications.

Anomalous Temperature Dependence of Piezo-Optic Birefrigence in Li2Ge7O15 Crystals

Peaks are observed in the temperature dependences of the photoelastic coefficients C zx and C zy at temperatures ∼ 279 K. These are attributed to the occurrence of paraelectric to ferroelectric phase transition and the spontaneous polarization along c-axis of the crystals Li 2 Ge 7 O 15 . A thermal hysteresis is observed in a cooling and a heating cycle besides a valley around 289 K. The results are interpreted in terms of the soft mode and relaxational mode related to the phase transition. The phase transition temperature appears to shift linearly downwards with the increase in the uniaxial stress along the crystallographic c-axis.

Piezooptic Dispersion of Li2Ge7015 Crystals

The piezooptic dispersion of the ferroelectric crystal Li 2 Ge 7 0 15 (LGO) was studied in the visible region of the spectrum at room temperatures between 278 K and 298 K. The results show an interesting photoelastic behavior. The piezooptic coefficient Cλ decreases with wavelength in the visible region up to a certain wavelength and then it shows a peak at sodium yellow wavelength and later on the usual reduction is observed. Thus there seems an optical zone in between 5400 Å and 6200 Å with an enhanced piezooptical behavior. This enhancement is observed to be very high, as high as 50%, when the crystal is stressesd along [001] and observation made along [100]. The height of this region is different for different orientations, though the width seems approximately the same.

Irradiation Effect on Piezooptic Dispersion of Li2Ge7O15 Crystals

The ferroelectric single crystals Li 2 Ge 7 O 15 (LGO) was irradiated by X-ray for one hour. The piezooptic dispersion of the ferroelectric crystal Li 2 Ge 7 O 15 (both un-irradiated and irradiated) were studied in the visible region of the spectrum at room temperature (298 K). The results show an interesting photoelastic behavior. The piezooptic coefficient Cλ decreases with wavelength in the visible region up to a certain wavelength and then it shows a peak at sodium yellow wavelength and later on the usual reduction is observed. Thus there seems an optical zone in between 5400 Å and 6200 Å with an enhanced piezooptical behavior. This enhancement is observed to be very high, as high as 50%, when the crystal (un-irradiated) is stressesd along [001] and observation made along [100]. The height of this region is different for different orientations, though the width seems approximately the same. The peak value increases about 25% when the crystal (irradiated) is stressed along [001] and observation made along [010] and that value decreases about 18% when the crystal (irradiated) is stressed along [001] and observation made along [100]. The height of the peak of this region remains almost same for all other orientation of the crystal (irradiated).

Optical Properties of Ferroelectrics and Measurement Procedures

It is well known that the optical properties of ferroelectric materials find wide ranging appli‐ cations in laser devices. Particularly in the recent years, there has been tremendous interest in the investigation of the nonlinear optical properties of ferroelectric thin films [1-5] for pla‐ nar waveguide and integrated –optic devices. A new class of thin film waveguides has been developed using BaTiO3 thin films deposited on MgO substrates [6]. Barium strontium tita‐ nate Ba1-xSrxTiO3 (BST) is one of the most interesting thin film ferroelectric materials due to its high dielectric constant, composition dependent Curie temperature and high optical non‐ linearity. The composition dependent Tc enables a maximum infrared response to be ob‐ tained at room temperature. The BST thin films in the paraelectric phase, have characteristics such as good chemical and thermal stability and good insulating properties, due to this nature they are often considered the most suitable capacitor dielectrics for suc‐ cessful fabrication of high density Giga bit (Gbit) scale dynamic random access memories (DRAMs). Compositionally graded ferroelectric films have exhibited properties not previ‐ ously observed in conventional ferroelectric materials. The most notable property of the graded ferroelectric devices or graded Functionally Devices (GFDs) is the large DC polariza‐ tion offset they develop when driven by an alternating electric field. Such GFDs can find ap‐ plications as tunable multilayer capacitors, waveguide phase shifters and filters [7]. Recently, BST thin films were used in the formation of graded ferroelectric devices by de‐ positing successive layers of BST with different Ba/Sr ratios [8].

Piezo-optic and Dielectric Behavior of the Ferroelectric Lithium Heptagermanate Crystals

It is well known that piezo-optic and electro-optic effects in crystals find wide ranging applications in laser devices. The photoelastic behavior of crystals forms a necessary prelude to study the electro-optical effect of ferroelectric crystals. Lithium heptagermanate Li2Ge7O15 (LGO) is regarded as a weak ferroelectric and its curie point Tc is 283.5K (Wada et al., 1981, 1983). Due to its intermediate behaviour between order-disorder and displacive types in a conventional grouping of ferroelectric materials LGO remains a subject of interest from both the theoretical and the application point of view. The paraelectric phase above Tc is orthorhombic 14 2h D ~ pbcn and below Tc the ferroelectric phase is 5 2v C ~ pbc21 with four formula units in a unit cell in both the phases. Below Tc LGO shows dielectric hysteresis loop and the permittivity shows a sharp peak at Tc (Preu, 1982; Wada et al., 1981, 1983). The Raman scattering spectrum shows a shoft mode whose frequency tends to zero as Tc is approached from below (Wada & Ishibashi, 1983). Below Tc the spontaneous polarization appears along the c-axis. The nature of the second order phase transition is not simple because according to Raman spectra the transition is suggested to be a displacive phase transition. But the temperature dependence of the permittivity ┝ is indicative of the order disorder character of the phase transition (Preu, 1982; Wada et al., 1981, 1983) and does not agree with the behaviour expected of a displacive phase transition. Many interesting physical properties of LGO such as birefringence (Kaminsky & HaussUhl, 1990), elastic (HaussUhl et al., 1980), thermal expansion (Wada & Ishibashi, 1983), dielectric susceptibility (Preu, 1982; Kudzin, 1994a, 1995b), electron paramagnetic resonance (EPR) of doped ions Mn2+ and Cr+3 (Trubitsyn et al., 1992; Bain, 1994) and photoluminescence (Bain, 1994) exhibit strong anomalies around Tc. However, the optical properties vary only to such a small degree that the transition could not be detected with the aid of a standard polarization microscope (Kaminsky & HaussUhl, 1990). Interestingly with the help of a high resolution polarization device, Kaminsky and HaussUhl (Kaminsky & HaussUhl, 1990) studied the birefringence in LGO near Tc and observed anomalies at the phase transition. The study of piezo-optic dispersion of LGO (un-irradiated and irradiated) in the visible region of the spectrum of light at room temperature (RT=298 K) shows an optical zone/window in between 5400A and 6200A with an enhanced piezo-optical behavior (Bain et al., 2008). The temperature dependence of the photoelastic coefficients of the ferroelectric

Study of Impedance in Ferroelectric Li2Ge7O15 Crystals

The a.c. electrical impedance (Z) was studied along the c-axis in ferroelectric Li2Ge7O15 [LGO] single crystals in 100 kHz–10,000 kHz frequency range in the temperature interval from 298 K to 273 K during cooling and heating process including Tc = 283.5 K. A temperature hysteresis of impedance is observed in a cooling and heating cycle at Tc = 283.5 K. This is attributed to the occurrence of paraelectric [PE] to ferroelectric [FE] phase transition and the spontaneous polarization along c-axis of the Li2Ge7O15 crystals. The results are interpreted in terms of generation of space charges inside the FE Li2Ge7O15 crystals during the heating process. The value of impedance decreases sharply with increasing frequency and tends to zero value at about the frequency of 10,000 kHz. So, in the application point of view, LGO is suitable for conductivity even at room temperature and frequency controlled switch.

Irradiation Effect on Photoelastic Coefficients in Ferroelectric Li2Ge7O15 Crystals

The Ferroelectric single crystals Li2Ge7O15 (LGO) was irradiated by X-ray for one hour. The photoelastic coefficients of the ferroelectric crystals Li2Ge7O15 (both un-irradiated and x-irradiated) were studied in a cooling and a heating cycle between room temperature and 273 K. The results show an interesting photoelastic behaviour. Peaks are observed in the temperature dependences of the photoelastic coefficients Czx and Czy at temperature ∼279 K. These are attributed to the occurrence of paraelectric to ferroelectric phase transition and the spontaneous polarization along c-axis of the Li2Ge7O15 crystals. Some interesting results are obtained in the case of X-irradiated LGO crystal. The peak value of Czy increases about 20% and the value of Czx decreases about 18% at the wave length λ = 5890 Å during cooling process of the crystal. The X-irradiation may produce internal stress and electric fields inside the Li2Ge7O15 crystals due to defects that can change the values of photoelastic coefficients.