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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

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