Selected Properties of Single Crystals with CGG Structure

3 Selected Properties of Single Crystals with CGG Structure 

Specific Electric Conductivity The specific electric conductivities a of LGS, LGN and LGT were measured using a four-point technique at temperatures between 795 and 1000 K on crystal slices cut perpendicular to the Z-axes. The temperature dependence of the conductivities in dielectric materials with a limited defect density are caused by localized electronic states, and thus the data yield a straight line in a plot of In c versus 1/T, the slope of which is a measure of the activation energy AE of the charge carriers in a Mott-type transition model. These activation energies were found to be 1.1 (LGS), 1.0 (LGN) and 0.9 (LGT) eV, respectively. The rather strong increase in electric conductivity (LGS 10 Q cm at 530°C, 5 X lO- Q- cm- at 730 °C LGT lO- fi- cm at 600 °C, 8 x 10- Q- cm- at 730 °C) appears to limit the high-temperature applications of these materials. 

Indices of Refraction The principal refractive indices (i) =n ) and rtp) (=W(e)) and their dispersions determined between 365 and 2325 nm wavelengths, can be fitted to a four-parametric SeUmeier equation of the form n X) = A + Bj(X -C) —DX (Table 8.7). 

Elastic Coefficients The elastic stiffness coefficients Cy can be calculated from the measured velocity of propagation of bulk acoustic ultrasonic waves, according to the Papadakis method (quartz transducer with center frequency of 20MHz) (Papadakis, 1967), on differently oriented bar-shaped samples using the equations given by Truell et al. (1969) and corrected for the piezoelectric contributions (Ljamov, 1983 Ikeda, 1990). The samples were oriented in axial directions XYZ, and 45° rotated against the X- and Y-axes, respectively. In order to obtain optimized values for the elastic materials parameters, the elastic stiffness coefficients Cy were used to calculate and critically compare the results of surface acoustic wave (SAW) measurements. 

The elastic stiffness components are considerably smaller for SNGS compared to LGS, LGN and LGT. In particular, the very small tensor component for SNGS has been confirmed by several independent measurements. Although the reason for this is not yet clear, it may concern the ordered arrangement of the atoms in the quaternary SNGS structure, as opposed to the statistical structure of the ternary compounds LGS, LGN, and LGT. 

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