X-RAY TOPOGRAPHIC STUDY OF TWINS IN NDXY(1-X)AL-3(BO3)(4) CRYSTAL

The twins in self-frequency-doubling laser crystal NdxY(1-x)Al-3(BO3)( 4) (NYAB) have been investigated by the transmission synchrotron topog raphy and conventional Lang topography. It has been found that the twi ns in NYAB are similar to Brazil twins in natural quartz, i.e. the twi n element is the mirror plane of type {<11(2)over bar 0>} normal to th e twofold (electric) axis. The twin is also discovered to have the sha pe of lamella and the twin boundaries are {<10(1)over bar 1>) type. It is known that the contrast of this kind of twin lamella originates fr om the phase difference alpha between the X-ray diffracted at the twin boundaries. Using the atomic coordinates of the NYAB crystal and twin ning structures, we have calculated the phase theta(H) and theta(T) fo r the host crystal and twinning, respectively. The calculations show t hat the phase angles theta(H) and theta(T) change with the reflections . When theta(H) and theta(T) are equal, the contrast duet to the twin boundaries is invisible. Otherwise, the contrast of twin lamella can b e observed.