ABSTRACT
The macroscopic symmetry of Pb(Mg1/3Nb2/3)1-xTixO3 (PMN-PT) crystals with x = 0.25-0.5 has been studied by optical microscopy. Precise data on the temperature dependence of the birefringence and optical extinction directions have been obtained. Two different low-symmetry phases separating rhombohedral and tetragonal phases have been observed in the compositional range x = 0.3-0.47. The optical extinction in the range x = 0.3-0.37 is consistent with Cm space group symmetry, while for x = 0.37-0.47 Pm symmetry is appropriate. Gradual rotation of the optical indicatrix has been found at temperatures just below Tc in crystals with x = 0.27-0.3. A refined phase diagram is presented.
ABSTRACT
In 1947 Kathleen Lonsdale conducted a series of experiments on X-ray diffraction using a divergent beam external to a crystal sample. Unlike the Kossel technique, where divergent X-rays are excited by the presence of fluorescing atoms within the crystal, the use of an external divergent source made it possible to study non-fluorescing crystals. The resulting photographs not only illustrated the complexity of X-ray diffraction from crystals in a truly beautiful way, but also demonstrated unprecedented experimental precision. This long-forgotten work is repeated here using a synchrotron radiation source and, once again, considerable merit is found in Lonsdale's technique. The results of this experiment suggest that, through the use of modern 'third-generation' synchrotron sources, divergent-beam diffraction could soon enjoy a renaissance for high-precision lattice-parameter determination and the study of crystal perfection.
