Formation and Control of Twin Domains in the Pyrochlore Oxide Cd₂Re₂O₇

Abstract

The successive phase transitions of the pyrochlore oxide Cd 2 Re 2 O 7 are studied by polarizing microscopy and magnetic susceptibility measurements. The formation of twin domains is visualized in the polarizing images of a pristine (111) crystal surface upon cooling through the cubic-to-tetragonal transition at T s1 ~ 200 K. Moreover, a dramatic change in the twinning pattern is observed at T s2 ~120 K, which suggests that the tetragonal c axis flips as the strain changes its direction at the tetragonal-to-tetragonal transition. Magnetic susceptibility measurements reveal significant domain alignment upon cooling across T s1 and T s2 in a magnetic field of 7 T, which are due to ~10% anisotropy in the magnetic susceptibility for the lowtemperature phases. Interestingly, the anisotropy is reversed at T s2 : χ c < χ a above T s2 and vice versa below T s2 .

Domain Control by Adjusting Anisotropic Stress in Pyrochlore Oxide Cd2Re2O7

“…In addition, POM observations on a natural (0 0 1) surface can give more straightforward information on the formation of twin domains than those on a (111) surface; previous POM observations gave no domain contrast on a polished crystal surface. 28) We were successful in preparing truncated-octahedral crystals with (0 0 1) facets, as shown in Fig. 1(a) in two steps via a modified synthesis route: first, octahedral crystals were grown by a chemical vapor transport (CVT) method starting from a mixture of CdO:ReO3:Re = 1.44:1:0.2 at a temperature gradient of 1000-1100 K for 100 h, and second, recrystallization was carried out by a CVT method at a temperature gradient of 900-960 K for 300 h. Occasionally, we obtained large crystals with a 1 mm edge of the (0 0 1) surface.…”

“…The observations were carried out using an optical microscope (Olympus, BXFM) in the reflection mode. 28) Images were recorded using a monochrome CCD camera (ST-402ME, SBIG). A pristine (0 0 1) crystal surface was observed in a crossed-Nicols setup.…”

“…4,5) Moreover, anisotropies in physical properties, which must be dominated by spin-split FSs, are to be examined. Nevertheless, such experiments have been difficult thus far because of complexity generated by the formation of tetragonal domains at the cubic-to-tetragonal transition, which was actually observed by polarizing optical microscopy (POM); 26,28) any anisotropy may be averaged over. In previous de Haas- van Alphen experiments, the FSs of phase III were deduced by taking into account the contributions from all domains.…”

“…Experimentally, such flipping was suggested from previous POM observations of the (1 1 1) surface showing a marked change in domain pattern at Ts2. 28) The tetragonality and its possible flip at Ts2 will be unequivocally determined by examining the responses of domains to anisotropic stresses.…”

Crystal structure of the inversion-breaking metal Cd2Re2O7

“…From this, it is clear that single crystal studies of the structure are needed to determine what its actual space group is, and also the displacements of the various ions from their cubic positions in order to ascertain their potential impact on the nature of the SHG signal (x-rays being more sensitive to the heavier Cd and Re ions, and neutrons requiring isotropically enriched Cd to mitigate absorption). This could be aided by forcing the sample into a single domain by application of a magnetic field, given the anisotropy seen in the magnetic susceptibility below T s1 [19].…”