Polarized Optical Study of Co-containing Double Layered Hydroxides

We report on the optical polarized microscopy study of semitransparent slabs pressed from the flakes-like nanoparticles of the Co( n)Al ( (n=2,3,4 - the Co/Al cations ratio) layered double hydroxides (LDHs) intercalated with carbonate anions. In these compounds, Co and Al cations occupy trigonally distorted octahedral positions in the way that the average space symmetry is R3m regardless of the relative Co content. The compacted nanoflakes of the Co(2)Al-CO
₃
and Co(3)Al-CO
₃
LDHs have been observed to compose optically anisotropic crystallite blocks which are well seen in polarized microscope as conoscopic figures, whereas no conoscopic figures have been detected in slabs of the Co(4)Al-CO
₃
LDH. We associate the observed difference in optical behaviour of the pressed slabs with difference in the diameter-to-thickness ratio of their nanoflakes. As-grown flakes of Co( n)Al LDH with n=2 have a ratio of the size along the trigonal axis (diameter) to the size in perpendicular direction (thickness) to be about 10:1 that favours their preferred orientation under uniaxial pressure, whereas for those with n=4 the ratio is an order of one. Using a quartz colour tint plate, the birefringence in the LDHs with n=2 and n=3 has been determined to be negative.