Optimized combination of intrinsic and form birefringence in oriented LaPO₄ nanorod assemblies

The birefringence of colloidal nematic suspensions and thin films of aligned LaPO₄ nanorods is quantitatively studied. Single crystalline hexagonal hydrated-LaPO₄ nanorods are synthesized with the c-axis along the rod long dimension. Films are prepared on a glass substrate by blade coating deposition from concentrated colloidal suspensions of rods, allowing for their alignment through the effect of shear stress. The obtained material exhibits a high degree of rod alignment and a resulting high birefringence (Δn-≈ 0.13). This remarkable value is shown to result from the optimized combination of both the intrinsic birefringence originating from the rod crystalline anisotropy and the form birefringence due to the high rod aspect ratio. The birefringence is measured over the spectral range between 250 nm and 2000 nm and is found to be almost constant while the material transmittance is close to unity. In the highly oriented soft state of colloidal rod suspension, the form birefringence, although reduced by a weakened refractive index contrast between the rods and the surrounding medium (solvent), still provides a significant contribution to the overall birefringence which can be exploited in electro-optical devices.