Emergence of dielectric anisotropy of crystalline Strontium Titanate due to temperature-dependent phase transitions

We have undertaken precision measurements of permittivity of single crystal Strontium Titanate from 294.6 K to 5.6 K by measuring the frequency of multiple microwave transverse electric and magnetic resonant modes. Compared with conflicting results reported previously, we unequivocally show that the permittivity has an isotropic value of $316.3\pm2.2$ at room temperature, consistent with its well-known cubic structure. The onset of dielectric anisotropy at cryogenic temperatures was determined and shows that the crystal becomes uniaxial anisotropic below 105 K with a tetragonal structure, and biaxial anisotropic below 51 K with an orthorhombic structure, when it undergoes several phase transitions.