文摘
Strontium ditantalum oxide SrTa2O6 exists in ¦Á-, ¦Â-, and ¦Â?polymorphs. Herein the crystal structures of the latter two were studied using synchrotron X-ray and constant-wavelength neutron powder diffraction. While ¦Â?SrTa2O6 [space group P4/mbm, a=12.47099(1) ?, c=3.898210(5) ?, V=606.271(2) ?3, Z=5] belongs to the regular tetragonal tungsten bronze (TTB) family, it contains locally disordered strontium atoms within the pentagonal channel. ¦Â-SrTa2O6 [space group Pnam, a=12.36603(2) ?, b=12.43467(2) ?, c=7.72403(1) ?, V=1187.705(4) ?3, Z=10] can be described as an orthorhombic modification of the TTB, where the octahedral tilting distortion effectively alleviates the bonding strains around TaO6 and SrO13 polyhedra. For comparison, rynersonite type ¦Á-SrTa2O6 [space group Pnma, a=11.00610(6) ?, b=7.63397(3) ?, c=5.62634(3) ?, V=472.727(5) ?3, and Z=4] is built from edge-shared dimer units of TaO6 octahedra. As measured by diffuse-reflection absorption spectroscopy, ¦Á-, ¦Â-, and ¦Â?SrTa2O6 have indirect band gap energies of 4.4, 4.0, and 3.8 eV, respectively.
