Crystal structure analysis of tungsten bronzes β-SrTa ₂O ₆ and β′-SrTa ₂O ₆ by synchrotron X-ray and neutron powder diffraction

Strontium ditantalum oxide SrTa ₂O ₆ exists in α-, β-, and β′-polymorphs. Herein the crystal structures of the latter two were studied using synchrotron X-ray and constant-wavelength neutron powder diffraction. While β′-SrTa ₂O ₆ [space group P4/mbm, a=12.47099(1) Å, c=3.898210(5) Å, V=606.271(2) Å ³, Z=5] belongs to the regular tetragonal tungsten bronze (TTB) family, it contains locally disordered strontium atoms within the pentagonal channel. β-SrTa ₂O ₆ [space group Pnam, a=12.36603(2) Å, b=12.43467(2) Å, c=7.72403(1) Å, V=1187.705(4) Å ³, Z=10] can be described as an orthorhombic modification of the TTB, where the octahedral tilting distortion effectively alleviates the bonding strains around TaO ₆ and SrO ₁₃ polyhedra. For comparison, rynersonite type α-SrTa ₂O ₆ [space group Pnma, a=11.00610(6) Å, b=7.63397(3) Å, c=5.62634(3) Å, V=472.727(5) Å ³, and Z=4] is built from edge-shared dimer units of TaO ₆ octahedra. As measured by diffuse-reflection absorption spectroscopy, α-, β-, and β′-SrTa ₂O ₆ have indirect band gap energies of 4.4, 4.0, and 3.8 eV, respectively.