Structure and luminescence of the α-LnNb3O9-type rare earth niobates

α-LnNb₃O₉ (Ln = La, Pr, Nd) compounds have been prepared hydrothermally from acidic solutions. In comparison to the previously reported orthorhombic β modifications, α-LnNb₃O₉ compounds are monoclinic. The structure of α-PrNb₃O₉ was determined with a = 5.3784(6), b = 7.602(2), c = 16.344(2) Å, and β = 92.21(1)°, space group $\text{P2}{}_1\text{c}$. It is built of double and single chains of corner-shared NbO₆ octahedra extended along the b axis. Praseodymium atoms reside in tunnels along the b axis and are in eight-coordination with oxygen. All α-LnNb₃O₉ compounds can be irreversibly converted to the β modification by heating in air to 1200°C. The X-ray excited luminescence of Sm-, Eu-, Tb-, and Dy-doped α-LaNb₃O₉ is also reported.