Structure Determination of Two Polymorphic Phases of La(NO 3) 3· 4H 2O from X-Ray Powder Diffraction

The structure of two polymorphic phases of lanthanum nitrate tetrahydrate, prepared from the thermal decomposition of the hexahydrate compound, have been determined from conventional monochromatic X-ray powder diffraction data. The α phase has a monoclinic unit cell with the parametersa= 6.7778(9) Å,b= 11.3673(1) Å,c= 6.5843(1) Å, β = 90.644(5)°, space groupP2₁/m. The β phase is orthorhombic with the unit cell parametersa= 11.834(1) Å,b= 12.973(1) Å,c= 13.531(1) Å, space groupPbca. For the α phase, atomic coordinates were found by interpretation of one Patterson and three difference Fourier maps, while for the β phase the starting model was the crystal structure of the related cerium compound. The structure models were refined by the Rietveld method. The structures consist of infinite chains along [001], built from 11-fold coordinated and 10-fold coordinated La atoms in the α and β phases, respectively, and linked by bridging nitrate groups. The cohesion of the structures is ensured by hydrogen bonds.