Novel High-Temperature Polymorphs of MgBr2 and MnBr2 - Limits of Powder Diffraction for Structure Determination

Article abstract of DOI:10.1107/S010876819200497X

High-temperature polymorphs of magnesium bromide and manganese bromide, hexagonal, R<*>m, Z=3.MgBr2, M_r=184.13, Cu Kα₁, λ=1.54051 Angstroem, T=973 K, a=3.9152(7), c=19.420(4) Angstroem, V=257.80(9) Angstroem³.MnBr2, M_r =214.76, neutron radiation, λ=1.594(2) Angstroem, T=773 K, a=3.92240(2), c=19.167(2) Angstroem, V=255.38(3) Angstroem³, Rietveld refinement, 10 <*> 2θ <*> 150 deg, R_wp=0.13.The hitherto unknown high-temperature polymorphs of both MgBr2 and MnBr2 crystallize in the CdCl2 form.The manganese compound is pseudocubic =11.0864(1) Angstroem at 773 K> above 623 K.Distinction between the rhombohedral layered CdCl2 structure and the cubic three-dimensional net λ-MnO2 structure is not possible by powder diffraction experiments alone.Additional observations must be employed.The JCPDS File Nos. for MnBr2 and MgBr2 are 43-1499 and 43-1500 respectively.