Structure of the fully ferroelectric/fully ferroelastic orthorhombic room-temperature phase of iron iodine boracite, Fe₃B₇O₁₃I

摘要

FeBOI, M=578.13, orthorhombic, Pca2, a=8.6540(3), b=8.6481(3), c=12.2336(5) A, v=915.57(6) A, Z=4, D=4.19Mg m, A(Mo Ka)=0.7107A, u08.31 mm, F(000)=1080, T=298K, R=0.03 for 1936 reflexions from an optically controlled single-domain crystal. The metal surroundings of the three Fe atoms are analysed in detail; two of three metal sites (Fe2, Fe3) have similar chemical environments and equal displacement parameters. The averaged difference between the two Fe-I distances is compared with those of other non-cubic boracites. It decreases linearly as the ionic radius of the halogen increases, except for boracites containing Cr and Cu atoms with degenerate electronic states (showing Jahn-Teller effect).