Crystals of Fe3R(BO3)4 (R = Y, La, Nd) have been grown. Their structure and properties show close relations. The structures of these materials have been established by single-crystal X-ray diffraction. They crystallize in the huntite, CaMg3(CO3)4, structure type, trigonal system, space group R32 (No. 155), Z = 3, with unit-cell parameters (A) a = 9.592(1), 9.578(1), and 9.563(2) and c = 7.597(7), 7.605(3), and 7.575(2), for R = La, Nd, and Y0.5Bi0.5, respectively. They contain RO6 trigonal prisms and smaller FeO6 octahedra forming layers and giving rise to Fe−R distances of ≃3.78 A. RO6 polyhedra are interconnected within the layers by corner sharing with triangular BO3 groups of two types and FeO6 octahedra. FeO6 octahedra form 1D helicoidal chains with relatively short Fe−Fe distances, ≃3.18 A. The temperature dependence of the magnetic susceptibility from 350 to 1.8 K for Fe3R(BO3)4 shows two maxima at about 260 and 35 K and, for R = Nd, another maximum at 6 K. The presence of magnetic order is attribu...

Crystal Structure, Magnetic Order, and Vibrational Behavior in Iron Rare-Earth Borates

It is shown that one of the main reasons for most failures of the methods for calculating distance-dependent bond strengths is related to the distortion of the coordination polyhedra. The charge distribution (CD) method which depends on only one universal empirical parameter (contraction parameter) is modified to include: (i) an iterative calculation of the effective coordination number (ECoN), to deal with structures containing very distorted coordination polyhedra; (ii) a specific contraction parameter to treat structures containing any type of hydrogen bond; (iii) scale factors for coordination subshells, to treat structures with hetero-ligand polyhedra. The contraction parameter for the hydrogen bonds was obtained from 119 well refined structures based on neutron diffraction data. Examples of the application of the iterative charge distribution (CD-IT) are presented to show the efficiency of the new method in dealing with distorted (including hydrogen bonding) and hetero-ligand polyhedra. In particular, analysis of a series of 74 structures with pentacoordinated cations shows that deviations from overall trends are related to structure instability. The possible failure of the method with polyionic structures and `dynamic' structures is discussed.

https://hal.archives-ouvertes.fr/hal-00130631/document

Charge distribution as a tool to investigate structural details. II. Extension to hydrogen bonds, distorted and hetero‐ligand polyhedra

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