MnB4O7is paramagnetic at room temperature, with effective paramagnetic moment of 5.84 mgr;B, and orders antiferromagnetically at about 3deg;K. The lattice constants of orthorhombic MnB4O7areaequals;8.62354plusmn;0.00003,bequals;14.0071plusmn;0.0001,cequals;8.07237plusmn;0.00002 Aring; at 298deg;K, with space groupPbcaand eight formulas in the unit cell. A total of 11 317 reflections were measured with PEXRAD resulting in 1060 symmetryhyphen;independent structure factors. The structure was solved by a combination of Patterson and Fourier series and refined by the method of least squares. The finalRfactor for a model with all atoms undergoing anisotropic motion is 0.046. The structure is shown by halfhyphen;normal probability plot analysis to be accurately isomorphic to that of CdB4O7. The B4O7group consists of two BO4tetrahedra with one oxygen atom in common, each sharing two other oxygen atoms with two triangular BO3groups. Each triangular group contains an oxygen atom from both tetrahedra. The dihedral angle between planes containing the two tetrahedral group boron and each of the triangular group boron atoms is 106.2deg;. The average tetrahedral Bsngbnd;O distance is 1.473 Aring;, and average triangular Bsngbnd;O distance is 1.368 Aring;. The BO3groups are exactly planar, within experimental error. The Mn2plus;ion occupies a distorted tetrahedron of oxygen atoms at an average distance of 2.109 Aring;: four others at distances ranging from 2.442 to 3.188 Aring; lie approximately along the normals to the tetrahedral faces. The secondary extinction coefficient in MnB4O7on gently heating the coldhyphen;worked crystal initially increases significantly, consistent with a temperaturehyphen;dependent increase in mosaic block dimension.
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