DOI: 10.1002/zaac.200700376 M 2 B 5 or M 2 B 4 ? A Reinvestigation of the Mo/B and W/B System Michael Frotscher a , Wilhelm Klein b,1) , Joseph Bauer c , Chang-Ming Fang c,2) , Jean-Fran ¸ois Halet c, *, Anatoliy Senyshyn d , Carsten Baehtz e , and Barbara Albert a, * a Darmstadt/Deutschland, Eduard Zintl-Institut für Anorganische und Physikalische Chemie der Technischen Universität b Hamburg/Deutschland, Institut für Anorganische und Angewandte Chemie der Universität c Rennes/France, Sciences Chimiques de Rennes, UMR 6226 du CNRS et de l’Universite ´ de Rennes 1 d Darmstadt/Deutschland, Fachbereich Material- und Geowissenschaften der Technischen Universität e Hamburg/Deutschland, Hasylab/DESY Received July 20th, 2007. Dedicated to Professor Rudolf Hoppe on the Occasion of his 85 th Birthday Abstract. The literature states different compositions (M/B = 1:2 vs. 2:5) and structures for diborides of molybdenum and tungsten. Using X-ray and neutron powder diffraction as well as energy and wavelength dispersive electron microprobe analysis, the Mo/B and W/B systems were now reinvestigated. Molybdenum diboride cry- stallizes as a stoichiometric compound Mo 2 B 4 (formerly described as Mo 2 B 5 ) in space group R3 ¯ m (No. 166, a, b = 3.01375(2) A ˚ , c = 20.9541(3) A ˚ ), and as a non-stoichiometric compound MoB 2-x (formerly described as MoB 2 ) in P6/mmm (No. 191, a, b = Introduction For the Mo/B and W/B system, compounds like MB, MB 2 , M 2 B 5 ,M 0.8 B 3 , and MB 4 are reported in the literature. Such compounds have interesting electrical and mechanical properties, which indicate possible applications as high-tem- perature ceramic materials [1-7]. Recently, Telle et al. showed that W 2 B 5 -based materials display unique fracture behaviours and delamination phenomena [6, 8-10]. The first one to structurally characterize the dimetal pentaborides Mo 2 B 5 and W 2 B 5 was Kiessling in 1947 [11]. The crystal structures of the Kiessling phases show alternat- ing layers of boron and metal atoms with different stacking sequences and c-axes for the two compounds (Fig. 1a and b). They were described in space groups R3 ¯ m (Mo 2 B 5 ) and P6 3 /mmc (W 2 B 5 ). Both structures show two types of boron atom layers, graphite-like planar layers and corrugated layers of condensed cyclohexane-like chairs with an ad- ditional boron atom at the center of the ring. The unusual coordination sphere of the additional, centering boron atom raised controversy in the literature. Further structural * Prof. Dr. Barbara Albert Technische Universität Darmstadt Eduard-Zintl-Institut D-64287 Darmstadt Fax: (+49) 6151-16-6029 E-mail: albert@ac.chemie.tu-darmstadt.de 2626  2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Z. Anorg. Allg. Chem. 2007, 633, 2626-2630 3.043(2) A ˚ , c = 3.067(2) A ˚ ), whereas stoichiometric tungsten di- boride W 2 B 4 (formerly described as W 2 B 5 ) is found to crystallize in space group P6 3 /mmc (No. 194, a, b = 2.9864(4) A ˚ ,c = 13.896(2) A ˚ ). These results seem to be supported by DFT calcula- tions which show the instability of a hypothetic W 2 B 5 . Keywords: Molybdenum; Tungsten; Borides; X-ray diffraction; Neutron diffraction; Microprobe; DFT calculations investigations led to contradictory results, including partial [12] and absent occupation of the boron atom position in question („B3“) [13, 14]. These investigations included X- ray and neutron diffraction measurements but no conclus- ive elemental analysis. A better formula to describe a compound with no occupation at “B3” would be M 2 B 4 (= MB 2 ) rather than M 2 B 5 .M 2 B 4 has to be distinguished from MB 2 , which crystallizes with an AlB 2 -type structure (Fig. 1c) [12, 15]. One main problem is common to almost all publications dealing with these phases, that is, the difficulty to synthesize a macroscopic, pure and mono-phasic sample which allows the application of several complementary methods of analy- sis. The synthesis of transition metal borides requires high temperatures and inert conditions, and elements from the material of the crucible are often accumulated as impurities in the samples. Other binary borides like MB, MB 2 or MB 4 are formed under very similar reaction conditions and can be observed as by-products. In addition, it is difficult to locate boron atoms next to heavy atoms with conventional X-ray analysis. 1) Present address: Forschungsneutronenquelle Heinz Maier-Leib- nitz (FRM II), Technische Universität München, Garching, Deutschland 2) Present address: Institute for Molecules and Materials, Radboud Universiteit Nijmegen, Nijmegen, The Netherlands