Hexagonal Molybdenum Trioxide-Known for 100 Years and Still a Fount of New Discoveries

In 1906, the preparation of molybdic acid hydrate was published by Arthur Rosenheim. Over the past 40 years, a multitude of isostructural compounds, which exist within a wide phase range of the system MoO3-NH3-H2O, have been published. The reported molecular formulas of hexagonal molybdenum oxide varied from MoO3 to MoO3 center dot 0.33NH(3) to MoO3 center dot nH(2)O (0.09 <= n <= 0.69) to MoO3 center dot mNH(3)center dot nH(2)O (0.09 <= m <= 0.20; 0.18 <= n <= 0.60). Samples, prepared by the acidification route were investigated using thermal analysis coupled online to a mass spectrometer for evolved gas analysis, X-ray powder diffraction, Fourier transform infrared, Raman, magic-angle-spinning H-1- and N-15 NMR spectroscopy, and incoherent inelastic neutron scattering. A comprehensive characterization of these samples will lead to a better understanding of their structure and physical properties as well as uncover the underlying relationship between the various compositions. The synthesized polymeric parent samples can be represented by the structural formula (NH4)(x infinity)(3)[Mo-y square(1-y)3(3y)(OH)(x)(H2O)(m-n)]center dot H2O with 0.10 <= x <= 0.14, 0.84 <= y <= 0.88, and m + n >= 3 - x - 3y. The X-ray study of a selected monocrystal confirmed the presence of the well-known 3D framework of edge- and comer-sharing MoO6 octahedra. The colorless monocrystal crystallizes in the hexagonal system with space group P6(3)/m, Z=6, and unit cell parameters of a = 10.527(1) angstrom, c=3.7245(7) angstrom, V=357.44(8) angstrom(3), and rho = 3.73 g.cm(-3). The structure of the prepared monocrystal can best be described by the structural formula (NH4)(0.13 infinity)(3)[Mo-0.86 square O-0.14(2.58)(OH)(0.13)(H2O)(0.29-n)]center dot nH(2)O, which is consistent with the existence of one vacancy (square) for six molybdenum sites. The sample MoO3 center dot 0.326NH(3)center dot 0.343H(2)O, prepared by the ammoniation of a partially dehydrated MoO3 center dot 0.170NH(3)center dot 0.153H(2)O with dry gaseous ammonia, accommodates NH3 in the hexagonal tunnels, in addition to [NH4](+) cations and H2O. The chimie douce reaction of MoO3 center dot 0.155NH(3)center dot 0.440H(2)O with a 1:1 mixture of NO/NO2 at 100 degrees C resulted in the synthesis of MoO3 center dot 0.539H(2)O: This material is of great interest as a host of various molecules and cations.