We report investigations into the dehydration pathway of the precursor material MoO2PO3OH·H2O to γ-(MoO2)2P2O7. The reaction occurs in three distinct stages via the formation of two new previously unidentified molybdenum phosphate phases, β-MoOPO4 and δ-(MoO2)2P2O7. Conditions for the isolation of these phases were identified by a whole powder pattern fitting technique to follow phase evolution versus time and temperature and later verified by full Rietveld refinement. Structural refinement of β-MoOPO4 was performed against X-ray and neutron data. The new phase has lattice parameters a = 7.4043(3) Å, b = 7.2128(3) Å, c = 7.2876(3) Å, β = 118.346(2)°, and volume 342.53(3) Å3 at room temperature, containing 7 unique atoms in space group Cc. δ-(MoO2)2P2O7 forms on slowly heating the precursor material to 793 K. Lattice parameters at room temperature are a = 16.2213(11) Å, b = 3.8936(3) Å, and c = 6.2772(4) Å and volume 396.46(5) Å3, in space group C2221. A transformation mechanism is proposed for the dehydration. Lithium intercalation into layered δ-(MoO2)2P2O7 has been shown.
We report the crystal structure and phase transitions of Mo2P4O15 which, despite a simple chemical formula, has 441 crystallographically unique atoms in its asymmetric unit and thus has the most complex structure of any extended oxide reported to date.
We report structural studies on Mo(2)P(4)O(15) over the temperature range 16-731 K, which show that it is considerably more complex than revealed by earlier work. Its low-temperature structure has lattice parameters a = 24.1134(6) A, b = 19.5324(5) A, c = 25.0854(6) A, beta = 100.015(1) degrees, and V = 11635.0(5) A(3) at 120 K, containing 441 unique atoms in space group Pn, a remarkably high number for a material with such a simple composition. Mo(2)P(4)O(15) undergoes a structural phase transition at approximately 520 K to a high-temperature phase in space group P1, with lattice parameters a = 17.947(3) A, b = 19.864(3) A, c = 21.899(3) A, alpha = 72.421(3) degrees, beta = 78.174(4) degrees, gamma = 68.315(4) degrees, and V = 6877.2(19) A(3) at 573 K. The high-temperature structure, with 253 unique atoms, retains much of the low-temperature complexity.
We report structural investigations into (MoO(2))(2)P(2)O(7) using a combination of X-ray, neutron and electron diffraction, and solid-state NMR supported by first principles quantum chemical calculations. These reveal a series of phase transitions on cooling at temperatures of 377 and 325 K. The high temperature gamma-phase has connectivity consistent with that proposed by Kierkegaard at room temperature (but with improved bond length distribution), and contains 13 unique atoms in space group Pnma with lattice parameters a = 12.6577(1) A, b = 6.3095(1) A, c = 10.4161(1) A, and volume 831.87(1) A(3) from synchrotron data at 423 K. The low temperature alpha-structure was indexed from electron diffraction data and contains 60 unique atoms in space group P2(1)/c with cell parameters a = 17.8161(3) A, b = 10.3672(1) A, c = 17.8089(3) A, beta = 90.2009(2) degrees, and volume 3289.34(7) A(3) at 250 K. First principles calculations of (31)P chemical shift and J couplings were used to establish correlation between local structure and observed NMR parameters, and 1D and 2D (31)P solid-state NMR used to validate the proposed crystal structures. The intermediate beta-phase is believed to adopt an incommensurately modulated structure; (31)P NMR suggests a smooth structural evolution in this region.
There has been a considerable amount of interest in negative thermal expansion (NTE) phases which are of importance, for example, in the manufacture of low or zero expansion composite materials. Cubic gamma-ZrMo(2)O(8) is of importance since it shows smooth NTE (alpha = -8 x 10(-6) K(-1) at 100 K) over a wide temperature range with no significant discontinuities as a function of temperature. The material has long been thought to be metastable at all temperatures and has previously only been produced under kinetic control by routes such as the careful dehydration of ZrMo(2)O(7)(OH)(2).2H(2)O. High-speed, high-temperature in situ diffraction studies carried out at beamline ID11 at the European Synchrotron Radiation Facility (ESRF) have shown that it is in fact possible to prepare gamma-ZrMo(2)O(8) directly from the constituent oxides in a narrow temperature window around 1400 K where it appears to be entropically stabilized. Under these conditions gamma-ZrMo(2)O(8) forms in a matter of seconds and can be quenched to room temperature. Full quantitative Rietveld analysis of diffraction patterns collected in as little as 0.25 s during the synthesis has allowed the reaction pathway to be followed.
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