Structural and Mechanistic Studies of the Dehydration of MoO₂PO₃OH·H₂O and the In situ Identification of Two New Molybdenum Phosphates

Abstract

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.

Layered Molybdenum (Oxy) Pyrophosphate (MoO2)2P2O7 as a Cathode Material for Sodium‐Ion Batteries

“…As can be seen in Figure a, the XRD pattern of as prepared MoO 2 HPO 4 precursor shows excellent fit with its standard pattern except for some difference in the intensity of several peaks, indicating a pure phase of the product but a preferred orientation. Meanwhile, the XRD pattern of (MoO 2 ) 2 P 2 O 7 is in good accordance with that in reference,, indicating that we synthesized the pure δ‐ (MoO 2 ) 2 P 2 O 7 . SEM graphic in Figure b shows the obtained (MoO 2 ) 2 P 2 O 7 is of one‐dimensional rod‐like morphology and has wide size distribution in the range from nanometers to micrometers, this character is in good correspondence with the morphology and size of the reported δ‐(MoO 2 ) 2 P 2 O 7 …”

Ultimate Limits to Intercalation Reactions for Lithium Batteries

“…As can be observed from Figure , molybdenum phosphates should be good cathode candidates if all three oxidation states can be accessed. The electrochemical behavior of one molybdenum pyrophosphate, δ-(MoO 2 )­P 2 O 7 , has been investigated by Wen et al; the originally synthesis procedure was followed . The electrochemical behavior is shown in Figure , and this indicates that although the capacity is relatively low the capacity retention in cycling is quite good.…”

New compounds and structures in the solid state

“…A series of sequential transitions have also been observed in the dehydration of MoO 2 PO 3 OHÁH 2 O to form g-(MoO 2 )P 2 O 7 leading to the identification of two new phases. 25 b-MoOPO 4 is formed by complete dehydration of the starting material and has a three-dimensional structure where all MoO 6 and PO 4 units are fully corner linked. Careful control of heating rate during the dehydration led to the formation of d-MoOPO 4 which contains layers built up from chains of MoO 6 which are linked by PO 4 groups.…”

“…Anion vacancies in perovskites are coupled with ordering of larger cations in YBaCo 2 O 5 and LaBaCo 2 O 5 and the use of NaH as a solid state reducing agent on these compounds at relatively low temperatures has produced highly reduced phases YBaCo 2 O 4.5 and LaBaCo 2 O 4. 25 . 6 This extraction of oxide from the structure gives low coordination numbers for cobalt with distorted tetrahedral CoO 4 units and square-based pyramidal CoO 5 in YBaCo 2 O 4.5 and an oxidation state of Co 2+ .…”