Crystal structure refinement of magnesium zinc divanadate, MgZnV2O7, from powder X-ray diffraction data

The crystal structure of magnesium zinc divanadate can be described by an alternate stacking of V2O7 layers and (Mg/Zn) atom layers. Each V2O7 layer consists of a V2O7 dimer and a V4O14 tetramer.


Chemical context
Mixed vanadium oxides with tetrahedrally coordinated pentavalent vanadium ions have been used as catalysts in the heterogeneous oxidation process (Chang & Wang, 1988). Since there is a strong correlation between the crystal structure and its properties, the phase relations of vanadates have been thoroughly investigated. During the course of studying the phase diagram in the MgO-ZnO-V 2 O 5 system, a new phase was identified by its X-ray diffraction pattern in the solid-solution range between (Mg 0.80 Zn 1.20 )V 2 O 7 and (Mg 1.16 Zn 0.84 )V 2 O 7 , which was completely different from Mg 2 V 2 O 7 or Zn 2 V 2 O 7 (Chang & Wang, 1988). The crystal structure of the new phase has not been reported to date. We present here the crystal structure of MgZnV 2 O 7 (Fig. 1), as determined and refined from laboratory powder X-ray diffraction data (Table 1).

Structural commentary
The crystal structure of magnesium zinc divanadate, MgZnV 2 O 7 , is isotypic with Mn 0.6 Zn 1.4 V 2 O 7 (Knowles et al., 2009), where statistically distributed Mg and Zn atoms (Mn and Zn for Mn 0.6 Zn 1.4 V 2 O 7 ) are located in disordered environments in the crystal structure. The unit-cell volume of MgZnV 2 O 7 is smaller than that of Mn 0.6 Zn 1.4 V 2 O 7 by 1.65%.
The crystal structure can be described as an alternate stacking of V 2 O 7 layers and (Mg/Zn) atom layers along [201] (Fig. 1b). Each V 2 O 7 layer consists of two groups: a V 2 O 7 dimer and a V 4 O 14 tetramer. For illustration, a slab of one V 2 O 7 layer and the adjacent (Mg/Zn) layer is shown in Fig. 1c, which is rotated by 90 from Fig. 1b

Figure 1
The crystal structure of MgZnV 2 O 7 with VO 4 tetrahedra, VO 5 trigonal bipyramids (light purple), and (Mg/Zn) atoms (green/yellow). (a) overview of the structure, (b) a selected slab of one V 2 O 7 layer and the adjacent (Mg/Zn) layer, and (c) a top view of the slab of (b) and magnified local structure of V 4 O 14 tetrameric and V 2 O 7 dimeric units.

Synthesis and crystallization
MgZnV 2 O 7 was synthesized by a solid-state reaction from a mixture of Mg(CH 3 COO) 2 Á4H 2 O (98.0-102.0%, Alfa-Aesar), ZnO (99.99%, Aldrich) and V 2 O 5 (99.99%, Aldrich) with a nominal composition of Mg:Zn:V = 1:1:2. The mixture was thoroughly ground in an agate mortar with acetone, dried, pressed into a pellet, heated in air at 673 K for 3 h, at 943 K for 6 h, and again at 1023 K for 6 h with intermediate grinding and pressing. For the powder X-ray diffraction measurement, the pellet was ground again in an agate mortar and the resultant powder was dispersed on a zero-background Si sample holder.

Figure 3
Powder X-ray diffraction Rietveld refinement profiles for MgZnV 2 O 7 from room-temperature data. Black dots mark experimental data, the solid red line represents the calculated profile, and the solid green line is the background. The bottom trace presents the difference curve (blue) and the ticks denote the expected Bragg reflection positions (magenta). only three vanadium atoms, and the Le Bail fit was applied in GSAS (step 5). Improved structure factors were then extracted, which were used for the refinement in CRYSTALS (step 6). This process (step 5 to 6) was repeated until a complete and satisfactory structural model was obtained. Finally, Rietveld refinement in GSAS was employed to complete the structure model, resulting in reasonable isotropic displacement parameters and agreement indices (step 7). The refinement parameters were scale factors, background, unitcell parameters, peak profile coefficients, atomic coordinates, occupancies for the two (Mg/Zn) sites, common U iso for the metal atoms, common U iso for the oxygen atoms, and a March-Dollase preferential orientation coefficient (<111> direction). For the final Rietveld refinement cycles, the Mg-O, Zn-O, and V-O bond lengths were restrained with a tolerance value of 0.01 Å with respect to the distances determined from CRYSTALS, which matched reasonably well with the radii sums of Shannon (1976). Atomic coordinates and labeling were finally adapted from isotypic Mn 0.6 Zn 1.4 V 2 O 7 (Knowles et al., 2009). The final Rietveld plot is displayed in Fig. 3.   (6)