Synthesis and crystal structure of a mixed alkaline-earth powellite, Ca0.84Sr0.16MoO4

The mixed alkaline-earth compound Ca0.84Sr0.16MoO4 has a typical CaMoO4 powellite structure. Its cell parameters fit well within the trends observed in similar powellites.


Chemical context
Powellite (CaMoO 4 ) is a naturally occurring mineral with the scheelite (CaWO 4 ) structure and has been studied for different applications including laser materials, phosphors, catalysts, electrodes, and radionuclide waste forms (Kato et al., 2005;Lei & Yan, 2008;Rabuffetti et al., 2014;Peterson et al., 2018;Ryu et al., 2007). Powellites doped with rare-earth elements have broad absorption bands and fluorescence emissions in the visible to near-infrared range (Kim & Kang, 2007;Lei & Yan, 2008;Schmidt et al., 2013), and isostructural BaMoO 4 and SrMoO 4 crystals have high photoluminescence emission in the visible spectral region (Bi et al., 2008;Lei et al., 2010). Powellite has been investigated for use in a potential electrode with Li cyclability for battery applications (Reddy et al., 2013). Alkaline-earth powellites crystallize during the development of the ceramic-waste forms for radionuclides in the high-level waste (HLW) raffinate stream from aqueous reprocessing of used nuclear fuel (Crum et al., 2019;Peterson et al., 2018).

Structural commentary
Powellite crystallizes in the tetragonal space group I4 1 /a and contains Ca 2+ cations coordinated by eight [MoO 4 ] 2À tetrahedra, sharing an oxygen atom with each tetrahedron. The crystal structure of Ca 0.84 Sr 0.16 MoO 4 is isostructural to powellite, but with larger unit-cell parameters and (Ca/Sr)-O bond distances compared to CaMoO 4 powellite because of the partial incorporation of the larger Sr 2+ cation into the Ca 2+ sites (Fig. 1). Similarly, the Ba-O and Sr-O bond distances in BaMoO 4 (Nassif et al., 1999;Panchal et al., 2006;Cavalcante et al., 2008) and SrMoO 4 (Egorov-Tismenko et al., 1967;Gü rmen et al., 1971;Nogueira et al., 2013) are longer than the Ca-O bond distance in CaMoO 4 (Aleksandrov et al., 1968;Gü rmen et al., 1971) or the (Ca/Sr)-O bond distance in this study. Fig. 2 shows a summary of unit-cell parameters (a and c), unit-cell volumes (V), and unit-cell densities () from the literature as well as the current composition including CaMoO 4 (Aleksandrov et al., 1968;Gü rmen et al., 1971;Wandahl & Christensen, 1987;Peterson et al., 2018), Ca 0.747 Sr 0.194 Ba 0.059 MoO 4 (Peterson et al., 2018), SrMoO 4 (Gü rmen et al., 1971;Egorov-Tismenko et al., 1967;Nogueira et al., 2013;Peterson et al., 2018) (Nogueira et al., 2013), and BaMoO 4 (Cavalcante et al., 2008;Panchal et al., 2006;Vegard & Refsum, 1927;Nogueira et al., 2013;Nassif et al., 1999;Bylichkina et al., 1970;Peterson et al., 2018). The structural parameters of Ca 0.84 Sr 0.16 MoO 4 fit well to the trendlines in Fig. 2, and the data show well-fit linear relationships for the unit cell and volume. For the density, a non-linear trendline was drawn based on the densities of end members, and a linear trendline was drawn using the densities from both end members and  Summary of (a) unit-cell parameter a, (b) unit-cell parameter c, (c) unitcell volume (V), and (d) density () as a function of the average ionic crystal radii in the structure (coordination number = 8) from Shannon (1976). Data for the end members include averages and standard deviations from multiple sources.  mixed powellites from the literature (Fig. 2d). Despite our expectation, the density values did not fit well into either trendline, and more density values from different chemistries of mixed alkaline-earth powellites would help to understand the behavior of densities in powellites. The trendlines show that the unit cells, volumes, and densities all increase with larger alkaline-earth cations. Details of unit cell parameters, volumes, and densities from literature and the current study are summarized in Table 1.

Synthesis and crystallization
The mixed alkaline-earth powellite, Ca 0.84 Sr 0.16 MoO 4 , was synthesized using the end-member powellites within a LiCl flux. The loss of mass due to dehydration for LiCl was measured by placing a given amount of LiCl (Alfa Aesar, >99%) into a furnace at 100 C and weighing daily for five days. For the synthesis of CaMoO 4 and SrMoO 4 , the stoichiometric amounts of CaCO 3 (Alfa Aesar, >99.5%), SrCO 3 (Sigma Aldrich, >99.9%), and MoO 3 (Alfa Aesar, >99.5%) were placed in Pt/10%Rh crucible and heated to 1500 C at 5 C min À1 , held for 30 min, ramped down to 1400 C at 1 C min À1 , held for 1 h, and then cooled down to room temperature at 1 C min À1 . Details of synthesis are provided elsewhere (Peterson et al., 2018). For the synthesis of Ca 0.84 Sr 0.16 MoO 4 , appropriate amounts of CaMoO 4 and SrMoO 4 powders were used as precursors and mixed together in Pt/10%Rh crucibles. Then, LiCl was added at a 1:1 ratio by mass, where the mass of CaMoO 4 + SrMoO 4 was equivalent to that of the LiCl. The crucible was covered with a tight-fitting Pt/10%Rh lid and heated according to a method described by Arora et al. (1983). The furnace was ramped up to 850 C, held for 2 h, abruptly decreased to 750 C, cooled to 550 C at a rate of 3 C h À1 , and then the furnace was shut off. Crystals were recovered after washing in a sonic bath and rinsing with deionized water.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. For the occupancy refinement of the Ca and Sr sites, the occupancy parameters of both Sr and Ca were refined with isotropic atomic displacement parameters while keeping the total occupancy as 1. The refined occupancy values were 0.86 for Ca and 0.14 for Sr after rounding, and then these values were fixed and anisotropic refinements were performed on all the atoms including Ca, Sr, Mo, and O. The final refinement converged at R 1 = 4.30%, and the goodness-of-fit was 1.44. The single crystals of Comparison between P-XRD pattern of ground Ca 0.84 Sr 0.16 MoO 4 single crystals and calculated pattern generated from the solved structure. (Parabar 10312, Hampton Research). Powder X-ray diffraction (P-XRD) was performed using a Bruker D8 Advance diffractometer on a zero-background quartz sample holder. The measured P-XRD pattern was compared to the calculated pattern from the solved structure, and they were in good agreement (see Fig. 3).

Calcium strontium molybdate
Crystal data Special details Refinement. F000 reported from JANA is 388.0 and calculated is 387.5 from CheckCIF.Both occupancies of Ca and Sr were refined with isotropic ADP while keeping the total occupancy at 1 and same position for both atoms, and their occupancy values were closed to 0.84 ±0.001 and 0.16±0.001 respectively between the refinements. Therefore, we fixed the occupancy to 0.84 and 0.16 with rounding off, and the anisotropic refinement was applied after fixing the occupancies.The difference in reported and caculated rho(max)is likely due to difference in how PLATON and JANA2006 calculate Fourier maps and take weights of reflections into Fourier calculations.