Crystal structure of apatite type Ca2.49Nd7.51(SiO4)6O1.75

Single crystals of Ca2.49Nd7.51(SiO4)6O1.75 have been synthesized from a mixture of Nd2O3, CaO and SiO2 at 1873 K rapidly quenched to room temperature after 24 h.


Chemical context
The study of calcium rare earth oxide silicates is important because they are usually observed in nuclear waste along with rare earth silicates. So far, the calcium rare earth oxide silicates of Nd (Fahey & Weber, 1982;Fahey et al., 1985), Sm (PDF 29-365;Smith, 1977), Eu (PDF 29-320; Smith, 1977), Gd (PDF 28-212;Smith, 1976), Tb (PDF 38-256; Lacout, 1986), and Ce (Skakle et al., 2000) have been studied. Fahey & Weber et al. (1982) and Fahey et al. (1985) published the structure and stoichiometry limits of the Ca 2+x Nd 8-x (SiO 4 ) 2-0.5x system using X-ray and neutron powder diffraction. In that study, the samples were synthesized at 1523 or 1873 K and cooled at a rate of 250 K per hour. However, such a slow cooling process may lead to undesired modifications of the obtained specimens since the solubility of calcium does not remain constant but decreases with decreasing temperature. This problem is avoided in the present work by rapid quenching of the Ca 2+x Nd 8-x (SiO 4 ) 6 O 2-0.5x samples in their equilibrium state at 1873 K to room temperature within a few seconds. Consequently, compositions of the samples can be preserved better.

Structural commentary
The single crystal structure determined from room temperature data was found to belong to the space group P6 3 /m and has the composition Ca 2.49 Nd 7.51 (SiO 4 ) 6 O 1.75 and is isotypic with natural apatite and the previously reported Ca 2 Nd 8 (SiO 4 ) 6 O 2 and Ca 2.2 Nd 7.8 (SiO 4 ) 6 O 1.9 (Fahey & Weber, 1982;Fahey et al., 1985). The solubility limit of calcium in the equilibrium state at 1873 K was found to occur at a composition of Ca 2+x Nd 8-x (SiO 4 ) 6 O 2-0.5x , where x = 0.49. ISSN 2056-9890 There are two metal positions in the asymmetric unit of the structure (Fig. 1) and both contain disordered Nd and Ca ions: Nd1/Ca1 occupies the lower symmetry site 6h and Nd2/Ca2 the higher symmetry site 4f. The occupancies of these metal sites were refined resulting in 0.887 (5)/0.113 (5) for Nd1/Ca1 and 0.546 (4)/0.454 (4) for Nd2/Ca2. The majority (80%) of calcium is situated at the 4f site. In the structures of Ca 2 Nd 8 (SiO 4 ) 6 O 2 and Ca 2.2 Nd 7.8 (SiO 4 ) 6 O 1.9 , these values are 89 and 73%, respectively (Fahey et al., 1985). The refined value of the amount of Nd in the structure gives a value of 0.49 for x in the equation Ca 2+x Nd 8-x (SiO 4 ) 6 O 2-0.5x . For charge-balance purposes, the occupancy of O 2À in the structure must be 2 À 0.5x or 1.755. Initially, the occupancy of the O 2À position O4 in the structure was allowed to refine freely and its value was close to what is required for charge balance; however, it was fixed at 0.146 as the refinement of heavy-atom positions is the most reliable and exact charge balance is required.
The O4 atom (O 2À ion) is coordinated to three different Nd1/Ca1 ions whilst the SiO 4 4À group has eight contacts to different Nd/Ca positions. The O1 atom coordinates one Nd1/Ca1 position and two Nd2/Ca2 positions, the O2 atom coordinates one Nd1/Ca1 position and two Nd2/Ca2 positions and the O3 position coordinates one Nd1/Ca1 and one Nd2/ Ca2 positions. These contacts generate the packing, which can be seen viewed down the c axis in Fig. 2.

Figure 2
View along the c axis of the packing arrangement.
was made by calcination of CaCO 3 at 1373 K for 12 h. The sample was then quenched in a cold-water bath to give a lightblue crystalline solid, from which a single crystal of the title compound was selected.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2. There are two metal positions in the structure and the Nd and Ca ions are disordered on both of these sites. Nd/Ca occupancy on each of the two positions was refined and the occupancy of Nd was found to be 88.7 (5)% for one site and 54.6 (4)% for the other, giving a value of 0.49 for x in Ca 2+x Nd 8-x (SiO 4 ) 6 O 2-0.5x . The occupancy of the anionic O atom was fixed at 2 À 0.5x. Constraints were applied so that the Nd and Ca on the same site had identical positional and displacement parameters.  (Agilent, 2012); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009) and publCIF (Westrip, 2010).