Crystal structure of Na2HfSi2O7 by Rietveld refinement

A new sodium hafnium disilicate was detected as an major intermediate product of a global reaction between borosilicate glass and hafnium. The composition of this phase was determined to be Na2HfSi2O7 by laboratory powder diffraction and Rietveld refinement.


Chemical context
Laboratory work in order to explore the chemistry of compounds with radioactive elements such as actinides is difficult because of the emission of ionizing radiation. To overcome this problem, these radionuclides are often replaced by a stable element having similar properties as the radioactive element, for instance by using elements with a similar ionic radius or with the same oxidation state. Hence actinides are often replaced by neodymium, zirconium, europium, or hafnium (Ramsey et al., 1995). The reactivity of uranium with an Na-Si-O glass at high temperatures was thus simulated by using hafnium instead of uranium. We have obtained samples with different phases among which was a sodium hafnium disilicate, similar to the sodium zirconium silicate already observed in a similar glass (Plaisted et al., 1999). The structure of the sodium hafnium disilicate is discussed in this paper.

Structural commentary
The Na 2 HfSi 2 O 7 phase is isostructural with the parakeldyshite phase (Voronkov et al., 1970;Fleischer et al., 1979). As reported in Table 1, the cell parameters of the Na 2 HfSi 2 O 7 phase are slightly smaller than those of parakeldyshite, and the volume of the cell is 0.8% smaller. For the Na 2 HfSi 2 O 7 phase, the Hf1O 6 octahedral and the Si2O 4 tetrahedral volumes are about the same as the analogous Zr octahedral and Si tetrahedral volumes in parakeldyshite. The Si1O 4 tetrahedral volume of the Na 2 HfSi 2 O 7 phase is about 5% smaller than that in parakeldyshite. It is thus in the latter tetrahedron that the bond lengths differ significantly whereas the other bond lengths are quite similar in both phases. The sodium coordination polyhedral volumes are quite similar in volume for the two phases, about 30.1 Å 3 . A polyhedral view of the Na 2 HfSi 2 O 7 structure is given in Fig. 1. The Na 2 ZrSi 2 O 7 phase is capable of ion exchange on the sodium site thanks to the sufficient dimension of the sodium tunnels in the [010] direction (Kostov-Kytin et al., 2008). Since these dimensions are the same in both phases, ion exchange should also be possible in the Na 2 HfSi 2 O 7 phase. A numerical comparison of the structures of the parakeldyshite and the Na 2 HfSi 2 O 7 phase was performed with COMPSTRU (de la Flor et al., 2016). The structures' similarities were estimated by different parameters such as the measure of similarity Á (Bergerhoff et al., 1999). This parameter was determined to be 0.018 for a maximum distance between paired atoms of 1 Å , indicating that structures are effectively isostructural. Since hafnium simulates uranium, the existence of the Na 2 USi 2 O 7 phase can also be supposed.

Database survey
The crystal chemistry of zirconosilicates can be described in terms of an MT framework with MO 6 octahedra and TO 4 tetrahedra (M = Zr, T = Si; Ilyushin & Blatov, 2002). The voids in the MT framework are filled with alkaline or alkaline earth elements coordinated in an eight-vertex polyhedron. The crystal system of sodium zirconosilicates can vary from triclinic (Na 2 ZrSi 2 O 7 ) to monoclinic (Na 2 ZrSi 4 O 11 ) or trigonal (Na 8 ZrSi 6 O 18 ). If we focus on the chemistry of zirconosilicates with Si 2 O 7 diortho groups and their analogs (Pekov et al., 2007), the triclinic phase is privileged such as the parakeldyshite Na 2 ZrSi 2 O 7 phase (Ferreira et al., 2001) or the keldyshite (Na,H) 2 ZrSi 2 O 7 phase (Khalilov et al., 1978). The potassium analogue, however, is monoclinic as in the case of khibinskite K 2 ZrSi 2 O 7 (Chernov et al., 1970;Nosyrev et al., 1976). Polyhedral representation of the Na 2 HfSi 2 O 7 phase with SiO 4 units (blue), HfO 6 units (green) and sodium (yellow) with displacement ellipsoids drawn at the 99% probability level. Table 1 Cell parameters, selected distances (Å ), angles ( ) and volumes (Å 3 ) for the title phase compared to parakeldyshite.

Synthesis and crystallization
The synthesis of sodium hafnium disilicate was based on the two-step synthesis protocol of parakeldyshite Na 2 ZrSi 2 O 7 (Lin et al., 1999;Ferreira et al., 2001). The first step was the synthesis of the Hf-petarasite phase Na 5 Zr 2 Si 6 O 18 (ClÁOH) 2 Á-H 2 O with zirconium totally substituted by hafnium. Adequate quantities of sodium silicate solution (27% SiO 2 , 8% Na 2 O), sodium chloride, hafnium chloride, potassium chloride, sodium hydroxide and water were mixed thoroughly in a polytetrafluoroethylene (PTFE) vessel at room temperature for 30 minutes. A gel was obtained with a pH value around 13. The PTFE vessel was put in a Parr digestion apparatus for a hydrothermal synthesis over 10 days at 523 K. The resulting powder was washed, filtered, and dried overnight at 393 K. In spite of the drying process, the powder was still hydrated. Powder X-ray diffraction showed the compound to be isostructural to petarasite. The second step was the calcination of Hf-petarasite over 15 h at 1373 K under air which lead to a white powder. SEM observation of the powder showed large grains with Na, Hf, Si and O and smaller grains with supplementary K. The chemical composition of the major phase was determined by EDS to have the following stoichiometry Na 1.7AE0.2 Hf 1.0 Si 2.3AE0.1 O 7.3AE0.9 as compared to the theoretical stoichiometry of Na 2 HfSi 2 O 7 . Thus the major phase is very close to the expected one. The sample was analysed by differential thermal analysis to determine its melting point. There was no thermal event indicating a melting until 1623 K and the sample was still in powder form. The Na 2 HfSi 2 O 7 phase therefore has a higher melting point than parakeldyshite which is below 1523 K (Ferreira et al., 2001).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Observed and calculated intensities for Na 2 HfSi 2 O 7 are shown in Fig. 2 along with the difference pattern. The reliability factors of the refinement were quite poor because of an amorphous bump attributed to the second minor phase. Hence the reliability factors were negatively impacted. The isotropic ADP's of the oxygen atoms were constrained to be equal in volume in order to avoid a slightly negative ADP value on O5. The residual electron density is about 2.4 e Å 3 , which is less than 10% of the electron density of a Hf atom. The occupancies of all atoms were fixed to unity.  Step 2 values ( ) 2 min = 8.013 2 max = 120.013 2 step = 0.017

Figure 2
Comparison of observed (red squares) and calculated (solid line) intensities for Na 2 HfSi 2 O 7 . The difference pattern appears below. Inset: focus on the 12-35 2 range.