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Crystal structure of Na2HfSi2O7 by Rietveld refinement

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aCEA, DEN, DTCD, Marcoule, BP17171, F-30207 Bagnols sur Ceze, France, and bCRPG, CNRS UMR-5873, Université de Lorraine, BP 20, F-54501 Vandoeuvre les Nancy cedex, France
*Correspondence e-mail: nicolas.massoni@cea.fr

Edited by A. Van der Lee, Université de Montpellier II, France (Received 1 September 2016; accepted 6 September 2016; online 16 September 2016)

The structure of triclinic disodium hafnium disilicate, Na2HfSi2O7, has been determined by laboratory powder X-ray diffraction and refined by the Rietveld refinement. The structure is a framework made of alternate layers of HfO6 octa­hedra and SiO4 tetra­hedra linked by common O atoms. Sodium atoms are located in the voids of the framework, aligned into tunnels along the [010] direction. Na2HfSi2O7 is isostructural with the parakeldyshite Na2ZrSi2O7 phase.

1. 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[Ramsey, W. G., Bibler, N. E. & Meaker, T. F. (1995). WM Symposia, Waste Management, 95, 23828-23907.]). 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[Plaisted, T., Hrma, P., Vienna, J. & Jiricka, A. (1999). Proceedings of MRS 608, 13-14.]). The structure of the sodium hafnium disilicate is discussed in this paper.

2. Structural commentary

The Na2HfSi2O7 phase is isostructural with the parakeldyshite phase (Voronkov et al., 1970[Voronkov, A. A., Shumyatskaya, N. G. & Pyatenko, Yu. A. (1970). Zh. Strukt. Khim. 11, 932-933.]; Fleischer et al., 1979[Fleischer, M., Chao, G. Y. & Mandarino, J. A. (1979). Am. Mineral. 64, 652-659.]). As reported in Table 1[link], the cell parameters of the Na2HfSi2O7 phase are slightly smaller than those of parakeldyshite, and the volume of the cell is 0.8% smaller. For the Na2HfSi2O7 phase, the Hf1O6 octa­hedral and the Si2O4 tetra­hedral volumes are about the same as the analogous Zr octa­hedral and Si tetra­hedral volumes in parakeldyshite. The Si1O4 tetra­hedral volume of the Na2HfSi2O7 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 Å3. A polyhedral view of the Na2HfSi2O7 structure is given in Fig. 1[link]. The Na2ZrSi2O7 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[Kostov-Kytin, V., Fujiwara, K., Nakayama, N. & Nikolova, R. (2008). Proc. Bulg. Geol. Soc. pp. 13-14.]). Since these dimensions are the same in both phases, ion exchange should also be possible in the Na2HfSi2O7 phase. A numerical comparison of the structures of the parakeldyshite and the Na2HfSi2O7 phase was performed with COMPSTRU (de la Flor et al., 2016[Flor, G. de la, Orobengoa, D., Tasci, E., Perez-Mato, J. M. & Aroyo, M. I. (2016). J. Appl. Cryst. 49, 653-664.]). The structures' similarities were estimated by different parameters such as the measure of similarity Δ (Bergerhoff et al., 1999[Bergerhoff, G., Berndt, M., Brandenburg, K. & Degen, T. (1999). Acta Cryst. B55, 147-156.]). 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 Na2USi2O7 phase can also be supposed.

Table 1
Cell parameters, selected distances (Å), angles (°) and volumes (Å3) for the title phase compared to parakeldyshite

Cell parameters of Na2ZrSi2O7 are from Ferreira et al. (2001[Ferreira, P., Ferreira, A., Rocha, J. & Soares, M. R. (2001). Chem. Mater. 13, 355-363.]).

Na2HfSi2O7   Na2ZrSi2O7  
a, b, c 6.6123 (2), 8.7948 (3), 5.41074 (15) a, b, c 6.6364 (4), 8.8120 (5), 5.4233 (3)
α, β, γ 92.603 (2), 94.084 (2), 71.326 (2) α, β, γ 92.697 (4), 94.204 (3), 71.355 (3)
Vcell 297.25 (2) Vcell 299.61 (3)
Hf1 octa­hedron   Zr octa­hedron  
O1—O7 4.15 (4) O1—O7 4.22 (2)
O4—O5 4.28 (3) O4—O5 4.24 (2)
O3—O6 4.22 (4) O3—O6 4.18 (2)
Hf1—O7 2.23 (3) Zr—O7 2.13 (2)
Hf1—O1 1.92 (3) Zr—O1 2.08 (2)
Hf1—O3 2.30 (3) Zr—O3 2.11 (2)
Hf1—O4 2.04 (2) Zr—O4 2.16 (3)
Hf1—O5 2.26 (3) Zr—O5 2.09 (3)
Hf1—O6 1.93 (2) Zr—O6 2.03 (2)
Hf1—O7—Si2 116.7 (6) Zr—O7—Si2 124.0 (4)
Polyhedron volume 12.5 Polyhedron volume 12.4
       
Si1 tetra­hedron   Si1 tetra­hedron  
Si1—O2i 1.55 (3) Si1—O2i 1.62 (2)
Si1—O3i 1.37 (4) Si1—O3i 1.58 (1)
Si1—O4i 1.66 (3) Si1—O4i 1.55 (1)
Si1—O1 1.68 (3) Si1—O1 1.57 (1)
Polyhedron volume 1.94 Polyhedron volume 2.02
       
Si2 tetra­hedron   Si2 tetra­hedron  
Si2—O2 1.77 (3) Si2—O2 1.67 (2)
Si2—O7i 1.61 (3) Si2—O7i 1.64 (1)
Si2—O5 1.61 (3) Si2—O5 1.62 (1)
Si2—O6 1.60 (3) Si2—O6 1.53 (1)
Polyhedron volume 2.12 Polyhedron volume 2.14
       
Si1—O—Si2 bridging angle 136.7 (5) Si1—O—Si2 bridging angle 130.9 (5)
Symmetry code: (i) −x, −y, −z.
[Figure 1]
Figure 1
Polyhedral representation of the Na2HfSi2O7 phase with SiO4 units (blue), HfO6 units (green) and sodium (yellow) with displacement ellipsoids drawn at the 99% probability level.

3. Database survey

The crystal chemistry of zirconosilicates can be described in terms of an MT framework with MO6 octa­hedra and TO4 tetra­hedra (M = Zr, T = Si; Ilyushin & Blatov, 2002[Ilyushin, G. D. & Blatov, V. A. (2002). Acta Cryst. B58, 198-218.]). 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 (Na2ZrSi2O7) to monoclinic (Na2ZrSi4O11) or trigonal (Na8ZrSi6O18). If we focus on the chemistry of zirconosilicates with Si2O7 diortho groups and their analogs (Pekov et al., 2007[Pekov, I. V., Zubkova, N. V., Pushcharovsky, D. Yu., Kolitsch, U. & Tillmanns, E. (2007). Crystallogr. Rep. 52, 1066-1071.]), the triclinic phase is privileged such as the parakeldyshite Na2ZrSi2O7 phase (Ferreira et al., 2001[Ferreira, P., Ferreira, A., Rocha, J. & Soares, M. R. (2001). Chem. Mater. 13, 355-363.]) or the keldyshite (Na,H)2ZrSi2O7 phase (Khalilov et al., 1978[Khalilov, A. D., Khomyakov, A. P. & Makhmudov, S. A. (1978). Dokl. Akad. Nauk SSSR, 238, 573-575.]). The potassium analogue, however, is monoclinic as in the case of khibinskite K2ZrSi2O7 (Chernov et al., 1970[Chernov, A. N., Maksimov, B. A., Ilyukhin, V. V. & Belov, N. V. (1970). Dokl. Akad. Nauk SSSR, 193, 1293-1296.]; Nosyrev et al., 1976[Nosyrev, N. A., Treushnikov, E. N. & Voronkov, A. A. (1976). Dokl. Akad. Nauk SSSR, 231, 1351-1353.]).

4. Synthesis and crystallization

The synthesis of sodium hafnium disilicate was based on the two-step synthesis protocol of parakeldyshite Na2ZrSi2O7 (Lin et al., 1999[Lin, Z., Rocha, J., Ferreira, P., Thursfield, A., Agger, J. R. & Anderson, M. W. (1999). J. Phys. Chem. B, 103, 957-963.]; Ferreira et al., 2001[Ferreira, P., Ferreira, A., Rocha, J. & Soares, M. R. (2001). Chem. Mater. 13, 355-363.]). The first step was the synthesis of the Hf–petarasite phase Na5Zr2Si6O18(Cl·OH)2·H2O with zirconium totally substituted by hafnium. Adequate qu­anti­ties of sodium silicate solution (27% SiO2, 8% Na2O), 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 Na1.7±0.2Hf1.0Si2.3±0.1O7.3±0.9 as compared to the theoretical stoichiometry of Na2HfSi2O7. 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 Na2HfSi2O7 phase therefore has a higher melting point than parakeldyshite which is below 1523 K (Ferreira et al., 2001[Ferreira, P., Ferreira, A., Rocha, J. & Soares, M. R. (2001). Chem. Mater. 13, 355-363.]).

5. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2[link]. Observed and calculated intensities for Na2HfSi2O7 are shown in Fig. 2[link] 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 Å3, which is less than 10% of the electron density of a Hf atom. The occupancies of all atoms were fixed to unity.

Table 2
Experimental details

Crystal data
Chemical formula Na2HfSi2O7
Mr 392.6
Crystal system, space group Triclinic, P[\overline{1}]
Temperature (K) 293
a, b, c (Å) 6.6123 (2), 8.7948 (3), 5.41074 (15)
α, β, γ (°) 92.603 (2), 94.0843 (18), 71.3262 (18)
V3) 297.25 (2)
Z 2
Radiation type Cu Kα1, λ = 1.540562, 1.544390 Å
Specimen shape, size (mm) Flat sheet, 25 × 25
 
Data collection
Diffractometer Panalytical XPert MPD Pro
Specimen mounting Packed powder pellet
Data collection mode Reflection
Scan method Step
2θ values (°) 2θmin = 8.013 2θmax = 120.013 2θstep = 0.017
 
Refinement
R factors and goodness of fit Rp = 0.024, Rwp = 0.032, Rexp = 0.015, R(F) = 0.024, χ2 = 4.973
No. of parameters 67
Computer programs: X'Pert Data Collector (PANalytical, 2011[PANalytical (2011). X'Pert Data Collector. PANalytical BV, Almelo, The Netherlands.]), JANA2006 (Petříček et al., 2014[Petříček, V., Dušek, M. & Palatinus, L. (2014). Z. Kristallogr. Cryst. Mater. 229, 345-352.]), VESTA (Momma & Izumi, 2011[Momma, K. & Izumi, F. (2011). J. Appl. Cryst. 44, 1272-1276.]) and publCIF (Westrip, 2010[Westrip, S. P. (2010). J. Appl. Cryst. 43, 920-925.]).
[Figure 2]
Figure 2
Comparison of observed (red squares) and calculated (solid line) intensities for Na2HfSi2O7. The difference pattern appears below. Inset: focus on the 12–35° 2θ range.

Supporting information


Computing details top

Data collection: X'Pert Data Collector (PANalytical, 2011); cell refinement: JANA2006 (Petříček et al., 2014); data reduction: JANA2006 (Petříček et al., 2014); program(s) used to solve structure: JANA2006 (Petříček et al., 2014); program(s) used to refine structure: JANA2006 (Petříček et al., 2014); molecular graphics: VESTA (Momma & Izumi, 2011); software used to prepare material for publication: publCIF (Westrip, 2010).

Disodium hafnium disilicate top
Crystal data top
Na2HfSi2O7Z = 2
Mr = 392.6F(000) = 356
Triclinic, P1Dx = 4.387 Mg m3
a = 6.6123 (2) ÅCu Kα1 radiation, λ = 1.540562, 1.544390 Å
b = 8.7948 (3) ÅT = 293 K
c = 5.41074 (15) ÅParticle morphology: plate-like
α = 92.603 (2)°white
β = 94.0843 (18)°flat_sheet, 25 × 25 mm
γ = 71.3262 (18)°Specimen preparation: Prepared at 1393 K and 100 kPa
V = 297.25 (2) Å3
Data collection top
Panalytical XPert MPD Pro
diffractometer
Data collection mode: reflection
Radiation source: sealed X-ray tubeScan method: step
Specimen mounting: packed powder pellet2θmin = 8.013°, 2θmax = 120.013°, 2θstep = 0.017°
Refinement top
Rp = 0.02467 parameters
Rwp = 0.0320 restraints
Rexp = 0.0156 constraints
R(F) = 0.024Weighting scheme based on measured s.u.'s
6423 data points(Δ/σ)max = 0.005
Profile function: pseudo-VoigtBackground function: Legendre polynoms
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Na10.8823 (15)0.0970 (12)0.2630 (18)0.035 (4)*
Na20.3456 (14)0.5024 (10)0.7608 (18)0.011 (3)*
Hf10.2880 (3)0.2710 (2)0.2201 (3)0.0158 (7)
Si10.6528 (13)0.1435 (9)0.7769 (13)0.010 (3)*
Si20.9381 (12)0.3380 (8)0.6861 (15)0.006 (2)*
O10.307 (2)0.0382 (17)0.172 (3)0.0030 (15)*
O20.865 (2)0.1777 (14)0.727 (2)0.0030 (15)*
O30.494 (2)0.2081 (15)0.530 (3)0.0030 (15)*
O40.562 (2)0.2522 (15)0.020 (3)0.0030 (15)*
O50.003 (2)0.3116 (14)0.401 (3)0.0030 (15)*
O60.120 (2)0.3300 (14)0.872 (3)0.0030 (15)*
O70.284 (2)0.5148 (14)0.288 (3)0.0030 (15)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Hf10.0148 (10)0.0177 (11)0.0126 (10)0.0003 (7)0.0035 (7)0.0111 (8)
Geometric parameters (Å, º) top
O1—Hf12.017 (15)Na1—Na1vii3.169 (13)
O1—Si1i1.568 (17)Na1—Na2v3.363 (13)
O2—Si11.567 (18)Na1—Hf1viii3.513 (12)
O2—Si21.660 (17)Na1—Si1ii2.919 (12)
O3—Hf12.060 (13)Na1—Si13.210 (13)
O3—Si11.644 (15)Na1—Si1vii3.140 (11)
O4—Na12.450 (15)Na1—Si23.132 (13)
O4—Si1ii1.622 (15)Na2—Na2v3.590 (14)
O5—Na1iii2.323 (18)Na2—Na2ix3.174 (13)
O5—Si2iii1.605 (16)Na2—Hf13.556 (9)
O6—Hf1iv2.115 (13)Na2—Hf1iv3.399 (10)
O6—Si2iii1.497 (16)Na2—Hf1v3.590 (10)
O7—Na2v2.441 (18)Na2—Si13.161 (10)
O7—Si2v1.625 (13)Na2—Si2v3.052 (11)
Na1—Na1vi3.453 (14)
Hf1—O1—Si1i161.7 (9)Na2ix—Na2—Si176.2 (3)
Si1—O2—Si2136.8 (8)Na2ix—Na2—Si2v153.7 (4)
Hf1—O3—Si1175.1 (10)Hf1—Na2—Hf1iv102.1 (3)
Na1—O4—Si1ii89.2 (6)Hf1—Na2—Hf1v119.7 (3)
Na1iii—O5—Si2iii104.3 (9)Hf1—Na2—Si166.6 (2)
Hf1iv—O6—Si2iii153.3 (10)Hf1—Na2—Si2v59.6 (2)
Na2v—O7—Si2v134.4 (9)Hf1iv—Na2—Hf1v126.1 (3)
O4—Na1—O5viii97.2 (6)Hf1iv—Na2—Si162.6 (2)
O4—Na1—Na1vi91.7 (4)Hf1iv—Na2—Si2v134.1 (4)
O4—Na1—Na1vii152.6 (6)Hf1v—Na2—Si1102.9 (3)
O4—Na1—Na2v51.1 (4)Hf1v—Na2—Si2v97.7 (3)
O4—Na1—Hf1viii109.2 (5)Si1—Na2—Si2v125.7 (4)
O4—Na1—Si1ii33.8 (4)O1—Hf1—O388.5 (5)
O4—Na1—Si195.1 (5)O1—Hf1—O6ii91.8 (5)
O4—Na1—Si1vii143.7 (5)O1—Hf1—Na1iii51.7 (4)
O4—Na1—Si2103.7 (5)O1—Hf1—Na2ii123.8 (4)
O5viii—Na1—Na1vi114.1 (6)O1—Hf1—Na2131.8 (4)
O5viii—Na1—Na1vii89.9 (5)O1—Hf1—Na2v136.4 (4)
O5viii—Na1—Na2v46.9 (4)O3—Hf1—O6ii171.1 (6)
O5viii—Na1—Hf1viii36.4 (4)O3—Hf1—Na1iii107.6 (5)
O5viii—Na1—Si1ii113.7 (5)O3—Hf1—Na2ii124.6 (5)
O5viii—Na1—Si185.7 (5)O3—Hf1—Na250.0 (4)
O5viii—Na1—Si1vii94.3 (5)O3—Hf1—Na2v71.4 (5)
O5viii—Na1—Si229.8 (4)O6ii—Hf1—Na1iii79.4 (4)
Na1vi—Na1—Na1vii109.5 (3)O6ii—Hf1—Na2ii48.7 (4)
Na1vi—Na1—Na2v116.9 (4)O6ii—Hf1—Na2133.2 (3)
Na1vi—Na1—Hf1viii79.2 (3)O6ii—Hf1—Na2v102.6 (4)
Na1vi—Na1—Si1ii58.3 (3)Na1iii—Hf1—Na2ii127.8 (2)
Na1vi—Na1—Si1158.2 (4)Na1iii—Hf1—Na2111.9 (2)
Na1vi—Na1—Si1vii52.3 (2)Na1iii—Hf1—Na2v171.0 (2)
Na1vi—Na1—Si2141.2 (4)Na2ii—Hf1—Na2102.1 (2)
Na1vii—Na1—Na2v125.8 (4)Na2ii—Hf1—Na2v53.9 (2)
Na1vii—Na1—Hf1viii92.0 (3)Na2—Hf1—Na2v60.3 (2)
Na1vii—Na1—Si1ii156.1 (5)O1i—Si1—O2111.9 (8)
Na1vii—Na1—Si159.0 (3)O1i—Si1—O3113.6 (9)
Na1vii—Na1—Si1vii61.2 (3)O1i—Si1—O4iv109.9 (9)
Na1vii—Na1—Si270.6 (3)O1i—Si1—Na197.9 (6)
Na2v—Na1—Hf1viii71.7 (3)O1i—Si1—Na1iv76.2 (6)
Na2v—Na1—Si1ii76.5 (3)O1i—Si1—Na1vii61.5 (6)
Na2v—Na1—Si183.1 (3)O1i—Si1—Na2150.4 (7)
Na2v—Na1—Si1vii135.6 (5)O2—Si1—O3104.6 (8)
Na2v—Na1—Si255.9 (3)O2—Si1—O4iv105.7 (9)
Hf1viii—Na1—Si1ii104.5 (4)O2—Si1—Na152.6 (6)
Hf1viii—Na1—Si1117.6 (3)O2—Si1—Na1iv77.6 (6)
Hf1viii—Na1—Si1vii64.0 (3)O2—Si1—Na1vii50.5 (5)
Hf1viii—Na1—Si262.2 (3)O2—Si1—Na297.4 (6)
Si1ii—Na1—Si1123.9 (4)O3—Si1—O4iv110.8 (7)
Si1ii—Na1—Si1vii110.6 (4)O3—Si1—Na164.3 (6)
Si1ii—Na1—Si2132.4 (4)O3—Si1—Na1iv167.4 (6)
Si1—Na1—Si1vii120.1 (3)O3—Si1—Na1vii121.8 (6)
Si1—Na1—Si256.5 (3)O3—Si1—Na259.9 (5)
Si1vii—Na1—Si2103.3 (3)O4iv—Si1—Na1150.5 (8)
O7v—Na2—Na1v99.3 (5)O4iv—Si1—Na1iv57.1 (5)
O7v—Na2—Na2v45.5 (4)O4iv—Si1—Na1vii125.7 (6)
O7v—Na2—Na2ix60.6 (4)O4iv—Si1—Na255.6 (5)
O7v—Na2—Hf196.4 (4)Na1—Si1—Na1iv123.9 (4)
O7v—Na2—Hf1iv113.5 (4)Na1—Si1—Na1vii59.9 (3)
O7v—Na2—Hf1v35.8 (3)Na1—Si1—Na2103.3 (3)
O7v—Na2—Si168.4 (4)Na1iv—Si1—Na1vii69.4 (3)
O7v—Na2—Si2v110.4 (5)Na1iv—Si1—Na2107.6 (3)
Na1v—Na2—Na2v91.8 (3)Na1vii—Si1—Na2147.9 (4)
Na1v—Na2—Na2ix97.3 (3)O2—Si2—O5viii100.7 (8)
Na1v—Na2—Hf1117.5 (3)O2—Si2—O6viii106.7 (8)
Na1v—Na2—Hf1iv124.7 (3)O2—Si2—O7v102.8 (8)
Na1v—Na2—Hf1v64.7 (3)O2—Si2—Na155.3 (5)
Na1v—Na2—Si1167.6 (4)O2—Si2—Na2v103.7 (6)
Na1v—Na2—Si2v58.2 (3)O5viii—Si2—O6viii116.1 (9)
Na2v—Na2—Na2ix106.1 (3)O5viii—Si2—O7v109.8 (8)
Na2v—Na2—Hf160.3 (2)O5viii—Si2—Na146.0 (6)
Na2v—Na2—Hf1iv142.8 (3)O5viii—Si2—Na2v53.3 (5)
Na2v—Na2—Hf1v59.4 (2)O6viii—Si2—O7v118.2 (8)
Na2v—Na2—Si180.2 (3)O6viii—Si2—Na1131.2 (6)
Na2v—Na2—Si2v68.4 (3)O6viii—Si2—Na2v149.4 (8)
Na2ix—Na2—Hf1141.7 (3)O7v—Si2—Na1110.3 (6)
Na2ix—Na2—Hf1iv66.1 (3)O7v—Si2—Na2v57.1 (6)
Na2ix—Na2—Hf1v60.0 (3)Na1—Si2—Na2v65.9 (3)
Symmetry codes: (i) x+1, y, z+1; (ii) x, y, z1; (iii) x1, y, z; (iv) x, y, z+1; (v) x+1, y+1, z+1; (vi) x+2, y, z; (vii) x+2, y, z+1; (viii) x+1, y, z; (ix) x+1, y+1, z+2.
Cell parameters, selected distances (Å), angles (°) and volumes (Å3) for the title phase compared to parakeldyshite top
Cell parameters of Na2ZrSi2O7 are from Ferreira et al. (2001).
Na2HfSi2O7Na2ZrSi2O7
a, b, c6.6123 (2), 8.7948 (3), 5.41074 (15)a, b, c6.6364 (4), 8.8120 (5), 5.4233 (3)
α, β, γ92.603 (2), 94.084 (2), 71.326 (2)α, β, γ92.697 (4), 94.204 (3), 71.355 (3)
Vcell297.25 (2)Vcell299.61 (3)
Hf1 octahedronZr octahedron
O1—O74.15 (4)O1—O74.22 (2)
O4—O54.28 (3)O4—O54.24 (2)
O3—O64.22 (4)O3—O64.18 (2)
Hf1—O72.23 (3)Zr—O72.13 (2)
Hf1—O11.92 (3)Zr—O12.08 (2)
Hf1—O32.30 (3)Zr—O32.11 (2)
Hf1—O42.04 (2)Zr—O42.16 (3)
Hf1—O52.26 (3)Zr—O52.09 (3)
Hf1—O61.93 (2)Zr—O62.03 (2)
Hf1—O7—Si2116.7 (6)Zr—O7—Si2124.0 (4)
Polyhedron volume12.5Polyhedron volume12.4
Si1 tetrahedronSi1 tetrahedron
Si1—O2i1.55 (3)Si1—O2i1.62 (2)
Si1—O3i1.37 (4)Si1—O3i1.58 (1)
Si1—O4i1.66 (3)Si1—O4i1.55 (1)
Si1—O11.68 (3)Si1—O11.57 (1)
Polyhedron volume1.94Polyhedron volume2.02
Si2 tetrahedronSi2 tetrahedron
Si2—O21.77 (3)Si2—O21.67 (2)
Si2—O7i1.61 (3)Si2—O7i1.64 (1)
Si2—O51.61 (3)Si2—O51.62 (1)
Si2—O61.60 (3)Si2—O61.53 (1)
Polyhedron volume2.12Polyhedron volume2.14
Symmetry code: (i) -x, -y, -z.
 

Acknowledgements

NM would like to thank Professor Philippe Deniard (Institut des Matériaux de Nantes) for his continuous support of the Rietveld refinements. The authors gratefully acknowledge AREVA and ANDRA for their financial support of this work.

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