Crystal structure of Na2HfSi2O7 by Rietveld refinement.

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 octa-hedra and SiO4 tetra-hedra 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.

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 Na2HfSi2O7 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 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 ▸. 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 ▸). 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 ▸). The structures’ similarities were estimated by different parameters such as the measure of similarity Δ (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 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 ▸).

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)
V cell	297.25 (2)	V cell	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

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.

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 ▸). 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 ▸), the triclinic phase is privileged such as the parakeldyshite Na2ZrSi2O7 phase (Ferreira et al., 2001 ▸) or the keldyshite (Na,H)2ZrSi2O7 phase (Khalilov et al., 1978 ▸). The potassium analogue, however, is monoclinic as in the case of khibinskite K2ZrSi2O7 (Chernov et al., 1970 ▸; Nosyrev et al., 1976 ▸).

Synthesis and crystallization

The synthesis of sodium hafnium disilicate was based on the two-step synthesis protocol of parakeldyshite Na2ZrSi2O7 (Lin et al., 1999 ▸; Ferreira et al., 2001 ▸). 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 ▸).

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2 ▸. Observed and calculated intensities for Na2HfSi2O7 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 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
M r	392.6
Crystal system, space group	Triclinic, P
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)
V (Å3)	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	R p = 0.024, R wp = 0.032, R exp = 0.015, R(F) = 0.024, χ2 = 4.973
No. of parameters	67

Computer programs: X’Pert Data Collector (PANalytical, 2011 ▸), JANA2006 (Petříček et al., 2014 ▸), VESTA (Momma & Izumi, 2011 ▸) and publCIF (Westrip, 2010 ▸).

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.

Crystal structure: contains datablock(s) nahfsio, I. DOI: 10.1107/S2056989016014225/vn2115sup1.cif

CCDC reference: 1502957

Additional supporting information: crystallographic information; 3D view; checkCIF report

Na2HfSi2O7	Z = 2
Mr = 392.6	F(000) = 356
Triclinic, P1	Dx = 4.387 Mg m−3
a = 6.6123 (2) Å	Cu Kα1 radiation, λ = 1.540562, 1.544390 Å
b = 8.7948 (3) Å	T = 293 K
c = 5.41074 (15) Å	Particle 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

Panalytical XPert MPD Pro diffractometer	Data collection mode: reflection
Radiation source: sealed X-ray tube	Scan method: step
Specimen mounting: packed powder pellet	2θmin = 8.013°, 2θmax = 120.013°, 2θstep = 0.017°

Rp = 0.024	67 parameters
Rwp = 0.032	0 restraints
Rexp = 0.015	6 constraints
R(F) = 0.024	Weighting scheme based on measured s.u.'s
6423 data points	(Δ/σ)max = 0.005
Profile function: pseudo-Voigt	Background function: Legendre polynoms

	x	y	z	Uiso*/Ueq
Na1	0.8823 (15)	0.0970 (12)	0.2630 (18)	0.035 (4)*
Na2	0.3456 (14)	0.5024 (10)	0.7608 (18)	0.011 (3)*
Hf1	0.2880 (3)	0.2710 (2)	0.2201 (3)	0.0158 (7)
Si1	0.6528 (13)	0.1435 (9)	0.7769 (13)	0.010 (3)*
Si2	0.9381 (12)	0.3380 (8)	0.6861 (15)	0.006 (2)*
O1	0.307 (2)	0.0382 (17)	0.172 (3)	0.0030 (15)*
O2	0.865 (2)	0.1777 (14)	0.727 (2)	0.0030 (15)*
O3	0.494 (2)	0.2081 (15)	0.530 (3)	0.0030 (15)*
O4	0.562 (2)	0.2522 (15)	0.020 (3)	0.0030 (15)*
O5	−0.003 (2)	0.3116 (14)	0.401 (3)	0.0030 (15)*
O6	0.120 (2)	0.3300 (14)	0.872 (3)	0.0030 (15)*
O7	0.284 (2)	0.5148 (14)	0.288 (3)	0.0030 (15)*

	U11	U22	U33	U12	U13	U23
Hf1	0.0148 (10)	0.0177 (11)	0.0126 (10)	0.0003 (7)	0.0035 (7)	0.0111 (8)

O1—Hf1	2.017 (15)	Na1—Na1vii	3.169 (13)
O1—Si1i	1.568 (17)	Na1—Na2v	3.363 (13)
O2—Si1	1.567 (18)	Na1—Hf1viii	3.513 (12)
O2—Si2	1.660 (17)	Na1—Si1ii	2.919 (12)
O3—Hf1	2.060 (13)	Na1—Si1	3.210 (13)
O3—Si1	1.644 (15)	Na1—Si1vii	3.140 (11)
O4—Na1	2.450 (15)	Na1—Si2	3.132 (13)
O4—Si1ii	1.622 (15)	Na2—Na2v	3.590 (14)
O5—Na1iii	2.323 (18)	Na2—Na2ix	3.174 (13)
O5—Si2iii	1.605 (16)	Na2—Hf1	3.556 (9)
O6—Hf1iv	2.115 (13)	Na2—Hf1iv	3.399 (10)
O6—Si2iii	1.497 (16)	Na2—Hf1v	3.590 (10)
O7—Na2v	2.441 (18)	Na2—Si1	3.161 (10)
O7—Si2v	1.625 (13)	Na2—Si2v	3.052 (11)
Na1—Na1vi	3.453 (14)
Hf1—O1—Si1i	161.7 (9)	Na2ix—Na2—Si1	76.2 (3)
Si1—O2—Si2	136.8 (8)	Na2ix—Na2—Si2v	153.7 (4)
Hf1—O3—Si1	175.1 (10)	Hf1—Na2—Hf1iv	102.1 (3)
Na1—O4—Si1ii	89.2 (6)	Hf1—Na2—Hf1v	119.7 (3)
Na1iii—O5—Si2iii	104.3 (9)	Hf1—Na2—Si1	66.6 (2)
Hf1iv—O6—Si2iii	153.3 (10)	Hf1—Na2—Si2v	59.6 (2)
Na2v—O7—Si2v	134.4 (9)	Hf1iv—Na2—Hf1v	126.1 (3)
O4—Na1—O5viii	97.2 (6)	Hf1iv—Na2—Si1	62.6 (2)
O4—Na1—Na1vi	91.7 (4)	Hf1iv—Na2—Si2v	134.1 (4)
O4—Na1—Na1vii	152.6 (6)	Hf1v—Na2—Si1	102.9 (3)
O4—Na1—Na2v	51.1 (4)	Hf1v—Na2—Si2v	97.7 (3)
O4—Na1—Hf1viii	109.2 (5)	Si1—Na2—Si2v	125.7 (4)
O4—Na1—Si1ii	33.8 (4)	O1—Hf1—O3	88.5 (5)
O4—Na1—Si1	95.1 (5)	O1—Hf1—O6ii	91.8 (5)
O4—Na1—Si1vii	143.7 (5)	O1—Hf1—Na1iii	51.7 (4)
O4—Na1—Si2	103.7 (5)	O1—Hf1—Na2ii	123.8 (4)
O5viii—Na1—Na1vi	114.1 (6)	O1—Hf1—Na2	131.8 (4)
O5viii—Na1—Na1vii	89.9 (5)	O1—Hf1—Na2v	136.4 (4)
O5viii—Na1—Na2v	46.9 (4)	O3—Hf1—O6ii	171.1 (6)
O5viii—Na1—Hf1viii	36.4 (4)	O3—Hf1—Na1iii	107.6 (5)
O5viii—Na1—Si1ii	113.7 (5)	O3—Hf1—Na2ii	124.6 (5)
O5viii—Na1—Si1	85.7 (5)	O3—Hf1—Na2	50.0 (4)
O5viii—Na1—Si1vii	94.3 (5)	O3—Hf1—Na2v	71.4 (5)
O5viii—Na1—Si2	29.8 (4)	O6ii—Hf1—Na1iii	79.4 (4)
Na1vi—Na1—Na1vii	109.5 (3)	O6ii—Hf1—Na2ii	48.7 (4)
Na1vi—Na1—Na2v	116.9 (4)	O6ii—Hf1—Na2	133.2 (3)
Na1vi—Na1—Hf1viii	79.2 (3)	O6ii—Hf1—Na2v	102.6 (4)
Na1vi—Na1—Si1ii	58.3 (3)	Na1iii—Hf1—Na2ii	127.8 (2)
Na1vi—Na1—Si1	158.2 (4)	Na1iii—Hf1—Na2	111.9 (2)
Na1vi—Na1—Si1vii	52.3 (2)	Na1iii—Hf1—Na2v	171.0 (2)
Na1vi—Na1—Si2	141.2 (4)	Na2ii—Hf1—Na2	102.1 (2)
Na1vii—Na1—Na2v	125.8 (4)	Na2ii—Hf1—Na2v	53.9 (2)
Na1vii—Na1—Hf1viii	92.0 (3)	Na2—Hf1—Na2v	60.3 (2)
Na1vii—Na1—Si1ii	156.1 (5)	O1i—Si1—O2	111.9 (8)
Na1vii—Na1—Si1	59.0 (3)	O1i—Si1—O3	113.6 (9)
Na1vii—Na1—Si1vii	61.2 (3)	O1i—Si1—O4iv	109.9 (9)
Na1vii—Na1—Si2	70.6 (3)	O1i—Si1—Na1	97.9 (6)
Na2v—Na1—Hf1viii	71.7 (3)	O1i—Si1—Na1iv	76.2 (6)
Na2v—Na1—Si1ii	76.5 (3)	O1i—Si1—Na1vii	61.5 (6)
Na2v—Na1—Si1	83.1 (3)	O1i—Si1—Na2	150.4 (7)
Na2v—Na1—Si1vii	135.6 (5)	O2—Si1—O3	104.6 (8)
Na2v—Na1—Si2	55.9 (3)	O2—Si1—O4iv	105.7 (9)
Hf1viii—Na1—Si1ii	104.5 (4)	O2—Si1—Na1	52.6 (6)
Hf1viii—Na1—Si1	117.6 (3)	O2—Si1—Na1iv	77.6 (6)
Hf1viii—Na1—Si1vii	64.0 (3)	O2—Si1—Na1vii	50.5 (5)
Hf1viii—Na1—Si2	62.2 (3)	O2—Si1—Na2	97.4 (6)
Si1ii—Na1—Si1	123.9 (4)	O3—Si1—O4iv	110.8 (7)
Si1ii—Na1—Si1vii	110.6 (4)	O3—Si1—Na1	64.3 (6)
Si1ii—Na1—Si2	132.4 (4)	O3—Si1—Na1iv	167.4 (6)
Si1—Na1—Si1vii	120.1 (3)	O3—Si1—Na1vii	121.8 (6)
Si1—Na1—Si2	56.5 (3)	O3—Si1—Na2	59.9 (5)
Si1vii—Na1—Si2	103.3 (3)	O4iv—Si1—Na1	150.5 (8)
O7v—Na2—Na1v	99.3 (5)	O4iv—Si1—Na1iv	57.1 (5)
O7v—Na2—Na2v	45.5 (4)	O4iv—Si1—Na1vii	125.7 (6)
O7v—Na2—Na2ix	60.6 (4)	O4iv—Si1—Na2	55.6 (5)
O7v—Na2—Hf1	96.4 (4)	Na1—Si1—Na1iv	123.9 (4)
O7v—Na2—Hf1iv	113.5 (4)	Na1—Si1—Na1vii	59.9 (3)
O7v—Na2—Hf1v	35.8 (3)	Na1—Si1—Na2	103.3 (3)
O7v—Na2—Si1	68.4 (4)	Na1iv—Si1—Na1vii	69.4 (3)
O7v—Na2—Si2v	110.4 (5)	Na1iv—Si1—Na2	107.6 (3)
Na1v—Na2—Na2v	91.8 (3)	Na1vii—Si1—Na2	147.9 (4)
Na1v—Na2—Na2ix	97.3 (3)	O2—Si2—O5viii	100.7 (8)
Na1v—Na2—Hf1	117.5 (3)	O2—Si2—O6viii	106.7 (8)
Na1v—Na2—Hf1iv	124.7 (3)	O2—Si2—O7v	102.8 (8)
Na1v—Na2—Hf1v	64.7 (3)	O2—Si2—Na1	55.3 (5)
Na1v—Na2—Si1	167.6 (4)	O2—Si2—Na2v	103.7 (6)
Na1v—Na2—Si2v	58.2 (3)	O5viii—Si2—O6viii	116.1 (9)
Na2v—Na2—Na2ix	106.1 (3)	O5viii—Si2—O7v	109.8 (8)
Na2v—Na2—Hf1	60.3 (2)	O5viii—Si2—Na1	46.0 (6)
Na2v—Na2—Hf1iv	142.8 (3)	O5viii—Si2—Na2v	53.3 (5)
Na2v—Na2—Hf1v	59.4 (2)	O6viii—Si2—O7v	118.2 (8)
Na2v—Na2—Si1	80.2 (3)	O6viii—Si2—Na1	131.2 (6)
Na2v—Na2—Si2v	68.4 (3)	O6viii—Si2—Na2v	149.4 (8)
Na2ix—Na2—Hf1	141.7 (3)	O7v—Si2—Na1	110.3 (6)
Na2ix—Na2—Hf1iv	66.1 (3)	O7v—Si2—Na2v	57.1 (6)
Na2ix—Na2—Hf1v	60.0 (3)	Na1—Si2—Na2v	65.9 (3)