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 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). 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.

2. 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 ų. 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 (ų) 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)
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—O2^i	1.55 (3)	Si1—O2^i	1.62 (2)
Si1—O3^i	1.37 (4)	Si1—O3^i	1.58 (1)
Si1—O4^i	1.66 (3)	Si1—O4^i	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—O7^i	1.61 (3)	Si2—O7^i	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.

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). 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).

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; 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 Na_1.7±0.2Hf_1.0Si_2.3±0.1O_7.3±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).

5. 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 ų, 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
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 (Å^3)	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, χ^2 = 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.

Supporting information

Computing details

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

Crystal data

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) Å^3

Data collection

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°

Refinement

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

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	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)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Hf1	0.0148 (10)	0.0177 (11)	0.0126 (10)	0.0003 (7)	0.0035 (7)	0.0111 (8)

Geometric parameters (Å, º)

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

Symmetry codes: (i) −x+1, −y, −z+1; (ii) x, y, z−1; (iii) x−1, 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.