Crystal structures and X-ray powder diffraction data for Cs₂NiSi₅O₁₂, RbGaSi₂O₆, and CsGaSi₂O₆ synthetic leucite analogues

Abstract

Leucites are tetrahedrally coordinated silicate framework structures with some of the silicon framework cations partially replaced by divalent or trivalent cations. These structures have general formulae A₂BSi₅O₁₂ and ACSi₂O₆; where A is a monovalent alkali metal cation, B is a divalent cation, and C is a trivalent cation. In this paper, we report the Rietveld refinements of three more synthetic leucite analogues with stoichiometries of Cs₂NiSi₅O₁₂, RbGaSi₂O₆, and CsGaSi₂O₆. Cs₂NiSi₅O₁₂ is Ia$\bar{3}$d cubic and is isostructural with Cs₂CuSi₅O₁₂. RbGaSi₂O₆ is I4₁/a tetragonal and is isostructural with KGaSi₂O₆. CsGaSi₂O₆ is $I\bar{4}3d$ cubic and is isostructural with RbBSi₂O₆.

I. INTRODUCTION

Synthetic anhydrous analogues of the silicate framework minerals leucite (KAlSi₂O₆) and pollucite (CsAlSi₂O₆) can be prepared with the general formulae A ₂BSi₅O₁₂ and ACSi₂O₆; where A is a monovalent alkali metal cation, B is a divalent cation, and C is a trivalent cation. These structures all have the same topology with B and C cations partially substituting onto tetrahedrally coordinated sites (T-sites) in the silicate framework and charge balancing A cations sitting in extra-framework channels. The A cations can be replaced by ion exchange, Cs containing silicate framework minerals are of potential technological interest as storage media for radioactive Cs from nuclear waste (Gatta et al., 2008).

We have used X-ray and neutron powder diffraction to determine and Rietveld refine the ambient temperature crystal structures of leucite analogues with the general formulae A ₂BSi₅O₁₂ and ACSi₂O₆. Crystal structures have been refined in the Ia $\bar{3}$d cubic and I4₁/a tetragonal space groups (A = K, Rb, Cs; B = Mg, Mn, Co, Cu, Zn; C = Fe³⁺, Ga; Bell and Henderson, 1994a, 1994b, 2018, 2020; Bell et al., 1994a, 2010). These structures all have disordered T-site cations and also have A cation sites fully occupied with either K, Rb, or Cs. Crystal structures have also been refined at ambient temperature for P2₁/c monoclinic crystal structures of leucite analogues with the general formulae A ₂BSi₅O₁₂ (A = K, B = Mg, Fe²⁺, Co, Zn; Bell et al., 1994a; Bell and Henderson, 2018) and also for Pbca orthorhombic (A = Rb; B = Mg, Mn, Ni, Cd; Bell and Henderson, 1996, 2009, 2016) and (A = Cs; B = Mg, Mn, Co, Ni, Cu, Zn, Cd; Bell et al., 1994b, 2010; Bell and Henderson, 1996, 2009). These structures all have ordered T-site cations and also have A cation sites fully occupied with either K, Rb, or Cs. Cs₂ZnSi₅O₁₂ undergoes a reversible phase transition from Pbca to $Pa\bar{3}$ on heating to 566 K (Bell and Henderson, 2012). K₂MgSi₅O₁₂ and K₂ZnSi₅O₁₂ both undergo phase transitions from P2₁/c to Pbca on heating to 622 K (K₂MgSi₅O₁₂; Redfern and Henderson, 1996) and over the temperature range 843–868 K (K₂ZnSi₅O₁₂; Bell et al., 2021). KGaSi₂O₆ undergoes a phase transition from I4₁/a to Ia $\bar{3}$d over the temperature range 673–970 K (Bell and Henderson, 2020). Cs₂X ²⁺Si₅O₁₂ (X = Cd, Cu, Zn) all retain the cation ordered Pbca orthorhombic structure below 10 K (Bell, 2021).

RbCsX ²⁺Si₅O₁₂ (X = Mg, Ni, Cd) leucite analogues, with two different extra-framework alkali metal cations, all have the Pbca structure with ordered T-site cations (Bell and Henderson, 2019). For X = Ni and Cd, these structures have disordered extra-framework cations, but, for X = Mg, there is partial extra-framework cation ordering.

In this paper, we report the Rietveld refinements (Rietveld, 1969) of three more T-site cation-disordered synthetic leucite analogues with stoichiometries of Cs₂NiSi₅O₁₂, RbGaSi₂O₆, and CsGaSi₂O₆.

II. EXPERIMENTAL

A. Sample synthesis

All three samples were prepared from appropriate stoichiometric mixtures of Rb₂CO₃, Cs₂CO₃, SiO₂, NiO, and Ga₂O₃. The starting mixture for Cs₂NiSi₅O₁₂ was loaded into a platinum crucible and heated for 24 h at 873 K to decompose the carbonate. The mixture was then heated at 1673 K for 90 min before quenching by dipping the base of the crucible in water. The resultant glass was then reground and heated at 1393 K for 5 days; this produced a dark blue powdered sample. The starting mixtures for RbGaSi₂O₆ and CsGaSi₂O₆ were also loaded into platinum crucibles. Both mixtures were heated at 10 K min⁻¹ from room temperature to 873 K; the temperature was then maintained at 873 K for 12 h, in order to decompose carbonates. The mixtures were then heated to 1473 K at 10 K min⁻¹ and the temperature was then maintained at 1473 K for 10 h. Each crucible was removed from the furnace and quenched by dipping the base of the crucible in water. The resultant glasses were then reground and heated up at 1473 K for 7 days before cooling at 2 K min⁻¹ to room temperature. This produced white powdered samples.

B. X-ray powder diffraction data collection

After heating the samples were removed from the metal capsules, ground with a mortar and pestle and then mounted on low-background silicon wafers with a drop of acetone prior to ambient temperature X-ray powder diffraction (XRD).

For Cs₂NiSi₅O₁₂, data were collected on a PANalytical Empyrean diffractometer using CoKα X-rays with an iron β-filter and a 3.3473° 2θ wide 255 channel PIXCEL-3D area detector. Data were collected in a single scan over 66 h using Data Collector 5.1a (PANalytical, 2014). These data were collected over the range 12–140° 2θ with a step width of 0.0131° 2θ and an effective counting time of 5998 s per point, the beam size was defined with a 20 mm mask, fixed divergence antiscatter (¼°) slit and automatic divergence slit with a 15-mm long beam footprint. These diffracted intensities were converted from the automatic divergence slit mode to the fixed divergence slit mode in HighScore Plus (PANalytical, 2009) prior to data analysis.

For the RbGaSi₂O₆ and CsGaSi₂O₆ samples, data were collected on a PANalytical X'Pert Pro MPD using CuKα X-rays, with a nickel β-filter and a 3.3473° 2θ wide 255 channel PIXCEL-1D area detector. These data were collected over the range 8–100° 2θ with a step width of 0.0131° 2θ using Data Collector 5.5a (PANalytical, 2017). The beam size was defined with a 20 mm mask, fixed antiscatter (¼°) and divergence (⅛°) slits. For RbGaSi₂O₆, a single-scan was collected lasting 33 h and an effective counting time of 4175 s per point. For CsGaSi₂O₆, a single-scan was collected lasting 24 h and an effective counting time of 3035 s per point.

No smoothing or α ₂ stripping was done on any of these data. Both diffractometers were calibrated with an external NIST 640e silicon standard.

C. XRD data analysis

All powder diffraction data were using HighScore Plus and the ICDD Powder Diffraction File. Analysis of the powder diffraction data for Cs₂NiSi₅O₁₂ showed that this sample was single-phase cubic and the position of the Bragg reflections in the powder diffraction data matched the cubic pattern PDF# 00-037-0335 for Cs₂NiSi₅O₁₂. However, analysis of the powder diffraction data for RbGaSi₂O₆ showed that this sample consisted of two phases. The main phase was I4₁/a tetragonal RbGaSi₂O₆ PDF# 00-037-0350 with C2/m monoclinic Ga₂O₃ PDF# 00-043-1012 as a minor phase. A similar analysis of CsGaSi₂O₆ also showed $I\bar{4}3d$ cubic pattern PDF# 00-050-0175 for CsGaSi₂O₆ as the main phase and C2/m monoclinic Ga₂O₃ PDF# 00-043-1012 as a minor phase.

Rietveld refinements were done using FULLPROF (Rodríguez-Carvajal, 1993). Backgrounds were fitted by linear interpolation between a set of background points with refinable heights. The Thompson-Cox-Hastings Pseudo-Voigt function (van Laar and Yelon, 1984), convoluted with asymmetry due to axial divergence (Finger et al., 1994), was used to model the profile shape.

The crystal structure of Cs₂NiSi₅O₁₂ was refined using the Ia $\bar{3}$d cubic structure of Cs₂CuSi₅O₁₂ (Bell et al., 2010) as the starting model. In this starting model, Ni replaced Cu on the disordered T-site. In this crystal structure, there is one Ia $\bar{3}$d 16b Wyckoff special position site which is 100% occupied by Cs, there is one 48 g special position site which is 1/6th occupied by Ni and 5/6th by Si (T-site occupancies were not refined) and there is a 96 h general position 100% occupied by O. A stoichiometry of Cs₂NiSi₅O₁₂ was assumed. The isotropic temperature factors of the T-site atoms Si and Ni were constrained to be the same. It should be noted that one of the authors (AMTB) published a Pbca orthorhombic structure for Cs₂NiSi₅O₁₂, with ordered T-site cations and a, b, and c being very close but slightly different (Bell and Henderson, 1996). However, the XRD data for this sample did not show the slight orthorhombic distortion that was seen in the synchrotron XRD data used for the earlier structure refinement.

The crystal structure of RbGaSi₂O₆ was refined using the I4₁/a structure of KGaSi₂O₆ (Bell and Henderson, 2020) as the starting model with Rb replacing K on the extra-framework cation site. In this crystal structure, all atoms were located on the I4₁/a 16f Wyckoff general position. There is one 16f position for Rb, three 16f positions for T-sites (disordered 1/3rd Ga and 2/3rd Si, T-site occupancies were not refined), and six 16f positions for O. The isotropic temperature factors of the T-site atoms Si and Ga were constrained to be the same on each T-site but were allowed to vary between different T-sites. All isotropic temperature factors for the six O sites were constrained to have the same value. As was done for KGaSi₂O₆ the T–O interatomic distances were soft constrained to be 1.68 ± 0.02 Å (the average bond distance for tetrahedral Si–O and Ga–O) assuming complete T-site disorder (1/3Ga:2/3Si on each T-site) as it was not possible to refine chemically sensible T-site occupancies. Rietveld refinements in noncubic leucite structures without soft interatomic distance constraints tend to give unrealistic interatomic T–O distances.

The crystal structure of CsGaSi₂O₆ was refined using the $I\bar{4}3d$ cubic structure of RbBSi₂O₆ (Filatov et al., 2011) as the starting model. This matched the space group assignment of the CsGaSi₂O₆ PDF# 00-050-0175. In this starting model, Ga replaced B on the disordered T-site, which is occupied by 1/3rd Ga and 2/3rd Si (T-site occupancies were not refined), and Cs replaced Rb on the extra-framework cation site. In this crystal structure, there is one $I\bar{4}3d$ 16c Wyckoff special position site which is 100% occupied by Cs. There are also three $I\bar{4}3d$ 48e Wyckoff general position sites, one of these is occupied by the disordered Ga/Si T-site and two are 100% occupied by O. The Rietveld refinements for RbGaSi₂O₆ and CsGaSi₂O₆ both used the crystal structure of Ga₂O₃ (da Silva et al., 2012) to fit the minor phase. The RbGaSi₂O₆ sample contained 11.8(1) wt.% Ga₂O₃ and the CsGaSi₂O₆ sample contained 9.2(1) wt.% Ga₂O₃.

VESTA (Momma and Izumi, 2011) was used to plot crystal structures.

III. RESULTS AND DISCUSSION

Crystal structures have been refined for Cs₂NiSi₅O₁₂, RbGaSi₂O₆, and CsGaSi₂O₆ leucite analogues from XRD data. All refined structures have disordered T-site cations. Cs₂NiSi₅O₁₂ is isostructural with Ia $\bar{3}$d cubic Cs₂CuSi₅O₁₂; RbGaSi₂O₆ is isostructural with I4₁/a tetragonal KGaSi₂O₆; and CsGaSi₂O₆ is isostructural with $I\bar{4}3d$ cubic RbBSi₂O₆. Table I shows the comparison of the refined lattice parameters for Cs₂NiSi₅O₁₂, RbGaSi₂O₆, and CsGaSi₂O₆ with the Cs₂CuSi₅O₁₂ (Bell et al., 2010), KGaSi₂O₆ (Bell and Henderson, 2020), and RbBSi₂O₆ (Filatov et al., 2011) starting structures used for Rietveld refinement. Tables II–VII similarly show refined interatomic distances and angles; the mean T–O distances are close to the constraint distances, the mean O–T–O angles are close to the ideal tetrahedral angle of 109.47°. Table VIII similarly shows the tetrahedral angle variances for the T-sites (Robinson et al., 1971) in the silicate framework structures.

TABLE I. Refined lattice parameters compared with those for starting structures

TABLE II. Refined interatomic distances, Ia $\bar{3}$d cubic structures

TABLE III. Refined interatomic angles, Ia $\bar{3}$d cubic structures

TABLE IV. Refined interatomic distances (Å)

I4₁/a tetragonal structures. AGaSi₂O₆ (A = K, Rb). T = disordered Si/Ga.

TABLE V. Refined interatomic angles (°)

I4₁/a tetragonal structures. AGaSi₂O₆ (A = K, Rb). T = disordered Si/Ga.

TABLE VI. Refined interatomic distances (Å)

$I\bar{4}3d$ cubic structures. A = Cs/Rb, T = disordered Si/Ga and Si/B.

TABLE VII. Refined interatomic angles (°)

$I\bar{4}3d$ cubic structures. A = Cs/Rb, T = disordered Si/Ga and Si/B

A. Cs₂NiSi₅O₁₂ structure

Figures 1 and 2 respectively show the Rietveld difference and the VESTA crystal structure plots for the refined crystal structure of Cs₂NiSi₅O₁₂. Table I shows that this crystal structure has a unit cell volume that is slightly larger than the isostructural Cs₂CuSi₅O₁₂ which was used as a starting model for Rietveld refinement. However, the ionic radius (Shannon, 1976) for Ni²⁺ (0.69 Å) is smaller than that for Cu²⁺ (0.71 Å), it would be expected that the smaller ionic radius for Ni²⁺ would result in a smaller unit cell compared with that for Cs₂CuSi₅O₁₂. This discrepancy may be due to Cs₂NiSi₅O₁₂ not having the assumed stoichiometry. Table II shows that one set of Cs–O distances is slightly larger for Cs₂NiSi₅O₁₂ compared with Cs₂CuSi₅O₁₂. However, the other set of Cs–O distances are equivalent between error limits. Table II also shows that both sets of T–O distances are equivalent between error limits for Cs₂NiSi₅O₁₂ and Cs₂CuSi₅O₁₂. Table III shows that two of the six O–T–O angles are different between error limits for Cs₂NiSi₅O₁₂ and Cs₂CuSi₅O₁₂, but the T–O–T angles are equivalent between error limits for Cs₂NiSi₅O₁₂ and Cs₂CuSi₅O₁₂. Table VIII shows that tetrahedral distortions for Cs₂NiSi₅O₁₂ and Cs₂CuSi₅O₁₂ are very close.

Figure 1. Rietveld difference plot for Cs₂NiSi₅O₁₂. Red circles represent observed data points, the blue line represents calculated data points, the green line represents difference curve, and black crosses represent positions of Bragg reflections. R-factors for this refinement were: R _p = 21.5314%, R _wp = 11.5115%, R _exp = 4.7035%, χ ² = 6.2111.

Figure 2. VESTA Ia $\bar{3}$d cubic structure plot for Cs₂Ni²⁺Si₅O₁₂, viewed down [111] showing a channel for extra-framework light blue Cs⁺ cations. Disordered (Si/Ni)O₄ tetrahedra are shown in gray with O²⁻ anions are shown in red.

B. RbGaSi₂O₆ structure

Figures 3 and 4 respectively show the Rietveld difference and the VESTA crystal structure plots for the refined crystal structure of RbGaSi₂O₆. Note that Figure 4 shows that this tetragonal crystal structure has a slightly collapsed silicate framework structure compared with the cubic structure in Figure 2. Table I shows that the crystal structure of RbGaSi₂O₆ has a larger unit cell volume than that of KGaSi₂O₆ which was used as a starting model for Rietveld refinement, this reflects the difference in the ionic radii for Rb⁺ (1.72 Å) and K⁺ (1.64 Å) cations (Shannon, 1976). However, the ambient temperature c/a ratio for RbGaSi₂O₆ is 1.032, which is smaller than the ambient temperature c/a ratio for KGaSi₂O₆, which is 1.053. When KGaSi₂O₆ is heated the c/a ratio decreases before a phase transition from I4₁/a to Ia $\bar{3}$d over the temperature range 673–970 K (Bell and Henderson, 2020). It would be interesting to do a high-temperature XRD experiment on RbGaSi₂O₆ as this smaller c/a ratio would suggest that this leucite analogue would undergo a phase transition from I4₁/a to Ia $\bar{3}$d at a lower temperature than KGaSi₂O₆. Table IV shows larger A–O distances for A = Rb compared with A = K, also reflecting the ionic radii difference. The mean T–O distances for RbGaSi₂O₆ and KGaSi₂O₆ are the same within error limits. Table IV shows that the mean O–T–O and T–O–T angles for RbGaSi₂O₆ and KGaSi₂O₆ are the same within error limits. Table VIII shows that mean tetrahedral distortion for KGaSi₂O₆ is larger than that for RbGaSi₂O₆, this reflects the greater collapse (Taylor and Henderson, 1968) of the silicate framework structure with the smaller K⁺ cation compared with the larger Rb⁺ cation.

Figure 3. Rietveld difference plot for RbGaSi₂O₆. Red circles represent observed data points, the blue line represents calculated data points, and the green line represents difference curve. The upper line of black crosses represent positions of Bragg reflections for RbGaSi₂O₆ and the lower line of black crosses represent positions of Bragg reflections for Ga₂O₃ impurity. R-factors for this refinement were: R _p = 10.1855%, R _wp = 8.2308%, R _exp = 2.7240%, χ ² = 9.2667.

Figure 4. VESTA I4₁/a tetragonal structure plot for RbGaSi₂O₆, viewed down [1–11] showing a channel for extra-framework pink Rb⁺ cations. Disordered (Si/Ga)O₄ tetrahedra are shown in light green with O²⁻ anions are shown in red.

TABLE VIII. Tetrahedral angle variance [σ ², deg²]: σ ²= Σ(θ–109.47)²/5 (Robinson et al., 1971), where θ is the O–T–O tetrahedral angle.

Mean variance and standard deviation is given for the three tetrahedral sites in each I4₁/a structure. Variance is given for the single tetrahedral site in each Ia $\bar{3}$d and $I\bar{4}3d$ structure.

C. CsGaSi₂O₆ structure

Figures 5 and 6 respectively show the Rietveld difference and the VESTA crystal structure plots for the refined crystal structure of CsGaSi₂O₆. Table I shows that the crystal structure of CsGaSi₂O₆ has a larger a unit-cell volume than that of RbBSi₂O₆ which was used as a starting model for Rietveld refinement, this reflects the differences in the ionic radii for Cs⁺ (2.02 Å), Ga³⁺ (0.61 Å), Rb⁺ (1.86 Å), and B³⁺ (0.25 Å) cations (Shannon, 1976). CsGaSi₂O₆ has the $I\bar{4}3d$ cubic crystal structure, unlike RbGaSi₂O₆, which has the I4₁/a tetragonal structure. These differences in cation size mean that the silicate framework for the RbGaSi₂O₆ is more collapsed (Taylor and Henderson, 1968) than for CsGaSi₂O₆, and consequently, there is a lowering of symmetry for the crystal structure. Table VI shows that for CsGaSi₂O₆ the A–O and T–O distances are larger than those for the RbBSi₂O₆ due to the differences in ionic radii for the cations present in these crystal structures. Table VII shows that three of the six O–T–O angles are different between error limits for CsGaSi₂O₆ and RbBSi₂O₆. The mean T–O–T angles for CsGaSi₂O₆ are smaller than those for RbBSi₂O₆, reflecting the greater framework collapse of RbBSi₂O₆ compared with CsGaSi₂O₆. Table VIII shows that tetrahedral distortion for CsGaSi₂O₆ is larger than that for RbBSi₂O₆. This reflects the greater distortion of the silicate framework structure by incorporation of the larger Ga³⁺ cation into the framework compared with the smaller B³⁺ cation.

Figure 5. Rietveld difference plot for CsGaSi₂O₆. Red circles represent observed data points, the blue line represents calculated data points, and the green line represents difference curve. The upper line of black crosses represent positions of Bragg reflections for CsGaSi₂O₆ and the lower line of black crosses represent positions of Bragg reflections for Ga₂O₃ impurity. R-factors for this refinement were: R _p = 9.9377%, R _wp = 8.0887%, R _exp = 2.5680%, χ ² = 10.4871.

Figure 6. VESTA $I\bar{4}3d$ cubic structure plot for CsGaSi₂O₆, viewed down [111] showing a channel for extra-framework light blue Cs⁺ cations. Disordered (Si/Ga)O₄ tetrahedra are shown in light green with O²⁻ anions are shown in red.

The R-factors for this refinement of the CsGaSi₂O₆ crystal structure in $I\bar{4}3d$ were: R _p = 9.9377%, R _wp = 8.0887%, R _exp = 2.5680%, χ ² = 10.4871. However, it should be noted that a refinement of the CsGaSi₂O₆ crystal structure in Ia $\bar{3}$d, using the cubic structure of CsAlSi₂O₆ (Yanase et al., 1997) as a starting structure, gave the following R-factors: R _p = 10.3604%, R _wp = 8.4084%, R _exp = 2.5708%, χ ² = 11.5234. These R-factors are only slightly worse than those for $I\bar{4}3d$, suggesting that the crystal structures in these two different space groups show some similarities. Ia $\bar{3}$d is a supergroup of $I\bar{4}3d$, it would also be interesting to try a high-temperature XRD experiment on CsGaSi₂O₆ to see if there might be a phase transition from $I\bar{4}3d$ to Ia $\bar{3}$d.

IV. CONCLUSIONS

Crystal structures have been refined for Cs₂NiSi₅O₁₂, RbGaSi₂O₆, and CsGaSi₂O₆ synthetic leucite analogues. All refined structures have disordered T-site cations. Cs₂NiSi₅O₁₂ is isostructural with Ia $\bar{3}$d cubic Cs₂CuSi₅O₁₂; RbGaSi₂O₆ is isostructural with I4₁/a tetragonal KGaSi₂O₆, and CsGaSi₂O₆ is isostructural with $I\bar{4}3d$ cubic RbBSi₂O₆.

V. DEPOSITED DATA

CIF files with information related to crystal structure, interatomic distances, and angles, and powder diffraction data for Cs₂NiSi₅O₁₂, RbGaSi₂O₆, and CsGaSi₂O₆ synthetic leucite analogues were deposited with the ICDD. You may request these data from ICDD at [email protected].