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

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 (KAlSi 2 O 6 ) and pollucite (CsAlSi 2 O 6 ) can be prepared with the general formulae A 2 BSi 5 O 12 and ACSi 2 O 6 ; 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 2 BSi 5 O 12 and ACSi 2 O 6 . Crystal structures have been refined in the Ia 3d cubic and I4 1 /a tetragonal space groups (A = K, Rb, Cs; B = Mg, Mn, Co, Cu, Zn; C = Fe 3+ , Ga; , 2018, 2020Bell et al., , 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 P2 1 /c monoclinic crystal structures of leucite analogues with the general formulae A 2 BSi 5 O 12 (A = K, B = Mg, Fe 2+ , Co, Zn; 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., , 2010Bell andHenderson, 1996, 2009). These structures all have ordered T-site cations and also have A cation sites fully occupied with either K, Rb, or Cs. Cs 2 ZnSi 5 O 12 undergoes a reversible phase transition from Pbca to Pa 3 on heating to 566 K (Bell and Henderson, 2012). K 2 MgSi 5 O 12 and K 2 ZnSi 5 O 12 both undergo phase transitions from P2 1 /c to Pbca on heating to 622 K (K 2 MgSi 5 O 12 ; Redfern and Henderson, 1996) and over the temperature range 843-868 K (K 2 ZnSi 5 O 12 ; Bell et al., 2021). KGaSi 2 O 6 undergoes a phase transition from I4 1 /a to Ia 3d over the temperature range 673-970 K (Bell and Henderson, 2020). Cs 2 X 2+ Si 5 O 12 (X = Cd, Cu, Zn) all retain the cation ordered Pbca orthorhombic structure below 10 K (Bell, 2021).
RbCsX 2+ Si 5 O 12 (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.

A. Sample synthesis
All three samples were prepared from appropriate stoichiometric mixtures of Rb 2 CO 3 , Cs 2 CO 3 , SiO 2 , NiO, and Ga 2 O 3 . The starting mixture for Cs 2 NiSi 5 O 12 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 a) Author to whom correspondence should be addressed. Electronic mail: anthony.bell@shu.ac.uk powdered sample. The starting mixtures for RbGaSi 2 O 6 and CsGaSi 2 O 6 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 Cs 2 NiSi 5 O 12 , 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 RbGaSi 2 O 6 and CsGaSi 2 O 6 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 RbGaSi 2 O 6 , a single-scan was collected lasting 33 h and an effective counting time of 4175 s per point. For CsGaSi 2 O 6 , 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 Cs 2 NiSi 5 O 12 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 Cs 2 NiSi 5 O 12 . However, analysis of the powder diffraction data for RbGaSi 2 O 6 showed that this sample consisted of two phases. The main phase was I4 1 /a tetragonal RbGaSi 2 O 6 PDF# 00-037-0350 with C2/m monoclinic Ga 2 O 3 PDF# 00-043-1012 as a minor phase. A similar analysis of CsGaSi 2 O 6 also showed I 43d cubic pattern PDF# 00-050-0175 for CsGaSi 2 O 6 as the main phase and C2/m monoclinic Ga 2 O 3 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 Cs 2 NiSi 5 O 12 was refined using the Ia 3d cubic structure of Cs 2 CuSi 5 O 12 (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 3d 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 Cs 2 NiSi 5 O 12 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 Cs 2 NiSi 5 O 12 , 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 RbGaSi 2 O 6 was refined using the I4 1 /a structure of KGaSi 2 O 6 (Bell and Henderson, 2020) as the starting model with Rb replacing K on the extraframework cation site. In this crystal structure, all atoms were located on the I4 1 /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 KGaSi 2 O 6 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 CsGaSi 2 O 6 was refined using the I 43d cubic structure of RbBSi 2 O 6 (Filatov et al., 2011) as the starting model. This matched the space group assignment of the CsGaSi 2 O 6 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 43d 16c Wyckoff special position site which is 100% occupied by Cs. There are also three I 43d 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 (Bell et al., 2010), KGaSi 2 O 6 (Bell and Henderson, 2020), and RbBSi 2 O 6 (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.
A. Cs 2 NiSi 5 O 12 structure Figures 1 and 2 respectively show the Rietveld difference and the VESTA crystal structure plots for the refined crystal structure of Cs 2 NiSi 5 O 12 . Table I shows that this crystal structure has a unit cell volume that is slightly larger than the isostructural Cs 2 CuSi 5 O 12 which was used as a starting model for Rietveld refinement. However, the ionic radius (Shannon, 1976) for Ni 2+ (0.69 Å) is smaller than that for Cu 2+ (0.71 Å), it would be expected that the smaller ionic radius for Ni 2+ would result in a smaller unit cell compared with that for Cs 2 CuSi 5 O 12 . This discrepancy may be due to Cs 2 NiSi 5 O 12 not having the assumed stoichiometry. Table II shows that one set of Cs-O distances is slightly larger for Cs 2 NiSi 5 O 12 compared with Cs 2 CuSi 5 O 12 . However, the other set of Cs-O distances are equivalent between error limits. Figures 3 and 4 respectively show the Rietveld difference and the VESTA crystal structure plots for the refined crystal structure of RbGaSi 2 O 6 . 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 RbGaSi 2 O 6 has a larger unit cell volume than that of KGaSi 2 O 6 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 RbGaSi 2 O 6 is 1.032, which is smaller than the ambient temperature c/a ratio for KGaSi 2 O 6 , which is 1.053. When KGaSi 2 O 6 is heated the c/a ratio decreases before a phase transition from I4 1 /a to Ia 3d over the temperature range 673-970 K (Bell and Henderson, 2020). It would be interesting to do a hightemperature XRD experiment on RbGaSi 2 O 6 as this smaller c/a ratio would suggest that this leucite analogue would undergo a phase transition from I4 1 /a to Ia 3d at a lower temperature than KGaSi 2 O 6 .  (Taylor and Henderson, 1968) of the silicate framework structure with the smaller K + cation compared with the larger Rb + cation.

C. CsGaSi 2 O 6 structure
Figures 5 and 6 respectively show the Rietveld difference and the VESTA crystal structure plots for the refined crystal structure of CsGaSi 2 O 6 . Table I shows that the crystal structure of CsGaSi 2 O 6 has a larger a unit-cell volume than that of RbBSi 2 O 6 which was used as a starting model for Rietveld refinement, this reflects the differences in the ionic radii for Cs + (2.02 Å), Ga 3+ (0.61 Å), Rb + (1.86 Å), and B 3+ (0.25 Å) cations (Shannon, 1976). CsGaSi 2 O 6 has the I 43d cubic crystal structure, unlike RbGaSi 2 O 6 , which has the I4 1 /a tetragonal structure. These differences in cation size mean that the silicate framework for the RbGaSi 2 O 6 is more collapsed (Taylor and Henderson, 1968) than for CsGaSi 2 O 6 , and consequently, there is a lowering of symmetry for the crystal structure. Table VI shows that for CsGaSi 2 O 6 the A-O and T-O distances are larger than those for the RbBSi 2 O 6 due to the differences in ionic radii for the cations present in these crystal structures.    different between error limits for CsGaSi 2 O 6 and RbBSi 2 O 6 . The mean T-O-T angles for CsGaSi 2 O 6 are smaller than those for RbBSi 2 O 6 , reflecting the greater framework collapse of RbBSi 2 O 6 compared with CsGaSi 2 O 6 . Table VIII shows that tetrahedral distortion for CsGaSi 2 O 6 is larger than that for RbBSi 2 O 6 . This reflects the greater distortion of the silicate framework structure by incorporation of the larger Ga 3+ cation into the framework compared with the smaller B 3+ cation. The R-factors for this refinement of the CsGaSi 2 O 6 crystal structure in I 43d 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 CsGaSi 2 O 6 crystal structure in Ia 3d, using the cubic structure of CsAlSi 2 O 6 (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 43d, suggesting that the crystal structures in these two different space groups show some similarities. Ia 3d is a supergroup of I 43d, it would also be interesting to try a high-temperature XRD experiment on CsGaSi 2 O 6 to see if there might be a phase transition from I 43d to Ia 3d.
V. DEPOSITED DATA CIF files with information related to crystal structure, interatomic distances, and angles, and powder diffraction data for Cs 2 NiSi 5 O 12 , RbGaSi 2 O 6 , and CsGaSi 2 O 6 synthetic leucite analogues were deposited with the ICDD. You may request these data from ICDD at info@icdd.com.