Orthorhombic-Cubic Phase Transition in Rb₂CoSi₅O₁₂ Leucite Analogue

Abstract

An Rb₂CoSi₅O₁₂ leucite analogue has been synthesized. An ambient temperature X-ray powder diffraction study shows that this analogue has the Pbca orthorhombic structure of Cs₂CdSi₅O₁₂. A high temperature X-ray powder diffraction study on this analogue shows a Pbca orthorhombic to $Pa\overline{3}$ cubic phase transition at 457 K. The Rb₂CoSi₅O₁₂ unit cell volume initially decreases with increasing temperature on passing through this phase transition.

1. Introduction

Synthetic analogues of the silicate framework minerals leucite KAlSi₂O₆ [1] and pollucite CsAlSi₂O₆ [2] can be prepared with the general formulae ABSi₂O₆ and A₂CSi₅O₁₂. A is an alkali metal cation (K, Rb, Cs), B is a trivalent cation (Al, B, Fe³⁺, Ga) and C is a divalent cation (Be, Mg, Mn, Fe²⁺, Co, Ni, Cu, Zn, Cd). These structures have tetrahedrally coordinated silicate frameworks with B or C cations partially substituting for Si on the tetrahedrally coordinated silicon sites (T-sites). A cations sit in the extra framework channels; these A cations can be removed by ion exchange which makes them of technological interest as possible storage media for radioactive Cs from nuclear waste [3].

Leucite analogues with high symmetry structures such as I4₁/a tetragonal KGaSi₂O₆ [4] and Ia-3d cubic Rb₂ZnSi₅O₁₂ [5] have B and C cations disordered over the T-sites. However, lower symmetry leucite structures are known where C cations are ordered onto separate T-sites. The P2₁/c monoclinic K₂MgSi₅O₁₂ [6] structure has twelve crystallographically distinct and fully ordered T-sites, ten of these are fully occupied by Si and two are fully occupied by Mg. Three more K₂CSi₅O₁₂ (C = Fe²⁺, Co, Zn) structures [7] are known which are isostructural with P2₁/c monoclinic K₂MgSi₅O₁₂ and have fully ordered T-sites. The Pbca orthorhombic structure of Cs₂CdSi₅O₁₂ [8] has six crystallographically distinct and fully ordered T-sites, five of these are fully occupied by Si and one is fully occupied by Cd. Five more structures with the general formula Cs₂CSi₅O₁₂ (C = Mg, Mn, Co, Cu, Zn) [9,10,11], four structures with the general formula Rb₂CSi₅O₁₂ (C = Mg, Mn, Ni, Cd) [9,10,12] and three structures with the general formula RbCsCSi₅O₁₂ (C = Mg, Ni, Cd) [13] are all isostructural with the fully T-site cation ordered structure of Cs₂CdSi₅O₁₂. However, Nuclear Magnetic Resonance spectroscopy [14] and high-resolution synchrotron X-ray powder diffraction [10] studies on Cs₂ZnSi₅O₁₂ described a Pbca structure where Zn is partially disordered over two of the six T-sites.

High temperature neutron and X-ray powder diffraction studies on KAlSi₂O₆, RbAlSi₂O₆ and KFe³⁺Si₂O₆ [15] and KGaSi₂O₆ [4] showed first-order phase transitions from I4₁/a to Ia-3d (isostructural with pollucite CsAlSi₂O₆). High temperature X-ray powder diffraction studies on K₂MgSi₅O₁₂ [16] and K₂ZnSi₅O₁₂ [17] showed first-order phase transitions from P2₁/c to Pbca (isostructural with Cs₂CdSi₅O₁₂.)

A high temperature study from 295 to 1173 K [18] has also been done on three Cs₂CSi₅O₁₂ (C = Cu, Cd, Zn) leucite analogues using lower resolution synchrotron X-ray powder diffraction. For Cs₂ZnSi₅O₁₂, the ambient temperature crystal structure shows (unlike for the high-resolution synchrotron X-ray powder diffraction study) that the Pbca structure is also isostructural with Cs₂CdSi₅O₁₂ with complete T-site cation ordering. However, the sample with C = Zn shows evidence for a transition to a previously unknown $Pa\overline{3}$ cubic structure, with some T-site cation disorder, at 566 K, on heating. This transition is reversible on cooling to 633 K.

Rb₂CSi₅O₁₂ leucite structures are known where C = Cd, Ni, Mg, Mn and Zn [5,7,9,10,12] as is the structure of Cs₂CoSi₅O₁₂ [9]. A cubic lattice parameter of 13.4(1)Å for Rb₂CoSi₅O₁₂ has been reported [19] but there is no published crystal structure for this leucite analogue. Therefore, a sample of Rb₂CoSi₅O₁₂ leucite analogue has been synthesized. An ambient temperature X-ray powder diffraction study on this analogue has been undertaken to determine the crystal structure for Rb₂CoSi₅O₁₂. A high temperature X-ray powder diffraction study has also been done to look for any phase transitions.

2. Materials and Methods

A sample of Rb₂CoSi₅O₁₂ was synthesized from a stoichiometric mixture of Rb₂CO₃ (Alfa Aesar, Heysham, United Kingdom, 99.5%); CoO (Sigma-Aldrich, St. Louis, MO, USA, 99.98%) and SiO₂ (Better Equipped purified white silica sand, Nantwich, United Kingdom). The mixture was then heated overnight at 873 K to decompose the carbonates and melted in a platinum crucible at 1673 K for 1.5 h before quenching to form a glass. The Rb₂CoSi₅O_l2 glass was then dry crystallized at ambient pressure and 1393 K for 5 days. These were the same sample preparation conditions as were used to synthesise Cs₂CoSi₅O_l2 leucite analogue [9].

This sample was then mounted on a low-background silicon wafer with a drop of acetone. Ambient temperature X-ray powder diffraction was then collected on this sample using a PANalytical Empyrean X-ray powder diffractometer (PANalytical, Almelo, Netherlands). Data were collected using CoKa X-rays over the range from 12 to 100 °2θ, the scan time was 19 h.

This Rb₂CoSi₅O_l2 sample was then loaded into a platinum flat plate sample holder which was loaded into an Anton HTK1200N high temperature stage mounted on a PANalytical X’Pert MPD X-ray powder diffractometer (PANalytical, Almelo, Netherlands). Data were collected using CuKa X-rays over the range from 10 to 80 °2θ. Scans were done at ambient temperature (scan time 30 min) and then in 50 K increments (scan time 2.5 h) from 323 to 1373 K. A second series of high temperature scans were done Rb₂CoSi₅O_l2 on using the HTK1200N over the range from 10 to 80 °2θ. Scans were done in 10 K increments from 423 to 523 K, the scan time at each temperature was 8 h. A final scan on cooling to ambient temperature was also measured over the range from 10 to 80 °2θ, the scan time was 30 min.

High temperature X-ray powder diffraction data were also collected on a sample of MgO (99.99%, Acros Organics, New Jersey, USA) to calibrate the temperature of the HTK1200N. This was loaded into a platinum flat plate sample holder which was loaded onto the same Anton HTK1200N high temperature stage. Data were collected over the range from 33 to 111 °2θ, the scan time at each temperature was 10 min. Scans were done at ambient temperature and then in 50 K increments from 323 to 1473 K.

Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) were collected on a sample of Rb₂CoSi₅O_l2 from ambient temperature to 1673 K. Data were collected in a TA Instruments SDT 650 simultaneous TGA/DSC (New Castle, DE, USA), under a flow of compressed air. The instrument was suitably calibrated, an empty alumina crucible was used for the sample holder and reference sample, with no background corrections.

3. Results

3.1. Ambient Temperature X-ray Powder Diffraction

Analysis of the ambient temperature powder diffraction data collected on Rb₂CoSi₅O_l2 showed that the observed powder diffraction data were a close match to the Powder Diffraction File (http://www.icdd.com/ (accessed on 5 April 2022)) pattern 70-0007 for Rb₂NiSi₅O_l2. Therefore, the Pbca crystal structure for Rb₂NiSi₅O_l2 [12], with Co replacing Ni on one of the T-sites, was used as a starting model for Rietveld refinement [20] using GSAS-II [21]. Si-O and Co-O distances for T-sites were restrained to those in the crystal structure for Cs₂CoSi₅O_l2 [9].

Figure 1 shows the Rietveld difference plot for ambient temperature Rb₂CoSi₅O_l2. Note that the X-ray powder diffraction data collected for this figure were collected in automatic divergence slit mode. These data were converted to fixed slit mode for Rietveld refinement, hence the very high background at low 2θ angles. Figure 2 shows a VESTA [22] crystal structure plot for the refined ambient temperature Pbca crystal structure of Rb₂CoSi₅O_l2.

Table 1. Refined atomic coordinates for ambient temperature Rb₂CoSi₅O_l2.

	Rb2CoSi5O12 Pbca Orthorhombic. All Sites Fully Occupied and all Atoms on 8c Wyckoff Position
	a (Å)	b (Å)	c (Å)	V (Å3)
	13.370(4)	13.639(4)	13.497(4)	2461.4(20)
Atom	x	y	z	Uiso
Rb1	0.1455(9)	0.1306(11)	0.1371(17)	0.270(6)
Rb2	0.3751(12)	0.3606(11)	0.3787(17)	0.270(6)
Co1	0.3468(22)	0.8405(22)	0.9184(16)	0.203(14)
Si1	0.1223(29)	0.628(3)	0.610(3)	0.117(7)
Si2	0.6105(30)	0.1033(25)	0.6364(30)	0.117(7)
Si3	0.6625(27)	0.5965(31)	0.115(3)	0.117(7)
Si4	0.9115(24)	0.355(3)	0.8258(31)	0.117(7)
Si5	0.8438(29)	0.9494(27)	0.363(4)	0.117(7)
O1	0.4908(29)	0.418(3)	0.122(5)	0.036(5)
O2	0.1957(31)	0.4815(29)	0.380(5)	0.036(5)
O3	0.3650(30)	0.1469(25)	0.490(3)	0.036(5)
O4	0.7292(24)	0.4557(22)	0.615(4)	0.036(5)
O5	0.6223(26)	0.757(4)	0.356(4)	0.036(5)
O6	0.380(3)	0.594(3)	0.7432(29)	0.036(5)
O7	0.9792(31)	0.8219(30)	0.635(4)	0.036(5)
O8	0.5673(27)	0.968(3)	0.860(5)	0.036(5)
O9	0.885(3)	0.6312(31)	0.9438(19)	0.036(5)
O10	0.2217(30)	0.9414(30)	0.1311(31)	0.036(5)
O11	0.1722(23)	0.2036(27)	0.886(3)	0.036(5)
O12	0.9090(31)	0.142(4)	0.2032(31)	0.036(5)

shows the refined ambient temperature coordinates for Rb₂CoSi₅O_l2.

Table 2. Refined interatomic distances (Å) for ambient temperature Rb₂CoSi₅O_l2.

Rb1	O1	3.44(5)	Co1	O4	1.926(6)
Rb1	O2	3.85(6)	Co1	O7	1.925(6)
Rb1	O3	4.13(4)	Co1	O9	1.966(6)
Rb1	O4	3.72(6)	Co1	O11	1.935(6)
Rb1	O5	3.55(4)	Si1	O1	1.642(6)
Rb1	O6	4.04(5)	Si1	O3	1.643(6)
Rb1	O7	3.56(6)	Si1	O5	1.639(6)
Rb1	O8	4.07(4)	Si1	O10	1.654(6)
Rb1	O9	3.45(4)	Si2	O1	1.638(6)
Rb1	O10	2.78(4)	Si2	O2	1.639(6)
Rb1	O11	3.55(6)	Si2	O6	1.635(6)
Rb1	O12	3.29(5)	Si2	O11	1.625(6)
Rb2	O1	3.88(6)	Si3	O2	1.631(6)
Rb2	O2	2.91(5)	Si3	O3	1.627(6)
Rb2	O3	3.28(4)	Si3	O4	1.614(6)
Rb2	O4	2.87(3)	Si3	O12	1.645(6)
Rb2	O5	3.46(5)	Si4	O5	1.642(6)
Rb2	O6	3.72(5)	Si4	O7	1.620(6)
Rb2	O7	4.00(6)	Si4	O8	1.631(6)
Rb2	O8	3.49(4)	Si4	O12	1.656(6)
Rb2	O9	3.33(5)	Si5	O6	1.627(6)
Rb2	O10	3.75(5)	Si5	O8	1.641(6)
Rb2	O11	2.85(4)	Si5	O9	1.640(6)
Rb2	O12	3.21(6)	Si5	O10	1.639(6)
mean Rb1-O		3.62	mean Co-O		1.938
stdev Rb1-O		0.38	mean Si-O		1.636
mean Rb2-O		3.40
stdev Rb2-O		0.39
mean Rb-O		3.51
stdev Rb-O		0.40

and

Table 3. Refined interatomic angles (°) for ambient temperature Rb₂CoSi₅O_l2.

O4	Co1	O7	120.4(26)	Si1	O1	Si2	148(4)
O4	Co1	O9	101.3(27)	Si2	O2	Si3	120(4)
O7	Co1	O9	98.1(20)	Si1	O3	Si3	145(3)
O4	Co1	O11	131.7(23)	Co1	O4	Si3	96.6(26)
O7	Co1	O11	84.8(24)	Si1	O5	Si4	164(3)
O9	Co1	O11	116.0(21)	Si2	O6	Si5	158(4)
O1	Si1	O3	104.5(30)	Co1	O7	Si4	156(3)
O1	Si1	O5	109(4)	Si4	O8	Si5	122.2(28)
O3	Si1	O5	97(4)	Co1	O9	Si5	132(4)
O1	Si1	O10	121(4)	Si1	O10	Si5	148(4)
O3	Si1	O10	100.3(31)	Co1	O11	Si2	141.9(28)
O5	Si1	O10	120(4)	Si3	O12	Si4	138(3)
O1	Si2	O2	123(3)	mean Si-O-Si			142.9
O1	Si2	O6	100(4)	mean Co-O-Si			131.6
O2	Si2	O6	91(3)
O1	Si2	O11	128(3)
O2	Si2	O11	102.5(30)
O6	Si2	O11	102(3)
O2	Si3	O3	120(4)
O2	Si3	O4	48.0(18)
O3	Si3	O4	113(4)
O2	Si3	O12	120(5)
O3	Si3	O12	109.8(22)
O4	Si3	O12	134(4)
O5	Si4	O7	84.8(25)
O5	Si4	O8	149(4)
O7	Si4	O8	91(3)
O5	Si4	O12	105(4)
O7	Si4	O12	110.7(30)
O8	Si4	O12	105(4)
O6	Si5	O8	90.4(28)
O6	Si5	O9	104.1(26)
O8	Si5	O9	104(4)
O6	Si5	O10	108(4)
O8	Si5	O10	140.3(29)
O9	Si5	O10	105(3)
mean O-Co-O			108.7
mean O-Si-O			108.0

show the interatomic distances and angles for ambient temperature Rb₂CoSi₅O_l2.

3.2. High Temperature X-ray Powder Diffraction

3.2.1. First High Temperature Scans

The first series of high temperature scans were done in 50 K increments from 323 to 1373 K. The strongest Bragg reflections in the Pbca orthorhombic powder diffraction patterns for Rb₂CoSi₅O_l2 were three closely overlapped 004, 040 and 400 Bragg reflections. As the temperature increases, these three reflections merge to form a single peak by 473 K, suggesting a phase transition from orthorhombic to cubic. This single peak is retained to 1123 K. However, at higher temperatures extra peaks appear in the powder diffraction pattern before all the Rb₂CoSi₅O_l2 peaks disappear at 1323 K; it was not possible to identify any phase(s) causing these extra peaks as the Rb₂CoSi₅O_l2 started to decompose. When the sample was removed from the high temperature stage, on cooling to ambient temperature, the sample appeared to have partially melted. Figure 3 shows all the powder diffraction data collected on heating 298–1123 K. Figure 4 shows how the closely overlapped 004, 040 and 400 Bragg reflections merge to a single peak on heating 298–1123 K.

3.2.2. Second High Temperature Scans

The second series of high temperature scans were done in 10 K increments from 423 to 523 K to determine more precisely the orthorhombic to cubic transition temperature. Some Bragg reflections from the platinum sample holder were also observed in the powder diffraction data from the second series of high temperature scans, these platinum peaks were excluded from the Rietveld refinements. Figure 5 shows all the powder diffraction data collected. Figure 6 shows how the closely overlapped 004, 040 and 400 Bragg reflections merge to a single peak on heating 423–523 K.

3.2.3. MgO Temperature Calibration Scans

The $Fm\overline{3}m$ cubic structure of MgO [23] was used as a starting model for Rietveld refinements of the MgO crystal structure over the whole temperature range. The refined MgO lattice parameters determined at each temperature and thermal expansion parameters for MgO [24] were then used to calibrate the temperature of the HTK1200N. The second high temperature scan data, collected in 10 K increments from 423 to 523 K, were calibrated by interpolating between the calibrated temperatures from the MgO data collected at 423, 473 and 523 K.

3.2.4. TGA/DSC Analyses

No significant changes were detected by TGA/DSC on heating Rb₂CoSi₅O_l2 around the phase transition temperature.

3.3. Rietveld Refinement Data Analysis

All high temperature powder diffraction data from the two series of scans up to 443 K could be refined using the Pbca orthorhombic ambient temperature structure for Rb₂CoSi₅O_l2 as a starting model. All high temperature powder diffraction data from the two series of scans from 453 K to 1123 K could be refined using a $Pa\overline{3}$ cubic structure isostructural with the high temperature crystal structure for Cs₂ZnSi₅O₁₂ [18]. The ambient temperature structure for Rb₂CoSi₅O_l2 after heating reverts to Pbca orthorhombic.

Table 4. Variation of lattice parameters with calibrated temperatures for Rb₂CoSi₅O_l2.

T (K)	T—MgO (K)	a(Å)	b(Å)	c(Å)	V(Å3)
298	+	13.370(4)	13.639(4)	13.497(4)	2461.4(20)
298	++	13.444(13)	13.612(9)	13.457(16)	2463(5)
298	303	13.416(8)	13.620(8)	13.495(8)	2466(4)
323	324	13.427(4)	13.612(4)	13.508(4)	2469(2)
373	376	13.455(3)	13.587(3)	13.518(3)	2471(2)
423	427	13.492(3)	13.604(2)	13.507(3)	2479(1)
433	437	13.494(3)	13.603(2)	13.509(3)	2480(1)
443	447	13.496(3)	13.606(2)	13.516(3)	2482(1)
453	457	13.532(1)			2477.7(6)
463	467	13.533(1)			2478.4(6(
473	477	13.536(1)			2480.2(6)
483	487	13.541(1)			2482.8(5)
493	497	13.544(1)			2484.4(5)
503	507	13.5448(9)			2485.0(5)
513	517	13.5478(9)			2486.6(5)
523	528	13.5498(9)			2487.7(5)
573	581	13.5495(9)			2487.5(5)
623	632	13.5545(9)			2490.3(5)
673	684	13.5609(8)			2493.8(5)
723	736	13.5686(8)			2498.1(4)
773	785	13.5709(7)			2499.3(4)
823	835	13.5775(7)			2503.0(4)
873	883	13.5820(7)			2505.5(4)
923	935	13.5853(6)			2507.3(3)
973	986	13.5885(6)			2509.07(31)
1023	1036	13.5919(5(			2510.95(27)
1073	1084	13.5968(5)			2513.67(26)
1123	1133	13.6008(5)			2515.88(26)

+ = ambient temperature scan outside high temperature stage. ++ = ambient temperature scan in high temperature stage after heating.

shows how the Rb₂CoSi₅O_l2 lattice parameters vary with MgO calibrated temperature. Figure 7 and Figure 8 show how the lattice parameters and unit cell volumes vary with MgO calibrated temperature.

To study the changes in the Rb₂CoSi₅O_l2 structure with temperature the variance of the intratetrahedral O-T-O (T = Co and Si) angles (compared with the ideal tetrahedral angle of 109.5°) in the 6 Pbca orthorhombic T-sites and 2 $Pa\overline{3}$ cubic T-sites have been determined using the following equation [25]

n

θσ_(tet)² = Σ (θ_i − 109.5)²/(n − 1)

i = 1

where n is the number of different O-T-O angles (n = 6) and θ is the O-T-O angle in each TO₄ unit. Figure 9 shows how these T-site distortion parameters vary with temperature for the structures of Pbca orthorhombic and $Pa\overline{3}$ cubic Rb₂CoSi₅O_l2.

Figure 10 shows a VESTA [22] plot and Figure 11 shows the Rietveld difference plot for the 1133 K (MgO calibrated temperature) $Pa\overline{3}$ cubic structure of Rb₂CoSi₅O_l2.

Table 5. Refined atomic coordinates for 1133 K Rb₂CoSi₅O_l2.

Rb2CoSi5Ol2 $\mathit{P}\mathit{a}\overline{3}$ Cubic
a = 13.6008(5)Å		V = 2515.88(26)Å3
Atom	x	y	Z	occ.	Uiso	Wyckoff
Rb1	0.1363(5)	0.1363(5)	0.1363(5)	1.0000	0.142(6)	8c
Rb2	0.3871(6)	0.3871(6)	0.3871(6)	1.0000	0.124(5)	8c
Si1	0.36116(31)	0.82652(27)	0.9199(3)	0.6667	0.092(6)	24d
Co1	0.36116(31)	0.82652(27)	0.9199(3)	0.3333	0.092(6)	24d
Si2	0.1280(8)	0.6550(7)	0.5936(9)	1.0000	0.026(4)	24d
O1	0.4759(10)	0.3756(16)	0.1415(18)	1.0000	0.098(4)	24d
O2	0.1365(13)	0.7276(8)	0.1045(15)	1.0000	0.098(4)	24d
O3	0.9685(6)	0.8798(6)	0.6183(12)	1.0000	0.098(4)	24d
O4	0.6428(12)	0.20079(23)	0.5761(11)	1.0000	0.098(4)	24d

shows refined 1133 K (MgO calibrated temperature) coordinates for Rb₂CoSi₅O_l2.

Table 6. Refined interatomic distances (Å) for 1133 K Rb₂CoSi₅O_l2.

Rb1	O1	3.862(22)	Rb2	O1	3.555(25)	Si1/Co1	O2	1.7110(31)
Rb1	O1	3.862(24)	Rb2	O1	3.555(30)	Si1/Co1	O3	1.711(3)
Rb1	O1	3.862(23)	Rb2	O1	3.555(26)	Si1/Co1	O3	1.712(3)
Rb1	O2	3.358(18)	Rb2	O2	3.358(22)	Si1/Co1	O4	1.7117(30)
Rb1	O2	3.358(20)	Rb2	O2	3.358(26)	Si2	O1	1.610(3)
Rb1	O2	3.358(22)	Rb2	O2	3.358(21)	Si2	O1	1.613(3)
Rb1	O3	3.635(18)	Rb2	O3	3.358(9)	Si2	O2	1.608(3)
Rb1	O3	3.635(19)	Rb2	O3	3.358(12)	Si2	O4	1.606(3)
Rb1	O3	3.635(18)	Rb2	O3	3.358(9)	mean (Si1/Co1)-O		1.711
Rb1	O4	3.021(15)	Rb2	O4	3.567(17)	mean Si2-O		1.609
Rb1	O4	3.021(16)	Rb2	O4	3.567(17)
Rb1	O4	3.021(15)	Rb2	O4	3.567(17)
mean Rb1-O		3.469	mean Rb2-O		3.460
stdev Rb1-O		0.328	stdev Rb2-O		0.106
			mean Rb-O		3.464
			stdev Rb-O		0.239

and

Table 7. Refined interatomic angles (°) for 1133 K Rb₂CoSi₅O_l2. T = Si or Co.

	O2	Si1/Co1	O3	103.5(7)
	O2	Si1/Co1	O3	105.2(11)
	O3	Si1/Co1	O3	87.5(8)
	O2	Si1/Co1	O4	122.7(9)
	O3	Si1/Co1	O4	117.4(6)
	O3	Si1/Co1	O4	114.5(9)
	O1	Si2	O1	110.3(18)
	O1	Si2	O2	106.4(13)
	O1	Si2	O2	101.9(9)
	O1	Si2	O4	101.6(9)
	O1	Si2	O4	110.9(14)
	O2	Si2	O4	125.5(11)
	Si2	O1	Si2	155.4(17)
	Si1/Co1	O2	Si2	136.0(12)
	Si1/Co1	O3	Si1/Co1	145.8(4)
	Si1/Co1	O4	Si2	125.6(9)
mean	O	Si1/Co1	O	108.5
mean	O	Si2	O	109.4
mean	O	T	O	109.0
mean	T	O	T	140.7

show interatomic distances and angles for 1133 K (MgO calibrated temperature) Rb₂CoSi₅O_l2.

4. Discussion

All temperatures referred to in this section are now MgO calibrated temperatures.

Analysis of the high temperature X-ray powder diffraction data for Rb₂CoSi₅O_l2 showed a first-order phase transition from Pbca orthorhombic to $Pa\overline{3}$ cubic at 457 K. However, Table 4 and Figure 7 and Figure 8 show that the unit cell volume decreases with increasing temperature after the phase transition. The Pbca structure for Rb₂CoSi₅O_l2 has the tetrahedrally coordinated Co and Si atoms ordered onto separate T-sites (1 Co and 5 Si sites). However, the $Pa\overline{3}$ structure for Rb₂CoSi₅O_l2 has some partial T-site disorder. One T site is fully occupied by Si, the other T site is 2/3 occupied by Si and 1/3 occupied by Co. Figure 9 shows how the T-site distortions for Rb₂CoSi₅O_l2 vary with temperature for the Pbca orthorhombic and $Pa\overline{3}$ cubic phases. There is a much wider spread of T-site distortions for Pbca orthorhombic compared with $Pa\overline{3}$ cubic. A plot of the 447 K Pbca T-site ordered crystal structure (Figure 12) shows that the central channel (which contains different sized SiO₄ and CoO₄ tetrahedra) is much more distorted than the central channel of the partially T-site disordered 457 K $Pa\overline{3}$ structure (Figure 13). This agrees with the plot of T-site distortions shown in Figure 9. In the $Pa\overline{3}$ structure, there is a smaller size difference between the SiO₄ and (Si_2/3Co_1/3)O₄ tetrahedra compared with the size difference between the Pbca SiO₄ and CoO₄ tetrahedra. The smaller size difference between the TO₄ (T = Si or Co) tetrahedra in the $Pa\overline{3}$ structure compared with the Pbca structure means that the $Pa\overline{3}$ structure is less distorted than the Pbca structure. This decrease in distortion of the crystal structure through the phase transition is why the central channel contracts through the phase transition. This is reflected in the decrease in unit cell volume.

Such a phase transition with a unit cell contraction has not been observed before in a leucite mineral analogue. However, similar transitions have been observed in other materials such as BiNiO₃ [26] and Hf_0.86Ta_0.14Fe₂ [27].

Thermal expansion due to the increase in temperature means that the cubic unit cell volume eventually becomes greater than the orthorhombic unit cell volume at 497 K, 40 K higher than the phase transition temperature. The $Pa\overline{3}$ cubic structure is retained up to 1133 K. Above this temperature the Rb₂CoSi₅O_l2 sample starts to decompose before partially melting at 1323 K.

Table 8. Thermal expansion coefficients (TEC) for leucite analogues.

Stoichiometry	SG	T (K)	Mean TEC (× 105 K−1)	Reference
Rb2CoSi5O12	Pbca	303–447	4(1)	This work
Rb2CoSi5O12	$Pa\overline{3}$	457–1133	4(2)	This work
KGaSi2O6	I41/a	298–873	5.2(4)	[4]
KGaSi2O6	Ia-3d	673–1473	8(2)	[4]
K2ZnSi5O12	P21/c	773–843	10(1)	[17]
K2ZnSi5O12	Pbca	868–973	6(3)	[17]
Cs2CdSi5O12	Pbca	295–1173	1.8(4)	[18]
Cs2CuSi5O12	Pbca	295–333	6(3)	[18]
Cs2CuSi5O12	Pbca	333–1173	0.8(3)	[18]
Cs2ZnSi5O12	Pbca	295–543	2(1)	[18]
Cs2ZnSi5O12	$Pa\overline{3}$	573–1173	1.1(3)	[18]
K2MgSi5O12	P21/c	293–647	5(1)	[16]
K2MgSi5O12	Pbca	668–842	2(1)	[16]
CsAlSi2O6	I41/a	20–368	5.1(1.4)	[15]
CsAlSi2O6	Ia-3d	373–573	3.0(2.2)	[15]
RbAlSi2O6	I41/a	298–743	7.1(7)	[15]
RbAlSi2O6	Ia-3d	753–953	3(1)	[15]
KAlSi2O6	I41/a	4–923	5(1)	[15]
KAlSi2O6	Ia-3d	943–1023	4.1(6)	[15]
KFeSi2O6	I41/a	298–843	6.2(8)	[15]
KFeSi2O6	Ia-3d	853–1050	5(1)	[15]
Nat. leucite	I41/a	298–963	8.562	[28]
Nat. leucite	Ia-3d	963–1193	0.877	[28]
KAlSi2O6	I41/a	298–878	8.548	[28]
KAlSi2O6	I41/a	878–1093	6.323	[28]
KAlSi2O6	Ia-3d	1093–1193	0.921	[28]
RbAlSi2O6	I41/a	298–583	7.98	[28]
RbAlSi2O6	I41/a	583–793	4.653	[28]
RbAlSi2O6	Ia-3d	793–1193	1.215	[28]
CsAlSi2O6	Ia-3d	298–463	4.789	[28]

shows the thermal expansion coefficients (TEC) for the two Rb₂CoSi₅O_l2 phases. TEC for other leucite phases are given as comparison. It can be seen that the TEC for Rb₂CoSi₅O_l2 are in the same order of magnitude as those reported for other leucites.

The Pbca orthorhombic to $Pa\overline{3}$ cubic leucite phase transition was first observed in Cs₂ZnSi₅O₁₂ [18]. There was no unit cell contraction noticed through the phase transition in Cs₂ZnSi₅O₁₂ and this was assumed to be a second-order phase transition. However, the high temperature X-ray powder diffraction data for Cs₂ZnSi₅O₁₂ were collected every 30 K, a larger temperature increment than was used for Rb₂CoSi₅O_l2. It would be interesting to repeat the high temperature XRD study on Cs₂ZnSi₅O₁₂ with a smaller temperature increment to see if a unit cell contraction could be observed for this phase transition.

5. Conclusions

• The ambient temperature crystal structure of the Rb₂CoSi₅O_l2 leucite analogue has been determined as isostructural with the Pbca orthorhombic T-site ordered crystal structure of Cs₂CdSi₅O₁₂.
• High temperature X-ray powder diffraction on Rb₂CoSi₅O_l2 leucite analogue shows a Pbca orthorhombic to $Pa\overline{3}$ cubic phase transition at 457 K, like that reported for Cs₂ZnSi₅O₁₂. The decrease in the size difference between the TO₄ (T = Si and Co) units before and after this transition means that the central channel of the cubic leucite structure is less distorted than that for the orthorhombic structure. The decrease in structural distortion means that there is an initial unit cell contraction with increasing temperature after the phase transition.
• The $Pa\overline{3}$ structure for Rb₂CoSi₅O_l2 leucite analogue is retained up to 1133 K; above this temperature the sample starts to decompose and then partially melts at 1323 K.