Crystal structure of langbeinite-related Rb0.743K0.845Co0.293Ti1.707(PO4)3.

Potassium rubidium cobalt(II)/titanium(IV) tris-(orthophosphate), Rb0.743K0.845Co0.293Ti1.707(PO4)3, has been obtained using a high-temperature crystallization method. The obtained compound has a langbeinite-type structure. The three-dimensional framework is built up from mixed-occupied (Co/Ti(IV))O6 octa-hedra (point group symmetry .3.) and PO4 tetra-hedra. The K(+) and Rb(+) cations are statistically distributed over two distinct sites (both with site symmetry .3.) in the large cavities of the framework. They are surrounded by 12 O atoms.

Chemical context

Nowadays, there are a number of reports on the synthesis and investigation of langbeinite-related complex phosphates, which exhibit inter­esting properties such as magnetic (Ogorodnyk et al., 2006 ▸), luminescence (Zhang et al., 2013 ▸; Chawla et al., 2013 ▸) and phase transitions (Hikita et al., 1977 ▸). It should be noted that compounds with a langbeinite-type structure are prospects for use as a matrix for the storage of nuclear waste (Orlova et al., 2011 ▸). Zaripov et al. (2009 ▸) and Ogorodnyk et al. (2007a ▸) proved that caesium can be introduced into the cavity of a langbeinite framework that can be used for the immobil­ization of 137Cs in an inert matrix for safe disposal.

A large number of compounds with a langbeinite framework based on a variety of different valence elements are known. Three major types of substitutions of the elements are known as well as their combinations. They are: metal substitution in octa­hedra, element substitution in anion tetra­hedra, and substitution of ions in cavities. Among these compounds, potassium-containing langbeinites are the most studied (Ogorodnyk et al., 2006 ▸, 2007b ▸,c ▸; Norberg, 2002 ▸; Orlova et al., 2003 ▸). However, several reports concerning phosphate langbeinites with Rb+ in the cavities of the framework are known: Rb2FeZr(PO4)3 (Trubach et al., 2004 ▸), Rb2YbTi(PO4)3 (Gustafsson et al., 2005 ▸) and Rb2TiY(PO4)3 (Gustafsson et al., 2006 ▸).

Herein, the structure of Rb0.743K0.845Co0.293Ti1.707(PO4)3, potassium rubidium cobalt(II)/titanium(IV) tris­(ortho­phos­phate) is reported.

Structural commentary

The asymmetric unit of Rb0.743K0.845Co0.293Ti1.707(PO4)3 consists of two mixed-occupied (Co/TiIV), two (Rb/K), one P and four oxygen positions (Fig. 1 ▸). The structure of the title compound is built up from mixed (Co/TiIV)O6 octa­hedra and PO4 tetra­hedra, which are connected via common O-atom vertices. Each octa­hedron is linked to six adjacent tetra­hedra and reciprocally, each tetra­hedron is connected to four neighboring octa­hedra into a three-dimensional rigid framework (Fig. 2 ▸).

Figure 1

The asymmetric unit of Rb0.743K0.845Co0.293Ti1.707(PO4)3, showing displace­ment ellipsoids at the 50% probability level.

Figure 2

Two-dimensional net and three-dimensional framework for Rb0.743K0.845Co0.293Ti1.707(PO4)3.

The oxygen environment of the metal atoms in the (Co/TiIV)1O6 octa­hedra is slightly distorted, with M—O bonds of 1.940 (2) and 1.966 (2) Å. These distances are close to the corresponding bond lengths in K2Ti2(PO4)3 [d(Ti—O) = 1.877 (10)–1.965 (10) Å; Masse et al., 1972 ▸], which could be explained by the small occupancy of cobalt in the mixed (Co/TiIV)1 [occupancy = 0.1307 (9)] and (Co/TiIV)2 [occupancy = 0.162 (3)] sites. It should be noted that (Co/TiIV)2—O distances [1.949 (2) and 1.969 (2) Å] are slightly shorter than those in K2Co0.5Ti1.5(PO4)3 (Ogorodnyk et al., 2006 ▸).

The orthophosphate tetra­hedra are also slightly distorted with P—O bond lengths ranging from 1.525 (2) to 1.531 (2) Å. These distances are almost identical to the corresponding ones in K2Co0.5Ti1.5(PO4)3 [d(P—O) =1.525 (2)–1.529 (9) Å; Ogorodnyk et al., 2006 ▸). A comparison of the corresponding inter­atomic distances for the octa­hedra and tetra­hedra in Rb0.743K0.845Co0.293Ti1.707(PO4)3 and K2Co0.5Ti1.5(PO4)3 shows that partial substitution of K+ by Rb+ and decreasing the amount of cobalt slightly influences the distances in the polyhedra for Rb0.743K0.845Co0.293Ti1.707(PO4)3.

The K+ and Rb+ cations are located in large cavities of the three-dimensional framework in Rb0.743K0.845Co0.293Ti1.707(PO4)3. They are statistically distributed over two distinct sites in which they have partial occupancies of 0.540 (9) and 0.330 (18) for Rb1 and K1, respectively, and 0.203 (8) and 0.514 (17) for Rb2 and K2, respectively. For the determination of the (Rb/K)1 and (Rb/K)2 coordination numbers (CN), Voronoi–Dirichlet polyhedra (VDP) were built using the DIRICHLET program included in the TOPOS package (Blatov et al., 1995 ▸). Analysis of the solid-angle (Ω) distribution revealed twelve (Rb/K)—O contacts for both the (Rb/K)1 and (Rb/K)2 sites (cut-off distance of 4.0 Å, neglecting those corresponding to Ω < 1.5%; Blatov et al., 1998 ▸). The results of the construction of the Voronoi–Dirichlet polyhedra (Blatov et al., 1995 ▸) indicated that the coordination scheme for (Rb/K)1 is described as [9 + 3] [nine meaning ‘ion–covalent’ bonds are in the range 2.896 (2)–3.095 (2) Å which have Ω > 5.0% and three (Rb/K)1—O distances equal to 3.438 (8) Å with Ω = 2.42%]. The (Rb/K)—O distances in the [(Rb/K)2O12]-polyhedra are in the range 2.870 (2)–3.219 (2) Å (4.91% < Ω < 9.5%).

The corresponding K1—O contacts in K2Co0.5Ti1.5(PO4)3 (Ogorodnyk et al., 2006 ▸) are in the range 2.872 (2)–3.231 (3) Å while the K2—O distances in the K2O12 polyhedra are in the range 2.855 (2)–3.473 (3) Å, slightly longer than those in Rb0.743K0.845Co0.293Ti1.707(PO4)3. These results indicate that the substitution of K+ cations by Rb+ cations in Rb0.743K0.845Co0.293Ti1.707(PO4)3 caused a decrease of the (Rb/K)—O bond length. This fact confirms the rigidity of the framework and the suitability of the cavity dimensions to accommodate different sized ions whose size and nature insignificantly influence the framework.

Synthesis and crystallization

The title compound was prepared during crystallization of a self-flux in the Rb2O–K2O–P2O5–TiO2–CoO system. The starting components RbH2PO4 (4.0 g), KPO3 (2.4 g), TiO2 (0.532 g) and CoO (0.50 g) were ground in an agate mortar, placed in a platinum crucible and H3PO4 (85%, 0.42 g) was added. The mixture was heated up to 1273 K. The melt was kept at this temperature for one h. After that, the temperature was decreased to 873 K at a rate of 10 K h−1. The crystals of Rb0.743K0.845Co0.293Ti1.707(PO4)3 were separated from the rest flux by washing in hot water. The chemical composition of a single crystal was verified using EDX analysis. Analysis found: K 6.72, Rb 13.85, Co 3.74, Ti 16.86, P 19.96 and O 38.87 at%, while Rb0.743K0.845Co0.293Ti1.707(PO4)3 requires K 6.86, Rb 13.15, Co 3.60, Ti 17.06, P 19.36 and O 39.97 at%.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1 ▸. The O-atom sites were determined from difference Fourier maps. It was assumed that both types of alkaline ions occupy cavity sites while the transition metals occupy framework sites. The occupancies were refined using linear combinations of free variables taking into account the total charge of the cell.

Table 1

Experimental details

Crystal data
Chemical formula	Rb0.743K0.845Co0.293Ti1.707(PO4)3
M r	480.40
Crystal system, space group	Cubic, P213
Temperature (K)	293
a ()	9.8527(1)
V (3)	956.46(2)
Z	4
Radiation type	Mo K
(mm1)	6.63
Crystal size (mm)	0.10 0.07 0.05
Data collection
Diffractometer	Oxford Diffraction Xcalibur-3
Absorption correction	Multi-scan (Blessing, 1995 ▸)
T min, T max	0.559, 0.734
No. of measured, independent and observed [I > 2(I)] reflections	1414, 1414, 1312
R int	0.025
(sin /)max (1)	0.804
Refinement
R[F 2 > 2(F 2)], wR(F 2), S	0.026, 0.051, 1.05
No. of reflections	1414
No. of parameters	67
No. of restraints	3
max, min (e 3)	0.37, 0.36
Absolute structure	Flack (1983 ▸), 612 Friedel pairs
Absolute structure parameter	0.024(10)

Computer programs: CrysAlis CCD and CrysAlis RED (Oxford Diffraction, 2006 ▸), SHELXS97 and SHELXL97 (Sheldrick, 2008 ▸), DIAMOND (Brandenburg, 1999 ▸), WinGX (Farrugia, 2012 ▸) and enCIFer (Allen et al., 2004 ▸).

Crystal structure: contains datablock(s) global, I. DOI: 10.1107/S2056989015001826/br2246sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989015001826/br2246Isup2.hkl

CCDC reference: 1045876

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

Rb0.743K0.845Co0.293Ti1.707(PO4)3	Dx = 3.336 Mg m−3
Mr = 480.40	Mo Kα radiation, λ = 0.71073 Å
Cubic, P213	Cell parameters from 1414 reflections
Hall symbol: P 2ac 2ab 3	θ = 2.9–34.9°
a = 9.8527 (1) Å	µ = 6.63 mm−1
V = 956.46 (2) Å3	T = 293 K
Z = 4	Tetrahedron, dark red
F(000) = 920	0.1 × 0.07 × 0.05 mm

Oxford Diffraction Xcalibur-3 diffractometer	1414 independent reflections
Radiation source: fine focus sealed tube	1312 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.025
φ and ω scans	θmax = 34.9°, θmin = 2.9°
Absorption correction: multi-scan (Blessing, 1995)	h = −10→10
Tmin = 0.559, Tmax = 0.734	k = 0→11
1414 measured reflections	l = 1→15

Refinement on F2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	w = 1/[σ2(Fo2) + (0.0183P)2] where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.026	(Δ/σ)max = 0.051
wR(F2) = 0.051	Δρmax = 0.37 e Å−3
S = 1.05	Δρmin = −0.36 e Å−3
1414 reflections	Extinction correction: SHELXL97 (Sheldrick, 2008)
67 parameters	Extinction coefficient: 0.0026 (6)
3 restraints	Absolute structure: Flack (1983), 612 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.024 (10)

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

	x	y	z	Uiso*/Ueq	Occ. (<1)
Rb1	0.71155 (4)	0.71155 (4)	0.71155 (4)	0.02169 (18)	0.540 (9)
K1	0.71155 (4)	0.71155 (4)	0.71155 (4)	0.02169 (18)	0.330 (18)
Rb2	0.93045 (5)	0.93045 (5)	0.93045 (5)	0.0199 (3)	0.203 (8)
K2	0.93045 (5)	0.93045 (5)	0.93045 (5)	0.0199 (3)	0.514 (17)
Ti1	0.14135 (4)	0.14135 (4)	0.14135 (4)	0.00760 (12)	0.8693 (9)
Co1	0.14135 (4)	0.14135 (4)	0.14135 (4)	0.00760 (12)	0.1307 (9)
Ti2	0.41386 (3)	0.41386 (3)	0.41386 (3)	0.00709 (12)	0.838 (3)
Co2	0.41386 (3)	0.41386 (3)	0.41386 (3)	0.00709 (12)	0.162 (3)
P1	0.45604 (5)	0.22826 (5)	0.12582 (5)	0.00682 (10)
O1	0.30739 (16)	0.23395 (16)	0.08086 (17)	0.0141 (3)
O2	0.54329 (18)	0.29756 (17)	0.01814 (17)	0.0179 (3)
O3	0.50157 (16)	0.08190 (16)	0.14744 (18)	0.0168 (3)
O4	0.47835 (17)	0.30686 (19)	0.25786 (18)	0.0190 (4)

	U11	U22	U33	U12	U13	U23
Rb1	0.02169 (18)	0.02169 (18)	0.02169 (18)	0.00120 (13)	0.00120 (13)	0.00120 (13)
K1	0.02169 (18)	0.02169 (18)	0.02169 (18)	0.00120 (13)	0.00120 (13)	0.00120 (13)
Rb2	0.0199 (3)	0.0199 (3)	0.0199 (3)	−0.00172 (19)	−0.00172 (19)	−0.00172 (19)
K2	0.0199 (3)	0.0199 (3)	0.0199 (3)	−0.00172 (19)	−0.00172 (19)	−0.00172 (19)
Ti1	0.00760 (12)	0.00760 (12)	0.00760 (12)	0.00051 (11)	0.00051 (11)	0.00051 (11)
Co1	0.00760 (12)	0.00760 (12)	0.00760 (12)	0.00051 (11)	0.00051 (11)	0.00051 (11)
Ti2	0.00709 (12)	0.00709 (12)	0.00709 (12)	−0.00033 (11)	−0.00033 (11)	−0.00033 (11)
Co2	0.00709 (12)	0.00709 (12)	0.00709 (12)	−0.00033 (11)	−0.00033 (11)	−0.00033 (11)
P1	0.0059 (2)	0.0076 (2)	0.0070 (2)	−0.00019 (16)	0.00100 (16)	−0.00054 (17)
O1	0.0088 (7)	0.0167 (8)	0.0169 (8)	0.0001 (5)	−0.0032 (6)	0.0021 (6)
O2	0.0183 (9)	0.0185 (8)	0.0170 (8)	0.0001 (7)	0.0075 (6)	0.0049 (6)
O3	0.0160 (8)	0.0130 (8)	0.0214 (8)	0.0064 (6)	0.0022 (6)	0.0034 (6)
O4	0.0196 (9)	0.0229 (10)	0.0146 (8)	−0.0027 (7)	0.0001 (7)	−0.0099 (6)

Rb1—O1i	2.8956 (17)	Rb2—O4ix	3.219 (2)
Rb1—O1ii	2.8956 (17)	Rb2—O4viii	3.219 (2)
Rb1—O1iii	2.8956 (17)	Ti1—O2x	1.9404 (17)
Rb1—O2iv	3.0780 (19)	Ti1—O2xi	1.9404 (17)
Rb1—O2v	3.0780 (19)	Ti1—O2xii	1.9404 (17)
Rb1—O2vi	3.0780 (19)	Ti1—O1xiii	1.9657 (16)
Rb1—O4iv	3.0945 (18)	Ti1—O1	1.9657 (16)
Rb1—O4vi	3.0945 (18)	Ti1—O1xiv	1.9657 (16)
Rb1—O4v	3.0945 (18)	Ti2—O3ii	1.9494 (16)
Rb2—O3iv	2.8703 (18)	Ti2—O3iii	1.9494 (16)
Rb2—O3vi	2.8703 (18)	Ti2—O3i	1.9494 (16)
Rb2—O3v	2.8703 (18)	Ti2—O4xiii	1.9691 (17)
Rb2—O2vii	2.9452 (19)	Ti2—O4	1.9691 (17)
Rb2—O2viii	2.9452 (19)	Ti2—O4xiv	1.9691 (17)
Rb2—O2ix	2.9452 (19)	P1—O3	1.5252 (17)
Rb2—O4iv	3.028 (2)	P1—O2	1.5266 (17)
Rb2—O4vi	3.028 (2)	P1—O4	1.5299 (17)
Rb2—O4v	3.028 (2)	P1—O1	1.5312 (16)
Rb2—O4vii	3.219 (2)
O1i—Rb1—O1ii	90.95 (5)	O2ix—Rb2—O4v	94.29 (5)
O1i—Rb1—O1iii	90.95 (5)	O4iv—Rb2—O4v	87.83 (5)
O1ii—Rb1—O1iii	90.95 (5)	O4vi—Rb2—O4v	87.83 (5)
O1i—Rb1—O2iv	145.70 (5)	O3iv—Rb2—O4vii	86.01 (4)
O1ii—Rb1—O2iv	82.60 (4)	O3vi—Rb2—O4vii	55.94 (4)
O1iii—Rb1—O2iv	55.72 (4)	O3v—Rb2—O4vii	157.18 (5)
O1i—Rb1—O2v	55.72 (4)	O2vii—Rb2—O4vii	46.55 (5)
O1ii—Rb1—O2v	145.70 (5)	O2viii—Rb2—O4vii	86.90 (5)
O1iii—Rb1—O2v	82.60 (4)	O2ix—Rb2—O4vii	101.29 (5)
O2iv—Rb1—O2v	119.386 (9)	O4iv—Rb2—O4vii	53.03 (6)
O1i—Rb1—O2vi	82.60 (4)	O4vi—Rb2—O4vii	104.695 (9)
O1ii—Rb1—O2vi	55.72 (4)	O4v—Rb2—O4vii	137.38 (4)
O1iii—Rb1—O2vi	145.70 (5)	O3iv—Rb2—O4ix	55.94 (4)
O2iv—Rb1—O2vi	119.386 (9)	O3vi—Rb2—O4ix	157.18 (5)
O2v—Rb1—O2vi	119.386 (9)	O3v—Rb2—O4ix	86.01 (4)
O1i—Rb1—O4iv	165.33 (5)	O2vii—Rb2—O4ix	86.90 (5)
O1ii—Rb1—O4iv	82.87 (5)	O2viii—Rb2—O4ix	101.29 (5)
O1iii—Rb1—O4iv	102.41 (5)	O2ix—Rb2—O4ix	46.55 (5)
O2iv—Rb1—O4iv	46.75 (5)	O4iv—Rb2—O4ix	104.695 (9)
O2v—Rb1—O4iv	131.42 (5)	O4vi—Rb2—O4ix	137.38 (4)
O2vi—Rb1—O4iv	82.92 (5)	O4v—Rb2—O4ix	53.03 (6)
O1i—Rb1—O4vi	82.87 (5)	O4vii—Rb2—O4ix	115.47 (2)
O1ii—Rb1—O4vi	102.41 (5)	O3iv—Rb2—O4viii	157.18 (5)
O1iii—Rb1—O4vi	165.33 (5)	O3vi—Rb2—O4viii	86.01 (4)
O2iv—Rb1—O4vi	131.42 (5)	O3v—Rb2—O4viii	55.94 (4)
O2v—Rb1—O4vi	82.92 (5)	O2vii—Rb2—O4viii	101.29 (5)
O2vi—Rb1—O4vi	46.75 (5)	O2viii—Rb2—O4viii	46.55 (5)
O4iv—Rb1—O4vi	85.47 (6)	O2ix—Rb2—O4viii	86.90 (5)
O1i—Rb1—O4v	102.41 (5)	O4iv—Rb2—O4viii	137.38 (4)
O1ii—Rb1—O4v	165.33 (5)	O4vi—Rb2—O4viii	53.03 (6)
O1iii—Rb1—O4v	82.87 (5)	O4v—Rb2—O4viii	104.695 (9)
O2iv—Rb1—O4v	82.92 (5)	O4vii—Rb2—O4viii	115.47 (2)
O2v—Rb1—O4v	46.75 (5)	O4ix—Rb2—O4viii	115.47 (2)
O2vi—Rb1—O4v	131.42 (5)	O2x—Ti1—O2xi	90.14 (7)
O4iv—Rb1—O4v	85.47 (6)	O2x—Ti1—O2xii	90.14 (7)
O4vi—Rb1—O4v	85.47 (6)	O2xi—Ti1—O2xii	90.14 (7)
O3iv—Rb2—O3vi	101.27 (5)	O2x—Ti1—O1xiii	91.43 (7)
O3iv—Rb2—O3v	101.27 (5)	O2xi—Ti1—O1xiii	88.09 (7)
O3vi—Rb2—O3v	101.27 (5)	O2xii—Ti1—O1xiii	177.64 (7)
O3iv—Rb2—O2vii	99.30 (5)	O2x—Ti1—O1	88.09 (7)
O3vi—Rb2—O2vii	96.75 (5)	O2xi—Ti1—O1	177.64 (7)
O3v—Rb2—O2vii	149.30 (5)	O2xii—Ti1—O1	91.43 (7)
O3iv—Rb2—O2viii	149.30 (5)	O1xiii—Ti1—O1	90.39 (7)
O3vi—Rb2—O2viii	99.30 (5)	O2x—Ti1—O1xiv	177.64 (7)
O3v—Rb2—O2viii	96.75 (5)	O2xi—Ti1—O1xiv	91.43 (7)
O2vii—Rb2—O2viii	55.61 (6)	O2xii—Ti1—O1xiv	88.09 (7)
O3iv—Rb2—O2ix	96.75 (5)	O1xiii—Ti1—O1xiv	90.39 (7)
O3vi—Rb2—O2ix	149.30 (5)	O1—Ti1—O1xiv	90.39 (7)
O3v—Rb2—O2ix	99.30 (5)	O3ii—Ti2—O3iii	91.87 (7)
O2vii—Rb2—O2ix	55.61 (6)	O3ii—Ti2—O3i	91.87 (7)
O2viii—Rb2—O2ix	55.61 (6)	O3iii—Ti2—O3i	91.87 (7)
O3iv—Rb2—O4iv	49.64 (5)	O3ii—Ti2—O4xiii	94.30 (7)
O3vi—Rb2—O4iv	52.65 (5)	O3iii—Ti2—O4xiii	172.63 (8)
O3v—Rb2—O4iv	116.41 (6)	O3i—Ti2—O4xiii	83.90 (7)
O2vii—Rb2—O4iv	94.29 (5)	O3ii—Ti2—O4	83.90 (7)
O2viii—Rb2—O4iv	138.73 (5)	O3iii—Ti2—O4	94.30 (7)
O2ix—Rb2—O4iv	133.42 (5)	O3i—Ti2—O4	172.63 (8)
O3iv—Rb2—O4vi	116.41 (6)	O4xiii—Ti2—O4	90.39 (7)
O3vi—Rb2—O4vi	49.64 (5)	O3ii—Ti2—O4xiv	172.63 (8)
O3v—Rb2—O4vi	52.65 (5)	O3iii—Ti2—O4xiv	83.90 (7)
O2vii—Rb2—O4vi	133.42 (5)	O3i—Ti2—O4xiv	94.30 (7)
O2viii—Rb2—O4vi	94.29 (5)	O4xiii—Ti2—O4xiv	90.39 (7)
O2ix—Rb2—O4vi	138.73 (5)	O4—Ti2—O4xiv	90.39 (7)
O4iv—Rb2—O4vi	87.83 (5)	O3—P1—O2	110.75 (9)
O3iv—Rb2—O4v	52.65 (5)	O3—P1—O4	108.52 (11)
O3vi—Rb2—O4v	116.41 (6)	O2—P1—O4	106.48 (10)
O3v—Rb2—O4v	49.64 (5)	O3—P1—O1	110.87 (9)
O2vii—Rb2—O4v	138.73 (5)	O2—P1—O1	108.74 (10)
O2viii—Rb2—O4v	133.42 (5)	O4—P1—O1	111.40 (10)