β-Rubidium antimonide, Rb₄Sb₄

β-RbSb crystallizes with the LiAs structure type. As in the α phase (NaP type), Sb⁻ forms approximate 4₁ helical chains (2₁ crystallographic symmetry), with Sb—Sb distances of 2.838 (1) and 2.862 (1) Å. In contrast to the α phase, the helices have different chirality.

S1. Comment

The monopentelides A^IM^V of the alkaline metals A have been known for a long time and are well characterized. They crystallize in two structure types, which both contain infinite helical chains of M^- as common building units: In each pentelid series, the compounds having a smaller ratio of cation to anion radius, i.e. LiP (Hönle & von Schnering, 1981), LiAs (Cromer, 1959a), NaSb (Cromer, 1959b) and KSb (Hönle & von Schnering, 1981) form the LiAs structure type, whereas higher ionic radius ratios for example in NaP, KP (von Schnering & Hönle, 1979), NaAs, KAs, RbAs (Hönle & von Schnering, 1978), RbSb and CsSb (von Schnering et al., 1979) give rise to the formation of the NaSb structure type. As already expected by von Schnering & Hönle (1979), both structure types may be stable at the border of the ratio ranges.

The second form of RbSb, β-RbSb, crystallizes with the LiAs structure type (Cromer, 1959a). Sb^- atoms form infinite chains arranged around crystallographic 2₁ screw axes with approximate 4₁ symmetry (Fig. 1) running parallel to the [010] direction (Fig. 2). The distances and angles within the chains are in accordance with those observed in KSb (LiAs structure type) and α-RbSb (NaP structure type): The Sb—Sb distances are of two lengths, 2.838 (1) and 2.862 (1) Å [KSb: 2.829 (1) and 2.852 (1) Å; α-RbSb: 2.852 (3) and 2.864 (3) Å]. The interbond angles at the two crystallographically independent Sb atoms are 115.59 (3)° [Sb(1)] and 110.36 (3)° [Sb(2)], respectively, [KSb: 113.95 (4) and 109.38 (4)°; α-RbSb: 116.93 (8) and 109.72 (6)°]. The values of the dihedral angles [56.9 (1) and 71.6 (1)°] are also in the same range as in KSb and the α form of the title compound. The further coordination of the Sb atoms consists of six Rb cations with distances Sb—Rb ranging from 3.641 (2) Å to 3.794 (2) Å. The main difference of the two forms of RbSb is the chirality of the Sb^- screw chains: While in the α form only one chirality is present (acentric space group), the β form described here contains both screw directions side by side, i.e. the Sb chains at (0,y,1/4) and (0,y,3/4) are of different chirality (Fig. 2). We have observed a related form of dimorphism for the corresponding Cs compound, but the β form of CsSb (Hirschle et al., 2000) crystallizes with a superstructure of the LiAs structure. In the series of the ASb structures, the Sb bond angle and the rise per turn of the helical chains increases with the radius of the A cation [NaSb: 6.34 (2) Å; CsSb 7.32 (2) Å], where the β forms show smaller values [RbSb: α: 7.197 (4) Å, β: 7.134 (1) Å; CsSb: α: 7.345 (5) β: 7.32 (2)]. Both the α and the β phase where prepared below the melting point of RbSb at 883 K (Dorn & Klemm 1961). While the α phase (von Schnering et al., 1979) was synthezized by treatment of Sb with Rb at 843 K for 140 h, the β phase is formed at a lower temperature of 793 K. For this reason we assume, that the new modification reported here is the low temperature form of RbSb. This is in accordance with the unit-cell volumes per formular unit (α: 84.3 ų, β: 82.5 ų). Details concerning the thermodynamic and kinetic aspects of the formation of the two phases are currently under study.

Related literature

For related literature, see: Cromer (1959a, 1959b); Dorn & Klemm (1961); Hönle & von Schnering (1978, 1981); Hirschle, Emmerling & Röhr (2000); Schnering & Hönle (1979); Schnering et al. (1979).

S2. Experimental

Liquid rubidium (Alkalimetallhandelsgesellschaft Bonn, 99.9%) (678.8 mg, 7.93 mmol) was reacted with Sb (ABCR, 99.999%) (322.0 mg, 2.64 mmol) in tantalum crucibles under an argon atmosphere. The mixture was heated up to 793 K within 5 h and cooled to 733 K with a rate of 2 K h^-1 and after that to room temperature by switching off the furnace. Almost black metallic crystals in the form of flattened needles of β-RbSb up to a length of 0.3 mm grow in the matrix of elemental Rb.

Computing details

Data collection: CAD-4 Software (Enraf-Nonius, 1989); cell refinement: CAD-4 Software; data reduction: HELENA (Spek, 1993); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP (Johnson, 1968) and DRAWxtl (Finger & Kroeker, 1999); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. ORTEP (Johnson, 1968) view of the ^1∞[Sb]^- chains (75% probability ellipsoids) with the coordination polyhedra of one Sb1 and one Sb2 atom.
	Fig. 2. Unit cell of β-RbSb (light gray spheres: Rb; dark gray balls: Sb).

Rubidium antimonide

Crystal data

Rb4Sb4	F(000) = 704
Mr = 828.88	Dx = 4.172 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71070 Å
a = 7.3566 (8) Å	Cell parameters from 25 reflections
b = 7.1341 (9) Å	θ = 12.3–26.9°
c = 13.7930 (13) Å	µ = 22.70 mm^−1
β = 114.28 (2)°	T = 293 K
V = 659.86 (13) Å^3	Prism, dark metallic
Z = 2	0.3 × 0.1 × 0.08 mm

Data collection

Enraf-Nonius CAD-4 diffractometer	Rint = 0.048
Radiation source: fine-focus sealed tube	θmax = 29.0°, θmin = 4.0°
Graphite monochromator	h = 0→10
ω/2Θ scans	k = 0→9
Absorption correction: ψ scan (North et al., 1968)	l = −18→17
Tmin = 0.051, Tmax = 0.163	3 standard reflections every 120 min
1863 measured reflections	intensity decay: none
1740 independent reflections

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.048	Secondary atom site location: difference Fourier map
wR(F^2) = 0.127	w = 1/[σ^2(Fo^2) + (0.0591P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.02	(Δ/σ)max < 0.001
1627 reflections	Δρmax = 1.97 e Å^−3
37 parameters	Δρmin = −3.01 e Å^−3

Special details

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.

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

	x	y	z	Uiso*/Ueq
Sb1	0.17113 (12)	0.59690 (12)	0.21589 (6)	0.0250 (2)
Sb2	0.17081 (13)	0.33027 (12)	0.37083 (6)	0.0247 (2)
Rb1	0.28587 (19)	0.09726 (19)	0.16633 (9)	0.0305 (3)
Rb2	0.7414 (2)	0.3301 (2)	0.02977 (9)	0.0362 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sb1	0.0195 (4)	0.0274 (4)	0.0289 (4)	−0.0013 (4)	0.0107 (3)	−0.0012 (3)
Sb2	0.0223 (4)	0.0261 (4)	0.0204 (4)	0.0041 (4)	0.0033 (3)	−0.0014 (3)
Rb1	0.0248 (6)	0.0329 (7)	0.0336 (6)	−0.0049 (6)	0.0117 (5)	−0.0059 (5)
Rb2	0.0392 (8)	0.0333 (7)	0.0288 (6)	−0.0054 (6)	0.0066 (5)	0.0019 (5)

Geometric parameters (Å, º)

Sb1—Sb2^i	2.8383 (13)	Rb1—Sb2	3.6605 (15)
Sb1—Sb2	2.8617 (12)	Rb1—Sb2^vi	3.7097 (16)
Sb1—Rb1^ii	3.6414 (17)	Rb1—Sb1	3.7894 (17)
Sb1—Rb2^iii	3.6788 (17)	Rb1—Sb1^ix	3.7943 (17)
Sb1—Rb2^ii	3.6920 (15)	Rb1—Sb2^x	3.8417 (15)
Sb1—Rb2^iv	3.7365 (16)	Rb1—Rb2^iii	4.018 (2)
Sb1—Rb1	3.7894 (17)	Rb1—Rb2^xi	4.026 (2)
Sb1—Rb1^v	3.7943 (17)	Rb2—Sb1^xii	3.6788 (17)
Sb1—Rb1^i	4.2933 (17)	Rb2—Sb1^vii	3.6921 (15)
Sb2—Sb1^vi	2.8383 (13)	Rb2—Sb1^iv	3.7364 (16)
Sb2—Sb1	2.8617 (12)	Rb2—Sb2^vii	3.7793 (16)
Sb2—Rb1	3.6605 (15)	Rb2—Sb2^ii	3.7817 (17)
Sb2—Rb1^i	3.7096 (16)	Rb2—Sb2^x	4.0207 (18)
Sb2—Rb2^ii	3.7793 (16)	Rb2—Rb1^xi	4.026 (2)
Sb2—Rb2^vii	3.7817 (17)	Rb2—Rb2^iv	4.095 (3)
Sb2—Rb1^viii	3.8417 (15)	Rb2—Rb1^ii	4.6833 (19)
Sb2—Rb2^viii	4.0207 (18)	Rb2—Rb1^x	4.776 (2)
Rb1—Sb1^vii	3.6414 (17)
Sb2^i—Sb1—Sb2	115.59 (3)	Rb2^iii—Rb1—Sb1^vi	52.62 (3)
Sb2^i—Sb1—Rb1^ii	143.82 (4)	Rb2^xi—Rb1—Sb1^vi	115.05 (4)
Sb2—Sb1—Rb1^ii	90.28 (4)	Sb1^vii—Rb1—Rb2^vii	92.48 (4)
Sb2^i—Sb1—Rb2^iii	69.57 (4)	Sb2—Rb1—Rb2^vii	52.16 (3)
Sb2—Sb1—Rb2^iii	85.00 (4)	Sb2^vi—Rb1—Rb2^vii	62.94 (3)
Rb1^ii—Sb1—Rb2^iii	141.44 (4)	Sb1—Rb1—Rb2^vii	97.28 (3)
Sb2^i—Sb1—Rb2^ii	85.08 (4)	Sb1^ix—Rb1—Rb2^vii	50.31 (3)
Sb2—Sb1—Rb2^ii	69.10 (3)	Sb2^x—Rb1—Rb2^vii	158.82 (4)
Rb1^ii—Sb1—Rb2^ii	81.09 (4)	Rb2^iii—Rb1—Rb2^vii	100.75 (3)
Rb2^iii—Sb1—Rb2^ii	131.32 (3)	Rb2^xi—Rb1—Rb2^vii	106.53 (3)
Sb2^i—Sb1—Rb2^iv	89.39 (4)	Sb1^vi—Rb1—Rb2^vii	48.14 (3)
Sb2—Sb1—Rb2^iv	145.41 (4)	Sb1^vii—Rb1—Rb1^ii	52.36 (4)
Rb1^ii—Sb1—Rb2^iv	80.68 (4)	Sb2—Rb1—Rb1^ii	66.22 (3)
Rb2^iii—Sb1—Rb2^iv	81.86 (4)	Sb2^vi—Rb1—Rb1^ii	147.72 (5)
Rb2^ii—Sb1—Rb2^iv	140.45 (3)	Sb1—Rb1—Rb1^ii	49.474 (18)
Sb2^i—Sb1—Rb1	134.45 (4)	Sb1^ix—Rb1—Rb1^ii	141.36 (4)
Sb2—Sb1—Rb1	65.05 (3)	Sb2^x—Rb1—Rb1^ii	103.30 (4)
Rb1^ii—Sb1—Rb1	78.24 (3)	Rb2^iii—Rb1—Rb1^ii	104.76 (3)
Rb2^iii—Sb1—Rb1	65.08 (3)	Rb2^xi—Rb1—Rb1^ii	138.11 (5)
Rb2^ii—Sb1—Rb1	129.09 (4)	Sb1^vi—Rb1—Rb1^ii	106.64 (4)
Rb2^iv—Sb1—Rb1	80.40 (3)	Rb2^vii—Rb1—Rb1^ii	97.84 (4)
Sb2^i—Sb1—Rb1^v	66.20 (3)	Sb1^vii—Rb1—Rb1^vii	52.28 (4)
Sb2—Sb1—Rb1^v	145.94 (4)	Sb2—Rb1—Rb1^vii	106.71 (4)
Rb1^ii—Sb1—Rb1^v	78.18 (3)	Sb2^vi—Rb1—Rb1^vii	93.69 (3)
Rb2^iii—Sb1—Rb1^v	123.51 (4)	Sb1—Rb1—Rb1^vii	141.35 (4)
Rb2^ii—Sb1—Rb1^v	77.44 (3)	Sb1^ix—Rb1—Rb1^vii	49.459 (18)
Rb2^iv—Sb1—Rb1^v	64.62 (3)	Sb2^x—Rb1—Rb1^vii	115.77 (4)
Rb1—Sb1—Rb1^v	140.34 (4)	Rb2^iii—Rb1—Rb1^vii	152.07 (4)
Sb2^i—Sb1—Rb1^i	57.52 (3)	Rb2^xi—Rb1—Rb1^vii	65.95 (3)
Sb2—Sb1—Rb1^i	58.44 (3)	Sb1^vi—Rb1—Rb1^vii	106.58 (4)
Rb1^ii—Sb1—Rb1^i	135.83 (4)	Rb2^vii—Rb1—Rb1^vii	61.14 (3)
Rb2^iii—Sb1—Rb1^i	71.48 (3)	Rb1^ii—Rb1—Rb1^vii	99.03 (5)
Rb2^ii—Sb1—Rb1^i	59.86 (3)	Sb1^xii—Rb2—Sb1^vii	78.82 (3)
Rb2^iv—Sb1—Rb1^i	142.82 (4)	Sb1^xii—Rb2—Sb1^iv	98.14 (4)
Rb1—Sb1—Rb1^i	109.63 (4)	Sb1^vii—Rb2—Sb1^iv	153.73 (5)
Rb1^v—Sb1—Rb1^i	109.54 (4)	Sb1^xii—Rb2—Sb2^vii	105.39 (4)
Sb1^vi—Sb2—Sb1	110.36 (3)	Sb1^vii—Rb2—Sb2^vii	45.02 (2)
Sb1^vi—Sb2—Rb1	81.64 (3)	Sb1^iv—Rb2—Sb2^vii	113.14 (4)
Sb1—Sb2—Rb1	69.82 (3)	Sb1^xii—Rb2—Sb2^ii	44.70 (2)
Sb1^vi—Sb2—Rb1^i	69.37 (3)	Sb1^vii—Rb2—Sb2^ii	98.26 (3)
Sb1—Sb2—Rb1^i	80.46 (3)	Sb1^iv—Rb2—Sb2^ii	97.46 (4)
Rb1—Sb2—Rb1^i	127.58 (3)	Sb2^vii—Rb2—Sb2^ii	141.31 (4)
Sb1^vi—Sb2—Rb2^ii	134.11 (4)	Sb1^xii—Rb2—Rb1^xii	58.79 (3)
Sb1—Sb2—Rb2^ii	65.87 (3)	Sb1^vii—Rb2—Rb1^xii	67.52 (3)
Rb1—Sb2—Rb2^ii	130.45 (4)	Sb1^iv—Rb2—Rb1^xii	88.42 (4)
Rb1^i—Sb2—Rb2^ii	64.89 (3)	Sb2^vii—Rb2—Rb1^xii	56.72 (3)
Sb1^vi—Sb2—Rb2^vii	65.73 (3)	Sb2^ii—Rb2—Rb1^xii	103.38 (4)
Sb1—Sb2—Rb2^vii	147.72 (4)	Sb1^xii—Rb2—Sb2^x	158.85 (5)
Rb1—Sb2—Rb2^vii	77.98 (3)	Sb1^vii—Rb2—Sb2^x	97.17 (4)
Rb1^i—Sb2—Rb2^vii	123.00 (4)	Sb1^iv—Rb2—Sb2^x	94.34 (3)
Rb2^ii—Sb2—Rb2^vii	141.31 (4)	Sb2^vii—Rb2—Sb2^x	85.05 (4)
Sb1^vi—Sb2—Rb1^viii	107.25 (4)	Sb2^ii—Rb2—Sb2^x	116.78 (4)
Sb1—Sb2—Rb1^viii	129.45 (4)	Rb1^xii—Rb2—Sb2^x	138.93 (4)
Rb1—Sb2—Rb1^viii	149.18 (4)	Sb1^xii—Rb2—Rb1^xi	130.68 (5)
Rb1^i—Sb2—Rb1^viii	82.50 (4)	Sb1^vii—Rb2—Rb1^xi	103.78 (4)
Rb2^ii—Sb2—Rb1^viii	63.76 (3)	Sb1^iv—Rb2—Rb1^xi	58.38 (3)
Rb2^vii—Sb2—Rb1^viii	79.17 (3)	Sb2^vii—Rb2—Rb1^xi	58.87 (3)
Sb1^vi—Sb2—Rb2^viii	127.48 (4)	Sb2^ii—Rb2—Rb1^xi	155.79 (5)
Sb1—Sb2—Rb2^viii	105.97 (4)	Rb1^xii—Rb2—Rb1^xi	76.50 (4)
Rb1—Sb2—Rb2^viii	76.74 (3)	Sb2^x—Rb2—Rb1^xi	70.47 (3)
Rb1^i—Sb2—Rb2^viii	154.76 (3)	Sb1^xii—Rb2—Rb2^iv	105.03 (5)
Rb2^ii—Sb2—Rb2^viii	94.95 (4)	Sb1^vii—Rb2—Rb2^iv	104.79 (5)
Rb2^vii—Sb2—Rb2^viii	63.22 (4)	Sb1^iv—Rb2—Rb2^iv	101.20 (5)
Rb1^viii—Sb2—Rb2^viii	74.61 (3)	Sb2^vii—Rb2—Rb2^iv	129.51 (6)
Sb1^vii—Rb1—Sb2	102.44 (4)	Sb2^ii—Rb2—Rb2^iv	61.24 (4)
Sb1^vii—Rb1—Sb2^vi	145.72 (4)	Rb1^xii—Rb2—Rb2^iv	162.55 (5)
Sb2—Rb1—Sb2^vi	81.74 (3)	Sb2^x—Rb2—Rb2^iv	55.54 (4)
Sb1^vii—Rb1—Sb1	101.83 (4)	Rb1^xi—Rb2—Rb2^iv	120.94 (5)
Sb2—Rb1—Sb1	45.14 (2)	Sb1^xii—Rb2—Rb1^ii	60.37 (3)
Sb2^vi—Rb1—Sb1	104.57 (4)	Sb1^vii—Rb2—Rb1^ii	52.26 (3)
Sb1^vii—Rb1—Sb1^ix	101.74 (4)	Sb1^iv—Rb2—Rb1^ii	147.32 (4)
Sb2—Rb1—Sb1^ix	98.59 (4)	Sb2^vii—Rb2—Rb1^ii	97.07 (3)
Sb2^vi—Rb1—Sb1^ix	44.43 (2)	Sb2^ii—Rb2—Rb1^ii	49.86 (3)
Sb1—Rb1—Sb1^ix	140.34 (4)	Rb1^xii—Rb2—Rb1^ii	98.63 (4)
Sb1^vii—Rb1—Sb2^x	101.31 (4)	Sb2^x—Rb2—Rb1^ii	100.63 (4)
Sb2—Rb1—Sb2^x	137.44 (4)	Rb1^xi—Rb2—Rb1^ii	154.28 (4)
Sb2^vi—Rb1—Sb2^x	97.50 (4)	Rb2^iv—Rb2—Rb1^ii	65.53 (4)
Sb1—Rb1—Sb2^x	95.54 (4)	Sb1^xii—Rb2—Rb1	116.52 (4)
Sb1^ix—Rb1—Sb2^x	110.45 (4)	Sb1^vii—Rb2—Rb1	48.99 (3)
Sb1^vii—Rb1—Rb2^iii	155.32 (5)	Sb1^iv—Rb2—Rb1	144.89 (4)
Sb2—Rb1—Rb2^iii	70.83 (3)	Sb2^vii—Rb2—Rb1	64.60 (3)
Sb2^vi—Rb1—Rb2^iii	58.39 (3)	Sb2^ii—Rb2—Rb1	103.21 (4)
Sb1—Rb1—Rb2^iii	56.13 (3)	Rb1^xii—Rb2—Rb1	113.45 (4)
Sb1^ix—Rb1—Rb2^iii	102.74 (4)	Sb2^x—Rb2—Rb1	50.98 (3)
Sb2^x—Rb1—Rb2^iii	72.83 (4)	Rb1^xi—Rb2—Rb1	98.78 (4)
Sb1^vii—Rb1—Rb2^xi	92.39 (4)	Rb2^iv—Rb2—Rb1	65.98 (4)
Sb2—Rb1—Rb2^xi	153.93 (5)	Rb1^ii—Rb2—Rb1	59.50 (3)
Sb2^vi—Rb1—Rb2^xi	74.11 (4)	Sb1^xii—Rb2—Rb1^x	134.99 (4)
Sb1—Rb1—Rb2^xi	151.69 (4)	Sb1^vii—Rb2—Rb1^x	144.99 (4)
Sb1^ix—Rb1—Rb2^xi	56.99 (3)	Sb1^iv—Rb2—Rb1^x	48.79 (2)
Sb2^x—Rb1—Rb2^xi	57.36 (3)	Sb2^vii—Rb2—Rb1^x	115.13 (4)
Rb2^iii—Rb1—Rb2^xi	103.50 (4)	Sb2^ii—Rb2—Rb1^x	102.69 (4)
Sb1^vii—Rb1—Sb1^vi	135.83 (4)	Rb1^xii—Rb2—Rb1^x	132.13 (5)
Sb2—Rb1—Sb1^vi	40.85 (2)	Sb2^x—Rb2—Rb1^x	48.24 (3)
Sb2^vi—Rb1—Sb1^vi	41.10 (2)	Rb1^xi—Rb2—Rb1^x	63.73 (2)
Sb1—Rb1—Sb1^vi	70.44 (3)	Rb2^iv—Rb2—Rb1^x	63.18 (4)
Sb1^ix—Rb1—Sb1^vi	70.39 (3)	Rb1^ii—Rb2—Rb1^x	128.71 (4)
Sb2^x—Rb1—Sb1^vi	122.43 (4)	Rb1—Rb2—Rb1^x	98.55 (4)

Symmetry codes: (i) −x, y+1/2, −z+1/2; (ii) −x+1, y+1/2, −z+1/2; (iii) x−1, y, z; (iv) −x+1, −y+1, −z; (v) x, y+1, z; (vi) −x, y−1/2, −z+1/2; (vii) −x+1, y−1/2, −z+1/2; (viii) x, −y+1/2, z+1/2; (ix) x, y−1, z; (x) x, −y+1/2, z−1/2; (xi) −x+1, −y, −z; (xii) x+1, y, z.