SrZn₁₁: a new binary compound with the BaCd₁₁ structure

Single crystals of strontium undeca­zinc, SrZn₁₁, were obtained when decomposing SrZn₂ under conditions of high pressure and high temperature. The new binary Sr-Zn compound crystallizes in the space group I4₁/amd (BaCd₁₁ structure type) with one Sr position (m2) and three Zn sites (m2, .2/m., 1). The structure is described in terms of all-face-capped Zn₈ tetra­hedra as the central building unit, defined by the Zn atoms on .2/m. and 1. The building units are condensed into chains by the central tetra­hedra sharing edges, and the chains are inter­connected by shared capping atoms. The resulting three-dimensional framework of Zn atoms yields channels that are occupied by Sr and Zn atoms on the high-symmetry m2 positions.

S1. Comment

Alkaline earth and rare earth metals (M) display remarkably intricate phase diagrams with zinc (Massalski, 1996). This is particularly true on the Zn-rich side (>80 at.% Zn), where often phases with similar compositions and complex structures occur (e.g. M₃Zn₂₂, M₂Zn₁₇, MZn₁₁, MZn₁₂ and MZn₁₃). For example, if M is Ca or an early rare earth metal (La, Ce, Pr, Nd or Eu), M–Zn systems contain both MZn₁₁ (91.7 at.% Zn) and MZn₁₃ (92.8 at.% Zn) with the BaCd₁₁ or NaZn₁₃ structure, respectively, as the most Zn-rich phases (Sanderson & Baenziger, 1953; Lott & Chiotti, 1966; Iandelli & Palenzona, 1967). If M is Sr or Ba, MZn₁₁ is missing, and MZn₅ (83.3 at.% Zn) and MZn₁₃ represent the most Zn-rich phases (Bruzzone & Merlo, 1983; Bruzzone et al., 1985). We obtained SrZn₁₁ unintentionally when exposing SrZn₂ to 7 GPa and 1273 K by means of multi-anvil high-pressure techniques. Instead of transforming into a high-pressure phase, SrZn₂ decomposed into a Zn-rich phase, SrZn₁₁, and presumably a Sr-rich melt which oxidized under the conditions applied.

The crystal structure of SrZn₁₁ corresponds to the tetragonal BaCd₁₁ type (space group I4₁/amd). Many structures of Mg- and Zn-rich compounds with M can be described using an all-face-capped tetrahedron [tetrahedral star (TS)] as a central building unit which can be linked or condensed in many different ways (Häussermann et al., 1998). In SrZn₁₁, the central tetrahedron of a TS unit is formed by the Zn atoms (Zn3) occupying the general 32i position (Fig. 1). These tetrahedra are centred around the 16 g position (1/4, 1/4, 0) and form strands by sharing edges. Within a strand, each atom from a central tetrahedron also acts as a capping atom for the neighbouring TS unit, and vice versa (Fig. 2). The remaining capping atoms correspond to the Zn atoms occupying the site 8 d (Zn2). The translational period of a TS strand consists of four units and the translational direction coincides with a 4₁ or 4₃ axis. The unit cell contains four tetragonally arranged TS strands, which are linked by the 8 d capping atoms (Fig. 3). The resulting framework of Zn atoms features channels around (0, 0, z) and (1/2, 1/2, z), which are filled alternately by the Sr and the remaining Zn atoms (Zn1) on positions 4a and 4 b, respectively (Figs. 4 and 5).

The Zn—Zn distances within the TS strands range from 2.68031 (15) to 2.963 (2) Å (Table 1). The longest Zn···Zn distances occur within central tetrahedra (Zn3···Zn3). The distances between atom Zn1 on 4 b and neighbouring Zn atoms are 2.8223 (8) Å (to Zn2^iv) and 2.9048 (12) Å (to Zn3^v) [symmetry codes: (iv) ?; (v) ? Please complete]. The Sr—Zn distances are 3.3350 (14) and 3.4396 (14) Å (to Zn3ⁱ and Zn3ⁱⁱ, respectively), and 3.7302 (12) Å to Zn2^iv [symmetry codes: (i) ?; (ii) ? Please complete]. The unit-cell volume of SrZn₁₁ [797.1 (6) ų] is clearly the highest among binary Zn compounds with the BaCd₁₁ structure [M = Eu (789.8 ų), La (785.7 ų), Ca (781.8 ų), Ce (779.5 ų), Yb (771.9 ų), Pr (771.8 ų) and Nd (768.4 ų)]. It has been argued that the TS framework in BaCd₁₁-type compounds is rather rigid, giving little flexibility for the size of the voids around positions 4a and 4 b (Sanderson & Baenziger, 1953; Iandelli & Palenzona, 1967). Therefore, the size of M is an important factor for the stability of MZn₁₁ phases.

Sr and Ba have larger sizes than the M metals displaying phases MZn₁₁ in their M–Zn phase diagrams. This might explain why MZn₁₁ is absent in the Sr–Zn and Ba–Zn systems, and why the synthesis of SrZn₁₁ requires high-pressure conditions.

S2. Experimental

The starting material, SrZn₂, was synthesized from the elements strontium (crystalline dendritic pieces, Alfa Aesar, 99.95%) and zinc (shots, Alfa Aesar, 99.9999%), which were weighed in the appropriate atomic ratio and sealed in a tantalum tube in an argon atmosphere. The tantalum tube was protected from air by a silica jacket sealed under vacuum, then heated at 973 K for 2 h and quenched in water. Subsequently, the sample was reheated and annealed at 773 K for 21 d, followed by quenching in water. Powder X-ray diffraction showed that the sample corresponded to phase-pure CeCu₂-type SrZn₂ (Bergman & Shlichta, 1964). SrZn₁₁ was obtained by subjecting SrZn₂ to a pressure of 7 GPa and a temperature of 1273 K using a 6–8 Walker-type multi-anvil high-pressure module. SrZn₂ was ground in a mortar and a sample (57 mg) was loaded into a boron nitride (BN) capsule (4.555 mm diameter × 2.555 mm long). Thereafter, the BN capsule was positioned with a graphite furnace and a zirconia insulating sleeve in a magnesia octahedron with 14 mm e dge length (Leinenweber & Parize, 1995). The sample was pressurized [to 7 GPa?] and then heated at approximately 400 K min⁻¹ [to 1273 K and kept at this temperature for 1 h?]. After 1 h, the sample was quenched isobarically by turning off power to the furnace, and then slowly decompressed. The powder X-ray diffraction pattern of the recovered sample revealed SrZn₁₁ as the main product. The by-product(s) could not be identified, but may correspond to oxidation products of a Sr-rich melt also formed upon decomposing SrZn₂. The recovery of the high-pressure treated sample was not performed under air-free conditions. Single crystals investigated from the recovered sample corresponded to SrZn₁₁.

S3. Refinement

The origin choice 1 was used for correspondence with a standardized structure type comparison according to the STRUCTURE TIDY program (Gelato & Parthé, 1987). Refinement was performed on F² for all reflections. The weighted R-factor wR₂ 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 > 2σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement.

Computing details

Data collection: SMART (Bruker, 2003); cell refinement: SAINT (Bruker, 2003); data reduction: SAINT (Bruker, 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: SHELXTL (Sheldrick, 2003b).

Figures

	Fig. 1. The tetrahedral star (TS) unit in SrZn11. Atoms Zn2 and Zn3 are denoted by black and medium grey spheres, respectively.
	Fig. 2. Two TS units connected by sharing a common edge. Black spheres are Zn2 and medium-grey spheres are Zn3.
	Fig. 3. A strand of edge-sharing TS units. The repeat unit consists of four TS units and each unit cell is made up of four such repeat units. Black spheres are Zn2 and medium-grey spheres are Zn3.
	Fig. 4. A view of the TS arrangement in an SrZn11 unit cell, along the [001] projection. Zn1, Zn2 and Zn3 are white hatched, black and medium-grey spheres, respectively, and Sr1 is represented as large dark-grey spheres.
	Fig. 5. The channel arrangement of Sr and Zn atoms in SrZn11 along the high-symmetry 4m2 positions, showing 95% probability displacement ellipsoids. Sr1 is dark grey, Zn1 white hatched, Zn2 black and Zn3 medium grey.

strontium undecazinc

Crystal data

SrZn11	Dx = 6.722 Mg m^−3
Mr = 806.69	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I41/amd	Cell parameters from 843 reflections
Hall symbol: -I 4bd 2	θ = 7.0–53.9°
a = 10.749 (3) Å	µ = 38.97 mm^−1
c = 6.899 (4) Å	T = 298 K
V = 797.1 (6) Å^3	Block, metallic grey
Z = 4	0.09 × 0.06 × 0.06 mm
F(000) = 1472

Data collection

Bruker SMART APEX diffractometer	258 independent reflections
Radiation source: fine-focus sealed tube	214 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.113
ω scans	θmax = 27.5°, θmin = 3.5°
Absorption correction: empirical (using intensity measurements) (SADABS; Sheldrick, 2003a)	h = −13→13
Tmin = 0.075, Tmax = 0.101	k = −13→13
3644 measured reflections	l = −8→8

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)] = 0.034	Secondary atom site location: difference Fourier map
wR(F^2) = 0.077	w = 1/[s^2(Fo^2) + (0.0305P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.08	(Δ/σ)max < 0.001
258 reflections	Δρmax = 1.04 e Å^−3
18 parameters	Δρmin = −1.21 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.

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

	x	y	z	Uiso*/Ueq
Sr1	0.0000	0.0000	0.0000	0.0126 (5)
Zn1	0.0000	0.0000	0.5000	0.0191 (7)
Zn2	0.0000	0.2500	0.6250	0.0168 (5)
Zn3	0.12224 (10)	0.20651 (9)	0.30640 (14)	0.0158 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sr1	0.0131 (7)	0.0131 (7)	0.0115 (11)	0.000	0.000	0.000
Zn1	0.0188 (10)	0.0188 (10)	0.0196 (15)	0.000	0.000	0.000
Zn2	0.0121 (10)	0.0263 (12)	0.0119 (10)	0.000	0.000	−0.0026 (8)
Zn3	0.0160 (6)	0.0176 (6)	0.0139 (5)	0.0008 (4)	0.0016 (4)	−0.0013 (4)

Geometric parameters (Å, º)

Sr1—Zn3^i	3.3350 (14)	Zn3—Zn3^vi	2.628 (2)
Sr1—Zn3^ii	3.4396 (14)	Zn3—Zn3^viii	2.672 (2)
Sr1—Zn2^iii	3.7302 (12)	Zn3—Zn3^ix	2.6803 (14)
Zn1—Zn2^iv	2.8223 (8)	Zn3—Zn3^x	2.717 (2)
Zn1—Zn3^v	2.9048 (12)	Zn3—Zn3^xi	2.905 (2)
Zn2—Zn3^vi	2.6031 (15)	Zn3—Zn3^v	2.963 (2)
Zn2—Zn3^vii	2.6528 (13)
Zn3^i—Sr1—Zn3^vi	113.69 (3)	Zn2—Zn3—Zn3^ix	60.26 (4)
Zn3^i—Sr1—Zn3^xii	46.41 (4)	Zn3^vi—Zn3—Zn3^ix	109.75 (3)
Zn3^vi—Sr1—Zn3^xii	82.95 (5)	Zn2^xvii—Zn3—Zn3^ix	106.56 (3)
Zn3^vi—Sr1—Zn3^xiii	101.33 (5)	Zn3^viii—Zn3—Zn3^ix	111.25 (6)
Zn3^xii—Sr1—Zn3^xiii	157.85 (4)	Zn3^xvii—Zn3—Zn3^ix	114.46 (3)
Zn3^xii—Sr1—Zn3^xiv	83.46 (4)	Zn2—Zn3—Zn3^x	117.52 (5)
Zn3^i—Sr1—Zn3^ii	46.58 (3)	Zn3^vi—Zn3—Zn3^x	132.647 (12)
Zn3^vi—Sr1—Zn3^ii	122.12 (3)	Zn2^xvii—Zn3—Zn3^x	105.50 (2)
Zn3^xii—Sr1—Zn3^ii	92.029 (19)	Zn3^viii—Zn3—Zn3^x	59.64 (3)
Zn3^xiii—Sr1—Zn3^ii	67.15 (3)	Zn3^xvii—Zn3—Zn3^x	65.12 (4)
Zn3^xiv—Sr1—Zn3^ii	122.95 (4)	Zn3^ix—Zn3—Zn3^x	59.34 (6)
Zn3^xv—Sr1—Zn3^ii	46.43 (4)	Zn2—Zn3—Zn1	61.36 (3)
Zn3^xvi—Sr1—Zn3^ii	79.699 (16)	Zn3^vi—Zn3—Zn1	63.11 (2)
Zn3—Sr1—Zn3^ii	146.38 (2)	Zn2^xvii—Zn3—Zn1	60.84 (2)
Zn3^ii—Sr1—Zn3^xvii	135.53 (3)	Zn3^viii—Zn3—Zn1	134.28 (5)
Zn3^ii—Sr1—Zn3^viii	167.01 (3)	Zn3^xvii—Zn3—Zn1	110.38 (5)
Zn3^xvii—Sr1—Zn3^viii	46.53 (3)	Zn3^ix—Zn3—Zn1	112.45 (4)
Zn2^iv—Zn1—Zn2	95.357 (7)	Zn3^x—Zn3—Zn1	162.91 (4)
Zn2^iv—Zn1—Zn2^xvii	144.42 (2)	Zn2—Zn3—Zn3^xi	101.66 (5)
Zn2^iv—Zn1—Zn3^v	150.24 (2)	Zn3^vi—Zn3—Zn3^xi	160.99 (4)
Zn2—Zn1—Zn3^v	55.17 (2)	Zn2^xvii—Zn3—Zn3^xi	56.80 (2)
Zn2^xvii—Zn1—Zn3^v	54.05 (3)	Zn3^viii—Zn3—Zn3^xi	107.77 (4)
Zn2^xv—Zn1—Zn3^v	106.87 (3)	Zn3^xvii—Zn3—Zn3^xi	63.92 (5)
Zn3^v—Zn1—Zn3^xv	154.52 (4)	Zn3^ix—Zn3—Zn3^xi	58.05 (4)
Zn2^iv—Zn1—Zn3^xiii	106.87 (2)	Zn3^x—Zn3—Zn3^xi	56.82 (2)
Zn3^v—Zn1—Zn3^xiii	102.21 (2)	Zn1—Zn3—Zn3^xi	106.13 (4)
Zn3^xv—Zn1—Zn3^xiii	53.79 (4)	Zn2—Zn3—Zn3^v	56.49 (4)
Zn3^xiii—Zn1—Zn3^xviii	61.32 (5)	Zn3^vi—Zn3—Zn3^v	107.80 (3)
Zn3^v—Zn1—Zn3^xix	125.25 (5)	Zn2^xvii—Zn3—Zn3^v	54.90 (3)
Zn3^xviii—Zn1—Zn3^xix	99.67 (4)	Zn3^viii—Zn3—Zn3^v	162.11 (2)
Zn3^vi—Zn2—Zn3^xx	119.37 (6)	Zn3^xvii—Zn3—Zn3^v	105.48 (6)
Zn3^xxi—Zn2—Zn3^xx	60.63 (6)	Zn3^ix—Zn3—Zn3^v	61.73 (4)
Zn3^vi—Zn2—Zn3^vii	68.61 (4)	Zn3^x—Zn3—Zn3^v	104.97 (4)
Zn3^xxi—Zn2—Zn3^vii	111.39 (4)	Zn1—Zn3—Zn3^v	59.34 (2)
Zn3^xx—Zn2—Zn3^vii	61.31 (3)	Zn3^xi—Zn3—Zn3^v	54.35 (3)
Zn3—Zn2—Zn3^vii	118.69 (3)	Zn2—Zn3—Sr1	117.12 (4)
Zn3^vii—Zn2—Zn3^xxii	66.40 (4)	Zn3^vi—Zn3—Sr1	66.797 (19)
Zn3^ix—Zn2—Zn3^xxii	113.60 (4)	Zn2^xvii—Zn3—Sr1	76.12 (3)
Zn3^vi—Zn2—Zn1	64.59 (2)	Zn3^viii—Zn3—Sr1	68.85 (5)
Zn3^xxi—Zn2—Zn1	115.41 (2)	Zn3^xvii—Zn3—Sr1	68.76 (4)
Zn3^vii—Zn2—Zn1	64.00 (2)	Zn3^ix—Zn3—Sr1	176.52 (4)
Zn3^ix—Zn2—Zn1	116.00 (2)	Zn3^x—Zn3—Sr1	122.43 (6)
Zn3^vi—Zn2—Sr1^iii	62.86 (3)	Zn1—Zn3—Sr1	66.71 (4)
Zn3^xxi—Zn2—Sr1^iii	117.14 (3)	Zn3^xi—Zn3—Sr1	125.38 (3)
Zn3^vii—Zn2—Sr1^iii	119.78 (2)	Zn3^v—Zn3—Sr1	119.24 (4)
Zn3^ix—Zn2—Sr1^iii	60.22 (2)	Zn2—Zn3—Sr1^iii	74.81 (3)
Zn1—Zn2—Sr1^iii	118.30 (3)	Zn3^vi—Zn3—Sr1^iii	67.542 (17)
Zn1^iii—Zn2—Sr1^iii	61.70 (3)	Zn2^xvii—Zn3—Sr1^iii	170.21 (4)
Zn2—Zn3—Zn3^vi	59.68 (3)	Zn3^viii—Zn3—Sr1^iii	64.73 (4)
Zn2—Zn3—Zn2^xvii	105.13 (4)	Zn3^xvii—Zn3—Sr1^iii	119.97 (3)
Zn3^vi—Zn3—Zn2^xvii	121.18 (2)	Zn3^ix—Zn3—Sr1^iii	64.66 (4)
Zn2—Zn3—Zn3^viii	136.73 (5)	Zn3^x—Zn3—Sr1^iii	66.734 (17)
Zn2^xvii—Zn3—Zn3^viii	117.38 (5)	Zn1—Zn3—Sr1^iii	125.46 (3)
Zn2—Zn3—Zn3^xvii	161.94 (4)	Zn3^xi—Zn3—Sr1^iii	113.455 (17)
Zn3^vi—Zn3—Zn3^xvii	133.38 (3)	Zn3^v—Zn3—Sr1^iii	119.95 (4)
Zn2^xvii—Zn3—Zn3^xvii	58.43 (4)	Sr1—Zn3—Sr1^iii	112.85 (3)
Zn3^viii—Zn3—Zn3^xvii	61.02 (4)

Symmetry codes: (i) −y, −x, −z; (ii) −x, y−1/2, −z+1/4; (iii) −y, x+1/2, z+1/4; (iv) y−1/2, −x, z−1/4; (v) y, x, −z+1; (vi) −x, y, z; (vii) −y, x, −z+1; (viii) x, −y+1/2, −z+1/4; (ix) y, −x+1/2, z+1/4; (x) −y+1/2, −x+1/2, −z+1/2; (xi) −x+1/2, y, −z+3/4; (xii) −y, x, −z; (xiii) x, −y, z; (xiv) y, x, −z; (xv) −x, −y, z; (xvi) y, −x, −z; (xvii) −y+1/2, x, z−1/4; (xviii) y, −x, −z+1; (xix) −y, −x, −z+1; (xx) −x, −y+1/2, −z+5/4; (xxi) x, −y+1/2, −z+5/4; (xxii) −y, −x+1/2, z+1/4.