Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270199015553/br1274sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270199015553/br1274Isup2.hkl |
HT-LaAs2 was obtained as a by-product from a reaction of powders of elemental lanthanum, tin, and arsenic in the ratio 2:1:5, heated in an evacuated fused-silica tube at 1223 K for 4 d, and followed by cooling to room temperature over 1 d. Powder X-ray diffraction analysis revealed the major phase to be HT-LaAs2, along with LaAs and SnAs. The needle-shaped crystals were verified to contain La and As in the ratio 1:2 according to an EDX (energy-dispersive X-ray) analysis on a Hitachi F2700 scanning electron microscope.
Crystal integrity was verified by Weissenberg photographs, which show monoclinic symmetry and systematic absences consistent with the space groups C2/c or Cc. After the structure was successfully solved and refined in Cc, it was standardized with the program STRUCTURE TIDY (Gelato & Parthé, 1987). While analysis of the final refined structure by ADDSYM (Le Page, 1988) in the PLATON suite of programs (Spek, 1997) indicated the presence of a fourfold screw axis and an inversion centre as pseudosymmetry elements, the deviation of two of the atoms, As2 and As5 (0.243 Å), is large and close to the generous default limit (0.25 Å) for positional errors set by the program. These distortions of the As2 and As5 positions result in the formation of the asymmetric As57− anionic units and are largely responsible for the lowering to monoclinic symmetry and the choice of a noncentrosymmetric space group.
Data collection: CAD-4-PC (Enraf-Nonius, 1992); cell refinement: CAD-4-PC; data reduction: XCAD4 (Harms & Wocadlo, 1996); program(s) used to solve structure: SHELXTL (Sheldrick, 1997); program(s) used to refine structure: SHELXTL; molecular graphics: ATOMS (Dowty, 1997); software used to prepare material for publication: SHELXTL.
LaAs2 | F(000) = 1968 |
Mr = 288.75 | Dx = 6.419 Mg m−3 |
Monoclinic, Cc | Mo Kα radiation, λ = 0.71073 Å |
a = 12.864 (3) Å | Cell parameters from 24 reflections |
b = 14.422 (3) Å | θ = 11.1–15.1° |
c = 9.0830 (18) Å | µ = 35.93 mm−1 |
β = 134.83 (3)° | T = 295 K |
V = 1195.2 (4) Å3 | Needle, black |
Z = 16 | 0.20 × 0.03 × 0.02 mm |
Enraf-Nonius CAD4 diffractometer | 2990 reflections with F2 > 2σ(F2) |
Radiation source: fine-focus sealed tube | Rint = 0.077 |
Graphite monochromator | θmax = 30.0°, θmin = 2.6° |
θ–2θ scans | h = −18→18 |
Absorption correction: numerical (SHELXTL; Sheldrick, 1997) | k = −20→20 |
Tmin = 0.052, Tmax = 0.534 | l = −12→12 |
6855 measured reflections | 3 standard reflections every 250 min |
3494 independent reflections | intensity decay: none |
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | w = 1/[σ2(Fo2) + (0.0331P)2] where P = (Fo2 + 2Fc2)/3 |
R[F2 > 2σ(F2)] = 0.048 | (Δ/σ)max = 0.001 |
wR(F2) = 0.104 | Δρmax = 2.50 e Å−3 |
S = 1.01 | Δρmin = −2.23 e Å−3 |
3494 reflections | Extinction correction: SHELXTL (Sheldrick, 1997), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
110 parameters | Extinction coefficient: 0.00045 (3) |
2 restraints | Absolute structure: Flack H D (1983), Acta Cryst. A39, 876-881 |
Primary atom site location: structure-invariant direct methods | Absolute structure parameter: 0.03 (4) |
LaAs2 | V = 1195.2 (4) Å3 |
Mr = 288.75 | Z = 16 |
Monoclinic, Cc | Mo Kα radiation |
a = 12.864 (3) Å | µ = 35.93 mm−1 |
b = 14.422 (3) Å | T = 295 K |
c = 9.0830 (18) Å | 0.20 × 0.03 × 0.02 mm |
β = 134.83 (3)° |
Enraf-Nonius CAD4 diffractometer | 2990 reflections with F2 > 2σ(F2) |
Absorption correction: numerical (SHELXTL; Sheldrick, 1997) | Rint = 0.077 |
Tmin = 0.052, Tmax = 0.534 | 3 standard reflections every 250 min |
6855 measured reflections | intensity decay: none |
3494 independent reflections |
R[F2 > 2σ(F2)] = 0.048 | 2 restraints |
wR(F2) = 0.104 | Δρmax = 2.50 e Å−3 |
S = 1.01 | Δρmin = −2.23 e Å−3 |
3494 reflections | Absolute structure: Flack H D (1983), Acta Cryst. A39, 876-881 |
110 parameters | Absolute structure parameter: 0.03 (4) |
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 | ||
La1 | 0.20913 (10) | 0.45944 (7) | 0.07692 (14) | 0.0075 (2) | |
La2 | 0.31390 (10) | 0.04212 (7) | 0.38156 (14) | 0.00727 (19) | |
La3 | 0.39355 (11) | 0.27908 (7) | 0.68047 (15) | 0.00710 (19) | |
La4 | 0.59953 (10) | 0.30229 (7) | 0.28369 (14) | 0.0078 (2) | |
As1 | 0.0002 (2) | 0.12726 (13) | 0.0001 (3) | 0.0091 (4) | |
As2 | 0.0209 (2) | 0.02337 (12) | 0.2495 (3) | 0.0100 (4) | |
As3 | 0.1048 (2) | 0.37244 (14) | 0.2733 (3) | 0.0089 (4) | |
As4 | 0.18993 (19) | 0.11817 (14) | 0.5662 (3) | 0.0076 (4) | |
As5 | 0.2010 (2) | 0.24884 (13) | 0.2091 (3) | 0.0105 (4) | |
As6 | 0.2907 (2) | 0.62784 (15) | 0.3638 (3) | 0.0100 (4) | |
As7 | 0.41172 (19) | 0.12441 (14) | 0.1747 (3) | 0.0089 (4) | |
As8 | 0.5272 (2) | 0.12585 (12) | 0.0398 (3) | 0.0086 (4) |
U11 | U22 | U33 | U12 | U13 | U23 | |
La1 | 0.0087 (4) | 0.0061 (4) | 0.0104 (5) | −0.0001 (4) | 0.0078 (4) | −0.0006 (4) |
La2 | 0.0074 (4) | 0.0060 (4) | 0.0098 (5) | 0.0003 (4) | 0.0066 (4) | 0.0000 (4) |
La3 | 0.0096 (4) | 0.0048 (4) | 0.0085 (4) | −0.0003 (4) | 0.0070 (4) | −0.0007 (4) |
La4 | 0.0104 (4) | 0.0060 (4) | 0.0090 (5) | −0.0002 (4) | 0.0076 (4) | −0.0003 (4) |
As1 | 0.0098 (8) | 0.0112 (9) | 0.0087 (9) | −0.0024 (7) | 0.0074 (8) | −0.0017 (7) |
As2 | 0.0101 (8) | 0.0068 (8) | 0.0152 (9) | 0.0014 (7) | 0.0096 (8) | 0.0008 (7) |
As3 | 0.0100 (8) | 0.0074 (8) | 0.0100 (8) | 0.0003 (7) | 0.0074 (7) | −0.0005 (7) |
As4 | 0.0093 (9) | 0.0065 (7) | 0.0094 (8) | −0.0012 (7) | 0.0074 (8) | −0.0004 (7) |
As5 | 0.0161 (9) | 0.0078 (8) | 0.0118 (8) | 0.0035 (7) | 0.0113 (8) | 0.0024 (7) |
As6 | 0.0108 (9) | 0.0103 (8) | 0.0115 (9) | −0.0016 (6) | 0.0088 (8) | −0.0007 (7) |
As7 | 0.0092 (9) | 0.0092 (9) | 0.0103 (8) | 0.0003 (7) | 0.0075 (8) | 0.0012 (7) |
As8 | 0.0088 (8) | 0.0052 (8) | 0.0123 (9) | 0.0002 (6) | 0.0076 (8) | 0.0003 (6) |
La1—As6i | 3.049 (2) | La4—As7 | 3.167 (2) |
La1—As7ii | 3.121 (3) | La4—As5iii | 3.173 (2) |
La1—As6 | 3.152 (2) | La4—As1iii | 3.173 (2) |
La1—As3 | 3.152 (2) | La4—As6xii | 3.223 (2) |
La1—As1iii | 3.172 (3) | La4—As2v | 3.293 (2) |
La1—As3i | 3.173 (2) | La4—La3xiii | 4.085 (2) |
La1—As8iv | 3.200 (2) | La4—La2v | 4.1112 (16) |
La1—As2v | 3.245 (2) | As1—As5 | 2.537 (3) |
La1—As5 | 3.295 (2) | As1—As2 | 2.569 (3) |
La1—La2ii | 4.0349 (18) | As1—La3ii | 3.092 (2) |
La1—La3i | 4.1937 (16) | As1—La1ii | 3.172 (3) |
La2—As2 | 3.057 (2) | As1—La4ii | 3.173 (2) |
La2—As3iii | 3.067 (3) | As2—As4 | 2.460 (3) |
La2—As4vi | 3.107 (2) | As2—La3ii | 3.122 (2) |
La2—As7vii | 3.117 (2) | As2—La1viii | 3.245 (2) |
La2—As7 | 3.139 (2) | As2—La4viii | 3.293 (2) |
La2—As8vii | 3.147 (2) | As3—As5 | 2.463 (3) |
La2—As1 | 3.176 (3) | As3—La2ii | 3.067 (3) |
La2—As5 | 3.200 (2) | As3—La3ii | 3.105 (2) |
La2—As4 | 3.214 (2) | As3—La1ix | 3.173 (2) |
La2—La3 | 4.0222 (16) | As4—La2vii | 3.107 (2) |
La2—La1iii | 4.0349 (18) | As4—La4xiv | 3.132 (2) |
La2—La4viii | 4.1112 (16) | As4—La3ii | 3.190 (3) |
La3—As6ix | 3.071 (2) | As5—La4ii | 3.173 (2) |
La3—As4 | 3.081 (2) | As6—As8iv | 2.468 (3) |
La3—As1iii | 3.092 (2) | As6—La1ix | 3.049 (2) |
La3—As3iii | 3.105 (2) | As6—La3i | 3.071 (2) |
La3—As5 | 3.115 (3) | As6—La4ix | 3.159 (3) |
La3—As2iii | 3.122 (2) | As6—La4iv | 3.223 (2) |
La3—As3 | 3.150 (3) | As7—As8 | 2.505 (3) |
La3—As4iii | 3.190 (3) | As7—La2vi | 3.117 (2) |
La3—As8x | 3.253 (2) | As7—La1iii | 3.121 (3) |
La3—La4x | 4.085 (2) | As7—La4ii | 3.122 (3) |
La3—La1ix | 4.1937 (16) | As8—As6xii | 2.468 (3) |
La4—As8 | 3.057 (2) | As8—La2vi | 3.147 (2) |
La4—As7iii | 3.122 (3) | As8—La1xii | 3.200 (2) |
La4—As4xi | 3.132 (2) | As8—La3xiii | 3.253 (2) |
La4—As6i | 3.159 (3) | ||
As6i—La1—As7ii | 77.49 (6) | As2iii—La3—La1ix | 50.07 (4) |
As6i—La1—As6 | 141.26 (7) | As3—La3—La1ix | 48.69 (4) |
As7ii—La1—As6 | 133.26 (6) | As4iii—La3—La1ix | 82.98 (5) |
As6i—La1—As3 | 132.02 (7) | As8x—La3—La1ix | 130.63 (5) |
As7ii—La1—As3 | 81.28 (6) | La2—La3—La1ix | 141.08 (4) |
As6—La1—As3 | 82.26 (6) | La4x—La3—La1ix | 87.51 (3) |
As6i—La1—As1iii | 88.05 (6) | As8—La4—As7iii | 126.63 (6) |
As7ii—La1—As1iii | 133.73 (7) | As8—La4—As4xi | 84.64 (6) |
As6—La1—As1iii | 83.19 (6) | As7iii—La4—As4xi | 80.89 (6) |
As3—La1—As1iii | 76.54 (6) | As8—La4—As6i | 84.79 (6) |
As6i—La1—As3i | 83.56 (6) | As7iii—La4—As6i | 141.22 (7) |
As7ii—La1—As3i | 81.97 (6) | As4xi—La4—As6i | 80.32 (6) |
As6—La1—As3i | 79.63 (6) | As8—La4—As7 | 47.43 (5) |
As3—La1—As3i | 135.07 (7) | As7iii—La4—As7 | 135.72 (6) |
As1iii—La1—As3i | 140.28 (6) | As4xi—La4—As7 | 129.81 (6) |
As6i—La1—As8iv | 142.78 (6) | As6i—La4—As7 | 81.26 (6) |
As7ii—La1—As8iv | 88.00 (6) | As8—La4—As5iii | 110.05 (6) |
As6—La1—As8iv | 45.71 (6) | As7iii—La4—As5iii | 66.33 (6) |
As3—La1—As8iv | 77.67 (6) | As4xi—La4—As5iii | 146.76 (6) |
As1iii—La1—As8iv | 125.04 (6) | As6i—La4—As5iii | 129.19 (6) |
As3i—La1—As8iv | 60.35 (5) | As7—La4—As5iii | 76.15 (6) |
As6i—La1—As2v | 64.06 (6) | As8—La4—As1iii | 131.73 (6) |
As7ii—La1—As2v | 138.88 (6) | As7iii—La4—As1iii | 86.78 (6) |
As6—La1—As2v | 78.69 (6) | As4xi—La4—As1iii | 139.83 (6) |
As3—La1—As2v | 135.58 (6) | As6i—La4—As1iii | 86.14 (6) |
As1iii—La1—As2v | 61.68 (6) | As7—La4—As1iii | 84.34 (6) |
As3i—La1—As2v | 79.88 (6) | As5iii—La4—As1iii | 47.13 (5) |
As8iv—La1—As2v | 113.64 (6) | As8—La4—As6xii | 46.19 (6) |
As6i—La1—As5 | 87.96 (6) | As7iii—La4—As6xii | 80.94 (6) |
As7ii—La1—As5 | 75.03 (6) | As4xi—La4—As6xii | 82.23 (5) |
As6—La1—As5 | 119.12 (6) | As6i—La4—As6xii | 129.17 (7) |
As3—La1—As5 | 44.86 (5) | As7—La4—As6xii | 74.38 (6) |
As1iii—La1—As5 | 60.57 (5) | As5iii—La4—As6xii | 87.15 (6) |
As3i—La1—As5 | 156.72 (6) | As1iii—La4—As6xii | 133.36 (6) |
As8iv—La1—As5 | 121.40 (6) | As8—La4—As2v | 145.23 (6) |
As2v—La1—As5 | 115.59 (6) | As7iii—La4—As2v | 80.95 (6) |
As6i—La1—La2ii | 127.55 (5) | As4xi—La4—As2v | 79.15 (6) |
As7ii—La1—La2ii | 50.07 (5) | As6i—La4—As2v | 62.36 (5) |
As6—La1—La2ii | 87.12 (5) | As7—La4—As2v | 129.72 (5) |
As3—La1—La2ii | 48.64 (5) | As5iii—La4—As2v | 100.09 (6) |
As1iii—La1—La2ii | 125.14 (5) | As1iii—La4—As2v | 61.14 (5) |
As3i—La1—La2ii | 89.55 (5) | As6xii—La4—As2v | 155.80 (6) |
As8iv—La1—La2ii | 49.94 (4) | As8—La4—La3xiii | 51.78 (4) |
As2v—La1—La2ii | 163.59 (5) | As7iii—La4—La3xiii | 130.65 (5) |
As5—La1—La2ii | 78.53 (4) | As4xi—La4—La3xiii | 50.37 (4) |
As6i—La1—La3i | 96.90 (5) | As6i—La4—La3xiii | 48.11 (5) |
As7ii—La1—La3i | 130.08 (5) | As7—La4—La3xiii | 83.32 (5) |
As6—La1—La3i | 46.82 (4) | As5iii—La4—La3xiii | 159.30 (5) |
As3—La1—La3i | 129.08 (5) | As1iii—La4—La3xiii | 133.89 (5) |
As1iii—La1—La3i | 94.87 (5) | As6xii—La4—La3xiii | 84.78 (5) |
As3i—La1—La3i | 48.21 (5) | As2v—La4—La3xiii | 95.00 (5) |
As8iv—La1—La3i | 66.76 (4) | As8—La4—La2v | 132.24 (5) |
As2v—La1—La3i | 47.55 (4) | As7iii—La4—La2v | 48.73 (4) |
As5—La1—La3i | 154.89 (5) | As4xi—La4—La2v | 48.51 (4) |
La2ii—La1—La3i | 116.22 (3) | As6i—La4—La2v | 94.23 (5) |
As2—La2—As3iii | 130.48 (7) | As7—La4—La2v | 175.48 (5) |
As2—La2—As4vi | 83.27 (6) | As5iii—La4—La2v | 107.15 (5) |
As3iii—La2—As4vi | 141.44 (6) | As1iii—La4—La2v | 95.65 (5) |
As2—La2—As7vii | 84.87 (6) | As6xii—La4—La2v | 108.62 (5) |
As3iii—La2—As7vii | 83.74 (6) | As2v—La4—La2v | 47.19 (4) |
As4vi—La2—As7vii | 81.37 (6) | La3xiii—La4—La2v | 93.50 (4) |
As2—La2—As7 | 134.60 (6) | As5—As1—As2 | 108.98 (10) |
As3iii—La2—As7 | 82.33 (6) | As5—As1—La3ii | 84.23 (7) |
As4vi—La2—As7 | 83.76 (6) | As2—As1—La3ii | 66.19 (6) |
As7vii—La2—As7 | 135.37 (7) | As5—As1—La1ii | 152.68 (9) |
As2—La2—As8vii | 123.23 (6) | As2—As1—La1ii | 97.97 (8) |
As3iii—La2—As8vii | 79.73 (6) | La3ii—As1—La1ii | 103.22 (6) |
As4vi—La2—As8vii | 64.14 (6) | As5—As1—La4ii | 66.43 (7) |
As7vii—La2—As8vii | 47.14 (5) | As2—As1—La4ii | 153.68 (9) |
As7—La2—As8vii | 88.63 (6) | La3ii—As1—La4ii | 135.33 (7) |
As2—La2—As1 | 48.63 (6) | La1ii—As1—La4ii | 91.29 (6) |
As3iii—La2—As1 | 133.09 (7) | As5—As1—La2 | 67.06 (7) |
As4vi—La2—As1 | 81.38 (6) | As2—As1—La2 | 63.27 (6) |
As7vii—La2—As1 | 131.85 (6) | La3ii—As1—La2 | 106.73 (6) |
As7—La2—As1 | 86.45 (6) | La1ii—As1—La2 | 132.54 (8) |
As8vii—La2—As1 | 145.50 (6) | La4ii—As1—La2 | 92.44 (6) |
As2—La2—As5 | 83.21 (5) | As4—As2—As1 | 99.43 (9) |
As3iii—La2—As5 | 87.43 (6) | As4—As2—La2 | 70.31 (7) |
As4vi—La2—As5 | 118.75 (6) | As1—As2—La2 | 68.11 (7) |
As7vii—La2—As5 | 155.01 (6) | As4—As2—La3ii | 68.52 (7) |
As7—La2—As5 | 65.80 (5) | As1—As2—La3ii | 64.98 (6) |
As8vii—La2—As5 | 152.80 (6) | La2—As2—La3ii | 108.99 (6) |
As1—La2—As5 | 46.89 (5) | As4—As2—La1viii | 119.69 (9) |
As2—La2—As4 | 46.12 (5) | As1—As2—La1viii | 114.54 (9) |
As3iii—La2—As4 | 84.60 (6) | La2—As2—La1viii | 167.67 (7) |
As4vi—La2—As4 | 127.88 (6) | La3ii—As2—La1viii | 82.38 (5) |
As7vii—La2—As4 | 82.37 (6) | As4—As2—La4viii | 117.92 (8) |
As7—La2—As4 | 137.65 (6) | As1—As2—La4viii | 118.82 (8) |
As8vii—La2—As4 | 128.17 (6) | La2—As2—La4viii | 80.61 (5) |
As1—La2—As4 | 73.80 (6) | La3ii—As2—La4viii | 170.22 (7) |
As5—La2—As4 | 73.54 (5) | La1viii—As2—La4viii | 87.90 (5) |
As2—La2—La3 | 89.83 (5) | As5—As3—La2ii | 114.95 (9) |
As3iii—La2—La3 | 49.74 (4) | As5—As3—La3ii | 85.18 (8) |
As4vi—La2—La3 | 167.30 (5) | La2ii—As3—La3ii | 81.34 (6) |
As7vii—La2—La3 | 108.71 (5) | As5—As3—La3 | 66.11 (7) |
As7—La2—La3 | 93.67 (5) | La2ii—As3—La3 | 176.71 (8) |
As8vii—La2—La3 | 128.36 (5) | La3ii—As3—La3 | 95.74 (6) |
As1—La2—La3 | 86.05 (5) | As5—As3—La1 | 70.64 (7) |
As5—La2—La3 | 49.51 (5) | La2ii—As3—La1 | 80.89 (6) |
As4—La2—La3 | 48.84 (4) | La3ii—As3—La1 | 140.08 (8) |
As2—La2—La1iii | 174.26 (5) | La3—As3—La1 | 102.37 (6) |
As3iii—La2—La1iii | 50.47 (5) | As5—As3—La1ix | 141.57 (9) |
As4vi—La2—La1iii | 94.01 (5) | La2ii—As3—La1ix | 97.12 (7) |
As7vii—La2—La1iii | 89.73 (5) | La3ii—As3—La1ix | 121.80 (7) |
As7—La2—La1iii | 49.67 (5) | La3—As3—La1ix | 83.10 (6) |
As8vii—La2—La1iii | 51.11 (4) | La1—As3—La1ix | 95.71 (6) |
As1—La2—La1iii | 136.07 (5) | As2—As4—La3 | 130.78 (9) |
As5—La2—La1iii | 102.53 (5) | As2—As4—La2vii | 96.93 (8) |
As4—La2—La1iii | 135.02 (5) | La3—As4—La2vii | 119.21 (7) |
La3—La2—La1iii | 93.76 (4) | As2—As4—La4xiv | 121.73 (8) |
As2—La2—La4viii | 52.20 (4) | La3—As4—La4xiv | 96.73 (6) |
As3iii—La2—La4viii | 132.27 (5) | La2vii—As4—La4xiv | 82.44 (6) |
As4vi—La2—La4viii | 49.04 (4) | As2—As4—La3ii | 65.61 (7) |
As7vii—La2—La4viii | 48.84 (5) | La3—As4—La3ii | 95.40 (6) |
As7—La2—La4viii | 132.80 (5) | La2vii—As4—La3ii | 142.90 (7) |
As8vii—La2—La4viii | 71.76 (5) | La4xiv—As4—La3ii | 80.49 (6) |
As1—La2—La4viii | 86.94 (5) | As2—As4—La2 | 63.57 (7) |
As5—La2—La4viii | 132.38 (5) | La3—As4—La2 | 79.40 (5) |
As4—La2—La4viii | 84.03 (4) | La2vii—As4—La2 | 96.10 (6) |
La3—La2—La4viii | 132.35 (3) | La4xiv—As4—La2 | 174.59 (7) |
La1iii—La2—La4viii | 122.38 (3) | La3ii—As4—La2 | 103.53 (6) |
As6ix—La3—As4 | 85.58 (6) | As3—As5—As1 | 105.12 (10) |
As6ix—La3—As1iii | 128.18 (6) | As3—As5—La3 | 67.59 (7) |
As4—La3—As1iii | 137.73 (6) | As1—As5—La3 | 121.94 (9) |
As6ix—La3—As3iii | 142.14 (7) | As3—As5—La4ii | 127.77 (9) |
As4—La3—As3iii | 86.26 (6) | As1—As5—La4ii | 66.44 (7) |
As1iii—La3—As3iii | 79.70 (6) | La3—As5—La4ii | 162.21 (7) |
As6ix—La3—As5 | 125.26 (6) | As3—As5—La2 | 133.53 (9) |
As4—La3—As5 | 76.59 (6) | As1—As5—La2 | 66.04 (7) |
As1iii—La3—As5 | 63.44 (6) | La3—As5—La2 | 79.10 (5) |
As3iii—La3—As5 | 88.30 (6) | La4ii—As5—La2 | 91.99 (6) |
As6ix—La3—As2iii | 81.83 (6) | As3—As5—La1 | 64.50 (7) |
As4—La3—As2iii | 163.00 (6) | As1—As5—La1 | 129.90 (9) |
As1iii—La3—As2iii | 48.83 (5) | La3—As5—La1 | 99.96 (6) |
As3iii—La3—As2iii | 110.67 (6) | La4ii—As5—La1 | 82.06 (5) |
As5—La3—As2iii | 101.54 (6) | La2—As5—La1 | 156.49 (7) |
As6ix—La3—As3 | 81.23 (6) | As8iv—As6—La1ix | 87.37 (8) |
As4—La3—As3 | 84.69 (7) | As8iv—As6—La3i | 96.49 (8) |
As1iii—La3—As3 | 77.74 (6) | La1ix—As6—La3i | 175.81 (8) |
As3iii—La3—As3 | 134.57 (7) | As8iv—As6—La1 | 68.17 (7) |
As5—La3—As3 | 46.30 (6) | La1ix—As6—La1 | 98.26 (6) |
As2iii—La3—As3 | 82.14 (6) | La3i—As6—La1 | 84.72 (6) |
As6ix—La3—As4iii | 80.77 (6) | As8iv—As6—La4ix | 160.86 (10) |
As4—La3—As4iii | 142.57 (7) | La1ix—As6—La4ix | 93.90 (6) |
As1iii—La3—As4iii | 75.28 (6) | La3i—As6—La4ix | 81.92 (6) |
As3iii—La3—As4iii | 83.63 (6) | La1—As6—La4ix | 130.26 (7) |
As5—La3—As4iii | 138.71 (6) | As8iv—As6—La4iv | 63.36 (6) |
As2iii—La3—As4iii | 45.87 (5) | La1ix—As6—La4iv | 85.20 (6) |
As3—La3—As4iii | 126.75 (7) | La3i—As6—La4iv | 95.06 (6) |
As6ix—La3—As8x | 82.98 (6) | La1—As6—La4iv | 131.19 (7) |
As4—La3—As8x | 63.17 (6) | La4ix—As6—La4iv | 97.68 (6) |
As1iii—La3—As8x | 135.27 (6) | As8—As7—La2vi | 67.05 (7) |
As3iii—La3—As8x | 60.46 (6) | As8—As7—La1iii | 85.16 (7) |
As5—La3—As8x | 129.02 (6) | La2vi—As7—La1iii | 97.18 (7) |
As2iii—La3—As8x | 125.96 (6) | As8—As7—La4ii | 99.87 (8) |
As3—La3—As8x | 145.10 (6) | La2vi—As7—La4ii | 82.43 (6) |
As4iii—La3—As8x | 80.59 (6) | La1iii—As7—La4ii | 174.28 (7) |
As6ix—La3—La2 | 137.33 (5) | As8—As7—La2 | 157.23 (9) |
As4—La3—La2 | 51.76 (4) | La2vi—As7—La2 | 97.42 (6) |
As1iii—La3—La2 | 90.60 (5) | La1iii—As7—La2 | 80.26 (6) |
As3iii—La3—La2 | 48.93 (5) | La4ii—As7—La2 | 94.12 (6) |
As5—La3—La2 | 51.39 (4) | As8—As7—La4 | 63.98 (6) |
As2iii—La3—La2 | 139.36 (5) | La2vi—As7—La4 | 130.61 (7) |
As3—La3—La2 | 92.42 (5) | La1iii—As7—La4 | 84.98 (6) |
As4iii—La3—La2 | 132.42 (5) | La4ii—As7—La4 | 99.64 (6) |
As8x—La3—La2 | 78.64 (5) | La2—As7—La4 | 131.19 (7) |
As6ix—La3—La4x | 49.97 (5) | As6xii—As8—As7 | 101.91 (10) |
As4—La3—La4x | 96.15 (5) | As6xii—As8—La4 | 70.45 (7) |
As1iii—La3—La4x | 124.37 (5) | As7—As8—La4 | 68.59 (7) |
As3iii—La3—La4x | 94.44 (5) | As6xii—As8—La2vi | 125.18 (9) |
As5—La3—La4x | 172.07 (5) | As7—As8—La2vi | 65.81 (7) |
As2iii—La3—La4x | 84.46 (5) | La4—As8—La2vi | 133.94 (7) |
As3—La3—La4x | 130.76 (6) | As6xii—As8—La1xii | 66.12 (7) |
As4iii—La3—La4x | 49.14 (5) | As7—As8—La1xii | 125.74 (8) |
As8x—La3—La4x | 47.59 (4) | La4—As8—La1xii | 136.19 (7) |
La2—La3—La4x | 126.21 (3) | La2vi—As8—La1xii | 78.94 (5) |
As6ix—La3—La1ix | 48.45 (4) | As6xii—As8—La3xiii | 119.92 (9) |
As4—La3—La1ix | 112.93 (5) | As7—As8—La3xiii | 114.86 (8) |
As1iii—La3—La1ix | 83.13 (5) | La4—As8—La3xiii | 80.63 (5) |
As3iii—La3—La1ix | 160.43 (5) | La2vi—As8—La3xiii | 113.01 (7) |
As5—La3—La1ix | 92.39 (5) | La1xii—As8—La3xiii | 116.44 (7) |
Symmetry codes: (i) x, −y+1, z−1/2; (ii) x−1/2, −y+1/2, z−1/2; (iii) x+1/2, −y+1/2, z+1/2; (iv) x−1/2, y+1/2, z; (v) x+1/2, y+1/2, z; (vi) x, −y, z−1/2; (vii) x, −y, z+1/2; (viii) x−1/2, y−1/2, z; (ix) x, −y+1, z+1/2; (x) x, y, z+1; (xi) x+1/2, −y+1/2, z−1/2; (xii) x+1/2, y−1/2, z; (xiii) x, y, z−1; (xiv) x−1/2, −y+1/2, z+1/2. |
Experimental details
Crystal data | |
Chemical formula | LaAs2 |
Mr | 288.75 |
Crystal system, space group | Monoclinic, Cc |
Temperature (K) | 295 |
a, b, c (Å) | 12.864 (3), 14.422 (3), 9.0830 (18) |
β (°) | 134.83 (3) |
V (Å3) | 1195.2 (4) |
Z | 16 |
Radiation type | Mo Kα |
µ (mm−1) | 35.93 |
Crystal size (mm) | 0.20 × 0.03 × 0.02 |
Data collection | |
Diffractometer | Enraf-Nonius CAD4 diffractometer |
Absorption correction | Numerical (SHELXTL; Sheldrick, 1997) |
Tmin, Tmax | 0.052, 0.534 |
No. of measured, independent and observed [F2 > 2σ(F2)] reflections | 6855, 3494, 2990 |
Rint | 0.077 |
(sin θ/λ)max (Å−1) | 0.703 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.048, 0.104, 1.01 |
No. of reflections | 3494 |
No. of parameters | 110 |
No. of restraints | 2 |
Δρmax, Δρmin (e Å−3) | 2.50, −2.23 |
Absolute structure | Flack H D (1983), Acta Cryst. A39, 876-881 |
Absolute structure parameter | 0.03 (4) |
Computer programs: CAD-4-PC (Enraf-Nonius, 1992), CAD-4-PC, XCAD4 (Harms & Wocadlo, 1996), SHELXTL (Sheldrick, 1997), SHELXTL, ATOMS (Dowty, 1997).
La1—As6i | 3.049 (2) | La3—As3iii | 3.105 (2) |
La1—As7ii | 3.121 (3) | La3—As5 | 3.115 (3) |
La1—As6 | 3.152 (2) | La3—As2iii | 3.122 (2) |
La1—As3 | 3.152 (2) | La3—As3 | 3.150 (3) |
La1—As1iii | 3.172 (3) | La3—As4iii | 3.190 (3) |
La1—As3i | 3.173 (2) | La3—As8ix | 3.253 (2) |
La1—As8iv | 3.200 (2) | La4—As8 | 3.057 (2) |
La1—As2v | 3.245 (2) | La4—As7iii | 3.122 (3) |
La1—As5 | 3.295 (2) | La4—As4x | 3.132 (2) |
La2—As2 | 3.057 (2) | La4—As6i | 3.159 (3) |
La2—As3iii | 3.067 (3) | La4—As7 | 3.167 (2) |
La2—As4vi | 3.107 (2) | La4—As5iii | 3.173 (2) |
La2—As7vii | 3.117 (2) | La4—As1iii | 3.173 (2) |
La2—As7 | 3.139 (2) | La4—As6xi | 3.223 (2) |
La2—As8vii | 3.147 (2) | La4—As2v | 3.293 (2) |
La2—As1 | 3.176 (3) | As1—As5 | 2.537 (3) |
La2—As5 | 3.200 (2) | As1—As2 | 2.569 (3) |
La2—As4 | 3.214 (2) | As2—As4 | 2.460 (3) |
La3—As6viii | 3.071 (2) | As3—As5 | 2.463 (3) |
La3—As4 | 3.081 (2) | As6—As8iv | 2.468 (3) |
La3—As1iii | 3.092 (2) | As7—As8 | 2.505 (3) |
As5—As1—As2 | 108.98 (10) | As3—As5—As1 | 105.12 (10) |
As4—As2—As1 | 99.43 (9) | As6xi—As8—As7 | 101.91 (10) |
Symmetry codes: (i) x, −y+1, z−1/2; (ii) x−1/2, −y+1/2, z−1/2; (iii) x+1/2, −y+1/2, z+1/2; (iv) x−1/2, y+1/2, z; (v) x+1/2, y+1/2, z; (vi) x, −y, z−1/2; (vii) x, −y, z+1/2; (viii) x, −y+1, z+1/2; (ix) x, y, z+1; (x) x+1/2, −y+1/2, z−1/2; (xi) x+1/2, y−1/2, z. |
LaAs2 was reported to transform reversibly at 1023 K from its low-temperature NdAs2-type structure (P21/c; Wang et al., 1978) to a unique high-temperature structure (Ono et al., 1970). Precession photography on HT-LaAs2 originally suggested the monoclinic space group B2/b (an alternate setting of C2/c) with a = 12.891, b = 9.140, c = 14.450 Å, and γ = 135.16°. A misleading tetragonal or orthorhombic indexing (a ~b = 9.07, c = 14.31 Å) was also proposed for HT-LaAs2, and subsequently for LaP2 and HT-CeP2 (Hayakawa et al., 1976). An independent single-crystal structure determination on LaP2 confirmed its monoclinic symmetry, but in the noncentrosymmetric space group Ia (an alternate setting of Cc). By extension, the powder pattern of HT-LaAs2 calculated based on the LaP2 structure was found to match the observed pattern, with monoclinic cell parameters a = 9.140, b = 14.450, c = 9.090 Å, and β = 90.0° (von Schnering et al., 1975). Because no single-crystal structure determination on HT-LaAs2 has been forthcoming since that time (Ono et al., 1970), a detailed study seemed warranted.
The structure of HT-LaAs2 was determined here in standard setting Cc. It contains two kinds of discrete chain polyarsenide anions, As35− and As57−. They pack in an alternating fashion along the b direction, with their mean planes roughly perpendicular to each other (Figs. 1 and 2. The bond angles in the As35− (V-shaped) and As57− (fragment of a boat conformation of a six-membered ring) ions are smaller [99.43 (9)–108.98 (10)°] than those of the corresponding polyphosphide ions in LaP2 (102.6–113.6°) (von Schnering et al., 1975), reflecting the greater p character in the As–As bonds. The order of short-long-long-short bond lengths in the As57− ion also contrasts with short-long-short-short in the P57− ion in LaP2. The four inequivalent types of La3+ ions intervene between the polyarsenide ions, and are each coordinated by nine arsenic atoms in a distorted tricapped trigonal prism.
Consistent with the Zintl concept, assignment of formal charges of 1- for the bridging and 2- for the terminal As atoms leads to the formulation La3+As1−As2−, with the expectation of semiconducting or possibly semimetallic properties. HT-LaAs2 does not appear to bear a simple structural relationship to LT-LaAs2, which possesses As46− fragments instead (Wang et al., 1978).
A similarity to the ThSi2-type structure (I41/amd), which contains Si-centred Th6 trigonal prisms (Pearson, 1972), was previously suggested (Ono et al., 1970), but the relationship is remote. (A portion of the HT-LaAs2 structure, which can be described as As5La4□6, bears an antitype relationship to ThSi2, but the pattern of vacancies removes the fourfold symmetry. Then, additional As atoms are inserted into this highly distorted framework to yield the LaAs2 structure.)