Abstract
The quaternary gold arsenide oxides Ce9Au5−xAs8O6 and Pr9Au5−xAs8O6 were synthesized from the rare earth elements (RE), rare earth oxides, arsenic and gold powder at maximum annealing temperatures of 1173 K. The structures were refined from single crystal X-ray diffractometer data: Pnnm, a=1321.64(6) pm, b=4073.0(3), c=423.96(2), wR2=0.0842, 3106 F2 values, 160 variables for Ce9Au4.91(4)As8O6 and Pnnm, a=1315.01(4), b=4052.87(8), c=420.68(1) pm, wR2=0.0865, 5313 F2 values, 160 variables for Pr9Au4.75(1)As8O6. They represent a new structure type and show a further extension of pnictide oxide crystal chemistry. A complex polyanionic gold arsenide network [Au5As8]15− (with some disorder in the gold substructure) is charge compensated with polycationic strands of condensed edge-sharing O@RE4/4 and O@RE4/3 tetrahedra ([RE4O3]212+) as well as RE3+ cations in cavities.
1 Introduction
The crystal chemistry of pnictide (Pn) oxides [1], [2], [3], [4], [5] is dominated by compounds with the ZrCuSiAs-type structure. More than 150 phases with this structure type have been reported. The most prominent examples are the superconductors LaFeAsO1−xFx (x=0.05–0.12; TC=26 K) [6] and SmFeAsO1−xFx (x=0.1; TC=55 K) [7]. Besides the intensive studies on the iron based phases, such pnictide oxides have also been reported with manganese, cobalt, zinc or ruthenium [2]. Comparatively few representatives are known with the coinage metals, i.e. UCuPO [8], [9], [10], ThCuPO [11], ThAgPO [12], [13] and ThCuAsO [11]. For an electron-precise description one needs a tetravalent cation besides Cu+ and Ag+ and this is possible with uranium and especially thorium. Electronic structure calculations [14] show substantial covalent bonding within the [Th2O2] and [Cu2Pn2] layers which are held together by ionic interactions. The charge transfers range from 1.94 to 2.08 e−, much smaller than expected from a formal charge splitting (Th2O2)4+ (Cu2Pn2)4−.
Polycrystalline and single crystalline UCuPO samples show a high Néel temperature of TN=220 K [8], [9], [10]. The spin alignment (AF I type; +−+−) was determined from neutron diffraction data. Isotypic NpCuPO [10] was obtained by reaction of neptunium metal, CuO and phosphorus. Resistivity measurements indicate long-range magnetic ordering around 90 K which has been confirmed by DFT calculations.
Besides the ZrCuSiAs-type phases, U2Cu2P3O (formerly reported as a ternary phosphide U4Cu4P7 [15], [16], [17], [18] with a half-occupied phosphorus site) and U2Cu2As3O [19] have been reported. These structures contain similar layers of condensed OU4/4 and CuPn4/4 tetrahedra. U2Cu2P3O orders antiferromagnetically at TN=146 K [16]. Resistivity measurements on single crystals (grown with iodine as transport agent) show distinctly anisotropic transport behavior [17].
The rare earth-based phosphide oxides RE3Cu4P4O2 (RE=La–Nd, Sm) [20], [21], [22], [23] and arsenide oxides RE3Cu4As4O2 (RE=La–Pr) [24] also exhibit layers of condensed CuPn4/4 tetrahedra, however, they are related by a mirror plane, leading to the formation of P2 and As2 dumbbells with single bond character. Incorporation of purely ionic LaOCl slabs into La3Cu4P4O2 and La3Cu4As4O2 was observed for La5Cu4P4O4Cl2 (≡ La3Cu4P4O2⋅2LaOCl) [23] and isotypic La5Cu4As4O4Cl2 (≡ La3Cu4As4O2⋅2LaOCl) [25], nicely extending the structural chemistry of these pnictide oxides.
The gold-based pnictide oxides show different crystal chemistry. Nd10Au3As8O10 and Sm10Au3As8O10 [26] are remarkable compounds with Au(I) in almost square-planar coordination by arsenic dumbbells. These polyanionic layers are stacked and charge-compensated by polycationic layers of condensed edge-sharing ORE4/4 tetrahedra. Almost trigonal-planar coordination of Au(I) by two P3− and one P24− dumbbell (end-on) occurs in the phosphide oxides RE2AuP2O (RE=La–Nd) [27], [28]. Both the polyanionic and the polycationic substructures are one-dimensional and arranged in the motif of a hexagonal rod packing, an entirely new motif in pnictide oxide structural chemistry.
In continuation of our phase analytical studies of the quaternary systems RE-Au-Pn-O we obtained well-shaped single crystals of Ce9Au5−xAs8O6 and Pr9Au5−xAs8O6 from NaCl/KCl salt flux synthesis. The structural chemistry of these arsenide oxides is reported herein.
2 Experimental
2.1 Synthesis
Starting materials for the syntheses were cerium (Sigma-Aldrich, >99.9%), praseodymium (Sigma-Aldrich, >99.9%), cerium(IV) oxide (ChemPur, >99.99%), praseodymium (III,IV) oxide (ChemPur, >99.9%), gold (Agosi, >99.9%) and arsenic granules (Ventron). Filings of cerium and praseodymium were prepared under dried paraffin oil (sodium wire), washed with cyclohexane and stored under argon prior to synthesis. Argon was purified with titanium sponge (870 K), silica gel and molecular sieves. Gold powder was obtained by dissolving gold pieces in aqua regia and subsequent precipitation using (NH4)2Fe(SO4)2·xH2O (VWR, >99%). Arsenic was resublimed, stored under argon and pulverized prior synthesis.
Black, polycrystalline samples were prepared via mixing filings of the rare earth elements (RE), their oxides, gold powder and powder of arsenic in the molar ratios 6:3:5:8 (Ce9Au5As8O6) and 63/11:6/11:5:8 (Pr9Au5As8O6). Amounts of 0.5 g were cold-pressed to pellets, sealed in evacuated silica ampoules and heated in a resistance furnace; first up to 873 K (24 h annealing) and finally at 1173 K (72 h annealing). Heating rates were 100 K h−1, cooling was done by shutting off the furnace. Except for a small impurity of elemental gold and with regard to the limit of detection via XRD, the praseodymium compound was obtained almost phase pure (see Fig. 1). The cerium compound on the contrary could not be synthesized without rare earth oxide and arsenide impurities.
Suitable crystals for structure determination were prepared via salt flux synthesis [29]. Polycrystalline RE9Au5−xAs8O6 (200–300 mg) and an equimolar NaCl/KCl mixture (~1 g) were sealed in evacuated silica ampoules, heated up to 1223 K (72 h, 100 K h−1), slowly cooled down to 773 K (2 K h−1) and finally to room temperature by shutting off the furnace. Slat-shaped single crystals (especially for the praseodymium compound, see Fig. 2) with metallic luster could be isolated from the reaction mixture after dissolving the NaCl/KCl flux with demineralised water.
2.2 X-ray image plate data and data collections
The polycrystalline RE9Au5−xAs8O6 samples were characterized by Guinier powder patterns: Enraf Nonius FR 552 camera, image plate system Fuji film, BAS-1800, CuKα1 radiation and α-quartz (a=491.30, c=540.46 pm) as an internal standard. The orthorhombic lattice parameters (a=1321.7(3), b=4073.5(6), c=423.9(1) pm, V=2.2823 nm3 for the Ce9Au5−xAs8O6 and a=1316.1(2), b=4051.4(6), c=421.1(1) pm, V=2.2453 nm3 for the Pr9Au5−xAs8O6 sample) were deduced from least-squares refinements. Correct indexing was facilitated by intensity calculations with LazyPulverix [30].
Well-shaped single crystals of both compounds were glued to thin quartz fibers using bees wax and first tested by Laue photographs on a Buerger camera (white molybdenum radiation, image plate technique, Fujifilm, BAS-1800). Intensity data were collected at ambient temperature by use of a Stoe StadiVari diffractometer equipped with a Mo micro focus source and a Pilatus detection system. Due to a Gaussian-shaped profile of the X-ray source, scaling was applied along with the numerical absorption corrections. Details of the data collections and the structure refinements are listed in Table 1.
Compound | Ce9Au4.91(4)As8O6 | Pr9Au4.75(1)As8O6 | Pr9Au4.87(2)As8O6 | Pr9Au4.86(1)As8O6 |
---|---|---|---|---|
Unit cell dimensions (single crystal data) | ||||
a, pm | 1321.64(6) | 1315.45(9) | 1315.63(7) | 1315.01(4) |
b, pm | 4073.0(3) | 4047.9(2) | 4049.8(1) | 4052.87(8) |
c, pm | 423.96(2) | 421.44(2) | 421.40(1) | 420.68(1) |
Cell volume V, nm3 | 2.2821 | 2.2441 | 2.2452 | 2.2420 |
Molar mass M, g mol−1 | 2923.6 | 2899.1 | 2922.8 | 2920.8 |
Calculated density, g cm−3 | 8.51 | 8.58 | 8.65 | 8.64 |
Detector distance, mm | 110 | 120 | 110 | 90 |
Absorption coefficient, mm−1 | 60.5 | 61.7 | 62.5 | 62.4 |
Integr. param. A/B/EMS | 12.0/1.8/0.010 | 7.0/−4.7/0.019 | 7.8/−5.8/0.011 | 7.7/−5.5/0.010 |
F(000), e | 4887 | 4872 | 4911 | 4908 |
Crystal size, µm3 | 113×8×4 | 85×15×3 | 39×12×7 | 39×12×7 |
Transm. ratio (min /max) | 0.10/0.76 | 0.36/0.83 | 0.38/0.67 | 0.50/0.74 |
θ range, deg | 1.6–27.9 | 1.0–35.6 | 2.5–29.4 | 3.1–30.8 |
Range in hkl | ±17, ±53, ±5 | ±21, ±66, ±6 | ±18, ±55, ±5 | ±18, ±57, ±5 |
Total no. reflections | 9886 | 51017 | 40029 | 4812 |
Independent reflections/Rint | 3106/0.1234 | 5313/0.0827 | 3358/0.1645 | 2673/0.0469 |
Reflections with I>3 σ(I)/Rσ | 895/0.3062 | 2570/0.0936 | 970/0.2651 | 1155/0.1497 |
Data/ref. parameters | 3106/160 | 5313/160 | 3358/160 | 2673/160 |
Goodness-of-fit on F2 | 0.59 | 1.25 | 0.73 | 0.81 |
R1/wR2 for I>3 σ(I) | 0.0276/0.0521 | 0.0357/0.0711 | 0.0313/0.0616 | 0.0325/0.0606 |
R1/wR2 for all data | 0.1270/0.0842 | 0.0741/0.0865 | 0.0931/0.0748 | 0.0893/0.0753 |
Extinction coefficient | 164(7) | 500(20) | 140(20) | 20(20) |
Largest diff. peak/hole, e Å−3 | 3.19/−3.26 | 4.60/−4.18 | 0.81/−0.92 | 2.45/−2.45 |
Refined occupancies of relevant gold sites in % | ||||
Au3 | 49(4) | 27(1) | 37(2) | 41(1) |
Au4 | 42(3) | 48(1) | 50(2) | 45(1) |
2.3 EDX data
The single crystals measured on the diffractometer were analyzed semi-quantitatively using a Zeiss EVO MA10 scanning electron microscope with CeO2, PrF3, Au, InAs and SiO2 as standards. No impurity elements heavier than sodium (detection limit of the instrument) were observed. The experimentally determined RE:Au:As ratios were in close agreement with the compositions obtained from the structure refinements. The oxygen content could not be obtained reliably due to the limitation of the instrument’s resolution.
3 Results and discussion
3.1 Structure refinements
Analyses of the four data sets revealed primitive orthorhombic lattices and the systematic extinctions were compatible with the centrosymmetric space group Pnnm. The starting atomic parameters were deduced with the Superflip algorithm [31] and the structures were refined with Jana2006 (full-matrix least-squares on Fo2) [32] with anisotropic displacement parameters for the RE, Au and As atoms and isotropic ones for the oxygen atoms. Except for the partially occupied Au3 and Au4 sites, all positions were fully occupied within two standard deviations. The partial occupancies were refined as least-squares variables in the final cycles. The final difference Fourier syntheses were flat. The refined positional parameters, displacement parameters and interatomic distances are exemplarily listed in Tables 2 and 3 for Pr9Au4.75(1)As8O6. The other compounds show slightly differing partial gold occupancies and these crystallographic data have been deposited.
Atom | x | y | U11 | U22 | U33 | U12 | Ueq |
---|---|---|---|---|---|---|---|
Pr1 | 0.08267(8) | 0.09253(2) | 195(4) | 182(4) | 175(5) | 4(3) | 184(2) |
Pr2 | 0.04567(8) | 0.19326(2) | 212(4) | 184(4) | 170(5) | 10(3) | 188(3) |
Pr3 | 0.84494(8) | 0.00319(2) | 204(4) | 173(4) | 166(5) | −2(3) | 181(2) |
Pr4 | 0.01942(8) | 0.28784(2) | 212(4) | 167(4) | 184(5) | 10(4) | 188(2) |
Pr5 | 0.72082(8) | 0.25560(2) | 216(4) | 166(4) | 183(5) | −2(3) | 188(3) |
Pr6 | 0.17616(8) | 0.37413(2) | 246(5) | 174(4) | 185(5) | 2(4) | 202(3) |
Pr7 | 0.73336(8) | 0.34930(2) | 210(4) | 159(4) | 178(5) | 1(3) | 182(2) |
Pr8 | 0.11788(8) | 0.46946(2) | 193(4) | 177(4) | 175(5) | −4(3) | 182(2) |
Pr9 | 0.78814(8) | 0.44304(2) | 201(4) | 165(4) | 168(5) | −9(3) | 178(3) |
Au1 | 0.52612(7) | 0.08196(2) | 303(4) | 221(3) | 300(4) | 42(3) | 274(2) |
Au2 | 0.48128(7) | 0.15315(2) | 391(4) | 228(3) | 304(4) | 1(3) | 308(2) |
Au3a | 0.8319(3) | 0.1555(1) | 240(20) | 450(30) | 189(17) | −107(18) | 293(13) |
Au4a | 0.8716(2) | 0.1385(1) | 405(15) | 580(20) | 279(12) | −82(14) | 420(9) |
Au5 | 0.31101(9) | 0.30764(3) | 382(6) | 252(4) | 1466(15) | 64(4) | 700(5) |
Au6 | 0.36989(7) | 0.42554(2) | 295(4) | 232(3) | 562(6) | −31(3) | 363(3) |
As1 | 0.4375(1) | 0.02216(4) | 218(9) | 190(8) | 155(9) | 20(7) | 188(5) |
As2 | 0.3444(1) | 0.20034(4) | 200(8) | 182(7) | 198(9) | −10(7) | 193(5) |
As3 | 0.7268(2) | 0.07290(4) | 240(9) | 192(8) | 180(9) | 1(7) | 204(5) |
As4 | 0.6643(2) | 0.18130(4) | 308(10) | 179(8) | 175(9) | −3(7) | 221(5) |
As5 | 0.4190(2) | 0.25679(4) | 231(9) | 167(7) | 216(10) | −2(7) | 205(5) |
As6 | 0.4294(2) | 0.36433(6) | 349(13) | 234(10) | 940(20) | 5(10) | 506(10) |
As7 | 0.9445(2) | 0.38580(5) | 197(9) | 223(8) | 240(10) | 3(7) | 220(5) |
As8 | 0.5112(1) | 0.46847(4) | 197(8) | 186(8) | 190(9) | 1(7) | 191(5) |
O1 | 0.213(1) | 0.0530(3) | – | – | – | – | 180(20) |
O2 | 0.154(1) | 0.1463(3) | – | – | – | – | 200(20) |
O3 | 0.134(1) | 0.2458(3) | – | – | – | – | 170(20) |
O4 | 0.625(1) | 0.3037(3) | – | – | – | – | 230(30) |
O5 | 0.667(1) | 0.4014(3) | – | – | – | – | 200(20) |
O6 | 0.259(1) | 0.5008(3) | – | – | – | – | 160(20) |
aThe Au3 and Au4 sites show small defects (Table 1).
Pr1: | 1 | O1 | 235(1) | Pr7: | 1 | O5 | 228(1) | Au5: | 1 | As5 | 250.3(2) | As5: | 1 | As2 | 248.9(3) |
1 | O2 | 237(1) | 1 | O4 | 234(1) | 2 | Au3 | 259.5(3) | 1 | Au5 | 250.3(2) | ||||
2 | O5 | 239.5(6) | 2 | O2 | 235.6(6) | 1 | As6 | 277.5(3) | 2 | Pr4 | 307.4(2) | ||||
1 | Au4 | 334.5(4) | 1 | As7 | 314.8(2) | 2 | As4 | 289.2(2) | 1 | O4 | 331(1) | ||||
2 | As8 | 338.0(2) | 2 | As2 | 325.8(2) | 2 | Au4 | 313.5(3) | 2 | Pr2 | 336.3(2) | ||||
2 | As6 | 340.0(2) | Pr8: | 1 | O6 | 225(1) | 1 | Pr6 | 322.6(2) | 2 | Pr5 | 338.9(2) | |||
2 | Au6 | 357.9(1) | 2 | As3 | 307.1(2) | 1 | O3 | 342(1) | As6: | 2 | Au4 | 224.2(2) | |||
Pr2: | 2 | O4 | 235.2(6) | 2 | As1 | 308.6(2) | 2 | Pr5 | 352.2(1) | 2 | Au3 | 259.4(3) | |||
1 | O2 | 238(1) | 2 | As1 | 319.1(2) | Au6: | 1 | As8 | 254.7(2) | 1 | Au6 | 260.0(3) | |||
1 | O3 | 243(1) | 2 | Au1 | 319.8(1) | 1 | As6 | 260.0(3) | 1 | Au5 | 277.5(3) | ||||
1 | Au4 | 319.0(4) | Pr9: | 1 | O5 | 232(1) | 2 | As3 | 282.5(2) | 1 | Pr6 | 335.6(3) | |||
1 | Au3 | 320.2(5) | 2 | O1 | 233.0(5) | 1 | Pr6 | 329.2(2) | 2 | Pr1 | 340.0(2) | ||||
2 | As5 | 336.3(2) | 1 | O6 | 236(1) | 2 | Au4 | 334.1(3) | 1 | O5 | 347(1) | ||||
2 | As6 | 349.6(2) | 1 | As7 | 310.1(2) | 1 | O6 | 338(1) | 2 | Pr2 | 349.6(2) | ||||
Pr3: | 2 | O6 | 239.8(6) | 2 | As1 | 320.7(2) | 2 | Pr1 | 357.9(1) | As7: | 2 | Au2 | 267.5(1) | ||
1 | O1 | 240(1) | Au1: | 1 | As3 | 266.7(2) | 2 | Pr3 | 358.9(1) | 2 | Au1 | 270.0(1) | |||
2 | As8 | 316.1(2) | 1 | As1 | 268.9(2) | As1: | 1 | As1 | 243.5(3) | 1 | Pr6 | 308.5(2) | |||
1 | As3 | 322.4(2) | 2 | As7 | 270.0(1) | 1 | Au1 | 268.9(2) | 1 | Pr9 | 310.1(2) | ||||
2 | As8 | 324.7(2) | 1 | Au2 | 294.4(1) | 2 | Pr8 | 308.6(2) | 1 | Pr7 | 314.8(2) | ||||
2 | Au6 | 358.9(1) | 2 | Pr8 | 319.8(1) | 2 | Pr8 | 319.1(2) | As8: | 1 | Au6 | 254.7(2) | |||
Pr4: | 1 | O3 | 228(1) | 2 | Pr6 | 339.1(1) | 2 | Pr9 | 320.7(2) | 1 | As8 | 257.2(3) | |||
2 | As5 | 307.4(2) | Au2: | 1 | As2 | 262.7(2) | As2: | 1 | As5 | 248.9(3) | 2 | Pr3 | 316.1(2) | ||
2 | As4 | 310.3(2) | 1 | As4 | 266.5(2) | 1 | Au2 | 262.7(2) | 2 | Pr3 | 324.7(2) | ||||
2 | As2 | 315.8(2) | 2 | As7 | 267.5(1) | 2 | Pr4 | 315.8(2) | 1 | O6 | 328(1) | ||||
2 | Au2 | 322.5(1) | 1 | Au1 | 294.4(1) | 2 | Pr5 | 320.4(2) | 2 | Pr1 | 338.0(2) | ||||
Pr5: | 1 | O4 | 232(1) | 2 | Pr4 | 322.5(1) | 2 | Pr7 | 325.8(2) | O1: | 2 | Pr9 | 233.0(5) | ||
2 | O3 | 239.5(6) | 2 | Pr6 | 349.7(1) | As3: | 1 | Au1 | 266.7(2) | 1 | Pr1 | 235(1) | |||
1 | As4 | 310.1(2) | Au3: | 1 | Au4 | 86.5(6)a | 2 | Au6 | 282.5(2) | 1 | Pr3 | 240(1) | |||
2 | As2 | 320.4(2) | 1 | As4 | 244.0(5)a | 2 | Pr8 | 307.1(2) | O2: | 2 | Pr7 | 235.6(6) | |||
2 | As5 | 338.9(2) | 2 | As6 | 259.4(3) | 2 | Pr6 | 308.0(2) | 1 | Pr1 | 237(1) | ||||
2 | Au5 | 352.2(1) | 2 | Au5 | 259.5(3) | 1 | Pr3 | 322.4(2) | 1 | Pr2 | 238(1) | ||||
Pr6: | 2 | As3 | 308.0(2) | 2 | Pr6 | 317.4(4) | 1 | Au4 | 327.0(4) | O3: | 1 | Pr4 | 228(1) | ||
2 | As4 | 308.2(2) | 1 | Pr2 | 320.2(5) | As4: | 1 | Au3 | 244.0(5) | 2 | Pr5 | 239.5(6) | |||
1 | As7 | 308.5(2) | Au4: | 1 | Au3 | 86.5(6)a | 1 | Au2 | 266.5(2) | 1 | Pr2 | 243(1) | |||
2 | Au3 | 317.4(4) | 2 | As6 | 224.2(2)a | 2 | Au5 | 289.2(2) | O4: | 1 | Pr5 | 232(1) | |||
1 | Au5 | 322.6(2) | 2 | Au5 | 313.5(3) | 2 | Pr6 | 308.2(2) | 1 | Pr7 | 234(1) | ||||
1 | Au6 | 329.2(2) | 1 | Pr2 | 319.0(4) | 1 | Pr5 | 310.1(2) | 2 | Pr2 | 235.2(6) | ||||
1 | As6 | 335.6(3) | 1 | As4 | 323.2(4) | 2 | Pr4 | 310.3(2) | O5: | 1 | Pr7 | 228(1) | |||
2 | Au4 | 336.3(3) | 1 | As3 | 327.0(4) | 1 | Au4 | 323.2(4) | 1 | Pr9 | 232(1) | ||||
2 | Au1 | 339.1(1) | 2 | Au6 | 334.1(3) | 2 | Pr1 | 239.5(6) | |||||||
2 | Au2 | 349.7(1) | 1 | Pr1 | 334.5(4) | O6: | 1 | Pr8 | 225(1) | ||||||
2 | Pr6 | 336.3(3) | 1 | Pr9 | 236(1) | ||||||||||
2 | Pr3 | 239.8(6) |
All distances within the first coordination spheres are listed. Standard deviations are given in parentheses. aDistances affected by the split positions.
Further details of the crystal structure investigations may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/request_for_deposited_data.html) on quoting the deposition numbers CSD-431599 (Ce9Au4.91As8O6), CSD-431600 (Pr9Au4.75As8O6), CSD-431601 (Pr9Au4.86As8O6), and CSD-431602 (Pr9Au4.87As8O6).
3.2 Crystal chemistry
The quaternary arsenide oxides Ce9Au5−xAs8O6 and Pr9Au5−xAs8O6 show new structural motifs in the crystal chemistry of pnictide oxides. Besides the tetragonal arsenides Nd10Au3As8O10 and Sm10Au3As8O10 [26] and polytypic Ca10(FeAs)10(Pt3As8) [33], [34] they are among the structurally most complex phases in this family of materials. Ce9Au5−xAs8O6 and Pr9Au5−xAs8O6 crystallize with their own structure type, space group Pnnm, Pearson symbol oP116 and Wyckoff sequence g29.
A projection of the Pr9Au5−xAs8O6 structure along the short unit cell axis is presented in Fig. 3. We start the crystal chemical discussion with the cationic unit. The six crystallographically independent oxygen atoms have distorted tetrahedral praseodymium coordination with a broad range of O–Pr distances from 225 to 243 pm. These six OPr4 tetrahedra are condensed via common edges in b direction and the resulting chains are further condensed in c direction forming strands. Only the praseodymium atoms Pr1–Pr5 and Pr7–Pr9 take part in this polycationic unit. The Pr6 atoms have no Pr–O contact. They are located within the gold-arsenide network. As a consequence of the structurally complex anionic unit we observe a slight bending of the polycationic [Pr8O6]12+ units, leading to the broader range of O–Pr distances. In tetragonal, ZrCuSiAs type α-PrZnPO [35] and PrZnSbO [36] the planar [PrO]+ layers contain only one crystallographically independent praseodymium and oxygen atoms and all O–Pr distances are equal, i.e. 238 pm in PrZnSbO and 233 pm in α-PrZnPO. The gross structural features of Pr9Au5−xAs8O6 are similar to those of the recently reported germanide oxide Ce4Ag3Ge4O0.5 [37], where a chain of trans-edge-sharing OCe4 tetrahedra is embedded within the polyanionic [Ag3Ge4] network.
Now we turn to the structural description of the polyanionic network. The latter is by far more complex than the simple layers of edge-sharing tetrahedra in the ZrCuSiAs family of compounds [2]. The polyanionic network contains six crystallographically independent gold sites (Au3 and Au4 show partial occupancy) and eight arsenic sites. Half of the arsenic atoms are isolated (no As–As bonding) and can be considered as As3− Zintl anions. As1, As2, As5 and As8 form As2 dumbbells with As–As distances of 244–257 pm, similar to Nd10Au3As8O10 (255 pm As–As) [26] and the Zintl phase CaAs (256 pm As–As) [38]. These are typical single bond distances and we can assume As24− Zintl anions (isoelectronic with bromine).
The gold substructure of Pr9Au5−xAs8O6 is the complicated part of this structure type. Au1, Au2, Au5, and Au6 have distorted tetrahedral arsenic coordination (by As3− and end-on coordinated As24−) with Au–As distances ranging from 224 to 289 pm. In Nd10Au3As8O10 [26] with square-planar and rectangular coordinated gold atoms the Au–As distances show a much smaller range of 256–259 pm, a consequence of the higher site symmetry. The shorter Au4–As6 distance of 224 pm in Pr9Au5−xAs8O6 is a consequence of the disorder in the gold substructure discussed below. A molecular pendant to these pnictide oxides is the diiodobis(o-phenylenebis(dimethylarsine))gold(III) ion [39], [40], [41] with Au–As distances of 243–246 pm in almost square-planar coordination. Further examples are the arsonium salts [(C6H5)3PAu]4As+BF4−⋅3CH2Cl2 and [(C6H5)3PAu]4As+BF4−⋅4CH2Cl2 with average Au–As distances of 250 and 249 pm [42], respectively. An overview of various gold complexes with heavy group V elements is given by Laguna [43].
Layers of edge-sharing AuAs4 tetrahedra with Au–As distances ranging from 272 to 274 pm occur in PrAuAs2 [44]. The latter are slightly longer than the sum of the covalent radii for Au+As of 255 pm [45].
Au3 and Au4 correspond to a split position (Fig. 3) with partial occupancies ranging from 27 to 50% (Table 1), manifesting a small homogeneity range for the two compounds. Consequently we observe a physically impossibly short Au3–Au4 distance of 87 pm. Thus, only one of these positions can be occupied in an ordered model. The Au5 and As6 atoms react on this split position with enhanced displacement parameters U33. The four data sets were carefully checked with respect to superstructure reflections, which might enable gold ordering in a lower-symmetry space group. No additional reflections were observed. Also refinements in translationengleiche subgroups did not resolve the disorder. A possible ordering pattern is presented in the lower right-hand part of Fig. 3. In such a model, only one of the two split positions can be occupied and the neighboring Au5 and As6 atoms show an ordered displacement in c direction to allow for reasonable distances. In the second domain the adjacent split position is occupied and the Au5 and As6 atoms are displaced in the opposite direction emphasized by dashed red lines in the figure. Probably the disordered cages around the Pr6 atoms are too far away to enable long-range ordering.
Besides the split positions we need to mention the slightly lower occupancy parameters especially for the Au3 sites. The local gold-arsenic substructure around the Pr6 atoms is closely related to the ZrCuSiAs and ThCr2Si2 phases, which often show ordering of defects [2], [46]. This is also similar to CeAu0.985As2 [47] and CeAu0.838Sb2 [48].
A further possibility hindering the correct structure solution might be twinning. Indications for merohedry, partial merohedry or especially pseudo-merohedry would be elongated reflections for high diffraction angles along with the large lattice parameter b, pretending high orthorhombic Laue symmetry (mmm) typically for monoclinic lattices with β angles close to 90° [49]. However, suitable models could not be found up to now.
Assuming the ideal composition Pr9Au5As8O6 (neglecting the partial occupancies of Au3 and Au4) we can write up an electron precise formula: (9Pr3+)(5Au+)(4As3−)(2As24−)(6O2−). This formula is only a first approximation. Besides the vacancies in the Au3 and Au4 substructure, the compound exhibits a variety of Au–Au interactions with Au–Au distances ranging from 260 to 334 pm (the shorter ones are affected by the partial occupancy and do not occur in the ordered model), comparable to that in fcc gold (288 pm) [50].
Acknowledgments
We thank Dipl.-Ing. U. Ch. Rodewald for collecting the single-crystal diffractometer data. This work was financially supported by the Deutsche Forschungsgemeinschaft through SPP 1458 Hochtemperatursupraleitung in Eisenpniktiden.
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