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Publicly Available Published by De Gruyter April 20, 2016

V18P9C2: a complex phosphide carbide

Herbert Boller EMAIL logo and Herta Effenberger

Abstract

V18P9C2 crystallizes in the orthorhombic space group Pmma with the lattice parameters a = 17.044(3), b = 3.2219(7), and c = 13.030(2) Å, Z = 2. The crystal structure is composed of 19 symmetry-independent atoms. The crystal structure is considered as a network formed by the transition metal atoms exhibiting cubic, trigonal prismatic, and octahedral voids centered by V, P, and C atoms, respectively. Vice versa, the V and P atoms form a three-dimensional network. The two CV6 octahedra are edge- and corner-connected to chains running parallel to [010]. The five unique P atoms are trigonal prismatically coordinated by V atoms with one to three faces capped again by a V atom. The V atoms have mainly cubic environments formed solely by V or by V and P atoms. V18P9C2 exhibits some structural relations to other compounds of the ternary system V–P–C as well as to other intermetallic phases. Despite the low carbon content, V18P9C2 is considered as a ternary compound rather than an interstitially stabilized (binary) phosphide in view of its special structural features.

1 Introduction

In the system V–P–C, the binary V4P3 and five ternary compounds V2PC, V3PC, V4P2C, V5+xP3C1–x, and V~6P3C1–x were described earlier [1]. Four of these ternary phases have been structurally characterized. The crystal structures of V4P2C and V3PC were determined from X-ray powder patterns [1, 2]. V5+xP3C1–x is isotypic with V5+xP3N1–x [3] and crystallizes in the hexagonal structure type of Mn5Si3 (space group P63/mcm) with partly occupied interstitial positions. The atomic arrangement of V2PC is isotypic with that of V2AsC [2]; it was later refined by Bouhemadou et al. [4]. The crystal structure of V~6P3C1–x (mentioned as compound X) has not been investigated up to now. The present work deals with its crystal structure determination based on single-crystal X-ray data and gives a topological survey of the V–P–C phases. The ideal composition of the title compound was found to be V18P9C2. The compound is of interest due to its low carbon content. The atomic arrangements of the ternary V–P–C compounds are compared topologically; in addition, basic structural building blocks are compared.

2 Results and discussion

Nineteen crystallographically independent atoms characterize the crystal structure of V18P9C2: 12 vanadium, 5 phosphorus, and 2 carbon atoms. All atoms are located at mirror planes in y = 0 or ½ and exhibit the site symmetries. 2/m., mm2, or.m., respectively.

Topologically, the crystal structure is characterized by a complex three-dimensional network formed by vanadium atoms. All P and C atoms are centered in trigonal prismatic and octahedral holes, respectively. The vanadium atoms V1, V2, V5, and V6 are 8-coordinated by V atoms like in a bcc environment (Fig. 1). Each of these polyhedra shares faces parallel to [010]; V5V8 and V6V8 share faces in (001) among each other.

Fig. 1: The net of V atoms in V18P9C2. The trigonal prismatic coordination polyhedra hosting the P atoms are shaded in yellow, cubes of V atoms centred by a V atom are shaded in green. The C atoms are located in faces of the V5V8 cube.
Fig. 1:

The net of V atoms in V18P9C2. The trigonal prismatic coordination polyhedra hosting the P atoms are shaded in yellow, cubes of V atoms centred by a V atom are shaded in green. The C atoms are located in faces of the V5V8 cube.

The four VV8 cubes in V18P9C2 have distinct environments. The V1V8 and V2V8 cubes share four faces with prism faces of four trigonal prismatic PV6 polyhedra. Around the V1V8 cubes, each of the two P3V6 and P4V6 prisms are oriented parallel to [010]; around the V2V8 cubes, two P1V6 prisms are oriented again parallel to [010], whereas the two P2V6 prisms are oriented vertically. The V5V8 and V6V8 cubes are coordinated by two (P3V6) and three (P4V6 twice and P5V6) prisms, respectively. The carbon atoms are centered in two opposite faces of the V5V8 polyhedron; one of them is sharing the face with the V6V8 cube. Analogous geometries are known from the crystal structure of Mo5As4 (Ti5Te4 type of structure; Fig. 2) [57] or V4P2C (Fig. 3) [1] and also encountered in other pnictides, e.g. in V4P3 [1] (isotypic to V4As3 [8]) or in V3As2 [9].

Fig. 2: The crystal structure of Mo5As4 (structure type Ti5Te4); the MoMo8 cubes are shaded.
Fig. 2:

The crystal structure of Mo5As4 (structure type Ti5Te4); the MoMo8 cubes are shaded.

Fig. 3: The crystal structure of V4P2C. The trigonal prismatic coordination polyhedra hosting the P atoms are shaded in yellow; the mono-capped VV8 cubes are shaded in green.
Fig. 3:

The crystal structure of V4P2C. The trigonal prismatic coordination polyhedra hosting the P atoms are shaded in yellow; the mono-capped VV8 cubes are shaded in green.

The C1V6 octahedra share two opposite V5–V6 edges, and the C2V6 octahedra share two V10–V10 edges forming each a chain. These chains are corner-connected via the V5 atoms; thus, each V5 atom is linked to three C atoms. The resulting lath-like ribbons have the composition C2V7 (Fig. 4).

Fig. 4: The linking of the CV6 octahedra in V18P9C2 to a lath-like arrangement.
Fig. 4:

The linking of the CV6 octahedra in V18P9C2 to a lath-like arrangement.

Edge-sharing CV6 octahedra occur in all ternary V–P–C compounds known so far; however, their connection schemes differ. In V4P2C [1], the CV6 octahedra are linked to single chains of CV4 composition only. In V18P9C2 and V4P2C, the V:C ratio is relatively large and amounts to 9:1 and 4:1, respectively. In the two V–P–C phases exhibiting the smallest V:P ratios of 3:1 and 2:1 going along with an increased carbon content, these rows are linked to layers. In V3PC, corner linkage of the edge-shared chains results in layers with composition CV3 [2]; in V2PC, brucite-like layers with composition CV2 are formed due to edge linkage in three directions [100], [010], and [110] [3, 4]. In V2PC, V3PC, and V4P2C, the linked CV6 octahedra are interconnected by P atoms only. In contrast, the significantly higher V content in the title compound causes the C2V7 chains to be embedded in a network of vanadium and phosphorus atoms (Fig. 5).

Fig. 5: The edge and corner connection of CV6 octahedra to rows and layers in vanadium carbide phosphides.
Fig. 5:

The edge and corner connection of CV6 octahedra to rows and layers in vanadium carbide phosphides.

A common feature of all V–P–C compounds is the unusually short translation period of roughly 3.1 to 3.2 Å. They appear between the two opposite shared edges of the CV6 octahedra, i.e. parallel to [100] in V3PC, parallel to [010] in V18P9C2, parallel to [001] in V4P2C, and parallel to [100], [010], and [110] in V2PC (Fig. 2). It should be mentioned that edge sharing of the CV6 octahedra does not solely control this short translation period. Similar short cell parameters are known from many structure types of transition metals and transition metal pnictides, among them also from VP2 [10, 11] or VP [1214]. The often observed high point symmetries of the individual atoms going along with these short translation periods result in an orientation of the coordination figures parallel to crystallographic axes.

Alternatively, the crystal structure of the title compound exhibits a three-dimensional network formed by the V and P atoms; channels along [010] host the carbon atoms (Fig. 6). The interatomic distances between the vanadium atoms (2.649–3.068 Å) are slightly larger than in elementary bcc vanadium (2.62 and 3.03 Å). The V atoms in V18P9C2 feature coordination figures that are based on cubes (atoms V1–V7 and V12) and capped trigonal prisms (atoms V10 and V11). The corners are V and partly P atoms. Of special interest are the C atoms that center a face (atoms V4, V5, and V7) or an edge (atoms V5 and V6); thus, the coordination polyhedron around the V atom and the CV6 octahedron penetrate each other, and the distance between the central V atoms and those involved in the CV6 octahedron are significantly enlarged. The coordination figures around the atoms V8 and V9 are irregular.

Fig. 6: The three-dimensional network of the V and P atoms in the crystal structure of V18P9C2 hosting the one-dimensionally linked CV6 octahedra.
Fig. 6:

The three-dimensional network of the V and P atoms in the crystal structure of V18P9C2 hosting the one-dimensionally linked CV6 octahedra.

The first coordination spheres of the five crystallographically different P atoms consist of V atoms only (Fig. 1). The coordination figures are mono-capped (atom P2), bi-capped (atom P4), and tri-capped (atoms P1, P3, P5) trigonal prisms; capped are only the prism faces. The V-P distances vary between 2.301 and 2.747 Å. Such coordination figures are common for phosphides. The top and bottom faces of the P1V6 and P3V6 to P6V6 prisms are parallel to (010). Because all atoms occupy special positions, the height of these prisms is equal to c. Only the P2V6 prism in the title compound has the base and top faces vertical to (010). The P3V9 polyhedron shares a common face with the C2V6 octahedron; thus, the P–C bond length is only 2.888 Å. The next nearest neighbors of all the five P atoms are again P atoms in a distance of 3.221 Å (identical within limits of error for neighbors in (010) and parallel to [010]; the latter corresponds to the translation period along [010]). The C[6]–V bond lengths as well as the V–C[6]–V bond angles vary only moderately (1.910 to 2.121 Å and 81.07° to 101.08°).

3 Experimental section

3.1 Sample preparation

For synthesis, a mixture of fine powders of the pure elements vanadium (H. C. Starck), red phosphorus (Merck KGaA), and graphite (reactor quality), with nominal composition of 62 mol% V, 31 mol% P, and 7 mol% C, respectively, was inserted in evacuated and sealed silica tubes and heated to 1000°C for 3 days. After grinding of the powdered products, a second heat treatment was applied for further homogenization and then rapidly cooled. An X-ray powder diagram proved the presence of the pure title compound. To achieve crystallization, the sample was melted in an induction furnace. A needle-like crystal could be isolated from the smashed regulus.

3.2 X-ray structure determination

For structure investigation by single-crystal X-ray techniques, a small crystal was carefully checked for sufficient quality of the reflection spots. The data collection was performed on a Nonius four-circle diffractometer equipped with a Ccd detector and a 300-μm capillary optics collimator (Mo tube, graphite monochromator). Unit cell parameters were obtained by least-squares refinements of the 2θ values of the Bragg reflections. Corrections for Lorentz, polarization, and absorption effects (multi-scan method) were performed (program Collect [15, 16]).

The extinction symbol is Pa and includes the space groups Pm2a, Pm21a, and Pmma. The determination of the crystal structure by Direct Methods and subsequent difference Fourier summations (programs Shelxs-97 and Shelxl-97 [1719]) was successful, assuming centro-symmetry. Because the anisotropic displacement parameters did not show any conspicuous values, Pmma is considered the correct space group. For the refinements, complex scattering functions were used. Details of data collection, structure refinements, and results, atomic coordinates, and interatomic bond distances are given in Tables 13. To draw the figures, the program Atoms [20] was used.

Table 1:

Single-crystal data and numbers pertinent to data collection and structure refinement of the compound V18P9C2.

Crystal dimensions, μm326 × 45 × 105
Crystal systemOrthorhombic
Space groupPmma (no. 51)
a, Å17.044(3)
b, Å3.2219(7)
c, Å13.030(2)
V, Å3715.5
Z2
ρcalcd, g cm−35.66
μ(MoKα), mm−112.1
hkl range±27, ±5, ±20
2θ-Range data collection, °3 < 2θ < 70
Number of images/rotation angle per image, °618/2
Scan mode (at 11 distinct ω angles)φ scans
Scan time, s per deg/frame size (binned mode)190/621 × 576 pixels
Detector-to-sample distance, mm40
Measured reflections11482
Unique reflections (n)1837
Rinta0.050
Reflections with Fo > 4 σ(Fo) (nobs)1495
Extinction parameterb0.00091(12)
R1c (all data/Fo > 4 σ(Fo))0.035/0.022
wR2d, e0.043
GooFf1.070
Max Δ/σ/number of ref. parameters (p)< 0.001/98
Final difference Fourier map, e Å−3–1.21 to 1.42

aRint = Σ|Fo2Fo2(mean)|/ΣFo2; bFc* = Fc · k[1+0.001·Fc2λ3/sin(2θ)]−1/4; cR1 = Σ||Fo| – |Fc||/Σ|Fo|; dwR2 = [Σw(Fo2Fc2)2w(Fo2)2]1/2; ew = [σ2(Fo2) + (0.012P)2+0.88P]−1, where P = (max(Fo2, 0) + 2Fc2)/3; fGooF = S = [Σw(Fo2Fc2)2/(nobsp)]1/2.

Table 2:

Fractional atomic coordinates and displacement parameters of V18P9C2.

Wyckoff letterSite symmetryxyzUequiv/UisoU11U22U33U13
V12b.2/m.0½00.00528(11)0.0043(3)0.0064(3)0.0052(2)0.0000(2)
V22c.2/m.00½0.00593(11)0.0050(3)0.0077(3)0.0050(2)–0.0006(2)
V32emm2¼00.56126(5)0.00565(12)0.0049(3)0.0071(3)0.0050(3)0
V42fmm2¼½0.39589(5)0.00848(12)0.0073(3)0.0066(3)0.0115(3)0
V52fmm2¼½0.10046(5)0.00721(12)0.0055(3)0.0066(3)0.0095(3)0
V62fmm2¼½–0.10549(5)0.00597(12)0.0045(3)0.0075(3)0.0059(3)0
V74i.m.0.13239(3)00.00447(3)0.00547(8)0.00496(18)0.00566(19)0.00579(17)–0.00011(15)
V84i.m.0.00071(3)00.16136(3)0.00531(8)0.00439(18)0.00543(19)0.00610(18)0.00019(15)
V94i.m.0.16242(3)00.75946(3)0.00555(9)0.00525(18)0.0056(2)0.00577(18)0.00031(15)
V104i.m.0.16911(3)00.24450(3)0.00715(9)0.00532(19)0.0110(2)0.00516(18)–0.00005(15)
V114j.m.0.05973(3)½0.34415(3)0.00688(9)0.00732(19)0.0060(2)0.00727(19)–0.00104(15)
V124j.m.0.12232(3)½0.57177(3)0.00630(9)0.00673(19)0.0060(2)0.00612(18)0.00041(15)
P14i.m.0.14271(4)00.43328(5)0.00548(12)0.0056(3)0.0047(3)0.0061(3)–0.0002(2)
P24i.m.0.04580(4)00.67049(5)0.00642(13)0.0059(3)0.0076(3)0.0058(3)–0.0002(2)
P34j.m.0.10229(4)½0.14079(5)0.00659(13)0.0066(3)0.0054(3)0.0077(3)0.0007(2)
P44j.m.0.10365(4)½0.87077(5)0.00553(12)0.0058(3)0.0050(3)0.0058(3)0.0006(2)
P52fmm2¼½0.69098(7)0.00598(17)0.0062(4)0.0049(4)0.0068(4)0
C12emm2¼0–0.0013(3)0.0056(6)0.0020(14)0.0091(16)0.0055(14)0
C22fmm2¼½0.2493(4)0.0270(11)0.024(2)0.030(3)0.027(2)0

The anisotropic displacement parameters are defined as exp [–2π2 Σ3i=1 Σ3j=1Uija*ia*jhihj]; for all atoms U23 = U12 = 0.

Table 3:

Interatomic distances in V18P9C2.

V1–P42.4405(7)V8–P22.3299(8)P1–V112.4381(7)
V1–P32.5307(8)V8–P32.3801(7)P1–V122.4438(7)
V1–V82.6487(5)V8–P42.4359(7)P1–V32.4749(8)
V1–V72.7731(5)V8–V12.6487(5)P1–V42.4853(6)
V8–V92.9656(8)P1–V102.5007(9)
V2–P22.3547(7)V8–V73.0359(7)P1–V22.5830(8)
V2–P12.5830(8)
V2–V112.7848(5)V9–P22.3011(9)P2–V92.3011(9)
V2–V122.7957(5)V9–P52.3705(6)P2–V82.3299(8)
V9–P42.3879(6)P2–V22.3547(7)
V3–P52.3350(9)V9–V62.8142(6)P2–V112.4220(7)
V3–P12.4749(8)V9–V82.9656(8)P2–V122.4394(7)
V3–V42.6905(8)V9–V32.9828(8)
V3–V122.7111(5)V9–V123.0072(6)P3–V82.3801(7)
V3–V92.9828(8)P3–V102.3914(7)
V10–C22.1213(5)P3–V72.4522(7)
V4–C21.910(5)V10–P32.3914(7)P3–V12.5307(8)
V4–P12.4853(6)V10–P12.5007(9)P3–V52.5718(8)
V4–V32.6905(8)V10–V102.7572(10)P3–V112.7473(9)
V4–V102.8960(7)V10–V112.7852(6)P3–C22.888(3)
V10–V52.8316(6)
V5–C21.940(5)V10–V42.8960(7)P4–V92.3879(6)
V5–C12.087(2)V10–V83.0680(8)P4–V72.4228(7)
V5–P32.5718(8)P4–V82.4359(7)
V5–V62.6836(10)V11–P22.4220(7)P4–V12.4405(7)
V5–V102.8316(6)V11–P12.4381(7)P4–V62.5135(8)
V5–V72.8598(6)V11–P32.7473(9)
V11–V22.7848(5)P5–V32.3350(9)
V6–C12.107(2)V11–V102.7852(6)P5–V92.3705(6)
V6–P42.5135(8)V11–V83.0463(6)P5–V62.6520(12)
V6–P52.6520(12)P5–V122.6738(8)
V6–V52.6836(10)V12–P22.4394(7)
V6–V92.8142(6)V12–P12.4438(7)C1–V72.0060(6)
V6–V72.9439(6)V12–P52.6738(8)C1–V52.087(2)
V12–V32.7111(5)C1–V62.107(2)
V7–C12.0060(6)V12–V22.7957(5)
V7–P42.4228(7)V12–V93.0072(6)C2–V41.910(5)
V7–V12.7731(5)C2–V51.940(5)
V7–V52.8598(6)C2–V102.1213(4)
V7–V62.9439(6)C2–P32.888(3)
V7–V83.0359(7)

Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: ; http://www.fiz-informationsdienste.de/en/DB/icsd/depot_anforderung.html) (deposition number CSD-430670).


Dedicated to: Professor Wolfgang Jeitschko on the occasion of his 80th birthday.


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Received: 2016-1-11
Accepted: 2016-1-22
Published Online: 2016-4-20
Published in Print: 2016-5-1

©2016 by De Gruyter

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