The novel high-pressure/high-temperature compound Co12P7 determined from synchrotron data

Co12P7, synthesized at high pressure/temperature conditions, crystallizes isotypically with ordered Cr12P7 in space-group type P .

There are few structures reported in the literature for transition-metal phosphides with the composition M 12 P 7 . Baurecht et al. (1971) first examined Cr 12 P 7 and determined that it adopts a hexagonal lattice with space group P6, Z = 1.
The structure consists of columns of alternating tetrahedral and pyramidal polyhedra and columns of stacked triangularprismatic polyhedra extending along the c-axis direction. Chromium atoms occupy half of all possible tetrahedral and pyramidal sites while the triangular-prismatic sites are empty (Baurecht et al., 1971). The polyhedra in the unit cell can be described as Cr 9 P Cr 3 T [] 2 Pr P 7 (P = pyramidal, T = tetrahedral, Pr = trigonal-prismatic, [] = empty site) (Maaref et al., 1981). Coupled disordering of two half-atoms of the corresponding metal with two half-atoms of phosphorus within the tetrahedral and pyramidal sites has been observed in this structure for compounds Th 7 S 12 , V 12 P 7 , and Cr 12 P 7 , increasing the symmetry to the P6 3 /m space group (Zachariasen, 1949;Olofsson & Ganglberger 1970;Chun & Carpenter, 1979).
At ambient conditions the M 12 P 7 composition is not observed in the binary systems with M = Co, Ni, Fe. Dhahri (1996) concluded that Co 12 P 7 , Ni 12 P 7 and Fe 12 P 7 do not occur in the Cr 12 P 7 structure type at ambient conditions because, unlike Cr and V, the elements Co, Ni and Fe do not preferentially occupy pyramidal sites. In support of this conclusion, the Zn 2 Fe 12 P 7 structure type (P6, Z = 1) with many structural similarities to the Cr 12 P 7 structure type, has been observed in Ln 2 M 12 P 7 (Ln = rare-earth element; M = Co, Ni, Fe) compounds where the pyramidal-to-tetrahedral site ratio is 1:3 (Jeitschko et al., 1978;Jeitschko & Jaberg, 1980;Reehuis & Jeitschko, 1989). Ordering is present in the Co-, Fe-, Ni-rich Zn 2 Fe 12 P 7 isomorphs (Jeitschko et al., 1984). No other struc-ture types for the composition M 12 P 7 (M = Co, Ni, Fe) have been reported so far.
The effect of pressure and temperature on stabilizing Co in both the tetrahedral and pyramidal sites and ordering of Co and P in the Cr 12 P 7 -type structure has not been examined previously. In the current study, we report the synthesis of a Co 12 P 7 phase at 27 GPa and 1750 K, and at 48 GPa and 1790 K; both phases are isostructural and crystallize in space group P6. Structure refinements revealed that Co and P sites are ordered in the high P-T structure and Co atoms occupy tetrahedral and pyramidal coordinations. Using single-crystal diffraction techniques, we report refined atomic coordinate sites of Co 12 P 7 at 48 GPa and 15 GPa.

Structural commentary
Refinement of the structure confirms that Co 12 P 7 assumes the ordered Cr 12 P 7 structure type (Baurecht et al., 1971;Chun & Carpenter, 1979). Two of the Co sites (Co0, Co1) occupy Wyckoff position 3 j (point group symmetry m..), the other two Co sites (Co2, Co3) Wyckoff position 3 k (m..), one P site (P5) Wyckoff position 3 j, one P site (P4) Wyckoff position 3 k, and one P site (P6) Wyckoff position 1 a (6..). The Co sites occupy tetrahedral (cyan) and pyramidal (violet) sites as imaged in Fig. 1. Chains of edge-sharing CoP 5 square pyramids and chains of corner-sharing CoP 4 tetrahedra build up the framework with trigonal-prismatic channels running parallel to the c axis.
Ranges of interatomic Co-P distances and polyhedral volumes are provided in Table 1 and Fig. 2 with CoP 4 tetrahedra represented by a cyan polyhedron and CoP 5 pyramids represented by violet polyhedra. Co0 atoms occupy a distorted tetrahedral site with one P atom at a short distance, two at 1666 Zurkowski et al. Co 12 P 7 and Co 12 P 7 Acta Cryst. (2020). E76, 1665-1668 research communications Table 1 Selected structural parameters for Co 12 P 7 at 48 GPa.

Figure 2
Co-P polyhedra as observed in the Co 12 P 7 structure (48 GPa data set) showing varying degrees of volume and distortion, quantified in Table 1. CoP 4 tetrahedra are shaded in cyan and CoP 5 square pyramids are shaded in violet. Displacement ellipsoids are drawn at the 50% probability level.

Figure 1
Crystal structure of Co 12 P 7 based on the 48 GPa data set with atoms of the asymmetric unit labeled. CoP 4 tetrahedra are shaded in cyan and CoP 5 square pyramids are shaded in violet.
intermediate distances and one at a long distance (Table 1, Fig. 2). Co1 and Co2 atoms occupy square pyramids with two intermediate and two long interatomic distances at the base. Co3 atoms occupy a less distorted square pyramid with two elongated and two truncated bonds at the base (Fig. 2). Interatomic distances at 48 GPa range from 2.063 (2)-2.102 (2) Å in the tetrahedral polyhedra, 2.147 (4)-2.220 (4) Å for Co1-P polyhedra, 2.197 (4)-2.317 (2) Å for Co2-P polyhedra and 2.194 (3)-2.219 (3) Å for Co3-P polyhedra (Table 1). These interatomic distances are comparable to those observed in Co 2 P and CoP (Rundqvist 1960(Rundqvist , 1962. A grain of Co 12 P 7 was decompressed to ambient conditions where 44 total reflections were identified in reciprocal space and indexed to a unit cell of a = 8.47 (1) Å , c = 3.37 (1) Å . These unit-cell parameters are in agreement with the pressure-volume trend observed, but peak broadening and loss of reflections at high angles may reflect the onset of phase instability on decompression.

Synthesis and crystallization
The synthesis of Co 12 P 7 was performed at high pressures and temperatures in a laser-heated diamond anvil cell (LHDAC). Two samples were loaded for this study in which Co 12 P 7 was synthesized at 26.9 (8) GPa and 1740 (110) K and 48.2 (5) GPa and 1790 (200) K, respectively. Pressure was generated in BX-90-type (70 angular opening) diamond anvil cells (DACs) with 300 mm culet, Boehler-Almax type diamonds and seats. Co-P samples and a ruby sphere for pressure calibration were loaded into a sample chamber drilled from a rhenium gasket. The chamber was subsequently filled with compressed neon gas . Pressure was determined using the ruby fluorescence scale and the Ne equation of state (Mao & Bell, 1976;Fei et al., 2007).
Samples were heated from both sides with 100W Yb-doped fiber lasers at beamline 13-ID-D (GeoSoilEnviroCARS) of the Advanced Photon Source (APS), Argonne National Laboratory. Heating cycles typically lasted $15 minutes at target temperatures prior to quench. The lasers were shaped with $15 mm flat tops and temperature was measured spectroradiometrically from a 6 mm central region of the laser heated spot using a gray body approximation (Heinz & Jeanloz, 1987). Axial temperature gradients through the sample were accounted for by applying a 3% correction on temperature measurements (Campbell et al., 2007(Campbell et al., , 2009. Upon quench from high temperatures, high-pressure samples consisted of agglomerates of Co 12 P 7 and Pnma Co 2 P (Rundqvist, 1960) crystals of variable grain sizes up to $5 mm in diameter. Grains of target phases were identified in reciprocal space and sorted out from the scattering contribution of other grains, neon and diamond. Diffraction data were processed using Dioptas (Prescher & Prakapenka, 2015) and CrysAlis Pro (Rigaku OD, 2018). Decompression data were collected for both samples in two experimental stations; here we report two selected refinements of the Co 12 P 7 structure at 48.2 (5) GPa and 15.4 (2) GPa.

Refinement
Crystal data, data collection and structure refinement details at 48 GPa and 15 GPa are summarized in Table 2. Monochromatic X-ray diffraction measurements took place at beamlines 13-ID-D (2 mm x 3 mm beam, = 0.2952 Å ) and 13-BM-D (5 mm Â 8 mm beam, = 0.3344 Å ) at APS (Table 2). Diffraction measurements were collected at synthesis pressures and upon decompression. At target pressure steps, 10 x 10 mm still image maps were collected in 2 mm steps around the heated region. At selected map locations exhibiting the largest crystallites, rotation images were collected spanning AE30 at a rate of 1s per 0.5 step.
Grains of Co 12 P 7 identified in reciprocal space were indexed to a primitive hexagonal lattice. Analysis of systematic absences indicated space group P6 with Z = 1. Two grains from distinct loadings and measured at different beamlines were selected for structural refinements as they showed the largest number of observed reflections and good statistical parameters (Table 2). Structure factors measured in microdiffraction in the LHDAC show some well-known limitations, such as limited resolution and redundancy, reflections overlapped by parasitic scattering, diamond diffraction (Loveday et al., 1990) and, more notably, variable volume of illuminated crystal during rotation. As could be expected, we identified eight and five outlier reflections in the refinements for the 48 GPa and 15 GPa data sets, respectively, and omitted them in the final calculations. Based on the ratio 'observed reflections/refined parameters' and statistical tests (Hamilton, 1965), we concluded that the P sites should be refined with isotropic displacement parameters (U iso ) whereas the Co sites could be refined with anisotropic displacement parameters. After convergence, site occupancies of Co atoms and P atoms were released in alternate runs. Within uncertainty (< 1.2% for Co and < 1.3% for P), all sites are fully occupied. Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.