Acta Cryst. (2007). E63, i180-i181 [ doi:10.1107/S1600536807044765 ]
Nonacalcium chromium(III) heptakis(orthophosphate) has been obtained from a melt in the system Cs2O-CaO-Cr2O3 using a polyphosphate flux. The three-dimensional framework is related to the whitlockite structure [![[beta]](http://journals.iucr.org/logos/entities/beta_rmgif.gif)
-Ca3(PO4)3] and is built up from CaO8 and CaO9 polyhedra sharing vertices, edges and faces, further connected by PO4 tetrahedra and CrO6 octahedra. The Cr, one P and one O atom are located on threefold rotation axes. Ca9Cr(PO4)7 is isotypic with Ca9Fe(PO4)7. The crystal studied was an inversion twin.
The title compound was prepared in a flux in the system Cs2O—P2O5—CaO-Cr2O3. A mixture of CsPO3 (5.0 g), CaCO3 (0.708 g) and Cr2O3 (0.270 g) was ground in an agate mortar, placed into a platinum crucible and heated up to 1273 K. The melt was kept at this temperature until it became homogenous (2 h). The temperature was then decreased to 1053 K at a rate of 30 K h-1, and at this temperature the remaining flux was decantated. Finally, the crucible was cooled down to room temperature. The solidified melt was leached out with deionized water and light-green crystals of Ca9Cr(PO4)7 were recovered.
The measured crystal was racemically twinned (Flack parameter 0.59 (4)). The highest remaining peak in the final Fourier map is 0.82 Å from atom P1.
Data collection: CrysAlis CCD (Oxford Diffraction, 2005); cell refinement: CrysAlis CCD (Oxford Diffraction, 2005); data reduction: CrysAlis RED (Oxford Diffraction, 2005); 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: WinGX (Farrugia, 1999).
![[Figure 1]](./wm2144fig1thm.gif)
| Fig. 1. Unit cell with the three types of calcium layers in the crystal structure of (I). Colour code: Pink plane – Ca1 layer, green plane – Ca2 layer, red plane – Ca3 layer; green octahedra – CrO6 polyhedra, purple tetrahedra – PO4). |
![[Figure 2]](./wm2144fig2thm.gif)
| Fig. 2. The Ca2+ cations with their oxygen neighbours, displayed with anisotropic displacement ellipsoids at the 70% probability level [Symmetry code: (ii) −x + y, −x, z; (v) 2/3 − x + y, 1/3 + y, −1/6 + z; (vi) 1/3 + x, 2/3 + x-y, 1/6 + z; (vii) 1/3 − x + y, −1/3 + y, 1/6 + z; (viii) 2/3 − y, 1/3 − x, −1/6 + z; (x) 2/3 − y, 1/3 + x-y, −1/3 + z; (xi) 1/3 − y, 2/3 − y, 1/6 + z.]. |
![[Figure 3]](./wm2144fig3thm.gif)
| Fig. 3. Fragment of the crystal structure of (I) showing the coordination of the Cr3+ cation. The CaO8 polyhedra are displayed with grey shading, PO4 tetrahedra with purple shading and Cr atom as green circle. |
| Ca9Cr(PO4)7 | Z = 6 |
| Mr = 1077.51 | F000 = 3198 |
| Trigonal, R3c | Dx = 3.13 Mg m−3 |
| Hall symbol: R 3 -2"c | Mo Kα radiation λ = 0.71073 Å |
| a = 10.3272 (5) Å | Cell parameters from 8460 reflections |
| b = 10.3272 (5) Å | θ = 3.2–30.0º |
| c = 37.132 (2) Å | µ = 3.14 mm−1 |
| α = 90º | T = 293 (2) K |
| β = 90º | Prism, green |
| γ = 120º | 0.07 × 0.07 × 0.05 mm |
| V = 3429.6 (3) Å3 |
| XCalibur-3 CCD (Oxford Diffraction) diffractometer | 2161 independent reflections |
| Radiation source: fine-focus sealed tube | 1814 reflections with I > 2σ(I) |
| Monochromator: graphite | Rint = 0.035 |
| T = 293(2) K | θmax = 30.0º |
| φ and ω scans | θmin = 3.2º |
| Absorption correction: multi-scan MULABS (Blessing, 1995) | h = −14→14 |
| Tmin = 0.810, Tmax = 0.859 | k = −14→14 |
| 8460 measured reflections | l = −46→52 |
| Refinement on F2 | w = 1/[σ2(Fo2) + (0.0394P)2] where P = (Fo2 + 2Fc2)/3 |
| Least-squares matrix: full | (Δ/σ)max < 0.001 |
| R[F2 > 2σ(F2)] = 0.030 | Δρmax = 1.78 e Å−3 |
| wR(F2) = 0.073 | Δρmin = −0.84 e Å−3 |
| S = 1.10 | Extinction correction: none |
| 2161 reflections | Absolute structure: Flack (1983), 1040 Friedel pairs |
| 137 parameters | Flack parameter: 0.59 (4) |
| 1 restraint |
| Ca9Cr(PO4)7 | γ = 120º |
| Mr = 1077.51 | V = 3429.6 (3) Å3 |
| Trigonal, R3c | Z = 6 |
| a = 10.3272 (5) Å | Mo Kα |
| b = 10.3272 (5) Å | µ = 3.14 mm−1 |
| c = 37.132 (2) Å | T = 293 (2) K |
| α = 90º | 0.07 × 0.07 × 0.05 mm |
| β = 90º |
| XCalibur-3 CCD (Oxford Diffraction) diffractometer | 2161 independent reflections |
| Absorption correction: multi-scan MULABS (Blessing, 1995) | 1814 reflections with I > 2σ(I) |
| Tmin = 0.810, Tmax = 0.859 | Rint = 0.035 |
| 8460 measured reflections |
| R[F2 > 2σ(F2)] = 0.030 | 1 restraint |
| wR(F2) = 0.073 | Δρmax = 1.78 e Å−3 |
| S = 1.10 | Δρmin = −0.84 e Å−3 |
| 2161 reflections | Absolute structure: Flack (1983), 1040 Friedel pairs |
| 137 parameters | Flack parameter: 0.59 (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. |
| x | y | z | Uiso*/Ueq | ||
| Ca1 | 0.05547 (9) | 0.52061 (10) | 0.100572 (19) | 0.00617 (17) | |
| Ca2 | 0.20187 (10) | 0.37769 (10) | 0.23313 (2) | 0.00654 (17) | |
| Ca3 | 0.38876 (10) | 0.20333 (9) | 0.16032 (2) | 0.01017 (17) | |
| Cr1 | 0 | 0 | 0.00199 (3) | 0.00405 (18) | |
| P1 | 0 | 0 | 0.27062 (5) | 0.0106 (4) | |
| P2 | 0.17511 (12) | 0.31774 (12) | 0.13731 (3) | 0.0042 (2) | |
| P3 | 0.18862 (12) | 0.34253 (14) | 0.03226 (3) | 0.0052 (2) | |
| O1 | 0 | 0 | 0.31204 (16) | 0.0148 (11) | |
| O2 | 0.1360 (3) | 0.1467 (3) | 0.25799 (8) | 0.0111 (6) | |
| O3 | 0.1821 (4) | 0.2731 (4) | 0.17561 (8) | 0.0121 (6) | |
| O4 | 0.0164 (3) | 0.2525 (4) | 0.12245 (8) | 0.0103 (6) | |
| O5 | 0.2744 (3) | 0.2782 (3) | 0.11421 (7) | 0.0076 (6) | |
| O6 | 0.2502 (4) | 0.4937 (3) | 0.13402 (8) | 0.0083 (6) | |
| O7 | 0.3479 (3) | 0.3948 (3) | 0.04562 (8) | 0.0097 (6) | |
| O8 | 0.1122 (3) | 0.4133 (3) | 0.05284 (8) | 0.0101 (6) | |
| O9 | 0.0940 (3) | 0.1692 (3) | 0.03804 (8) | 0.0076 (6) | |
| O10 | 0.1937 (3) | 0.3779 (3) | −0.00752 (7) | 0.0097 (6) |
| U11 | U22 | U33 | U12 | U13 | U23 | |
| Ca1 | 0.0074 (4) | 0.0061 (4) | 0.0053 (4) | 0.0036 (3) | −0.0003 (3) | −0.0012 (3) |
| Ca2 | 0.0062 (4) | 0.0071 (4) | 0.0060 (3) | 0.0032 (4) | 0.0011 (3) | 0.0020 (3) |
| Ca3 | 0.0151 (4) | 0.0082 (4) | 0.0091 (4) | 0.0072 (3) | −0.0035 (3) | 0.0001 (3) |
| Cr1 | 0.0040 (3) | 0.0040 (3) | 0.0042 (4) | 0.00199 (13) | 0 | 0 |
| P1 | 0.0060 (5) | 0.0060 (5) | 0.0199 (10) | 0.0030 (2) | 0 | 0 |
| P2 | 0.0037 (5) | 0.0030 (4) | 0.0058 (5) | 0.0016 (4) | 0.0007 (4) | −0.0006 (4) |
| P3 | 0.0060 (5) | 0.0049 (4) | 0.0046 (5) | 0.0026 (4) | 0.0020 (4) | 0.0008 (4) |
| O1 | 0.0126 (16) | 0.0126 (16) | 0.019 (3) | 0.0063 (8) | 0 | 0 |
| O2 | 0.0055 (13) | 0.0057 (14) | 0.0184 (18) | 0.0002 (10) | −0.0037 (11) | 0.0035 (11) |
| O3 | 0.0149 (15) | 0.0126 (15) | 0.0095 (15) | 0.0075 (12) | 0.0002 (12) | 0.0003 (11) |
| O4 | 0.0080 (15) | 0.0141 (15) | 0.0080 (15) | 0.0050 (12) | 0.0004 (11) | 0.0001 (11) |
| O5 | 0.0073 (14) | 0.0053 (13) | 0.0098 (14) | 0.0028 (11) | −0.0017 (10) | −0.0026 (11) |
| O6 | 0.0081 (14) | 0.0075 (14) | 0.0107 (14) | 0.0050 (13) | 0.0007 (11) | 0.0001 (10) |
| O7 | 0.0031 (13) | 0.0131 (15) | 0.0111 (15) | 0.0026 (12) | −0.0008 (11) | 0.0012 (12) |
| O8 | 0.0142 (15) | 0.0103 (14) | 0.0099 (14) | 0.0091 (13) | 0.0025 (11) | −0.0023 (11) |
| O9 | 0.0058 (14) | 0.0030 (13) | 0.0129 (14) | 0.0014 (12) | −0.0001 (11) | 0.0010 (11) |
| O10 | 0.0128 (15) | 0.0097 (14) | 0.0031 (13) | 0.0031 (12) | −0.0016 (11) | 0.0005 (10) |
| Ca1—O8 | 2.316 (3) | Ca3—P3vi | 3.0244 (14) |
| Ca1—O10i | 2.352 (3) | Ca3—P2 | 3.0987 (14) |
| Ca1—O2ii | 2.404 (3) | Ca3—P3v | 3.1244 (14) |
| Ca1—O7iii | 2.477 (3) | Cr1—O6ix | 2.012 (3) |
| Ca1—O5iii | 2.480 (3) | Cr1—O6viii | 2.012 (3) |
| Ca1—O6 | 2.495 (3) | Cr1—O6ii | 2.012 (3) |
| Ca1—O6iii | 2.513 (3) | Cr1—O9x | 2.023 (3) |
| Ca1—O4 | 2.716 (3) | Cr1—O9vii | 2.023 (3) |
| Ca1—P2iii | 3.1171 (14) | Cr1—O9 | 2.023 (3) |
| Ca1—P2 | 3.2195 (14) | P1—O2x | 1.536 (3) |
| Ca1—P1iv | 3.4804 (9) | P1—O2 | 1.536 (3) |
| Ca1—Ca2ii | 3.4861 (10) | P1—O2vii | 1.536 (3) |
| Ca2—O2 | 2.320 (3) | P1—O1 | 1.538 (6) |
| Ca2—O3 | 2.356 (3) | P1—Ca3i | 3.2507 (15) |
| Ca2—O5i | 2.386 (3) | P1—Ca3xi | 3.2507 (16) |
| Ca2—O9i | 2.471 (3) | P1—Ca3vi | 3.2508 (15) |
| Ca2—O4v | 2.474 (3) | P1—Ca1xii | 3.4803 (9) |
| Ca2—O9v | 2.503 (3) | P1—Ca1v | 3.4804 (9) |
| Ca2—O7i | 2.590 (3) | P1—Ca1xiii | 3.4804 (9) |
| Ca2—O8v | 2.630 (3) | P2—O3 | 1.508 (3) |
| Ca2—P3i | 3.1141 (15) | P2—O4 | 1.530 (3) |
| Ca2—P3v | 3.1424 (14) | P2—O5 | 1.540 (3) |
| Ca2—Ca1v | 3.4862 (10) | P2—O6 | 1.584 (3) |
| Ca2—Cr1i | 3.5254 (12) | P2—Ca1xiv | 3.1171 (14) |
| Ca3—O7vi | 2.398 (3) | P2—Ca3x | 3.5400 (13) |
| Ca3—O5 | 2.417 (3) | P3—O10 | 1.516 (3) |
| Ca3—O4vii | 2.457 (3) | P3—O8 | 1.522 (3) |
| Ca3—O10v | 2.497 (3) | P3—O7 | 1.535 (3) |
| Ca3—O1viii | 2.5484 (16) | P3—O9 | 1.567 (3) |
| Ca3—O10vi | 2.577 (3) | P3—Ca3xv | 3.0244 (14) |
| Ca3—O3 | 2.633 (3) | P3—Ca2viii | 3.1141 (15) |
| Ca3—O8v | 2.645 (3) | P3—Ca3ii | 3.1244 (14) |
| Ca3—O2viii | 2.913 (3) | P3—Ca2ii | 3.1424 (14) |
| O8—Ca1—O10i | 142.88 (11) | O1viii—Ca3—O2viii | 53.59 (14) |
| O8—Ca1—O2ii | 84.88 (11) | O10vi—Ca3—O2viii | 65.06 (9) |
| O10i—Ca1—O2ii | 77.14 (10) | O3—Ca3—O2viii | 130.12 (10) |
| O8—Ca1—O7iii | 73.68 (10) | O8v—Ca3—O2viii | 141.78 (9) |
| O10i—Ca1—O7iii | 138.10 (10) | O6ix—Cr1—O6viii | 83.53 (13) |
| O2ii—Ca1—O7iii | 91.57 (10) | O6ix—Cr1—O6ii | 83.53 (13) |
| O8—Ca1—O5iii | 141.13 (11) | O6viii—Cr1—O6ii | 83.53 (13) |
| O10i—Ca1—O5iii | 74.37 (10) | O6ix—Cr1—O9x | 98.93 (13) |
| O2ii—Ca1—O5iii | 98.61 (10) | O6viii—Cr1—O9x | 177.54 (15) |
| O7iii—Ca1—O5iii | 67.56 (9) | O6ii—Cr1—O9x | 96.57 (12) |
| O8—Ca1—O6 | 85.21 (11) | O6ix—Cr1—O9vii | 96.57 (12) |
| O10i—Ca1—O6 | 79.07 (10) | O6viii—Cr1—O9vii | 98.93 (13) |
| O2ii—Ca1—O6 | 124.72 (10) | O6ii—Cr1—O9vii | 177.54 (15) |
| O7iii—Ca1—O6 | 136.30 (10) | O9x—Cr1—O9vii | 80.98 (13) |
| O5iii—Ca1—O6 | 121.53 (10) | O6ix—Cr1—O9 | 177.54 (15) |
| O8—Ca1—O6iii | 124.59 (11) | O6viii—Cr1—O9 | 96.57 (12) |
| O10i—Ca1—O6iii | 77.97 (10) | O6ii—Cr1—O9 | 98.93 (13) |
| O2ii—Ca1—O6iii | 150.53 (10) | O9x—Cr1—O9 | 80.98 (13) |
| O7iii—Ca1—O6iii | 96.50 (10) | O9vii—Cr1—O9 | 80.98 (13) |
| O5iii—Ca1—O6iii | 59.32 (10) | O2x—P1—O2 | 111.11 (12) |
| O6—Ca1—O6iii | 64.72 (14) | O2x—P1—O2vii | 111.11 (12) |
| O8—Ca1—O4 | 71.64 (10) | O2—P1—O2vii | 111.11 (12) |
| O10i—Ca1—O4 | 71.58 (10) | O2x—P1—O1 | 107.78 (13) |
| O2ii—Ca1—O4 | 68.34 (10) | O2—P1—O1 | 107.78 (13) |
| O7iii—Ca1—O4 | 141.05 (10) | O2vii—P1—O1 | 107.78 (13) |
| O5iii—Ca1—O4 | 145.46 (10) | O3—P2—O4 | 114.19 (18) |
| O6—Ca1—O4 | 56.93 (10) | O3—P2—O5 | 107.73 (17) |
| O6iii—Ca1—O4 | 117.61 (10) | O4—P2—O5 | 113.00 (17) |
| O8—Ca1—P2iii | 143.40 (9) | O3—P2—O6 | 110.64 (18) |
| O2—Ca2—O3 | 88.61 (11) | O4—P2—O6 | 106.27 (17) |
| O2—Ca2—O5i | 84.27 (11) | O5—P2—O6 | 104.58 (16) |
| O3—Ca2—O5i | 142.23 (11) | P1—O1—Ca3i | 102.58 (13) |
| O2—Ca2—O9i | 157.39 (11) | P1—O1—Ca3xi | 102.58 (13) |
| O3—Ca2—O9i | 88.49 (11) | Ca3i—O1—Ca3xi | 115.40 (9) |
| O5i—Ca2—O9i | 84.36 (10) | P1—O1—Ca3vi | 102.58 (13) |
| O2—Ca2—O4v | 73.98 (10) | Ca3i—O1—Ca3vi | 115.39 (9) |
| O3—Ca2—O4v | 139.97 (12) | Ca3xi—O1—Ca3vi | 115.39 (9) |
| O5i—Ca2—O4v | 72.72 (11) | P1—O2—Ca2 | 142.12 (19) |
| O9i—Ca2—O4v | 120.70 (11) | P1—O2—Ca1v | 122.56 (17) |
| O2—Ca2—O9v | 137.71 (11) | Ca2—O2—Ca1v | 95.10 (10) |
| O3—Ca2—O9v | 81.44 (10) | P1—O2—Ca3i | 88.21 (14) |
| O5i—Ca2—O9v | 126.71 (10) | Ca2—O2—Ca3i | 86.29 (9) |
| O9i—Ca2—O9v | 63.75 (14) | Ca1v—O2—Ca3i | 92.17 (9) |
| O4v—Ca2—O9v | 87.73 (10) | P2—O3—Ca2 | 136.17 (19) |
| O2—Ca2—O7i | 98.25 (10) | P2—O3—Ca3 | 92.84 (15) |
| O3—Ca2—O7i | 77.37 (11) | Ca2—O3—Ca3 | 115.12 (13) |
| O5i—Ca2—O7i | 67.12 (10) | P2—O4—Ca3x | 123.57 (16) |
| O9i—Ca2—O7i | 59.26 (10) | P2—O4—Ca2ii | 142.26 (17) |
| O4v—Ca2—O7i | 139.70 (10) | Ca3x—O4—Ca2ii | 93.97 (11) |
| O9v—Ca2—O7i | 119.04 (10) | P2—O4—Ca1 | 94.49 (15) |
| O2—Ca2—O8v | 79.84 (10) | Ca3x—O4—Ca1 | 95.96 (11) |
| O3—Ca2—O8v | 70.70 (11) | Ca2ii—O4—Ca1 | 84.28 (9) |
| O5i—Ca2—O8v | 143.05 (10) | P2—O5—Ca2viii | 147.59 (18) |
| O9i—Ca2—O8v | 120.07 (10) | P2—O5—Ca3 | 100.77 (14) |
| O4v—Ca2—O8v | 70.88 (10) | Ca2viii—O5—Ca3 | 97.29 (11) |
| O9v—Ca2—O8v | 58.03 (9) | P2—O5—Ca1xiv | 98.99 (14) |
| O7i—Ca2—O8v | 148.04 (10) | Ca2viii—O5—Ca1xiv | 103.64 (11) |
| O7vi—Ca3—O5 | 153.05 (12) | Ca3—O5—Ca1xiv | 101.18 (11) |
| O7vi—Ca3—O4vii | 94.92 (11) | P2—O6—Cr1i | 135.74 (18) |
| O5—Ca3—O4vii | 72.50 (10) | P2—O6—Ca1 | 101.92 (15) |
| O7vi—Ca3—O10v | 123.00 (10) | Cr1i—O6—Ca1 | 103.22 (13) |
| O5—Ca3—O10v | 72.96 (10) | P2—O6—Ca1xiv | 96.44 (15) |
| O4vii—Ca3—O10v | 142.08 (10) | Cr1i—O6—Ca1xiv | 102.60 (13) |
| O7vi—Ca3—O1viii | 100.65 (13) | Ca1—O6—Ca1xiv | 118.58 (12) |
| O5—Ca3—O1viii | 106.20 (13) | P3—O7—Ca3xv | 98.15 (14) |
| O4vii—Ca3—O1viii | 113.31 (12) | P3—O7—Ca1xiv | 130.55 (17) |
| O10v—Ca3—O1viii | 62.91 (7) | Ca3xv—O7—Ca1xiv | 109.05 (12) |
| O7vi—Ca3—O10vi | 59.84 (9) | P3—O7—Ca2viii | 94.56 (14) |
| O5—Ca3—O10vi | 133.01 (9) | Ca3xv—O7—Ca2viii | 130.06 (13) |
| O4vii—Ca3—O10vi | 72.52 (10) | Ca1xiv—O7—Ca2viii | 98.01 (10) |
| O10v—Ca3—O10vi | 123.73 (12) | P3—O8—Ca1 | 158.44 (19) |
| O1viii—Ca3—O10vi | 61.84 (7) | P3—O8—Ca2ii | 94.58 (15) |
| O7vi—Ca3—O3 | 101.93 (11) | Ca1—O8—Ca2ii | 89.39 (10) |
| O5—Ca3—O3 | 58.15 (10) | P3—O8—Ca3ii | 93.18 (14) |
| O4vii—Ca3—O3 | 98.18 (11) | Ca1—O8—Ca3ii | 106.09 (12) |
| O10v—Ca3—O3 | 76.22 (10) | Ca2ii—O8—Ca3ii | 105.99 (11) |
| O1viii—Ca3—O3 | 139.12 (8) | P3—O9—Cr1 | 130.69 (17) |
| O10vi—Ca3—O3 | 157.60 (11) | P3—O9—Ca2viii | 98.43 (14) |
| O7vi—Ca3—O8v | 69.36 (10) | Cr1—O9—Ca2viii | 102.88 (13) |
| O5—Ca3—O8v | 112.01 (10) | P3—O9—Ca2ii | 98.45 (14) |
| O4vii—Ca3—O8v | 153.80 (11) | Cr1—O9—Ca2ii | 101.80 (12) |
| O10v—Ca3—O8v | 57.68 (9) | Ca2viii—O9—Ca2ii | 128.60 (12) |
| O1viii—Ca3—O8v | 90.72 (13) | P3—O10—Ca1viii | 145.17 (19) |
| O10vi—Ca3—O8v | 113.31 (9) | P3—O10—Ca3ii | 99.33 (15) |
| O3—Ca3—O8v | 66.48 (10) | Ca1viii—O10—Ca3ii | 102.53 (10) |
| O7vi—Ca3—O2viii | 124.77 (10) | P3—O10—Ca3xv | 91.55 (13) |
| O5—Ca3—O2viii | 71.97 (10) | Ca1viii—O10—Ca3xv | 102.52 (11) |
| O4vii—Ca3—O2viii | 64.38 (9) | Ca3ii—O10—Ca3xv | 116.21 (12) |
| O10v—Ca3—O2viii | 90.31 (9) |
| Symmetry codes: (i) −y+1/3, −x+2/3, z+1/6; (ii) x−1/3, x−y+1/3, z−1/6; (iii) −x+y, −x+1, z; (iv) −y−1/3, −x+1/3, z−1/6; (v) x+1/3, x−y+2/3, z+1/6; (vi) −x+y+1/3, y−1/3, z+1/6; (vii) −x+y, −x, z; (viii) −y+2/3, −x+1/3, z−1/6; (ix) −x+y−1/3, y−2/3, z−1/6; (x) −y, x−y, z; (xi) x−2/3, x−y−1/3, z+1/6; (xii) −x+y−2/3, y−1/3, z+1/6; (xiii) −y+1/3, −x−1/3, z+1/6; (xiv) −y+1, x−y+1, z; (xv) −x+y+2/3, y+1/3, z−1/6. |
| Ca1—O8 | 2.316 (3) | Ca3—O10iv | 2.497 (3) |
| Ca1—O10i | 2.352 (3) | Ca3—O1vii | 2.5484 (16) |
| Ca1—O2ii | 2.404 (3) | Ca3—O10v | 2.577 (3) |
| Ca1—O7iii | 2.477 (3) | Ca3—O3 | 2.633 (3) |
| Ca1—O5iii | 2.480 (3) | Ca3—O8iv | 2.645 (3) |
| Ca1—O6 | 2.495 (3) | Ca3—O2vii | 2.913 (3) |
| Ca1—O6iii | 2.513 (3) | Cr1—O6viii | 2.012 (3) |
| Ca1—O4 | 2.716 (3) | Cr1—O9 | 2.023 (3) |
| Ca2—O2 | 2.320 (3) | P1—O2 | 1.536 (3) |
| Ca2—O3 | 2.356 (3) | P1—O1 | 1.538 (6) |
| Ca2—O5i | 2.386 (3) | P2—O3 | 1.508 (3) |
| Ca2—O9i | 2.471 (3) | P2—O4 | 1.530 (3) |
| Ca2—O4iv | 2.474 (3) | P2—O5 | 1.540 (3) |
| Ca2—O9iv | 2.503 (3) | P2—O6 | 1.584 (3) |
| Ca2—O7i | 2.590 (3) | P3—O10 | 1.516 (3) |
| Ca2—O8iv | 2.630 (3) | P3—O8 | 1.522 (3) |
| Ca3—O7v | 2.398 (3) | P3—O7 | 1.535 (3) |
| Ca3—O5 | 2.417 (3) | P3—O9 | 1.567 (3) |
| Ca3—O4vi | 2.457 (3) |
| Symmetry codes: (i) −y+1/3, −x+2/3, z+1/6; (ii) x−1/3, x−y+1/3, z−1/6; (iii) −x+y, −x+1, z; (iv) x+1/3, x−y+2/3, z+1/6; (v) −x+y+1/3, y−1/3, z+1/6; (vi) −x+y, −x, z; (vii) −y+2/3, −x+1/3, z−1/6; (viii) −x+y−1/3, y−2/3, z−1/6. |
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Partial substitution of alkaline earth metal atoms in MII3(PO4)2 (MII = Ca, Sr) whitlockite-type structures (Dickens et al., 1974) by monovalent, bivalent and tetravalent metals provides possibilities for obtaining new compounds with useful properties. This group of orthophosphates and their solid solutions have been intensively studied and are interesting in aspects of applications. For example, Ca9In(PO4)7 (Morozov et al., 2002), Ca9RE(PO4)7 (RE = rare-earth metals) (Teterskii et al., 2005) and Ca9Fe(PO4)7 (Lazoryak et al., 2004) exhibit interesting dielectric properties and large second-harmonic generation (SHG) effects; the solid solutions Sr9.2Co1.3(PO4)7 (Belik et al., 2006), Ca3 − xCox(PO4)2 (Legrouri et al., 1996) and Ca3 − xCux(PO4)2 (Benarafa et al., 2000) possess catalytic activity; Ca9Fe(PO4)7 (Lazoryak et al., 1996) can be used as a sensor material and for removing H2 from gas mixtures.
We report here the flux-growth synthesis and structural characterization of the whitlockite-related phosphate Ca9Cr(PO4)7, (I), which is isotypic with Ca9Fe(PO4)7 (Lazoryak et al., 2004).
The structure of (I) contains three types of layers, which are formed by Ca atoms in positions Ca1, Ca2 and Ca3, respectively (Fig. 1). The Ca1O8, Ca2O8 and Ca3O9 polyhedra, with Ca–O distances ranging from 2.316 (3) to 2.913 (3) Å (Table), are linked together via vertices, edges and faces. The polyhedral network is additionally linked by three different corner- or edge-sharing PO4 tetrahedra and a CrO6 octahedron. The PO4 tetrahedra are quite regular with P–O bond lengths ranging from 1.508 (3) to 1.584 (3) Å, and O–P–O angles spreading over the range 104.58 (16)–114.19 (18)°. The coordination of the Cr3+ cation is slightly distorted octahedral with two different Cr–O distances of 2.012 (3) and 2.023 (3) Å, respectively (Fig. 3).