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Crystal Structures of Incommensurately Modulated Ln(PO3)3 (Ln = Tb−Yb) and Commensurate Gd(PO3)3 and Lu(PO3)3

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posted on 30.04.2007, 00:00 by Henning A. Höppe, Stefan J. Sedlmaier
The crystal structure of the late lanthanoids' catena-polyphosphates Ln(PO3)3 (Ln = Tb−Yb) is incommensurately modulated (Dy(PO3)3:  space group Cc(0β0)0; Z = 4; a = 1417.4(4), b = 670.96(14), c = 1009.5(3) pm; β = 127.62(2)°, q = 0.364b*; Rall = 0.057, wRall = 0.071; 293 K) and consists of infinite chains of corner-sharing PO4 tetrahedra. The cations are coordinated 6-fold in an almost octahedral arrangement over the whole modulation period. All atoms comprise a sinoidal positional modulation. The basic structure can be derived from a fcc packing which explains the pseudo-face-centering observed in the diffraction patterns. The crystal structure of Lu(PO3)3 is isotypic with C-type phosphates (Cc; Z = 12; a = 1397.2(1), b = 2001.8(2), c = 995.56(9) pm; β = 127.351(6)°; R1 = 0.042, wR2 = 0.097, 293 K), and Gd(PO3)3 crystallizes in a new structure type (I2/a; Z = 16; a = 2601.7(2), b = 1351.1(1), c = 1008.4(1) pm; β = 119.311(6)°; R1 = 0.039, wR2 = 0.092; 293 K). Both can be described in terms of superstructures of the basic structure unit cell of the incommensurate phases, and thus, a consistent structural description of many polyphosphates is provided. Tb(PO3)3 was obtained as single phase adopting a novel synthesis under reducing conditions. The absence of an inversion center in the incommensurate phases and Lu(PO3)3 was proved by a SHG experiment. The vibrational spectra are also discussed.