The phase transition of the incommensurate phases image image, crystal structures of image image and image

The incommensurately modulated room-temperature phases β-Ln(PO3)3β-Ln(PO3)3(Ln=Y,Tb…Yb)(Ln=Y,Tb…Yb) undergo a topotactic phase transition monitored by vibrational spectroscopy below 180 K leading to α-Ln(PO3)3α-Ln(PO3)3(Ln=Y,Dy…Yb)(Ln=Y,Dy…Yb), above 200 K the incommensurate phases are reobtained. The low-temperature phases exhibit a new structure type View the MathML source(α-Dy(PO3)3,P21/c,Z=12,a=14.1422(6),b=20.0793(9),c=10.1018(4)A˚,β=127.532(3)∘). α-Tb(PO3)3α-Tb(PO3)3 is isotypic with Gd(PO3)3Gd(PO3)3View the MathML source(α-Tb(PO3)3,I2/a,Z=16,a=25.875(6),b=13.460(3),c=10.044(2)A˚,β=119.13(3)∘). The symmetry relations between the involved phases of the phase transition are discussed. The crystal structure of Sc(PO3)3Sc(PO3)3 is isotypic with that of Lu(PO3)3Lu(PO3)3 and CC-type phosphates. The polyphosphates consist of infinite zig-zag chains of corner-sharing PO4PO4 tetrahedra, the cations are coordinated sixfold in an almost octahedral arrangement. To confirm the quality of the determined crystal structures the deviation of the phosphate tetrahedra from ideal symmetry was determined and discussed.