The phase transition of the incommensurate phases β - Ln ( PO 3 ) 3 ( Ln = Y , Tb … Yb ) , crystal structures of α - Ln ( PO 3 ) 3 ( Ln = Y , Tb … Yb ) and Sc ( PO 3 ) 3

The incommensurately modulated room-temperature phases β - Ln ( PO 3 ) 3 ( Ln = Y , Tb … Yb ) undergo a topotactic phase transition monitored by vibrational spectroscopy below 180 K leading to α - Ln ( PO 3 ) 3 ( Ln = Y , Dy … Yb ) , above 200 K the incommensurate phases are reobtained. The low-temperature phases exhibit a new structure type ( α - Dy ( PO 3 ) 3 , P 2 1 / c , Z = 12 , a = 14.1422 ( 6 ) , b = 20.0793 ( 9 ) , c = 10.1018 ( 4 ) A ˚ , β = 127.532 ( 3 ) ∘ ) . α - Tb ( PO 3 ) 3 is isotypic with Gd ( PO 3 ) 3 ( α - Tb ( PO 3 ) 3 , I 2 / 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 ( PO 3 ) 3 is isotypic with that of Lu ( PO 3 ) 3 and C -type phosphates. The polyphosphates consist of infinite zig-zag chains of corner-sharing PO 4 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. Basic structure from which all crystal structures of the late lanthanoids’ polyphosphates at room temperature and below can be derived.