Magnetic and vibrational properties and crystal structure of Sr9.2Co1.3(PO4)7 with disordered arrangements of some strontium, cobalt, and phosphate ions

Solid solutions of Sr9+xCo1.5−x(PO4)7 were found in the compositional range of 0.05⩽x⩽0.300.05⩽x⩽0.30. The structure of Sr9.2Co1.3(PO4)7 (x=0.2x=0.2) was determined from single crystal X-ray diffraction (space group R3¯m (No. 166); Z=3Z=3; a=10.6100(5)Å and c=19.6960(5)Å; V=1920.17(14)Å3; ρcal=4.014gcm-3; μ=20.377mm-1) and refined to R1=0.0343R1=0.0343 and wR2=0.0633wR2=0.0633 for 586 reflections with I>2σ(I)I>2σ(I). Sr9.2Co1.3(PO4)7 is structurally related to β-Ca3(PO4)2 and Sr3(PO4)2 and has disordered arrangements of some Sr2+, Co2+, and PO43− ions. Sr2+ ions at a 9e site are statistically disordered among four positions near the center of symmetry. Co2+ and Sr2+ ions are split along the c-axis to occupy a 6c site that is 75% vacant. The P1O4 tetrahedra are orientationally disordered. Sr2+ ions at an 8-fold coordinated 18h site, Co2+ ions at an octahedral 3a site, and the P2O4 tetrahedra are ordered in the structure of Sr9.2Co1.3(PO4)7. Features of Raman spectra are discussed in relation to the crystallographic structure of Sr9.2Co1.3(PO4)7 and in comparison with Raman spectra of β-Ca3(PO4)2-type and Sr3(PO4)2-type compounds. Sr9.2Co1.3(PO4)7 is paramagnetic between 2 and 300 K with an effective magnetic moment of 4.98μB per Co2+ ion.

Disordered structural elements in Sr9.2Co1.3(PO4)7: the P1O4 tetrahedron and the Sr31, Sr32, Sr4, and Co4 sites. The Co4−O21 bonds and ordered P2O4 and Co5O6 polyhedra are also shown.Figure optionsDownload as PowerPoint slide