COMPLETION OF THE ALKALI-METAL TITANIUM (III) PYROPHOSPHATE SERIES - SYNTHESIS AND STRUCTURE OF A1TIP2O7(A1=K,RB,CS)

The structure of a new titanium (III) pyrophosphate RbTiP2O7 has been determined. It crystallizes in a monoclinic unit cell ( P21 c, Z = 4) with dimensions a = 7.542 (7) Å, b = 10.256 (2) Å, c = 8.270 (3) Å, β = 105.59 (5)°, V = 616.2 (6) Å3. The single crystal structure refinement gives a final structure solution with R index on F20 of 0.046 for 101 variables and GOF = 1.76. RbTiP2O7 is isostructural with KAlP2O7, whose structure is also adopted by trivalent transition metal containing pyrophosphates; namely MIMoP2O7 (MI = K, Rb, and Cs) and KFeP2O7. The potassium and cesium analogues are also synthesized. The structure differences between LiTiP2O7, NaTiP2O7 (β-phase), and (K,Rb,Cs)TiP2O7 are contrasted by the connectivity of the TiO6 octahedra and P2O7 pyrophosphate groups. The structural relationship between RbTiP2O7 and previously reported BaTi2(P2O7)2 versus the role of electropositive cations in the formation of Ti(P2O7)nO6-2n units (n is 1 for Rb+ and 2 for Ba2+) are discussed. © 1991.