THE V(III) MONOPHOSPHATE SERIES AV(2)O(PO4)(2) WITH A=CD, CA, SR - STRUCTURE AND MAGNETISM

The strontium vanadophosphate, belonging to the series AV(2)O(PO4)(2) with A = Cd, Ca, Sr has been synthesized. Its single-crystal X-Ray stu dy shows its isotypism with the cadmium and calcium phases. It crystal lizes in the space group Pnma with a = 14.220(1) Angstrom, b = 6.5138( 6) Angstrom, and c = 7.5166(8) Angstrom, Z = 4, and R = 0.039. In its structure, built up from [(VO2)-O-III](infinity) rutile chains connect ed through VO6 octahedra and PO4 tetrahedra, the geometry of the polyh edra is very close to that obtained for the Cd and Ca phases. The main difference deals with the V(2)-V(2) intrachains distances that increa se significantly with the size of the A cation. The study of the magne tic susceptibility of the three monophosphates (A = Cd, Ca, Sr) eviden ces a narrow transition of 20 K from a paramagnetic to antiferromagnet ic state. This behavior is in contrast with that of the (VPO4)-P-III p hase that also exhibits isolated rutile chains but shows a broad trans ition spreading over 100 K. This difference is explained by the presen ce of additional VO6 octahedra that may weaken the unidimensional char acter of these phosphates compared to VPO4. (C) 1996 Academic Press.