Structural and magnetochemical studies at the ternary phosphates Ba2MII(PO4)(2) (M-II = Mn, Co) and refinement of the crystal structure of BaNi2(PO4)(2)

Single crystals of the following phosphates were grown by the floating zone technique using a mirror furnace and their crystal structures refined (0.0 2 < R-1 < 0.04 and 0,04 < wR(2) < 0,10, resp.): Ba2Mn(PO4)(2) (a=531.1(1), b = 896.8(1), c = 1625.6(3) pm, beta = 90,26(1)degrees), Ba2Co(POS)p (a = 5 29.8(1), b = 884.4(1), c = 1614.4(3) pm, beta = 90.68(2)degrees) and BaNi2( PO4)(2) (a = 480.0(1), c = 2327.3(5)pm, Z = 3, space group R (3) over bar). Both compounds Ba2MII(PO4)(2) crystallize with Z = 4 in space group P2(1)/ n of the monoclinic Ba2Ni(PO4)(2) type; BaNi2(PO4)(2) has the hexagonal-rho mbohedral structure of the BaNi2(AsO4)(2) type. Magnetic measurements of po wders of Ba2Mn(PO4)(2) and Ba2Co(PO4)(2) yielded room temperature moments o f mu (eff) = 5.73 and 4.93 mu (B), resp., but only the manganese compound o beys the Curie-Weiss law down to flow temperatures. Weak antiferromagnetic interactions at both compounds only near T-M = 5 K lead to a reciprocal sus ceptibility minimum.