Synthesis and Structure of Bismuth(III)-Containing Noncentrosymmetric Phosphates, Cs3KBi2M4(PO4)6Cl (M = Mn, Fe). Monoclinic (Cc) and Tetragonal (P43) Polymorphs Templated by Chlorine-Centered Cl(Bi2Cs) Acentric Units

Single crystals of three new noncentrosymmetric (NCS) phosphates, alpha (1) and beta (2) forms of Cs3KBi2Mn4(PO4)6Cl and alpha-Cs3KBi2Fe4(PO4)(6)Cl (3), were grown in a reactive CsCl/KCl molten-salt media. Their structures were determined by single-crystal X-ray diffraction methods showing that the a form crystallizes in the space group Cc (No. 9), which is in one of the 10 NCS polar crystal classes, m (2/m) while the beta form crystallizes in P4(3) (No. 78) of another polar class, 4 (4/m). The unit cell parameters of the alpha form can be approximately correlated with that of the beta form via the 3 X 3 orientation matrix [0.5, 0.5, 0; -0.5, 0.5, 0; 0, 0, 2 sin beta]. The structures of these otherwise complicated phosphates exhibit two types of channels with circular and elliptical windows where the Cl-centered Cl(Bi2Cs) acentric unit is located. The neighboring acentric units are arranged in a parallel fashion in the alpha form, resulting in the monoclinic (Cc) lattice, but "antiparallel" in the beta form, thus giving the tetragonal (P4(3)) unit cell. 1-3 feature the compatible M-O-P unit that contains four crystallographically independent MOx (x = 4, 5) polyhedra, which are connected to the Cl(Bi2Cs) acentric unit through one short and one long M-II center dot center dot center dot Cl bond. The compositions of 1 and 2 consist of three Mn2+ (d(5)) and one Mn3+ (d(4)) per formula unit and that of 3 has three Fe2+ (d(6)) and one Fe3+ (ds). Bond valence sums reveal that, in the alpha phase, the trivalent site adopts distorted tetrahedral M(1)O-3+(4) coordination and, in the beta phase, distorted trigonal-bipyramidal M(4)O-3+(5). Thus far, the iron phase has only been isolated in the alpha form presumably because of little extra stabilization energy gain if the Fe2+ d(6) ion were to occupy the M(1)O-4 site. The possible origins pertaining to the structural differences in the alpha and beta forms are discussed.