Synthesis and Structure of Bismuth(III)-Containing Noncentrosymmetric Phosphates, Cs<sub>3</sub>KBi<sub>2</sub>M<sub>4</sub>(PO<sub>4</sub>)<sub>6</sub>Cl (M = Mn, Fe). Monoclinic (<i>Cc</i>) and Tetragonal (<i>P</i>4<sub>3</sub>) Polymorphs Templated by Chlorine-Centered Cl(Bi<sub>2</sub>Cs) Acentric Units
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Single crystals of three new noncentrosymmetric (NCS) phosphates, α (1) and β (2) forms of Cs3KBi2Mn4(PO4)6Cl and α-Cs3KBi2Fe4(PO4)6Cl (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 α 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 β form crystallizes in P43 (No. 78) of another polar class, 4 (4/m). The unit cell parameters of the α form can be approximately correlated with that of the β form via the 3 × 3 orientation matrix [0.5, 0.5, 0; −0.5, 0.5, 0; 0, 0, 2 sin β]. 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 α form, resulting in the monoclinic (Cc) lattice, but “antiparallel” in the β form, thus giving the tetragonal (P43) 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 MII···Cl bond. The compositions of 1 and 2 consist of three Mn2+ (d5) and one Mn3+ (d4) per formula unit and that of 3 has three Fe2+ (d6) and one Fe3+ (d5). Bond valence sums reveal that, in the α phase, the trivalent site adopts distorted tetrahedral M(1)3+O4 coordination and, in the β phase, distorted trigonal-bipyramidal M(4)3+O5. Thus far, the iron phase has only been isolated in the α form presumably because of little extra stabilization energy gain if the Fe2+ d6 ion were to occupy the M(1)O4 site. The possible origins pertaining to the structural differences in the α and β forms are discussed.
