Journal of Solid State Chemistry 175 (2003) 27–33 Alignment of acentric MoO 3 F 3 3 anions in a polar material: (Ag 3 MoO 3 F 3 )(Ag 3 MoO 4 )Cl PaulA.Maggard, a TiffanyS.Nault, b CharlotteL.Stern, b andKennethR.Poeppelmeier b, a Department of Chemistry, 2620 Yarbrough Drive, North Carolina State University, Raleigh, NC 27695-8204, USA b Department of Chemistry, 2145 Sheridan Road, Northwestern University, Evanston, IL 60208-3113, USA Received 24 September 2002; received in revised form 10 January 2003; accepted 18 January 2003 Abstract (Ag 3 MoO 3 F 3 )(Ag 3 MoO 4 )Cl was synthesized by hydro(solvato)thermal methods and characterized by single-crystal X-ray diffraction (P3m1; No.156, Z ¼ 1; a ¼ 7:4488ð6Þ A ˚ , c ¼ 5:9190ð7Þ A ˚ ).Thetransparentcolorlesscrystalsarecomprisedofchainsof distorted fac-MoO 3 F 3 3 octahedra and MoO 4 2 tetrahedra anions, as suggested by the formulas Ag 3 MoO 3 F 3 and Ag 3 MoO 4 + , and areconnectedthroughAg + cationsinapolaralignmentalongthe c-axis.OneCl  anionperformulaunitservesasachargebalance and connects the two types of chains in a staggered fashion, offset by B 1 2  c: In MoO 4 2 , the Mo atom displaces towards a single oxide vertex, and in MoO 3 F 3 3 , the Mo displaces towards the three oxide ligands. The ordered oxide–fluoride ligands on the MoO 3 F 3 3 anion is important to prevent local inversion centers, while the polar organization is directed by the Cl  anion and interchain dipole–dipole interactions. The dipole moments of MoO 3 F 3 3 andMoO 4 2 aligninthenegative c-axisdirection,togivea polar structure with no cancellation of the individual moments. The direction and magnitude of the dipole moments for MoO 3 F 3 3 and MoO 4 2 were calculated from bond valence analyses and are 6.1 and 1.9debye (10 18 esucm) respectively, compared to 4.4debye for polar NbO 6 octahedra in LiNbO 3 , and 4.5debye for polar TiO 6 octahedra in KTiOPO 4 (KTP). r 2003 Elsevier Science (USA). All rights reserved. Keywords: Polar compound; Dipole moment calculation; Metal oxyfluoride anions 1. Introduction The synthesis of polar materials, one subcategory of the noncentrosymmetric symmetries [1], has been noted increasingly in the literature as a significant goal, or achievement, for the observation of physical properties such as pyroelectricity, ferroelectricity, piezoelectricity, or second harmonic generation (SHG) in a compound. For a material to exhibit one of the two former properties, polar symmetries (crystal classes 1; 2; 3; 4; 6; m; mm2; 3m; 4mm; 6mm) are a necessary condition, and for the latter two properties, polar symmetry is sufficient, but not necessary [2]. However, symmetry restrictions do not place a limitation on the variety of chemical components or synthetic pathways a chemist could utilize, and several bottom-up approaches for buildingpolarstructureshavebeendescribed.Thelistof current synthetic strategies includes utilizing molecular host frameworks (guanidinium sulfate sheets, mm2) [3], the topology of a metal-organic coordination network (bis(nicotinato)zinc, mm2) [4], the packing of chemical species with active lone-pairs (iodate, mm2 in RbMoO 3 (IO 3 ), or tellurite, mm2 in Te 2 SeO 7 ) [5,6], or with an inherently polar anion (dichromate, mm2 in Cu(py) 4 Cr 2 O 7 ) [7], or an inherently polar ligand arrangement (metal oxide-fluorides, MoO 3 F 3 3 ,3m in (Ag 3 MoO 3 F 3 )(Ag 3 MoO 4 )Cl here). The electrostatic dipole–dipole interaction is a com- mon structure-directing feature, regardless of whether the polar chemical species are molecules, layers, or networks. Near-neighbor dipole–dipole interactions, either repulsive (+/+, /) or attractive (+/), will determine whether individual polar units combine to form a polar or nonpolar structure (see discussion). These structures can be topologically compared by analyzing the packing and electrostatic interaction of constituent dipole moments. The problem is that while thedirectionofthe net dipolemomentisusuallyknown (i.e., because it is restricted to lie on a particular ARTICLE IN PRESS  Corresponding author. E-mail address: krp@northwestern.edu (K.R. Poeppelmeier). 0022-4596/03/$-see front matter r 2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0022-4596(03)00090-2