ABSTRACT
The valence matching principle is used to explain the loss of inversion symmetry in the noncentrosymmetric (NCS) polymorph of KNaNbOF5 in comparison to its centrosymmetric (CS) polymorph. The [NbOF5](2-) anion has five contacts to both potassium and sodium in the NCS polymorph, whereas in the CS polymorph there are only four contacts to potassium and six contacts to sodium. The lower average Lewis acidity of the cationic framework in the NCS polymorph relative to the CS polymorph reflects the loss of inversion symmetry. This lower average Lewis acidity is achieved during hydrothermal synthesis with a potassium-rich solution when the K:Na ratio in the reaction is greater than 1:1, as the Lewis acidity of potassium is lower than that of sodium. The contrasting coordination environments are manifested in secondary distortions that weaken the primary NbâO interaction and lengthen the NbâO bond in the NCS polymorph. An unusual heat-induced phase transition from the CS to the NCS polymorph was studied with in situ powder X-ray diffraction. The transition to the NCS polymorph upon cooling occurs through an intermediate phase(s).
ABSTRACT
Complicated structures where oxygen and fluorine are found together in one framework, where deviations from Pauling's second crystal rule (PSCR) are expected, often result in structures with important physical properties. The [NbOF5]2- anion and therefore all the individual Nb-O and Nb-F bonds are ordered in noncentrosymmetric KNaNbOF5 and centrosymmetric CsNaNbOF5. The Na/K- and Na/Cs-O/F interactions in these phases, in particular the expected deviations from PSCR and the bond valence model, reveal the essential role of the small potassium cations in the acentric packing of the [NbOF5]2- anion. KNaNbOF5 crystallizes in the orthorhombic and polar space group Pna21 (No. 33) with lattice constants a = 11.8653(11) A, b = 5.8826(6) A, c = 8.1258(8) A, and Z = 4, while CsNaNbOF5 crystallizes in the orthorhombic space group Pbcn (No. 60) with lattice constants a = 8.3155(7), b = 13.3176(11), c = 11.1314(9), and Z = 8.
ABSTRACT
Pb2Cu3B4O11 crystallizes in the monoclinic space group P2/n (No. 13) with a = 6.8016(15) A, b = 4.7123(10) A, c = 14.614(3) A, beta = 97.089(3) degrees, and Z = 2. The crystal structure consists of infinite [Cu3O8]10- zigzag chains of alternating dimers and monomers. The magnetic susceptibility and specific heat capacity show spin-gap and Curie-Weiss behaviors that can be explained by a model of Cu(2)-Cu(2) dimers and isolated or weakly coupled Cu(1) monomers.
ABSTRACT
Single crystals of Li(6)CuB(4)O(10) have been synthesized, and its crystal structure has been determined. Li(6)CuB(4)O(10) crystallizes in the non-centrosymmetric triclinic space group P1 (No. 1). The structure consists of isolated [CuB(4)O(10)](6)(-) polyanions that are bridged by six LiO(4) tetrahedra. Li(6)CuB(4)O(10) is a congruently melting compound. It produces SHG intensity similar to that produced by KH(2)PO(4) and is phase-matchable.
