New open-framework nonlinear optical materials. Crystal structure of Na[sub 1/2](H[sub 3]O)[sub 1/2]Nb[sub 2]PO[sub 8] and physical properties of the solid solution K[sub 2/3]Li[sub 1/3]Nb[sub 2[minus]x]Ta[sub x]PO[sub 8]

The syntheses, structures, and physical properties of several members of a new family of noncentrosymmetric, open-framework, ion-exchangeable phases which have a substantial nonlinear optical response (Nd:YAG 1064 nm) are described. Single-crystal (to 2 mm) hydrothermal synthesis of Na[sub 1/2](H[sub 3]O)[sub 1/2]Nb[sub 2]PO[sub 8] was carried out at 30,000-40,000 psi and 650-700[degrees]C. The structure of Na[sub 1/2](H[sub 3]O)[sub 1/2]Nb[sub 2]PO[sub 8], as determined from single-crystal X-ray data consists of an octahedral (NbO[sub 6])/tetrahedral (PO[sub 4]) framework, connected via Nb-O-P and Nb-O-Nb bonds, with cavities and channels occupied by the Na[sup +] and H[sub 3]O[sup +] cations. Members of the solid solution K[sub 2/3]Li[sub 1/3]Nb[sub 2[minus]x]Ta[sub x]PO[sub 8] (0 < x < 2) have been synthesized as powders by solid-state methods and are all isomorphous with Na[sub 1/2](H[sub 3]O)[sub 1/2]Nb[sub 2]PO[sub 8]. Various ion-exchange reactions with NH[sub 4][sup +], Rb[sup +], and Ag[sup +] are possible. These materials have been characterized by X-ray powder diffraction, TGA, [sup 31]P MAS NMR spectroscopy, second-harmonic generation measurements, and UV/visible spectroscopy. Crystal data: Na[sub 1/2](H[sub 3]O)[sub 1/2]Nb[sub 2]PO[sub 8], M[sub r] = 364.94, rhombohedral, R32 (No. 155), a = 8.4962 [angstrom], [alpha] = 104.775[degrees], V = 538.77 [angstrom][sup 3], Z = 3, R = 2.88%, R[sub w] = 3.56% for 66 parameters and 743 observations [I > 3[sigma](I)]. 23 refs., 10 figs., 5 tabs.