Isomorphous substitution in non-linear optical KTiOPO4. Powder diffraction and magic angle spinning nuclear magnetic resonance study of (K½Na½)TiOPO4 and (Rb½Na½)TiOPO4

Abstract

The structures of (K_½Na_½)TiOPO₄ and (Rb_½Na_½)TiOPO₄ have been determined from Rietveld analysis of powder diffraction data, the former from a joint analysis of time-of-flight neutron and X-ray data, the latter from constant wavelength neutron data: Pn2₁a, (K_½Na_½)TiOPO₄a= 12.75613(7)Å, b= 10.60426(6)Å, c= 6.31869(4)Å; (Rb_½Na_½)TiOPO₄a= 12.8273(6)Å, b= 10.6052(5)Å, c= 6.3347(3)Å. ³¹P and ²³Na magic angle spinning NMR spectra have also been obtained for the title compounds and the appropriate end members. Complete ordering of Na in the K(1) site and K or Rb in the K(2) site is demonstrated by both the diffraction and the NMR results. The second-harmonic generation properties of these compounds relative to KTiOPO₄ are discussed.