crystallizing in a non-centrosymmetric (space group P421rn) trirutile structure, exhibit second harmonic generation (SHG) of 1064 n m radiation with efficiencies 15-45 times that of a-quartz; interestingly, the SHG response is retained by the protonated derivatives HMVMV106.xH20, and their n-alkylamine intercalates as well.Transition-metal oxides containing octahedrally coordinated do cations exhibit several novel structural and electronic properties, such as crystallographic shear,I ferroelectricity2 and nonlinear optical (NLO) response334 (e.g. second harmonic generation), which could be traced to out-of-centre distortions of the metal-oxygen octahedra. These distortions arise from a secondorder Jahn-Teller effect5.6 that involves mixing of empty cation d orbitals with filled anion p orbitals resulting in removal of degeneracy and consequent spontaneous structural distortion. KTiOP04 (KTP) is one such material exhibiting efficient second harmonic generation (SHG) of 1064 nm radiation from an Nd : YAG laser;7 the 3d0 electronic configuration of Titv and the associated distortion of Ti06 octahedra play a crucial role toward the SHG property of this material.*,9 The importance of do electronic configuration is revealed by the synthesis of the S n I V 4dI" analogue of KTP, KSnOP04, where the distortion of Snob octahedra as well as the SHG response decrease dramatically.lO We have recently demonstratedll,l* the role of do cations towards the SHG property of this structure by synthesizing several KTP derivatives, KMvo.sTi1110.50P0~ and K0.5Mvo STi~V0.50P04 (M = Nb, Ta), of which only the latter show SHG response comparable to KTP.Oxides of the general formula LiMVMvIO6 (Mv = Nb, Ta; Mv1 = Mo. W), crystallizing in a non-centrosymmetric (P?21ni) rutile related structure in which Li, MX' and MV' atoms are ordered in layers perpendicular to the c direction, show interesting ion-exchange and intercalation behaviour. [13][14][15][16][17] The crystal structure of one of the members, LiNbWO6, has been determined.18 The structure (Fig. 1) shows the presence of distorted octahedral environment around the do cations as well as lithium. The lithium atoms in these oxides, LiMVMV1O6, are topochemically exchanged with protons in aqueous H N 0 3 to yield HMvMV106..~H20.15-17 The latter are strong Bransted acids intercalating readily a wide variety of organic bases including pyridine, aniline and pyrrole. Both the protonated derivatives and their organic intercalates retain the rutile-like MvMVI06 sheets. [15][16][17] Considering that LiNbW06 crystallizes in a non-centrosymmetric structure consisting of distorted Nb06 and W 0 6 octahedra. we envisaged that oxides of this family and their derivatives would be potential candidates for SHG. Our investigations, reported here, show that not only the parent materials but also the protonated derivatives as well as their amine intercalates show a distinct SHG response toward 1064 nm radiation. We believe that this is the first report of SHG response of an isostructural series of layered oxid...
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