Li2Ca1.5Nb3O10 from X-ray powder data

Lithium calcium niobium oxide (2/1.5/3/10), Li2Ca1.5Nb3O10, has been synthesized by conventional solid-state reaction. Its structure consists of triple-layer perovskite slabs of corner-sharing NbO6 octahedra interleaved with lithium ions; Ca cations partially occupy the perovskite A sites at 75% occupancy probability. All eight atoms in the asymmetric unit are on special positions: one Nb atom has site symmetry 4/mmm; the second Nb, both K, the Sr and two O atoms have site symmetry 4mm; the remaining two O atoms have site symmetries 2mm. and mmm., respectively.

Lithium calcium niobium oxide (2/1.5/3/10), Li 2 Ca 1.5 Nb 3 O 10 , has been synthesized by conventional solid-state reaction. Its structure consists of triple-layer perovskite slabs of cornersharing NbO 6 octahedra interleaved with lithium ions; Ca cations partially occupy the perovskite A sites at 75% occupancy probability. All eight atoms in the asymmetric unit are on special positions: one Nb atom has site symmetry 4/ mmm; the second Nb, both K, the Sr and two O atoms have site symmetry 4mm; the remaining two O atoms have site symmetries 2mm. and mmm., respectively.

2002)
, where B is a small transition metal cation, A is a larger s-, d-, or f-block cation and A' is always an alkali cation.
The Ruddlesden-Popper phases which are intergrowths of the perovskite and rocksalt structures posses a wide variety of interesting properties including superconductivity, colossal magnetoresistance, ferroelectricity, and catalytic activity. Related crystal structures of A sites deficiency three-layer Ruddlesden-Popper phases have been reported for K 2 Sr 1.5 Ta 3 O 10 (Le Berre et al., 2002), Li 2 La 1.78 Nb 0.66 Ti 2.34 O 10 ( Bhuvanesh et al., 1999b), and Li 4 Sr 3 Nb 6 O 20 ( Bhuvanesh et al., 1999a). Fig. 1 shows the observed, calculated and difference plots of the Rietveld refinement. We applied the March-Dollase formalism for a correction of the 00l preferential orientation which is frequently observed in Rietveld refinement of layered perovskites.
The structure of the compound is illustrated in Fig. 2. It is formed from two differently stacked NbO 6 octahedra thick slabs cut along the c direction. Two successive layers are shifted by (a+b)/2 with Ca cations partially occupying the 12-coordinated sites. The Li cations occupy the interlayer spacing at Wyckoff site 8f and not the 4e site since the distance between two adjacent layers is short. Ca cations partially occupy the perovskite A sites at 75% occupancy probability. The Nb cations are coordinated by six oxygen atoms to form NbO 6 octahedra with Nb-O distances ranging from 1.689 (8)

Experimental
The sample was prepared by conventional solid-state reaction. Stoichiometric amounts of Li 2 CO 3 ,CaCO 3 and Nb 2 O 5 were mixed, ground, and calcined at 1423 K for 6 h with one intermediate grid. An excess amount of Li 2 CO 3 (20 mol%) was added to compensate for the loss due to the volatilization of alkali metal carbonate.

Refinement
The crystal structures of Li 4 Sr 3 Nb 6 O 20 (Bhuvanesh et al., 1999a) and Li 2 CaTa 2 O 7 (Liang et al., 2008) were used as a starting model for the Rietveld refinement. The X-ray powder diffraction patterns of Li 2 Ca 1.5 Nb 3 O 10 were indexed in a body-centered tetragonal space group I4/mmm. Structure refinement was carried out by the Rietveld method using the GSAS profile refinement program (Larson & Von Dreele, 2004). The site occupancy factors of Ca and Li were set at 0.75 and 0.50, respectively in view of the close ressemblance of the cell parameters with those of the related structures and they were supplementary materials sup-2 not further refined. The corresponding isotropic atomic displacement parameters of all oxygen atoms and niobium atoms were constrained to be equal, respectively. The March-Dollase option in the EXPGUI program (Toby, 2001) was applied to correct 00l preferential orientation. Fig. 1. Experimental and calculated X-ray diffraction pattern of Li 2 Ca 1.5 Nb 3 O 10 . The difference profile is given at the bottom. The Bragg positions are indicated by the vertical markers below the observed pattern.