Date Published: June 01, 2017
Publisher: International Union of Crystallography
Author(s): S. Huband, D. S. Keeble, N. Zhang, A. M. Glazer, A. Bartasyte, P. A. Thomas.
The structure of lithium niobate-tantalate has been studied using powder and single-crystal X-ray diffraction focusing on the composition and temperature induced zero-birefringence points.
Lithium niobate (LN) and lithium tantalate (LT) have both been used extensively for their optical properties in a range of devices: including optical modulators, Pockels cells, optical parametric oscillators, waveguides etc. LN and LT are isostructural and ferroelectric at room temperature, with space group R3c. Their physical properties, however, differ from one another. For example, at room temperature LN has a negative birefringence and congruent LT is positive. Congruent crystals form with the same composition as the initial materials, which does not occur at the stoichiometric (equal Li and Nb or Ta content) composition for LN or LT. In these materials the congruent compositions are Li-deficient with compositions around 48.45 mol% Li2O (O’Bryan et al., 1985 ▸; Kushibiki & Ohashi, 2006 ▸).
Powder LNT samples with a mass of 15 g were prepared by a solid-state reaction between Li2CO3 (99.99%), Nb2O5 (99.99%) and Ta2O5 (99.85%). The starting materials were weighed according to the desired ratios and then ball-milled in isopropanol, placed in a platinum crucible with a lid and sintered for 140 h at 1433 K, following the method used by Bartasyte et al. (2012 ▸). The main set of samples were made with compositions (LiNb1 − xTaxO3) of x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1, with a further focused region every 0.01 between 0.9 and 1. Variable-temperature measurements were made on compositions with x = 0.94, 0.96, 0.98, 0.99 and 1, along with high-temperature measurements only on x = 0.92.
The Li content of a LV flux-grown LT crystal was calculated to be 49.65 (10) and 49.6 (1) mol% Li2O using measurements of the Curie point and zero-birefringence temperature, respectively. The zero-birefringent temperature determined using flux-grown crystals increases as the LT content decreases, with a zero-birefringence temperature between 300 and 400 K for crystals with greater than 90 mol% LT. XRD and SHG measurements do not show any changes through these zero-birefringence points, confirming that the material remains polar at this temperature. At higher temperatures, the lattice parameters determined using high-resolution powder XRD measurements can be used to determine the transition temperature to the paraelectric phase.