X-ray diffraction and vibrational-spectroscopy study of the structure of La 2 Ti 2 O 7
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N. G. Eror Oregon Graduate Center, Beaverton, Oregon 97006 (Received 31 May 1988; accepted 26 July 1989)
The compound La 2 Ti 2 0 7 was prepared by a liquid mix technique and examined at room temperature by x-ray diffraction, infrared absorption, and Raman spectroscopy. The examinations confirm and supplement the earlier observation by x-ray diffraction that La 2 Ti 2 0 7 crystallizes in the monoclinic structure without an inversion symmetry. The energy shifts (cm 1 ) that were observed in the vibrational spectra may be explained by a sixfold coordination for the titanium and a ninefold coordination for the lanthanum in the structure. This type of coordination satisfies Pauling's electrostatic valence rule regarding the electrostatic bond strengths at the shared anions.
I. INTRODUCTION Most of the rare-earth titanates form compounds with the general formula Ln 2 Ti 2 0 7 (where Ln = Sm to Lu and Y) that are isostructural to the mineral pyrochlore (NaCa) (NbTa)O6F. The space group of the ideal pyrochlore structure is Fd3m(Oh1) and the crystallographic unit cell contains eight molecular formula units. Ln 2 Ti 2 0 7 compounds are predominantly cubic and ionic. However, compounds with Ln = La, Ce, Pr, and Nd do not crystallize in the pyrochlore structure. These latter compounds have recently found extensive application in various branches of technology. Because their ferroelectric properties result from the characteristic features of their crystal structure, data on the structure of these compounds are of considerable interest.' The dielectric constant2 of La 2 Ti 2 0 7 was found to be 37. The formation of a needle-like La2Ti2O7 phase is reported in the system PbO-TiO 2 -La 2 O 33 and in lanthanum-doped SrTiO3.4 La 2 Ti 2 0 7 is considered to be isostructural with the compound Ca 2 Nb 2 0 7 . 5 " 7 In the Ca 2 Nb 2 0 7 structure (space group C22, P2 l7 z = 4), there are 129 (=3n - 3) normal modes of vibration represented by 65 A species and 64 B species and all these modes are active both in Raman scattering and infrared absorption. Ross8 observed 27 absorption bands in the IR spectrum and 26 bands in the Raman spectrum and suggested that niobium was octahedrally coordinated. La 2 Ti 2 0 7 has two modifications, one of which has the symmetry of P1X and the other of Pbnli.7'9 The monoclinic La 2 Ti 2 0 7 with space group P2X transforms its structure into one with orthorhombic space group CmC2l at - 7 8 0 °C. Infrared absorption and Raman scattering characteristics of the rare-earth titanates and zirconates that form the pyrochlore structure have been discussed in detail.1011 The IR spectra of nonpyrochlore compounds of the type Ln 2 Ti 2 0 7 have been recorded along with several other pyrochlore materials by Klee and Weitz,12 but the non-
pyrochlore compounds were excluded from the discussion because of their complex spectra. The present paper describes a study of the vibrational characteristics and x-ray diffraction of the compound La2Ti2O7. The purpose of this work is to supplement the available x-ray diffraction d
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