Structural transformation in (Ta 2 O 5 ) 12x (TiO 2 ) x ceramics P.S. Dobal a , R.S. Katiyar a, * , Y. Jiang b , R. Guo b , A.S. Bhalla b a Department of Physics, University of Puerto Rico, San Juan PR 00931-3343, Puerto Rico b Material Research Laboratory, Pennsylvania State University, University Park, PA 16802, USA Received 2 February 2000; accepted 28 March 2000 Abstract Bulk (Ta 2 O 5 ) 12x (TiO 2 ) x ceramics for x  0.05, 0.08, and 0.11 were prepared by the conventional solid state reaction technique and the temperature dependent Raman scattering in the range 70±1173 K was used to study the structural transformations in them. The lowest frequency phonon modes, at about 35±38 cm 21 , in these ceramics were assigned as Ta±Ti vibrations originating from the interaction between TiO 6 octahedra and TaO 522n n or Ta 6 O 16 12 clusters. Raman spectra exhibited a softening of this mode with increasing temperature, followed by a structural transformation. A structural transition from triclinic to monoclinic phase was observed in the ceramics at about 360, 450, and 5408C for compositions x  0.05, 0.08, and 0.11, respectively. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramics; C. Raman spectroscopy; D. Phase transitions 1. Introduction Higher dielectric constant materials are necessary to keep the capacitor cells structure simple and manufacturable in order to meet the 1 Gbit requirement of dynamic random access memory (DRAM) devices. Among the new dielec- trics, tantalum pentoxide (Ta 2 O 5 ) has received considerable attention [1] because of its high-dielectric constant (,35), high-refractive index (2.1±2.2), high resistivity, low-inter- nal stress, and good dielectric breakdown strength [2,3] with a promise of compatibility with microelectronic processing. Ta 2 O 5 has been investigated as the most promising for 250 Mbit and 1 Gbit DRAMs [4] and seems to be the most appropriate and compatible material for the conven- tional DRAM process [5]. Recently, it has been reported that the dielectric constant of Ta 2 O 5 can further be enhanced, from 35 to 126, through an addition of 8% titanium oxide TiO 2 [6]. In our previous communication [7], we attempted a Raman spectroscopic study to understand the origin responsible for the enhanced dielectric constant of Ta 2 O 5 powder through 8% substitution with TiO 2 . To our knowl- edge, no analogous Raman study has been performed to study the structural transformations in these ceramics. In various materials, the low-frequency Raman modes which are characterized by different symmetries re¯ect the change in lattice symmetry at the structural phase transitions. Among such modes there is at least one mode, known as soft-mode, whose frequency decreases as one approaches the transition temperature. Such a mode is responsible for the instability of the lattice, and hence, the structural phase transition occurs. In our case, the lowest frequency phonon modes about 35±38 cm 21 soften with increasing tempera- ture followed by a structural transition. While induced by incident radiation in any polarizable material, dielectric ¯uctuations may also occur simultaneously in crystals undergoing structural changes. The structural phase transi- tion studies can therefore be useful in understanding the dielectric anomalies in such materials. In this note, the results of temperature dependent micro-Raman study are presented in the range 70±1173 K and the ®rst Raman evidence of structural phase transition in (Ta 2 O 5 ) 12x (TiO 2 ) x bulk ceramics is reported. 2. Experimental (Ta 2 O 5 ) 12x (TiO 2 ) x ceramics for compositions x  0.05, 0.08, and 0.11 were obtained from the powders prepared by the conventional solid state reaction technique. High purity Ta 2 O 5 and TiO 2 reagents were ®rst mixed in ethanol. The mixed powders were mechanically ground, sieved and Journal of Physics and Chemistry of Solids 61 (2000) 1805±1808 0022-3697/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S0022-3697(00)00058-5 www.elsevier.nl/locate/jpcs * Corresponding author. Tel.: 11-787-751-4210; fax: 11-787- 751-0625.