Dielectric anomaly in LiCa 2 V 5 O 15 ceramics Banarji Behera a , P. Nayak a , R.N.P. Choudhary b, ⁎ a P.G. Department of Physics, Sambalpur University, Sambalpur 768 019, India b Department of Physics and Meteorology, IIT, Kharagpur 721 302, India Received 2 October 2006; accepted 22 December 2006 Available online 30 December 2006 Abstract A polycrystalline sample of LiCa 2 V 5 O 15 (LCV) was prepared using a mixed oxide method at low temperature (i.e., at 630 °C). X-ray structural analysis shows the single-phase formation of the compound in the orthorhombic crystal system at room temperature. A study on the surface morphology of the compound showed uniform grain distribution on the surface and in the bulk of the sample with less porosity. A dielectric anomaly suggests that the compound has a transition temperature at 274 °C. The activation energy, calculated from the temperature dependence of ac conductivity (dielectric data), of the compound was found to be 0.67 eV at 10 kHz. The nature of the variation of conductivity and value of activation energy in different regions, suggest that the conduction process is of mixed type (i.e., ionic–polaronic and space charge generated from the oxygen ion vacancies). © 2006 Elsevier B.V. All rights reserved. Keywords: Ceramics; Crystal structure; Dielectrics; Electron microscopy 1. Introduction Though a large number of ferroelectric oxides of the perovskite structural family have been studied in the past, some oxides with tungsten bronze structure (TB) have been found useful for device applications such as actuators [1], transducers [2], capacitors [3], electro-optic [4], and ferroelec- tric random access memory [5]. The materials with TB structure have some advantages for devices because of their high dielectric constant, and low tangent loss [6–8]. The TB structure has a general chemical formula [(A 1 ) 2 (A 2 ) 4 (C) 4 ] [(B 1 ) 2 (B 2 ) 8 ]O 30 , where the A site is usually filled by divalent or trivalent cations, and the B sites by V +5 atoms. This structure consists of a complex array of distorted BO 6 octahedra sharing corners in such a way that three different types of interstices (A 1 ,A 2 ,B 1 ,B 2 and C) are available for cations out of which the C site is generally empty. Though a lot of work has been reported on complex TB compounds [9–12], no ferroelectric/ dielectric studies on alkali alkaline vanadates of a general formula AB 2 V 5 O 15 (A = alkali, B = alkaline) have so far been reported. In this paper, a phase transition based on dielectric studies of LiCa 2 V 5 O 15 is reported. 2. Experimental procedure The polycrystalline sample of LiCa 2 V 5 O 15 was prepared by a mixed oxide method at low temperature using high-purity ingredients: Li 2 CO 3 (99.9%, M/S. Sarabhai M. Chemicals, India), CaCO 3 (99.9%, M/S. Sarabhai M. Chemicals, India) and V 2 O 5 (99%, Koch Light Ltd, England) in the desired stoichi- ometry. The oxides and carbonate were mixed thoroughly; first in air for 1 h and then in alcohol for 1 h. Thermogravimetric analysis (TGA) of the homogeneous physical mixture was carried out with the help of Mettler Toledo Star System at a heating rate of 15 °C/min to find the calcination temperature and structural stability. As suggested by TGA, the mixed powder was calcined at 630 °C for 7 h. The process of grinding and calcination was repeated several times till the desired compound was formed. The formation of the compound was studied by an X-ray diffraction (XRD) technique at room temperature with an X-ray powder diffractometer (Rigaku Miniflex, Japan) using CuK α radiation (λ = 1.5405 Å) in a wide range of Bragg angles 2θ (20° ≤ 2θ ≤ 80°) with a scanning rate of 4°/min. The calcined powder was then cold pressed into cylindrical pellets of 10 mm Materials Letters 61 (2007) 3859 – 3862 www.elsevier.com/locate/matlet ⁎ Corresponding author. Tel.: +91 3222 283814; fax: +91 3222 255303. E-mail address: crnpfl@gmail.com (R.N.P. Choudhary). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.12.048