Negative thermal expansion change of different stoichiometric proportions calcium-doped strontium barium niobate crystals Ch.Y. Gao a, ⁎, H.R. Xia a , J.Q. Xu a , C.L. Zhou a , H.J. Zhang b , J.Y. Wang b a School of Physics Shandong University, Jinan, 250100, People's Republic of China b Institute of Crystal Materials, Shandong University, Jinan, 250100, People's Republic of China abstract article info Article history: Received 16 June 2008 Accepted 15 September 2008 Available online 26 September 2008 Keywords: Thermal properties Ferroelectrics The polarization Raman spectra were measured for the calcium-doped strontium barium niobate crystals ((Ca 0.28 Ba 0.75 ) x (Sr 0.60 Ba 0.40 ) 1 - x Nb 2 O 6 (x = 0.25,0.50 or 0.75, CSBN) to determine the negative thermal expansion(NTE) change induced by different stoichiometric proportions calcium-doped concentration. The different count-intensity (CI) of Raman peaks with the scattering geometry Z(XY)Z À , corresponding to the symmetry species B 2 , are attributed to the different vacancy rate(VR) in A2 sites of tetragonal tungsten bronze-type structure, which can result in the change of the short-range molecular force (SRMF) between ions filled in A2 sites and Nb–O octahedron. Since the thermal properties of materials direct responding to the vibration of crystal lattices, the NTE change of CSBN crystals are considered as arising due to the geometry change for the different calcium-doped concentration. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Calcium-doped CSBN crystals grown by the Institute of Crystal Materials of Shandong University are ferroelectric and photoelectric crystals having the structure of ferroelectric tetragonal tungsten bronze-type (TB). The starting materials are 99.99% CaCO 3 , SrCO 3 , BaCO 3 , and Nb 2 O 5 . The main constituents in stoichiometric propor- tions were mixed. According to different proportion of the materials, the grown crystals can be divided into three types: CSBN75 (CaCO 3 10.5%, x = 0.75), CSBN50 (CaCO 3 7%, x = 0.50) and CSBN25 (CaCO 3 3.5%, x =0.25). In addition, compared with the lower phase transition temperature of Sr 1 - x Ba x Nb 2 O 6 (0.25 b x b 0.75), [1–5] CSBN crystals exhibit a higher Curie temperature at 400 K,450 K and 480 K, respectively. For CSBN crystals whose structure is similar to the SBN (Fig. 1(a)), two independent Nb–O6 octahedrons are joined by oxygen corners into a three-dimensional network. There are three structural channels that connect these octahedrons along the c axis. The channel with a triangular cross section is empty, whereas the square channels are occupied by Sr 2+ ions and the larger pentagonal channels are randomly filled by Ba 2+ and Ca 2+ ions. The non-centrosymmetrical distribution of the electrostatic potential around Nb–O6 clusters is further disturbed as a result of the Ca 2+ partly replacing the Ba 2+ at A2 site of TB structure [6]. Different stoichiometric proportions calcium- doped may trigger different distortion of lattice which is enough to affect the thermal properties of crystal, while the thermal properties are directly responsive to the vibration of crystal lattices. The thermal expansion of materials is very important in the design and fabrication of devices that exploit their electro-optic or photo- refractive properties. Thermal expansion studies have generated enormous interest with the recent discovery of NTE in ZrW 2 O 8 over a large temperature range [7]. Most materials exhibit positive thermal expansion, but it has been known for some time that some materials, such as SBN, ZrMo 2 O 8 and CdTe exhibit NTE at low temperatures [8–10]. As shown in our recent literature [11], the NTE was observed along the c-axes below the Curie temperature in CSBN25 crystal and was attributed to the deviation of the Nb site from the center of oxygen octahedrons and the disorder in the chains along Materials Letters 63 (2009) 139–141 ⁎ Corresponding author. Tel.: +86 531 88508820; fax: +86 531 88392262. E-mail address: gchy@sdu.edu.cn (C.Y. Gao). Fig. 1. (a) Schematic diagram of the structure of TB structure (b) distortion of Nb–O octahedron. 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.09.037 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet