New quaternary oxides with both families of second-order Jahn–Teller (SOJT) distortive cations: Solid-state synthesis, structure determination, and characterization of YNbTe 2 O 8 and YNbSe 2 O 8 Yeong Hun Kim a , Beom-Yong Jeon b , Tae-Soo You b , Kang Min Ok a,⇑ a Department of Chemistry, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 156-756, Republic of Korea b Department of Chemistry, Chungbuk National University, Cheongju, Chungbuk 361-763, Republic of Korea article info Article history: Received 23 October 2014 Received in revised form 27 January 2015 Accepted 27 February 2015 Available online 5 March 2015 Keywords: Synthesis Structure Tellurite Selenite Second-order Jahn–Teller (SOJT) distortion X-ray diffraction abstract Two novel quaternary mixed metal tellurite and selenite, YNbTe 2 O 8 and YNbSe 2 O 8 , respectively, have been synthesized through standard solid-state reactions using Y 2 O 3 , Nb 2 O 5 , TeO 2 or SeO 2 as reagents. Single crystal X-ray and powder neutron diffraction analyses have been utilized to determine the structures of the reported materials. YNbTe 2 O 8 and YNbSe 2 O 8 are isostructural to each other and crystal- lize in the monoclinic centrosymmetric space group, C2/m (No. 12). Due to the two families of constituent second-order Jahn–Teller (SOJT) distortive cations, i.e., Nb 5+ and Te 4+ /Se 4+ , local asymmetric environ- ments occur from the three-dimensional frameworks. Intra-octahedral distortions along the local C 4 direction and asymmetric trigonal pyramidal coordination moieties generated by stereoactive lone pairs are observed from the NbO 6 octahedra and TeO 3 (or SeO 3 ) polyhedra, respectively. Thermogravimetric analysis, infrared and UV–vis diffuse reflectance spectroscopies, elemental analysis, out-of-center distor- tions, dipole moment calculations, and electronic structure calculations for the reported materials are presented. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Mixed metal oxides composed of second-order Jahn–Teller (SOJT) distortive cations [1–8] have been of great interest to the broad field of materials science owing to their important struc- ture-driven characteristics. The unsymmetrical coordination environment of constituent SOJT cations often facilitate the com- pounds crystallizing in macroscopic noncentrosymmetric (NCS) space groups, which may exhibit important materials’ properties such as piezoelectricity, second-harmonic generation (SHG), pyroelectricity, ferroelectricity, and multiferroicity [9–13]. The SOJT effects are generally observed from two kinds of cation fami- lies, i.e., d 0 transition metal cations under octahedral environment (Ti 4+ , Nb 5+ ,W 6+ , etc.) and cations with stereoactive lone pairs (Tl + , Pb 2+ , Bi 3+ , Te 4+ , etc.). With the former, the SOJT distortion occurs through the mixing between the empty metal d-orbitals and the filled ligand p-orbitals. With the latter, however, the lone pair for- mation arises from the interaction of the cation s- and p-orbitals with the anion p-orbitals. In general, the observed octahedral distortions occur along one of the three directions from the center: the local C 2 direction toward an edge, the local C 3 direction toward a face, and the local C 4 direction toward a corner [14,15]. The dis- tortion of lone pair cations is toward the nonbonded electron pair. In addition to these directional primary distortions, secondary dis- placements such as lattice distortions have been also observed from the interaction between two different families of SOJT cations [8,16]. Moreover, the inherently asymmetric building units can suggest a rich structural chemistry with unprecedented chains, layers, and 3D frameworks in extended structures. During the exploratory synthetic investigation in the Y 3+ –Nb 5+ –Te 4+ /Se 4+ -oxide system, we were able to discover two new quaternary nio- bium tellurium/selenium oxides containing SOJT distortive cations. Several stoichiometrically similar quaternary vanadium tellurites/ selenites, M 3+ V 5+ Q 4+ 2 O 8 (M = Bi, Eu, Gd, Tb, In, Sc, and Y; Q = Se and Te) with various structural backbones have been reported thus far [17–21]. In particular, NCS InVSe 2 O 8 and YVSe 2 O 8 reveal moderate SHG efficiencies as well. Herein, we add two quaternary niobium tellurite and selenite, YNbTe 2 O 8 and YNbSe 2 O 8 , to the series. Detailed synthetic procedures along with structure determinations, thorough characterization, and calculations of the novel materials are presented. http://dx.doi.org/10.1016/j.jallcom.2015.02.202 0925-8388/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +82 2 820 5197; fax: +82 2 825 4736. E-mail address: kmok@cau.ac.kr (K.M. Ok). Journal of Alloys and Compounds 637 (2015) 155–161 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom