Thermally induced structural changes in modulated Ca_0.28Ba_0.72Nb₂O₆(CBN28)

Abstract

One of the goals of crystal engineering is to make it possible to design and synthesize MOFs with predetermined topology and properties, usually through judicious selection of multitopic organic ligands as the spacers and metal ions or clusters as the junctures [1]. As functional metal centers, rare earth metals are attracting more and more attention due to their fantastic coordination properties and special chemical characteristics arising from 4f electrons [2], [3]. Many coordination polymers based on rare earths have been synthesized, and most of them exhibit amazing optical and magnetic properties, enabling them as fluorescent probes and electroluminescence devices [4], [5]. We report herein a family of 11 isostructural 3D metal-organic compounds, {[Ln(µ 3-pmdc)(µ-ox) 0.5 (H 2 O) 2 ]•2.25H 2 O} n

Changes of the crystal structure at the relaxor ferroelectric phase transition of strontium barium niobate (SBN53)

“…Differences in size of the cations might lead to the distortions responsible for the large U 11 and U 22 adp. Furthermore, a wave‐like distribution of cations and vacancies causes density modulation wave at the Me2 site and additional displacive modulations of the adjacent oxygen atoms .…”

“…The amplitude of the modulation increases with the Sr 2+ substitution rate . The positional modulation of the oxygen atoms seems to originate from a wave‐like occupation of the Me2 site .…”

Incommensurate modulation of calcium barium niobate (CBN28 and Ce:CBN28)

“…The Me1 site houses less foreign incorporation and shows hardly any modulation. The structural modulations persist at temperatures above the transition from the ferroelectric to the paraelectric state (Schneck et al, 1981;Graetsch et al, 2011) so that it might be assumed that the modulations do not disappear until the temperature is high enough to enable enhanced diffusion of the cations. This could finally destroy the wave-like ordered occupation of the Me2 site and the associated displacive modulations of the neighbouring O atoms.…”

Incommensurate modulations of relaxor ferroelectric Ca0.24Ba0.76Nb2O6(CBN24) and Ca0.31Ba0.69Nb2O6(CBN31)

“…In SBN the amplitudes of the positional modulation increase as a function of the strontium substitution rate x (Schefer et al, 2008). The origin of the displacive modulation of the O atoms seems to lie in a modulation of the occupation of the Me2 site (Schefer et al, 2008;Graetsch, Pandey et al, 2012).…”

“…As there are two Me1 sites and four Me2 sites per unit cell for five large cations (Z = 5), both sites are incompletely occupied. Most of the Ba 2+ ions are located on the larger Me2 site and most Sr 2+ or Ca 2+ on the smaller Me1 site (Jamieson et al, 1968;Chernaya et al, 2000;Podlozhenov et al, 2006;Woike et al, 2003;Graetsch, Pandey et al, 2012).…”