Relaxor-like behaviour of pyrochlores

2–5 pyrochlore relaxor ferroelectric Cd2Nb2O7 and its Fe2+/Fe3+ modifications

“…In addition, in Cd 2 Nb 2 O 7 these ions are located within (CdO 8 ) nϪ scalenohedra, while in CdNb 2 O 6 they are located within (CdO 6 ) nϪ octahedra. The pyrochlore structure can be described as two interpenetrating networks formed by (NbO 6 ) nϪ and (CdO 8 ) nϪ polyhedra respectively, 7,10,15 with the former being ferroelectrically active. 16,20 The columbite structure is described as a hexagonal closely packed network of oxygen ions with the (CdO 6 ) nϪ octahedra-formed layer and two (NbO 6 ) nϪ octahedra-formed layers alternating along the z axis.…”

“…Moreover, unlike perovskites, in Cd 2 Nb 2 O 7 a sequence of the disordered states of different nature ͑relaxor, incommensurable, glassy͒ develops in the ferroelectric phase from T C down to 4 K, with the correlation between these states still unclear. [8][9][10] Their origination is ascribed to a dynamical location of the Cd 2ϩ ions reoriented at a fast rate Ͼ10 7 Hz over the six off-center equivalent positions at room temperature ͑RT͒. 7,10 However, so far no immediate evidence for the high-frequency dipole relaxation in the system at RT ͑a centrosymmetric paraelectric phase͒ has been presented, which remains the role of the (CdO 8 ) nϪ network in the process ascertainable.…”

“…[8][9][10] Their origination is ascribed to a dynamical location of the Cd 2ϩ ions reoriented at a fast rate Ͼ10 7 Hz over the six off-center equivalent positions at room temperature ͑RT͒. 7,10 However, so far no immediate evidence for the high-frequency dipole relaxation in the system at RT ͑a centrosymmetric paraelectric phase͒ has been presented, which remains the role of the (CdO 8 ) nϪ network in the process ascertainable. It should be noted that in former works the dielectric relaxation studies of Cd 2 Nb 2 O 7 were restricted by the frequency range from 100 Hz to 1 MHz.…”

“…It should be noted that in former works the dielectric relaxation studies of Cd 2 Nb 2 O 7 were restricted by the frequency range from 100 Hz to 1 MHz. [7][8][9][10][11][12][13] The fact that random impurities and defects on the Cd site ͑A-site ions͒ enhance the relaxor properties of this system indicates that its behavior is influenced by a structural disorder in the (CdO 8 ) nϪ network. 8,11,12 The structural A-site disorder can result from a chemical heterogeneity ͑different ionic radii, valency, polarizability, ion deficiency͒, from a heterogeneous positional distribution of the ions included and from offcenter location of ions.…”

“…[1][2][3][4][5][6] Although the perovskite relaxors identified so far are mixed A͑X,Z͒O 3 or (A,M͒XO 3 -type systems, Cd 2 Nb 2 O 7 which belongs to the 2-5 undiluted pyrochlores,A 2 2ϩ B 2 5ϩ O 7 , also exhibits a relaxor behavior associated with the presence of polar nanoregions. [7][8][9] In perovskites, a relaxor state transforms to a ferroelectric state either spontaneously or in an external dc electric field. 1,3,4 On the contrary, in Cd 2 Nb 2 O 7 pyrochlore a relaxor state and a ferroelectric state exist simultaneously below T C and T s ͑T C ϭ196 K and T s ϭ205 K are the ferroelectric and ferroelastic phase transition temperatures, respectively͒.…”