Optical and electric investigations of the phase transitions in pyrochlore Cd2Nb2O7

Ye, Zuo‐Guang; Колпакова, Н. Н.; Rivera, Jean-Pierre; Schmid, Hans

doi:10.1080/00150199108209451

Cited by 39 publications

(28 citation statements)

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“…27,28 The material undergoes a phase transition at approximately 204 K, evidenced by small peaks in heat capacity and dielectric measurements. 29 This has been described as a second order 29 and improper ferroelastic 26 as well as an improper ferroelectric 30 transition, with reports of tetragonal, 31 orthorhombic, 32 and rhombohedral 33 crystal structures. In addition, a series of phase transitions have been observed around 190 K, leading to conclusions of relaxor behavior due to broad signatures in the heat capacity and dielectric data.…”

Section: Introductionmentioning

confidence: 99%

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Covalency-driven Structural Evolution in the Polar Pyrochlore Series Cd₂Nb₂O_7–xS_x

et al. 2019

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The arrangement of cations on the triangular pyrochlore lattice leads to a wealth of interesting physical phenomena influenced by geometric frustration. Although uncommon, several pyrochlore materials overcome this frustration and exhibit polar structures. Unraveling the origin of such behavior is key to understanding how broken inversion symmetry arises in complex crystal structures. Here we investigate the effect of varying degrees of covalency in the pyrochlore lattice through a detailed structural and lattice dynamical analysis of the pyrochlore oxysulfide series Cd 2 Nb 2 O 7−x S x above and below the ferroelectric transition temperatures (T C) using synchrotron x-ray diffraction (SXRD) and first principles calculations. All compositions exhibit the cubic F d3m pyrochlore aristotype above T C , whereas the amplitude and character of various structural distortions are found to be composition dependent below T C. For x = 0, large Cd and Nb cation displacements occur to produce the polar Ima2 structure accompanied by a change in translational symmetry. Our symmetry and lattice dynamical calculations indicate Cd 2 Nb 2 O 7 undergoes a proper ferroelectric transition through T C. Analysis of the sulfur-substituted niobates indicates that although the polar space group F dd2 is adopted by the nominal x = 0.25 sample, the transition into the polar phase is improper. For the nominally x = 0.7 composition, the lattice remains nearly cubic, but exhibits a high degree of structural disorder in the pyrochlore channel, with a deviation from the linear Cd-X'-Cd bond by nearly 15 • to accommodate the large size of S while preventing extreme stretching of the Nb-O bond. This highly distorted Cd-X' network is accompanied by a highly distorted NbO 6 network, which is accommodated by the polarizable NbO 6 coordination environment. This sheds light on the limited existence of oxysulfide pyrochlores, for example, the lack of reported S substitution in the case of the similar yet less-polarizable Cd 2 Ta 2 O 7. Our work both provides new understanding of how inversion-symmetry lifting displacements arise and how anion substitution, which tunes covalent cation-anion interactions, is a useful strategy for manipulating polar behavior in the pyrochlore lattice.

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Section: Introductionmentioning

confidence: 99%

“…were considered. Low temperatures structures have been reported from tetragonal,31 orthorhombic,32 and rhombohedral33 systems, with Ima2 31 and I4m2 40 from powder refinements. As the material has been widely reported as polar through various techniques, the I4m2 space group was not considered.…”

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Covalency-driven Structural Evolution in the Polar Pyrochlore Series Cd₂Nb₂O_7–xS_x

et al. 2019

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“…Symmetric Nb-O bond distances prevail parallel to [100] at this latter site. At the transition to the Ima2 phase from higher temperatures, domains with boundaries parallel to h110i appear (Salaev et al, 1983(Salaev et al, , 1992Ye et al, 1991). Over a limited temperature interval these domains coexist with the h100i domains that appear below T 1 .…”

Section: Introductionmentioning

confidence: 99%

The ferroelectric properties of Cd₂Nb₂O₇: a Monte Carlo simulation study

Malcherek

2011

J Appl Cryst

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The order–disorder contributions to the ferroelectric properties of Cd2Nb2O7 (CNO) have been studied by Monte Carlo simulation of a 12‐state modified Potts model on the pyrochlore lattice. Spin configurations obtained by these simulations are mapped to local Nb displacements. Secondary Cd displacements normal to the Nb displacement directions are considered as well. The model correctly reproduces diffuse scattering experimentally observed in CNO. A first‐order phase transition is observed for kTp/J = 0.3891 (k is the Boltzmann constant, Tp is the model phase transition temperature and J is the interaction energy). To further adapt the model to the properties of CNO, coupling of local Nb displacements to the T2u soft mode is simulated via the addition of an appropriate field term in the model Hamiltonian. The critical temperature Tc of the soft mode is scaled to kTc/J= 0.3704. Similarities to experimental observations, i.e. the occurrence of stable domains with {100} boundaries, as well as spontaneous polarization along the cubic 〈100〉 and 〈110〉 directions, indicate that Tp can be associated with the transition temperature T1 = 205 K in CNO. Frequency dispersion of the dielectric permittivity of CNO can be attributed to the low‐frequency switching of correlated chains of Nb displacement that remain partially disordered in the temperature range between 195 and ∼100 K.

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“…At 205 K, Cd 2 Nb 2 O 7 undergoes a ferroelastic transition to an orthorhombic phase, followed by a ferroelectric phase with relaxor behavior. [13][14][15] In addition, a tetragonal distortion from the cubic pyrochlore structure has been suggested for Cd 2 Nb 2 O 7 below 185 K. 3,10 The isometric pyrochlore, Cd 2 Nb 2 O 7 , is a derivative of the fluorite structure, AX 2 , but with two cation sites and one-eighth fewer anions, and both cations and anion vacancies are ordered on their sublattices. The crystal structure of Cd 2 Nb 2 O 7 at room temperature can be considered as a framework of corner-sharing niobiumoxygen octahedra.…”

Section: Introductionmentioning

confidence: 99%

Pressure-induced structural transitions and phase decomposition in theCd2Nb2O7pyrochlore

et al. 2006

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The structural transitions of the pyrochlore, Cd 2 Nb 2 O 7 , at pressures up to 32.5 GPa have been investigated by in situ Raman scattering and angle-dispersive x-ray diffraction ͑ADXRD͒ methods. The x-ray diffraction results reveal that small amounts ͑ϳ7%͒ of metallic cadmium form by chemical decomposition at pressures greater than 4 GPa. Both Raman and XRD results indicate that a pressure-induced structural distortion from pyrochlore to defect fluorite occurs in pyrochlore Cd 2 Nb 2 O 7 at pressures of 12-14 GPa. Subsequently, a new high-pressure phase formed and the phase transition was complete at ϳ27 GPa. The high-pressure phase is either monoclinic or orthorhombic and transforms to either the pyrochlore ͑or defect fluorite͒ structure or the amorphous state when quenched to ambient conditions. Energy dispersive spectroscopy ͑EDS͒ analysis and high-resolution transmission electron microscopy ͑HRTEM͒ observation of the quenched sample confirmed the Cd loss and resulting mixture of ordered pyrochlore, defect fluorite, high-pressure phase, as well as amorphous domains.

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Optical and electric investigations of the phase transitions in pyrochlore Cd₂Nb₂O₇

Cited by 39 publications

References 5 publications

Covalency-driven Structural Evolution in the Polar Pyrochlore Series Cd₂Nb₂O_7–xS_x

Covalency-driven Structural Evolution in the Polar Pyrochlore Series Cd₂Nb₂O_7–xS_x

The ferroelectric properties of Cd₂Nb₂O₇: a Monte Carlo simulation study

Pressure-induced structural transitions and phase decomposition in theCd2Nb2O7pyrochlore

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Optical and electric investigations of the phase transitions in pyrochlore Cd2Nb2O7

Cited by 39 publications

References 5 publications

Covalency-driven Structural Evolution in the Polar Pyrochlore Series Cd2Nb2O7–xSx

Covalency-driven Structural Evolution in the Polar Pyrochlore Series Cd2Nb2O7–xSx

The ferroelectric properties of Cd2Nb2O7: a Monte Carlo simulation study

Pressure-induced structural transitions and phase decomposition in theCd2Nb2O7pyrochlore

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Optical and electric investigations of the phase transitions in pyrochlore Cd₂Nb₂O₇

Covalency-driven Structural Evolution in the Polar Pyrochlore Series Cd₂Nb₂O_7–xS_x

Covalency-driven Structural Evolution in the Polar Pyrochlore Series Cd₂Nb₂O_7–xS_x

The ferroelectric properties of Cd₂Nb₂O₇: a Monte Carlo simulation study