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Measurements and ab initio molecular dynamics simulations of the high temperature ferroelectric transition in hexagonal RMnO3
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10.1063/1.3656698
/content/aip/journal/jap/110/8/10.1063/1.3656698
http://aip.metastore.ingenta.com/content/aip/journal/jap/110/8/10.1063/1.3656698

Figures

Image of FIG. 1.
FIG. 1.

(Color online) Measured temperature dependent XANES spectra of HoMnO3 are shown in (a). The simulated XANES spectra for the high temperature (thin line), low temperature (thick line) and intermediate structure (no tilting of polyhedral but buckled Ho layers, dashed line) are given in (b). Panel (c) gives the low temperature crystal structure of hexagonal HoMnO3 showing the tilted MnO5 bipyramids.

Image of FIG. 2.
FIG. 2.

(Color online) Measured temperature dependent XANES spectra of YMnO3 revealing same trend as found in Fig. 1(a) for HoMnO3.

Image of FIG. 3.
FIG. 3.

(Color online) Temperature dependent lattice parameters of HoMnO3 showing a change in slope occurring near ∼900 K as the possible onset of the ferroelectric state. Note that the c-axis is reduced with increased temperature.

Image of FIG. 4.
FIG. 4.

(Color online) HoMnO3 XAFS structure functions for temperatures from 298 K to 1038 K for structure about the average Mn and Ho sites. Note that the buckling of the Ho layer is maintained for the entire temperature range studied.

Image of FIG. 5.
FIG. 5.

(Color online) HoMnO3 local structure derived (XAFS) temperature dependent bond distances are shown in (a) and the thermal factors with Einstein model fits are given in (b) and (c) for the <Mn−Mn>, <Ho−O>, <Ho−Mn>, and <Ho−Ho> distances.

Image of FIG. 6.
FIG. 6.

(Color online) HoMnO3 local structure derived (XAFS) temperature dependent bond distances about Mn are shown in (a) and the thermal factors with Einstein model fits are given in (b) for the <Mn−Oa> and <Mn−Ob> distances.

Image of FIG. 7.
FIG. 7.

(Color online) The temperature dependence of the expanded preedge feature in HoMnO3 (for 1s to “Mn np-hybridized with Mn 3d” region) shown in Fig. 1.

Image of FIG. 8.
FIG. 8.

(Color online) Average structural changes in the 120 atoms HoMnO3 supercell from ab initio molecular dynamics at temperatures of 500, 700, 900, 1100, 1250, and 1400 K. The structure shown is for the 300 K reference structure with arrows from atomic center indicating displacements from that structure at the given temperature. The red (small), purple (medium) and blue (large)spheres correspond to O, Mn, and Ho ions, respectively.

Image of FIG. 9.
FIG. 9.

(Color online) Panel (a) gives change occurring in the 900 K structure manifested mainly as z-axis displacements of the O3, O4, and Ho ions. In (b) the 1400 K structure is shown with rotation of the polyhedral. The final structure at 1400 K is given in (c).

Image of FIG. 10.
FIG. 10.

(Color online) Calculated HoMnO3 polarization magnitude indicating that start to decrease at low temperatures and vanishes near ∼900 K, before the MnO6 polyhedra rotate significantly and before the Ho planes loses their buckling. The polarization is along the z-axis.

Tables

Generic image for table
Table I.

Born effective charge tensor.a

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/content/aip/journal/jap/110/8/10.1063/1.3656698
2011-10-31
2014-04-23
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Measurements and ab initio molecular dynamics simulations of the high temperature ferroelectric transition in hexagonal RMnO3
http://aip.metastore.ingenta.com/content/aip/journal/jap/110/8/10.1063/1.3656698
10.1063/1.3656698
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