High-pressure neutron-diffraction studies of KH2PO4-type phase transitions as Tctends to ok

Nelmes, R. J.; McMahon, M. I.; Piltz, Ross O.; Wright, Nick

doi:10.1080/00150199108209465

Cited by 39 publications

(55 citation statements)

References 15 publications

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“…In both systems, we eventually expect the collective tunnel splitting to converge to zero as N → ∞, signaling long-range ferroelectricity; the tunnel splitting should conversely remain nonzero in the quantum paraelectric state, attainable at high pressure. [5,17] Remarkably, deuteration at fixed geometry does not affect the fact that the tunnel splittings in large clusters [16] are exceedingly smaller than k B T c , which renders T c practically insensitive. In addition, the effective mass change is reduced by the involvement of the heavy nuclei.…”

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confidence: 99%

Ferroelectricity and Isotope Effects in Hydrogen-Bonded KDP Crystals

et al. 2002

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Based on an accurate first principles description of the energetics in H-bonded KDP, we conduct a first study of nuclear quantum effects and of the changes brought about by deuteration. Cluster tunneling involving also heavy ions is allowed, the main effect of deuteration being a depletion of the proton probability density at the O-H-O bridge center, which in turn weakens its protonmediated covalent bonding. The ensuing lattice expansion couples selfconsistently with the proton off-centering, thus explaining both the giant isotope effect, and its close connection with geometrical effects.Potassium dihydrogen phosphate (KH 2 PO 4 , or KDP) belongs to a family of ferroelectric (FE) crystals where molecular units are linked by hydrogen bonds, the ferroelectricity being connected to proton off-center ordering in the bonds. A characteristic feature of this family is the large increase in the Curie temperature T c upon deuteration. In this particular case, it goes from ≃ 122 K in KDP to ≃ 229 K in the deuterated compound (DKDP). The origin of this huge isotope effect is still controversial, and has been mostly understood in terms of the quantum tunneling model proposed by Blinc, [1] later modified by inclusion of the coupling between proton motion and the K-PO 4 dynamics.[2] While direct experimental indications of tunneling have recently emerged [3], the connection between proton tunneling and isotope effect remains unclear. There is in fact strong evidence, [4,5] that isotope substitution acts rather through a geometrical modification of the hydrogen bonds, [6] with an expansion of the O-H-O distance. The proton off-centering, and thus the corresponding increase of lattice parameter upon deuteration, appear to be remarkably correlated to the increase of order parameter and of T c . These findings stimulated new theoretical work where some of these facts could be addressed without invoking tunneling, [7] however so far only at a rather phenomenological level.In the first part of this letter we investigate, using electronic structure calculations within Density Functional Theory (DFT), the relationship between proton ordering, polarization, and geometry in KDP. In the second part, we introduce a study of energy and subsequently the quantization of the collective ion displacements in small KDP clusters, embedded in a host paraelectric lattice. These calculations demonstrate the difference between deuterated and protonated KDP, the more delocalized proton bridging the oxygens and pulling them together more effectively than the deuteron. This phenomenon, which is at the root of the geometric effect, is further illustrated by solving in the last part a selfconsistent nonlinear model.For the DFT calculations we use two different approaches: one employing a basis set of confined pseudoatomic orbitals (SIESTA), [8] another a plane wave (PPW) representation.[9] For the first we choose a double-zeta basis set with polarization functions, and an orbital confinement energy E c = 50 meV. In the second, we set the energy cutoff to...

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confidence: 99%

Ferroelectricity and Isotope Effects in Hydrogen-Bonded KDP Crystals

et al. 2002

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“…For the pressure at which the peak separation δ for DKDP coincides with that of KDP, both would have the same T c according to the observed phenomenology. (62) Moreover, this T c equality is observed for any pressure applied to KDP and correspondingly higher pressure applied to DKDP such that their δ's are brought in coincidence. However the δ coincidence at the high pressures used in the experiments is achieved in our calculations only by taking into account the mutual self-consistent arrangement between D(H) distribution and host structure (the geometrical effect), modeled as explained in Subsection 3.6 at the different imposed pressures.…”

Section: Additional Ab Initio Results For Kdp 371 Pressure Effectsmentioning

confidence: 86%

“…The consequent barrier-height increase would couple selfconsistently with tunneling in the way described above leading to the large isotope effect observed in ADP. (62) This mechanism is expected to be universal for the family of H-bonded ferroelectrics exhibiting large isotope effects.…”

Section: The Nonlinear Self-consistent Phenomenon and The Isotope Effmentioning

confidence: 99%

Ab Initio Studies of H-Bonded Systems: The Cases of Ferroelectric KH2PO4 and Antiferroelectric NH4H2PO4

Koval¹,

Lasave²,

Migoni³

et al. 2011

Ferroelectrics - Characterization and Modeling

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“…However, it is known that the transition temperature in NH 4 H 2 PO 4 and ND 4 D 2 PO 4 , as well as in KH 2 PO 4 and KD 2 PO 4 , is a linear function of the H-site distance δ -a separation between two possible positions of a hydrogen on the bond O-H. . .O -and turns to zero in NH 4 H 2 PO 4 at δ c = 0.2Å [12][13][14]. Furthermore, at equal δ with an accuracy of 0.01Å the transition temperatures in…”

Section: Introductionmentioning

confidence: 99%

“…These changes allow us to describe the experiment for the pressure dependences of dielectric characteristics of a ND 4 D 2 AsO 4 crystal 1 [9], reduce the number of free parameters and to reproduce within the proton ordering model the universality of the transition temperature vs H-site distance dependence in several crystals of this family [12][13][14].…”

Section: Introductionmentioning

confidence: 99%

The Influence of Hydrostatic Pressure on Dadp-Type Antiferroelectrics

Levitskii¹,

Moina²

1998

Condens. Matter Phys.

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We study the effects of hydrostatic pressures on deuterated ND 4 D 2 PO 4 type antiferroelectric crystals. Within the four-particle cluster approximation the free energy of the crystals is calculated, and an equation for the transition temperature and explicit expressions for static dielectric permittivities of the crystals are derived. Model pressure dependences of the effective dipole moments of unit cells are proposed. Some peculiarities of the dipole moment formation in these hydrogen bonded crystals are elucidated. In particular, we show the importance of mutual polarization of the dipole moments. The theory parameters providing a satisfactory description of the experimental data for pressure dependences of transition temperature and dielectric characteristics of the considered crystals are found.

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High-pressure neutron-diffraction studies of KH₂PO₄-type phase transitions as T_ctends to ok

Cited by 39 publications

References 15 publications

Ferroelectricity and Isotope Effects in Hydrogen-Bonded KDP Crystals

Ferroelectricity and Isotope Effects in Hydrogen-Bonded KDP Crystals

Ab Initio Studies of H-Bonded Systems: The Cases of Ferroelectric KH2PO4 and Antiferroelectric NH4H2PO4

The Influence of Hydrostatic Pressure on Dadp-Type Antiferroelectrics

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