Mechanisms of the paraelectric to ferroelectric phase transition in RbH2PO4probed by purely resonant x-ray diffraction

Abstract

Resonant x-ray diffraction was used to study the proton jumps in hydrogen-bonded rubidium dihydrogen phosphate (RDP) crystals. In the paraelectric RDP phase, hydrogen is delocalized between two crystallographically equivalent positions. At lower temperatures, this symmetry can be broken, which defines the processes that lead to the para-to ferroelectric phase transition. We have measured the energy spectra of the forbidden reflections 006 and 550 at incident radiation energies close to the Rb K edge in a wide temperature range, down to the temperature of the ferroelectric phase transition. In the paraelectric phase, we observed a growth of integrated intensity for both forbidden reflections with temperature. This behavior is opposite to conventional nonresonant Bragg reflections, where intensity decreases in accordance with the Debye-Waller factor. The developed theoretical model explains this effect with the thermal motion induced (TMI) scattering mechanism and also confirms the adiabatic approximation stating that electrons instantly follow the nuclei movements. In the 550 energy spectra, we have observed an additional contribution to the resonant structure factor, which could be associated with the presence of transient Slater-type proton configurations (PC) in the half-filled hydrogen position.

Proton configurations in the hydrogen bonds ofKH2PO4as seen by resonant x-ray diffraction

“…A method to calculate the PC contributions has been reported 20 : it consists of calculating the various tensor components f C αβγ for each configuration according to Eq. (5) after relaxing the structure in the chosen configuration.…”

“…In Ref. 20, only the Slater configurations were evidenced: the polar configurations were found to contribute to the experimental spectra and the Takagi configurations were neglected due to their higher energy. In the case of KDP, it turns out that all three types of configurations are needed to explain the experimental spectra, which are much more complex than those of RDP.…”

Sample chamber for synchrotron based in-situ X-ray diffraction experiments under electric fields and temperatures between 100 K and 1250 K

“…For instance, the low-temperature regime allows the separation of thermal-motion-induced contributions (Ovchinnikova et al, 2005(Ovchinnikova et al, , 2010 and point-defect-induced contributions (Dmitrienko & Ovchinnikova, 2003;Ovchinnikova et al, 2005) to the reflection intensity. This separation is particularly important while measuring RXD at 'forbidden' reflections, which is a powerful technique to study atomic displacements (Richter et al, 2014(Richter et al, , 2018 and the local electronic structure of selected atomic positions in crystals (Zschornak et al, 2014).…”

Helicity-dependent resonant X-ray scattering in CuB2O4

“…Interestingly, forbidden reflections that originate from interference between different scattering channels (e.g. E1.E1 with E1.E2) have been experimentally observed (Kirfel et al, 2002;Collins et al, 2003;Beutier et al, 2014Beutier et al, , 2015Richter et al, 2014;Kokubun et al, 2001;Oreshko et al, 2012). However, local chirality could not be fully investigated in these experiments since linearly polarized X-rays were used.…”