Dynamics of Hydride Ions in Metal Hydride-Reduced BaTiO₃ Samples Investigated with Quasielastic Neutron Scattering

Abstract

Perovskite-type oxyhydrides, BaTiO3–x H x , have been recently shown to exhibit hydride-ion (H–) conductivity at elevated temperatures, but the underlying mechanism of hydride-ion conduction and how it depends on temperature and oxygen vacancy concentration remains unclear. Here, we investigate, through the use of quasielastic neutron scattering techniques, the nature of the hydride-ion dynamics in three metal hydride-reduced BaTiO3 samples that are characterized by the simultaneous presence of hydride ions and oxygen vacancies. Measurements of elastic fixed window scans upon heating reveal the presence of quasielastic scattering due to hydride-ion dynamics for temperatures above ca. 200 K. Analyses of quasielastic spectra measured at low (225 and 250 K) and high (400–700 K) temperature show that the dynamics can be adequately described by established models of jump diffusion. At low temperature, ≤250 K, all of the models feature a characteristic jump distance of about 2.8 Å, thus of the order of the distance between neighboring oxygen atoms or oxygen vacancies of the perovskite lattice and a mean residence time between successive jumps of the order of 0.1 ns. At higher temperatures, >400 K, the jump distance increases to about 4 Å, thus of the order of the distance between next-nearest neighboring oxygen atoms or oxygen vacancies, with a mean residence time of the order of picoseconds. A diffusion constant D was computed from the data measured at low and high temperatures, respectively, and takes on values of about 0.4 × 10–6 cm–2 s–1 at the lowest applied temperature of 225 K and between ca. 20 × 10–6 and 100 × 10–6 cm–2 s–1 at temperatures between 400 and 700 K. Activation energies E a were derived from the measurements at high temperatures and take on values of about 0.1 eV and show a slight increase with increasing oxygen vacancy concentration.

The role of oxygen vacancies on the vibrational motions of hydride ions in the oxyhydride of barium titanate

“…We found that in BaTiO 3 the required energy for this process is 2.4 eV, which, in agreement with previous results, 7,8 is considerably larger than the typical activation energy for hydride-ion conduction in this system (<0.5 eV). 4,8 Thus, we have no reason to expect that the indirect interstitial mechanism takes place in BaTiO 3 , which is in full accordance with the absence of any signature of O-H species in the vibrational spectra.…”

“…The results of the average structural and thermal characterisation of CA1, CA2 and NAB were reported and discussed elsewhere. 4,13 The characterisation of CA3, together with that of CA2 and NAB, can be found in Fig. 1(a-c) shows the INS spectra of CA1, CA2, CA3, and NAB at T < 10 K, as measured on (a) MERLIN (CA2 and CA3), (b) TOSCA (CA1, CA2 and CA3), and (c) VISION (NAB), respectively.…”

“…[1][2][3] Understanding the defect chemistry and hydride ion conduction mechanism in these materials is of great importance for developing efficient synthesis routes that allow the development of new oxyhydrides with tailored ionic and electronic conductivity for certain applications, but neither the defect chemistry nor hydride ion conduction mechanism are fully understood. Recent results from quasielastic neutron scattering (QENS), 4 isotope exchange, 5,6 combined isotope exchange with depth proling analysis 7 and rst-principles calculations 7,8 suggest a vacancy-assisted diffusion mechanism. However, a mechanism based on the transformation into a proton and then interstitial diffusion has been also discussed in the past.…”

Bandvs.polaron: vibrational motion and chemical expansion of hydride ions as signatures for the electronic character in oxyhydride barium titanate

“…The details of the sample preparation can be found in ref. 41 and the present work makes use of the sample denoted as CA1 in ref. 41.…”

“…Powder X-ray diffraction (PXRD) measurements conrmed a simple cubic structure for the sample with lattice parameter a ¼ 4.0055Å at ambient temperature. 41 Thermogravimetric analysis (TGA) of the sample showed that the total defect concentration x, as in BaTiO 3Àx H y , xÀy , was x ¼ 0.18, 41 where , denotes oxygen vacancies in the oxyhydride. The hydride ion concentration was determined to be y ¼ 0.10 using nuclear magnetic resonance (NMR) spectroscopy, 41 which implies an oxygen vacancy concentration of (x À y) ¼ 0.08.…”