Structural and thermoelastic study of the protonic conducting perovskite SrCe0.95Yb0.05Oξ (ξ ∼ 3) between 373 K and 1273 K

Abstract

The temperature dependence of the crystal structure of the protonic conducting perovskite SrCe 0.95 Yb 0.05 O ξ (ξ∼3) has been determined from Rietveld refinement of neutron time-of-flight powder diffraction data in 25 K steps from 373 K to 1273 K. In contrast to most SrB IV O 3 perovskites which show a sequence of structural phase transitions from Pmcn -Incn -I4/mcm -Pm3m with increasing temperature, SrCe 0.95 Yb 0.05 O ξ remains orthorhombic with space group Pmcn from 4.2 K to the highest temperature measured. Crystallographic results are presented graphically in the form of the magnitude of the seven unique symmetry adapted basis vectors of a hypothetical aristotype perovskite phase. Thermoelastic properties of SrCe 0.95 Yb 0.05 O ξ deduced from the temperature variation of the unit cell volume and Debye-Waller factors are compared with literature values derived from independent calorimetric, Raman and elastic measurements. Thermodynamically, SrCe 0.95 Yb 0.05 O ξ behaves as a Debye-like solid with an average Grüneisen parameter in the range 1.4-1.5, but with two characteristic temperatures, ∼200 K and ∼600 K, the lower temperature being more related to the cation vibrations, the higher to the anion vibrations.

Low temperature thermoelastic and structural properties of LaGaO3 perovskite in the Pbnm phase

“…The two anions show similar zero point contributions and vibrational Debye temperatures, 3.11(5) Â 10 À 3Å2 and 776(8) K, and 3.16(4) Â 10 À 3Å2 and 689 (5) [44], and to a lesser extent in CaTiO 3 [32], Knight found that the vibrational Debye temperatures of the cations were of a similar magnitude, but distinctly lower than those of the anions, which were also found to be approximately equal in magnitude. The average vibrational Debye temperatures of the 9.…”

Thermoelastic and structural properties of ionically conducting cerate perovskites: (II) SrCeO3 between 1273 K and 1723 K

“…In the crystallographic study of the Yb-doped SrCeO 3 between 373 K and 1273 K, the isotropic atomic displacement parameters for the two cations were found to exhibit a linear temperature dependence 12 as would be expected for harmonic behaviour at temperatures greater than the corresponding vibrational Debye temperatures. For the anions, the atomic displacements were represented by the equivalent isotropic displacement parameters, and these exhibited a more quadratic temperature dependence that was attributed to the averaging of the markedly anisotropic principal axes magnitudes.…”

“…No evidence was found for changes in the fundamental reflection multiplicities, and no class of superlattice reflection was lost in the temperature interval 1273 K to 1723 K, hence the space group of SrCeO 3 (Pbnm) is unchanged at high temperature from that determined under ambient conditions 9,10 . The 1273 K data set of SrCeO 3 was fitted first using the structural model of SrCe 0.95 Yb 0.05 O  ( ~ 3) that had been determined from data collected at the identical temperature 12 . Convergence from this trial solution was rapid for a model with isotropic atomic displacement parameters for the cations, and anisotropic atomic displacement parameters for the anions.…”

“…no evidence for incipient structural phase transitions, in agreement with conclusions drawn from isobaric heat capacity measurements made at higher temperatures. Vibrational Debye temperatures calculated from fitting the temperature-variations of the isotropic, or isotropic equivalent atomic displacement parameters, has shown that the two cations have similar characteristic temperatures (Sr 222(1) K; Ce 265(2) K) and that these are significantly separated in energy from those determined for the two crystallographically inequivalent anion sites (O1, 4c, 494(4) K; O2, 8d, 479(3) K) 12 . The Debye temperature of SrCeO 3 has been estimated at 359 K from measurements of the longitudinal and transverse sound velocities 6 , however this value was calculated assuming a bulk modulus of 64 GPa, which is far removed from the value determined directly from the equation-of-state (110.1 GPa) 13 .…”

“…The isobaric heat capacity has been determined on several occasions; between 5 K and 1500 K 14 , 298.15 K and 1700 K 15 , and 270 K to 900 K 16 , with no evidence being found for structural phase transitions in the temperature interval 5 K -1700 K. Fitting the isobaric heat capacity to a variety of Debye-based vibrational density of states modelsinsufficient to characterise the material, and that below 300 K, SrCeO 3 behaves as a Debye-like solid, but with two characteristic temperatures, 206(6) K, and 604(7) K 12 . The observation of two characteristic Debye temperatures in the approximated vibrational density of states, and two, wellseparated vibrational Debye temperatures for the cations, and anions, appears to be common in perovskite-structured compounds being observed in CaTiO 3 18 , LaGaO 3 19 and BaCeO 3 20 suggesting a commonality in the phonon density of states for simple ionic perovskite-structured phases.…”