Crystal structure of Re-substituted lanthanum tungstate La_5.4W_1−yRe_yO_12–δ(0 ≤y≤ 0.2) studied by neutron diffraction

Abstract

A precise determination of sample composition and water uptake of La6−xWO12−δ (0.4 ≤ x ≤ 0.8) and Re‐substituted La5.4W1−yReyO12−δ (0 ≤ y ≤ 0.2) lanthanum tungstate is carried out. Sample compositions and water uptake were determined by electron probe micro‐analysis and thermogravimetry, respectively. A single‐phase region of Re‐substituted lanthanum tungstates is reported. The crystal structure of two selected specimens produced by the citrate‐complexation route based on the Pechini method, namely La5.4WO12−δ and La5.4W0.8Re0.2O12−δ, was investigated by neutron diffraction in the temperature range 1.5 ≤ T ≤ 1200 K. The structural model for lanthanum tungstates is validated, according to which the Wyckoff site shared by La and W ( space group, 24d site) is split with half‐site occupancies ( space group, 48h site). Replacement of W by up to 20 mol% Re does not change the lattice structure, and Re atoms substitute for W statistically in both 4a and 48h Wyckoff sites of the space group, as shown by combining the average neutron scattering length procedure, thermogravimetry and electron probe micro‐analysis. Using the Willis and Pryor approach to anisotropic displacement parameters it is shown that the remaining static disorder in the unit cell found in La5.4WO12−δ and La5.4W0.8Re0.2O12−δ structures is comparable, when the space group with split 48h site is employed. Through the estimation of the Debye temperature for both compounds, extracted from the analysis of thermal expansion coefficients and from the Willis and Pryor approach, anion anharmonic vibrations like those in yttria‐stabilized zirconia are proven to exist in LaWO for the first time.

Relation between composition and vacant oxygen sites in the mixed ionic-electronic conductors La5.4W1−MO12− (M= Mo, Re; 0 ≤y≤ 0.2) and their mother compound La6−WO12− (0.4 ≤x≤ 0.8)

“…This has been rationalized as a larger oxidation state of Re compared to W, i.e. Re 7+ compared to W 6+ [26]. This result will be shortly recalled below.…”

“…Finally, an agreement on the crystal structure has been recently reached [19,20,26,27]. In 2016, the first publications on the detailed crystal structure of Mo/Re-LWO are appearing [19,26,27]. However, a systematic study on the water uptake and a comparison between theory and observation on LWO and Mo/Re-LWO systems is still missing.…”

“…% of La2O3 for LWO56, respectively. Further details on the quantification of secondary phases in the LWO and LWReO systems investigated here are reported elsewhere [26]. The secondary phase peaks are shown with asterisks and sharps in Fig.…”

“…The determination of the structure details of lanthanum tungstates drew much attention in the recent years due to subtle distortions difficult to resolve univocally [18][19][20][21][22][23][24][25]. Finally, an agreement on the crystal structure has been recently reached [19,20,26,27]. In 2016, the first publications on the detailed crystal structure of Mo/Re-LWO are appearing [19,26,27].…”

“…Hence, the 3 � space group with doubled lattice parameter as used for LWO in the literature is rationalized. For detailed structural studies on LWO, the reader is referred to [19,26,27] and to the Supporting Information, where the resulting crystal structure of La5.6WO12-δ is reported (see Fig. S2, SI †).Single-peak fitting and pattern simulations give an upper limit of 1 wt.…”