We investigated moderate-temperature oxygen diffusion mechanisms in Sr 2 ScGaO 5 with Brownmillerite structure type. From oxygen isotope 18 O− 16 O exchange experiments we determined that oxygen mobility sets in above 550 °C. Temperature-dependent neutron and X-ray (synchrotron) diffraction experiments allowed us to correlate the oxygen mobility with a subtle phase transition of the orthorhombic room-temperature structure with I2mb space group toward Imma, going along with a disorder of the (GaO 4 ) ∞ -tetrahedral chains. From lattice dynamical simulations we could clearly evidence that dynamic switching of the (GaO 4 ) ∞ -tetrahedral chains from its R to L configuration sets in at 600 °C, thus correlating oxygen diffusion with the dynamic disorder. Oxygen ion diffusion pathways are thus constrained along the onedimensional oxygen vacancy channels, which is a different diffusion mechanism compared to that of the isostructural CaFeO 2.5 , where diffusion of the apical oxygen atoms into the vacant lattice sites are equally involved in the diffusion pathway. The proposed ordered room-temperature structure in I2mb is strongly supported by 17 O, 45 Sc, and 71 Ga NMR measurements, which indicate the presence of crystallographically unique sites and the absence of local disordering effects below the phase transition. The electric field gradient tensor components measured at the nuclear sites are found to be in excellent agreement with calculated values using the WIEN2k program. The oxygen site assignment has been independently confirmed by 17 O{ 45 Sc} transfer of adiabatic populations double-resonance experiments.

Metastable β-As2Te3 (R3̅m, a = 4.047 Å and c = 29.492 Å at 300 K) is isostructural to layered Bi2Te3 and is known for similarly displaying good thermoelectric properties around 400 K. Crystallizing glassy-As2Te3 leads to multiphase samples, while β-As2Te3 could indeed be synthesized with good phase purity (97%) by melt quenching. As expected, β-As2Te3 reconstructively transforms into stable α-As2Te3 (C2/m, a = 14.337 Å, b = 4.015 Å, c = 9.887 Å, and β = 95.06°) at 480 K. This β → α transformation can be seen as the displacement of part of the As atoms from their As2Te3 layers into the van der Waals bonding interspace. Upon cooling, β-As2Te3 displacively transforms in two steps below T(S1) = 205-210 K and T(S2) = 193-197 K into a new β'-As2Te3 allotrope. These reversible and first-order phase transitions give rise to anomalies in the resistance and in the calorimetry measurements. The new monoclinic β'-As2Te3 crystal structure (P2(1)/m, a = 6.982 Å, b = 16.187 Å, c = 10.232 Å, β = 103.46° at 20 K) was solved from Rietveld refinements of X-ray and neutron powder patterns collected at low temperatures. These analyses showed that the distortion undergone by β-As2Te3 is accompanied by a 4-fold modulation along its b axis. In agreement with our experimental results, electronic structure calculations indicate that all three structures are semiconducting with the α-phase being the most stable one and the β'-phase being more stable than the β-phase. These calculations also confirm the occurrence of a van der Waals interspace between covalently bonded As2Te3 layers in all three structures.