Temperature-Induced Phase Transition in a Feldspar-Related Compound BaZn₂As₂O₈∙H₂O

The high-temperature (HT) behavior of BaAs ₂Zn ₂O ₈∙H ₂O was studied by in situ single-crystal X-ray diffraction (SCXRD) and hot stage Raman spectroscopy (HTRS) up to dehydration and the associated phase transition. During heating, the studied compound undergoes the dehydration process with the formation of BaAs ₂Zn ₂O ₈, which is stable up to at least 525 °C. The evolution of the fourteen main Raman bands was traced during heating. The abrupt shift of all Raman bands in the 70–1100 cm ⁻¹ spectral region was detected at 150 °C, whereas in the spectral region 3000–3600 cm ⁻¹ all the bands disappeared, which confirms the dehydration process of BaAs ₂Zn ₂O ₈∙H ₂O. The transition from BaAs ₂Zn ₂O ₈∙H ₂O to BaAs ₂Zn ₂O ₈ is accompanied by symmetry increasing from P2 ₁ to P2 ₁/c with the preservation of the framework topology. Depending on the research method, the temperature of the phase transition is 150 °C (HTRS) or 300 °C (HT SCXRD). According to the HT SCXRD data, in the temperature range 25–300 °C the studied compound demonstrates anisotropic thermal expansion (α _max/α _min = 9.4), which is explained by flexible crankshaft chains of TO ₄ (T = As, Zn) tetrahedra. Additionally, we discussed some crystal-chemical aspects of minerals with both (ZnO _n) and (AsO _m) polyhedra (n = 4, 5, 6; m = 3, 4) as main structural units.