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

The high-temperature (HT) behavior of BaAs2Zn2O8 center dot H2O 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 BaAs2Zn2O8, which is stable up to at least 525 degrees 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 degrees C, whereas in the spectral region 3000-3600 cm(-1) all the bands disappeared, which confirms the dehydration process of BaAs2Zn2O8 center dot H2O. The transition from BaAs2Zn2O8 center dot H2O to BaAs2Zn2O8 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 degrees C (HTRS) or 300 degrees C (HT SCXRD). According to the HT SCXRD data, in the temperature range 25-300 degrees C the studied compound demonstrates anisotropic thermal expansion (alpha(max)/alpha(min) = 9.4), which is explained by flexible crankshaft chains of TO4 (T = As, Zn) tetrahedra. Additionally, we discussed some crystal-chemical aspects of minerals with both (ZnOn) and (AsOm) polyhedra (n = 4, 5, 6; m = 3, 4) as main structural units.

Automated summary

The high-temperature behavior of BaAs2Zn2O8·H2O was studied using in situ single-crystal X-ray diffraction and hot stage Raman spectroscopy. The compound undergoes dehydration and forms BaAs2Zn2O8, which is stable up to 525 degrees C. The phase transition from BaAs2Zn2O8·H2O to BaAs2Zn2O8 is accompanied by a change in symmetry and is observed at different temperatures depending on the research method.