Synthesis And Cationic Doping-induced Structural Changes Of New Stuffed Tridymite Oxides

The stuffed tridymite compounds are known to show interesting ferroic properties and a great variety of different polymorphic forms.Because Ga³⁺may have a flexibility of coordination to either 4,5 or 6 oxygen atoms and combined the single electron pair effect of Pb²⁺,as a consequence,it might adapt interstitial oxygen atoms within this open-framework structure?under some certain circumstances?and lead to the possible application in oxygen conductivity.In addition to this,we use Ca²⁺which was much differ with Pb²⁺and Ga³⁺,to increase the possibility of generating new compounds.The main results are given as following:1.Temperature-induced phase transitions for stuffed tridymite AGa₂O₄?A=Sr,Ca?were investigated by experimental and theoretical calculations.?-SrGa₂O₄ was stable under 1200°C,and transform to the?phase when heated to 1200°C.A similar phenomenon was found for CaGa₂O₄,and the temperature boundary was about 1350°C.Rietveld refinements provided detailed structural information for these polymorphs and suggest that the driving force of these phase transitions is the under-bonded nature of the alkaline earth cations.2.We combined temperature with composition to study the solid solution Ca_1-xSr_xGa₂O₄?0?x?1.0?.We found a new compound,which adopt a beryllonite-type framework with a mixed UDUDUD+UUDDUD topology.Both the temperature and composition are crucial for the formation of these phases.Finally,in the high temperature range,for the composition of 0.2?x?0.95 they exist in the structure of Ca₂SrGa₆O₁₂.3.For the solid solution Ca_1-xBa_xGa₂O₄?0?x?1.0?,we found a new structure.And with the increasing content of Ba²⁺,the structure experience the changes from?-CaGa₂O₄,Ca₂BaGa₆O₁₂,CaBaGa₄O₈,new structure to BaGa₂O₄.It was related to the bigger size of Ba²⁺,which need bigger holes,so the structure changed.In other words,the needing of larger space for Ba²⁺in according phase forces the 6-membered ring channel expand through the arrangement of the oxygen atoms.4.The structure of Ca₂PbGa₈O₁₅ was stable in a range.When we changed the ratio of Ca²⁺and Pb²⁺,in a small range it can keep the Ca₂PbGa₈O₁₅ structure.And when we doped Ba²⁺,it formed the solid solution of Ca₂Pb_1-xBa_xGa₈O₁₅?0?x?1.0?,and they could keep the Ca₂PbGa₈O₁₅ structure,in the whole range.While,for the doping of Sr²⁺,only in a small range we can get the pure phase of Ca₂PbGa₈O₁₅.The different size of Sr²⁺and Ba²⁺results in the different result of doping.In the Ca₂PbGa₈O₁₅ structure,the hole needs relatively bigger size ions.On the other hand,the single electron pair effect of Pb²⁺is also a factor.5.The structure of CaPbGa₄O₈ is sensible to the size of ions.And compared with it,the structure of Ca₂PbGa₆O₁₂ could exist in a wider range.It's the size of these ions that result in the transforming of the two structures.