| High entropy fluorite oxides have high hardness,low thermal conductivity,and good thermal stability,which have broad application prospects in the field of thermal protection.The preparation and expansion of its systems have always been great concern of researchers.However,currently,the atom ratio is equal in most composition of high entropy fluorite oxides,which does not take advantage of the huge composition space of high entropy oxides.In addition,due to the limited number of available tetravalent oxides,it is necessary to use oxides lower than tetravalent,resulting in oxygen vacancies in the vast majority of high entropy oxides.Based on this,this article conducts research on the composition design and synthesis of high entropy fluorite oxides that meet the stoichiometric ratio based on the "valence state combination" strategy.Firstly,taking the high entropy fluorite oxide with five main components as an example,the possible number of valence state combinations was calculated.The results indicate that when the valence states of cation ions are limited to the range of+2 to+6,there are only 86 valence state combinations.The entropy of mixing of each valence state combination is calculated according to the composition interval of 1%.It is found that the maximum entropy of mixing of 56 valence state combinations can reach 1.50R,which can be used as a candidate for the composition design of high entropy fluorite oxide.On the basis of valence state combination,select a specific cation combination,and calculate the average cation radius （?） and entropy of mixing ΔSmix of all composition points with 1%interval,and build a relationship diagram between （?） and ΔSmix（referred to as （?）-S diagram）to assist in the design of high entropy fluorite oxide composition,and give （?）-S diagrams of multiple cation combination systems.Then,several cationic systems were designed and preliminary synthesis was carried out to judge the possibility of preparing single-phase high-entropy fluorite oxides.It was found that the（Ca,Ce,Zr,Hf,Nb）O2 system,（Yb,Ce,Zr,Hf,Nb）O2 system,and（Yb,Ce,Zr,Sn,Nb）O2 system have a higher possibility of synthesizing single-phase high entropy fluorite oxides.Subsequently,composition selection and synthesis experiments were conducted based on （?）-S diagrams of（Ca,Ce,Zr,Hf,Nb）O2 and（Yb,Ce,Zr,Sn,Nb）O2 systems.The results show that the（Ca,Ce,Zr,Hf,Nb）O2 system is prone to monoclinic phase because the ratio of Ca2+ to Nb5+ is 1:2,and the high content of Nb5+leads to the high content of monoclinic phase in the system.All the five selected components fail to produce single-phase high-entropy fluorite oxides.In the（Yb,Ce,Zr,Sn,Nb）O2 system,all samples contained undissolved SnO2,and increasing the synthesis temperature would lead to serious evaporation of SnO2.Finally,a study was conducted on the composition design and synthesis of high entropy fluorite oxides in the（Yb,Ce,Zr,Hf,Nb）O2 system based on （?）-S diagrams.It is found that during the formation of high entropy fluorite oxides,some Yb2O3 and Nb2O5 would first form YbNbO4,and with the increase of temperature,YbNbO4 would gradually dissolve into the fluorite matrix.Under the same entropy and synthesis temperature,the content of Yb2O3 and Nb2O5 determines the amount of the second phase YbNbO4,the more Yb2O3 and Nb2O5,the greater the enthalpy ΔHmix of the system;if the product of synthesis temperature T and system mixing entropy ΔSmix cannot offset the effect of mixing enthalpy,it will leave a second phase YbNbO4 in the sample.The phase structure of highentropy fluorite oxide is related to the mean cationic radius.When the mean cationic radius is larger,the content of Ce,Yb and Nb elements is more,and the cubic fluorite phase is easier to form.When the mean cationic radius is smaller,the two elements Zr and Hf are more,and the tequatrous phase is easier to form.The high entropy fluorite oxide is mainly based on electron conductance,and there is no oxygen vacancy,which is consistent with the design idea.Based on the above understanding,single-phase high-entropy cubic fluorite oxides,single-phase high-entropy tetrophilic fluorite oxaides,and high-entropy fluorite oxides with different proportions of tetrophilic and cubic phases can be prepared by composition fine-tuning.Is proposed in this paper to build （?）-S figure to guide the composition design of high entropy fluorite oxide,and its practicability is verified by the experiments.The lack of related also are analyzed and discussed,and also produced a number of high entropy fluorite oxide,which will provide the design of high entropy fluorite oxide preparation with important reference value. |
