We take up one of BLSF bismuth layered ferroelectric crystals called SrBi2Ta2O9 (SBT) as a topological insulator.
From the firing temperature, crystal structure and fine composition analysis of SrBi 2Ta2O9 (SBT) crystal, the mechanism of expression of topological insulator is discussed based on red-ox potential.
Since SBT has less membrane fatigue due to polarization reversal and can extend the life of ferroelectric memory.
It was studied in 1996. However, leakage current flowed during hydrogen gas treatment and insulation characteristics decreased, making it a material hard to use as a memory, and practical application was limited.
Considering why hydrogen gas reduces insulation,
It is easy to explain using the topological idea of the 2016 Nobel Prize in Physics.
Here, "topological" refers to a physical phenomenon in which only the surface of the crystal grain is altered in the film which is a cluster of crystal grains.
The interior of the SBT crystal is not reduced with hydrogen gas, and it is almost in its original form.
Assuming that the oxidation-reduction potential of hydrogen gas is 0 V,
Base metal lithium is 3V
Strontium is 2.8 V
Titanium is -1.6V
Tantalum is - 0.8 V
Lead is 0.1V
Bismuth + 0.3V
Copper close to precious metal is +0.34 V
And the oxidation-reduction potential in an aqueous solution.
The electrode uses precious metal platinum.
Precious metals are prone to metal
Base metals tend to become oxides and are reduced and hard to become metal.
Probably SBT is an oxide that is more base than lead + 0.1V lead than bismuth + 0.3V.
In estimation, since it has an oxidation-reduction potential of about -0.8 V
The inside of the crystal is not reduced with hydrogen gas.
In Real, the main SBT crystal is not destroyed by hydrogen gas heating.
However, from firing temperature and crystal structure analysis
Microcrystals structure(low temperature phase) of the Bismuth oxide Films are not reduced much.
Large crystals (high temperature phase) of the SBT structure are not reduced to the inside.
Among the intermediate phase pyrochlore microcrystals,
Resistance-reducing pyrochlore (low-temperature type) is similar to fluolite and has characteristics that it is not reduced much.
It can be analyzed that its composition ratio is 0.7: 2.3: 2 to 0.9: 2.1: 2 for S: B: T.
The easily reducible pyrochlore (high temperature type) is prone to be specifically reduced, and further has the property of oxidizing itself and reducing the contacted crystal.
It can be analyzed that S: B: T is about 0.7: 1.5: 2.
In particular, the defect of bismuth is characteristic, and the defects of strontium and tantalum are not clear.
This is presumed that bismuth is caused by movement to the inside and outside of the membrane called migration at a particularly low melting point.
When this easily reducible pyrochlore (high temperature type) appears,
This is easy to reduce.
However, it is reduced unilaterally and not a metal.
There is an event that can be explained by assuming a mechanism that pyrochlore is oxidized again and reduces the SBT crystal in contact with it.
It is SBT crystal film fired at 750 ℃.
Although SBT crystals occupy about 80%, it has been confirmed that about 20% high temperature type pyrochlore exhibits a crystal structure distributed in gaps.
The leakage characteristic at that time has a very large current value,
It also has low fatigue resistance.
Although SBT has a similar crystal,
The characteristics do not show the inherent insulation and fatigue resistance of the SBT.
Even with films with low temperature pyrochlore, insulation is overwhelmingly low and fatigue properties are poor.
To illustrate this,
It is presumed that the crystal surface of SBT is topologically conductive.
We also believe that its topological conductivity is responsible for metallic precipitates such as bismuth and alteration of SBT surface.
The mechanism by which the surface alteration of SBT occurs.
It is inferred that the aforementioned high-temperature type pyrochlore is a bismuth deficient type, that is, a base oxide rich in tantalum strontium, and because of the nature of tantalum, it is easy to reoxidize, that is, the surface of the contacted SBT crystal is easily reduced .
In other words, the easily reducible pyrochlore has an oxidation-reduction potential of between -0.9 and -2.8 V.
There is bismuth of + 0.3 V partly,
The difference is expected to be large.
That is, a part to be easily reduced and a part which is oxidized from reduction to reduce partner appear.
From the above,
1) SBT leakage current is caused by SBT crystal surface topological conductivity.
2) The surface topological conductivity of the SBT is caused by the mechanism caused by the force of reducing the surroundings once the bismuth defective pyrochlore around it is once reduced.
Guess the model
Therefore, it can be concluded that making a bismuth deficient pyrochlore will help reduce leakage current.
In addition, it is thought that it is necessary to make the composition ratio and constituent elements that do not become easily reducible pyrochlore.
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