Có ai biết v́ sao cái mạng lưới tinh thể khi sắp xếp các quả cấu lại gần theo kiểu sắp xếp khit lục phương th́ nó lại có chử T+ và T- không th́ bày cho ḿnh với nghe ,cảm ơn các bạn nghe
Q: What is perovskite structure?
The general expression of a perovskite structure can be written as ABX 3. It is a structure consisting of 12-coordinated A +2 atoms (can be cerium, calcium, sodium, strontium, lead and/or various rare earth metals) on the corners of a cube, octahedral X ions (like O, F and Cl, etc.) on the faces, and tetrahedral B +4 ions (usually titanium, niobium and/or iron) in the center (Fig. 1). The name comes from the mineral CaTiO 3 which exhibits this structure. Perovskite structures have the curious property that the central atom does not "touch" its coordination neighbors, in violation of Pauling's rules. This gives the structure the property of piezoelectricity and sometimes ferroelectricity. Oxide such as barium titanate (BaTiO3), KFeF3 and CsAuCl3 are known as representative compounds. Moreover, there are also defect perovskite, or complex perovskite such as Pb(Mg1/3Nb2/3)O3, Pb(Zr0.3Ti0.7)O3 and solid solutions of them.
Fig.1 Perovskite structure (Grey atoms: A, Black: B, Blue: O)
Why is such a structure important?
From consideration of the geometry, there is a “perfect fit” relationship between the ionic radii of A, B and O as R A+R O=2(R B+R O). However, for many compounds having the perovskite structure, the relationship will not hold exactly because of small variations in the sized of the A and B ions. In that case, small lattice distortions will occur in order to minimize lattice energy. It is exactly such distortions that contributes to the materials’ dielectric properties and makes them a very important part of the modern technical world.
Use BTO as an example, above its Curie point (approximately 130 ° C) the unit cell is cubic; below the Curie point the structure is slightly distorted to the tetragonal form with a dipole moment along the c direction. The dipole is generated because of the central Ti 4+ ions are closer to the central oxygen ions as shown in Fig. 2.
What are the applications for such materials?
Such a spontaneous polarizability enables these piezoceramics to be widely used for: 1) the generation of voltages; 2) electromechanical actuation; 3) frequency control; 4) the generation and detection of acoustic and ultrasonic energy. Furthermore, put in outer electric field, the direction change of the spontaneous polarization highly resembles the behavior of a ferromagnet in the magnetic field (such a property is therefore called ferroelectricity). The two stable polarization states +P and –P could be used to encode the 1 and 0 of the Boolean algebra that forms the basis of memory and logic circuitry in all modern computers. Thus, the ferroelectrics like Pb(ZrTi)O3 and (BaSr)TiO3 etc. are among the favorite materials for non-volatile random access memories (NV-RAMs). In NV-RAMs, the stored information is retained even if power to the chip is interrupted. The ferroelectrics serve not just as capacitors, but as the memory element itself.
Fig. 2 Lattice distortion and dipole generation in BTO below Tc
Why make the materials patterned and with small size?
Generally, it is for making the materials readily integrated to compact array for modern device use. For the applications in RAMs or pyroelectric sensors, the miniaturization is especially significant in order to achieve very high-density memories or fast response and higher sensitivity.
What is DPN?
The shortened form for “dip-pen nanolithography”. A newly developed method of patterning small features in a serial way. You can think of it as “writing” with a nanometer sized “pen” using organic based “ink” on some substrates. The writing process can be controlled through an atomic force microscope (AFM) and the “pen” is your AFM tip. For more information, please refer to Ming Su’s webpage.
What is MIMIC?
The shortened form for “Micromolding in Capillaries”. Micropatterns are first transferred by casting a silicone rubber, usually poly-(dimethylsiloxane) (PDMS), against a master structure. The PDMS is then peeled off, cut, and used as a mold that forms microcapillaries (shown in the top part of Fig. 3) on a substrate which can be filled with a liquid. Because of the capillary force, the filling of such microchannels is spontaneously carried out between two surfaces (PDMS and substrate) in conformal contact. The fluid may be a liquid prepolymer, a solution or suspension of the materials to be formed, or precursors of these materials, and in our case it is the ceramic sol or the nanoparticle suspension. After filling, the PDMS mold is left on the substrate until the liquid in the channels is dried and solid phase materials are left in exactly the same pattern. By this way fabricating micrometer- and submicrometer-scale structures of organic, inorganic, and biological materials can be readily realized. In my experiment, PZT line features on SiOx/Si substrate were fabricated by this method (Fig.4)
Fig. 3 MIMIC Scheme
Fig. 4 PZT patterns formed by MIMIC on SiOx/Si substrate, before heating (A: SEM image, B: AFM topographic image and C: AFM cross-section analysis)
|Ionic crystals have at least two atoms in their base which are ionized. Charge neutrality demands that the total charge in the base must be zero; so we always need ions with opposing charge.|
|The binding between the ions is mostly electrostatic and rather strong (binding energies around 1000 kJ/mol); it has no directionality.|
|Ionic crystals thus can be described as an ensemble of hard spheres which try to occupy a minimum volume while minimizing electrostatic energy at the same time (i.e. having charge neutrality in small volumes, too).|
|There are no free electrons, ionic crystals are insulators.|
|Ionic crystals come in simple and more complicated lattice types; the latter is true in particular for oxides which are often counted among ionic crystals. Some prominent lattice types follow|
|The lattice is face centered cubic (fcc), with two atoms in the base: one at (0, 0, 0), the other one at (½, 0, 0)|
|Many salts and oxides have this structure, e.g. KCl, AgBr, KBr, PbS, ...|
MgO, FeO, ...
|The lattice is cubic primitive with two atoms in the base at (0,0,0) and (½, ½, ½). It is a common error to mistake it for a bcc lattice.|
|Intermetallic compounds (not necessarily ionic crystals), but also common salts assume this structure; e.g. |
CsCl, TlJ, ...,
or AlNi, CuZn,
|The "zinc blende" lattice is face centered cubic (fcc) with two atoms in the base at (0,0,0) and (¼, ¼, ¼).|
|It is not only an important lattice for other ionic crystals like ZnS, which gave it its name, but also the typical lattice of covalently bonded group IV semiconductors (C (diamond form), Si, Ge) or III-V compounds semiconductors (GaAs, GaP, InSb, InP, ..)|
|The ZnS lattice is easily confused with the ZrO2 lattice below.|
|The lattice is face centered cubic (fcc) with three atoms in the base, one kind (the cations) at (0,0,0), and the other two (anions of the same kind) at (¼, ¼, ¼), and (¼, ¾, ¼).|
|It is often just called the "fluorite structure".|
|The lattice is essentially cubic primitive, but may be distorted to some extent and then becomes orthorhombic or worse. It is also known as the BaTiO3 or CaTiO3 lattice and has three different atoms in the base. In the example it would be Ba at (0,0,0), O at (½, ½, ,0) and Ti at (½, ½, ½).|
|A particular interesting perovskite (at high pressures) is MgSiO3. It is assumed to form the bulk of the mantle of the earth, so it is the most abundant stuff on this planet, neglecting its Fe/Ni core. The mechanical properties (including the movement of dislocations) of this (and related) minerals are essential for geotectonics - forming the continents, making and quenching volcanoes, earthquakes - quite interesting stuff!|
|The spinel structure (sometimes called garnet structure) is named after the mineral spinel (MgAl2O4); the general composition is AB2O4. It is essentially cubic, with the O - ions forming a fcc lattice. The cations (usually metals) occupy 1/8 of the tetrahedral sites and 1/2 of the octahedral sites and there are 32 O-ions in the unit cell.|
|This sounds complicated, but it is not as bad as it could be; look at the drawing. We "simply" have two types of cubic building units inside a big fcc O-ion lattice, filling all 8 octants.|
|The spinel structure is very flexible with respect to the cations it can incorporate; there are over 100 known compounds. In particular, the A and B cations can mix! In other words, the composition with respect to one unit cell can be|
|A few examples (in regular chemical symbols)
|The spinel structure is also interesting because it may contain vacancies as regular part of the crystal. For example, if magnetite is slowly oxidized by lying around a couple of billion years, or when rocks cool, Fe2+ will turn into Fe3+ (oxidation, in chemical terms, means you take electrons away). If all Fe2+ is converted into Fe3+, charge balance requires a net formula of Fe21,67O32 per unit cell and this means that 2,33 sites must be vacant - we have what is called a defect spinel. In a way, the composition is now Fe21,67Vac2,33O3; having lots of vacancies as an integral part of the structure.|
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