Lanthanum Aluminate

Aluminum, oxygen, and lanthanum comprise lanthanum aluminate. Lanthanum is a rare earth metal that is silvery white in appearance and very ductile. The metal is so soft that the blade of a knife can cut it in two with relative ease. Lanthanum was discovered in the late 1830s by C. G. Mosander, but was unable to be successfully isolated until the following century in the 1920s. The name of the element was coined from the Greek lanthanein, which means “to be concealed.” The moniker reflects the substantial difficulty involved in isolating the element in a considerably pure form.

Lanthanum Aluminate High Magnification

Lanthanum aluminate is a member of a class of ceramics known as perovskites. Though perovskites can be found in nature, the perovskites utilized for various commercial and research purposes are generally synthesized in the laboratory. The name of this collection of ceramics stems from their similarity of structure to the mineral perovskite, which was discovered in the 1830s by the German mineralogist and crystallographer Gustav Rose in a sample obtained in the Ural Mountains. He coined the moniker of the mineral, which has the formula CaTiO₃, as a tribute to Count Lev Aleksevich von Perovski, a mineralogist from Russia.

Similar to the mineral perovskite, the general formula of perovskite ceramics is ABO₃, with the A atom having a +2 charge and the B atom having a +4 charge. The formula for lanthanum aluminate, LaAlO₃, is a prime example. The atoms of perovskites are arranged in a pseudocubic, or orthorhombic, crystal structure. In an idealized perovskite, the structure is roughly a face-centered cube with A atoms marking the corners of the cube, O ions along the faces of the cube, and a B atom, small atomic radius located at the center. Depending on the specific sizes of the A and B cations, distortions in form may occur. Also, the position of the central atom can be shifted by the application of an electric field to a perovskite.

Lanthanum Aluminate Low Magnification

When the position of the central atom of a perovskite is altered, the symmetry of the ceramic is changed so that an electrostatic dipole is produced. Such a dipole is characterized by the alignment of positive and negative charges toward opposing ends of the structure. Electrostatic dipoles in perovskites can produce interesting ferromagnetic properties that make some of the compounds useful in certain applications, especially in the temporary electric energy storage devices known as capacitors. In fact, because several perovskites can be readily transformed into highly effective dielectrics, estimates indicate that more than 90 percent of all capacitors employ perovskites or similar ceramics.

Many perovskites are also found in piezoelectric devices. These devices produce electrical signals in response to vibrations or other application of mechanical pressure. Examples of devices that often function due to piezoelectricity include sensors, motors, transducers, pressure gauges, communications equipment, and actuators. The perovskites that are suitable for such devices acquire their distinctive properties through alterations from their typical cubic structure.

Lanthanum Aluminate High Magnification Twinning

Perovskites are perhaps best known for their use as superconductors. Superconductivity was first observed in 1911 by Heike Kamerlingh Onnes when he found that the resistance to the flow of electricity in rod of mercury suddenly disappeared when its temperature was dropped to only a few Kelvins above absolute zero. Onnes is also credited with discovering that a superconductor can be returned to its non-superconducting state by the application of a strong magnetic field or electrical current. His work garnered him the Nobel Prize for Physics in 1913, but because of the expense involved in lowering materials to the incredibly low temperatures then necessary to achieve superconduction, practical use of superconductors was farfetched.

An announcement made by IBM in 1986 that two researchers, Georg Bednorz and Alex Müller, had developed a perovskite material that entered the superconducting state at the then remarkably high temperature of 35 K spurred fresh hope that someday superconductors could be employed in variety of applications. The numerous studies inspired by the announcement rapidly led to new perovskite ceramics with even higher critical temperatures (the temperature at which a material loses resistance to electrical currents). By 1988, the highest critical temperature of a perovskite was more than 100 Kelvins greater than it had been just two years earlier. Subsequent research has resulted in several more minor increases in perovskite critical temperatures, but ever since the important milestone of 77 K was surpassed, widespread use of superconducting materials has become a much more practical and seemingly obtainable goal. This is because 77 K is the boiling point of liquid nitrogen, which is considerably more common and cheaper than the liquid helium that had been necessary to cool superconducting materials with lower critical temperatures.

Stairstep Twinning in Lanthanum Aluminate

Though many perovskites are able to be used as superconductors, all of them cannot achieve a superconducting state. Lanthanum aluminate, for example, is a perovskite that is not a superconductor. The ceramic, however, is of considerable interest to researchers working in the field of superconductivity because it makes an excellent substrate upon which thin films of superconducting materials can be deposited. For example, the 60-carbon alkenes known as buckminsterfullerenes or buckyballs that have gained significant scientific attention in recent years can be grown on substrates comprised of lanthanum aluminate.

A variety of techniques have been developed for growing superconducting crystals on lanthanum aluminate or other substrates. Two of the most common methods of producing thin films are metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). MOCVD (also known as metalorganic vapour phase epitaxy, organometallic vapour phase epitaxy, or organometallic chemical vapour deposition) and other means of chemical deposition involve a precursor that undergoes a chemical change at the surface of a substrate so that a solid layer is produced. Specifically with MOCVD an organometallic gas serves as the precursor. Because the gas completely surrounds the substrate, when the chemical change occurs, the material is deposited simultaneously along all surfaces.

Twinning in Lanthanum Aluminate

In contrast, MBE is a process that can be considered to rely upon both chemical and physical means of deposition. MBE is carried out in an ultra-high vacuum and entails directing a stream of one or metals or other materials onto a substrate one layer of atoms at a time. The method takes on a chemical aspect during the deposition of compounds such as gallium arsenide which are generally formed by alternatively depositing layers of one element and then the other. Thin films of gallium arsenide were first formed in the late 1960 by American scientist John Arthur.

Whenever MOCVD, MBE, or other methods are utilized to generate thin films, the goal is to create a smooth, flawless final product with a surface similar to a mirror. An important step toward achieving this goal is finding a good match between the deposited material and a substrate. Good matches are usually found between substrates formed of perfect crystalline lattices with structures similar to the materials layered upon them. During the deposition process, the overlying crystal adopts the ordering of the substrate so that any imperfections in the lattice of the substrate result in imperfections of the thin film. Commonly crystallization problems lead to deformations such as wrinkling, clumping, and striations. Deformed thin films are not suitable for most applications.

Lanthanum Aluminate Twinning

Although perfect crystals of lanthanum aluminate make excellent substrates for thin films of certain superconducting materials, creating perfect crystal of lanthanum aluminate is no easy task. Naturally formed crystals of lanthanum aluminate rarely exhibit a perfect crystalline lattice. The ceramic tends to form imperfect, twinned crystals, a phenomenon that is also common when it is synthesized in the laboratory. Twinning, in mineralogy, refers to the patterned intergrowth of crystal grains that are mirror or rotated images of one another. Many types of crystals exhibit twinning, which usually develops at the beginning of crystal growth.

Stair-step twinning of lanthanum aluminate often occurs when the material is cooled to rapidly from the elevated temperatures of annealed crystallite formation. The rapid cooling does not allow the material sufficient time to settle into a flawless crystalline lattice and the imperfection becomes locked into the lanthanum aluminate structure. Some studies indicate that gravity can also have an adverse effect on the crystallization of lanthanum aluminate. As a result, attempts to produce perfect lanthanum aluminate crystals under gravity-free conditions are underway and could help lead to important advances in superconductor production. Perfect lanthanum aluminate substrates are of significant import because superconductors built upon flawed surfaces do not perform in a predictable, consistent manner and are unable to handle strong electric currents. Poorly performing superconductors are of little commercial use.

Stairstep Twinning in Lanthanum Aluminate

In addition to the structural benefits of utilizing lanthanum aluminate as a substrate when it is in its perfect crystalline form, other properties of the material make it well suited for use with superconductors as well. Notably, at superconducting temperatures lanthanum aluminate exhibits a low loss tangent and low dielectric constant. The perovskite also does not appreciably interact with most superconducting materials. Thus it can be successfully utilized as an insulating spacer layer in multilayered superconductors, which have considerable potential uses.

Not only is lanthanum aluminate useful in the superconductor industry, recently the material has been found to show great promise as a gate dielectric in metal oxide semiconductor field-effect transistors (MOSFETs). MOSFETs are the most common field-effect transistors in circuits, both digital and analog. Silicon dioxide is typically used as the gate dielectric in MOSFETs, but as the material approaches the limits of its capacity, alternatives have been very actively sought. Accordingly, lanthanum aluminate research has become even more important to the future of technology.

Perhaps even more critical to humanity is the role that lanthanum aluminate and other perovskites may one day play in environmental clean up. Studies show that the ceramics may provide a means of safely encapsulating radioactive wastes. Research into this area is still in its early stages, but any advances could have a profound impact on the way that radioactive materials are handled by future generations.