Structure of barium titanate
time：2020-11-30
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Barium titanate is a consistent melting compound with a melting point of 1618℃. Below this temperature, barium titanate crystallized above 1460℃ belongs to the non-ferroelectric hexagonal 6/ MMM dot group. At this point, the hexagonal system is stable. Barium titanate is converted into a cubic perovskite structure at 1460~130℃. In this structure, Ti4+(titanium ion) resides in the center of the oxygen octahedron composed of O2-(oxygen ion), while Ba2+(barium ion) is in the void enclosed by eight oxygen octahedron. In this case, the barium titanate crystal structure is extremely symmetrical, so there is no dipole moment, no ferroelectric properties, and no piezoelectric properties.
As the temperature drops, the symmetry of the crystal decreases. When the temperature dropped to 130℃, barium titanate underwent paramagnetic - ferroelectric phase transition. In the temperature range of 130~5℃, barium titanate is a tetragonal 4mm point group, which has significant subway electrical properties. Its spontaneous polarization intensity is in the direction of C axis, namely [001] direction. When barium titanate is transformed from cubic to tetragonal, its structure changes little. From the point of view of the cell, it is only elongated along one axis (C axis) of the protocubic crystal system and shortened along the other two axes.
When the temperature drops below 5℃, the barium titanate crystal is transformed into the orbicular mM2 point group within the temperature range of 5~-90℃. At this time, the crystal is still ferroelectric, and its spontaneous polarization intensity follows the direction of the plane diagonal [011] of the original cubic cell. For convenience, the monoclinic parameters of orthorhombic systems are usually used to describe orthorhombic cells. The advantage of doing this is that we can easily see spontaneous polarization in a single cell. Barium titanate was transformed from tetragonal to orthorhombic, and its structure did not change much. From the point of view of cell, one plane diagonal of the original cubic crystal system lengthened, and the other plane diagonal shortened, while the C axis remained unchanged.
Crystal phase transition
When the temperature continues to drop below -90℃, the crystal changes from the orbicular system to the trigonal system 3m point group. At this point, the crystal is still ferroelectric and its spontaneous polarization strength is parallel to the diagonal direction of the body of the original cubic cell. Barium titanate was transformed from an orthorhombic system to a trigonal system, and its structure did not change much. From the point of view of the cell, one body equivalent to the protocubic cell elongates diagonally, while the other body shrinks diagonally.
To sum up, in the whole temperature region (<1618℃), barium titanate has five crystal structures, namely hexagonal, cubic, quadrangular, orthogonal and tripartite. With the decrease of temperature, the crystal symmetry becomes lower and lower. Above 130 ° C (ie Curie point), barium titanate crystals exhibit paramectric properties, while below 130 ° C they exhibit ferroelectric properties.
As the temperature drops, the symmetry of the crystal decreases. When the temperature dropped to 130℃, barium titanate underwent paramagnetic - ferroelectric phase transition. In the temperature range of 130~5℃, barium titanate is a tetragonal 4mm point group, which has significant subway electrical properties. Its spontaneous polarization intensity is in the direction of C axis, namely [001] direction. When barium titanate is transformed from cubic to tetragonal, its structure changes little. From the point of view of the cell, it is only elongated along one axis (C axis) of the protocubic crystal system and shortened along the other two axes.
When the temperature drops below 5℃, the barium titanate crystal is transformed into the orbicular mM2 point group within the temperature range of 5~-90℃. At this time, the crystal is still ferroelectric, and its spontaneous polarization intensity follows the direction of the plane diagonal [011] of the original cubic cell. For convenience, the monoclinic parameters of orthorhombic systems are usually used to describe orthorhombic cells. The advantage of doing this is that we can easily see spontaneous polarization in a single cell. Barium titanate was transformed from tetragonal to orthorhombic, and its structure did not change much. From the point of view of cell, one plane diagonal of the original cubic crystal system lengthened, and the other plane diagonal shortened, while the C axis remained unchanged.
Crystal phase transition
When the temperature continues to drop below -90℃, the crystal changes from the orbicular system to the trigonal system 3m point group. At this point, the crystal is still ferroelectric and its spontaneous polarization strength is parallel to the diagonal direction of the body of the original cubic cell. Barium titanate was transformed from an orthorhombic system to a trigonal system, and its structure did not change much. From the point of view of the cell, one body equivalent to the protocubic cell elongates diagonally, while the other body shrinks diagonally.
To sum up, in the whole temperature region (<1618℃), barium titanate has five crystal structures, namely hexagonal, cubic, quadrangular, orthogonal and tripartite. With the decrease of temperature, the crystal symmetry becomes lower and lower. Above 130 ° C (ie Curie point), barium titanate crystals exhibit paramectric properties, while below 130 ° C they exhibit ferroelectric properties.
