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Langbeinites
Langbeinites are a family of crystalline substances based on the structure of langbeinite with basic formula M2M'2(SO4)3. M is a large univalent cation such as potassium, rubidium, caesium, or ammonium). M' is a small divalent cation for example (magnesium, calcium, manganese, iron, cobalt, nickel, copper, zinc or cadmium). The sulfate group SO42− can be substituted by other tetrahedral anions with a double negative charge such as tetrafluoroberyllate BeF42−, selenate (SeO42−), chromate (CrO42−), molybdate (MO4), or tungstates. Although monofluorophosphates are predicted, they have not been described.
At higher temperatures the crystal structure is cubic P213.〔 However the crystal structure may change to lower symmetries at lower temperatures, for example P21 P1 P212121.〔 Usually this temperature is well below room temperature, but in a few cases the substance must be heated to have the cubic structure.
==Crystal structure==
The crystal structure of langbeinites is formed by a skeleton of tetrahedron shaped anions (such as the sulfate) held together by the other ions. In the basic formula M2M'2(XO4)3 The XO4 units are the tetrahedral anions. There are also two octahedral sites filled with M' atoms, and two nine-coordinated sites with the M atoms.〔 The unit cell contains four of the formula. In the cubic form the tetrahedron anions have slightly random orientations. If cooled the randomness ceases and the ions adopt more coherent orientations with lower energy, changing the crystal to a lower symmetry.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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