Langbeinites are a family of crystalline substances based on the structure of
langbeinite
Langbeinite is a potassium magnesium sulfate mineral with the chemical formula K2Mg2(SO4)3. Langbeinite crystallizes in the isometric-tetartoidal (cubic) system as transparent colorless or white with pale tints of yellow to green and violet cry ...
with general formula , where M is a large univalent cation (such as
potassium
Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosph ...
,
rubidium,
caesium, or ammonium), and M' is a small divalent cation (for example,
magnesium
Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
,
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...
,
manganese
Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
,
iron
Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
,
cobalt
Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, p ...
,
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
,
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
,
zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
or
cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of ...
). The sulfate group, , can be substituted by other tetrahedral anions with a double negative charge such as
tetrafluoroberyllate (),
selenate
The selenate ion is .
Selenates are analogous to sulfates and have similar chemistry. They are highly soluble in aqueous solutions at ambient temperatures.
Unlike sulfate, selenate is a somewhat good oxidizer; it can be reduced to selenite o ...
(),
chromate (),
molybdate
In chemistry a molybdate is a compound containing an oxoanion with molybdenum in its highest oxidation state of 6. Molybdenum can form a very large range of such oxoanions which can be discrete structures or polymeric extended structures, althou ...
(), or
tungstate
In chemistry, a tungstate is a compound that contains an oxyanion of tungsten or is a mixed oxide containing tungsten. The simplest tungstate ion is , "orthotungstate". Many other tungstates belong to a large group of polyatomic ions that are ...
s. Although
monofluorophosphates are predicted, they have not been described. By redistributing charges other anions with the same shape such as
phosphate
In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid .
The phosphate or orthophosphate ion is derived from phosph ...
also form langbeinite structures. In these the M' atom must have a greater charge to balance the extra three negative charges.
At higher temperatures the crystal structure is cubic P2
13.
However, the crystal structure may change to lower symmetries at lower temperatures, for example, P2
1, P1, or P2
12
12
1.
Usually this temperature is well below room temperature, but in a few cases the substance must be heated to acquire the cubic structure.
Crystal structure
The crystal structures of langbeinites consist of a network of oxygen vertex-connected tetrahedral
polyanions
Polyelectrolytes are polymers whose repeating units bear an electrolyte group. Polycations and polyanions are polyelectrolytes. These groups dissociate in aqueous solutions (water), making the polymers charged. Polyelectrolyte properties are th ...
(such as sulfate) and distorted metal ion-oxygen octahedra.
[ The unit cell contains four formula units. In the cubic form the tetrahedral anions are slightly rotated from the main crystal axes. When cooled, this rotation disappears and the tetrahedra align, resulting in lower energy as well as lower crystal symmetry.
]
Examples
Sulfates include dithallium dicadmium sulfate, Dirubidium dicadmium sulfate dipotassium dicadmium sulfate, dithallium manganese sulfate. dirubidium dicalcium trisulfate.
Selenates include diammonium dimanganese selenate. A diammonium dicadmium selenate langbeinite could not be crystallised from water, but a trihydrate exists.
Chromate based langbeinites include dicaesium dimanganese chromate.
Molybdates include . Potassium members are absent, as are zinc and copper containing solids, which all crystallize in different forms. Manganese, magnesium, cadmium and some nickel double molybdates exist as langbeinites.
Double tungstates of the form are predicted to exist in the langbeinite form.
An examples with tetrafluroberyllate is dipotassium dimanganese tetrafluoroberyllate (). Other tetrafluoroberyllates may include: ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;; ; ; ;
.
The phosphate containing langbeinites were found in 1972 with the discovery of , and since then a few more phosphates that also contain titanium have been found such as and . By substituting metals in , A from (K, Rb, Cs), and M from (Cr, Fe, V), other langbeinites are made. The NASICON
NASICON is an acronym for sodium (Na) Super Ionic CONductor, which usually refers to a family of solids with the chemical formula Na1+xZr2SixP3−xO12, 0 < x < 3. In a broader sense, it is also used for similar compounds where Na, Zr and/or Si ...
-type structure competes for these kinds of phosphates, so not all possibilities are langbeinites.
Other phosphate based substances include , , , , , . Sodium barium diiron tris-(phosphate) () is yet another variation with the same structure but differently charged ions. Most phosphates of this kind of formula do not form langbeinites, instead crystallise in the NASICON structure with archetype .
A langbeinite with arsenate
The arsenate ion is .
An arsenate (compound) is any compound that contains this ion. Arsenates are salts or esters of arsenic acid.
The arsenic atom in arsenate has a valency of 5 and is also known as pentavalent arsenic or As(V).
Arsenate res ...
is known to exist by way of .
Properties
Physical properties
Langbeinite crystals can show ferroelectric
Ferroelectricity is a characteristic of certain materials that have a spontaneous electric polarization that can be reversed by the application of an external electric field. All ferroelectrics are also piezoelectric and pyroelectric, with the ad ...
or ferroelastic properties. Diammonium dicadmium sulfate identified by Jona and Pepinsky with a unit cell size of 10.35 Å becomes ferroelectric when the temperature drops below 95 K. The phase transition temperature is not fixed, and can vary depending on the crystal or history of temperature change. So for example the phase transition in diammonium dicadmium sulfate can occur between 89 and 95 K. Under pressure the highest phase transition temperature increases. ∂T/∂P = 0.0035 degrees/bar. At 824 bars there is a triple point with yet another transition diverging at a slope of ∂T/∂P = 0.103 degrees/bar. For dipotassium dimanganese sulfate pressure causes the transition to rise at the rate of 6.86 °C/kbar. The latent heat of the transition is 456 cal/mol.
Dithallium dicadmium sulfate was shown to be ferroelectric in 1972.
Dipotassium dicadmium sulfate is thermoluminescent
Thermoluminescence is a form of luminescence that is exhibited by certain crystalline materials, such as some minerals, when previously absorbed energy from electromagnetic radiation or other ionizing radiation is re-emitted as light upon hea ...
with stronger outputs of light at 350 and 475 K. This light output can be boosted forty times with a trace amount of samarium. Dipotassium dimagnesium sulfate doped with dysprosium develops thermoluminescence
Thermoluminescence is a form of luminescence that is exhibited by certain crystalline materials, such as some minerals, when previously absorbed energy from electromagnetic radiation or other ionizing radiation is re-emitted as light upon he ...
and mechanoluminescence Mechanoluminescence is light emission resulting from any mechanical action on a solid. It can be produced through ultrasound, or through other means.
* Fractoluminescence is caused by stress that results in the formation of fractures.
* Piezolum ...
after being irradiated with gamma rays. Since gamma rays occur naturally, this radiation induced thermoluminescence can be used to date evaporite
An evaporite () is a water- soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocean ...
s in which langbeinite can be a constituent.
At higher temperatures the crystals take on cubic form In mathematics, a cubic form is a homogeneous polynomial of degree 3, and a cubic hypersurface is the zero set of a cubic form. In the case of a cubic form in three variables, the zero set is a cubic plane curve.
In , Boris Delone and Dmitry Fa ...
, whereas at the lowest temperatures they can transform to an orthorhombic crystal group. For some types there are two more phases, and as the crystal is cooled it goes from cubic, to monoclinic
In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...
, to triclinic
180px, Triclinic (a ≠ b ≠ c and α ≠ β ≠ γ )
In crystallography, the triclinic (or anorthic) crystal system is one of the 7 crystal systems. A crystal system is described by three basis vectors. In the triclinic system, the crystal i ...
to orthorhombic. This change to higher symmetry on cooling is very unusual in solids. For some langbeinites only the cubic form is known, but that may be because it has not been studied at low enough temperatures yet. Those that have three phase transitions go through these crystallographic point group
In crystallography, a crystallographic point group is a set of symmetry operations, corresponding to one of the point groups in three dimensions, such that each operation (perhaps followed by a translation) would leave the structure of a crystal u ...
s: P213 – P21 – P1 – P212121, whereas the single phase change crystals only have P213 – P212121.
has a transition temperature above room temperature, so that it is ferroelectric in standard conditions. The orthorhombic cell size is a=10.2082 Å, b=10.2837 Å, c=10.1661 Å.
Where the crystals change phase there is a discontinuity in the heat capacity. The transitions may show thermal hysteresis.
Different cations can be substituted so that for example and can form solid solutions for all ratios of thallium and potassium. Properties such as the phase transition temperature and unit cell sizes vary smoothly with the composition.
Langbeinites containing transition metals can be coloured. For example, cobalt langbeinite shows a broad absorption around 555 nm due to the cobalt 4T1g(F)→4T1g(P) electronic transition.
The enthalpy of formation (ΔfHm) for solid at 298.2 K is , and for it is .
Sulfates
Fluoroberyllates
Phosphates
Phosphate silicates
Mixed anion phosphates
Vanadates
The orthovanadates have four formula per cell, with a slightly distorted cell that has orthorhombic symmetry.
Arsenates
Selenates
Langbeinite structured double selenates are difficult to make, perhaps because selenate ions arranged around the dication leave space for water, so hydrates crystallise from double selenate solutions. For example, when ammonia selenate and cadmium selenate solution is crystallized it forms diammonium dicadmium selenate trihydrate: and when heated it loses both water and ammonia to form a pyroselenate rather than a langbeinite.
Molybdates
Tungstates
Preparation
Diammonium dicadmium sulfate can be made by evaporating a solution of ammonium sulfate
Ammonium sulfate (American English and international scientific usage; ammonium sulphate in British English); (NH4)2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen a ...
and cadmium sulfate
Cadmium sulfate is the name of a series of related inorganic compounds with the formula CdSO4·H2O. The most common form is the monohydrate CdSO4·H2O, but two other forms are known CdSO4·H2O and the anhydrous salt (CdSO4). All salts are colourl ...
. Dithallium dicadmium sulfate can be made by evaporating a water solution at 85 °C. Other substances may be formed during crystallisation from water such as Tutton's salts or competing compounds like .
Potassium and ammonium nickel langbeinite can be made from nickel sulfate and the other sulfates by evaporating a water solution at 85 °C.
Dipotassium dizinc sulfate can be formed into large crystals by melting zinc sulfate
Zinc sulfate is an inorganic compound. It is used as a dietary supplement to treat zinc deficiency and to prevent the condition in those at high risk. Side effects of excess supplementation may include abdominal pain, vomiting, headache, and ti ...
and potassium sulfate
Potassium sulfate (US) or potassium sulphate (UK), also called sulphate of potash (SOP), arcanite, or archaically potash of sulfur, is the inorganic compound with formula K2SO4, a white water-soluble solid. It is commonly used in fertilizers, prov ...
together at 753 K. A crystal can be slowly drawn out of the melt from a rotating crucible at about 1.2 mm every hour.
can be made by heating , , , water and hydrochloric acid to 180 °C for eight days under pressure.
converts to on heating to 200 °C.
The sol-gel method produces a gel from a solution mixture, which is then heated. can be made by mixing solutions of , , , and dripping in . The gel produced was dried out at 95 °C and then baked at various temperatures from 400 to 1100 °C.
Langbeinites crystals can be made by the Bridgman technique, Czochralski process or flux technique.
A Tutton's salt may be heat treated and dehydrate, e.g. can be made from heated to 100 °C, forming as a side product. Similarly the ammonium vanadium Tutton's salt, , heated to 160 °C in a closed tube produces . At lower temperatures a hydroxy compound is formed.[
]
Use
Few uses have been made of these substances. Langbeinite itself can be used as an "organic" fertiliser with potassium, magnesium and sulfur, all needed for plant growth. Electrooptic devices could be made from some of these crystals, particularly those that have cubic transition temperatures as temperatures above room temperature. Research continues into this. Ferroelectric crystals could store information in the location of domain walls.
The phosphate langbeinites are insoluble, stable against heat, and can accommodate a large number of different ions, and have been considered for immobilizing unwanted radioactive waste
Radioactive waste is a type of hazardous waste that contains radioactive material. Radioactive waste is a result of many activities, including nuclear medicine, nuclear research, nuclear power generation, rare-earth mining, and nuclear weapons r ...
.
Zirconium phosphate langbeinites containing rare earth metals have been investigated for use in white LEDs and plasma displays.[ Langbeinites that contain bismuth are photoluminescent.][
In case of iron-containing ones complex magnetic behavior may be found.]
References
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Crystals
Ferroelectric materials
*
Crystal structure types