Langbeinites are a family of crystalline substances based on the structure of langbeinite with general formula , where M is a large univalent cation (such as potassium, rubidium, caesium, or ammonium), and M' is a small divalent cation (for example, magnesium, calcium, manganese, iron, cobalt, nickel, copper, zinc or cadmium). The sulfate group, , can be substituted by other tetrahedral anions with a double negative charge such as tetrafluoroberyllate (), selenate (), chromate (), molybdate (), or tungstates. Although monofluorophosphates are predicted, they have not been described. By redistributing charges other anions with the same shape such as phosphate 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 P213. However, the crystal structure may change to lower symmetries at lower temperatures, for example, P21, P1, or P212121. 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 (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, and dirubidium dicalcium trisulfate. Dipotassium dinickel sulfate can also be synthesized.
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-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 is known to exist by way of .
Properties
Physical properties Langbeinite-family crystals can show ferroelectric 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 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 and mechanoluminescence after being irradiated with gamma rays. Since gamma rays occur naturally, this radiation induced thermoluminescence can be used to date evaporites in which langbeinite can be a constituent.
At higher temperatures the crystals take on cubic form, 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, to triclinic 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 groups: 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 + Properties of langbeinites with sulfate anions Formula Weight (g/mol) Comment / Symmetries Transition temperature (K) Density Cell size (Å) Refractive index 1 2 3382.783 phases, 1–2, >3250350575414.994 phases langbeinite5154.963.82.8329.92111.536507.73made3.36710.00511.556602.61no compound372.87Efremovite2412202.499.979745.56≥3 phase227.8330.8430.77made2.72310.16621.525446.544 phases calciolangbeinite4572.69 2.68310.429Å a=10.334 b=10.501 c=10.186Nα=1.522 Nβ=1.526 Nγ=1.527539.282 phases1833.03410.56871.520634.153.41710.72131.549no compound404.42made1582.29710.53601.532colour clear green2.76NIMS search result10.089476.26 manganolangbeiniteBellanca, A. (1947). Sulla simmetria della manganolangbeinite/ Atti Accad. Nazi. Lincei Rend. Classe Sci. Fis. Mat. Nat. 2, 451–455.2 phasespale pink1913.0210.014(orthorhombic)a=10.081, b=10.108, c=10.048 Å1.576569made3.54610.21471.590663.87predicted434.14made2.7210.1908806.83made5.01510.22361.722478.07made?130predicted808.64exists435.95mineral ferroefremovite2.8410.0681.574484.252 phasesdeep purple1263.2809.93131.608576.99made3.80710.02041.602671.87442.13made2.949.997813.82made5.36110.03121.775483.77made light greenish yellow0.741.143.3699.84361.620576.51made3.9219.92171.636671.39predicted441.65made1603.029.904814.34predictedpredictedpredict notpredicted497.14 phases751383.3769.92471.592predictedpredict notpredicted591.212 phases4322.615 3.677a=10.212 b=10.280 c=10.171Nα=1.588 Nγ=1.592683.954 phases661031294.06010.38101.590549.094 phases953.28810.3511921.784 phases921201325.46710.38411.730
Fluoroberyllates + Properties of langbeinites with fluoroberyllate () anion Formula Weight (g/mol) Cell size (Å) Volume Density Comment4 phases transition at 2139.875962.81.599.9681.379.9331.729.9711.919.9972.859.8881.869.9742.199.9933.139.9639881.8210.0521.6110.0612.1410.0783.0510.1152.289.9321.8910.0361.6710.0352.2010.0603.1410.1022.3610.1021.7210.1871.8210.2171.5010.2432.0010.2552.8710.3272.1210.3762.2610.1331.9210.2202.0410.1331.9210.3802.2810.4512.4110.3421.8710.3852.3210.4023.1610.4742.4310.5582.5310.5012.1510.4801.7410.5831.8610.6721.98does not exist
Phosphates + Properties of langbeinites with phosphate () anion Formula Weight (g/mol) Cell size (Å) Density Commentref735.4810.59454.108non-linear optical578.2510.10533.192584.0310.0943.722499.8910.13183.772462.989.80013.267417.719.9384458.849.8688Also ; dark blue, purplish551.589.9115789.419.93869.8379.7759.9033.162dark brown9.8443.233dark brown1113.99÷29.9386437.969.76413.125colourlessK2TiYb(PO4)3481.24NaZr2(PO4)3980,7110.20883.06125negative thermal expansion 25-500 °C594.45...629.310.29668602.9210.1199510.1810.0554dark grey Note is not a langbeinite.543.2410.3346random and 611.5810.3457random and 698.8510.31794.226630.5110.30753.824684.8710.1993750.58510.3310.2417508.789.7989.874110.04609.92110.150988.119.72621063.169.83151074.709.909210.3262986.669.9092795.857842.227889.665603.9910.103508.53556.1675.6523.329.8093.68yellowish697.69.78184.9129.855green9.9223.54at high temperature > 950 °C dark grey9.873greenish yellow9.884grey602.59501.549.8527663.3210.3036503.25 R = La, Nd, Sm, Eu, Gd, Dy, Y666.07525.039.8732 (at 4 K)517.339.7890511.569.8892 has NZP structureK5/3MgTi4/3(PO4)39.836485.519.914 crystallises in kosnarite form555.399.8540 in kosnarite form9.954 does not form601.769.9090 does not formK5/3MgZr4/3(PO4)310.176479.1610.165598.7410.111528.8710.106525.5310.218645.1110.178575.2410.178572.9710.561over 1250 °C forms kosnarite phase10.11291185.589.88254.08yellow-green9.73329.8325beige529.29 (half)10.1572light yellow660.192 (half)10.122014.228yellowlilacPb2CoFe(PO4)39.7933Ba2FeCo(PO4)39.9081orangeRbSrSc2(PO4)310.02879Cs2UIVCe(PO4)3928.8910.85884.819Cs2UIVPr(PO4)3929.6710.83924.849Cs2UIVNd(PO4)3933.0110.82874.88Cs2UIVSm(PO4)3939.1110.80694.942Cs2UIVEu(PO4)3940.7310.78224.985Cs2UIVGd(PO4)3946.0210.77285.026Cs2UIVTb(PO4)3947.6910.75915.054Cs2UIVDy(PO4)3951.2610.74285.096Cs2UIVHo(PO4)3953.7010.72765.131Cs2UIVEr(PO4)3956.0310.71275.165Cs2UIVTm(PO4)3957.6910.68885.209Cs2UIVYb(PO4)3961.8110.67875.246Cs2UIVLu(PO4)3963.7410.66195.282
Phosphate silicates substanceformula weightunit cell edge ÅdensitycommentrefK2Sn2(PO4)2SiO4Stable to 650 °CK2Zr2(PO4)2SiO4Stable to 1000 °CRb2Zr2(PO4)2SiO4Cs2Zr2(PO4)2SiO4CsKZr2(PO4)2SiO4KBaZrY(PO4)2SiO4KBaZrLa(PO4)2SiO4KBaZrNd(PO4)2SiO4KBaZrSm(PO4)2SiO4KBaZrEu(PO4)2SiO4
Mixed anion phosphates substanceformula weightunit cell edge ÅdensitycommentrefK2MgTi(SO4)(PO4)2K2Fe2(MoO4)(PO4)2K2Sc2(MoO4)(PO4)2K2Sc2(WO4)(PO4)2
Vanadates The orthovanadates have four formula per cell, with a slightly distorted cell that has orthorhombic symmetry. formula weightcommentCell dimensions ÅVolumedensityrefractiveFormulag/molsymmetriesabcindex593.08Orthorhombic9.9810.529.519984.02609.13Orthorhombic9.9910.529.5310024.09694.00Orthorhombic10.0210.539.5310054.62
Arsenates substanceformula weightunit cell edge Ådensity658.6210.3927632.55810.5323.379
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. substanceformula weightunit cell edge Ådensitynote574.8310.533.26forms continuous series with too
Molybdates substanceformula weightunit cell edge Ådensityref970.511.239768.7Cs2Fe2(MoO4)310.9112 863.0110.7538627.7510.8713KMn0.25Mg0.25Co0.25Zn0.25InMo3O12Tl1.85K0.15Mg2(MoO4)310.6851Tl1.85Rb0.15Mg2(MoO4)310.69463
Tungstates substanceformula weightunit cell edge Ådensity963.0610.7661057.9310.878
Preparation