Transformation of Ammonium Dicyanamide into Dicyandiamide in the Solid

Ammonium dicyanamide NH4[N(CN)2] was synthesized through aqueous ion exchange. The crystal structure was investigated by single-crystal X-ray diffraction (P21/c, a = 378.67(6) pm, b = 1240.9(3) pm, c = 911.84(14) pm, β = 91.488(18)°, Z = 4). It derives from the CsCl structure type. Medium strong hydrogen bonds between NH4+ and [N(CN)2]- ions are indicative of the observed formation of dicyandiamide H4C2N4 during heating. According to DSC and temperature-dependent X-ray powder diffractometry, this isomerization is exothermic and occurs between 102 and 106°C in the solid. The reaction represents the isolobal analogue to the classical synthesis of urea by heating NH4OCN. While other alkali and alkaline earth dicyanamides undergo trimerization or polymerization of their anions during heating, ammonium dicyanamide thus shows a different reactivity

On new alkaline‐earth hexafluorogermanates as host structures for UV phosphors

Two new hexafluorogermanate dihydrates, viz. SrGeF6 ⋅ 2H2O and CaGeF6 ⋅ 2H2O, were synthesised from aqueous solutions; their crystal structures were refined in space group P21/n (SrGeF6 ⋅ 2H2O: a=5.9605(2) Å, b=9.6428(3) Å, a=10.9866(3) Å and β=99.1590(10), Z=4, 5122 refl., 104 param., R1=0.0195, wR2=0.0470; CaGeF6 ⋅ 2H2O: a=5.8472(5) Å, b=10.5099(9) Å, c=9.6267(9) Å and β=103.521(3), Z=4, 4756 refl., 101 param., R1=0.0224, wR2=0.0616). Upon doping with Eu2+ luminescence in the NUV regime was observed. The crystal structures of CaGeF6 and MgGeF6 could be solved and refined via Rietveld refinement from powder samples gained by thermal decomposition of the respective hydrates; both compounds adopt the LiSbF6 structure type (CaGeF6: a=5.4099(5) Å, c=13.9835(13) Å, Z=3, Rwp=0.0291, RBragg=0.0142; MgGeF6: a=5.1219(2) Å, c=13.0851(7), Z=3, Rwp=0.034, RBragg=0.01). Further, the luminescence of MgGeF6 ⋅ 6H2O:Eu2+, which emits light in the violet to blue regime, is reported

Silicate-analogous borosulfates featuring promising luminescence and frequency-doubling (SHG) properties based on a rich crystal chemistry

Our contribution adresses important features of the emerging compound class of silicate-analogous borosulfates, i.e. their rich crystal chemistry, their exciting optical properties and of course their syntheses – the chemistry behind. Silicate-analogous materials comprise tetrahedral anionic basic building units lacking an inversion centre enhancing the chance of non-centrosymmetric surroundings of metal ions promoting excellent optical properties. Since the very first characterization of crystalline borosulfates in 2012 over sixty members have been found. Therein, the reaction of boric and sulfuric acid yields supertetrahedral BX4 (X=SO4) moieties giving rise to a rich crystal chemistry from non-condensed [B(SO4)4]5– anions via band (see Fig.) and layer structures to anionic frameworks [B(SO4)2]– – which can be understood by principles well known from silicates (see Fig.). The selective synthesis of borosulfates can be challenging but we meanwhile found some basic principles helping to selectively synthesize new compounds as phase-pure samples. Great impact is ascribed to the nature of the boron source, the metal (salt) employed and the amount of oleum added. On one hand, borosulfates feature a low coordination strength which is beneficial for the luminescence and UV-Vis properties of compounds containing lanthanide and transition metal ions, such as Ce3+ (see Fig.), Eu3+, Tb3+ or Co2+ and Ni2+. On the other hand, borosulfates frequently adopt non-centrosysmmetric structures yielding optical properties like SHG (second harmonic generation) which – in combination with large band-gaps – makes them highly promising materials for frequency doubling in the high energy regime. Also ionic conductivity was observed recently

High-temperature synthesis, crystal structure, optical properties, and magnetism of the carbidonitridosilicates Ho2[Si4N6C] and Tb2[Si4N6C]

The novel carbidonitridosilicates Ho2[Si4N6C] and Tb2[Si4N6C] were obtained by the reaction of the respective lanthanoid metal with carbon and Si(NH)2 in a radiofrequency furnace at the temperature of 1700°C. According to the single-crystal structure analysis of Ho2[Si4N6C] (P21/c, Z = 4, a = 593.14(1), b = 989.74(1), c = 1188.59(2) pm, β = 119.58(4)°, R1 = 0.0355, wR2 = 0.0879, 2187 F2 values, 119 parameters) the compound contains a condensed network of corner-sharing star like [C(SiN3)4] units. The holmium ions are situated in channels along [100]. The UV–VIS absorption spectrum of Ho2[Si4N6C] shows the typical Ho3+ absorption bands. The spectroscopic results show that the 4f states remain almost unaffected by the coordination sphere and thus it is impossible to distinguish between the two crystallographic sites of Ho3+ in the UV–VIS spectrum. Magnetic susceptibility measurements of Tb2[Si4N6C] and Ho2[Si4N6C] show Curie–Weiss behaviour above 150 K with experimental magnetic moments of 9.57(6) μB/Tb and 10.27(4) μB/Ho. The Weiss constants are −15(1) K and −11(1) K for the terbium and holmium compounds, respectively. Down to 2 K no magnetic ordering could be detected. The magnetization curves at 2 K show an increase of the magnetization with increasing flux density, indicating partial parallel spin alignments. At 5 T the magnetizations reach values of 4.15(5) μB/Tb and 4.75(5) μB/Ho, respectively

Cd[B2(SO4)4] and H2[B2(SO4)4] – a phyllosilicate-analogous borosulfate and its homeotypic heteropolyacid

Borosulfates consist of heteropolyanionic networks of corner-shared (SO4)- and (BO4)-tetrahedra charge compensated by metal or non-metal cations. The anionic substructures differ significantly, depending on the different branching of the silicate-analogous borosulfate building blocks. However, only one acid has been characterized by single crystal X-ray diffraction so far. Herein, we present H-2[B-2(SO4)(4)] as the first phyllosilicate analogue representative, together with the homeotypic representative Cd[B-2(SO4)(4)]. The latter can be considered the cadmium salt of the former. Their crystal structures and crystallographic relationship are elucidated. For H-2[B-2(SO4)(4)], the bonding situation is examined using Hirshfeld-surface analysis. Further, the optical and thermal properties of Cd[B-2(SO4)(4)] are investigated by FTIR and UV-Vis spectroscopy, thermogravimetry, as well as temperature-programmed powder X-ray diffraction

Synthesis, crystal structures and spectroscopic properties of pure YSb2O4Br and YSb2O4Cl as well as Eu3+- and Tb3+-doped samples

The quaternary halide-containing yttrium(iii) oxidoantimonates(iii) YSb(2)O(4)Cl and YSb(2)O(4)Br were synthesised through solid-state reactions from the binary components (Y(2)O(3), Sb(2)O(3) and YX(3), X = Cl and Br) at 750 °C in evacuated fused silica ampoules with eutectic mixtures of NaX and CsX (X = Cl and Br) as fluxing agents. YSb(2)O(4)Cl crystallizes tetragonally in the non-centrosymmetric space group P42(1)2 with unit-cell parameters of a = 773.56(4) pm and c = 878.91(6) pm, whereas YSb(2)O(4)Br is monoclinic (space group: P2(1)/c) with a = 896.54(6) pm, b = 780.23(5) pm, c = 779.61(5) pm and β = 91.398(3)°, both for Z = 4. The two new YSb(2)O(4)X compounds contain [YO(8)](13−) polyhedra, which are connected via four common edges to form [Image: see text] layers (d(Y(3+)–O(2−)) = 225–254 pm) without any Y(3+)⋯X(−) bonds (d(Y(3+)⋯X(−)) > 400 pm). Moreover, all oxygen atoms belong to ψ(1)-tetrahedral [SbO(3)](3−) units, which are either connected to four-membered rings [Sb(4)O(8)](4−) in the chloride (Y(2)[Sb(4)O(8)]Cl(2) for Z = 2) or endless chains in the bromide (Y(1/2)(SbO(2))Br(1/2) for Z = 8) by common vertices. With distances of 307 pm in YSb(2)O(4)Cl and 326 pm in YSb(2)O(4)Br there are not even substantial bonding Sb(3+)⋯X(−) (X = Cl and Br) interactions at work. Luminescence spectroscopy on samples doped with trivalent europium and terbium showed an energy transfer from the oxidoantimonate(iii) moieties as the sensitizer in the host structure onto the lanthanoid activators