K3Mo2O5.6F3.4 and K3V2O3.3F5.7 - exploring transition metal cation valence and anion distribution in oxyfluorides

Oxyfluorides come in many different structures and are highly adaptable in composition, not least because of their mixed-anionic nature. Slight changes, unless specifically looked for, can easily go unnoticed. In this paper, we present two oxyfluorides, K3Mo2O5.6F3.4 and K3V2O3.3F5.7, synthesized under high-pressure/high-temperature conditions, and demonstrate the importance of careful analysis of composition, oxidation state and O/F anion distribution for an accurate description of oxyfluorides. Their crystal structures were determined by single-crystal X-ray diffraction and the transition metal cation valences analyzed by X-ray photoelectron spectroscopy (XPS). The O/F anion ratio was calculated using the principle of charge neutrality and the local distribution within the crystallographic framework was studied using bond valence (BV) and charge distribution (CHARDI) calculations. Madelung Part of Lattice Energy (MAPLE) calculations and magnetic measurements provide insight into phase stability and corroborate the mixed-valent nature of the compounds.

Experimental and computational study of the exchange interaction between the V(III) centers in the vanadium-cyclal dimer

[V2(HCyclal)2] is prepared by controlled oxidation of vanadium nanoparticles at 50 °C in toluene. The V(0) nanoparticles are synthesized in THF by reduction of VCl3 with lithium naphthalenide. They exhibit very small particle sizes of 1.2 ± 0.2 nm and a high reactivity (e.g. with air or water). By reaction of V(0) nanoparticles with the azacrown ether H4Cyclal, [V2(HCyclal)2] is obtained with deep green crystals and high yield. The title compound exhibits a V(III) dimer (V⋯V: 304.1(1) pm) with two deprotonated [HCyclal]3- ligands as anions. V(0) nanoparticles as well as the sole coordination of V(III) by a crown ether as the ligand and nitrogen as sole coordinating atom are shown for the first time. Magnetic measurements and computational results point to antiferromagnetic coupling within the V(III) couple, establishing an antiferromagnetic spin S = 1 dimer with the magnetic susceptibility determined by the thermal population of the total spin ranging from ST = 0 to ST = 2.

Salt-flux growth of HoCuMg4 single crystals

Polycrystalline samples of the magnesium-rich intermetallic compounds RECuMg₄ (RE = Dy, Ho, Er, Tm) were synthesized by reaction of the elements in sealed tantalum ampoules heated in a high-frequency induction furnace. Phase purity of the RECuMg₄ phases was ascertained by powder X-ray diffraction patterns. Well-shaped single crystals of HoCuMg₄ could be grown in a NaCl/KCl salt flux and the crystal structure was refined from single crystal X-ray diffraction data: TbCuMg₄ structure-type, space group Cmmm, a = 1361.4(2), b = 2039.3(4), c = 384.62(6) pm. The crystal structure of the RECuMg₄ phases can be understood as a complex intergrowth variant of CsCl and AlB₂ related slabs. The remarkable crystal chemical motif concerns the orthorhombically distorted bcc-like magnesium cubes with Mg-Mg distances ranging from 306 to 334 pm. At high temperatures DyCuMg₄ and ErCuMg₄ are Curie-Weiss paramagnets with paramagnetic Curie-Weiss temperatures of -15 K and -2 K for RE = Dy and Er, respectively. The effective magnetic moments, 10.66μ_B for RE = Dy and 9.65μ_B for RE = Er prove stable trivalent ground states for the rare earth cations. Magnetic susceptibility and heat capacity measurements reveal long-range antiferromagnetic ordering at low temperatures (<21 K). Whereas DyCuMg₄ exhibits two subsequent antiferromagnetic transitions at T_N = 21 and 7.9 K which successively remove half of the entropy of a doublet crystal field ground state of Dy, ErCuMg₄ shows a single, possibly broadened, antiferromagnetic transition at 8.6 K. The successive antiferromagnetic transitions are discussed with respect to magnetic frustration in the tetrameric units present in the crystal structure.

Trimorphic TaCrP – A diffraction and 31P solid state NMR spectroscopic study

The metal-rich phosphide TaCrP forms from the elements by step-wise solid state reaction in an alumina crucible (maximum annealing temperature 1180 K). TaCrP is trimorphic. The structural data of the hexagonal ZrNiAl high-temperature phase (space group P 6 ‾ 2 m $P\overline{6}2m$ ) was deduced from a Rietveld refinement. At room temperature TaCrP crystallizes with the TiNiSi type (Pnma, a = 623.86(5), b = 349.12(3), c = 736.78(6) pm, wR = 0.0419, 401 F 2 values, 20 variables) and shows a Peierls type transition below ca. 280 K to the monoclinic low-temperature modification (P121/c1, a = 630.09(3), b = 740.3(4), c = 928.94(4) pm, β = 132.589(5)°, wR = 0.0580, 1378 F 2 values, 57 variables). The latter phase transition is driven by pairwise Cr–Cr bond formation out of an equidistant chain in o-TaCrP. The phase transition was monitored via different analytical tools: differential scanning calorimetry, powder synchrotron X-ray diffraction, magnetic susceptibility measurements and 31P solid state NMR spectroscopy.

Magnesium-rich intermetallic compounds Gd5Cu5Mg13 and Tb5Cu5Mg13 – intergrowth variants with CsCl and AlB2 related slabs

The magnesium-rich intermetallic compounds Gd5Cu5Mg13 and Tb5Cu5Mg13 were obtained from direct reactions of the elements (induction melting) in sealed tantalum ampoules. Both compounds crystallize with the orthorhombic Y5Cu5Mg13 type structure, space group Cmcm and Z = 4. The polycrystalline samples were characterized by powder X-ray diffraction. The structure of the gadolinium compound was refined from single crystal X-ray diffraction data: a = 414.78(2), b = 1921.87(12), c = 2573.89(16) pm, wR2 = 0.0492, 1611 F 2 values and 77 variables. Refinement of the occupancy parameters revealed a small degree of Gd/Mg mixing for the Gd3 site, leading to the composition Gd4.93(1)Cu5Mg13.07(1) for the studied crystal. The Gd5Cu5Mg13 structure contains slabs of equiatomic GdCuMg, which are embedded in a magnesium matrix. From a geometrical point of view, one can describe the Gd5Cu5Mg13 and Tb5Cu5Mg13 structures as intergrowth variants of distorted W/CsCl and AlB2 related slabs. The most remarkable crystal chemical feature concerns the bcc like magnesium slabs with short Mg–Mg distances ranging from 300 to 342 pm. Temperature dependent magnetic susceptibility measurements show Curie-Weiss paramagnetism for Tb5Cu5Mg13 (10.5(1) μ B Tb atom−1 and Θ P = −11.6(1) K). Antiferromagnetic ordering was detected below the Néel temperatures of T N = 30.5(3) K.

TiNiSi-type YbIrMg with trivalent ytterbium

YbIrMg was obtained from a reaction of the elements in an equiatomic ratio in a sealed tantalum ampoule in a muffle furnace. The TiNiSi-type structure of YbIrMg was refined from single crystal X-ray diffractometer data: Pnma, a = 695.72(3), b = 405.84(6), c = 840.57(7) pm, wR = 0.0431, 511 F 2 values and 21 variables. A small degree of Mg/Ir mixing within the three-dimensional [IrMg] network leads to the refined composition YbIr1.024(4)Mg0.976(4). The course of the cell volume in the REIrMg series (Iandelli plot) points to trivalent ytterbium in YbIrMg.

Magnesium and barium in two substructures: BaTMg2 (T = Pd, Ag, Pt, Au) and the isotypic cadmium compound BaAuCd2 with MgCuAl2 type structure

The intermetallic barium compounds BaTMg2 (T = Pd, Ag, Pt, Au) and BaAuCd2 were synthesized by reactions of the elements in sealed tantalum ampoules in muffle furnaces. The five compounds crystallize with the orthorhombic MgCuAl2 type structure, space group Cmcm, with small differences in chemical bonding between the magnesium and cadmium series. All samples were characterized through their Guinier powder diffraction patterns. The structures of BaPdMg2 (a = 444.57(4), b = 1174.67(10), c = 827.58(7) pm, wR2 = 0.0460, 475 F 2 values, 16 variables), BaAuMg2 (a = 450.27(6), b = 1183.94(16), c = 838.76(11) pm, wR2 = 0.0355, 473 F 2 values, 16 variables) and BaAuCd2 (a = 463.31(5), b = 1112.79(12), c = 826.63(8) pm, wR2 = 0.0453, 469 F 2 values, 16 variables) were refined from single crystal X-ray diffraction data. The large barium atoms push the [TMg2] respectively [AuCd2] substructures apart. This allows fast moisture attack and leads to fast hydrolyzes of the samples when they get in contact with water. The influence of the difference in electronegativity between magnesium and cadmium is reflected for the pair of compounds BaAuMg2 and BaAuCd2. The magnesium compound shows the higher auridic character, while the cadmium compound shows a tendency towards a three-dimensional cadmium substructure.

bcc superstructures: RE2RuIn with RE = Sc, Y, Dy-Tm and Lu

The rare earth-rich intermetallic phases RE₂RuIn with RE = Sc, Y, Dy-Tm and Lu were synthesized by reactions of the elements in sealed tantalum ampoules in an induction furnace. The samples were characterized through Guinier powder patterns and the structures of Sc₂RuIn and Er₂RuIn were refined from single crystal X-ray diffraction data. The indides crystallize with the Pt₂ZnCd type space group P4/mmm. The RE₂RuIn phases are superstructures of the bcc packing and can be explained as intergrowth variants of tetragonally distorted, CsCl derived slabs of compositions RERu and REIn. Chemical bonding is discussed for Sc₂RuIn and Sc₂RuMg in comparison with the binaries ScRu, ScMg and ScIn. The Ru/Mg respectively Ru/In ordering leads to an increase of Sc-Sc bonding for the slab with the shorter Sc-Sc distances, while the Sc-Ru bond strength values remain similar. The strongest bonding interactions occur within the magnesium and indium square nets. Magnetic susceptibility measurements reveal Pauli paramagnetism for Lu₂RuIn while Dy₂RuIn, Ho₂RuIn, Er₂RuIn and Tm₂RuIn are Curie-Weiss paramagnets. Antiferromagnetic ordering occurs at 13.1, 5.3 and 2.9 K for Dy₂RuIn, Er₂RuIn and Tm₂RuIn, respectively. Dy₂RuIn and Er₂RuIn show metamagnetic transitions at critical fields of 4.6 and 3.2 T.

Lu26 T 17–x In x (T = Rh, Ir, Pt) – first indium intermetallics with Sm26Co11Ga6-type structure

Several Lu26 T 17−x In x (T = Rh, Ir, Pt) phases with varying x values were obtained by induction-melting reactions of the elements in sealed tantalum ampoules. All samples were characterized through their X-ray powder diffraction patterns. These compounds crystallize with the tetragonal Sm26Co11Ga6-type structure, space group P4/mbm and Z = 2. The structure of Lu26Pt7.55In9.45 has been refined from single-crystal X-ray diffractometer data: a = 1165.32(5), c = 1547.46(6) pm, wR2 = 0.0895, 2173 F 2 values and 70 variables. The Lu26 T 17−x In x phases are the first indium representatives of the Sm26Co11Ga6 type. They belong to the n = 4 members of Parthé’s A 5n+6 B 3n+5 series, which show different stacking sequences of square prisms and antiprisms. Lu26Pt7.55In9.45 as a lutetium-rich intermetallic compound exhibits a broader range of shorter Lu–Lu distances (341–375 pm). Temperature dependent magnetic susceptibility measurements show Pauli paramagnetism for Lu26Ir8.5In8.5 and Lu26Rh7In10 with room temperature values of the magnetic susceptibility of χ = 1.53 × 10−3 emu mol−1 for Lu26Rh7In10 and χ = 6.54 × 10−4 emu mol−1 for Lu26Ir8.5In8.5.

Intermetallic compounds RE 2Ga2Mg (RE = Tb–Tm, Lu) with Mo2B2Fe-type structure

The rare earth intermetallic compounds RE 2Ga2Mg with RE = Tb–Tm and Lu were synthesized from the elements in sealed tantalum ampoules in a high-frequency furnace. These rare earth-rich phases crystallize with the tetragonal Mo2B2Fe-type structure, space group P4/mbm and Z = 2. The polycrystalline samples were characterized through their Guinier powder patterns. The structures of Er2Ga2.092(1)Mg0.908(1), Tm2Ga2.037(1)Mg0.963(1) and Lu2Ga2.176(1)Mg0.824(1) have been refined from single crystal X-ray diffractometer data. The refinements revealed small homogeneity ranges (small degrees of Mg/Ga mixing on the 2a sites). The magnesium atoms show square planar coordination by Ga2 dumbbells (282 pm Mg–Ga and 257 pm Ga–Ga in the lutetium compound). Geometrically one can describe the RE 2Ga2Mg phases as 1:1 intergrowth structures of CsCl and AlB2-related slabs of compositions REMg and REGa2. From DFT based calculations, charge transfer from the rare earth and magnesium atoms towards gallium can be illustrated in electron localization function ELF slice planes showing strong localization around gallium in the basal plane as well as along the tetragonal c axis signaling Ga–Ga pair interactions. The site-projected density of states DOS and COOP data further quantify this observation. Temperature dependent magnetic susceptibility measurements show Pauli paramagnetism for Sc2Ga2Mg and Lu2Ga2Mg with low room temperature susceptibility values of 2.1(1) × 10−4 and 1.1(1) × 10−4 emu mol−1, respectively. Ho2Ga2Mg, Er2Ga2Mg and Tm2Ga2Mg are Curie-Weiss paramagnets with stable trivalent rare earth ground states. Antiferromagnetic ordering was detected below the Néel temperatures of T N = 18.6(1) (RE = Ho), 11.9(1) (RE = Er) and 6.4(1) K (RE = Tm). The three compounds show metamagnetic transitions in their 3 K magnetization isotherms. Tm2Ga2Mg exhibits a square loop behavior with small hysteresis.

Tm4IrIn and Lu4PtIn – In4 tetrahedra embedded in RE 22 polyhedra

The rare earth-rich indides Tm4IrIn and Lu4PtIn were synthesized by reaction of the elements in sealed tantalum ampules in an induction furnace. Tm4IrIn (a = 1340.77(4) pm) and Lu4PtIn (a = 1338.0(1) pm) crystallize with the Gd4RhIn-type structure, space group F 4 ‾ 3 m $F&#x203e;{4}3m$ . The Lu4PtIn structure was refined from single crystal X-ray diffractometer data: wR = 0.0524, 517 F 2 values and 20 variables. The striking crystal chemical motif is the fcc packing of In4 tetrahedra with 318 pm In–In. The Lu4PtIn structure is closely related to the structures of Lu13Ni6In, Lu14Pd3In3 and Lu20Ir5In3 which all show icosahedral indium coordination and different condensation patterns that build up the indium substructure that consists of a dumbbell in Lu14Pd3In3 and a triangle in Lu20Ir5In3. The results of magnetic susceptibility measurements indicate Curie-Weiss paramagnetism for Tm4IrIn (7.76(1) µB per thulium atom) without magnetic ordering down to 2.5 K. Lu4PtIn is Pauli-paramagnetic.

EuTMg2 (T = Pd, Ag, Ir, Pt, Au), EuTCd2 (T = Pd, Pt, Au) and CaRhMg2 – intermetallic compounds with orthorhombically distorted tetrahedral magnesium (cadmium) substructures

The magnesium- and cadmium-rich intermetallic phases EuTMg2 (T = Rh, Pd, Ag, Ir, Pt, Au), EuTCd2 (T = Pd, Pt, Au) and CaRhMg2 were synthesized from the elements in sealed niobium or tantalum ampoules and with heat treatments in muffle or induction furnaces. The samples were characterized by powder X-ray diffraction and the structures were refined from single crystal X-ray diffractometer data. EuTMg2 (T = Pd, Ag, Pt, Au) and EuTCd2 (T = Pd, Pt, Au) crystallize with the MgCuAl2 type, space group Cmcm, while EuRhMg2, EuIrMg2 and CaRhMg2 adopt the YSiPd2 type, space group Pnma. The striking crystal chemical motif of both series of compounds are networks of puckered Mg(Cd) hexagons in ABAB stacking sequence that derive from the aristotype AlB2; however, with different tiling. Temperature dependent magnetic susceptibility and 151Eu Mössbauer spectroscopic measurements indicate stable divalent europium. Antiferromagnetic ordering sets in at 20.2 (EuIrMg2), 22.3 (EuPdMg2), 21.3 (EuAgMg2), 10.9 (EuPdCd2) and 15.5 K (EuPtCd2), respectively. The stable antiferromagnetic ground states are substantiated by metamagnetic transitions. The 151Eu isomer shifts show a linear correlation with the valence electron count for the whole series of EuTMg2, EuTCd2, EuTIn2 and EuTSn2 phases.

A europium kagome lattice in the solid solution Eu3−х Sr х Pt4Zn12 – first zinc representatives of the Gd3Ru4Al12 type

Eu3Pt4Zn12 and Sr3Pt4Zn12 form a complete solid solution Eu3−x Sr x Pt4Zn12. Samples with x = 0, 0.5, 1, 1.5, 2, 2.5 and 3 were synthesized from the elements in sealed tantalum ampoules in an induction furnace. All samples were characterized by powder X-ray diffraction and the structures of Sr3Pt3.93Zn12.07, Eu1.80Sr1.20Pt4Zn12 and Eu3Pt3.68Zn12.32 were refined from single crystal X-ray diffractometer data. The new compounds are isotypic with Gd3Ru4Al12, space group P63/mmc. The striking building units in these phases are the kagome networks occupied by the europium and strontium atoms and Pt1@Zn8 and Pt2@Zn8 distorted cubes. Besides the Eu/Sr mixing within the solid solution, the structure refinements indicated small homogeneity ranges induced by Pt/Zn mixing. The europium containing samples of the solid solution Eu3−x Sr x Pt4Zn12 are Curie–Weiss paramagnets and the experimental magnetic moments manifest stable divalent europium. The samples with x = 0, 0.5 and 2 order magnetically: T N = 15.4(1) K for x = 0, T C = 12.4(1) K for x = 0.5 and T N = 4.0(1) K for x = 2. The 3 K magnetization isotherms tend toward Brillouin type behavior with increasing europium dilution. The divalent ground state of Eu3Pt4Zn12 is further confirmed by 151Eu Mössbauer spectroscopy with an isomer shift of −9.66(2) mm s−1 at 78 K. In the magnetically ordered state Eu3Pt4Zn12 shows full magnetic hyperfine field splitting (23.0(1) T).

The stannides Ca1.692Pt2Sn3.308, SrPtSn2 and EuAuSn2

Polycrystalline samples of the stannides Ca1.692Pt2Sn3.308, SrPtSn2 and EuAuSn2 were synthesized directly from the elements, using sealed tantalum ampoules as crucible material. The reactions were performed in muffle or induction furnaces. The phase purity of the samples was studied by X-ray powder diffraction (Guinier technique). The structures of Ca1.692Pt2Sn3.308 and SrPtSn2 were refined from single-crystal X-ray diffractometer data: NdRh2Sn4 type, Pnma, a = 1887.22(13), b = 441.22(3), c = 742.89(4) pm, wR = 0.0626, 1325 F 2 values, 45 variables for Ca1.692(8) Pt2Sn3.308(8) and CeNiSi2 type, Cmcm, a = 462.59(5), b = 1932.8(2), c = 458.00(5) pm, wR = 0.0549, 481 F 2 values, 18 variables for SrPtSn2. The calcium compound shows a homogeneity range Ca1+x Pt2Sn4−x with substantial Sn4/Ca2 mixing on one of the 4c Wyckoff positions. The [PtSn2] network is characterized by Pt–Sn (269–281 pm) and Sn–Sn (306–336 pm) bonding interactions. SrPtSn2 contains two different tin substructures: (i) Sn1–Sn1 zig-zag chains (282 pm) and (ii) orthorhombically distorted Sn2 squares (326 pm) with stronger and weaker Sn–Sn bonding. Together, the platinum and tin atoms build up a three-dimensional [PtSn2] network in which the platinum atoms have a distorted square-pyramidal tin coordination with Pt–Sn distances ranging from 261–270 pm. EuAuSn2 also crystallizes with the CeNiSi2-type structure with the lattice parameters a = 453.9(1), b = 2018.9(5) and c = 456.8(1) pm. Temperature dependent magnetic susceptibility studies indicate europium(II) with an experimental magnetic moment of 8.28(2) µB per Eu atom. EuAuSn2 is ordered antiferromagnetically at T N  = 14.8(2) K. 151Eu Mössbauer spectra confirm the oxidation state +2 for europium (isomer shift δ = −11.17(2) mm s−1) and the magnetic ordering at low temperature (21.8 T magnetic hyperfine field at 6 K).

The germanides APtGe2 (A = Ca, Sr, Eu)

The germanides APtGe2 (A = Ca, Sr, Eu) were synthesized from the elements in sealed tantalum ampoules in an induction furnace followed by annealing. The polycrystalline samples were characterized by powder X-ray diffraction (Guinier patterns). The structures of CaPtGe2 (CeRhSn2 type, Cmcm, a = 443.45(3), b = 1593.03(12), c = 886.15(6) pm, wR = 0.0464, 673 F 2 values, 30 variables) and EuPt1.043(5)Ge1.957(5) (CeNiSi2 type, Cmcm, a = 445.23(4), b = 1752.5(2), c = 429.63(4) pm, wR = 0.0415, 389 F 2 values, 19 variables) were refined from single crystal X-ray diffractometer data. One of the germanium site in EuPt1.043(5)Ge1.957(5) showed a small Ge/Pt mixed occupancy. SrPtGe2 (a = 451.13(6), b = 1764.8(2), c = 429.60(5) pm) is isotypic with EuPtGe2. The platinum atoms in both germanides have trigonal prismatic coordination: Pt@Ca4Ge2 and Pt@Eu4Ge2. The germanium substructures differ significantly: Ge2 dumb-bells with 246 and 262 pm Ge–Ge distances in CaPtGe2 versus germanium zig-zag chains (253 pm Ge–Ge distances) and a distorted square net (309 pm Ge–Ge distances) in EuPtGe2. CaPtGe2 and SrPtGe2 are diamagnetic. EuPtGe2 is a Curie–Weiss paramagnet with an experimental magnetic moment of 7.88(1) µB Eu atom−1. The purely divalent character of europium is manifested by a single signal at δ = −11.38(1) mm s−1 in the 151Eu Mössbauer spectrum at 78 K. EuPtGe2 orders antiferromagnetically at T N = 5.4(1) K.

Magnesium intermetallics: synthesis and structure of Eu2Pt2Mg and Sr2Pt2Mg

The intermetallic phases Sr2Pt2Mg and Eu2Pt2Mg were obtained by reaction of the elements in sealed tantalum tubes at high temperature. Sr2Pt2Mg crystallizes with the monoclinic Ca2Ir2Si type (C2/c, a = 1020.7(7), b = 597.7(4), c = 827.0(4) pm, β = 103.37(5)°), while Eu2Pt2Mg adopts the orthorhombic W2CoB2-type structure (Immm, a = 440.31(5), b = 582.20(6), c = 914.11(9) pm, wR = 0.0359, 277 F 2 values, 14 variables). The magnesium atoms in both structures are coordinated by four Pt2 dumb-bells with a rectangular planar coordination in Eu2Pt2Mg (268 pm Pt–Mg) and a distorted tetrahedral one in Sr2Pt2Mg (273–275 pm Pt–Mg). The Pt–Pt distances are 277 pm in the europium and 269 pm in the strontium compound. The polyanionic [Pt2Mg] units are planar in Eu2Pt2Mg and separated by the europium atoms. The Sr2Pt2Mg structure shows the motif of hexagonal rod packing for the [Pt2Mg] rows that are embedded in a strontium matrix. Chemical bonding and the influence of the valence electron count on the formation of the structure types are discussed.