Revisiting Y2Si2O7 and Y2SiO5 polymorphic structures by Y MAS-NMR spectroscopy

This paper describes the ⁸⁹Y MAS-NMR spectra for all the established polymorphs of Y₂Si₂O₇ (y, α, β, γ and δ) and Y₂SiO₅ (X1 and X2). The combination of our spectroscopic data with the structural information published up to now from diffraction data permits the revision and correction of mistakes which appear in the literature. Finally, the influence of different structural factors, such as yttrium coordination number and Y-O distances on the ⁸⁹Y NMR isotropic chemical shift is analyzed.     

Mechanochemical reactions between Ag2O and V2O5 to form crystalline silver vanadates

Solid-state reactions between Ag₂O and V₂O₅ were studied under ball-milling conditions. Single-phase crystalline Ag₄V₂O₇ was formed from the mixture of Ag₂O and V₂O₅ of corresponding (2:1) composition. The main component in the product when the Ag₂O mole fraction is less than V₂O₅ is amorphous AgVO₃, which is crystallized into needle-like α-AgVO₃ in the presence of water. Excess V₂O₅ was hydrated into V₂O₅·nH₂O intercalated with Ag⁺ ions. The mixtures with more than two parts of Ag₂O relative to V₂O₅ are composite materials of Ag₄V₂O₇ and Ag₃VO₄, together with Ag₂O. The crystalline phases in these systems resist attack by water.     

Structural and electrical properties of the 2Bi2O3·3ZrO2 system

Powder mixtures of α-Bi₂O₃ (bismite) and monoclinic m-ZrO₂ (baddeleyite) in the molar ratio 2:3 were mechanochemically and thermally treated with the goal to examine the phases, which may appear during such procedures. The prepared samples were characterized by X-ray powder diffraction, differential scanning calorimetry (DSC), electrical measurements, as well as scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The mechanochemical reaction leads to the gradual formation of a nanocrystalline phase, which resembles δ-Bi₂O₃, a high-temperature Bi₂O₃ polymorph. Isothermal sintering in air at a temperature of 820 °C for 24 h followed by quenching to room temperature yielded a mixture of ZrO₂-stabilized β-Bi₂O₃ and m-ZrO₂ phases, whereas in slowly cooled products, the complete separation of the initial α-Bi₂O₃ and m-ZrO₂ constituents was observed. The dielectric permittivity of the sintered samples significantly depended on the temperature. The sintered and quenched samples exhibited a hysteresis dependence of the dielectric shift, showing that the ZrO₂-doped β-Bi₂O₃ phase possess ferroelectric properties, which were detected for the first time. This fact, together with Rietveld refinement of the β-Bi₂O₃/m-ZrO₂ mixture based on neutron powder diffraction data showed that ZrO₂-doped β-Bi₂O₃ has a non-centrosymmetric structure with as the true space group. The ZrO₂ content in the doped β-Bi₂O₃ and the crystal chemical reasons for the stabilization of the β-Bi₂O₃ phase by the addition of m-ZrO₂ are discussed.     

Phase transitions of LiAlO2 at high pressure and high temperature

This work presents a comprehensive study on phase transitions in LiAlO₂ system at high pressures and temperatures (0.5-5.0 GPa and 300-1873 K, respectively), as well as the phase stability for polymeric phases of LiAlO₂ in the studied P-T space by X-ray diffraction (XRD). Besides the previously described polymorphic hexagonal α-phase, orthorhombic β-phase and tetragonal δ-phase, a possible new phase of LiAlO₂ was observed after the tetragonal γ-LiAlO₂ sample was treated at 5.0 GPa and 389 K. The stable regimes of these high-pressure phases were defined through the observation of coexistence points of the polymeric phases. Our results revealed that LiAlO₂ could experience structural phase transitions from γ-LiAlO₂ to its polymorphs at lower pressures and temperatures compared to the reported results. Hexagonal α-LiAlO₂ with highly (003) preferential orientation was prepared at 5.0 GPa and 1873 K.     

Über fluorit- und tysonitverwandte Ordnungsphasen im System CaF2YF3

A systematic phase study has been made on the CaF₂YF₃ system with particular emphasis on the attainment of equilibrium at low temperatures. Long annealing has produced three ordered phases: Ca₂YF₇ (t), Ca₉Y₅E₃₃ (rhα), and Ca_8?δY_5+δF_31+δ = Ca_7+δY_6?δF_32?δ (rhβ). These phases crystallize as anion-excess fluorite-related superstructures and can be described as members of the homologous series (Ca, Y)_mF_2m+5 with m = 15, 14, and 13. In the fluorine-rich region of the system are found two high-temperature phases: a solid solution with tysonite structure (Z = 2) and a related, monoclinic, ordered phase with the formula Ca₃Y₇F₂₇.     

Magnetic properties of Mn2V2O7 single crystals

Magnetic properties of Mn₂V₂O₇ single crystals are investigated by means of magnetic susceptibility, magnetization, and heat capacity measurements. A structural phase transition of the α-β forms is clearly observed at the temperature range of 200-250 K and an antiferromagnetic ordering with magnetic anisotropy is observed below 20 K. A spin-flop transition is observed with magnetic field applied along the [110] axis of β-Mn₂V₂O₇, of which corresponds to the [001] axis of α-Mn₂V₂O₇, suggesting that the spins of Mn²⁺ ions locate within honeycomb layers which point likely in the [110] direction of β-Mn₂V₂O₇ or the [001] axis of α-Mn₂V₂O₇. However, a rather small jump of magnetization at spin-flop transition suggests a possible partition of crystal to some domains through β-to-α transition on cooling or much complex spin structure in honeycomb lattice with some frustration.     

Transformation of three-connected silicon nets in CaSi2

Up to 40 kbar and 1100°C, CaSi₂ is dimorphic. Trigonal/rhombohedral CaSi₂I (CaSi₂-type structure) with corrugated layers of three-connected Si atoms can be transformed by a high pressure-high temperature treatment into tetragonal CaSi₂II (α-ThSi₂-type structure) with a three-dimensional net of three-connected Si atoms. The silicon net of CaSi₂II is slightly distorted from the topologically simplest tetragonal three-dimensional three-connected net derived on a geometrical basis. In order to correlate crystal chemical with thermochemical data the transformation between both polymorphs of CaSi₂ has been studied at equilibrium and nonequilibrium conditions. The pressure-temperature phase diagram of CaSi₂ has been investigated by X-ray technique in quenched samples. From the slope of the equilibrium line and the change in molar volume the approximate values of the entropy and energy of transformation CaSi₂(I-II) have been determined ΔS = 3.2 e.u., ΔU = 4.9 kcal/mole. Under nonequilibrium conditions the transformation CaSi₂(II-I) yielded ΔH = ?4.2 kcal/mole at 500°C and ambient pressure in a DTA apparatus. Complete transformation of metastable CaSi₂II can be achieved within 5 min at a heating rate of 20°C/min. Due to the relatively high speed of transformation simple structural relations between both polymorphs of CaSi₂ are discussed.     

Synthesis, Structures, and Thermal Expansion of the La2W2−xMoxO9 Series

The La₂W_2−xMo_xO₉ series has been synthesized by the ceramic method. An alternative synthesis using microwave radiation is also reported. La₂W₂O₉ has two polymorphs and the low-temperature phase (α) transforms to the high-temperature form (β) at 1077°C. The influence of the W/Mo substitution in this phase transition has been investigated by DTA. The β structure for x≥0.7 compositions can be prepared as single phase at any cooling rate. The β phase for 0.3≤x≤0.7 compounds can be prepared as single phase by quenching, whereas a mixture of α and β phases is obtained by slow cooling. The W/Mo ratio in both coexisting phases is different with the β-phase having a higher Mo content. The x=0.1 and 0.2 compounds have been prepared as mixtures of phases. The room temperature structure of β-La₂W_1.7Mo_0.3O₉ has been analyzed by the Rietveld method in P2₁3 space group. The final R-factors were R_WP=9.0% and R_F=5.6% with a structure similar to that of β-La₂Mo₂O₉. Finally, the thermal expansion of both types of structures has been determined from a thermodiffractometric study. The thermal expansion coefficients were 2.9×10⁻⁶ and 9.7×10⁻⁶°C⁻¹ for α-La₂W₂O₉ and β-La₂W_1.2Mo_0.8O₉, respectively.     

Synthesis, crystal structure and magnetic characterization of Na2Cu5(Si2O7)2: An inorganic ferrimagnetic chain

The hydrothermal synthesis, crystal structure and magnetic properties of the new copper silicate Na₂Cu₅(Si₂O₇)₂, are reported. The crystal structure was determined through synchrotron powder diffraction data. The unit cell was indexed to a triclinic cell, space group P-1 (n° 2) with unit cell parameters a=5.71075(2) Å, b=7.68266(3) Å, c=7.96742(3) Å, α=64.2972(2)°, β=88.4860(2)° and γ=70.5958(2)° with Z=1. A structural model was obtained through a combination of a direct-space Monte-Carlo approach and Rietveld refinement. The crystal structure contains parallel chains consisting of zig-zag copper dimers and trimers. All silicon atoms are present as part of a [Si₂O₇]⁶⁻ anion that connects the chains; therefore the compound belongs to the sorosilicate mineral family. The magnetic susceptibility was measured and shows a behavior typical of one-dimensional ferrimagnetism, in agreement with the observed structure.     

Facile access to (Z)-alkene-containing diketopiperazine mimetics utilizing organocopper-mediated anti-SN2′ reactions

Regio- and stereoselective anti-S_N2′ alkylation of γ-phosphoryloxy-α,β-unsaturated-δ-lactams with organocopper reagents allowing the preparation of N-alkylated-α,δ-substituted-β,γ-unsaturated-δ-lactams as highly functionalized diketopiperazine mimetics is presented.     

Polymorphism in mercury(I) selenite(IV): preparation, crystal structures of α-, β-and γ-Hg2SeO3, and thermal behavior of the α- and β-modification

Mercury(I) selenite(IV) is polymorphic and crystallizes at least in three modifications, named α-, β-and γ-Hg₂SeO₃. Polycrystalline β-Hg₂SeO₃ was prepared by precipitation of a concentrated mercurous nitrate solution with selenous acid. Hydrothermal treatment of the colorless β-Hg₂SeO₃ powder in demineralized water at 250°C (10 days) yields light-yellow single crystals of α-Hg₂SeO₃ which show the highest density of the three modifications. Colorless needle-shaped single crystals of β-Hg₂SeO₃ and very few single crystals of γ-Hg₂SeO₃ co-crystallize from strongly diluted Hg₂(NO₃)₂ and H₂SeO₃ solutions and were grown by a diffusion technique. All crystal structures were solved and refined from single crystal diffractometer data sets and are based on Hg₂²⁺ dumbbells and trigonal pyramidal SeO₃²⁻ anions as the main building units. A common structural feature of all modifications is the formation of open channels extending parallel to the shortest crystallographic axis. The non-bonding orbitals of the Se^IV atoms are stereochemically active and protrude into the channels. Upon heating in an open system under N₂ atmosphere, both α- and β-Hg₂SeO₃ decompose in a well-separated three-step mechanism. The first step (T > 250°C) involves disproportionation into elementary mercury and α-HgSeO₃ which at ca. 400°C subsequently transforms into β-HgSeO₃. The second step between T = 400 and 500°C is accompanied by a loss of Hg and SeO₂ and the formation of the basic salt Hg₃SeO₆. In the third step, at temperatures between T = 500° and 600°C, this material decomposes completely. Upon heating in a closed system (sealed silica capillaries), β-Hg₂SeO₃ transforms between 320-340°C into the more dense α-Hg₂SeO₃ which on further heating likewise converts into elementary mercury and β-HgSeO₃.     

Crystal structures and photocatalysis of the triclinic polymorphs of BiNbO4 and BiTaO4

The high-temperature polymorphs of two photocatalytic materials, BiNbO₄ and BiTaO₄ were synthesized by the ceramic method. The crystal structures of these materials were determined by single-crystal X-ray diffraction. BiNbO₄ and BiTaO₄ crystallize into the triclinic system P1¯ (No. 2), with a=5.5376(4) Å, b=7.6184(3) Å, c=7.9324(36) Å, α=102.565(3)°, β=90.143(2)°, γ=92.788 (4)°, V=326.21 (5) Å³, Z=4 and a=5.931 (1) Å, b=7.672 (2) Å, c=7.786 (2) Å, α=102.94 (3)°, β=90.04 (3)° γ=93.53 (3)°, V=344.59 (1) Å³ and Z=4, respectively. The structures along the c-axis, consist of layers of [Bi₂O₂] units separated by puckered sheets of (Nb/Ta)O₆ octahedra. Photocatalytic studies on the degradation of dyes indicate selectivity of BiNbO₄ towards aromatics containing quinonic and azo functional groups.     

Phase stabilities of monoclinic oxoborates LaB3O6 and GdB3O6 in C121 and I12/a1 phase—Energetics and chemical bonds derived from first-principles calculations

The computational study on the relative phase stabilities between two monoclinic polymorphs of the lanthanide-containing oxoborates LaB₃O₆ and GdB₃O₆ is presented at the first-principles density functional theory gradient-corrected B3PW level. The hypothetical monoclinic α-BiB₃O₆-like C121 non-centrosymmetric crystal structures were assumed for LaB₃O₆ and GdB₃O₆ and the corresponding geometries were calculated and compared with their monoclinic I12/a1 centrosymmetric structures. The enthalpy-pressure correlations were calculated and the first-principles chemical bonds based on the crystal orbital overlapping population were quantitatively analyzed for LaB₃O₆ and GdB₃O₆. The chemical bonds between the central cation (La (III)/Gd (III)) and coordinated oxygen atoms rather than the B-O bonds in the borate units are found to stabilize the I12/a1 centrosymmetric LaB₃O₆ and GdB₃O₆ structures with respect to the C121 non-centrosymmetric counterparts.     

Phase transitions in Ca3(BN2)2 and Sr3(BN2)2

α-Ca₃(BN₂)₂ crystallizes in the cubic system (space group: ) with one type of calcium ions disordered over of equivalent (8c) positions. An ordered low-temperature phase (β-Ca₃(BN₂)₂) was prepared and found to crystallize in the orthorhombic system (space group: Cmca) with lattice parameters: , , and . Structure refinements on the basis of X-ray powder data have revealed that orthorhombic β-Ca₃(BN₂)₂ corresponds to an ordered super-structure of cubic α-Ca₃(BN₂)₂. The space group Cmca assigned for β-Ca₃(BN₂)₂ is derived from by a group-subgroup relationship.DSC measurements and temperature-dependent in situ X-ray powder diffraction studies showed reversible phase transitions between β- and α-Ca₃(BN₂)₂ with transition temperatures between 215 and 240 °C.The structure Sr₃(BN₂)₂ was reported isotypic with α-Ca₃(BN₂)₂ () with one type of strontium ions being disordered over of equivalent (2c) positions. In addition, a primitive () structure has been reported for Sr₃(BN₂)₂. Phase stability studies on Sr₃(BN₂)₂ revealed a phase transition between a primitive and a body-centred lattice around 820 °C. The experiments showed that both previously published structures are correct and can be assigned as α-Sr₃(BN₂)₂ (, high-temperature phase), and β-Sr₃(BN₂)₂ (, low-temperature phase).A comparison of Ca₃(BN₂)₂ and Sr₃(BN₂)₂ phases reveals that the different types of cation disordering present in both of the cubic α-phases () have a directing influence on the formation of two distinct (orthorhombic and cubic) low-temperature phases.     

Synthesis and structural investigation of the compounds containing HF2 anions: Ca(HF2)2, Ba4F4(HF2)(PF6)3 and Pb2F2(HF2)(PF6)

Three new compounds Ca(HF₂)₂, Ba₄F₄(HF₂)(PF₆)₃ and Pb₂F₂(HF₂)(PF₆) were obtained in the system metal(II) fluoride and anhydrous HF (aHF) acidified with excessive PF₅. The obtained polymeric solids are slightly soluble in aHF and they crystallize out of their aHF solutions. Ca(HF₂)₂ was prepared by simply dissolving CaF₂ in a neutral aHF. It represents the second known compound with homoleptic HF environment of the central atom besides Ba(H₃F₄)₂. The compounds Ba₄F₄(HF₂)(PF₆)₃ and Pb₂F₂(HF₂)(PF₆) represent two additional examples of the formation of a polymeric zigzag ladder or ribbon composed of metal cation and fluoride anion (MF⁺)_n besides PbF(AsF₆), the first isolated compound with such zigzag ladder. The obtained new compounds were characterized by X-ray single crystal diffraction method and partly by Raman spectroscopy. Ba₄F₄(HF₂)(PF₆)₃ crystallizes in a triclinic space group P1¯ with a=4.5870(2) Å, b=8.8327(3) Å, c=11.2489(3) Å, α=67.758(9)°, β=84.722(12), γ=78.283(12)°, V=413.00(3) Å³ at 200 K, Z=1 and R=0.0588. Pb₂F₂(HF₂)(PF₆) at 200 K: space group P1¯, a=4.5722(19) Å, b=4.763(2) Å, c=8.818(4) Å, α=86.967(10)°, β=76.774(10)°, γ=83.230(12)°, V=185.55(14) ų, Z=1 and R=0.0937. Pb₂F₂(HF₂)(PF₆) at 293 K: space group P1¯, a=4.586(2) Å, b=4.781(3) Å, c=8.831(5) Å, α=87.106(13)°, β=76.830(13)°, γ=83.531(11)°, V=187.27(18) Å³, Z=1 and R=0.072. Ca(HF₂)₂ crystallizes in an orthorhombic Fddd space group with a=5.5709(6) Å, b=10.1111(9) Å, c=10.5945(10) Å, V=596.77(10) Å³ at 200 K, Z=8 and R=0.028.     

Crystal chemistry of Co-doped Zn7Sb2O12

Zn₇Sb₂O₁₂ forms a full range of Co-containing α solid solutions, Zn_7−xCo_xSb₂O₁₂, with an inverse-spinel structure at high temperature. At low temperatures for x<2, the solid solutions transform into the low temperature β-polymorph. For x=0, the β→α transition occurs at 1225±25 °C; the transition temperature decreases with increasing x. At high x and low temperatures, α solid solutions are formed but are non-stoichiometric; the (Zn+Co):Sb ratio is >7:2 and the compensation for the deficiency in Sb is attributed to the partial oxidation of Co²⁺ to Co³⁺. From Rietveld refinements using ND data, Co occupies both octahedral and tetrahedral sites at intermediate values of x, but an octahedral preference attributed to crystal field stabilisation, causes the lattice parameter plot to deviate negatively from the Vegard's law. Sub-solidus compatibility relations in the ternary system ZnO-Sb₂O₅-CoO have been determined at 1100 °C for the compositions containing ?50% Sb₂O₅.     

Synthesis, structure, and characterization of hybrid materials: [Ce(H2O)]2[O2C(CH2)2CO2]3 and [Sm(H2O)]2[O2C(CH2)2CO2]3·H2O

The rare-earth dicarboxylate hybrid materials [Ce(H₂O)]₂[O₂C(CH₂)₂CO₂]₃ ([Ce(Suc)]) and [Sm(H₂O)]₂[O₂C(CH₂)₂CO₂]₃·H₂O ([Sm(Suc)]) have been hydrothermally synthesized (200°C, 3 days) under autogenus pressure. [Ce(Suc)] is triclinic, a=7.961 (3) Å, b=8.176 (5) Å, c=14.32 (2) Å, α=97.07° (7), β=96.75° (8), γ=103.73° (6), and z=2. The crystal structure of this compound has been determined using 3120 unique single crystal data. The final refinements let the agreement factors R₁ and wR₂(F²) converge to 0.0138 and 0.0363, respectively. [Ce(Suc)] is built up from infinite chains of edge-sharing nine-fold coordinated cerium atoms running along [100]. These chains are interconnected by the carbon atoms of the succinate anions, leading to a three-dimensional hybrid framework. The cell constants of [Sm(Suc)], isotypic with monoclinic C2/c [Pr(H₂O)]₂[O₂C(CH₂)₂CO₂]₃·H₂O ([Pr(Suc)]), were refined starting from X-ray powder data: a=20.275 (3) Å, b=7.919 (6) Å, c=14.130 (3) Å, and β=121.45° (1). Despite its lower symmetry, [Ce(Suc)] presents an important structural filiation with [Sm(Suc)]     

Effect of metal doping on the low-temperature structural behavior of thermoelectric β-Zn4Sb3

The low-temperature structural phase transitions of Bi, Pb, In and Sn-doped samples of thermoelectric Zn₄Sb₃ have been characterized on crystals grown from molten metal fluxes, using electrical resistance and single crystal X-ray diffraction measurements. Room temperature stable, disordered, β-Zn₄Sb₃ undergoes two phase transitions at 254 and 235 K to the consecutively higher ordered phases α and α′, respectively. The ideal crystallographic composition of α-Zn₄Sb₃ is Zn₁₃Sb₁₀. The α-α′ transformation is triggered by a slight and homogenous Zn deficiency with respect to this composition and introduces a compositional modulation in the α-Zn₄Sb₃ structure. When preparing β-Zn₄Sb₃ in the presence of metals with low melting points (Bi, Sn, In, Pb) the additional metal atoms are unavoidably incorporated in small concentrations (0.04-1.3 at%) and act as dopants. This incorporation alters the subtle balance between Zn disorder and Zn deficiency in Zn₄Sb₃ and has dramatic consequences for its low-temperature structural behavior. From molten metal flux synthesis it is possible to obtain (doped) Zn₄Sb₃ samples which (1) only display a β-α transition, (2) only display a β-α′ transition, or (3) do not display any low-temperature phase transition at all. Case (2) provided diffraction data with a sufficient quality to obtain a structural model for highly complex, compositionally modulated, α′-Zn₄Sb₃. The crystallographic composition of this phase is Zn₈₄Sb₆₅.     

Crystal structure and phase transitions of strontium zirconium diorthophosphate, SrZr(PO4)2

The crystal structure of SrZr(PO₄)₂ at 298 K was determined from conventional X-ray powder diffraction data using direct methods, and it was further refined by the Rietveld method. The structure was triclinic (space group , Z=2) with a=0.77508(4) nm, b=0.78887(5) nm, c=0.51251(3) nm, α=95.754(3)°, β=90.228(2)°, γ=92.474(2)°, and V=0.31149(3) nm³. Final reliability indices were R_wp=8.51%, R_p=6.07%, and R_B=2.46%. The powder specimens were also examined by high-temperature XRD and differential thermal analysis to reveal the occurrence of phase transitions from triclinic to monoclinic at 405 K, then to hexagonal (or trigonal) at 1196 K during heating. Upon cooling, the reverse change of the latter transition occurred at 1175 K. The subsequent monoclinic-to-triclinic transition was martensitic and incomplete during further cooling to 298 K. The monoclinic phase is most probably isostructural with yavapaiite. The present paper has described, for the first time, the higher- and lower-temperature polymorphs of the yavapaiite-type structure.     

Single-crystal growth of Tl2Ru2O7 pyrochlore using high-pressure and flux method

Single crystals of the thallium ruthenium pyrochlore have been grown by flux method under high oxygen pressure. The growth conditions were determined by direct observations using in situ powder X-ray diffraction (XRD) method under high pressure and high temperature. The crystals were grown using NaCl-KCl flux at 1350 °C and B₂O₃ flux at 1150 °C. High growth temperature of 1350 °C for the NaCl-KCl flux caused Pt contamination from the crucible and oxygen deficiency for the crystals obtained. The crystal growth using B₂O₃ flux proceeded at lower temperature by grain growth with material transfer through B₂O₃. The crystal obtained was characterized by single-crystal XRD method, and was found to have a stoichiometric composition, Tl₂Ru₂O_7−δ (δ=0), with a structural phase transition around 120 K. The grain growth technique with B₂O₃ is efficient for high-temperature single-crystal growth under high pressure.