Phase Transition of Rubidium Lead Tetrathiophosphate RbPbPS4

The single crystal structure of RbPbPS₄ was determined at 293 K. The compound crystallizes in the orthorhombic space group Pnma (No. 62) with a = 17.486(1) Å, b = 6.7127(5) Å, c = 6.4191(5) Å, V = 753.5(1) ų, Z = 4. Differential scanning calorimetry shows a reversible structural phase transition at 182 K on cooling and at 184 K on heating. The phase transition is attributed to the displacement of the lead atoms which are located on the mirror planes at room temperature. In the low temperature modification, the Pb²⁺ ions are moved away from the mirror plane thus changing the coordination number from seven at low temperature to six at room temperature. The low temperature phase of RbPbPS₄ is non‐centrosymmetric with space group P2₁2₁2₁, which is a maximal translationengleiche subgroup index 2 (t2) of Pnma. The analysis demonstrates that the phase transition is of second order, or at least nearly so.     

Partial ordering of tripivaloylmethane at 110 K

Tripivaloylmethane [systematic name: 4‐(2,2‐dimethylpropanoyl)‐2,2,6,6‐tetramethylheptane‐3,5‐dione], C₁₆H₂₈O₃, is known to crystallize at room temperature in the space group R3m with three molecules in the unit cell. The molecules are conformationally chiral and pack so that each molecular site is occupied with equal probability by the two enantiomers. Upon cooling to 110 K, the structure partially orders; two molecules in the unit cell order into two different conformations of opposite chirality, while the third remains disordered. The symmetry of the resulting crystal is P3, with each of the molecules lying about a different threefold rotation axis. This paper describes an unusual case of order–disorder phase transition in which the structure partially orders by changes of molecular conformation in the single crystals. Such behaviour is of interest in the study of phase transitions and molecular motion in the solid state.     

Composition dependence of the photophysical and photocatalytic properties of (AgNbO3)1−x(NaNbO3)x solid solutions

A series of orthorhombic photocatalysts (AgNbO₃)_1−x(NaNbO₃)_x solid solutions have been synthesized by a solid-state reaction method. The composition dependence of the photophysical and photocatalytic properties of synthesized solid solutions has been investigated systematically. With an increase in the content of NaNbO₃, we found that (1) the lattice parameters decreased; (2) the Nb-O bond length in NbO₆ octahedron reduced; (3) the band gap increased; and (4) the mean particle size decreased while the Brunauer-Emmett-Teller (BET) surface area increased. Photocatalytic activities of the (AgNbO₃)_1−x(NaNbO₃)_x (0?x?0.5) samples were evaluated from gaseous 2-propanol (IPA) decomposition into acetone and CO₂ under visible-light irradiation emitted from blue-light-emitting diodes (BLEDs; light intensity: 0.01 mW cm⁻²). Among all the samples, the (AgNbO₃)_0.6(NaNbO₃)_0.4 sample showed the highest photocatalytic activity.     

Phase transitions and twinned low‐temperature structures of tetraethylammonium tetrachloridoferrate(III)

The title compound, [(C₂H₅)₄N][FeCl₄], has at room temperature a disordered structure in the high‐hexagonal space group P6₃mc. At 230 K, the structure is merohedrally twinned in the low‐hexagonal space group P6₃. The volume has increased by a factor of 9 with respect to the room‐temperature structure. At 170 and 110 K, the structure is identical in the orthorhombic space group Pca2₁ and twinned by reticular pseudomerohedry. The volume has doubled with respect to the room‐temperature structure. All three space groups, viz. P6₃mc, P6₃ and Pca2₁, are polar and the direction of the polar axis is not affected by the twinning. In the P6₃ and Pca2₁ structures, all cations and anions are well ordered.     

The Crystal Structures of the Room Temperature and the Low Temperature Phase of Dimethylammonium Trifluoromethanesulfonate

Dimethylammonium trifluoromethanesulfonate 1 was synthesized by reaction of trifluoromethanesulfonic acid with an excess of dimethylamine. A temperature variable synchrotron measurement on the polycrystalline substance reveals that 1 passes through a phase transition below room temperature. The transition occurs in the temperature range of 282–285 K on heating and 272–280 K on cooling as determined by DSC. The room temperature phase crystallizes in space group Cmca (a = 11.031(6) Å, b = 18.466(14) Å, c = 8.173(9) Å, V = 1665(2) ų, Z = 8) and the low temperature phase in space group P 2₁/c (a = 8.8717(18) Å, b = 8.0838(16) Å, c = 10.968(2) Å, β = 92.128(4)°, V = 786.0(3) ų, Z = 4). The structures of both phases were determined by single crystal X‐ray diffraction, but refinement did not yield satisfactory residuals for the low temperature phase because of twinning of the crystal. It was, therefore, independently solved from the synchrotron powder diffraction data using rigid body models of the constituent ions and ab‐initio direct space methods. Both, the CF₃ group and the SO₃ group of the triflate ion, are rotationally disordered around the S–C bond, in the room temperature phase. In the low temperature phase, the triflate ion is well localized. Like in the alkali metal triflates, the triflate ions are arranged in double layers with the hydrophobic trifluoromethyl groups and the sulfonate groups, respectively, pointing towards each other. The dimethylammonium ion is located closer to the sulfonate group with contacts indicating hydrogen bonding. The packing in both phases is of the topological CsCl structure type.     

Enhanced photocatalytic activity of La and Zr-codoped AgNbO3 for rhodamine B and methylene blue degradation

In this study, AgNbO₃, 1%, 5%, and 10% La, Zr-codoped AgNbO₃ photocatalyst were synthesized by the solid state method. The synthesized AgNbO₃ was codoped with La and Zr to enhance its effectiveness in photocatalytic dye degradation. The as-synthesized materials were also characterized via XRD and SEM analysis to determine their structural and morphological properties. The vibrational bands of the photocatalysts observed in FT-IR spectroscopy confirmed the presence of NbO₆ octahedra which can be found in AgNbO₃. Furthermore, all the photocatalysts showed high crystallinity and are single-phase materials with an orthorhombic crystal structure. It was observed that the particle size decreased with the increasing concentration of La and Zr codopants. The calculated lattice parameters showed that codoping AgNbO₃ with 10% La and Zr had caused its unit cell volume to expand. The UV–Vis absorption spectra showed a slight band gap widening and a decrease in surface plasmonic resonance (SPR) effect with increasing La, Zr-codoping. The photocatalytic RhB and MB degradation results of undoped AgNbO₃ and La, Zr-codoped AgNbO₃ were compared and they showed that there are improvements in the photocatalytic performance. The highest degradation (98.1%) of RhB was achieved by 5% La, Zr-codoped AgNbO₃ while the highest MB degradation (48.3%) was achieved by 1% La, Zr-codoped AgNbO₃. Last but not least, La, Zr-codoped AgNbO₃ is a promising material for water remediation application as it showed enhanced performance for photocatalytic dye degradation under visible light irradiation.     

AgNbO<Subscript>3</Subscript> ceramics synthesized by aqueous solution-gel method

AgNbO₃ powders and ceramics were prepared by aqueous solution-gel method. The phase evolution of the powders was investigated by TG/DSC and XRD. The results showed that the pure AgNbO₃ phase was obtained at 600 °C without special treatment. The sintering behavior and dielectric properties of the AgNbO₃ ceramics were also investigated. It showed the dense ceramics were obtained as lower as 925 °C, which had the excellent dielectric properties with the permittivity of 291 and dielectric loss of about 1.7% at 1 MHz. The coarse grains were observed for the sample sintered over 975 °C, and then they decreased with the sintering temperature further increasing to 1,050 °C.     

Synthesis,Characterization, and Chemical Vapor Transport of Solid Solutions in the MgMoO4‐NiMoO4 System

Two solid solution series exist in the system MgMoO₄‐NiMoO₄. The α‐Ni_1–yMg_yMoO₄ solution series, isostructural to α‐NiMoO₄, is thermodynamically stable at ambient conditions for compositions between 0 % and about 75 % magnesium content. The solution series β‐Mg_1–xNi_xMoO₄, isostructural to MgMoO₄ and the high temperature β modification of NiMoO₄, is thermodynamically stable at ambient conditions for compositions with < 25 % nickel content. A complete solid solution series β‐Mg_1–xNi_xMoO₄ exists at higher temperatures (> 823 K). The transition temperature for the α → β transition decreases with increasing magnesium content. The coexistence of both polymorphs at room temperature in samples with a wide range of composition is a result of the kinetic inhibition of the phase transition β → α. The chemical vapor transport of β‐Mg_1–xNi_xMoO₄ solid solutions with chlorine was investigated. Crystals with a nickel content up to 25 % were synthesized in temperature gradients 1273 K → 1223 K or 1273 K → 1173 K. Deposited nickel richer crystals are destroyed during cooling down to room temperature due to the phase transition. The observed distinctive nickel enrichment during the transport process is in good agreement with predictions by thermodynamic modeling.     

Crystal Structure and Ionic Conductivity of Cesium Trifluoromethyl Sulfonate,CsSO3CF3

The crystal structures of the room and the high temperature modifications of cesium trifluoromethyl sulfonate were solved from high resolution X‐ray powder diffraction data. At room temperature, α‐CsSO₃CF₃ crystallizes in the monoclinic space group P2₁ with lattice parameters a = 9.7406(2) Å, b = 6.1640(1) Å, c = 5.4798(1) Å, and β = 104.998(1)°; Z = 2. At temperatures above T = 380 K, a second order phase transformation towards a disordered C‐centered orthorhombic phase in space group Cmcm occurs with lattice parameters at T = 492 K of a = 5.5074(3) Å, b = 19.4346(14) Å, and c = 6.2978(4) Å; Z = 4. Within the crystal structures, the triflate anions are arranged in double layers with the apolar CF₃‐groups pointing towards each other. The cesium ions are located between the SO₃‐groups. CsSO₃CF₃ shows a specific ion conductivity ranging from σ = 1.06·10^?8 Scm^?1 at T = 393 K to σ = 5.18·10^?4 Scm^?1 at T = 519 K.     

Few‐Layer Nanosheets of n‐Type SnSe2

Layered p‐block metal chalcogenides are renowned for thermoelectric energy conversion due to their low thermal conductivity caused by bonding asymmetry and anharmonicity. Recently, single crystalline layered SnSe has created sensation in thermoelectrics due to its ultralow thermal conductivity and high thermoelectric figure of merit. Tin diselenide (SnSe₂), an additional layered compound belonging to the Sn‐Se phase diagram, possesses a CdI₂‐type structure. However, synthesis of pure‐phase bulk SnSe₂ by a conventional solid‐state route is still remains challenging. A simple solution‐based low‐temperature synthesis is presented of ultrathin (3–5 nm) few layers (4–6 layers) nanosheets of Cl‐doped SnSe₂, which possess n‐type carrier concentration of 2×10¹⁸ cm^?3 with carrier mobility of about 30 cm² V^?1 s^?1 at room temperature. SnSe₂ has a band gap of about 1.6 eV and semiconducting electronic transport in the 300–630 K range. An ultralow thermal conductivity of about 0.67 Wm^?1 K^?1 was achieved at room temperature in a hot‐pressed dense pellet of Cl‐doped SnSe₂ nanosheets due to the anisotropic layered structure, which gives rise to effective phonon scattering.     

Jahn‐Teller‐Ordnung in Mangan(III)‐fluoridsulfaten. II. Phasenübergang und Verzwillingung bei K2[MnF3(SO4)] und 1D Magnetismus in Verbindungen A2[MnF3(SO4)] (A = K,NH4, Rb,Cs)

Jahn‐Teller Ordering in Manganese(III) Fluoride Sulfates. II. Phase Transition and Twinning of K₂[MnF₃(SO₄)] and 1D Magnetism in Compounds A₂[MnF₃(SO₄)] (A = K, NH₄, Rb, Cs) According to single‐crystal X‐ray investigations, K₂[MnF₃(SO₄)] crystallizes at low temperature, like the isostructural Rb, NH₄, and Cs analogues in space group P2₁/c, Z = 4, e.g. at 100 K with a = 7.197, b = 10.704, c = 8.427Å, β = 91.84°. Below about 300 K, the crystals are found to be [001] axis twins. Using a new integration method for area detector records, nearly complete intensity data could be gained allowing for structure refinements of similar quality as for untwinned crystals (e.g. at 100 K: wR₂ = 0.050, R = 0.020 for all reflections). With rising temperature, the monoclinic angle approaches continuously 90°. For an ordering parameter Δβ = β?90° a 2^nd‐order phase transition is observed with an exponent λ = 0.17. At the transition temperature of 280 K resulting from the fit, the monoclinic structure changes – with delay – to orthorhombic with the minimum super‐group Pnca, a = 7.243, b = 10.763, c = 8.457Å, R = 0.024, as found in an early structure determination at room temperature by Edwards 1971. In the chain‐like [MnF₃(SO₄)]^2? anions, manganese(III) is octahedrally coordinated by two trans‐terminal and two trans‐bridging fluorine ligands as well as by the O atoms of two trans‐bridging sulfate ligands. At low temperature, the octahedral elongation by the Jahn‐Teller effect alternates between a F–Mn–F and an O–Mn–O axis (antiferrodistortive ordering). All bridges are asymmetric. From about 320 K on they become symmetric. Due to 2D dynamical Jahn‐Teller effect all octahedra appear compressed. All compounds A₂[MnF₃(SO₄)] show 1D antiferromagnetism. The antiferrodistortive Jahn‐Teller order at low temperatures and the small bridge angles explain the much lower magnetic exchange energies and their inverse relation to the bridge angles as compared with other fluoromanganate(III) chain compounds with the usual ferrodistortive ordering.     

Decoupled Spin Crossover and Structural Phase Transition in a Molecular Iron(II) Complex

Crystalline [Fe(bppSMe)₂][BF₄]₂ ( 1 ; bppSMe=4‐(methylsulfanyl)‐2,6‐di(pyrazol‐1‐yl)pyridine) undergoes an abrupt spin‐crossover (SCO) event at 265±5 K. The crystals also undergo a separate phase transition near 205 K, involving a contraction of the unit‐cell a axis to one‐third of its original value (high‐temperature phase 1; Pbcn, Z=12; low‐temperature phase 2; Pbcn, Z=4). The SCO‐active phase 1 contains two unique molecular environments, one of which appears to undergo SCO more gradually than the other. In contrast, powder samples of 1 retain phase 1 between 140–300 K, although their SCO behaviour is essentially identical to the single crystals. The compounds [Fe(bppBr)₂][BF₄]₂ ( 2 ; bppBr=4‐bromo‐2,6‐di(pyrazol‐1‐yl)pyridine) and [Fe(bppI)₂][BF₄]₂ ( 3 ; bppI=4‐iodo‐2,6‐di(pyrazol‐1‐yl)‐pyridine) exhibit more gradual SCO near room temperature, and adopt phase 2 in both spin states. Comparison of 1 – 3 reveals that the more cooperative spin transition in 1 , and its separate crystallographic phase transition, can both be attributed to an intermolecular steric interaction involving the methylsulfanyl substituents. All three compounds exhibit the light‐induced excited‐spin‐state trapping (LIESST) effect with T(LIESST=70–80 K), but show complicated LIESST relaxation kinetics involving both weakly cooperative (exponential) and strongly cooperative (sigmoidal) components.     

Structure of Plastic Crystalline Succinonitrile: High‐Resolution in situ Powder Diffraction

The temperature dependent (150–290 K) crystal structure of the low‐temperature α‐phase, and high temperature β‐phase, of succinonitrile has been determined by high resolution in situ powder diffraction. The α‐phase has a monoclinic unit cell that contains four gauche molecules and belongs to the P2₁/a space group. The crystal undergoes a reversible first‐order phase transition at 233 K into the high temperature β‐phase. The lattice parameters increase with temperature and the phase transition leads to an abrupt 6.7 % increase in volume. The β‐phase crystallizes into a bcc‐structure that belongs to the space group. The high temperature phase; however, is a highly disordered plastic crystal at room temperature that contains both gauche and trans molecules. The non‐linearity in the overall isotropic temperature‐factor indicates other possible phase transitions in the temperature range of 233–250 K.     

Pyroelectric and piezoelectric properties of new lead-free ceramics with composition Ba1-xNaxTi1-xO3

Ceramics with composition Ba_1-xNa_xTi_1-xNb_xO₃ are of either classical ferroelectric (0 ≤ × < 0.075) or relaxor ferroelectric types (0.075 ≤ x ≤0.55), and ferro- or antiferroelectric for compositions 0.55 < × ≤ 1. The dielectric study of ceramics with compositions close to NaNbO₃ showed a sharp peak of ε'_r without frequency dispersion. The value of T_c is decreasing as composition deviates from NaNbO₃. Ceramic samples are tetragonal at room temperature; they could be polarized and then show pyroelectric and piezoelectric properties up to 400K (p = 25nC.cm⁻².K⁻¹ and d₃₁30pC.N⁻¹ at 300K). This study aims at the preparation of environment-friendly lead-free relaxor ceramics which present a transition temperature close to room temperature.     

Synthesis and structure analysis of (K[DB18 C6])4(C60)5·12THF containing C60 in three different bonding states

A new fulleride, (K[DB18C6])₄(C₆₀)₅?12 THF, was prepared in solution using the “break‐and‐seal” approach by reacting potassium, fullerene, and dibenzo[18]crown‐6 in tetrahydrofuran. Single crystals were grown from solution by the modified “temperature difference method”. X‐ray analysis was performed revealing a reversible phase transition occurring on cooling. Three different crystal structures of the title compound at different temperatures of data acquisition are addressed in detail: the “high‐temperature phase” at 225 K (C2, Z=2, a=49.055(1), b=15.075(3), c=18.312(4) Å, β=97.89(3)°), the “transitional phase” at 175 K (C2 m, Z=2, a=48.436(5), b=15.128(1), c=18.280(2) Å, β=97.90(1)°), and the “low‐temperature phase” at 125 K (Cc, Z=4, a=56.239(1), b=15.112(3), c=36.425(7) Å, β=121.99(1)°). On cooling, partial radical recombination of C₆₀^.? into the (C₆₀)₂^2? dimeric dianion occurs; this is first time that the fully ordered dimer has been observed. Further cooling leads to formation of a superstructure with doubled cell volume in a different space group. Below 125 K, C₆₀ exists in the structure in three different bonding states: in the form of C₆₀^.? radical ions, (C₆₀)₂^2? dianions, and neutral C₆₀, this being without precedent in the fullerene chemistry, as well. Experimental observations of one conformation exclusively of the fullerene dimer in the crystal structure are further explained on the basis of DFT calculations considering charge distribution patterns. Temperature‐dependent measurements of magnetic susceptibility at different magnetic fields confirm the phase transition occurring at about 220 K as observed crystallographically, and enable for unambiguous charge assignment to the different C₆₀ species in the title fulleride.     

Synthesis,Characterization and Thermodynamic Study of the Solid State Coordination Compound Ni(Nic)2·H2O(s)(Nic=Nicotinate)

A novel compound‐monohydrated nickel nicotinate was synthesized by the method of room temperature solid phase synthesis and ball grinder. FTIR, chemical and elemental analysis, TG/DTG, and X‐ray powder diffraction technique were applied to characterize the structure and composition of the coordination compound. Low‐temperature heat capacities of the solid coordination compound have been measured by a precision automated adiabatic calorimeter over the temperature range from 78 to 386 K. A solid‐solid phase transition occurred in the temperature range of 328–358 K in the heat capacity curve, and the peak temperature, the molar enthalpy and molar entropy of the phase transition were determined to be T_trs=(356.759±0.697) K, Δ_trsH_m=(13.650±0.408) kJ· mol^?1, and Δ_trsS_m= (38.279±0.086) J·K^?1·mol^?1, respectively. The experimental values of the molar heat capacities in the temperature ranges of 78–328 K and 358–386 K were fitted to two polynomials, respectively. The polynomial fitted values of the molar heat capacities and fundamental thermodynamic functions of the sample relative to the standard reference temperature 298.15 K were calculated and tabulated at the intervals of 5 K.     

The 293 K structure of tetradehydrohaliclonacyclamine A

The polycyclic title compound {systematic name: (1S,16S,17S,31S)‐3,20‐diazatetracyclo[15.15.0^1,17.1^3,31.1^16,20]tetratriaconta‐6,8,23,25‐tetraene}, C₃₂H₅₂N₂, has recently been isolated and characterized structurally, in solution by NMR spectroscopy and in the solid state by X‐ray crystallography. At 130 K the structure is monoclinic (P2₁, Z = 4) and comprises two molecules in the asymmetric unit with distinctly different conformations in the twelve‐C‐atom bridging chains. We report that, at 250 K, a phase change from monoclinic to orthorhombic (P22₁2₁, Z = 4) occurs. The higher‐temperature phase is structurally characterized herein at 293 K. The two different conformers resolved in the monoclinic low‐temperature form merge to give a single disordered molecule in the asymmetric unit of the high‐temperature phase.     

The structural phase transition in SrV6O11

Single‐crystal X‐ray diffraction and specific heat studies establish that strontium hexavanadium undecaoxide, SrV₆O₁₁, undergoes a P6₃/mmc to inversion twinned P6₃mc structural transition as the temperature is lowered through 322 K. The P6₃/mmc and P6₃mc structures have been determined at 353 K and at room temperature, respectively. For the room‐temperature structure, seven of the ten unique atoms lie on special positions, and for the 353 K structure all of the seven unique atoms sit on special positions. The P6₃/mmc to P6₃mc structural phase transition, accompanied by a magnetic transition, is a common characteristic of AV₆O₁₁ compounds, independent of the identity of the A cations.     

Lithium vanadate Li3+xV6O13 at low temperature

The structure of Li_3+xV₆O₁₃ [x = 0.24 (3)] at 95 K has been solved and refined using single‐crystal X‐ray diffraction. The refined lithium content corresponds to two fully occupied Li sites and one partially occupied Li site. A doubling of the c axis is observed upon cooling from room temperature, and this change is associated with shifts of the V atoms. The resulting space group is C2/c. The Li disorder present in the Li₃V₆O₁₃ phase at room temperature is also observed in the low‐temperature phase reported here.     

The high‐temperature modification of zinc catena‐polyphosphate, β‐Zn(PO3)2

Single crystals of the high‐temperature modification of zinc catena‐polyphosphate, β‐Zn(PO₃)₂, were grown from a melt and quenched from 1093 K to room temperature. The structure was solved from single‐crystal X‐ray diffraction data and is built of corrugated (PO₃)_∞ polyphosphate chains, which extend along the c direction with an eight‐tetrahedra repeat. Slightly distorted [ZnO₄] tetrahedra link the polyphos­phate chains into a three‐dimensional network.