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1.
Single crystals of SrAl2Si2 were synthesized by reaction of the elements in an aluminum flux at 1000 °C. SrAl2Si2 is isostructural to CaAl2Si2 and crystallizes in the hexagonal space group P-3m1 (90 K, a=4.1834 (2), c=7.4104 (2) Å, Z=1, R1=0.0156, wR2=0.0308). Thermal analysis shows that the compound melts at ∼1020 °C. Low-temperature resistivity on single crystals along the c-axis shows metallic behavior with room temperature resistivity value of ∼7.5 mΩ cm. High-temperature Seebeck, resistivity, and thermal conductivity measurements were made on hot-pressed pellets. The Seebeck coefficient shows negative values in entire temperature range decreasing from ∼−78 μV K−1 at room temperature to −34 μV K−1 at 1173 K. Seebeck coefficients are negative indicating n-type behavior; however, the temperature dependence is consistent with contribution from minority p-type carriers as well. The lattice contribution to the thermal conductivity is higher than for clathrate structures containing Al and Si, approximately 50 mW cm−1 K, and contributes to the overall low zT for this compound.  相似文献   

2.
The calcium cobalt oxide CaCo2O4 was synthesized for the first time and characterized from a powder X-ray diffraction study, measuring magnetic susceptibility, specific heat, electrical resistivity, and thermoelectric power. CaCo2O4 crystallizes in the CaFe2O4 (calcium ferrite)-type structure, consisting of an edge- and corner-shared CoO6 octahedral network. The structure of CaCo2O4 belongs to an orthorhombic system (space group: Pnma) with lattice parameters, a=8.789(2) Å, b=2.9006(7) Å and c=10.282(3) Å. Curie-Weiss-like behavior in magnetic susceptibility with the nearly trivalent cobalt low-spin state (Co3+, 3d, S=0), semiconductor-like temperature dependence of resistivity (ρ=3×10−1 Ω cm at 380 K) with dominant hopping conduction at low temperature, metallic-temperature-dependent large thermoelectric power (Seebeck coefficient: S=+147 μV/K at 380 K), and Schottky-type specific heat with a small Sommerfeld constant (γ=4.48(7) mJ/Co mol K2), were observed. These results suggest that the compound possesses a metallic electronic state with a small density of states at the Fermi level. The doped holes are localized at low temperatures due to disorder in the crystal. The carriers probably originate from slight off-stoichiometry of the phase. It was also found that S tends to increase even more beyond 380 K. The large S is possibly attributed to residual spin entropy and orbital degeneracy coupled with charges by strong electron correlation in the cobalt oxides.  相似文献   

3.
Subsolidus phase relationships in the In2O3-WO3 system at 800-1400°C were investigated using X-ray diffraction. Two binary-oxide phases—In6WO12 and In2(WO4)3—were found to be stable over the range 800-1200°C. Heating the binary-oxide phases above 1200°C resulted in the preferential volatilization of WO3. Rietveld refinement was performed on three structures using X-ray diffraction data from nominally phase-pure In6WO12 at room temperature and from nominally phase-pure In2(WO4)3 at 225°C and 310°C. The indium-rich phase, In6WO12, is rhombohedral, space group (rhombohedral), with Z=1, a=6.22390(4) Å, α=99.0338(2)° [hexagonal axes: aH=9.48298(6) Å, c=8.94276(6) Å, aH/c=0.9430(9)]. In6WO12 can be viewed as an anion-deficient fluorite structure in which 1/7 of the fluorite anion sites are vacant. Indium tungstate, In2(WO4)3, undergoes a monoclinic-orthorhombic transition around 250°C. The high-temperature polymorph is orthorhombic, space group Pnca, with a=9.7126(5) Å, b=13.3824(7) Å, c=9.6141(5) Å, and Z=4. The low-temperature polymorph is monoclinic, space group P21/a, with a=16.406(2) Å, b=9.9663(1) Å, c=19.099(2) Å, β=125.411(2)°, and Z=8. The structures of the two In2(WO4)3 polymorphs are similar, consisting of a network of corner sharing InO6 octahedra and WO4 tetrahedra.  相似文献   

4.
Garnet-structure related metal oxides with the nominal chemical composition of Li5La3Nb2O12, In-substituted Li5.5La3Nb1.75In0.25O12 and K-substituted Li5.5La2.75K0.25Nb2O12 were prepared by solid-state reactions at 900, 950, and 1000 °C using appropriate amounts of corresponding metal oxides, nitrates and carbonates. The powder XRD data reveal that the In- and K-doped compounds are isostructural with the parent compound Li5La3Nb2O12. The variation in the cubic lattice parameter was found to change with the size of the dopant ions, for example, substitution of larger In3+(rCN6: 0.79 Å) for smaller Nb5+ (rCN6: 0.64 Å) shows an increase in the lattice parameter from 12.8005(9) to 12.826(1) Å at 1000 °C. Samples prepared at higher temperatures (950, 1000 °C) show mainly bulk lithium ion conductivity in contrast to those synthesized at lower temperatures (900 °C). The activation energies for the ionic conductivities are comparable for all samples. Partial substitution of K+ for La3+ and In3+ for Nb5+ in Li5La3Nb2O12 exhibits slightly higher ionic conductivity than that of the parent compound over the investigated temperature regime 25-300 °C. Among the compounds investigated, the In-substituted Li5.5La3Nb1.75In0.25O12 exhibits the highest bulk lithium ion conductivity of 1.8×10−4 S/cm at 50 °C with an activation energy of 0.51 eV. The diffusivity (“component diffusion coefficient”) obtained from the AC conductivity and powder XRD data falls in the range 10−10-10−7 cm2/s over the temperature regime 50-200 °C, which is extraordinarily high and comparable with liquids. Substitution of Al, Co, and Ni for Nb in Li5La3Nb2O12 was found to be unsuccessful under the investigated conditions.  相似文献   

5.
Three novel metal polyphosphides, α-SrP3, BaP8, and LaP5, were prepared in BN crucibles by the reaction of the respective stoichiometric mixtures under a high pressure of 3 GPa at 950-1000°C. Their crystal structures were determined from single-crystal X-ray data (α-SrP3: space group C2/m, a=9.199(6) Å, b=7.288(3) Å, c=5.690(3) Å, β=113.45(4)°, Z=4, R1/wR2=0.0684/0.1180 for 471 observed reflections and 22 variables; BaP8: space group P−1, a=6.762(2) Å, b=7.233(2) Å, c=8.567(2) Å, α=86.32(2)°, β=84.31(2)°, γ=70.40(2)°, Z=2, R1/wR2=0.0476/0.1255 for 2702 observed reflections and 82 variables; LaP5: space group P21/m, a=4.885(1) Å, b=9.673(3) Å, c=5.577(2) Å, β=105.32(2)°, Z=2, R1/wR2=0.0391/0.1034 for 1272 observed reflections and 31 variables). α-SrP3 is isostructural with SrAs3 and the crystal structure consists of two-dimensional puckered polyanionic layers 2[P3]2− that stack along the c-axis yielding channels occupied by Sr2+ counterions. BaP8 crystallizes in a new structure type which contains a three-dimensional infinite polyanionic framework 3[P3]2−, with large channels hosting the barium cations. LaP5 is a layered compound containing 2[P5]3− polyanionic layers separated by La3+ ions. All three compounds exhibit expected diamagnetic behaviors.  相似文献   

6.
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) Å3, 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.  相似文献   

7.
Phase transitions in MgAl2O4 were examined at 21-27 GPa and 1400-2500 °C using a multianvil apparatus. A mixture of MgO and Al2O3 corundum that are high-pressure dissociation products of MgAl2O4 spinel combines into calcium-ferrite type MgAl2O4 at 26-27 GPa and 1400-2000 °C. At temperature above 2000 °C at pressure below 25.5 GPa, a mixture of Al2O3 corundum and a new phase with Mg2Al2O5 composition is stable. The transition boundary between the two fields has a strongly negative pressure-temperature slope. Structure analysis and Rietveld refinement on the basis of the powder X-ray diffraction profile of the Mg2Al2O5 phase indicated that the phase represented a new structure type with orthorhombic symmetry (Pbam), and the lattice parameters were determined as a=9.3710(6) Å, b=12.1952(6) Å, c=2.7916(2) Å, V=319.03(3) Å3, Z=4. The structure consists of edge-sharing and corner-sharing (Mg, Al)O6 octahedra, and contains chains of edge-sharing octahedra running along the c-axis. A part of Mg atoms are accommodated in six-coordinated trigonal prism sites in tunnels surrounded by the chains of edge-sharing (Mg, Al)O6 octahedra. The structure is related with that of ludwigite (Mg, Fe2+)2(Fe3+, Al)(BO3)O2. The molar volume of the Mg2Al2O5 phase is smaller by 0.18% than sum of molar volumes of 2MgO and Al2O3 corundum. High-pressure dissociation to the mixture of corundum-type phase and the phase with ludwigite-related structure has been found only in MgAl2O4 among various A2+B3+2O4 compounds.  相似文献   

8.
In this work we investigate the performance of high flux chemical vapour deposition (CVD) silica membranes for the separation of gas mixtures containing H2 and CO2 at various temperatures. The membranes were prepared by a counter diffusion CVD method where tetraethyl orthosilicate (TEOS) and O2 were used as reactants. Single gas permeation resulted in activated transport for the smaller kinetic diameter gases (H2 and He) whilst the larger kinetic diameter gases (CO2 and N2) showed negative activation energy. The single gas permeation of H2 increased from 5.1 × 10−7 to 7.0 × 10−7 mol m−2 s−1 Pa−1 in the temperature range 100–400 °C, and H2/CO2 and H2/N2 selectivities reached 36 and 57 at 400 °C, respectively. The H2 purity in the permeate stream also increased with temperature for H2:CO2 binary gas mixture, thus being beneficial for H2 diffusion. H2 competitively permeated through the membrane at a several range of gas mixtures, and a saturation level was achieved at H2:CO2 60:40 feed concentration, where the diffusion of CO2 molecules became negligible delivering ∼99% H2 purity in the permeate stream. These results substantiate that the counter diffusion CVD method produced thin silica film membranes with a very precise pore size control, in particular suggesting a narrow pore distribution with average pore radius of about 3.1 Å.  相似文献   

9.
Effect of surface fluorination and conductive additives on the charge/discharge behavior of lithium titanate (Li4/3Ti5/3O4) has been investigated using F2 gas and vapor grown carbon fiber (VGCF). Surface fluorination of Li4/3Ti5/3O4 was made using F2 gas (3 × 104 Pa) at 25-150 °C for 2 min. Charge capacities of Li4/3Ti5/3O4 samples fluorinated at 70 °C and 100 °C were larger than those for original sample at high current densities of 300 and 600 mA/g. Optimum fluorination temperatures of Li4/3Ti5/3O4 were 70 °C and 100 °C. Fibrous VGCF with a large surface area (17.7 m2/g) increased the utilization of available capacity of Li4/3Ti5/3O4 probably because it provided the better electrical contact than acetylene black (AB) between Li4/3Ti5/3O4 particles and nickel current collector.  相似文献   

10.
The reaction between lanthanum oxide and strontium carbonate was studied non-isothermally between 350 and 1150 °C at different heating rates, intermediates and the final solid product were characterized by X-ray diffractometry (XRD). The reaction proceeds through formation of lanthanum oxycarbonate La2O(CO3)2, lanthanum dioxycarbonate La2O2CO3, and non-stoichiometric strontium lanthanum oxide La2SrOx (x = 4 + δ). La4SrO7 was found to be the final product which begins to form at ∼700 °C. Li+ doping enhances the formation of the final product as well as commencement of the reactions at lower temperatures.  相似文献   

11.
Thermoelectric properties of polycrystalline La1−xSrxCoO3, where Sr2+ is substituted in La3+ site in perovskite-type LaCoO3, have been investigated. Sr-doping increases the electrical conductivity (σ) of La1−xSrxCoO3, and also decreases the Seebeck coefficient (S) for 0.01?x?0.40. A Hall coefficient measurement reveals that the increase in electrical conductivity arises from increases in both carrier concentration and the Hall mobility. The decrease in the Seebeck coefficient is caused by a decrease in carrier effective mass as well as increase in carrier concentration. The highest power factor (σS2) is 3.7×10−4 W m−1 K−2 at 250 K for x=0.10. The thermal conductivity (κ) is about 2 W m−1 K−1 at 300 K for 0?x?0.04, and increases for x?0.05 because of an increase in heat transport by conductive carrier. The thermoelectric properties of La1−xSrxCoO3 are improved by Sr-doping, and the figure of merit (Z=σS2 κ−1) reaches 1.6×10−4 K−1 for x=0.06 at 300 K (ZT=0.048). For heavily Sr-doped samples, the thermoelectric properties diminish mainly because of the decrease in the Seebeck coefficient and the increase in thermal conductivity.  相似文献   

12.
The compound NaAlF4 has been obtained in the form of thin fibrous crystals or fine colorless powder by condensation at 18 °C of vapors arising over chiolite Na5Al3F14 or NaCaAlF6, heated up to 800 °C. Thermal stability has been investigated by the methods of thermal analysis and high temperature X-ray diffraction. When heated in air, NaAlF4 is stable up to 390-400 °C, then there is an exothermal solid state decay into Na5Al3F14(s) and AlF3(s). At higher temperature Na5Al3F14(s) decays into Na3AlF6(s) and NaAlF4(g). The crystal structure (space group Cmcm, a=3.6124(1) Å, b=14.9469(7) Å, c=5.2617(3) Å, V=284.10 Å3) has been determined by X-ray powder diffraction method. In the crystal structure of NaAlF4 the octahedrons [AlF6] are joined through vertices and form corrugated layers, sodium ion layers being located between them. The distances between the atoms of Al-F are in the range 1.791-1.814 Å, and those for Na…F are in the range 2.297-2.439 Å. In spite of limited thermal stability of the crystal form, the compound NaAlF4 is the main component of the gas mixture over solid and molten salts in the ternary system NaF-AlF3-CaF2 and participates in chemical transformations between the phases at high temperature.  相似文献   

13.
The compound CeAu0.28Ge1.72 crystallizes in the ThSi2 structure type in the tetragonal space group I41/amd with lattice parameters a=b=4.2415(6) Å c=14.640(3) Å. CeAu0.28Ge1.72 is a polar intermetallic compound having a three-dimensional Ge/Au polyanion sub-network filled with Ce atoms. The magnetic susceptibility data show Curie-Weiss law behavior above 50 K. The compound orders ferromagnetically at ∼8 K with estimated magnetic moment of 2.48 μB/Ce. The ferromagnetic ordering is confirmed by the heat capacity data which show a rise at ∼8 K. The electronic specific heat coefficient (γ) value obtained from the paramagnetic temperature range 15-25 K is∼124(5) mJ/ mol K2. The entropy change due to the ferromagnetic transition is ∼4.2 J/mol K which is appreciably reduced compared to the value of R ln(2) expected for a crystal-field-split doublet ground state and/or Kondo exchange interactions.  相似文献   

14.
A new efficient blue phosphor, Eu2+ activated SrZnP2O7, has been synthesized at 1000 °C under reduced atmosphere and the crystal structure and photoluminescence properties have been investigated. The crystal structure of SrZnP2O7 was obtained via Rietveld refinement of powder X-ray diffraction (XRD) pattern. It was found that SrZnP2O7 crystallizes in space group of P21/n (no. 14), Z=4, and the unit cell dimensions are: a=5.30906(2) Å, b=8.21392(3) Å, c=12.73595(5) Å, β=90.1573(3)°, and V=555.390(3) Å3. Under ultraviolet excitation (200-400 nm), efficient Eu2+ emission peaked at 420 nm was observed, of which the luminescent efficiency at the optimal concentration of Eu2+ (4 mol%) was estimated to be 96% as that of BaMgAl10O17:Eu2+. Hence, the SrZnP2O7:Eu2+ exhibit great potential as a phosphor in different applications, such as ultraviolet light emitting diode and photo-therapy lamps.  相似文献   

15.
Highly crystalline phenyl 2,5-dichlorobenzenesulfonate (PDBS, Tmelt = 86-87 °C) and pentafluorophenyl 2,5-dichlorobenzenesulfonate (FPDBS, Tmelt = 120-122 °C) were synthesized. Single-crystal X-ray molecular structure determinations show that both compounds have similar three-dimensional molecular structures; however, PDBS crystals are thin platelets and FPDBS crystals form hexagonal tube-like structures that are predominately hollow at one end. PDBS crystals exhibit offset π-stacking of the phenoxy-rings that form complete two-dimensional layers each two molecules thick. Hydrogen-bonding interactions are calculated at ∼3.2 Å between the C6-hydrogen and the sulfonyl-oxygen of a neighboring molecule. On the other hand, for FPDBS, π-stacking of the dichloro-substituted ring as well as dipole-dipole interactions of the fluorinated-phenoxy rings appears to be the predominate intermolecular interactions. Neither structure exhibits any kind of side-on interaction of the phenyl rings. PDBS and FPDBS exhibit melting point depressions of 26 and 40 °C, respectively, in the presence of supercritical CO2. Although both sulfonates exhibit high solubility in CO2, much lower pressures are needed to dissolve FPDBS compared to PDBS. For example, at 100 °C FPDBS dissolves at 4750 psia and PDBS dissolves at 11,000 psia. The solubility data reinforce the observation that fluorinating a compound can significantly lower the conditions needed to dissolve that compound in CO2.  相似文献   

16.
The two new compounds, Sr4Cu3(AsO4)2(AsO3OH)4·3H2O (1) and Ba2Cu4(AsO4)2(AsO3OH)3(2), were synthesized under hydrothermal conditions. They represent previously unknown structure types and are the first compounds synthesized in the systems SrO/BaO-CuO-As2O5-H2O. Their crystal structures were determined by single-crystal X-ray diffraction [space group C2/c, a=18.536(4) Å, b=5.179(1) Å, c=24.898(5) Å, β=93.67(3)°, V=2344.0(8) Å3, Z=4 for 1; space group P42/n, a=7.775(1) Å, c=13.698(3) Å, V=828.1(2) Å3, Z=2 for 2]. The crystal structure of 1 is related to a group of compounds formed by Cu2+-(XO4)3− layers (X=P5+, As5+) linked by M cations (M=alkali, alkaline earth, Pb2+, or Ag+) and partly by hydrogen bonds. In 1, worth mentioning is the very short hydrogen bond length, D···A=2.477(3) Å. It is one of the examples of extremely short hydrogen bonds, where the donor and acceptor are crystallographically different. Compound 2 represents a layered structure consisting of Cu2O8 centrosymmetric dimers crosslinked by As1φ4 tetrahedra, where φ is O or OH, which are interconnected by Ba, As2 and hydrogen bonds to form a three-dimensional network. The layers are formed by Cu2O8 centrosymmetric dimers of CuO5 edge-sharing polyhedra, crosslinked by As1O4 tetrahedra. Vibrational spectra (FTIR and Raman) of both compounds are described. The spectroscopic manifestation of the very short hydrogen bond in 1, and ABC-like spectra in 2 were discussed.  相似文献   

17.
High-pressure synthesis experiments in the system Na2O-Y2O3-SiO2 revealed the existence of a previously unknown polymorph of NaYSi2O6 or Na3Y3[Si3O9]2 which was quenched from 3.0 GPa and 1000 °C. Structural investigations on this modification have been performed using single-crystal X-ray diffraction data collected at ambient conditions. Furthermore, unpolarized micro-Raman spectra have been obtained from single-crystal material. The high-P modification of NaYSi2O6 crystallizes in the centrosymmetric space group C2/c with 12 formula units per cell (a=8.2131(9) Å, b=10.3983(14) Å, c=17.6542(21) Å, β=100.804(9)°, V=1481.0(3) Å3, R(|F|)=0.033 for 1142 independent observed reflections) and belongs to the group of cyclo-silicates. Basic building units are isolated three-membered [Si3O9] rings located in layers parallel to (010). Within a single layer the rings are concentrated in strings parallel to [100]. The sequence of directedness of up (U) or down (D) pointing tetrahedra of a single ring is UUU or DDD, respectively. Stacking of the layers parallel to b results in the formation of a three-dimensional structure in which yttrium and sodium cations are incorporated for charge compensation. In more detail, four non-tetrahedral cation positions can be differentiated which are coordinated by 6 and 8 oxygen ligands. Refinements of the site occupancies did not reveal any indication for mixed Na-Y populations on these positions. Finally, several geometrical parameters of rings occurring in cyclo-trisilicate structures have been compiled and are discussed.  相似文献   

18.
Ba3MgSi2O8, a phosphor host examined for use in white-light devices and plant-growth lamps, was synthesized at 1225 °C in air. Its crystal structure has been determined and refined by a combined powder X-ray and neutron Rietveld method (, Z=3, a=9.72411(3) Å, c=7.27647(3) Å, V=595.870(5) Å3; Rp/Rwp=3.79%/5.03%, χ2=4.20). Superstructure reflections, observed only in the neutron diffraction data, provided the means to establish the true unit cell and a chemically reasonable structure. The structure contains three crystallographically distinct Ba atoms—Ba1 resides in a distorted octahedral site with S6 () symmetry, Ba2 in a nine-coordinate site with C3 (3) symmetry, and Ba3 in a ten-coordinate site with C1 (1) symmetry. The Mg atoms occupy distorted octahedral sites, and the Si atom occupies a distorted tetrahedral site.  相似文献   

19.
β-UP2O7 has been synthesized under hydrothermal conditions (θ=500°C, P=200 MPa), using UO2 and H3PO4. β-UP2O7 crystallizes in the orthorhombic space group Pn21a, with a=11.526 (2) Å, b=7.048 (2) Å, c=12.807 (2) Å and Z=4. Its structure has been determined through direct methods and difference Fourier synthesis and has been refined to R=0.0396. The structure is built on UO8 polyhedral chains along the b-axis. PO43− and P3O105− groups coexist in the structure and the latter groups form non-linear chains. Cohesion of the structure is made through the linkage of UO8 chains by PO4 and P3O10 groups leading to the formula U2(PO4)(P3O10) instead of β-UP2O7. Vibrational and optical spectra confirm the results obtained by X-ray diffraction. DTA-TGA measurements show that the transformation of U2(PO4)(P3O10) to the cubic α-UP2O7 occurs at θ=870°C.  相似文献   

20.
The compounds M[PO2(OH)2]2·2H2O (M=Mg, Mn, Fe, Co, Ni, Zn, Cd) were prepared from super-saturated aqueous solutions at room temperature. Single-crystal X-ray structure investigations of members with M=Ni, Zn, Cd were performed at 295 and 120 K. The space-group symmetry is P21/n, Z=2. The unit-cell parameters are at 295/120 K for M=Ni: a=7.240(2)/7.202(2), b=9.794(2)/9.799(2), c=5.313(1)/5.285(1) Å, β=94.81(1)/94.38(1)°, V=375.4/371.9 Å3; M=Zn: a=7.263(2)/7.221(2), b=9.893(2)/9.899(3), c=5.328(1)/5.296(2) Å, β=94.79(1)/94.31(2)°, V=381.5/377.5 Å3; M=Cd: a=7.356(2)/7.319(2), b=10.416(2)/10.423(3), c=5.407(1)/5.371(2) Å, β=93.85(1)/93.30(2)°, V=413.4/409.1 Å3. Layers of corner-shared MO6 octahedra and phosphate tetrahedra are linked by three of the four crystallographically different hydrogen bonds. The fourth hydrogen bond (located within the layer) is worth mentioning because of the short Oh?O bond distance of 2.57-2.61 Å at room temperature (2.56-2.57 Å at 120 K); only for M=Mg it is increased to 2.65 Å. Any marked temperature-dependent variation of the unit-cell dimension is observed only vertical to the layers. The analysis of the infrared (IR) spectroscopy data evidences that the internal PO4 vibrations are insensitive to the size and the electronic configuration of the M2+ ions. The slight strengthening of the intra-molecular P-O bonds in the Mg salt is caused by the more ionic character of the Mg-O bonds. All IR spectra exhibit the characteristic “ABC trio” for acidic salts: 2900-3180 cm−1 (A band), 2000-2450 cm−1 (B band) and 1550-1750 cm−1 (C band). Both the frequency and the intensity of the A band provide an evidence that the PO2(OH)2 groups in M[PO2(OH)2]2·2H2O compounds form weaker hydrogen bonds as compared with other acidic salts with comparable O?O bond distances of about 2.60 Å. The observed shift of the O-H stretching vibrations of the water molecule in the order M=Mg>Mn≈Fe≈Co>Ni>Zn≈Cd has been discussed with respect to the influence of both the character and the strength of M↔H2O interactions.  相似文献   

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