Monoclinic Compound Research Articles

Phonon modes of monoclinic BiB3O6

AbstractWe present a detailed study of the phonon modes of the monoclinic compound BiB3O6 based on polarized reflectivity measurements on single crystals.The spectra are analysed by means of a generalized Drude–Lorentz model, which allows us to resolve the modes of A and B symmetry. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic resonance studies of the low-dimensional magnet NaFe(WO4)2

Magnetic resonance studies of the low-dimensional monoclinic compound NaFe(WO4)2 are carried out in the frequency range 25–142 GHz and temperature range 1.8–300 K. The EPR data near the phase transition attest to the two-dimensionality of the magnetic structure of NaFe(WO4)2. The frequency-field relation of the AFMR spectrum shows that this compound is a biaxial antiferromagnet. The characteristic parameters of the AFMR energy spectrum are determined: the values of the energy gaps ν1=141 GHz and ν2=168.7 GHz, the anisotropy fields Ha1=10.5 kOe and Ha2=15 kOe, and the exchange field He=121 kOe. The ratio of the intralayer to the interlayer exchange is estimated. Additional absorption is observed due to local modes caused by destruction of the translational order of the magnetic structure.

Short-range charge order inRNiO3perovskites (R=Pr, Nd, Eu, Y) probed by x-ray-absorption spectroscopy

The short-range organization around Ni atoms in orthorhombic $R$NiO$_{3}$ ($R$=Pr,Nd,Eu) perovskites has been studied over a wide temperature range by Ni K-edge x-ray absorption spectroscopy. Our results demonstrate that two different Ni sites, with different average Ni-O bond lengths, coexist in those orthorhombic compounds and that important modifications in the Ni nearest neighbors environment take place across the metal-insulator transition. We report evidences for the existence of short-range charge-order in the insulating state, as found in the monoclinic compounds. Moreover, our results suggest that the two different Ni sites coexists even in the metallic state. The coexistence of two different Ni sites, independently on the $R$ ion, provides a common ground to describe these compounds and shed new light in the understanding of the phonon-assisted conduction mechanism and unusual antiferromagnetism present in all $R$NiO$_{3}$ compounds.

Large magnetoresistance in (AA′) 2FeReO 6 double perovskites

We review the main structural, magnetic and magnetotransport properties of the intriguing (AA′) 2FeReO 6 magnetic double perovskites. As the average cation size decreases, the crystallographic structure at room temperature evolves from cubic [(AA′) 2=Ba 2, Ba 1.5Sr 0.5, BaSr, Ba 0.5Sr 1.5] to tetragonal [(AA′) 2=Sr 2] and monoclinic [(AA′) 2=Ca 0.5Sr 1.5, CaSr, Ca 1.5Sr 0.5, Ca 2]. The Curie temperature increases anomalously from ≈303 K for Ba 2 to ≈522 K for Ca 2 in sharp contrast with the observed behaviour in the isostructural compounds (AA′) 2FeMoO 6. Other anomalous features in the (AA′) 2FeReO 6 series are: the large magnetic anisotropy, the large magnetoelastic coupling and the semiconducting behaviour of the monoclinic compounds. The monoclinic compounds undergo a structural/magnetic transition at T S below 125 K. Three different magnetoresistance mechanisms have been identified: the intergrain negative magnetoresistance effect, which is present across the whole series of compounds, and in the case of the monoclinic compounds below T S a negative magnetoresistance effect associated to the melting of the low-temperature phase and a positive magnetoresistance effect only present in (AA′) 2=Ca 2 below T ∼ 50 K .

Crystal matrix and crystal double matrix spectroscopy

Infrared and Raman spectra of matrix isolated ions in crystalline solids display the normal modes of these units with respect to the symmetry of the respective lattice sites. Hence, they give information on all structure changes of the incorporated entities (energetic distortion), the amount of intramolecular coupling of the respective stretching modes, and symmetry and potential at the host crystal lattice sites. In the case of neat compounds, this information is hidden by collective solid-state effects. Assignment of the modes of matrix isolated guest ions can be performed by Raman single crystal experiments. If two different matrix isolated entities in a crystalline matrix come into contact forming complexes (double matrix spectroscopy) symmetry and frequencies of the modes of the guest ions are additionally changed. In the case of solid hydrates, thus the influence of metal ions and anions on the strength of hydrogen bonds (cooperative, competitive, and synergetic effects) can be analysed in detail. Examples of crystal matrix and crystal double matrix spectroscopic experiments are presented on orthorhombic halates M( XO 3) 2, monoclinic halate monohydrates M( XO 3) 2·H 2O ( M=Sr, Ba, Pb and X=Cl, Br, I), and likewise monoclinic kieserite-type compounds MXO 4·H 2O ( M=Mn, Co, Ni, Zn, and X=S, Se) with matrix-isolated XO 3 −, XO 4 2−, and M 2+ guest ions and HDO guest molecules.

Chemical Conversion of Hg‐Clusters: Synthesis and Crystal Structure of $ {\rm [(DMSO)_2 Co(NC \overline{S)_4(Hg}-TePh)_2]_{\rm n} }$, the First Compound Containing a Polymeric Hg—Te Chain

AbstractThe six‐membered ring Hg3Te3 of [Hg3Cl3(μ‐TePh)3]·2 DMSO {(Ph = C6H5; DMSO = (CH3)2SO} was opened by redissolution with DMSO, reacting with Co[Hg(SCN)4] and affording polymeric $ {\rm [(DMSO)_2 Co(NC \overline{S)_4(Hg}-TePh)_2]_{\rm n} }$. The monoclinic novel compound belong to the space group P21/n and assembles in a bidimensional lattice tetrahedral HgII(SCN)2Te2‐ and octahedral CoII(NCS)4(DMSO)2‐chains linked trough SCN bridges along the crystallographic axis b and diagonally to the ac axes. The structure of [(DMSO)2Co(NCS)4(Hg—TePh)2]n is limited by the DMSO ligands in the axial positions of the Co‐octahedrons.

Impact of cation size on magnetic properties of(AA′)2FeReO6double perovskites

We study the influence of the cation size in the magnetic properties of ${(AA}^{\ensuremath{'}}{)}_{2}{\mathrm{FeReO}}_{6}$ ${(AA}^{\ensuremath{'}}={\mathrm{Ba}}_{2},{\mathrm{Ba}}_{1.5}{\mathrm{Sr}}_{0.5},{\mathrm{B}\mathrm{a}\mathrm{S}\mathrm{r},\mathrm{B}\mathrm{a}}_{0.5}{\mathrm{Sr}}_{1.5},{\mathrm{Sr}}_{2},{\mathrm{Ca}}_{0.5}{\mathrm{Sr}}_{1.5},{\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{r},\mathrm{C}\mathrm{a}}_{1.5}{\mathrm{Sr}}_{0.5},{\mathrm{Ca}}_{2})$ double perovskites. As the average cation size decreases, the crystallographic structure at room temperature evolves from cubic to tetragonal and monoclinic. The large lattice effects observed for the monoclinic compounds could be responsible for their anomalous behavior. The Curie temperature increases anomalously from \ensuremath{\approx}303 K for ${\mathrm{Ba}}_{2}$ to \ensuremath{\approx}522 K for ${\mathrm{Ca}}_{2},$ which seems to need an additional ferromagnetic coupling to the usual $(\mathrm{Fe}{t}_{2g}\ensuremath{-}\mathrm{Re}{t}_{2g})pdd\ensuremath{-}\ensuremath{\pi}$ coupling. At 5 K, the magnetization at 5 T is close to $3{\ensuremath{\mu}}_{B}/\mathrm{f}.\mathrm{u}.$ for those compounds with average cation size between that of ${\mathrm{Ba}}_{2}$ and that of ${\mathrm{Ca}}_{0.5}{\mathrm{Sr}}_{1.5}$ and the coercivity is found to be large in all cases (in the KOe range). The remaining compounds $({\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{r},\mathrm{C}\mathrm{a}}_{1.5}{\mathrm{Sr}}_{0.5},{\mathrm{Ca}}_{2})$ undergo a simultaneous structural and magnetic transition below 150 K which produces a huge increase in the coercivity and reduces the magnetization. Magnetotransport properties change accordingly. From our results, a novel strong magnetostructural coupling in these compounds is anticipated. These results are interpreted within a scenario where the Re orbital state plays a crucial role in the ground state.

Antiferromagnet-ferromagnet and structural phase transitions inLa0.88MnOxmanganites

The crystal and magnetic structure as well as elastic, magnetic, and electrotransport properties of ${\mathrm{La}}_{0.88}{\mathrm{MnO}}_{x}$ $(2.82l~xl~2.96)$ manganites have been investigated as a function of oxygen content by x-ray, neutron diffraction, Young's modulus, magnetization, and resistivity measurements. The crystal structure of the compounds has been found to be orthorhombic at $xl2.91$ and monoclinic at $xg~2.91.$ The gradual transition from an orbitally ordered state to an orbitally disordered one is observed with increasing temperature or oxygen content. The magnetic properties of the samples have been found to correlate with the type of orbital state. The dynamic orbital correlations favor ferromagnetic state, while A-type antiferromagnetic state is typical for the static Jahn-Teller distortions. The orthorhombic compounds have been found to exhibit a semiconducting behavior, and show a second-order transition into paramagnetic state. The metallic conductivity appears starting from the $x=2.92$ monoclinic compound and, simultaneously, a first-order paramagnet-ferromagnet magnetic phase transition is observed. The properties are discussed in the framework of structurally driven phase separation model.

Structural, magnetic and transport properties of CoZn/Cu electrodeposited multilayers

Abstract We present experimental results of (Co9.7Zn90.3/Cu)20 multilayers grown from electrochemical dual bath. X-ray diffraction patterns have shown that the CoZn structural lattice parameters are close to those of the monoclinic CoZn13 compound. The magnetic properties at room temperature reveal both superparamagnetic and ferromagnetic features. The magnetoresistance behaviour exhibits a broad, rounded maximum around H=0 and does not present any saturation.

Influence of V 2O 5 substitutions to Bi 2(Zn 1/3Nb 2/3) 2O 7 pyrochlore on sintering temperature and dielectric properties

The dielectric properties of the monoclinic zirconolite-like structure compound, Bi 2(Zn 1/3Nb 2/3 − x V x ) 2O 7 (0≤ x≤0.1), were investigated. We found a small vanadium substitution ( x) to Bi 2(Zn 1/3Nb 2/3) 2O 7 ceramics lowered sintering temperature from 950 to 850 °C. Low-temperature sintered Bi 2(Zn 1/3Nb 2/3 − x V x ) 2O 7 maintained high dielectric constant ( ε r), ∼80 and Qxf value, ∼3000 at 6 GHz. By increasing x, second phase, bismuth vanadate (Bi 4V 2O 11) was detected. The formation of second phase is accompanied by a significant decrease in dielectric loss. It was found that Bi 2(Zn 1/3Nb 2/3) 2O 7 had two phases, one was for the stoichiometric composition and the other was for Bi-deficient composition. The chemical compatibility of silver electrode and Bi 2(Zn 1/3Nb 2/3 − x V x ) 2O 7 ( x=0.005) has also been investigated using SEM micrograph and EDS line scan of the interface between a silver electrode and the ceramic body, for low-temperature cofired ceramics (LTCC) applications.

Magnetic properties and low temperature X-ray studies of the weak ferromagnetic monoclinic and trigonal chromium tellurides Cr 5Te 8

Monoclinic and trigonal Cr 5Te 8 show a transition into the ferromagnetic state with a Curie temperature T c, which sensitively depends on the actual composition. Monoclinic samples exhibit a lower T c despite their higher Cr content. This observation is explained on the basis of less effective ferromagnetic superexchange in the monoclinic compounds and the larger number of Cr atoms being antiferromagnetically coupled. Magnetization experiments performed at 5 K demonstrate that the compounds saturate already at rather low magnetic fields. In addition, small values are estimated for the coercitive field H c as well as for the remanence magnetization both being typical for weak ferromagnetic materials. The values for the saturation magnetization amount to about 72% and 65% for monoclinic and trigonal Cr 5Te 8, respectively. These low values can partially be explained on the basis of antiferromagnetically coupled Cr(III) d 3 centers. An additional spin canting is assumed to fully account for the reduced saturation moments. Above 300 K the susceptibilities follow a Curie–Weiss law with large positive values for the Weiss constant and magnetic moments in accordance with a Cr 3+ 3 d 3 spin configuration. Low temperature X-ray investigations reveal unusual thermal expansion of the a as well as b lattice parameters of the monoclinic and of the a lattice parameter of the trigonal sample, respectively. Near T c the slope of the thermal expansions changes significantly.

A single crystal X-ray and HRTEM study of new series of compounds DyCu x ( x=4.5, 4 and 3.5)

A series of monoclinic compounds DyCu x ( x=4.5,4 and 3.5) is described. It is constructed from structural blocks AB 5 (cubic AuBe 5 structure type) and AB 2 (cubic MgCu 2 structure type) by stacking nAB 5+ AB 2 and giving the compositions A 2 B 7, AB 4, A 4 B 17, A 5 B 22, A 6 B 27,…, AB 5. The resulting monoclinic superstructures can be derived from the cubic AuBe 5 type by introducing planar defects parallel to { hhh} that lead to a nearly orthogonal ≈( n+2)×( n+2)×( n+2−0.5) supercells. The present series is analogous to the monoclinic–hexagonal–trigonal–orthorhombic series A n+1 B 5 n+2 obtained by the stacking ( n–1) AB 5+ A 2 B 7 where AB 5 is of the hexagonal CaCu 5 structure type and A 2 B 7 is of the monoclinic Zr 2Ni 7 structure type.

(CuBr)3P4Se4: A Low Symmetric Variant of the (CuI)3P4Se4 Structure Type.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Powder pattern indexing using the weighted crosscorrelation and genetic algorithms.

X-ray diffraction is a powerful technique for investigating the structure of crystals and crystalline powders. Unfortunately, for powders, the first step in the structure elucidation process, retrieving the unit cell parameters (indexing), is still very critical. In the present article, an improved approach to powder pattern indexing is presented. The proposed method matches peak positions from experimental X-ray powder patterns with peak positions from trial cells using a recently published method for pattern comparison (weighted crosscorrelation). Trial cells are optimized with Genetic Algorithms. Patterns are not pretreated to remove any existing zero point shift, as this is determined during optimization. Another improvement is the peak assignment procedure. This assignment is needed for determining the similarity between lines from trial cells and experiment. It no longer allows calculated peaks to be assigned twice to different experimental peaks, which is beneficial for the indexing process. The procedure proves to be robust with respect to false peaks and accidental or systematic absensences of reflections, and is successfully applied to powder patterns originating from orthorhombic, monoclinic, and triclinic compounds measured with synchrotron as well as with conventional laboratory X-ray diffractometers.

Crystal structure and 4.2 K electronic spectrum of [Co(bpy) 3](CrO 4) 0.5NO 3·7H 2O, a double salt containing uncoordinated chromate ion

The novel Co II-tris bipyridine complex [Co(bpy) 3](CrO 4) 0.5NO 3·7H 2O has been synthesised and characterised with X-ray and spectroscopic methods. The monoclinic compound crystallises in the space group C2/ c with the lattice constants a=24.325 Å, b=13.974 Å, c=21.491 Å, α=90°, β=91.11°, γ=90.00°, and Z=8 at room temperature. In the electronic spectrum of single crystal at 4.2 K, beside the presence of Co II (d 7) d–d transitions, the spin forbidden chromate transitions 3T 1 and 3T 2 have been found.

(CuBr)3P4Se4: A Low Symmetric Variant of the (CuI)3P4Se4 Structure Type

AbstractBright orange (CuBr)3P4Se4 is obtained from the reaction of CuBr, P, and Se in stoichiometric amounts (CuBr : P : Se = 3 : 4 : 4). The composition and the crystal structure of the compound were determined from single crystal X‐ray diffraction data. Lattice constants are a = 33.627(2) Å, b = 6.402(1) Å, c = 19.059(1) Å, β = 90.19(3) °, V = 4103.2(3) Å3, and Z = 12. The compound crystallizes in a structure that is related to (CuI)3P4Se4. Cages of β‐P4Se4 are stacked along the b‐axis and are separated by columns of copper(I) bromide. However, the coordination of the β‐P4Se4 cage molecules to the copper atoms in the CuBr columns in (CuBr)3P4Se4 is quite different from (CuI)3P4Se4. The monoclinic compound (space group: P21, no. 4) has an almost orthorhombic metric in combination with a threefold superstructure in [100]. Structural aspects of (CuBr)3P4Se4 are discussed with respect to the heavier homologue (CuI)3P4Se4.

Polymorphism of Bi 1− xLn xO 1.5 phases (0< x

The Bi 1− x Ln x O 1.5 solid solutions (Ln=La, Pr, Nd), of the β 2/β 1/ε (Bi–Sr–O) structural type, have been investigated in their Ln-rich domains. For Ln=La, Pr, and Nd, the upper limits are 0.35, 0.35 and 0.33, respectively. The Bi 4Ln 2O 9 ε phase ( x=0.33) appears to be the single definite compound. For Bi 4La 2O 9, Bi 4Pr 2O 9 and Bi 4Nd 2O 9, the ε-type cells are respectively: a=9.484(4) Å, b=3.982(2) Å, c=7.030(3) Å, β=104.75(3)°; a=9.470(5) Å, b=3.945(2) Å, c=6.968(4) Å, β=104.73(3)° and a=9.439(3) Å, b=3.944(2) Å, c=6.923(2) Å, β=105.03(3)°. Upon heating, each monoclinic (ε) compound transforms successively into rhombohedral phases (β 2/β 1) and finally into a cubic fluorite-type phase. For La- and Pr-based compounds, all transitions are reversible; for Nd, depending on the thermal treatment, the reversibility of ε→β 2 can be incomplete. These transformations are characterized using X-ray thermodiffractometry, differential thermal analysis, dilatometry and impedance spectroscopy versus temperature. Examination of Bi 4(Ln, Ln′) 2O 9 samples allows to correlate the evolution of the thermal behavior and of the unit cell parameters, to the lanthanide size. A partial plot of the (Bi 2O 3) 1− x –(La 2O 3) x phase diagram (0≤ x≤0.40) is proposed.

New monoclinic compounds, Bi 3.24Ln2W 0.76O 10.14, having a pseudo-orthohexagonal cell based on a pseudo-fcc subcell in the systems Bi 2O 3– Ln2O 3–WO 3 ( Ln=La, Pr, and Nd)

New compounds, Bi 3.24 Ln 2W 0.76O 10.14, have been found in the systems Bi 2O 3– Ln 2O 3–WO 3 ( Ln=La, Pr, and Nd). They crystallize ostensibly in the orthorhombic (or orthohexagonal) symmetry, e.g. with a=6.9694(1) A ̊ , b=4.0242(1) A ̊ , c=9.3335(1) A ̊ , and Z=1 for Ln=La. At the same time, the lattice forms a superstructure based on a pseudo-fcc subcell with a′≈5.6 A ̊ , where the transformation matrix is (1/2,1/2,−1)/(−1/2,1/2,0)/(1,1,1). However, the cation configuration has proved that the true symmetry is monoclinic ( C2/ m) with β≈90°. All new compounds undergo two reversible polymorphic transitions at about 980°C and at 1097–1210°C depending on Ln. Their electrical conductivity exhibited lower values (about 10 −4 S cm −1 at 500°C) despite the pseudo-fcc subcell in relation to the δ-Bi 2O 3 type which is the well-known good oxide-ion conductor.

Partial frustration of the copper spins in RbCuCl 3

In order to investigate the magnetic order of the copper moments in the monoclinic (distorted hexagonal) compound RbCuCl 3 we carried out neutron diffraction experiments using single crystals and powder samples. Below the Néel temperature T N=19.3 K a helimagnetic structure with the propagation vector k mon=(0, 0.5985, 0) was observed. The magnetic moments lie in the monoclinic ab-plane and rotate along the b-axis with a turning angle of ϑ=108° between the adjacent planes (± 1/2 b). In the hexagonal representation the propagation vector corresponds to k hex=(0.2993, 0.2993, 0). This vector is close to k =(1/3, 1/3, 0) and therefore we deal with a distorted triangular antiferromagnet with partially released frustration. At 1.6 K the experimental magnetic moment of the copper atoms is μ exp=0.69(6) μ B. From susceptibility measurements with a SQUID magnetometer we derived the coupling constants of exchange interactions of the copper spins between ( J 0=25(1) K) and within ( J 1=−9(1) K) the stacked planes of triangles.

Crystal Chemistry and Physical Properties of Superconducting and Semiconducting Charge Transfer Salts of the Type (BEDT-TTF)4[AIMIII(C2O4)3]·PhCN (AI= H3O, NH4, K; MIII= Cr, Fe, Co, Al; BEDT-TTF = Bis(ethylenedithio)tetrathiafulvalene)

Synthesis, structure determination by single-crystal X-ray diffraction, and physical properties are reported and compared for superconducting and semiconducting molecular charge-transfer salts with stoichiometry (BEDT-TTF)(4)[A(I)M(III)(C(2)O(4))(3)].PhCN, where A(I) = H(3)O, NH(4), K; M(III) = Cr, Fe, Co, Al; BEDT-TTF = bis(ethylenedithio) tetrathiafulvalene. Attempts to substitute M(III) with Ti, Ru, Rh, or Gd are also described. New compounds with M = Co and Al are prepared and detailed structural comparisons are made across the whole series. Compounds with A = H(3)O(+) and M = Cr, Fe are monoclinic (space group C2/c), at 150, 120 K a = 10.240(1) A, 10.232(12) A; b = 19.965(1) A, 20.04(3) A; c = 34.905(1) A, 34.97(2) A; beta = 93.69(1) degrees, 93.25(11) degrees, respectively, both with Z = 4. These salts are metallic at room temperature, becoming superconducting at 5.5(5) or 8.5(5) K, respectively. A polymorph with A = H(3)O(+) and M = Cr is orthorhombic (Pbcn) with a = 10.371(2) A, b = 19.518(3) A, c = 35.646(3) A, and Z = 4 at 150 K. When A = NH(4)(+), M = Fe, Co, Al, the compounds are also orthorhombic (Pbcn), with a = 10.370(5) A, 10.340(1) A, 10.318(7) A; b = 19.588(12) A, 19.502(1) A, 19.460(4) A; c = 35.790(8) A, 35.768(1) A, 35.808(8) A at 150 K, respectively, with Z = 4. All of the Pbcn phases are semiconducting with activation energies between 0.15 and 0.22 eV. For those compounds which are thought to contain H(3)O(+), Raman spectroscopy or C=C and C-S bond lengths of the BEDT-TTF molecules confirm the presence of H(3)O(+) rather than H(2)O. In the monoclinic compounds the BEDT-TTF molecules adopt a beta' ' packing motif while in the orthorhombic phases (BEDT-TTF)(2) dimers are surrounded by monomers. Raman spectra and bond length analysis for the latter confirm that each molecule of the dimer has a charge of +1 while the remaining donors are neutral. All of the compounds contain approximately hexagonal honeycomb layers of [AM(C(2)O(4))(3)] and PhCN, with the solvent occupying a cavity bounded by [M(C(2)O(4))(3)](3-) and A. In the monoclinic series each layer contains one enantiomeric conformation of the chiral [M(C(2)O(4))(3)](3-) anions with alternate layers having opposite chirality, whereas in the orthorhombic series the enantiomers form chains within each layer. Analysis of the supramolecular organization at the interface between the cation and anion layers shows that this difference is responsible for the two different BEDT-TTF packing motifs, as a consequence of weak H-bonding interactions between the terminal ethylene groups in the donor and the [M(C(2)O(4))(3)](3-) oxygen atoms.