EPR g factors and tetragonal distortion for the isoelectronic Ni and Cu centers in the CuGaSe2 crystal

The EPR g factors, g_|| and g_⊥, for the isoelectronic 3d⁹ ions Ni⁺ and Cu²⁺ at the tetragonal Cu⁺ site of the CuGaSe₂ crystal are calculated from the high-order perturbation formulas based on a two-spin-orbit-parameter model. In the model, both the contributions to g factors from the spin-orbit parameter of central 3d⁹ ion and that of ligand ion are contained. The calculated results appear to be consistent with the experimental values. The tetragonal distortions (characterized by θ−θ₀, where θ is the angle between the metal-ligand bond and C₄ axis, and θ₀≈54.74° is the same angle in cubic symmetry) of Ni⁺ and Cu²⁺ centers, which are different from the corresponding angle in the host CuGaSe₂ crystal and from impurity to impurity, are obtained from the calculations. The difference of the sign of g_||−g_⊥ between the isoelectronic Ni⁺ and Cu²⁺ centers is found to be due to the different tetragonal distortions of both centers in the CuGaSe₂ crystal.     

Phase stratification in the Nd1 − x BaxCoO3 − δ (0.3 ≤ x ≤ 0.54) system

Slowly cooled Nd_{1 ? x} Ba_xCoO_{3 ? δ} samples were two-phase in the concentration interval 0.3 ≤ x ≤ 0.46. One of the phases had O-orthorhombic lattice distortions (Pbnm) characteristic of ferromagnetic samples with x ≤ 0.3, and the other phase had tetragonal distortions (P4/mmm) characteristic of samples with x ≥ 0.46. Tetragonal distortions were caused by ordering of Nd³⁺ and Ba²⁺ ions. Samples with ordered neodymium and barium ions (Nd_{1 ? y} Ba_{1 + y} Co₂O_{6 ? γ} at ?0.08 ≤ y ≤ 0.08) experienced metal-dielectric and orientation magnetic phase transitions.     

The superconducting bismuth-based mixed oxides

The present paper describes the synthesis, characterization of mixed-valence bismuthates with three- or two-dimensional perovskite-like structures and structural criteria that influence superconductivity in these compounds.Single-phase samples of Sr_1−xK_xBiO₃ were prepared for the broad range of K-content: 0.25⩽x⩽0.65. For these bismuthates the symmetry of the structure changes from monoclinic to orthorhombic and finally to tetragonal upon increasing the K-content thus resulting in the decrease of the Bi–O distances and reduction of the network distortions. Superconductivity with maximum T_c=12 K exists in the narrow range (x≈0.5–0.6) within the stability field of the tetragonal phase (0.33⩽x⩽0.65), when the three-dimensional octahedral framework has close to the ideal perovskite structure arrangement.The layered type (Ba,K)₃Bi₂O₇ and (Ba,K)₂BiO₄ bismuthates belonging to the A_n+1B_nO_3n+1 homologous series were investigated. Buckling of the (BiO₂) layers in the structure of the n=2 member occurs due to the ordering of alkaline- and alkaline-earth cations between two independent positions. The formation of the one-layer bismuthate was revealed by Electron Microscopy and XRPD studies. Both types of compounds are considered to be possible candidates for new superconducting materials among bismuthates.     

The heat capacity of the layer compounds (CH3CH2CH2NH3)2MnCl4 and (CH3CH2CH2NH3)2CdCl4 from 10 K TO 300 K

The heat capacity of the layer compounds tetrachlorobis (n-propylammonium) manganese II and tetrachlorobis (n-propylammonium) cadmium II, (CH₃CH₂CH₂NH₃)₂MnCl₄ and (CH₃CH₂CH₂NH₃)₂CdCl₄ respectively, has been measured over the temperature range 10 K ?T ? 300 K.Two known structural phase transitions were observed for the Mn compound in this temperature region: at T = 112.8 ± 0.1 K (ΔH_t= 586 ± 2 J mol^?1; ΔS_t = 5.47 ± 0.02 J K^?1mol^?1) and at T =164.3 ± (ΔH_t = 496 ± 7 J mol^?1; ΔS_t =3.29 ± 0.05 J K^?1mol^?1). The lower transition is known to be from a monoclinic structure to a tetragonal structure, while the upper is from the tetragonal phase to an orthorhombic one. From comparison with the results for the corresponding methyl Mn compound it is deduced that the lower transition primarily involves changes in H-bonding while the upper transition involves motion in the propyl chain.A new structural phase transition was observed in the Cd compound at T= 105.5 ± 0.1 K (ΔH_t= 1472.3 ± 0.1 J mol^?1; ΔS_t = 13.956 ± 0.001 J K^?1mol^?1), in addition to two transitions that have been observed previously by other techniques. The higher of these transitions(T = 178.7 ± 0.3 K; ΔH_t = 982 ± 4 J mol^?1 ΔS_t = 6.16 ± 0.02 J K^? mol^?1) is known to be between two orthorhombic structures, while the structural changes at the lower transition (T= 156.8 ± 0.2 K; ΔH_t = 598 ± 5 J mol^?1, ΔS_t = 3.85 ± 0.03 J K^?1 mol^?1) and at the new transition are not known. It is proposed that these two transitions correspond respectively to the tetragonal to orthorhombic and monoclinic to tetragonal transitions in the propyl Mn compounds.In addition to the structural phase transitions (CH₃CH₂CH₂NH₃)₂MnCl₄ magnetically orders at t? 130 K. The magnetic contribution to the heat capacity is deduced from the heat capacity of the corresponding diamagnetic Cd compound and is of the form expected for a quasi 2-dimensional Heisenberg antiferromagnet.     

Tetragonal distortion for AIBIIICVI2 chalcopyrite compounds

Pauli-force constants and Simons-Bloch radii for the atoms of groups IB, III and VI are reported. The latter have been used to define a dimensionless quantity ΔX_ch which is found to be correlated in an analytical way with the observed tetragonal distortion δ of A^IB^IIIC^VI₂ type chalcopyrite compounds.     

Vanadium phosphates on mesoporous supports: model catalysts for solid-state NMR studies of the selective oxidation of n-butane

SBA-15 was utilized as mesoporous support for the dispersion of vanadium phosphate (VPO) compounds. Loading of SBA-15 with VPO compounds was found to be accompanied by decreasing ²⁹Si MAS NMR signals of Q² (Si(2Si,2OH)) and Q³ (Si(3Si,1OH)) silicon species, which indicates coverage of the mesoporous support by the guest compounds. The ⁵¹V MAS NNR spectra of the activated VPO/SBA-15 catalysts consist of patterns typical for the α_II- and β-phases of vanadyl orthophosphate. In the ³¹P MAS NMR spectra of the activated VPO/SBA-15 catalysts, signals of β-, δ-, and α_II-VOPO₄ phases could be identified. Upon conversion of n-butane-¹³C₄, a strong decrease of the ³¹P MAS NMR signals characteristic for the δ-VOPO₄ phase occurred, while by ¹³C MAS NMR spectroscopy the formation of maleic anhydride, carbon monoxide, and carbon dioxide was observed. This finding supports the active role of the δ-VOPO₄ phase in the selective oxidation of n-butane on VPO/SBA-15 catalysts.     

Observation of the “bottleneck effect” for a non S-state ion in a d-band intermetallic : LuRh2 : Nd

We demonstrate the existence of bottleneck effect in the relaxation mechanism of Nd³⁺ localized moment in the d-band LuRh₂ intermetallic compound. The ESR data are analyzed using a phenomenological theory for the dynamic susceptibility. The resonance properties yield information about the spin-flip relaxation of the conduction electrons to the lattice δ_eL. The values of δ_eL are smaller than those observed previously for the analogous s-band compounds.     

Structural information from EPR powder spectra of Mn(II)-doped Rb2MgCl4, Rb2MgCl3Br and Rb2MgCl2Br2

The structure of the compounds Rb₂MgCl₄, Rb₂MgCl₃Br and Rb₂MgCl₂Br₂ has been determined to be the tetragonal K₂NiF₄ structure. EPR spectra at X and Q-band frequencies of the polycrystalline samples of the Mn(II)-doped compounds, indicate that the Br^? ions prefer to order at special sites of the structure, as concluded from the fact that only octahedral groups [MgCl₆] and tetragonal groups [MgCl₄Br₂] are found.     

Quadratic Jahn-Teller effect in octahedral clusters. An interpretation of the excitation spectrum of MgO: V2+

The tunneling between tetragonal and orthorhombic stable distortions in an O_h complex is considered. The results are discussed in view of a possible interpretation of the structure of the no-phonon line (⁴A_2g→⁴T_2g) in MgO: V²⁺.     

14N NMR in NH4I and NH4Br

¹⁴N magnetic resonance data show that the α → β transition in NH₄I is connected with a discontinuous change in the ¹⁴N chemical shift, Δδ = 20.5 ppm, whereas the order-disorder type β(O_h) → γ(D_4h) transition is accompanied by a small quadropole splitting of the ¹⁴N spectrum. A quadrupole splitting of the ¹⁴N line was as well found in the tetragonal “antiferromagnetically” ordered γ-phase of NH₄Br, but not in the “ferromagnetically” ordered δ(T_d) phase of NH₄Cl.     

Formation and lithuim ionic conduction of the antifluorite type solid electrolytes,Li2+x-y-z[(NH)1-x-y-zNxClyHz]

Antifluorite type solid electrolytes, Li_2+x-y-z[(NH)_1-x-y-zN_xCl_yH_z] have been synthesized from Li₃N, LiCl and LiCl·H₂O or $\text{Li}{}_2\text{[(}\text{NH}\text{)}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{N}{}_{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}\text{H}{}_{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$]. Either of the last two compounds introduces NH^2- ions into the product and is indispensable to form the antifluorite type structure. The lattice parameter decreases with increasing amount of NH^2- ions in the compound, reflecting the difference of anion sizes between Cl^- and NH^2- ions. Activation energies of Li⁺ diffusive motion, derived from both conductivity and NMR measurements, increase with concentration of NH^2- ions, although the values differ by a factor of ≈2 netween the both methods.     

Phase transitions in N—n-propylpyridinium-TCNQ2 and N—n-butylpyridinium-TCNQn at high pressures

The electrical resistivity of N-n-propylpyridinium-TCNQ₂ (NPPy-TCNQ₂) and N-n-butylpyridinium-TCNQ_n (NBPy-TCNQ_n) has been measured as a function of temperature and pressure. Phase transitions in these salts have been studied at high pressures. The transition temperature (T_c) in NPPy-TCNQ₂ at atmospheric pressure increased with increasing pressure at the rate of dT_c/dP = + 12.0 degkbar^?1. The value of volume change calculated from the Clapeylon-Clausius relation was + 4.4 cm³ mol^?1. The electrical resistivity along the a- and c-axis increased with increasing pressure below 7 kbar. This anomalous electrical behaviour is closely related to the crystal structure of NPPy-TCNQ₂. The resistivity dropped sharply at about 11 kbar. This abrupt change may be due to a new pressure induced phase transition.The T_c of the NBPy-TCNQ_n increased remarkably with increasing pressure up to 0.7 kbar, above which the phase transition disappeared. The phase transitions of N-n-alkyl-substituted pyridinium TCNQ salts depend strongly on the nature of cations.     

Mössbauer and magnetization studies of the NpM2Ge2(M=Cr,Fe, Co,Ni, Cu) intermetallic compounds

Magnetization and ²³⁷Np Mössbauer studies were performed on tetragonal NpM₂Ge₂ intermetallic compounds. All the compounds order antiferromagnetically. The Néel points and the magnetic hyperfine interactions of the NpM₂Ge₂ compounds are 62(3), 28(3), 36(5), 27(3), 34(3) K and 2860(40), 1800(40), 3040(40), 2600(40), 2330(90) Mc/sec for M=Cr, Fe, Co, Ni and Cu respectively.     

Tetragonal distortions of the octahedral environments of Cr3+, Fe3+ and Gd3+ ions in A2CdF4 (A = Rb,Cs) and ACdF3 crystals

Abstract

The tetragonal distortions of local octahedral environments of Cr³⁺, Fe³⁺ and Gd³⁺ ions in Rb₂CdF₄, Cs₂CdF₄, RbCdF₃ and CsCdF₃ crystals have been studied by analyzing their EPR spectra. From the studies, it is found that the tetragonal distortions for Cr³⁺ and Fe³⁺ impurity ions, which substitute Cd²⁺ and have nearly the same ionic radius, are close to each other, whereas that for Gd³⁺ impurity ion, having a larger ionic radius, is larger than those for Cr³⁺ and Fe³⁺ ions in the same crystal. It appears that not only the impurity-ligand distance, but also the tetragonal distortions of impurity centres in crystals are closely related to the size of impurity.     

Magnetic properties of the compounds N(CH3)4MnIIMIII(CN)6 ·8 H2O(MIII = Mn,Fe)

In the isostructural cyanobridged chain compounds N(CH₃)₄Mn^IIM^III(CN)₆ · 8H₂O high spin Mn(II) ions couple antiferromagnetically to low spin Mn(III) of Fe(III) ions. The Mn^II–Mn^III compound orders ferrimagnetically below T_N = 28.5 ± 1 K. The tetragonal a and b axes are easy ones for the magnetic moments. In the Mn^II–Fe^III compound antiferromagnetic order occurs below T_N = 9.3 K, with spins aligned along the tetragonal c axis. The compound undergoes a meta-magnetic transition from the antiferromagnetic to a ferrimagnetic phase. This occurs at 2 K for a field H_crit ≈ 1.2 T. The temperature dependence of H_crit, which vanishes at T_N, is followed. The tricritical temperature T¹ is ～ 5 K.     

Paramagnetic defects in manganese-doped lead tungstate

In manganese-doped PbWO₄ crystals, low-intensity signals of triclinic clusters Mn⁴⁺-V _O and Fe³⁺-V _Pb have been revealed in addition to signals of Mn²⁺ tetragonal centers. The Mn⁴⁺-V _O cluster is formed by a Mn⁴⁺ ion in the W⁶⁺ position, which is associated with a vacancy of the nearest neighbor O^2?ion, and the Fe³⁺-V _Pb cluster consists of a Fe³⁺ ion substituting for Pb²⁺ with a local compensation of by a lead vacancy. It has been shown that, in PbWO₄: Mn, there is also a small amount of Mn⁴⁺ tetragonal centers located in the Pb²⁺ position with a nonlocal compensation of an excess charge.     

Differential scanning calorimetry of phase transitions in (CnH2n+1NH3)2MnCl4 compounds

A second order phase transition between the space groups D¹⁸_2h and D¹⁷_4hleads to the high temperature tetragonal phase of (CH₃NH₃)₂MnCl₄. Similar transitions to tetragonal phases exist also in the ethyl- and propyl-compounds. Transition temperature increase with an increasing carbon chain length. Very low ΔH- and ΔS-values are compatible with a transition model obtained from nuclear resonance experiments. Further thermoanalytical results bear evidence on the complex role of alkyl-ammonium groups.     

Synthesis and characterization of a new series of BiOI1−x−yBrxCly pigments

New bismuth oxyhalides of the type BiOI_1?xBr_x, form a complete solid solution series and crystallize in a layered tetragonal matlockite structure Color varies from yellow to red as x decreases from 0 75 to 0 and layer separation, c, increases from 8 45 to 9 16 Å The compounds have large intrinsic optical absorption coefficients (α > 10⁵ cm^-1). Used as pigments, they are light-fast but show less color saturation than PbCrO₄ of the same hue.     

Phase transformations in perovskites TbBaCo2−x FexO5 + γ

The transport, magnetic, and elastic properties of TbBaCo_2?x Fe_xO_{5 + γ} are investigated. It is shown that these compounds exhibit first-order metal-insulator and antiferromagnet-weak ferromagnet transitions in the orthorhombic phase (x < 0.12), while these transitions are not observed in the tetragonal phase (x > 0.12). In the concentration range corresponding to the orthorhombic phase, doping with iron stabilizes the weakly ferromagnetic phase. However, the tetragonal phase is antiferromagnetic. Oxygen vacancies are assumed to be ordered in the orthorhombic case and disordered in the tetragonal phase. An analysis of Young’s modulus, magnetostriction, and effects of pressure and substitution of the O¹⁸ oxygen isotopes for O¹⁶ indicates a weak correlation between magnetic transformations and the crystal lattice.