Temperature dependence of the electric quadrupole interaction in PbHfO3

The temperature dependence of the Electric Field Gradient (EFG) in PbHfO₃ was studied in the temperature range 25–225°C by the Differential Perturbed Angular Correlation method. In the two anti-ferroelectric phases below 215°C, not too close to either transition temperature, the EFG decreases as the temperature increases toward T_c. Just above T_c an abrupt rise of EFG was observed indicating a critically behaving contribution to EFG. The results are interpreted in terms of a model based on the local static as well as time-dependent changes of the electric environment, at a lattice site. In accord with the predictions of this model the results exhibit qualitatively the P²_s temperature dependence of EFG far from T_c, where P_s is the sub-lattice polarization, while in close proximity to T_c the dominant contribution to EFG is due to the susceptibility X_q connected with the soft-mode fluctuations. The derived critical exponents are in agreement with previous experimental results on related compounds and with theoretical predictions. A hitherto unobserved additional component of V_zz was established, behaving critically at the antiferro-paraelectric transition at 215°C. This new component is interpreted to originate in local fluctuations connected with the central mode.     

V4+ in SrTiO3: A Jahn-Teller impurity

EPR results of V⁴⁺, with $\text{S =}\text{1}\text{2}$, in SrTiO₃ are reported. The tetragonal local symmetry of the impurity ion is related to strong T_2g × ?_g coupling as evidenced by intensity variations in the presence of stress. At 4.2 K the V⁴⁺ EPR behaviour is related to the intrinsic local strain in SrTiO₃.     

Al2O3:V3+晶体局域结构及其自旋哈密顿参量研究

提出了解释掺杂离子局域结构畸变的配体平面移动模型，建立了此模型下晶体微观结构与自旋哈密顿参量之间的定量关系.在考虑自旋与自旋、自旋与另一电子轨道和轨道与轨道作用等微小磁相互作用的基础上，采用全组态完全对角化方法，对Al₂O₃晶体中V³⁺的局域结构和自旋哈密顿参量进行了系统的研究.结果表明，V³⁺掺入Al₂O₃晶体后，上下配体氧平面间距离增大了0.0060 nm.从而成功地解释了Al₂O₃:V³⁺晶体的自旋哈密顿参量.在此基础上，研究了三角晶场下3d²离子自旋哈密顿参量的微观起源.研究发现，自旋三重态对自旋哈密顿参量的贡献是主要的，微小磁相互作用对自旋哈密顿参量的贡献只与自旋三重态有关.     

Domain configuration and dynamic processes in orthorhombic NaCN studied by 23Na NMR satellite lines and relaxation times

The orientational dependences of the ²³Na NMR spectra which were measured in a NaCN single crystal below T_c1 are related to the EFG tensor at the Na sites in different domains. The results are consistent with the assumption that 12 different domains may arise in the orthorhombic phase. Below T_c1 also the temperature dependences of the spin relaxation times T₁ and T_1ρ of the ²³Na nucleus were measured and related to dynamic processes. Particularly the activation energy and the frequency of the flip motions of the CN^--dipoles are determined.     

Li- and V-NMR study of LiVS2

⁷Li- and ⁵¹V-NMR have been measured to make clear the electronic state in a two-dimensional triangular lattice LiVS₂. Knight shift of both ⁷Li- and ⁵¹V-NMR is almost independent of temperature below the phase transition temperature T_c of about 310 K from the paramagnetic state to non-magnetic state. The ⁵¹V- spin-lattice relaxation rate 1/T₁ reveals an exponential temperature dependence below T_c, indicating a gap structure of electronic state. These results are consistent with a non-magnetic state with a trimer singlet of V³⁺ spins below T_c.     

Hyperfine interactions measurements on 181Ta probes in HfV2Hx compounds: (0⩽x≲4)

The electric field gradient (EFG) has been measured on ¹⁸¹Ta probe sites in HfV₂H_x alloys (0?x?4) using the time differential perturbed angular correlation technique. One observes that the main component V_zz of the EFG decreases by a factor of 2 for hydrogen concentrations increasing from x = 0 to x = 4 hydrogen atoms/formula. The opposite behavior was observed on ⁵¹V by using the NMR technique. At T = 4.2 K the EFG tensor is non-axially symmetric. The observed asymmetry factor for x = 0 is η = 0.58(3) and it slightly increases to η = 0.70 for x?4. The system is orthorhombic at low temperatures for all x and the experiments suggest that hydrogen would stabilize a tetragonal phase at higher temperatures. The relaxation effects are absent for x = 0 and vanishingly small for x ? 4 but they are observed at intermediate concentrations supporting the idea of a hydrogen ordering for x = 4.     

EPR of V2+ in SrCl2

The X-band EPR spectrum of SrCl₂:V has been measured at liquid nitrogen temperature. A signal associated with V²⁺ in a site of trigonal symmetry is observed. The EPR data have been explained using the spin hamiltonian $\textcolor{red}{}\text{= μ}{}_{\mathrm{\beta }}\text{H}\text{g}\text{?}\text{S}\text{+ D[S}{}^2{}_{\mathrm{z}}\text{?}\text{1}\text{3}\text{S(SH)] +}\text{S}\text{A}\text{?}\text{I}$, with D ? hv, g_∥ = 1.957 ± 0.004, g₆ = 1.954 ± 0.004, A_∥ = 230 ± 5 MHz, A₆ = 235 ± 5 MHz. This V²⁺ defect is similar to those previously reported in fluoride crystals with the fluorite structure.     

Investigation of phase transition mechanisms of CsH3(SeO3)2 single crystals by observation of nuclear magnetic resonance

The relaxation times of the ¹H and ¹³³Cs nuclei in CsH₃(SeO₃)₂ crystals were investigated using FT NMR. The ¹³³Cs spectrum does change from seven resonance lines to one resonance line near T_m (=350 K). The presence of only one ¹³³Cs signal is due to the liquid state resulting from the melting of the crystal. The variation in the separation of the ¹³³Cs resonance lines with temperature indicates that the EFG at the Cs sites produced by the (SeO₃)²⁻ groups varies with temperature, which in turn means that the atoms neighboring ¹³³Cs are displaced. And, the T₁ for ¹³³Cs is very long and undergoes significant changes near T_m. The change in the temperature dependence of T₁ at T_m for the ¹³³Cs nuclei coincides with the melting temperature. These results are compared with those obtained for MH₃(SeO₃)₂ (M=Na, K, and Cs) crystals.     

EPR study of a new low-temperature phase transition in Zn(ClO4)2·6H2O

From a temperature variation EPR study of Mn²⁺ doped single crystals of Zn(ClO₄)₂·6H₂O phase transition has been detected at T₂~290 K. The phase relationships in this crystal are as follows. Phase I transforms atT₂~346K to Phase II, which in turn transforms to Phase III at T₂ ~ 290K. The latter exists down to at least 220 K. The space group symmetry of crystal may be the same, i.e. Pmn2₁ both above and below T₂. The water-perchlorate sublattice symmetry below T₂ is found to be lower than the P6₃mc symmetry determined previously by X-ray measurements. The onset of a monoclinic or lower symmetry distortion of the water octahedron around a metal ion which starts just below T₂, is reflected through the observed temperature dependence of the rhombic distortion parameter E. It is felt that during this phase transition a change in the degree of configurational disorder associated with the perchlorate tetrahedra takes place, which in turn modifies the hydrogen bonded interaction in the crystal and consequently results in the onset of temperature dependent displacements of the mean positions of the oxygens of the water molecules.     

Single-crystal absorption and emission spectra of CsVCl3 and CsMgCl3:V2+

Single-crystal absorption and emission spectra of CsVCl₃ and CsMgCl₃:V²⁺ in the region of the first excited state were measured at temperatures down to 6 K. The ⁴A_2g?⁴T_2g transition shows a large trigonal splitting of the ⁴T_2g state and surprisingly well-resolved fine structure which can be analyzed in terms of electronic origins and vibronic side bands.     

Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa₂Co₂V₃O₁₂ and AgCa₂Ni₂V₃O₁₂ has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T_N=6.39 K for AgCa₂Co₂V₃O₁₂ and 7.21 K for AgCa₂Ni₂V₃O₁₂, respectively. The magnetic susceptibilities exhibit broad maximum, and these T_N correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χ_max). The magnetic entropy changes from zero to 20 K per mol Co²⁺ and Ni²⁺ ions are 5.31 J K⁻¹ mol-Co²⁺-ion⁻¹ and 6.85 J K⁻¹ mol-Ni²⁺-ion⁻¹, indicating S=1/2 for Co²⁺ ion and S=1 for Ni²⁺ ion. The magnetic susceptibility of AgCa₂Ni₂V₃O₁₂ shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ_eff=3.23 μ_B Ni²⁺-ion⁻¹ and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa₂Co₂V₃O₁₂. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state ²E(t₂⁶e) and the first excited state is spin quartet state ⁴T₁(t₂⁵e²) which locates extremely close to the ground state. The low spin state S=1/2 for Co²⁺ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.     

V4+ brownian motion in splat cooled amorphous V2O5 after water vapor adsorption

Splat cooling of molten V₂O₅ yields an amorphous oxide consisting of strongly entangled polymeric vanadium-oxygen chains. Upon hydration of this powder, a swelling process takes place and a Brownian motion of the fibres occurs. Vanadium pentoxide is usually non-stoichiometric and contains vanadium ions in a reduced oxidation state, namely V₄₊. These paramagnetic ions, which are part of the fibres constituents, can be used as spin probes to study the Brownian motion. X and Q band ESR spectra appear to be drastically modified when water is adsorbed on the powder at a given temperature. They have been analysed according to Kivelson's theory in the fast tumbling region between 293 and 373 K. An apparent activation energy of 3.8 kcal mole^-1 and a correlation time of about 10^?10 sec are observed. These results agree with the polymeric model proposed to explain the solubility of amorphous V₂O₅.     

Crystal growth,structure, electron paramagnetic resonance and magnetic properties of (NH4)2V3O8

A method to grow single crystals of ammonium vanadate (IV, V) (NH₄)₂V₃O₈ has been devised. The crystal structure is tetragonal P4bm; residual factor is R = 0.030. Cell parameters are $\text{a = 8.891 ± 0.004}\text{A}\text{?}$ and $\text{c = 5.582 + 0.002}\text{A}\text{?}$. The V⁵⁺ atom lies at the center of a triangular pyramide (VO₄ tetrahedron) while the V⁴⁺ atom is on A 4-fold rotation axis at the center of a square-based pyramide VO₅ whose symmetry point group is almost C_4v with the short V = O bond lying along the 4-fold axis parallel to the c edge of the tetragonal cell. Crystals are thin platlets with (001) cleavage planes. The platlets have very often a square or rectangular shape limited by {100} or {110} planes. Each single crystal was not large enough to record a good e.p.r. spectrum, but by sticking on the same quartz plate a score of them it was possible to gather enough crystals so to record correct spectra and by orienting the plate to obtain resonance lines separately for g_∥ = 1.9263 et g_τ = 1.9755. Measurements at 283 K on powder samples gave times for spin-spin relaxation T₂ = 0.4 × 10^?7s and for spin-lattice relaxation T₁ = 1.6 × 10^?7s. The magnetic structure is characterized by an exchange narrowing ω_e = 3 × 10¹⁰rad/s which corresponds to a transition temperature of about 0.5 K. Static susceptibility measurements at high magnetic field show a paramagnetic behaviour with an antiferromagnetic interaction which is interpreted in the magnetic space group P_2c4bm as the interaction between V⁴⁺ ions from consecutive planes parallel to (001).     

Investigation of TmFe4Al8 and DyFe4Al8 by means of 169Tm and 161Dy Mössbauer spectroscopy

We have performed ¹⁶⁹Tm and ¹⁶¹Dy Mössbauer spectroscopy on TmFe₄Al₈ and DyFe₄Al₈. From the temperature dependence of the electric quadrupole splitting of the ¹⁶⁹Tm spectra of TmFe₄Al₈ we have determined the second order crystal field potential V⁰₂ = (100 ± 10) K and the exchange field term g_Jμ_BH_M = (1 ± 1) K. The temperature dependence of the hyperfine field of the ¹⁶¹Dy spectrum of DyFe₄Al₈ gives g_Jμ_BH_M = (15 ± 3) K. With these exchange fields magnetic transition temperatures of the rare earth sublattices were found, which are consistent with experiment. The relaxation behaviour of the Tm sublattice below T_N = 187 K is discussed.     

Electric field gradient at 12 N implanted into ZnO

The nuclear quadrupole interaction of short-lived β-emitter ¹²N(I?=?1, T _1/2?=?11 ms) implanted into a ZnO single crystal has been studied by means of the β-NMR method. We have observed a quadrupole splitting of ¹²N in ZnO at room temperature, from which the electric field gradient (EFG) q?=?+?(8.6 ±1.1) ×10¹⁹ V/m² was deduced assuming axially symmetric EFG with respect to the crystalline c axis. A first principle calculation does not reproduce the data under assumption of the lattice location of ¹²N at the oxygen substitutional site.     

Magnetic ordering in GdI2

GdI₂ powder samples have been investigated by means of a.c. susceptibility and magnetisation measurements indicating magnetic ordering below T_C = 313 K. The structure consists of strongly coupled ferromagnetic layers of Gd³⁺ moments aligned perpendicular to the hexagonal crystal axis. The inter-layer coupling is much weaker and probably antiferromagnetic suggesting that GdI₂ is to a very good approximation a two-dimensional Heisenberg system.     

NMR spin echoes and quadrupole effects in nickel single crystals

NMR spin echo measurements of naturally abundant⁶¹Ni have been performed with Néel-type nickel single crystals at 4.2 K. The NMR excitation condition could properly be chosen in order to get signals either from nuclei situated within magnetic domains or within domain walls. In both cases a quadrupole splitting of the NMR line could be observed. By applying an external magnetic field, the direction of the domain magnetization could be varied with respect to the fcc crystal lattice. From the variation of the quadrupole splitting with the crystallographic direction, the tensor of the electric field gradient (EFG) has been derived. The quadrupole splittings and the corresponding field gradients in the principal axis system of the EFG are: Δv_Q kHz, Δv_Q kHz, Δv_Q kHz, V_<111>=(6.6±0.5)·10¹⁸ V/m², V_<110>=4.6±0.5)·10¹⁸ V/m², V_<112>=(2.0±0.5)·10¹⁸ V/m².     

Al NMR studies of NpPd5Al2

We present ²⁷Al NMR studies for a single crystal of the Np-based superconductor NpPd₅Al₂. We have observed a five-line ²⁷Al NMR spectrum with a center line and four satellite lines separated by first-order nuclear quadrupole splittings. The Knight shift clearly drops below T_c. The temperature dependence of the ²⁷Al nuclear spin-lattice relaxation rate shows no coherence peak below T_c, indicating that NpPd₅Al₂ is an unconventional superconductor with an anisotropic gap. The analysis of the present NMR data provides evidence for strong-coupling d-wave superconductivity in NpPd₅Al₂.     

Crystal-field hamiltonian and spin-orbit interaction for d and d ions at low C3 symmetry sites: V:Al2O3 and Ni:LiNbO3

The experimental studies have provided evidence of the occurrence of transitions from the ³T_1g(³F) ground state to the crystal-field levels ³T_2g(³F), ³T_1g(³P) and ³A_2g(³F) for the V³⁺ centres in Al₂O₃ crystal; and from the ³A_2g(³F) ground state to the crystal-field levels ³T_2g(³F), ³T_1g(³F) and ³T_1g(³P) for the Ni²⁺ centres in LiNbO₃ crystal (levels are assigned to irreps of the O_h point symmetry group). Using the experimental spectroscopic data, theoretical calculations of the crystal-field levels of V³⁺:Al₂O₃ and Ni²⁺:LiNbO₃ are carried out based on the Racah theory. The observed crystalline-field splittings of the V³⁺ and Ni²⁺ terms were accounted for using a C₃ symmetry Hamiltonian. The spin-orbit interaction was taken into account in this work. The Racah, crystal-field and spin-orbit parameters, which fit experimental and theoretical energy levels, have been reliably obtained. A good agreement between the theoretical and experimental results for the energy levels of V³⁺:Al₂O₃ and Ni²⁺:LiNbO₃ has been obtained.     

Anomalous reduced sound velocity of multiferroic TbMn2O5

The anomalous reduced sound velocity of multiferroic TbMn₂O₅ (TMO) has been studied using Green's function technique. To achieve this aim, the anharmonic phonon-phonon interaction and the spin-phonon interaction were used. It was shown that the reduced velocity of sound of TMO exhibits a kink at the ferroelectric phase transition temperature T_C. This can be explained as an effect of vanishing ferroelectric ordering above T_C. It was found that the reduced sound velocity increases with increasing V⁽³⁾ (the third-order atomic force constants of the anharmonic phonons) in the interval T?<?T_C, whereas the reduced sound velocity remains unchanged in the interval T_C?<?T?<?T_N. It was also found that the reduced sound velocity increases with the increase of V⁽⁴⁾ (the fourth-order atomic force constants of the anharmonic phonons) in the interval T?<?T_N. In addition, the ferroelectric phase transition temperature T_C decreases when V⁽⁴⁾ increases in the interval T?<?T_N. Those theoretical results are in agreement with the experimental data.