Nuclear magnetic resonance in [N(CH3)4]2CoCl4 single crystals: transferred hyperfine interaction and spin-lattice relaxation rate

The molecular susceptibility and paramagnetic shift of [N(CH₃)₄]₂CoCl₄ single crystals were measured, and from these experimental results we obtained the transferred hyperfine interaction, H_hf, due to the transfer of spin density from Co²⁺ ions to [N(CH₃)₄]⁺ ions. The transferred hyperfine interaction can be expressed as a linear equation, with H_hf increasing with increasing temperature. The remarkable change in H_hf near T_c5 (=192 K) corresponds to a phase transition. The proton spin-lattice relaxation times of [N(CH₃)₄]₂CoCl₄ single crystals were also investigated, and it was found that the relaxation process can be described by a single exponential function. The variation of the relaxation time with temperature undergoes a remarkable change near T_c5, confirming the presence of a phase transition at that temperature. From the above results, we conclude that the increase in H_hf with increasing temperature is large enough to allow the transfer of spin density between Co²⁺ ions and the nuclear spins of the nonmagnetic [N(CH₃)₄]⁺ ions in the lattice, and thus the increase in the relaxation time with temperature is attributed to an increase in the transferred hyperfine field.     

Cs nuclear magnetic resonance study in CsZnCl3 single crystals of perovskite ABX3 type

CsZnCl₃ single crystals were grown by the slow evaporation method, and the spin-lattice relaxation rates and resonance lines of the ¹³³Cs nuclei in the resulting crystals were investigated using FT NMR spectrometry. The temperature dependence of the relaxation rate of the ¹³³Cs nuclei in the CsZnCl₃ crystals was found to be continuous near T_C (=366 K), and was not affected by this phase transition. Our results for CsZnCl₃ are compared with those obtained previously for other CsBCl₃ (B=Mn, Cu, and Cd) perovskite crystals. The Cs relaxation time of CsCdCl₃ is longer than that of CsMnCl₃. The differences between the atomic weights of Mn, Cu, Zn, and Cd are responsible for the differences between the spin-lattice relaxation times of these single crystals. The influence of paramagnetic ions is also important in these crystals. The differences between the spin-lattice relaxation times of these crystals could also be due to differences between the electron structures of their metal ions, in particular the structures of the d electrons.     

EPR study of the low temperature ferroelectric phase transition in Cu doped Rb2ZnCl4 single crystals

Temperature dependent EPR measurements on copper doped Rb₂ZnCl₄ single crystals allowed us to evidence and study the P2₁cn↔C1c1 structural phase transition that takes place in this compound at 74.6 K. From the two types of Cu²⁺ centers localized at different anionic sites, called Cu²⁺(I) and Cu²⁺(II), which are formed in this compound, only the Cu²⁺(II) centers exhibit observable changes in their EPR spectra, attributable to the symmetry lowering. The observed changes have been related to the soft-mode responsible for the structural phase transition.     

A study on new phase transitions in Cs2CaCl4·2H2O single crystals by observation of Cs NMR

The ¹³³Cs 1/2→−1/2 spin-lattice relaxation rate, , and the spin-spin relaxation rate, , for a Cs₂CaCl₄·2H₂O single crystal have been measured in function of temperature. The dominant relaxation mechanism of this crystal over the whole temperature range investigated here proceeds via quadrupole interaction. The changes in the ¹³³Cs spin-lattice relaxation rate near 325 K (=T_c1) and 360 K (=T_c2) correspond to phase transitions in the crystal. The change in the spin-lattice relaxation rate at T_c1 is small because the crystal lattice does not change very much during this phase transition. The change in near T_c2 is due to the critical slowing down of the soft mode that typically occurs in structural phase transitions. The temperature dependence of the spin-lattice relaxation rate for this crystal has maximum values at about 240 K, which is attributable to the effect of molecular motion as described by Bloembergen-Purcell-Pound theory. The phase transition temperatures T_c1 and T_c2 obtained from the temperature dependence of the relaxation rate is also clear from data obtained using differential scanning calorimetry. Therefore, we know that previously unreported phase transitions occur at 325 and 360 K.     

Proton magnetic resonance study of the low-temperature phase transition in a LiNH4SO4 single crystal

The proton NMR line width and spin-lattice relaxation times for LiNH₄SO₄ single crystal were studied at low temperature range of 6 and 280 K. The changes in the proton relaxation behavior near the phase transition temperature indicates a change in the state of internal motion at the transition. The molecular motions obtained by the spin-lattice relaxation processes were found to be determined by molecular reorientation of the NH₄ ions in phases III, IV, and V. We also confirmed that the phase transitions occur at 26 and 133 K.     

Proton magnetic resonance study of the phase transitions of [N(CH3)4]2BCl4 (B=Co, Cu, Zn, and Cd) single crystals

The proton spin-lattice relaxation rates in [N(CH₃)₄]₂BCl₄ (B=⁵⁹Co, ⁶³Cu, ⁶⁷Zn, and ¹¹³Cd) single crystals grown using the slow evaporation method were investigated over the temperature range 120-400 K. It was found that the relaxation processes of ¹H for all the [N(CH₃)₄]₂BCl₄ crystals can be described with single exponential functions. The changes in the ¹H relaxation behavior in the neighborhood of the phase transition temperatures are used to detect changes in the state of internal motion. From the ¹H spin-lattice relaxation rate measurements for [N(CH₃)₄]₂BCl₄ crystals, the activation energies were calculated for each phase. The large values of the activation energies indicate that the N(CH₃)₄ groups are significantly affected during the transitions. Although these [N(CH₃)₄]₂BCl₄ crystals all belong to the group of A₂BX₄-type crystals, their ¹H spin-lattice relaxation rates have different temperature dependences and indicate the occurrence of different molecular motions within the crystals. We additionally show for the first time that the differences in ¹H spin-lattice relaxation rates among the [N(CH₃)₄]₂BCl₄ (B=⁵⁹Co, ⁶³Cu, ⁶⁷Zn, and ¹¹³Cd) single crystals arise from differences in the electron structures of the metal ions within the series.     

NMR evidence of two frequency scales of hydrogen jump motion in Ti2Ni-type compounds Ti2CoHx

Nuclear magnetic resonance measurements of the proton spin-lattice relaxation rate R₁ for Ti₂Ni-type compounds Ti₂CoH_x (x=0.56, 0.77 and 1.34) have been performed over the temperature range 20-510 K and the resonance frequency range 13-90 MHz. For Ti₂CoH_0.77 and Ti₂CoH_1.34 the temperature dependence of R₁ is found to exhibit an additional low-temperature peak near 280 K; the amplitude of this peak increases with increasing H content. These results give evidence for the coexistence of at least two types of hydrogen jump motion with different characteristic frequencies. For Ti₂CoH_0.56 no additional R₁ peak has been found. The concentration dependence of the additional peak is discussed in terms of the occupancy of inequivalent interstitial sites by hydrogen atoms.     

X-ray and thermoluminescence of LiKB4O7 single crystals

The thermoluminescent (TL) and X-ray luminescent (XL) spectra of undoped LiKB₄O₇ (LKBO) single crystals had been investigated in the temperature range 80-300 K. It was found that in LKBO crystals, there are two intensive TL peaks at 112 and 132 K. The only one band emission spectra of sharply defined Gaussian shape, confirming the same mechanism of XL and TL by the radiation annihilation of the strongly localized self-trapped excitons (STE), had been observed in the TL and XL spectra. The possible models of these localization centers STE have been discussed.     

Growth and spectral characterization of β-Ga2O3 single crystals

Beta gallium oxide (β-Ga₂O₃) single crystals were grown by the floating zone technique. The absorption spectra and the luminescence of the crystals were measured. The absorption spectra showed an intrinsic short cutoff edge around 260 nm with two shoulders at 270 and 300 nm. Not only the characteristic UV (395 nm), blue (471 nm) and green (559 nm) lights, but also the red (692 nm) light can be seen in the emission spectra. The deep UV light was attributed to the existing of quantum wells above the valence band and the red light was owed to the electron-hole recombination via the vicinity donors and acceptors.     

Raman study of phase transitions in KNbO3

The influence of grain size on the phase transitions of ferroelectric KNbO₃ was studied by micro Raman spectroscopy. It was found that the three transitions observed are not sharp for small particles (∼50 μm), indicating that they do not behave like bulk particles. The transition temperatures depend on the size and all particles show hysteresis. From these experiments we have obtained some evidence that in small particles monodomains of the rhombohedral and orthorhombic phases coexist in a range of temperatures.     

Sequence of structural phase transitions of CsInF4 crystal

In this work we employ calorimetric and dielectric techniques to study the sequence of structural phase transitions (SPTs) of CsInF₄ crystal in the temperature range from 450 to 250 K. Our results show three first-order SPTs. Based on these results and on direct interferometric observation of the domain patterns, we discuss the elastic state of CsInF₄ phases.     

Bias field effects on the dielectric properties of 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 single crystals with different orientations

The domain states and phase transitions in 0.67Pb(Mg_1/3Nb_2/3)O₃-0.33PbTiO₃ single crystals were investigated by studying their relative permittivity under various dc bias at constant heating and cooling rates. The orientation dependence of the bias field effect was revealed by examining the temperature dependence of relative permittivity as a function of crystal orientation (the 〈111〉, 〈011〉 and 〈001〉 directions) and dc bias field. The crystals basically have a macrodomain rhombohedral ferroelectric state in the ferroelectric phase under zero dc bias. External bias field could modulate the domain state and induce a stable macrodomain state in the crystals. Also, it is proposed that the dc bias applied along the 〈001〉 or 〈011〉 direction could induce a tetragonal ferroelectric phase or an orthorhombic ferroelectric phase, respectively, in an intermediate temperature range.     

Vibronic model for antiferroelectric phase transition in squaric acid, H2C4O4

The antiferroelectric phase transition in H₂C₄O₄ crystals is analyzed using the neutron diffraction and high resolution ¹³C, ¹⁷O NMR data, Hartree-Fock, Moller-Plesset and DFT calculations. Is shown that AFE phase transition is of mixed order-disorder and of displacive type represented within of the unified vibronic interaction model.     

H NMR study of the phase transitions of trissarcosine calcium chloride single crystals at low temperature

The ¹H NMR line-width and spin-lattice relaxation time T₁ of TSCC single crystals were studied. Variations in the temperature dependence of the spin-lattice relaxation time were observed near 65 and 130 K, indicating drastic alterations of the spin dynamics at the phase transition temperatures. The changes in the temperature dependence of T₁ near 65 and 130 K correspond to phase transitions of the crystal. The anomalous decrease in T₁ around 130 K is due to the critical slowing down of the soft mode. The abrupt change in relaxation time at 65 K is associated with a structural phase transition. The proton spin-lattice relaxation time of this crystal also has a minimum value in the vicinity of 185 K, which is governed by the reorientation of the CH₃ groups of the sarcosine molecules. From this result, we conclude that the two phase transitions at 65 and 130 K can be discerned from abrupt variations in the ¹H NMR relaxation behavior, and that ¹H nuclei play important roles in the phase transitions of the TSCC single crystal.     

Phase transition and ferroelastic property studied by using the Cs nuclear magnetic resonance in a CsHSO4 single crystal

The ¹³³Cs spin-lattice relaxation time in a CsHSO₄ single crystal was measured in the temperature range from 300 to 450 K. The changes in the ¹³³Cs spin-lattice relaxation rate near T_c1 (=333 K) and T_c2 (=415 K) correspond to phase transitions in the crystal. The small change in the spin-lattice relaxation time across the phase transition from II to III is due to the fact that during the phase transition, the crystal lattice does not change very much; thus, this transition is a second-order phase transition. The abrupt change of T₁ around T_c2 (II-I phase transition) is due to a structural phase transition from the monoclinic to the tetragonal phase; this transition is a first-order transition. The temperature dependences of the relaxation rates in phases I, II, and III are indicative of a single-phonon process and can be represented by T₁⁻¹=A+BT. In addition, from the stress-strain hysteresis loop and the ¹³³Cs nuclear magnetic resonance, we know that the CsHSO₄ crystal has ferroelastic characteristics in phases II and III.     

Geometric frustration in CuV2S4

A cubic spinel conductor CuV₂S₄, containing the corner-sharing tetrahedral lattice of V, was investigated. Above a structural transition at 90 K, the ⁵¹V nuclear spin-relaxation rate 1/T₁ is comparatively large in magnitude and tends to saturate at higher temperatures, indicating that the V lattice behaves as a nearly antiferromagnetic metal. By substituting nonmagnetic Sn for V, the susceptibility turns to be of Curie-Weiss type and temperature-independent, suggesting recovery of hidden spins as a result of a partial release of the frustration. These results lead to the conclusion that CuV₂S₄ is an itinerant-electron frustrated system as YMn₂. The CDW transition of CuV₂S₄ accompanied by a tetragonal distortion which may be understood as a result of lowering of interaction dimensionality inherent to the corner-sharing tetrahedral lattice.     

Crystal structure and structural transition caused by charge-transfer phase transition for iron mixed-valence complex (n-C3H7)4N[FeFe(dto)3] (dto=C2O2S2)

(n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] shows a new type of first order phase transition called charge-transfer phase transition around 120 K, where the charge transfer between Fe^II and Fe^III occurs reversibly. Recently, we have succeeded in obtaining single crystals of the title complex and determined the crystal structure at room temperature. Crystal data: space group P6₃, Z=2. Moreover, we have investigated the structural transition caused by the charge-transfer phase transition by means of powder X-ray diffraction measurement. When the temperature is decreased, the a-axis, which corresponds to the hexagonal ring size in two-dimensional honeycomb network structure of [Fe^IIFe^III(dto)₃]_∞, contracts by 0.1 Å at the charge-transfer transition temperature (T_CT), while the c-axis, perpendicular to the honeycomb network layer, elongates by 0.1 Å at T_CT. Consequently, when the temperature is decreased, the unit cell volume decreases without noticeable anomaly around T_CT, which is responsible for the quite small vibrational contribution to the entropy change, compared with usual spin crossover transition. Thus, the charge-transfer phase transition around 120 K for (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] is regarded as spin entropy driven phase transition.     

Proton NMR relaxation study of the CsHSO4 solid acid system

At 141 °C the solid acid CsHSO₄ is known to undergo transition to a superprotonic phase that is characterized by dramatic (several-order-of-magnitude) increases in hydrogen ion conductivity. Proton NMR spin-spin relaxation time T₂ measurements reported here for CsHSO₄ also reveal substantial increases (factors of 20-30) in the vicinity of the transition temperature. In the temperature range just below the transition (70-136 °C), T₂ increases by a factor of order 10 relative to the rigid-lattice regime, suggesting motional narrowing of the NMR resonance line. In the regime of motional narrowing, the activation energy barrier to diffusion is 0.40 eV, as determined from the present T₂ results. NMR spin-lattice relaxation T₁ measurements also show behavior consistent with transition to a regime of rapid hydrogen motion. In particular, proton T₁'s decrease with temperature (from 80 to 120 °C), and then drop sharply near the transition temperature. Above the transition temperature, T₁ exhibits a minimum in which the correlation time is found to be ∼2 ns.     

Nonvolatile hologram storage properties of tri-doped Zn:Ce:Cu:LiNbO3 crystals

Congruent Zn(7 mol%):Ce:Cu:LiNbO₃ single crystal was grown by the Czochralski method in air. The occupation mechanism of the Zn²⁺ was discussed by an infrared transmittance spectrum. The nonvolatile holographic recording in Zn(7 mol%):Ce:Cu:LiNbO₃ single crystal was measured by two-photon fixed method. Zn(7 mol%):Ce:Cu:LiNbO₃ single crystals present the faster recording time and higher light-induced scattering resistance ability comparing with Ce:Cu:LiNbO₃ single crystals.     

Magnetic resonance study of the two-dimensional spin diffusion in the Heisenberg paramaget (C18H37NH3)2MnCl4

¹H nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) techniques were employed to study the perovskite-type layered structure compound (C₁₈H₃₇NH₃)₂MnCl₄ undergoing structural phase transitions. The spin relaxation was found to sensitively reflect the two-dimensional electron spin diffusion.