H nuclear magnetic resonance study of the ferroelastic and superionic phase transitions of (NH4)4LiH3(SO4)4 single crystals

We investigated the temperature dependences of the line shape, spin-lattice relaxation time, T₁, and spin-spin relaxation time, T₂, of the ¹H nuclei in (NH₄)₄LiH₃(SO₄)₄ single crystals. On the basis of the data obtained, we were able to distinguish the “ammonium” and “hydrogen-bond” protons in the crystals. For both the ammonium and hydrogen-bond protons in (NH₄)₄LiH₃(SO₄)₄, the curves of T₁ and T₂ versus temperature changed significantly near the ferroelastic and superionic phase transitions at T_C (=232 K) and T_S (=405 K), respectively. In particular, near T_S, the ¹H signal due to the hydrogen-bond protons abruptly narrowed and the T₂ value for these protons abruptly increased, indicating that these protons play an important role in this superionic phase transition. The marked increase in the T₂ of the hydrogen-bond protons above T_S indicates that the breaking of O-H?O bonds and the formation of new H-bonds with HSO₄^- contribute significantly to the high-temperature conductivity of (NH₄)₄LiH₃(SO₄)₄ crystals.     

Li and K NMR relaxation study of a LiKSO4 single crystal

The ⁷Li and ³⁹K NMR relaxations in a LiKSO₄ single crystal grown by the slow evaporation method were investigated by employing a pulse NMR spectrometer. From the experimental data, the quadrupole coupling constant and asymmetry parameter were determined at the temperatures of 180 and 300 K. The relaxation processes of ⁷Li and ³⁹K were studied for the LiKSO₄ crystal, and the relaxation times for the ⁷Li and ³⁹K nuclei exhibit remarkable changes near T_c2 (=190 K). The activation energies for ⁷Li and ³⁹K were determined in phases I and III. The large change in the activation energy at 190 K indicates that the Li and K ions are significantly affected during this transition. The correlation time of the ⁷Li calculated from the spin-lattice relaxation time and quadrupole parameters was larger than that of the ³⁹K calculated using the same method. The reason for this is that the Li ion undergoes molecular motion as in the LiO₄ groups.     

Study of the ferroelectric–paraelectric phase transition of NaH3(SeO3)2 single crystals by H and Na NMR

The ¹H and ²³Na spin–lattice and spin–spin relaxation times of NaH₃(SeO₃)₂ single crystals grown using the slow-evaporation method were measured as functions of temperature and frequency in the ferroelectric and paraelectric phases. The changes in the symmetry of the (SeO₃)²⁻ dimers as a result of the ferroelectric–paraelectric phase transition are associated with large changes in the spin–lattice and spin–spin relaxation times, and in the number of resonance lines. The large changes in the relaxation times at 195 K indicate that the H and Na ions are significantly affected by this transition. The change in the number of resonance lines for the ¹H and ²³Na nuclei means that the orientations of the (SeO₃)²⁻ dimers and the environments of the Na ions change at T_C. Therefore, the orientations of the (SeO₃)²⁻ dimers and the environments of the Na ions play important roles in the phase transitions. In conclusion, the ferroelectric–paraelectric phase transition of NaH₃(SeO₃)₂ is accompanied by changes in hydrogen-bond structure and distortions of the (SeO₃)²⁻ and Na⁺ ion lattices, which form a slightly distorted octahedron.     

X-ray diffraction, Cs and H NMR and thermal studies of CdZrCs1.5(H3O)0.5(C2O4)4·xH2O displaying Cs and water dynamic behavior

A novel mixed cadmium zirconium cesium oxalate with an open architecture has been synthesized from precipitation methods at room pressure. It crystallizes with an hexagonal symmetry, space group P3₁12 (no. 151), a=9.105(5) Å, c=23.656(5) Å, V=1698(1) Å³ and Z=3. The structure displays a [CdZr(C₂O₄)₄]²⁻ helicoidal framework built from CdO₈ and ZrO₈ square-based antiprisms connected through bichelating oxalates, which generates channels along different directions. Cesium cations, hydronium ions and water molecules are located inside the voids of the anionic framework. They exhibit a dynamic disorder which has been further investigated by ¹H and ¹³³Cs solid-state NMR. Moreover a phase transition depending both upon ambient temperature and water vapor pressure was evidenced for the title compound. The thermal decomposition has been studied in situ by temperature-dependent X-ray diffraction and thermogravimetry. The final product is a mixture of cadmium oxide, zirconium oxide and cesium carbonate.     

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.     

Calorimetric and single crystal X-ray study of the phase transition of (PyH)2PdCl4

Polymorphic transition of pyridinium tetrachloropalladate(II) was investigated by heat capacity measurements and by single crystal X-ray structural analysis. A large λ-type anomaly was detected at 240 K in the temperature dependence of the heat capacity. The low-temperature phase (LTP) belongs to the triclinic space group with a=6.856(1), b=7.293(1), c=7.721(1) Å, α=75.180(2)°, β=71.081(2)°, γ=81.109(3)° at 100 K, and the high-temperature phase (HTP) to the same space group with a=7.217(2), b=7.470(2), c=7.880(2) Å, α=73.438(3)°, β=65.195(3)°, γ=82.727(4)° at 293 K. The pyridinium cations are ordered antiferroelectrically in LTP. In HTP, however, an orientational disorder of the cation was observed. The energy difference between potential wells for the reorientation of pyridinium ion in HTP is discussed referring to the results of the present single crystal X-ray and heat capacity as well as the previous ¹H NMR measurements. A five-site disorder model is shown to be consistent with both of the observations of ¹H NMR and X-ray study.     

NMR studies of magnetic superconductor RuSr2RECu2O8 (RE=Gd, Eu and Y)

We have carried out ^99/101Ru and ^63/65Cu nuclear magnetic resonance experiments in order to investigate magnetic and electronic properties of the magnetic superconductor RuSr₂RECu₂O₈ (RE=Gd, Eu and Y). The two kinds of ^99/101Ru signals were observed in the magnetically ordered state for each system, suggesting a charge segregation of Ru⁵⁺ (S=3/2) and Ru⁴⁺ (S=1) ions in the RuO₂ layers. The internal field at the Cu sites is revealed to be of the order of kilo Oe, indicating weak magnetic interactions between the CuO₂ and RuO₂ planes. The temperature dependence of nuclear spin-lattice relaxation time T₁ of ⁶³Cu in RE=Y shows a ‘spin gap’ like behavior, suggesting the system is under-doped.     

Nuclear magnetic resonance study of the relaxation mechanisms of the Be, Al, and Si nuclei in Cr-doped Be3Al2Si6O18 crystals

The variations with temperature of the relaxation mechanisms of the ⁹Be, ²⁷Al, and ²⁹Si nuclei in emerald (Be₃Al₂Si₆O₁₈:Cr³⁺) single crystals were investigated by using a pulse NMR spectrometer. The values of the spin-lattice relaxation rates for the three nuclei are different, and these differences are attributed to the differences between their Larmor frequencies and their local nuclear environments. The relaxation rates of the ⁹Be and ²⁷Al nuclei undergo no abrupt changes within the temperature range 180-400 K, which indicates that there are no phase transitions within this temperature range. The spin-lattice relaxation rates of ⁹Be and ²⁷Al were found to be proportional to temperature. Therefore, the nuclear spin-lattice relaxation processes of these two nuclei proceed via single-phonon processes.     

B NMR relaxation in superconductors: YRuB2 and LuRuB2

To investigate the electronic states in YRuB₂ and LuRuB₂, we have carried out ¹¹B NMR measurements. In the normal state, the spin-lattice relaxation rates 1/T₁'s in these compounds are proportional to the temperature T. 1/T₁'s show a small coherence peak just below the superconducting transition temperature T_c and decrease exponentially well below T_c. YRuB₂ and LuRuB₂ are found to be BCS superconductors with the energy gap 2Δ(0)=3.52 k_BT_c.     

Decoupling of the order-disorder and displacive contributions to the phase transition in NH4H(ClH2CCOO)2

The effects of hydrostatic pressure and substitution of Rb⁺for the ammonium cations on the ferroelectric phase transition temperature in NH₄H(ClH₂CCOO)₂ have been studied by electric permittivity measurements. The transition temperature (T_c) decreases with increasing pressure up to 800 MPa and the pressure coefficient dT_c/dp=−1.4×10⁻² [K/MPa] has been experimentally determined. The substitution of Rb⁺ for the ammonium cations has been shown to considerably lower the ferroelectric phase transition temperature T_c. In mixed crystals, additional electric permittivity anomaly has been clearly evidenced. The results are discussed assuming a model, which combines polarizability effects, related to the heavy ion units, with the pseudo-spin tunnelling.     

Molecular dynamics and phase transitions in paramagnetic [Mn(H2O)6][SiF6] investigated by H NMR

The temperature dependences of ²H NMR spectra and spin-lattice relaxation time T₁ have been measured for paramagnetic [Mn(H₂O)₆][SiF₆]. The obtained ²H NMR spectra were simulated by considering the quadrupole interaction and paramagnetic shift. The variation of the spectra measured in phase III was explained by the 180° flip of water molecules. The activation energy E_a and the jumping rate at infinite temperature k₀ for the 180° flip of H₂O were obtained as 35 kJ mol⁻¹ and 4×10¹⁴ s⁻¹, respectively. The spectral change in phases I and II was ascribed to the reorientation of [Mn(H₂O)₆]²⁺ around the C₃ axis where the E_a and k₀ values were estimated as 45 kJ mol⁻¹ and 1×10¹³ s⁻¹, respectively. From the almost temperature independent and short T₁ value, the correlation time for electron-spin flip-flops, τ_e, and the exchange coupling constant J were obtained as 3.0×10⁻¹⁰ s and 2.9×10⁻³ cm⁻¹, respectively. The II-III phase transition can be caused by the onset of the jumping motion of [Mn(H₂O)₆]²⁺ around the C₃ axis.     

Synthesis of ZnFe2O4 nanocrystallites by mechanochemical reaction

Mechanochemical reaction of ZnO and α-Fe₂O₃ in a planetary mill formed an amorphous precursor, which was subsequently heated to successfully produce zinc ferrite (ZnFe₂O₄) nanocrystallites. The amorphous precursor and nanocrystallites were characterized by differential thermal analysis (DTA), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Calcination of the precursor powder at 600 °C led to the formation of ZnFe₂O₄ nanocrystallites of about 22 nm in crystal size, and most of particle was about 10-50 nm in diameter. Effect of calcination temperature on the crystal size of the nanoparticles was investigated. The mechanism of nanocrystallite growth was primarily investigated. The activation energy of ZnFe₂O₄ nanocrystallite formation during thermal treatment was calculated to be 18.5 kJ/mol.     

Structure of the low-temperature phase of Rb3D(SeO4)2 by single-crystal neutron diffraction

Crystal structure of Rb₃D(SeO₄)₂ has been investigated at 25 K (below the transition temperature T_c=95.4 K) by single-crystal neutron diffraction. Accompanying the transition, the SeO₄ groups, which are all equivalent in the phase above the transition (space group A2/a), split into eight nonequivalent groups in a superlattice (a×2b×2c, space group A2) in the low-temperature phase. Based on the D atom positions obtained, each of the SeO₄ groups was identified to be in the state closer to a HSeO₄⁻ ion or to a SeO₄²⁻ ion and the dipole arrangement of SeO₄-D-SeO₄ dimer was revealed. This dipole arrangement has ‘ferri’ structure along the polar b-axis, but ‘antiferro’ structure in the plane perpendicular to the b-axis. These results are consistent with the characteristics found in the earlier dielectric measurements.     

A nuclear magnetic resonance study of the phase transitions and electric quadrupole Raman processes of M5H3(SO4)4·H2O (M=Na, K, Rb, and Cs) single crystals

The spin–lattice relaxation times and spin–spin relaxation times for ¹H and M in M₅H₃(SO₄)₄·H₂O (M=Na, K, Rb, and Cs) single crystals grown using the slow-evaporation method were measured as functions of temperature. Two kinds of protons were identified in the M₅H₃(SO₄)₄·H₂O structure: acid protons and water protons. Our experimental results show that the acid and water protons in Cs₅H₃(SO₄)₄·H₂O are involved in phase transitions of this crystal, whereas neither type of proton is involved in the phase transitions of the other three crystal type (M₅H₃(SO₄)₄·H₂O; M=Na, K, and Rb). Moreover, the relaxation times for the M (=Na, K, and Rb) nuclei in these crystals were found to decrease with increasing temperature and can be described with (k=2). The T₁ results for M (=Na, K, and Rb) in M₅H₃(SO₄)₄·H₂O crystals can be explained in terms of a relaxation mechanism in which the lattice vibrations are coupled to the nuclear electric quadrupole moments.     

Structure instability of A2Al2B2O7 (A=K, Na) crystals

A theoretical study on the stabilities of the crystal structures of K₂Al₂B₂O₇ (KABO) and Na₂Al₂B₂O₇ (NABO) has been carried out using density functional theory with generalized-gradient corrections. All structures have been optimized by minimizing the total energies with respect to lattice constants and to the atomic coordinates within the unit cell. In the case of KABO, the structure with space group P321 always takes energy advantage over the whole volume range studied, whereas on both volume expansion and contraction, the original NABO structure becomes unstable, taking a structure transition from its space group to the P321 space group of KABO structure. The theoretical predictions were well realized in the experimental results on the structure changes of the (K_1−xNa_x)₂Al₂B₂O₇ solid solution system.     

X-ray study of phase transitions in the system of solid solutions K2Pb4Nb10O30-Na2Pb4Nb10O30-K6W4Nb6O30

Boundaries of morphotropic phase transitions region in the system of solid solutions K₂Pb₄Nb₁₀O₃₀-Na₂Pb₄Nb₁₀O₃₀-K₆W₄Nb₆O₃₀ with the structure of the tetragonal tungsten bronze have been specified. Presence of the second morphotropic phase transition, perpendicular to the first one has been revealed. The temperature dependences of the structural parameters of some compounds have been investigated. The compounds with high values of Curie temperatures and working temperatures have been obtained.     

Raman spectroscopy study of BaI2 :Eu and SrI2 :Eu scintillator crystals

Europium-doped barium and strontium iodide crystals are high light yield scintillator materials with excellent energy resolution. In this communication, BaI₂:Eu and SrI₂:Eu single crystals with space groups of Pnma () and Pbca () respectively were grown from the vertical Bridgman method. The crystals were investigated with polarized Raman spectroscopy at temperatures varied from 77 to 300 K. The observed Raman-active modes for each crystal were assigned with the help of group theory analysis. It is found that the absence of the site symmetry leads to splitting of a spectral line in Pnma structure into two lines in the Pbca structure. Structural defects including dark spots formed during crystal growth and new species produced from hydration were characterized.     

Structure, phase transitions and molecular dynamics in 4-methylpyridinium tetrachloroantimonate(III), [4-CH3C5H4NH][SbCl4]

Crystal structure of the 4-methylpyridinium tetrachloroantimonate(III), [4-CH₃C₅H₄NH][SbCl₄], has been determined at 240 K by X-ray diffraction as monoclinic, space group, P2₁/n, Z=8. Differential scanning calorimetry and dilatometric studies indicate the presence of two reversible phase transitions of first order type, at 335/339 and 233/289 K (cooling/heating) with ΔS=0.68 and 2.2 J mol⁻¹ K⁻¹, respectively. Crystal dynamics is discussed on the basis of the temperature dependence of the ¹H NMR spin-lattice relaxation time T₁ and infrared spectroscopic studies. The low temperature phase transition at 233 K of an order-disorder type is interpreted in terms of a change in the motional state of the 4-methylpyridinium cations. The phase transition at 335 K, probably of a displacive type, is characterised by a complex mechanism involving the dynamics of both the cationic and anionic sublattice. The ¹H NMR studies show that the low temperature phase III is characterised only by the dynamics of the CH₃ groups.     

Study on the phase transitions by nuclear magnetic resonance of α-type RbAl(SO4)2·12H2O and β-type CsAl(SO4)2·12H2O single crystals

The thermodynamic properties, spin–lattice relaxation times, T₁, and spin–spin relaxation times, T₂, of the ²⁷Al, ⁸⁷Rb, and ¹³³Cs nuclei in MAl(SO₄)₂·12H₂O (M=Rb and Cs) crystals were investigated, and the two crystals were found to lose H₂O with increases in temperature. From our results for T₁ and T₂, we conclude that the discontinuities near T_d in the T₁ curves of the two crystals correspond to structural changes. In both crystals, below T_d the water molecules surrounding the Al³⁺ and M⁺ nuclei form distorted octahedra, whereas above T_d the water molecules around the Al³⁺ and M⁺ nuclei form regular octahedra and the environment of the Al³⁺ and M⁺ nuclei has cubic symmetry. Further, the T₁ for the ²⁷Al and ⁸⁷Rb nuclei in RbAl(SO₄)₂·12H₂O below T_d were found to increase with increasing temperature, whereas the T₁ for the ²⁷Al and ¹³³Cs nuclei in CsAl(SO₄)₂·12H₂O were found to decrease. It is possible that this difference is due to the different characteristics of α- and β-type crystals.     

Photoluminescence spectra of rare earth doped CaGa2S4 single crystals

Six kind CaGa₂S₄ single crystals doped with different rare earth (RE) elements are grown by the horizontal Bridgman method, and their photoluminescence (PL) spectra are measured in the temperature range from 10 to 300 K. The PL spectra of Ce or Eu doped crystals have broad line shapes due to the phonon assisted 4f-5d transitions. On the other hand, those of Pr³⁺, Tb³⁺, Er³⁺ or Tm³⁺ doped samples show narrow ones owing to the 4f-4f transitions. The assignments of the electronic levels are made in reference to the reported data of RE 4f multiplets observed in same materials.