Br NQR relaxation and successive phase transitions of CH3NH3HgBr3

The measurement of ⁸¹Br NQR in CH₃NH₃HgBr₃ has been carried out in the temperature range between 80 and 300 K using a pulse NQR method. The temperature dependence of ⁸¹Br NQR frequencies in CH₃NH₃HgBr₃ has revealed that it undergoes three characteristic successive phase transitions at T?=?123, 184 and 239 K. The phase transition temperature at T?=?239 K is the second-order type, whereas those at T?=?184 and 123 K are the first-order nature of the phase transitions. Each phase transition seems to be closely related to the motions of methyl ammonium cation as a partial or whole. The enhancement of 1/T ₁ at T?=?239 K indicates the onset of the molecular motion of the cation as a whole with increasing temperatures.     

121Sb and 123Sb NQR and the heterophase structure in the SbSI ferroelectric

The temperature dependences of NQR line frequencies and widths of ¹²¹Sb (for the ±1/2→±3/2 transition) and of ¹²³Sb (for the ±1/2→±3/2 and ±3/2→±5/2 transitions), as well as of the principal components and the asymmetry parameter of the electric-field-gradient tensor at the ¹²³Sb nucleus have been studied in a SbSI crystal in the 115–325 K range. The dynamic and static factors governing the character of these relations are discussed. The ±1/2→±3/2 line in the ¹²¹Sb NQR spectrum splits into a doublet within a narrow (0.5 K) temperature interval near the ferroelectric phase transition (T _c=293 K), which is associated with the formation of a macroscopic heterophase structure in the crystal. Fiz. Tverd. Tela (St. Petersburg) 41, 1286–1292 (July 1999)     

Quadrupole moment of the 19/2− isomer in115Sb

The quadrupole moment of the 19/2^? isomeric state in¹¹⁵Sb has been measured using the time differential perturbed angular distribution method. From the quadrupole coupling constante ² Qq/h=66(3) MHz atT=370 K in a Bi host the moment ratio is obtained relative to the stable ground state of¹²¹Sb as ¦Q(¹¹⁵Sb, 19/2^?)/Q(¹²¹Sb, 5/2⁺)¦ =1.45(7) resulting in a moment of ¦Q(¹¹⁵Sb, 19/2^?)¦=52(6) fm².     

Decay properties of the 19/2− isomers in113,117Sb

By means of the generalized centroid-shift method, the following half-lives were determined:T _1/2(3045 keV)=3.7±0.3 ns in¹¹³Sb using the reaction¹⁰⁴Pd(¹²C,p2n) as well asT _1/2 (1322.8 keV)=3.5±0.3 ns,T _1/2(2779.8 keV) =0.50±0.15ns,T _1/2(2874.9 keV)<0.2 ns andT _1/2(3072.9 keV)<0.1 ns in¹¹⁷Sb using the¹¹⁶In(α, 2n) reaction. Three-quasiparticle configurations of the type πd _5/2ν(h_11/2 d _3/2) in^113,117Sb are found to dominate in the wave functions of the 19/2^? states at 3045 and 2780 keV in¹¹³Sb and¹¹⁷Sb, respectively. TheB(E2, 19/2^? → 15/2^?) values in^113–119Sb are discussed.     

Study of 14N NQR response to SORC pulse sequence

The behavior of nuclear quadrupole resonance (NQR) signals between RF pulses of the strong off-resonance comb (SORC) as well as the spin-locking spin-echo (SLSE) pulse sequences was studied as for ¹⁴N NQR line ν _?+? of dimethylnitramine (CH₃)₂NNO₂ at 77 K. The periodic variation of the signal amplitude observed by using SORC pulse sequence could be reasonably explained by the theoretical expression reported in the literature.     

127I NQR and 1H NMR studies of 4-aminopyridinium tetraiodoantimonate(III); molecular motion and phase transition

The crystals of 4-NH₂PyHSbI₄ (Py = C₅H₄N) have been investigated by means of ¹²⁷I NQR, ¹H NMR T ₁ and DTA. The crystals can exist in two modifications of β and α(I) at room temperatures. The α(I)-phase is a metastable state which is obtained when the stable β form is heated. The α(I)-phase undergoes a first-order type phase transition of α(I) $\leftrightarrow \alpha $ (II) at 272 K (on heating), while the β-phase is stable down to 77 K. Four and two ¹²⁷I (m = ±1/2 $\leftrightarrow \pm $ 3/2) NQR lines have been found for the β- and α(II)-phases, respectively. One half of them is assignable to the terminal I atom(s) and the other to the bridging I atom(s) in each phase. All the resonance lines of the α(II)-phase underwent a disappearance above ca. 240 K and no resonance line was observed in the α(I)-phase. The second moment M ₂ value of ¹H NMR spectra with 8 G² at 290 K shows that the 4-NH₂PyH^?+? cations reside in the rigid lattice in the β-phase. In contrast, in the α(I)-phase the cation rotates about an axis more symmetric than pseudo threefold axis. The activation energy of 21 kJ mol^???1 was estimated for the reorientational motion in the α(I)-phase from the ¹H NMR T ₁ measurements. The nature of phase transitions in the 4-NH₂PyHSbI₄ is discussed in comparison with that in 4-NH₂PyHSbBr₄.     

Br NQR relaxation of the mixed-valence compound (NH4)4SbIIISbVBr12

⁸¹Br NQR measurements of the mixed-valence compound (NH₄)₄Sb^III Sb^VBr₁₂, which has Sb^VBr₆ ^? and Sb^IIIBr₆ ^3? octahedra in different oxidation states in the tetragonal pseudo-K₂PtCl₆ structure, were carried out by pulse method in the temperature range between 80 and 300 K. The phase transition temperature of T _c = 212 K was redetermined. The T ₁ values are quite different between Sb^VBr₆ ^? and Sb^IIIBr₆ ^3? octahedra. For all resonance lines T ₁ minima were observed at T _c. The T ₁ behavior at T _c was explained by a softening of the rotary lattice mode around a principal axis of each octahedron along the c axis of the crystal. The distinctive feature in the temperature dependence of both ⁸¹Br NQR frequencies and T ₁ values for each anion indicates that the static rotation may occur for Sb^VBr₆ ^? but not for Sb^IIIBr₆ ^3? at T _c in the low temperature phase.     

Fe-induced magnetic transition in YBa2(Cu1−x 57Fe x )3O6 by NQR and Mössbauer spectroscopy

Nuclear quadrupole resonance spectroscopy (around 30 MHz) on the chain site Cu(1) nuclei in oxygen deficient YBa₂(Cu_1?x Fe _x )₃O₆ doped with different amounts of⁵⁷Fe (x≤0.01) reveal an onset of magnetic order at low temperatures represented by a symmetrical doublet contribution to the nuclear quadrupole resonance (NQR) spectrum. The onset temperatureT _N2 depends on the concentration of Fe reaching 130 K forx=0.01. The splitting forx=0.01 at 100 K corresponds to a net internal field of 0.09 T with a distribution of ≈0.08 T representing about 70 percent of the Cu(1) nuclei.⁵⁷Fe and⁵⁷Co Mössbauer spectroscopy at 4.2 K with and without an external magnetic field of 5 T revealed that belowx=0.00015 Fe spins are decoupled from the Cu(2) moments in the antiferromagnetic state. Results are interpreted in terms of the magnetic model structure suggested by Kadowaki et al. [1].     

Microstructure and Mössbauer studies on magnetostrictive Tb0.3Dy0.7Fe1.95 − x Nb x (x = 0, 0.025, 0.05 and 0.075)

³⁵Cl NQR measurements were carried out between 77 and 209 K for 3-chlorothiophene, which has a glass transition in a stable crystalline state. An NQR signal with full widths of about 100 kHz at half maximum was observed in this temperature range. The spin-lattice relaxation time T ₁ was measured at the peak frequencies. The activation energy Δε _a obtained from the results of the T ₁ measurements showed a good agreement with those estimated from calorimetric measurements.     

Magnetic structure of the Sr<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>4</Subscript>Cl<Subscript>2</Subscript> two-subsystem antiferromagnet according to nuclear quadrupole resonance data

The magnetic structure of the Sr₂Cu₃O₄Cl₂ two-subsystem antiferromagnet is studied by the nuclear quadrupole resonance (NQR) method on the ^{63, 65}Cu and ³⁵Cl nuclei. The resonance spectrum above T _N2 = 40 K is determined by the Zeeman splitting of the levels of the ^{63, 65}Cu nuclei of the copper atoms at the Cu1 site with the first-order quadrupole perturbation. The magnetic field on the copper nuclei is equal to 93 kOe. The spectrum below n is significantly different: it includes a low-frequency part, which is associated with the ordering of the second magnetic subsystem Cu2. The splitting of the NQR lines of ³⁵Cl is observed above and below T _N2. This fact indicates the ferromagnetic ordering of the moments of the Cu1 subsystem, which are located along the c axis of the crystal, and makes it possible to determine the direction of the magnetic field on Cu1 copper as (110).     

Transition probabilities and the structure of some negative-parity states in126,130Xe and132Ba

The lifetimes of several negative-parity states in^126,130Xe and¹³²Ba have been determined by means of the generalized centroid-shift method. The reactions^124,128Te(α,2n) and¹²²Sn(¹³C, 3n) have been used. Following results were obtained:T _1/2(2758 keV)=1.3±0.2 ns in¹²⁶Xe,T _1/2(2060 keV)=0.20±0.10 ns,T _1/2(2104 keV)=0.50±0.10 ns,T _1/2(2376 keV)=0.30±0.10 ns andT _1/2(2973 keV)=4.6±0.4 ns in¹³⁰Xe as well asT _1/2(2120 keV)=0.40 _?0.10 ^+0.20 ns in¹³²Ba. A systematics of the B(E2; 7 _? ¹ ?5 ₁ ^? ) values in theN=76 nuclei is presented. Electric dipole and quadrupole transition rates are discussed in terms of octupole and quadrupole collectivity. The structure of the 5 ₁ ^? and 7 ₁ ^? states is considered. Nuclear reactions:^124,128Te(α, 2 _n ),E=26 MeV,¹²²Sn (¹³C, 3n),E=53 MeV; measuredE _γ I _γ , γ-r.f. DeducedT _1/2, B(σL) in^126,130Xe and¹³²Ba. Ge detectors. Generalized centroid-shift analysis.     

NQR, NMR and Crystal Structure Studies of [C(NH2)3]2HgX4 (X = Br, I)

The crystal structure of [C(NH₂)₃]₂HgBr₄ has been determined at room temperature: monoclinic, space group C2/c, with a = 10.035(2), b = 11.164(2), c = 13.358(3) Å, β = 111.67(3)°, and Z = 4. The crystal consists of planar [C(NH₂)₃]⁺ and distorted tetrahedral [HgBr₄]^2? ions. The Hg atom is located on a two-fold axis such that two sets of inequivalent Br atoms exist in an [HgBr₄]^2? ion. In accordance with the crystal structure, two ⁸¹Br NQR lines widely separated in frequency were observed between 77 and ca. 380 K. [C(NH₂)₃]₂HgI₄ yielded four ¹²⁷I NQR lines ascribable to m = ±1/2 ? ±3/2 transitions, indicating that its crystal structure is different from the bromide complex. The ¹H NMR T ₁ measurements showed a single minimum for the bromide but two minima for the iodide. The analyses based on the C₃ reorientations of the planar [C(NH₂)₃]⁺ ions gave the activation energies of 29.8 kJ mol^?1 for the bromide, and 30.2 and 40.0 kJ mol^?1 for the iodide.     

Magnetic interactions in119Sn substituted for Cu in antiferromagnetic and superconducting lamellar cuprates

Mössbauer Spectroscopy (MS) studies of¹¹⁹Sn were carried out in antiferromagnetic La₂(Cu_0.99Sn_0.01)O₄ (214) and in superconducting GdBa₂(Cu_0.99Sn_0.01)₃O₇(123). Non-magnetic Sn⁴⁺ substitutes for Cu if the right procedure for diffusing¹¹⁹SnO₂ in CuO is carried out. Studies performed in 214 show a large quadrupole splitting (QS) down to 120 K followed by an onset of a magnetic interaction reaching a saturation internal field ofH _eff=8.7(5) kOe atT=30K. From the combined magnetic-quadrupole interaction the angle θ formed byq _zz andH _eff, the η-parameter, and the sign ofQS were deduced and information on the local spin structure is derived. Studies conducted with the 123 material (T _c=90 K) reveal a broad unsplit line at temperatures down to 60 K followed by an abrupt onset of a magnetic interaction corresponding toH _eff (Sn)=8.3(1) kOe. The hyperfine fielddecreases with decreasing temperature reaching 6.0(1) kOe at 16 K. The onset of the magnetic interaction at the¹¹⁹Sn nucleus is explained as due to a local depletion of holes following the Sn⁴⁺ doping and a consequent quenching of the magnetic fluctuations in its vicinity.     

Hyperfine interactions in intermetallic rare earth-gallium compounds studied by 111 Cd PAC

Magnetic and electric hyperfine interaction of the nuclear probe ¹¹¹In/¹¹¹Cd in intermetallic compounds of the rare earth-gallium system have been investigated by perturbed angular correlation (PAC) spectroscopy. The PAC measurements, supported by X-ray diffraction, provide evidence for a marked phase preference of ¹¹¹In for hexagonal RGa₂ over orthorhombic RGa and of RGa₃ with the L12 structure over RGa₂. In the case of SmGa₂, the magnetic hyperfine field B_hf, the electric quadrupole interaction and the angle β between B_hf and the symmetry axis of the electric field gradient have been determined as a function of temperature. The angle β?=?0 is consistent with the results of previous magnetization studies. Up to T?≤?17 K the magnetic hyperfine field has a constant value of B_hf?=?3.0(2) T. The rapid decrease at higher T gives the impression of a first-order transition with an order temperature of T_N?=?19.5 K. In the RKKY model of indirect 4f interaction the ratio T_C/B_hf(0) is a measure of the coupling constant. For ¹¹¹Cd:SmGa₂ (T_C/B_hf(0)~6.5 K/T) this ratio is significantly smaller than for the same probe in other R intermetallics (SmAl₂ ~9.5 K/T, Sm₂In ~13.5 K/T).     

Magnetization and Mössbauer studies of 57Fe doped SrCoO3

⁵⁷Fe (1%) doped SrCoO₃ obtained by high-pressure method, has been investigated by magnetization and Mössbauer spectroscopy studies (MS) in the temperature range 4.2 K to 300 K. The ferromagnetic ordering temperature T _C obtained is 272(2) K. Isothermal magnetization curves have been measured at various temperatures, from which the saturation moments (M _sat) have been deduced. The ⁵⁷Fe MS spectra display standard six-line patterns with an isomer shift typical of Fe^3?+? and a very small quadrupole splitting (QS = 0.14(1) mm/s above T _C). The magnetic hyperfine field at 4.2 K is 276(1) kOe. The temperature dependencies of the iron hyperfine field and M _sat (1.83 µ _B at 5 K) are almost identical. This shows that the Fe^3?+? is replacing Co^4?+?, both of the same electronic configuration. They also interact similarly, namely the Fe–Co exchange is almost identical to the Co–Co exchange.     

NQR study of spin-freezing in superconducting La2−x Sr x CuO4: the example ofx = 0.06

Nuclear quadrupole resonance (NQR)¹³⁹La and⁶³Cu spin-lattice relaxation rateT ₁ ^t-1 measurements in a La_1.94Sr_0.06CuO₄ single crystal are described. Slowing-down of Cu²⁺ spin fluctuations is evidenced through a dramatic increase of¹³⁹ T ₁ ^?1 on cooling. While the onset of diamagnetism occurs atT _c = 8 K,¹³⁹ T ₁ ^?1 has a peak atT _g ? 5 K, when the characteristic frequency of magnetic fluctuations reaches the NQR frequencyv _Q ? 9 MHz. In agreement with a number of previous studies, these results show that the so-called “cluster spin-glass” phase persists in the superconducting regime. Issues concerning the coexistence of the two phases are discussed.     

Comment on the intensity and the temperature dependence of the nitrogen-broadened half-width of the P(6) line in the CO fundamental

The recent line-center absorption coefficient measurements on the P(6) line of the CO fundamental have been shown to be consistent with S_v(T) = 273(273/T)cm^-2atm^-1 and γ⁰(T) = 0.0652(300/T)^0.66 for the absolute intensity of the band and the nitrogen-broadened line width in the temperature range 300–800°K.     

Local and dynamic Jahn-Teller distortion in ulvöspinel Fe2TiO4

⁵⁷Fe Mössbauer spectroscopic study on ulvöspinel Fe₂TiO₄ has been conducted in a wide temperature range from 16 K to 500 K. The paramagnetic spectra are composed of several high spin Fe^2?+? doublets even at 500 K, which is rather strange because the point symmetry of the A-site is completely cubic (??43m). We explain the electric field gradient (EFG) at A-site by the local arrangement of Fe^2?+? and Ti^4?+? on the B-site. The spectra were successfully analyzed by four-subspectra model, which is based on the B-site arrangement. The model also fits rather well to the magnetically ordered spectra. Thus the temperature variations of the hyperfine parameters were obtained. The Néel temperature (T _N) is estimated to be about 125 K. The quadrupole coupling constants e ² qQ/2 of A-site subspectra show little change around cubic-tetragonal transition temperature (T _t?=?163 K), but rapidly increase below T _N. From the temperature variation of line width, we found local and dynamic Jahn-Teller distortions around A-site Fe^2?+? ions in the cubic phase.