Magnetic susceptibility studies of (CH2)3(NH3)2FeCl2Br2 and (CH2)6(NH3)2FeCl2Br2

The magnetic susceptibility of the layered compounds (CH₂)₃(NH₃)₂FeCl₂Br₂ and (CH₂)₆(NH₃)₂FeCl₂Br₂ has been measured in the range 80 < T < 300 K. The results follow a Curie-Weiss behavior in the range 120 < T < 300 K but are field dependent for T < 120 K. The results are interpreted in terms of a two-dimensional antiferromagnetic interaction which is canted. A comparison with the corresponding pure chloride compounds is given.     

Nonexponential proton relaxation in (NH4)2 HPO4

Nonexponential relaxation of protons in (NH₄)₂HPO₄ was found from 420 K to 110 K and was interpreted by the three-bath model below 167 K; inter-NH₄ group dipolar coupling above 167 K, and was attributed to non-equivalent NH₄ groups in a molecule.     

Two-dimensional critical behaviour in antiferrodistortive Al ur6(CIO4)3 and Ga ur6(ClO4)3

Critical behaviour with dimensionality d = 2 has been observed for the 300 K antiferrodistortive phase transition in Al ur₆(ClO₄)₃ and Ga ur₆(ClO₄)₃ by means of the temperature dependence of the ESR parameter D. The systems exhibited d = 2 behaviour in the static critical behaviour for T<T_c?40 K for T>T_c + 40 K. From the ESR data including line width measurements the local order parameter relaxation rate ω₁ has been obtained for various temperatures above T_c, with a lowest value of ω₁ = 150 MHz at T_c + 15 K     

Phase transitions in [Ni(NH3)6](NO3)2

Electric permittivity ^* = ′ − i″ of nickel-hexammino nitrate (NHN) has been measured within the range of temperature from 9 to 300 K at a frequency of 8.8 GHz (X-band). It has been found that the phase transitions at T_k1 = 247 K and T_k2 = 90 K are discontinuous structural transitions between centrosymmetric phases, whereas the transition at T_c = 63 K is a continuous phase transition (glass?).     

Magnetic ordering in NdRh2Si2 and ErRH2Si2

Neutron diffraction and magnetization study of polycrystalline NdRh₂Si₂ and ErRh₂Si₂ was performed in the temperature range from 4.2 to 293 K. Both compounds are of ThCr₂Si₂ type crystal structure and exhibit antiferromagnetic ordering below T_N = 53 K and T_N = 12.8 K respectively. The magnetic structure wave vector is τ = [0, 0, 1].     

NMR spectral densities and two dimensional diffusion in TiS2(NH3)1.0 and TaS2(NH3)x

NMR measurements of proton spin-lattice relaxation times T₁ and T_1? in the layered intercalation compounds TiS₂(NH₃)_1.0 and TaS₂(NH₃)_x (x = 0.8, 0.9, 1.0) are reported as functions of frequency and temperature (100 K – 300 K). These observations probe the spectral density of magnetic fluctuations due to motions of the intercalated molecules at frequencies accessible to the T₁ (4–90 MHz) and T_1? (1–100 kHz) measurements. Since the average molecular hopping time (τ) can be changed by varying temperature, different regions of the spectral density can be examined. For T > 200 K, both T^?1₁ and T^?1_1? vary logarithmically with frequency, reflecting the two dimensional character of the molecular diffusion. The temperature dependence of T₁ suggests that a more accurate picture of the short time dynamics is required. No dependence of relaxation rate on vacancy concentration is found.     

Absolute sense of pyroelectric p3 and piezoelectric d33 coefficients in 3La (IO3)3.HIO3.7H2O

The pyroelectric coefficient p₃ in 3La(IO₃)₃.HIO₃.7H₂O has an average value 2.0×10^-5 Cm^-2 in the temperature range 152 to 240 K. The resistivity decreases from 10¹² to 10¹⁰ ohm-cm between 258 and 338 K. At 298 K, the piezoelectric coefficient d₃₃  19×10^-12CN^-1. Positive polarity is generated on (001) by increasing temperature or tensile stress. A displacement toward (001) by La³⁺ or H₃O⁺ ions of 1×10^-4 Å per K or 10⁶Nm^-2, or rotation of the water molecule or iodate ion dipoles by about 5 arc minutes per K or 10⁶Nm^-2, produces the observed polarity.     

Neutron diffraction study of magnetic ordering in PrCu2Si2, PrCu2Ge2, PrFe2Ge2 and NdFe2Ge2

A neutron diffraction study of polycrystalline PrCu₂Si₂ [1], PrCu₂Ge₂ [2], PrFe₂Ge₂ [3] and NdFe₂Ge₂ [4] intermetallics carried out at liquid helium temperature shows the presence of a collinear antiferromagnetic order below T_N = 19 ± 1 K [1], T_N = 16 ± 1 K [2], T_N = 9 ± 1 K [3] and 13 ± 1 K [4]. Magnetic moment, parallel to the c-axis is localized on RE ions only. The magnetic structure of these compounds consists of ferromagnetic layers perpendicular to the c-axis coupled antiferromagnetically with sequence +-+- for PrCu₂Si₂ and PrCu₂Ge₂ and +--+ for PrFe₂Ge₂ and NdFe₂Ge₂. The RE moments amount close to the free ion values for Fe containing compounds but are smaller in those containing Cu suggesting a fairly strong influence of crystal field.     

Detailed infrared spectroscopic study of thermal transitions in new hybrid [(CH3)2CHNH3]4Cd3Cl10 crystal

Phase transitions of tetra(isopropylammonium)decachlorotricadmate(II) [(CH₃)₂CHNH₃]₄Cd₃Cl₁₀ crystal have been studied by infrared, far infrared and Raman measurements in wide temperature range, between 11 K and 388 K. The temperature changes of wavenumber, center of gravity, width and intensity of the bands were analyzed to clarify cationic and anionic contributions to the phase transitions mechanism. The results of investigation showed earlier by differential scanning calorimetry (DSC), thermal expansion and dielectric measurements clearly confirmed the sequence of phase transitions at T₁=353 K, T₂=294 K and T₃=260 K. The current results derived from DSC and infrared measurements revealed additional phase transition at T₄=120 K.     

Magnetic properties for the Cu2MnSnSe4 and Cu2FeSnSe4 compounds

Magnetic susceptibility χ measurements in the range from 2 to 300 K were carried out on samples of the Cu₂FeSnSe₄ and Cu₂MnSnSe₄ compounds. It was found that Cu₂FeSnSe₄ was antiferromagnetic showing ideal Curie-Weiss behavior with a Néel temperature T_N of about 19 K and Curie-Weiss temperature θ=−200 K, while for Cu₂MnSnSe₄ the behavior was spin-glass with a freezing temperature T_f of about 22 K and Curie-Weiss temperature θ=−25 K. The spin-glass order parameter q(T), determined from the susceptibility data, was found to be in agreement with the prediction of conventional spin-glass theory.     

Birefringence,X-ray and neutron diffraction measurements on the structural phase transitions of (CH3NH3)2 MnCl4 and (CH3NH3)2FeCl4

We used optical birefringence, X-ray and neutron diffraction methods with single crystals to study the structural phase transitions of the perowskite-type layer structures of (CH₃NH₃)₂MeCl₄ with Me=Mn, Fe. The Mn-compound shows the following structural transitions at 394 K — a continuous order-disorder phase transition from tetragonal symmetry $\text{I4}\text{mmm}$ to orthorhombic space group Abma (Cmca in reference 10); at 257 K — a discontinuous transition to a second tetragonal modification; at 95 K — a discontinuous transition to a monoclinic phase. For the Fe-compound the corresponding transition temperatures are 328 K and 231 K, respectively. A low temperature monoclinic phase could not be observed. The lattice parameters of the different modifications were determined as a function of temperature. The temperature dependent course of the order parameter has been investigated for the order—disorder transition. For both compounds, all the methods used gave the same value for the critical exponent of β = 0.315.     

Chlorine K X-ray absorption-edge structures of CIS-[Pt(NH3)2Cl2], trans-[Pt(NH3)2Cl2], trans-[Pd(NH3)2Cl2] and (NH4)2PdCl4

The K absorption-edge spectra of the ligand chlorine ion in square-planar complex compounds cis- and trans-[Pt(NH₃)₂Cl₂], trans-[Pd(NH₃)₂Cl₂], and (NH₄)₂PdCl₄ are reported and discussed in connection with the chlorine K absorption spectra of K₂PtCl₄ and K₂PdCl₄, reported previously. The observed chemical shift of a white line at the absorption threshold is interpreted in terms of the difference of the ligand-field splitting of electronic states for metal ions. The white line is attributed to the electronic transition from the Cl^? ls level to the lowest unoccupied antibonding molecular orbital (MO), which is specified by a $\text{MO}\text{b}{}_{\mathrm{1g}}\text{(σ}{}^1\text{)}$ in the square-planar complex with D_4h symmetry. The other absorption structures are regarded as continuum “shape resonances” of the outgoing electron trapped by the cage of the surrounding atoms. The effect of geometrical isomerism is found in the chlorine K absorption spectra of cis- and trans-[Pt(NH₃)₂Cl₂].     

Superconductivity in CeCu2Si2

Resistivity measurements of CeCu₂Si₂ are carried out under pressures p up to 12 kbars. Unlike polycrystalline samples, no traces of superconductivity have been observed in CeCu₂Si₂ at ambient pressure. When pressure is applied, CeCu₂Si₂ monocrystals become superconducting with anomalously large ratio H_c2(0)/T_c (0) = 34 K0e/K and with the derivative dH_c2/dT(T=T_c) = 140 K0e/K     

Two-consecutive magnetic transitions in (C6H5CH2NH3)2CuCl4

The a.c. magnetic susceptibility (χ) was measured for (C₆H₅CH₂NH₃)₂CuCl₄ (φC1Cl) above 4.2K, and two-consecutively anomalous behaviours of χ were observed at 6K and 6.9K. Brief discussions about these phenomena were given in relation to the two-dimensionality of φC1Cl system.     

Low temperature thermoelectric power of LaB6, PrB6 and NdB6

We have measured the thermoelectric power of LaB₆, PrB₆ and NdB₆ in the temperature range 2–20K. PrB₆ and NdB₆ order antiferromagnetically at T_N = 6.99K and 7.74K respectively, whereas LaB₆ is non-magnetic. The thermoelectric power data have a negative maximum near 5.5K in all three hexaborides. This negative peak is thought to be mainly due to phonon drag in LaB₆. In PrB₆ and NdB₆ there is an additional contribution due to magnon drag. It is expected that the high temperature thermoelectric power is mainly due to an electron diffusion term and a contribution due to spin disorder scattering. The thermoelectric power and the resistivity have been compared in the vicinity of T_N. A qualitative similarity in the temperature derivatives of these two quantities has been found for PrB₆ and NdB₆ near T_N.     

Thermal expansion of lT-TaS2 and 2H-NbSe2

The thermal expansion of the a and c axes of lT-TaS₂ and of the a axis of 2H-NbSe₂ have been measured between 4 K and 360 K. Discontinuities in the lattice parameters of TaS₂ were observed at the known charge density wave phase transitions near 200 K and 352 K, and a new transition was found near 283 K. These results are used to estimate the entropy changes occurring at the phase transitions. At the charge density wave onset temperature in NbSe₂ we find an upper limit to any discontinuity in the a axis of 2 × 10^-7 and to any discontinuity in the expansion coefficient of 3 × 10^-7 K^-1.     

Dielectric properties of PbSrWO4 and PbBaWO4 compounds

PbSrWO₄ and PbBaWO₄ have been synthesised by the solid state reaction technique XRD patterns show them to be tetragonal. Dielectric constant (K^/) and Dielectric loss (K ^//) of PbSrWO₄ and PbBaWO₄ have been measured at 1 kHz in the temperature range of 300 to 1050 K. The log K^/ vs T as well as log K ^// vs T plot of PbSrWO₄ and PbBaWO₄ shows rapid increase of dielectric constant above 590 K and 640 K, respectively.     

Adiabatic potential energies for Na2 and K2

Adiabatic potential energies (Σ states) were calculated for the systems Na₂ and K₂. Wells followed by bumps were observed for the ³Σ_u excited states dissociating to Na(3s) + Na(3p) and K(4s) + K(4p). Double minima for the ¹Σ_u states dissociating to Na(3s) + Na(4s) and K(4s) + K(5s) are reported.     

Lithium motion in Li0.33 TiS2 and Li0.94 TiS2

We have studied the motion of lithium ions in Li_xTiS₂ (x = 0.33, 0.94) using pulsed NMR techniques. The temperature dependences of the spin lattice relaxation in the rotating frame (T_1?) suggest comparable activation energies for lithium ion diffusion for both samples, 3370 K, but an appreciably longer hopping time for the x = 0.94 sample. Low temperature values of T₂ agree with calculated and measured second moments for both materials.