Electrical conduction studies on one-dimensional Cs2[Pt(CN)4](FHF)0.39 and Rb2[Pt(CN)4](FHF)0.40

Variable temperature (300-40 K) 4-probe d.c. conduction studies on Cs₂[Pt(CN)₄](FHF)_0.39 and Rb₂[Pt(CN)₄](FHF)_0.40 are described. In these salts T_3D occurs at a lower temperature than in K₂[Pt(CN)₄]Br_0.3·3H₂O and this is attributed to the absence of an inter-chain network of hydrogen bonded water molecules in the bifluorides.     

New quasi-one-dimensional tetracyanidoplatinate,Cs4[Pt(CN)4](CF3SO3)2: Synthesis,structure, and physical characterization

The synthesis and some physical properties of a new quasi-one-dimensional tetracyanidoplatinate, Cs₄[Pt(CN)₄](CF₃SO₃)₂ (CsCP(OTf)) are reported and described in comparison to the well-known K₂[Pt(CN)₄]Br_0.30·3.2H₂O (KCP). Single-crystal X-ray diffraction reveals Pt–Pt spacings to be greater than those of KCP by 5% longitudinal and 38% transverse, but much shorter than comparable spacings in other non-partially oxidized platinates. Anomalies are observed between temperatures 100 K and 200 K: (1) Longitudinal DC conductivity is two orders of magnitude higher and is non-monotonic with temperature, showing a minimum at around 170 K. (2) Nuclear magnetic resonance (NMR) longitudinal relaxation time T₁ is at least three orders of magnitude higher than that of KCP, and is also non-monotonic with temperature, showing a sharp peak at around 120 K. Since X-ray diffraction reveals no structural transition at 120 K, these suggest a possible lattice freezing or stiffening at around 120 K.     

Synthesis,structure and preliminary results on electrical and magnetic properties of (Perylene)2 [Pt(mnt)2]

In this paper we report the preparation, crystal structure and some physical properties of the conducting molecular (Perylene)₂ [Pt(S₄C₄(CN)₄)].The crystal structure consists of segregated stacks of perylene and Pt S₄C₄ (CN)₄ units, with a spacing of 3.3 Å between the carbon atoms in the perylenes. The temperature dependence of the electrical conductivity is metallic with a maximum at approximately 15 K. At this temperature a sharp transition to an insulating state occurs. Magnetic suceptibility measurements confirm the existence of a transition in this temperature range.     

Thermal expansion of the one-dimensional conductor K2Pt(CN)4Br0.3·xH2O and K2Pt(CN)4·xH2O

Measurements of the thermal expansion coefficients of K₂Pt(CN)₄Br_0.3· xH₂O and K₂Pt(CN)₄·xH₂O show a large anisotropy of the a-and c- directions. Their temperature dependence could be described by a simple Grüneisen theory. In the range from 80–330°K no anomaly indicating a Kohn-Peierls transition could be found.     

Coordination of acetonitrile (CH3CN) to platinum (111): Evidence for an η2(C,N) species

Acetonitrile (CH₃CN) coordination to a Pt(111) surface has been studied with electron energy loss vibrational spectroscopy (EELS), XPS, thermal desorption and work function measurements. We compare data for the surface states with known acetonitrile coordination complexes. For CH₃CN adsorbed on Pt(111) at 100 K, the molecule is rehybridized and adsorbs with the CN bond parallel or slightly inclined to the surface plane in an η²(C, N) configuration. The ν(CN) frequency is 1615 cm^?1 and the C ls and N ls binding energies are 284.6 eV and 397.2 eV respectively. By contrast, weakly adsorbed multilayer acetonitrile exhibits a ν(CN) vibrational frequency of 2270 cm^?1, and C ls and N ls binding energies of 286.9 eV and 400.1 eV respectively. Both the EELS and XPS results are consistent with rehybridization of the CN triple bond to a double bond with both C and N atoms of the CN group attached to the surface. In addition to this majority η²(C, N) monolayer state, evidence is found for a second, more strongly bound minority molecular state in thermal desorption spectra. As a result of the low coverage of this state, EELS was unable to spectroscopically identify it and we tentatively assign it as an η⁴(C, N) species associated with accidental step sites. By contrast to the surface complexes, almost all of the known platinum-nitrile coordination complexes are end-bonded via the N lone-pair orbital. Several cases of side-on bonding are known, however, and we compare the results with the known complex Fe₃(η²-NCCH₃)(CO)₉. The difference in the coordinative properties of a Pt(111) surface versus a single Pt atom must be due to the increased ability of multi-atom arrays to back-donate electrons into the π¹ system of acetonitrile. Previously published EELS and XPS results for monolayer acetonitrile on Ni(111) and polycrystalline films are almost identical to the present results on Pt(111). We believe that the monolayer of $\text{CH}{}_3\text{CN}\text{Ni}\text{(111)}$ is also an η²(C, N) species, not an end-bonded species previously proposed by Friend, Muetterties and Gland.     

Electrical properties of highly (111)-oriented lead zirconate thin films

Antiferroelectric PbZrO₃ thin films were grown on Pt/Ti/SiO₂/Si substrates with predominant (111) orientation using a sol-gel process. The Pt/PbZrO₃/Pt film capacitor showed well-saturated hysteresis loops at an applied voltage of 5 V with remanent polarisation (P_r) and coercive electric field (E_c) values of 8.97 μC/cm² and 162 kV/cm, respectively. The leakage current density of the highly (111)-oriented PbZrO₃ film was less than 1.0×10⁻⁷ A/cm² over electric field ranges from 0 to 105 kV/cm. The conduction current depended on the voltage polarity. The PbZrO₃/Pt interface forms a Schottky barrier at electric fields from 20 to 160 kV/cm. The dielectric relaxation current behaviour of Pt/PbZrO₃/Pt capacitor obeys the well-known Curie-Von Schweidler law at electric field of 20-80 kV/cm, the currents have contributions of both dielectric relaxation current and leakage current.     

Electron spin resonance spectroscopy of the organic conductor: (TMTSF)2PF6

The ESR g-factor, linewidth and spin susceptibility of (TMTSF)₂PF₆ are each found to have a distinct temperature dependence. The anisotropy of g and of the linewidth for static magnetic fields in the plane perpendicular to the highly conducting axis suggest that spin-phonon scattering is the dominant relaxation mechanism. The metal-insulator transition at 19 K is reflected particularly clearly in the magnetic properties.     

Identification of a bivalent flaw in n-GaAs from noise measurements

Magnetic susceptibility measurements of K₂Pt(CN)₄Br_0.3(H₂ O)_2.3 K₂Pt(CN)₄(H₂ O)₃ and K₂Pt(CN)₄Br₂ are reported as well as ESR measurements on the first compound. The results show that any metallic type of contribution to the susceptibility of the first material is at least an order of magnitude smaller than what is expected from simple band theory or from an interrupted strand model. A small anisotropic Curie type of susceptibility is attributed to ‘impurity’ Pt ions which because of crystaline imperfections have their d_xzd_yz doublet as the highest occupied orbitals.     

Studies on partially oxidised one-dimensional bis(oxalato)platinate salts containing divalent cations

The preparation and electrical conduction properties of the isostructural one-dimensional conductors Ni_0.84[Pt(C₂O₄)₂]·6H₂O(Ni-OP) and Mn_0.81[Pt(C₂O₄)₂]·6H₂O(Mn-OP) are described. Ni-OP exhibits a similar tem dependence of conductivity to the isostructural compounds Co_0.83[Pt(C₂O₄)₂]·6H₂O(Co-OP) and Zn_0.81[Pt(C₂O₄)₂]·6H₂O whereas the behaviour of Mn-OP is rather like that of K₂[Pt(CN)₄]Br_0.3·3H₂O. These differences are discussed in terms of the variation from compound to compound of the critical temperature for the formation of the “non-Peierls” superstructure (T_c) and the temperature at which the CDW/PD on adjacent conducting chains undergo three-dimensional ordering (T_3D). The variation of thermopower with temperature for Co-OP and Mg_0.82[Pt(C₂O₄)₂]·6H₂O is reported and related to the conduction properties and phase changes which have been observed for these compounds. For the isostructural series M_0.8[Pt(C₂O₄)₂]·6H₂O (M = Mg, Mn, Co, Ni or Zn) the variation of (T_3D) from compound to compound is related to differences in the polarizing power of the cations.     

Poly(3-methylthiophene)-based porous silicon substrates as a urea-sensitive electrode

Poly(3-methylthiophene) (P3MT)-based porous silicon (PS) substrates were fabricated and characterized by cyclic voltammetry, scanning electron microscopy, and auger electron spectroscopy. After doping urease (Urs) into the polymeric matrix, sensitivity and physicochemical properties of the P3MT-based PS substrate was investigated compared to planar silicon (PLS) and bulk Pt substrates. PS substrate was formed by electrochemical anodization in an etching solution composed of HF, H₂O, and ethanol. Subsequently, Ti and Pt thin-films were sputtered on the PS substrate. Effective working electrode area (A_eff) of the Pt-deposited PS substrate was determined from a redox reaction of Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox couple in which nearly reversible cyclic voltammograms were obtained. The i_p versus v^1/2 plots showed that A_eff of the PS-based Pt thin-film electrode was 1.62 times larger than that of the PLS-based electrode.Electropolymerization of P3MT on both types of electrodes were carried out by the anodic potential scanning under the given potential range. And then, urease molecules were doped to the P3MT film by the chronoamperometry. Direct electrochemistry of a Urs/P3MT/Pt/Ti/PS electrode in an acetonitrile solution containing 0.1 mol/L NaClO₄ was introduced compared to a P3MT/Pt/Ti/PS electrode at scan rates of 10 mV s⁻¹, 50 mV s⁻¹, and 100 mV s⁻¹.Amperometric sensitivity of the Urs/P3MT/Pt/Ti/PS electrode was ca. 1.67 μA mM⁻¹ per projected unit square centimeter, and that of the Urs/P3MT/Pt/Ti/PLS electrode was ca. 1.02 μA mM⁻¹ per projected unit square centimeter in a linear range of 1-100 mM urea concentrations. 1.6 times of sensitivity increase was coincident with the results from cyclic voltammetrc analysis.Surface morphology from scanning electron microscopy (SEM) images of Pt-deposited PS electrodes before and after the coating of Urs-doped P3MT films showed that pore diameter and depth were 2 μm and 10 μm, respectively. Multilayered-film structures composed of metals and organics for both electrodes were also confirmed by auger electron spectroscopy (AES) depth profiles.     

The chemistry of dimethyltetrazine on Pt(111)

As part of a program to further document the chemistry of CN on transition metal surfaces we have studied the decomposition of dimethyltetrazine on Pt(111). Products of the decomposition of dimethyltetrazine are H₂, N₂, HCN, C₂N₂ and small amounts of CH₃CN. Most of the methyl groups (>90%) are totally dehydrogenated leaving residual carbon on the surface. At low coverage the initial decomposition is CH bond cleavage. At higher coverage direct production of molecular N₂ at ~30°C is observed as the initial decomposition mode. Pretreatment of the Pt with H₂, shifts the high coverage decomposition to higher temperatures. Changes in the decomposition with coverage is explained as due to a change in bonding geometry. We suggest that at low coverage the molecular plane is parallel to the surface with the methyl groups in proximity to the surface, while at high coverage the molecule bonds on edge possibly through two adjacent nitrogens.     

Proton NMR in the one-dimensional conductor K2Pt(CN) 4Br0.33H2O

The orientation of the proton-proton vectors of the water molecules in the quasi-one-dimensional conductor K₂Pt (CN) ₄Br_0.33H₂O have been determined. Temperature dependent rotations of the water molecules have been observed.     

Electron scattering on centrosymmetric molecular dianions Pt(CN)4(2-) and Pt(CN)6(2-)

Electron scattering on stored Pt(CN)2-4 and Pt(CN)2-6 centrosymmetric molecular dianions has been performed at the electrostatic storage ring ELISA. The thresholds for production of neutral particles by electron bombardment were found to be 17.2 and 18.7 eV, respectively. The relatively high thresholds reflect the strong Coulomb repulsion in the incoming channel as well as a high energetic stability of the target electrons. A trianion resonance was identified with a positive energy of 17.0 eV for the Pt(CN)2-4 square-planar complex, while three trianion resonances were identified for the Pt(CN)2-6 octahedral complex with positive energies of 15.3, 18.1, and 20.1 eV.     

Lattice anharmonicities in K2Pt(CN)4Br0.3 · 3H2O

A difference of about 7 per cent between the velocities of first and zero sound has been observed in K₂Pt(CN)₄Br_0.3 · 3H₂O for longitudinal waves travelling along the direction of the Pt chains. Large temperature and pressure derivatives of the elastic module c₃₃ have been found.     

Identification of a mixed-valence triclinic product formed by the partial bromination of K2Pt(CN)4

X-ray diffraction measurements as well as the results of chemical analysis are presented as evidence that the partial bromination of K₂Pt(CN)₄ results in at least two products which differ in their structural features and stoichiometry. Triclinic crystals grow from acidic aqueous solution whereas the known tetragonal phase grows from basic solutions. The Pt-Pt spacing is 2.95 Å in the triclinic phase as compared to a Pt-Pt spacing of 2.88 Å in the tetragonal phase.     

Coupling of the CDW and the water mode in K2Pt(CN)4Br0.3⋯3.2H2O

In previous work we have observed the amplitude mode of the charge density wave (CDW) in K₂Pt(CN)₄Br_0.3?3.2H₂O (KCP) by means of Raman scattering. New measurements made on deuterated material, K₂Pt(CN)₄Br_0.3?3.2D₂O (KCP1), show the same mode but shifted from 44 to 38 cm_?1, maintaining the symmetry properties and temperature dependence of frequency and linewidth. This considerable isotope effect is interpreted in terms of a coupling of the CDW with the water stretching mode, which by the deuteration is shifted from 3494 cm^?1 in KCP to 2560 cm^?1 in KCP1 according to the change in atomic mass. Both of these modes exhibit A₁(z) symmetry. At 5 K the resulting decoupled frequency of the CDW amplitude mode is 57 cm^?1, and the coupling energy about 140 cm^?1. A discussion of the temperature dependence of various important quantities is given. The present results show that the water molecules, which are located in between the Pt chains are strongly involved in the eigenvector of the CDW amplitude mode.     

Adsorption and decomposition of H2O on a K-covered Pt(111) surface

The adsorption of H₂O on clean and K-covered Pt(111) was investigated by utilizing Auger, X-ray and ultra-violet photoemission spectroscopies. The adsorption on Pt(111) at 100–150 K was purely molecular (ice formation) in agreement with previous work. No dissociation of this adsorbed H₂O was noted on heating to higher temperatures. On the other hand, adsorption of H₂O on Pt(111) + K leads to dissociation and to the formation of OH species which were characterized by a work function increase, an O 1s binding energy of 530.9 eV and UPS peaks at 4.7 and 8.7 eV below the Fermi level. The amount of OH formed was proportional to the K coverage for θ_K > 0.06 whereas no OH could be detected for θ? 0.06. Dissociation of H₂O occurred already at T = 100 K, with a sequential appearance of O 1s peaks at 531 and 533 eV representing OH and adsorbed H₂O, respectively. At room temperature and above only the OH species was observed. Annealing of the surface covered with coadsorbed K/OH indicated the high stability of this OH species which could be detected spectroscopically up to 570 K. The adsorption energy of H₂O coadsorbed with K and OH on Pt(111) is increased relative to that of H₂O on Pt. The work function due to this adsorbed H₂O increases whereas it decreases for H₂O on Pt(111). The energy shifts of valence and O1s core levels of H₂O on Pt + K as deduced from a comparison of gas phase and adsorbate spectra are 2.8–4.2 eV compared to ≈ 1.3–2.3 eV for H₂O on Pt (111). This increased relaxation energy shift suggests a charge transfer screening process for H₂O on Pt + K possibly involving the unoccupied 4a₁ orbital of H₂O. The occurrence of this mode of screening would be consistent with the higher adsorption energy of H₂O on Pt + K and with its high propensity to dissociate into OH and H.     

Influence of hydrostatic pressure on the plasma reflection edge of K2Pt(CN)4Br0.3 · 3H2O

A blue shift of the optical plasma reflection edge of the one-dimensional metal K₂Pt(CN)₄Br_0.3 · 3H₂O under hydrostatic pressures up to 7.2 katm has been observed. The shift (75 cm^?1katm^?1±30%) is interpreted as an increase of the plasma frequency and is correlated with the changes in the interatomic distances under pressure.     

Study by X-ray crystallography and Mössbauer spectroscopy of CeFe(CN)6·5H2O and GdFe(CN)6·4H2O

From the X-ray crystallographic point of view, the environment of Fe in CeFe(CN)₆·5H₂O has higher octahedral symmetry than that in GdFe(CN)₆·4H₂O. The single Mössbauer absorption peak for CeFe(CN)₆·5H₂O indicates that the three t_2g levels are close to each other in energy due to a near-perfect octahedral Fe(CN)₆ unit. The doublet peak of of GdFe(CN)₆·4H₂O can be explained by assuming that the three t_2g levels are widely separated as a result of the distortion of the octahedral Fe(CN)₆ unit.     

Anisotropic thermopower in the quasi-one-dimensional conductor K2Pt(CN)4Br0.3·3H2O

The temperature-dependence of the Seebeck-coefficient has been measured along and perpendicular to the direction of high conductivity on single crystals of K₂Pt(CN)₄Br_0.3·3H₂O. This material has previously been shown to behave like a one-dimensional metallic conductor at optical frequencies. The thermopower is small but distinctly anisotropic and of negative sign.